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  • Wiley-Blackwell  (570)
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  • 1
    Electronic Resource
    Electronic Resource
    Origin of apparent swain-schaad deviations in criteria for tunneling (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 361-366 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Vibrational analysis calculations have been conducted in an effort to understand the origins of apparently anomalous relationships reported between kH/kT and kD/kT kinetic isotope effects. The actual isotopic substitutions used in previous work are more complex than a simple comparison between H/T and D/T isotope effects. The relationships between the actual isotope effects determined in these studies amount to tests of two standards of conventional isotope effect theory, the rule of the geometric mean (or the lack of isotope effects on isotope effects) and the Swain - Schaad rule. Model calculations illustrate the importance of violations of the rule of the geometric mean (over Swain - Schaad deviations) arising in models that incorporate both explicit reaction-coordinate coupling of two isotopic sites and reaction-coordinate tunneling. Implications for experimental studies of turneling using apparent Swain - Schaad exponents are discussed.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040607
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  • 2
    Electronic Resource
    Electronic Resource
    The schiff base between pyridoxal-5′-phosphate (PLP) and hexylamine. Formation of the unprotonated form of the imine by reaction of the unprotonated PLP and free amine (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 372-380 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Electrochemical and spectrophotometric studies of the pyridoxal-5′ -phosphate hexylamine Schiff base (PHSB) in strongly basic media were carried out. The equilibrium constant and rate constants of the formation and hydrolysis of the unprotonated imine from the unprotonated pyridoxal-5′ -phosphate and free amine were determined by linearscan cyclic voltammetry. The acid-base dissociation constant of the imine group was obtained by spectrophotometric measurements. Absorption bands for the free imine and the conjugate acid are characterized by a log-normal distribution. These studies are for general application to the quantitative characterization of non-enzymatic model compounds.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040609
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  • 3
    Electronic Resource
    Electronic Resource
    Factors affecting the reversibility of the electrophilic step in olefin bromination. The case of 5H-dibenzo [a, d] cycloheptene (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 387-398 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The tendency of ion-pair intermediates generated by treatment of trans-10-bromo-10, 11-dihydro-11-hydroxy-5H-dibenzo [a, d] cycloheptene (5) with HBr to release Br2 giving 5H-dibenzo [a, d,] cycloheptene (1) has been evaluated. The observed 9:1 ratio between ion-pair collapse to the trans-dibromide (7) and Br2 release to olefin 1, as compared with the 3:7 ratio found with the 5H-dibenz [b, f] azepine-5-carbonyl chloride system, has been interpreted as indicating a much lower propensity for reversibility of ion-pair formation in the bromination of 1. The structural parameters obtained by x-ray diffraction of the dibromide 7 show for this compound an angle strain at C-10 and C-11 analogous to that found in trans-10, 11-dibromo-10, 11-dihydro-5H-dibenz [b, f,] azepine-5-carbonyl chloride. Olefin 1 adds Br2 in 1,2-dichloroethane at 25°C according to a third-order rate law, with k3 = 30(3) M-2 s-1. About 10% of 5-bromo-5H,-dibenzo [a, d] cycloheptene, arising from collapse of a dibenzo [a, d] tropylium ion intermediate (9), is formed in this reaction besides the expected trans-dibromide 7. Spectral evidence for the formation of 9, arising by rearrangement of a first formed weakly bridged ionic intermediate in the reaction of 1 with Br2, is presented. the results obtained with the 5H-dibenzo [a, d,] cycloheptene system have been rationalized on the basis of the formation of very weakly bridged intermediates having a much lower tendency to revert back to olefin and Br2 with respect to the corresponding symmetrically bridged bromonium ions.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040611
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  • 4
    Electronic Resource
    Electronic Resource
    SRN1 Reactions of nucleophiles with radical clocks: Rate constants for some radical-nucleophile reactions (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 404-412 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Treatment of o-3-(but-3-enyloxy)benzene (1) with anions -SPh, -PPh2 and -PO(OEt)2 in ammonia, acetonitrile or dimethyl sulphoxide gives both direct substitution and cyclized products. The formation of the latter confirms the intermediacy of the o-3-(but-3-enyloxy)phenyl radical (2) and confirms that the reactions follow the SRN1 pathway. The kinetics of the coupling of the aryl radical 2 were determined by comparison with the known rate of cyclization of 2. Similar experiments with the neopentyl bromide, 6-bromo-5,5-dimethylhex-1-ene, and its 3-oxa derivative also gave cyclized and uncyclized products, thus confirming that the SRN1 mechanism applies to the reactions. Approximate rate constants for the coupling of a neopentyl radical with the nucleophiles -SPh and -PPh2 have thus been determined.
    Additional Material: 5 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040703
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  • 5
    Electronic Resource
    Electronic Resource
    Short Communications. Biphenyl-3,4′-Dinitrene (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 459-462 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Photolysis of biphenyl-3,4′ -diazide in a rigid glassy matrix at 77 K yields quintet state biphenyl-3,4′ -dinitrene with zero-field splitting parameters of |D/hc- = 0.153 cm-1 and |E/hc| = 0.019 cm-1 determined by electron spin resonance spectroscopic studies. Curie plot studies are consistent with assigning the quintet to be the ground state in this species. This finding confirms qualitative connectivity-based predictions for this general connectivity type of openshell system, and is in qualitative agreement with spectral INDO-CI computational predictions for both planar and twisted geometries of the dinitrene.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040708
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  • 6
    Electronic Resource
    Electronic Resource
    The α-bromocyclohexanone ring: The relationship between its conformation and the ultraviolet and infrared absorption of the keto group (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 420-438 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A series of diterpenoids whose C-ring was an α-bromocyclohexanone showed a discontinuity in the relationship between the Br—C—C=O torsion angle and the Br…O contact distance at about 50°, below which the Br…O distance remained effectively constant at about 3.0 Å. This effect is caused by the non-compressibility of the van der Waals radii of the Br and O atoms. Analysis of the parameters for all compounds in the Cambridge Data File containing an α-bromocyclohexanone ring confirmed this phenomenon. Once the Br…O distance approaches 3.0 Å, the Br—C—C and C—C=O angles are forced open to accommodate the Br…O compression strain as the torsion angle is further reduced. An approximate value of the Br…O distance, d in Å, can be estimated for any torsion angle, ø, by the empirical equation d2 = R - S cos φ + T cos2 φ where R = 11.73, S = 3.62 and T = 0.75. There is a linear relationship between the Br…O separation and both the infrared vibration frequency v and the ultraviolet absorption wavelength δmax of the C=O group: the greater the distance, the lower is the frequency v and longer the wavelength δmax. Thus measurement of the ultraviolet and infrared spectra of a compound containing an α-bromocyclohexanone system can yield information about the Br…O distance and hence the conformation of the six-membered ring. There are also systematic trends in the effect on the spectroscopic properties of the keto group that accompany changing the halogen from fluorine through to iodine. When the halogen is axial, its electronegativity has a maximum influence on the C=O vibration frequency by a ‘through-bond’ inductive process. When the halogen is equatorial, it exerts a steric compression on the non-bonded electrons in the 2p orbital of the oxygen atom. This enhances hybridization to the sp2 state and this simultaneously strengthens the C=O bond and enlarges the n → π* energy gap. This ‘through-space’ influence is simply a process of steric compression, dependent on the size of the halogen atom. It accounts well for all of the systematic trends in both infrared and ultraviolet frequencies of the C=O group when the halogen is equatorial and eclipses the C=O bond.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040705
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  • 7
    Electronic Resource
    Electronic Resource
    Masthead (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991) 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040801
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  • 8
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    Treatment of electrostatic effects within the molecular mechanics method. 3. Double bonds and conjugated systems (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 467-478 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The previously developed general method (IDME), which includes calculation of electrostatic and solvation effects on conformational equilibria, has been extended to molecules containing double bonds and conjugated systems. It was applied in connection with MM2 studies to the calculation of the charge distributions and dipole moments, and to the conformational energies of some β-substituted cyclohexenes, exo-methylenecyclohexanes and benzocycloheptenes. Conformational equilibria and dipole moments were well calculated. It was concluded that electrostatic interactions do not play a major role in determining conformational preferences in these compounds. On the other hand, the energies of the preferred conformations of the ten-membered ring in (E)-3β-acetoxy-5,10-seco-1(10)-cholesten-5-one are poorly calculated without the IDME procedure, and fairly well with it.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040802
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  • 9
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    Theoretical models for the reduction of arylmethyl halides by triorganotin hydridesPreliminary presentation of results at combined 45th Northwest-10th Rocky Mountain Regional Meeting of the American Chemical Society, Salt Lake City, UT, 13 June 1990. (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 485-491 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Two mechanistic pathways have been proposed for halogen atom transfer from the benzylic positions of halomethylarenes to triorganotin radicals. These are direct atom abstraction, which might involve an extremely polar transition state, and single electron transfer followed by bond cleavage. AM1 semi-empirical calculations have been utilized to model the rate-determining step of these processes. A wide range of related families of compounds have been studied, including substituted halomethylbenzes, selected halomethyl-substituted polycyclic aromatic hydrocarbons and oxygen- and nitrogen-containing chloromethyl-substituted heteroaromatic systems. Although these calculations are relatively simple, the present results corroborate the view that chlorine and bromine atom transfer from the benzylic position to triorganotin radicals involves a direct atom abstraction in the rate-determining step whereas reduction of the corresponding iodides proceeds via an electron-transfer mechanism.
    Additional Material: 8 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040804
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  • 10
    Electronic Resource
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    Ambident electrophilicity of alkyl aryl phosphates (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 516-522 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Hydrolysis of dimethyl aryl phosphates, (MeO)2P(O)OAr where the ArO group is derived from N-methyl-8-hydroxyquinolinium or 4-hydroxy-(N,N,N-trimethylanilinium) ion, proceeds with the cleavage of the P—OAr and Me—O bonds. The effect of external factors on the relative contributions of those two reaction pathways was studied. The increase in temperature favours, owing to the difference in activation entropies, the reaction at carbon. The addition acetone to the aqueous reaction medium slows down the reaction at carbon more rapidly than the reaction at phosphorus, indicating higher hydration requirements of the dealkylation transition state. In water-triffuoracetic acid mixtures only the P—OAr bond cleavage is acid catalysed; the catalytic effect is superimposed on the general, decelerating medium effect. Specific nucleophilic catalysis was demonstrated for the Me—O bond fission by thiosulphate ion (ca 60-fold acceleration), and for the reaction at phosphorus by fluoride ion (ca 500-fold acceleration).
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040807
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  • 11
    Electronic Resource
    Electronic Resource
    Masthead (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991) 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040901
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  • 12
    Electronic Resource
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    Short communication. Structural studies of bromine adducts of 1,2- and 1,4-bis(phenylseleno)benzenes. Evidence for the (TB, MC) structure of 1,2-(PhSeBr2)2C6H4 (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 523-525 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 1,2-Bis(phenylseleno)benzene reacted with equimolar amount of bromine to yield a trigonal bipyramidal (TB) adduct, in contrast to the molecular complex (MC) structure of selenanthrene with bromine. Whereas the structure of 1,4-(PhSeBr2)2C6H4 was (TB, TB) at the two selenium atoms, that of the 1,2-derivative was demonstrated to be (TB, MC), the first example of an MC structure of PhSeC6H4Y with Br2 [Y = 2-PhBr2Se(TB)]. Both electronic and steric effects of the group must play an important role in MC formation. Rapid structural exchange in this tetrabromide, (TB, MC) ⇄ (MC, TB), was also observed.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040808
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  • 13
    Electronic Resource
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    Conformations of pentan-1-OL, hexan-1-OL and their thio analogues (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 536-540 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Empirical linear relationships between standard Gibbs free energy, ΔG298°, and parachor and between standard entropy, S298°, and parachor are presented for the aliphatic series H(CH2)nX with X = CH3, CH=CH2, C≡CH, Cl, OH, SH, SCH3 and SC2H5. In the series with X = OH and SH significant deviations of points for n = 4 and 5 are observed. INDO quantum-chemical analysis points to the formation of cyclic conformers of butan-1-ol, pentan-1-ol and their corresponding thio analogues. The formation energy of the cyclic conformers (higher for thiols than for alcohols) is proportional to the sum of deviations from the linearity of the points under discussion.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040903
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  • 14
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    Review commentary. Solvation, the electrophilic driving force of ionic bromination of ethylenic compounds. The addition-solvolysis analogy (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 527-535 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Solvation is the main driving force of electrophilic bromination since it is impossible to obtain a bromonium ion from bromine and alkene in the gas phase, whereas it is a very fast reaction in solution. The role of a protic solvent in this addition was investigated experimentally by extended Winstein-Grunwald relationships, kinetic solvent isotope effects and R, the rate ratios in two solvents of similar ionizing powers but different nucleophilicities. It is shown that electrostatic medium effects and electrophilic assistance to bromide ion departure are the main rate-determining factors of the reaction. These two contributions are roughly independent of the double bond substituents. Nucleophilic solvent assistance to positive charge development is also found; however, it provides only a small acceleration, the magnitude of which depends on alkene structure. This nucleophilic solvent involvement is annulled when crowded substituents inhibit approach of the solvent to the cationic part of the transition state or when positive charge is delocalized by conjugated electron-donating groups. These several solvent roles are identical in nature and in magnitude with those observed in heterolytic solvolysis. In halogenated solvents, the driving force of bromination arises from catalysis by a second bromine molecule which assists heterolysis of the bromine-bromine bond, leading to the bromonium-tribromide ion pair. Similar halogen catalysis occurs also in some solvolyses. Finally, return is also found in both reactions; reversible formation of bromonium ions is observed when their nucleophilic attack, the productforming last step, is made energetically difficult either by steric inhibition or by poor nucleophilicity of the trapping nucleophiles. Similarities and differences between electrophilic bromine addition and limiting solvolysis are discussed in terms of respective intermediate stabilities and heats of formation.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610040902
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  • 15
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    Photochemical transformations. 49.For Part 48, see Ref. 1. Solvent effects on singlet ionic and triplet photoreactions of some bridged polycyclic chlorides: Comparison with ground-state reactionsDedicated to Professor Kurt Schaffner on his 60th birthday. (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 541-546 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Direct irradiations of 2-chloro-6,7 : 8,9-dibenzotricyclo [3.2.2.02,4] nona-6,8-diene (5-Cl) were conducted in cyclohexane, acetic acid and wet acetonitrile. The products are 1-methylfluoranthene (8) in all three solvents, the allylic chlorides 8-chloro-7-methylene-2,3 : 5,6-dibenzobicyclo [2.2.2] octa-2,5-diene (3-Cl) and 7-chloromethyl-2,3 : 5,6-dibenzobicyclo [2.2.2] octa-2,5,7-triene (2-Cl) in acetic acid and acetonitrile and solvolysis product amides in wet acetonitrile. Compound 5-Cl had previously been shown to be the product of triplet sensitization of 2-Cl and 3-Cl, so that the singlet reaction reverses that of the triplet. The formation of 8 from 5-Cl was quenched with piperylene, whereas that of the allylic chlorides was not. Quantum yields of products and singlet lifetimes in the three solvents were measured. The solvent effects are discussed. Deuterium-labeling results on the formation of 5-Cl from 2-Cl and of 8 from 5-Cl are reported.
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    URL: http://dx.doi.org/10.1002/poc.610040904
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  • 16
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    Steric effects of alkyl groups: A ‘cone angle’ approach (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 611-617 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A ‘cone angle’, θR, is defined for an alkyl group (R), which is proposed as a measure of the steric effect exerted by the group. The θR values for some 57 groups have been calculated mathematically by constructing the corresponding Corey - Pauling - Koltun molecular models. The variation in θR with the bulk of a group is satisfactory. However not all types of alkyl groups can be accommodated in this cone angle approach. In the case of the groups for which the cone angle can be measured, correspondences between θR and the two existing steric parameters (Taft's Es scale and Dubois' E′s scale) have been established. For some 23 alkyl groups θR has been used to explain the variation in the rate constant of the base-catalysed hydrolysis of RCOOEt with the nature of R. Our studies independently substantiate the basic assumptions of the Taft - Ingold hypothesis that, while the base-catalysed ester hydrolysis is mostly controlled by the steric and electronic effects of the R group, the acid-catalysed one is controlled almost solely be the steric effect.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610041005
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  • 17
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    Effect of hexadecyltrimethylammonium bromide micelles on the hydrolysis of substituted benzoate esters (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 13-18 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The alkaline hydrolysis of nine para-substituted phenyl esters of para-substituted benzoates was studied in the presence and absence of hexadecyltrimethylammonium bromide (CTAB) micelles. The second-order rate constant for alkaline hydrolysis in CTAB micelles (K2m) was calculated using a pseudo-phase ion-exchange model. the substituent effect on the second-order rate constant in the aqueous phase (k2w) and on k2m was analysed using Hammett's σ ρ plots. The calculated ρ values in micelles were 0.8 unit. larger than those in water. The effective low dielectric constant in the micellar surface may be responsible for the general effect of increasing the sensitivity of reactions to the electronic effect of substituents in micelles.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040104
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  • 18
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    1,5-Sigmatropic shifts of bromine over a cyclopentadiene ring (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 31-47 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The intrinsic mechanism of circumambulatory rearrangements of 5-bromo-5-methyl-1,2,3,4-tetramethoxycarbonylcyclopentadiene, 5-bromo-1,2,3,4 5-pentamethoxycarbonylcyclopentadien and 5-bromo- 1,2,3,4,5-pentaphenylcyclopentadiene due to sigmatropic shifts of bromine over the cyclopentadiene ring was proved, using the dynamic 13C and 1H NMR technique, to be governed by successive intramolecular 1,5-sigmatropic shifts. Semi-empirical AM1 and MINDO/3 calculations of reactions paths performed for fluoro-, chloro- and bromocyclopentadienes are in accord with the conclusion of a preference for a 1,5- over a 1,3-shift reaction path of halogen migration over a cyclopentadiene system. Intramolecular 1,5-sigmatropic shifts of chlorine in 5-chloro-5-methyl-1,2,3,4-tetramethoxycarbonylcyclopentadiene with the free energy barrier of ΔG298+ = 26.1 kcal mol-1 were studied using 1H NMR spectroscopy.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610040107
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  • 19
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    Masthead (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991) 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040201
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  • 20
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    Specific micellar rate effects on unimolecular decarboxylation and cyclization (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 71-76 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion and cyclization of o-3-halopropyloxyloxyphenoxide ion (PhY7) (Y = Br, I) are accelerated by aqueous cationic and zwitterionic micelles. For cationic micelles the rate enhancements (kM′kW′) increase with increasing bulk of the surfactant head groups but are largest for decarboxylation. There are good linear free energy relationships between micellar effects on these reactions and the plots of log kM′ for cyclization against log kM′ for decarboxylation have slopes of 0.46 and 0.64 for Y = Br and I, respectively. Surfactants that have twin tail or tri-N-alkylbenzyloxy head groups do not fit the relationship. Despite mechanistic differences between decarboxylation and cyclization (an intramolecular SN2 reaction), similar factors control micellar effects on these reactions. In the absence of surfactant there is only a qualitative relationship between free energies of activation of cyclization and decarboxylation in different solvents.
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    Aromaticity as a multi-dimensional phenomenon (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 163-169 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The classification of aromaticity criteria is studied. New aromaticity criteria are developed and discussed. A statistical analysis of such criteria is presented and compared with a similar analysis of the recent literature. It is shown that aromaticity is at least a two-dimensional phenomenon. The classification of the compounds depends on the type of criteria included in the analysis.
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    AB initio MO study of the halogen cation basicities of some organic bases (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 177-191 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Hartree - Fock calculations were performed to investigate the structure and relative stabilities of complexes between halogen cations and first- and second-row bases. It is shown by means of both a qualitative perturbation molecular orbital treatment and a topological analysis of the electronic charge density that second-row bases present enhanced halogen cation basicities compared with first-row bases. In this respect the results predict that although the fluorine cation basicity of water is smaller than its proton affinity, the fluorine cation basicities of SH2, thioether, phosphine and trimethylphosphine are considerably higher than their proton basicities. Similarly, phosphine and trimethylphosphine should have chlorine cation basicities greater than their proton affinities, in contrast with ammonia and trimethylamine. The results also show that fluorine and chlorine cation basicities are more sensitive to methyl substitution than proton affinities.
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    Micellar effects on the alkaline hydrolysis of N-alkyl-4-cyanopyridinium ions. An example of micelle-induced regiochemical selectivity (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 207-216 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rates of the reaction of N-alkyl-4-cyanopyridinium (RCP) ions [alkyl = CH3 (MCP), n-C4H9 (BCP), n-C8H17 (OCP), n-C12H25 (DCP)] with OH- ion in water are independent of chain length and RCP concentration up to 0.001 M. The ratios of the concentrations of the reaction products, N-alkyl-4-pyridone (P) and N-alkyl-4-carboxamidopyridinium (A), P/A, are similar for all substrates. In water the P/A ratios increase with pH, reaching a plateau value of ca 2 at pH ≥ 13. Added salts slightly decrease the reaction rate and do not affect the P/A ratios. In aqueous solutions, addition of dioxane increases both rates and P/A ratios. Micelles of hexadecyltrimethylammonium bromide (cetyltrimethylammonium bromide; CTAB) and 3-(N,N-dimethyl dodecyl ammonium)propane-1-sulphonate (SDP) catalyse the alkaline hydrolysis of OCP and DCP and increase the yield of pyridone with BCP, OCP and DCP. In micellar CTAB and SDP the attack of OH- on RCP occurs almost exclusively at the 4-position of the pyridinium ring. Sodium dodecyl sulphate micelles inhibit the reaction and produce small changes in the P/A ratio. The regiochemical selectivity produced by micelles, leading to a preference for the attack of OH- at the pyridinium ring, was rationalized in terms of the low effective dielectric constant at the micelle-water interface.
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    Structure, topological electron density analysis and aromaticity of 4-heterosubstituted methylenecyclopropenes: X= X=CH2, NH, O, SiH2, PH and S (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 242-250 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The structures of a series of heterosubstituted methylenecyclopropenes and methylenecyclopropanes were optimized at the HF/6-31G* level. All methylenecyclopropenes are planar except for the silicon analogue, which is bent at both C-3 and Si. The planar silicon structure is a transition state. The relative aromaticity of these compounds were evaluated using the C-1—C-3 bond length, the integrated charge on C-3 and the heteroatom, and delocalization energy. Second-row systems have slightly larger delocalization energies than their first-row counterparts, owing to their larger polarizability. Using these criteria, methylenecyclopropene and the silicon analogue are not aromatic and the N, O, P and S analogues are moderately aromatic. The planar silicon analogue is antiaromatic and bends from planarity to reduce this antiaromaticity.
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    Kinetics and mechanism of the alcoholyses of ethyl fluorosulphate (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 251-259 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The alcoholyses of ethyl fluorosulphate in methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl and n-pentyl alcohols in the initial concentrations and temperature ranges of (1-10) 10-3 M and - 15.00 to 22.00 °C, respectively, were studied. The reactions follow pseudo-first-order kinetic laws up to at least ca 75 per cent ester conversions. At 10°C the experimental rate constant for ethanolysis is similar to that of ethyl triflate but much smaller than for its hydrolysis. The reactivity of the ester is ca 104-105 times higher than that of alkyl halides and benzenesulphonates. In alcohol-benzene and alcohol-acetone mixtures the ΔH≠ and ΔS≠ activation parameters are only slightly affected by the media composition. The effect of the ‘lyate ion’ on the kinetics of these reactions were investigated and the effects of changes in the dielectric constant of the media were evaluated. It is concluded that the alcoholyses of the ester fulfil the requirements for predominant SN2-type mechanism, where the reaction ground states are more polar than the corresponding transition states.
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    NMR study of polycrystalline azoles (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 271-276 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Polycrystalline pyrazole, imidazole and benzotriazole were investigated by 1H and 2H NMR spectroscopy. In addition to the usual broad line of the 1H NMR signal due to the dipole-dipole interaction, a significantly narrowed component (δν = 1-2 kHz) of a complex triplet structure was detected in all three compounds. To interpret this observation, a mechanism with virtually complete averaging of the dipole-dipole interaction for a small part of the sample protons is suggested. The mechanism is based on proton interaction with the unpaired electron of a short-lived radical of the type Az· or (H-Az·-H).
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    Gas-phase acidity of some α-keto aldoximes: Experiment and theory (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 285-292 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Experimental gas-phase acidities of 2-oxopropanaloximes, XCH2COCH=NOH (X = H, CH3S, CH3SO, CH3SO2; compounds 1-4), were determined by Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. The values are δ Gacid° = 1401, 1381, 1360 and 1351 kJ mol-1 for 1, 2, 3, and 4, respectively. Molecular orbital calculations using the semi-empirical AM1 method provided information on the geometry and relative energy of neutrals species 1-4 and their conjugate bases, together with charge distributions and entropies of deprotonation. It is demonstrated that the proton abstraction occurs preferentially at the oxime function; the formation of an enolate as a conjugate base is unfavourable by 70-140 kJ mol-1. The large variation of the gas-phase acidities for 1-4 is explained in terms of the field/inductive empirical substituent constant σF. The variation of solution acidities appears to be comparatively strongly attenuated. This attenuation is attributed mainly to charge delocalization in the anion, which was confirmed by charge density calculations.
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    Depurination of (dien)Pt(II) complexes of purine deoxyribonucleosides. Comparison with the effects of (dien)Pd(II) ion complexing (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 301-309 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: First-order rate constants for the hydrolytic depurination of 2′-deoxyinosine and its various (dien)Pt(II) ion complexes were measured over the acidic pH range. The rate profiles obtained indicate that the uncomplexed nucleoside and its N1-(dien)Pt(II) complex are depurinated via mono- and di-protonated species, whereas the N7-(dien)Pt(II) and N1, N7-di(dien)Pt(II) complexes exhibit significant spontaneous hydrolysis, which competes with a markedly retarded acid-catalysed reaction. Rate constants for the various partial reactions were calculated and the results were employed to explain the effects that (dien)Pd(II) ion exerted on depurination rates of the same compounds. Similar measurements were carried out with the N1, N7-di(dien)Pt(II) complex of 2′-deoxyadenosine in order to further the understanding of the previously reported rate-enhancing effect of (dien)Pd(II) ion on the depurination of 2′-deoxyadenosine.
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    Crystallographic studies of intra- and inter- molecular interactions. Part VI. Crystal and molecular structure of N,N′-dimethyl-N′-phenylsulphonylformamidine. Equalization of CN bond lengths in the amidine fragment as a result of substituent effects due to push-pull (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 331-335 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The crystal and molecular structure of N,N-dimethyl-N′-phenylsulphonylformamidine is solved by direct methods and refined to R = 0.041 for 935 reflections, Equalization of CN bond lengths in the amidine fragment is discussed in terms of a push - pull effect operating between the N,N-dimethyl and SO2Ph groups. A significant role of the substituent at the functional carbon on the degree of π-electron delocalization on the NCN fragment is interpreted by use of the HOSE model.
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    Orientation of benzoic acid and terephthalic acid on an alumina surface and their reactivities observed by infrared spectroscopy (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 346-352 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The orientation of benzoic acid and terephthalic acid on alumina was ascertained by IR measurements. Benzoic acid was chemisorbed as a carboxylate anion with a vertical conformation of the benzene ring (0 〈 θ 〈 1.0) and as a mixture of the anion and the acid (1.0 〈 θ). Terephthalic acid was chemisorbed as a mixture of monocarboxylate anion (hydrogenterephthalate) and dicarboxylate anion (0 〈 θ 〈 0.3), as the monocarboxylate anion with a vertical conformation of benzene ring (0.3 〈 θ 〈 1.0) and a mixture of the monocarboxylate anion and terephthalic acid (1.0 〈 θ). The carboxyl group of expected hydrogenterephthalate adsorbed through the carboxylate anion on alumina (θ = 0.8 : 0.37 mmol g-1 alumina) readily reacted with dimethyl sulphate to form monomethyl terephthalate selectively.
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    Influence of some steric and electronic effects on the mechanism of SNAr reactions in dimethyl sulphoxide (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 367-371 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The reactions of 1-chloro- and 1-fluoro-2,4-dinitrobenzenes with aniline and 4-methylaniline in dimethyl sulphoxide are not base catalysed. When the nucleophile is 2-methylaniline, the reaction of the fluoro compound is base catalysed whereas that of the chloro compound is not. The reactions of 1-bromo-2-fluoro- and 1-bromo-2-chloro-3,5-dinitrobenzenes with both aniline and 2-methylaniline are not base catalysed. These results indicate that, in certain cases, the incidence or absence of base catalysis in aromatic nucleophilic substitution (SNAr) reactions in dipolar aprotic solvents results from an interplay of steric and electronic factors as they affect the magnitude of the (k2 + k3 [B])/k-1 ratio.
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    Controlling factors determining addition and reduction products in the grignard reaction of substituted benzophenones, acetophenone and fluorenone with ‘EtMgBr’ (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 381-386 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Addition (Add.) vs reduction (Red.) products in the reactions of substituted benzophenones, acetophenone and fluorenone with ‘EtMgBr’ were compared in diethyl ether, 1,2-dimethoxyethane and tetrahydrofuran. Plots of log (Add./Red.) vs σ-values or the oxidation potentials of ketones showed characteristic upward curves depending on the polarity of the solvents. The results are discussed on the basis of both the combined effects of the stability of the monomeric radical ion pair and the dimeric radical ion pair derived from ketone and ‘EtMgBr’, and the well known Schlenk equilibrium of ‘EtMgBr’ in solutions.
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    The influence of solvents on the basicity of dipolar amines (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 63-67 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The relative basicities of six monosubstituted dipolar trimethylamines (XCH2NMe2) were determined in nine different solvents from potentiometric titration. Solvation effects on basicity variations were analyzed by the solvatochromic parameters (π*, α and β). Non-specific solvation of the substituted dimethylammonium ions is not reflected adequately by the solvent's dipolarity-polarizability constant, π*. A better representation of non-specific interaction between solvents and the monosubstituted dipolar trimethylammonium ions is gained from the product of π* and the solvent dipole moment (μ). For these ions where the dipolar substituents are separated from the reaction center by one methylene unit, there is an interaction between the charge of the ammonium ion and the dipolar substituent. As a result, solvent molecules must not only solvate the dipolar substituents, but must also gain access between the lines of force of the dipolar substituent and the charge. Bulky solvents, such as nitrobenzene, cannot access adequately the region between the dipolar substituents and the reaction center to accomplish similar solvation as less bulky solvents.
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    Kinetic study of the formation and rupture of stable tetrahedral intermediates. C—O, C—N and C—S bond formation (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 80-89 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Measured pseudo-first-order rate constants for intramolecular formation of tetrahedral intermediates from N-2-hydroxyethylphthalimide, N-2-aminoethylphthalimide and N-2-thioethylphthalimide at pH 〉 6 are reported. The reaction is specific and general base catalysed, with β(Brønsted) values 0·44, 0·52 and 0·52 respectively. From a plot of log kb (general base rate constants) vs γ′ [the affinities of EtXH (X = O, NH, S) toward the carbonylic carbon], β′nuc values of 0·01 (with OH- as specific base), 0·25 (with imdidazole as general base) and 0·27 (with HPO42- as general base were obtained). The observed relationships pxy = ϑβ′/ - ϑpKa = -ϑ(Brønsted)/ϑγ′ = 0·03 is supported by the predictions of an energy contour diagram, which, on extrapolation to a non-stable tetrahedral intermediate, predicts a late and slightly protonated transition state for the cleavage process. At pH 〈 3, these intermediates cleave to yield only the corresponding diacylimides. These reactions are general base and acid catalysed with β 〉 0·3 and α 〈 0·1. A fast equilibrium between the intermediate and its N-protonated (amide) form is reached. The general base rupture of the latter is faster than that of the corresponding non-protonated intermediate by a factor of ca 109 - 1010-fold.
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    Single-parameter versus dual-parameter correlation for radical reactions. Addition of bromine atoms to α-methylstyrenes (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 96-104 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A rigorous procedure was developed for measuring the relative rates of addition of bromine atoms to eleven substituted α-methylstyrenes (1-Y, with Y = CF3, NO2, F, CN, Cl, Br, CO2Me, Me, COMe, OMe and SMe). The reaction was run in tetrahydrofuran in the presence of HBr, O2 and dibutyl peroxyoxalate at 30°C. All products were derived from the YC6H4CMeCH2Br adduct radicals, which were immediately intercepted by O2. Correlation analysis of all the data confirmed the proposition that in the absence of measurable steric effects, the relative rates for radical additions can be correlated only by a dual-parameter equation and not by a single-parameter equation. Among various combinations of σ* and σx, the (σJJ* + σmb) combination yields the best correlation.
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    Methyl transfer from MeCo(III)Pc to thiophenoxide (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 90-95 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Transfer of the cobalt-bound methyl in MeCo(III)Pc to thiophenoxide ion was studied (H2Pc is the planar macrocyclic phthalocyanine; the cobalt is held in the center in this plane). In dimethylacetamide solution, the reaction is rapid, requiring stopped flow for the kinetics, and yielding MeSPh and Co(I)Pc- in good yield. The kinetics are not simple second order, but instead approach a constant rate at high [PhS-], attributed to the reversible formation of an inert complex with PhS- occupying the vacant octahedral site in MeCo(III)Pc, on the other side of the phthalocyanine plane from the methyl group. The kinetics allow the estimation of the equilibrium constant, K, and the SN2 rate constant, k, which at 25°C have values of ca. 9·4 × 103 l mol-1 and 1·8 × 104 l mol-1, respectively. Although these values are rough, the ratio k/K is firm at 1·91 ± 0·02 s-1; this is the limit of the rate at high [PhS-]. An alternative mechanism, which is entirely consistent with the kinetics, involves a rate-determining homolysis of the Co—S bond of the same complex. The mechanism is not favored because the product yields are high for a radical combination process and alternative chain processes are kinetically unacceptable. Further, the rate constant is about what would be expected from the reactivity of other nucleophiles in SN2 reactions. Further arguments in favor of the SN2 mechanism are presented. This transfer of the methyl group from Co to S is part of the possible analogy to the vitamin B12-promoted methionine synthesis in nature. The other step in the biological, enzymatic process is the transfer of methyl from the nitrogen of N-methyltetrahydrofolate to cobalt. An attempt to model this with the very reactive N-methyl-2,6-dichloropyridinium ion was unsuccessful; the reaction took an entirely different course, presumably initiated by electron transfer, but leading to substantial loss of Cl- from the pyridine. No more than 0.5% methyl transfer took place. This system does mimic well the complete natural enzymatic process.
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    Announcements (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 116-116 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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    URL: http://dx.doi.org/10.1002/poc.610070209
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    Masthead (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994) 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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    URL: http://dx.doi.org/10.1002/poc.610070301
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    Protonation studies on isomeric tetrafluoro-2,11-dithia[3·3]cyclophanes, tetrafluoro[2·2]metaparacyclophane and their corresponding non-fluorinated analogs. Comparison with fluorinated and non-fluorinated[2·2]cyclophanes: A stable ion and semi-empirical (AM1, PM3) investigation (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 105-115 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Protonation of tetrafluoro-2,11-dithia [3·3]paracyclophane and tetrafluoro-2,11-dithia [3·3]metaparacyclophane in high-acidity super acid media, namely FSO3H.SbF5 (1:1) ‘magic acid’-SO2CIF, gave their corresponding acidic disulfonium ions. Additional ring protonation to give a disulfonium-monoarenium trication was not observed. With the non-fluorinated 2,11-dithia [3·3] cyclophanes, the disulfonium ions can be ring protonated in equilibrium to give a dynamic disulfonium-monoarenium trication. Tetrafluoro [2·2]-metaparacyclophane is monoprotonated at the meta ring and gives a complex mixture of conformational isomers. Multinuclear magnetic resonance data on the cyclophane precursors and their derived cations are compared and analysed. The energies, conformations and charge distributions of the isometric fluorinated and non-fluorinated [2·2] - and dithia [3·3] cyclophanes were calculated by the AM1 and PM3 methods, respectively. In all but one case the cyclophane arenium ions predicted by theory to be energetically most favoured are those observed in solution under stable ion conditions. In agreement with experiment, the instabilities of S,S,C-cyclophane trications are also theoretically predicted.
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    Announcement (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. I 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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    URL: http://dx.doi.org/10.1002/poc.610070302
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    Kinetics and mechanism of electron-transfer reaction of aquothallium(III) and coordinated thallium(III). Oxidation of metol with thallium(III) in acid perchlorate medium (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 117-121 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The kinetics of oxidation of metol (N-methyl-p-aminophenol) by thallium(III) have been studied in a perchloric acid medium. This organic compound is employed extensively in photographic work. The stoichiometry of the reaction is represented by: C6H4(OH)NHCH3 + TIIII → C6H4(O)NCH3 + TII + 2H+The kinetics indicate the rate law: \documentclass{article}\pagestyle{empty}\begin{document}$$k' = \frac{{Kk\left[{metal} \right]}}{{1 + K\left[{metal} \right]}}$$\end{document} where k′ is the pseudo-first-order rate constant. The rate constants and thermodynamic parameters for the rate-limiting step have been evaluated.
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    The volume profile for a cheletropic reaction (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 178-180 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The volumes of activation and of reaction were measured for the reaction between sulphur dioxide and 2,3-dimethylbuta-1,3-diene to form 3,4-dimethylsulpholene. Values of ΔV and ΔV* are -33 and -35 cm3 mol-1, respectively in accordance with a concerted reaction having a very product-like transition state.
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    Secondary α-deuterium isotpe effect on the hydration of p-nitrobenzaldehyde (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 192-195 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The secondary α-deuterium isotpe effect on the equiliburium constant (KD/KH) for the hydration of p-nitrobenzaldehyde is 1·39 ± 0·05. The Kinetic secondary α-deuterium isotope effect (KD/KH) for the addition of water to p-nitrobenzaldehyde is 1·18 ± 0·07. This result demonstrates that there is extensive carbon-oxygen bond formation in the transition state for addition of water to the carbonyl group of p-nitrobezaldehyde.
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    Masthead (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994) 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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    URL: http://dx.doi.org/10.1002/poc.610070501
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    Mechanism of the hydride transfer reaction of leuco crystal violet with cyanomethylene acceptors (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 218-220 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In the hydride transfer of Leuco Crystal Violet to form the Crystal Violet, cation, the role of cyanomethylene acceptors was found to be essentially different from that of p-benzoquinones, both previously believed to act as π-acceptors in the same manner.
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    Competition between decarboxylation and isomerization in the C3H5O2+ energy surface. Justification of the experimental results by molecular orbital calculations on the solvated ions (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 221-226 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In constrast with recent molecular orbital calculations on the decarboxylation of O-protonated 2-oxetanone, this experimental work indicates that no decarboxylation of this cation occurs in sulphuric acid solution up to 150°C, but instead a clean isomerization to protonated acrylic acid takes place. Parallel theoretical work shows that the gas-phase model is too crude to account successfully for the experimental facts obtained in acidic media. However, the latter are well reproduced when the effect of the solvent is taken into account. The present findings do not necessarily invalidate the reaction mechanism currently accepted to explain the rate enhancement and change of stereochemistry accompanying the decarboxylation of 3,4-disubstituted 2-oxetanones under acid catalysis.
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    Enolimine and geminaldiamine forms in the reaction of pyridoxal phosphate with ethylenediamine. An electrochemical and spectroscopic contribution (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 227-233 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The enol-keto tautomerism of the Schiff bases formed by pyridoxal 5'-phosphate (PLP) with ethylenediamine (Etd) and ethylamine (Et) was studied by electrochemical and spectrophotometric methods. The spectroscopic results revealed differences between the two reactions, i.e. the enolimine/ketoenamine ratio observed in PLP-Etd mixture is higher than that of PLP-Et. The differences observed in the electroreduction mechanism and stability of the Schiff bases formed provide additional evidence for an unusually high concentration of enolimine in PLP-Etd in buffered aqueous solutions.The results are consistent with a cyclic structure such as that previously proposed by Robitaille et al. [J. Am. Chem. Soc. 111, 3034-3047 (1989)] on the basis of spectroscopic data. Protonation of the terminal amino group of the Etd moiety in the Schiff base involves the formation of the cyclic species. The low basicity of ethylenediamine favours the formation of similar concentrations of the enol and keto tautomers of the Schiff base at neutral pH. This behaviour, also observed in the Schiff base formed by PLP and polylysine, may be involved in coenzyme-protein linkages.
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    Number and structure of solvolysis intermediates. Part 3.For Part 2, see Ref. 1.SN1 solvolysis of 2,2-Dimethyl-1-(p-Methoxyphenyl)Propyl p-Nitrobenzoate: Mechanism of the common ion salt effects arising at the stage of the second ion-pair intermediate (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 234-243 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 2,2-Dimethyl-1-(p-methoxyphenyl)propyl p-nitrobenzoate (ROPNB) was subjected to solvolysis in phenol in the presence of tetrabutylammonium [carboxy-13C]-p-nitrobenzoate, in which the ‘common ion rate depression’ was confirmed to arise at the stage of the second ion-pair intermediate (Int-2). The unchanged substrate recovered at 46% reaction contained the isotopically labelled leaving group, indicating the occurrence of common ion exchange to the extent of 41-46%. In the solvolysis of the optically active substrate under identical conditions, the unchanged substrate was recovered with 51·4% racemization and ROPh was produced with slightly (1·56%) retained configuration, similarly to the solvolysis in the absence of the common ion salt. These isotope-tracer and stereochemical outcomes indicate that the common ion exchange in this solvolysis system should be attributable to the retentive nucleophilic attack on Int-2 by the common ion salt via a quadrupole (four-centre ion pair) transition state, accompanying the common ion rate depression, and they suggest that the special salt effect also should proceed by an analogous anion-exchange mechanism to that for the common ion effects.
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    Relative thermodynamic stabilities of the isomeric propenylbenzenes (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 256-258 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The relative thermodynamic stabilities of 2-propenylbenzene (allylbenzene), and the E and Z forms of 1-propenylbenzene were determined over the temperature range 50-170°C by chemical equilibration in DMSO solution with t-BuOK as catalyst. The values of the thermodynamic parameters ΔG⊖, ΔH⊖ and ΔS⊖ at 298·15 K for each isomerization reaction between the title compounds were evaluated.
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    Hydride transfer reactions of Michler's hydride with different π-accetors (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 309-315 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The hydride transfer reactions of 4,4′-bis(dimethylaminophenyl)methane (Michler's hydride) with p-benzoquinones were studied. The rate of formation of Michler's Hydrol Blue was followed spectrophotometrically. The second-order rate constants and the activation parameters were estimated. The formation of a charge-transfer complex was observed at low temperatures. Stable and unstable paramagnetic species formed during the reactions were assigned by ESR spectroscopy. The observed kinetic behaviour and the stoichiometry were in line with those previously obtained for the systems involving Leuco Crystal Violet, Leuco Malachite Green and Leuco Bindschedler's Green. Hence the reaction is considered to proceed according to the so-far accepted multi-step mechanism. However, when tetracyano-p-quinodimethane or tetracyanoethylene was used as a π-acceptor, a comparatively stable radical ion pair was formed as a result of a one-electron transfer, followed by the gradual formation of Michler's Hydrol Blue. The kinetic behaviour and the stoichiometry of the reaction were examined, together with the enhanced kinetic effects of added triethylamine. A modified mechanism for these systems is proposed. The role of cyanomethylenes was found to be essentially different from that of p-benzoquinones, and therefore π-acceptors are divided into two groups of cyanomethylenes and p-benzoquinones.
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    Vinyl ether hydrolysis. 30. Effect of β-carboxy and β-carbomethoxy substitution (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 316-322 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rates of hydrolysis of the vinyl ether functional groups of (Z)- and (E)-β-methoxyacrylic acid and (Z)-and (E)-β-methoxymethacrylic acid and their methyl esters were measured in aqueous perchloric acid solution. Additional rate measurements were also made for one substrate, (Z)-β-methoxymethacrylic acid, in buffer solutions down to pH 7, and a rate profile was constructed. The results show that the β-carboxy and β-carbomethoxy substituents produce strong rate retardations, ranging from 2000- to 25 000-fold, for both Z- and E-isomers in both the acrylic and methacrylic acid series. The rate profile for (Z)-β-methoxymethacrylic acid indicates that ionization of this substrate to the carboxylate ion form rises the rate of hydrolysis by a factor of 240. It is argued that this difference in reactivity of ionized and non-ionized forms of the substrate is due to conjugative and inductive effects of the substituents, rather than β-lactone formation as suggested in an earlier observation of the same phenomenon in a different system.
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    Masthead (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994) 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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    URL: http://dx.doi.org/10.1002/poc.610070701
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    Editorial (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. ii 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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    URL: http://dx.doi.org/10.1002/poc.610070702
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    Newer aspects of electron transfer in electrophilic Aromatic Nitration. Radical pairs as reactive intermediates by time-resolved spectroscopy (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 325-351 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Aromatic nitration is effectively carried out by the deliberate irradiation of the charge-transfer absorption band (hvCT) of the transient electron donor-acceptor complex [ArH, NO2Y+], where NO2Y+ represents common nitrating agents. Time-resolved spectroscopy demonstrates that the charge-transfer nitration effected in this way proceeds via an electron-transfer mechanism, in which the collapse of the reactive triad, [ArH+ ·, NO2·, Y] is critical to the formation of the Wheland intermediate. Comparative product analyses of toluene and anisole nitrations together with the ‘non-conventional’ products arising from NO+ catalysis, ipso attack and addition/elimination show that the activation process leading to charge-transfer (photochemical) nitration is indistinguishable from that leading to electrophilic (thermal) aromatic nitration.
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    Stereochemistry and kinetics of methoxide ion substitution in (Z)- and (E)-O-methylbenzohydroximoyl cyanides (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 352-358 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The stereochemistry and kinetics of the methoxide ion substitution reactions (Z)- and (E)-methylbenzohydroximoyl cyanidc [PhC(CN)=NOCH3] were investigated. The reaction of the (Z)-hydroximoyl cyanide with sodium methoxide in DMSO-methanol (9:1) solution at 44·8°C gives a mixture of methyl (Z)-O-methylbenzohydroximate [PhC(OCH3)=NOCH3] and the O-methyloxime of α-ketophenylacetamide PhC(CONH2)=NOCH3. The (E)-hydroximoyl cyanide undergoes methoxide ion-catalyzed isomerization to the E-isomer faster than it undergoes nucleophilic substitution. These observations were interpreted in terms of an addition-elimination mechanism in which the rate-limiting step is elimination of the nucleofuge (AN + DN#).
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    Kinetic studies on the aminolysis of 1-(trimethylsilyl)ethyl arenesulphonates in acetonitrile and methanol (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 359-363 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Nucleophilic substitution reactions of 1-(trimethylsilyl)ethyl arenesulphonates with anilines and benzylamines in acetonitrile and methanol at 65·0°C were studied. The cross-interaction constants, ρXZ, between substituents in the nucleophile (X) and leaving group (Z) are relatively small (0·10 for XC6H4NH2 in MeCN) but similar to those for other SN2 processes at a secondary carbon atom. This provides further evidence for an approximately constant, loose SN2 transition state at a secondary carbon regardless of the size of the Cα substituent. The transition-state variations with substituents X and Z are in accord with that expected from the positive ρXZ value observed: a stronger nucleophile and/or nucleofuge leads to an earlier transition state, i.e. a lower degree of bond making and breaking.
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    Molecular mechanics calculations (MM3) on alkyl iodides (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 420-430 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The MM3 force field has been extended to cover alkyl iodides. Structures, vibrational spectra and heats of formation have been well fitted to available experimental data. The experimental and ab initio conformational relationships are also satisfactorily reproduced.
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    Kinetics and mechanism of the imidazole-catalysed hydrolysis of substituted N-benzoylimidazoles (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 431-436 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Imidazole (Imz)-catalysed hydrolysis of benzoate esters proceeds via the intermediate formation of N-benzoylimidazoles. This paper considers the second step of this reaction, viz., Imz-catalysed hydrolysis of N-(4-X-benzoyl)imidazoles, X = CH3, H, Cl, CN and NO2, and N-(disubstituted benzoyl)imidazoles, 2-chloro-4-nitro, 2, 4-dinitro and 3,5-dinitro, in water-acetonitrile mixtures (10% or 14%, v/v, in organic solvent). On the basis of catalytic rate constants and the kinetic solvent isotope effect, it is shown that catalysis by Imz is of the general-base type. Unexpectedly, the hydrolysis of N-(2,4-dinitrobenzoyl)imidazole was found to be slower than that of N-(4-nitrobenzoyl)imidazole. It is shown that this reactivity order is due to a combination of a steric effect and stabilization of the reactant state due to a donor-acceptor interaction between the Imz moiety and the 2,4-dinitrophenyl ring.
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    Micellar catalysis of organic reactions. Part 35. Kinetic determination of the critical micelle concentration of cationic micelles in the presence of additives (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 437-441 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The critical micelle concentration of solutions of cetyltrimethylammonium bromide and of tetradecyltrimethylammonium bromides were determined by a kinetic method. This involved the determination of the rates of the hydroxydehalogenation of some activated aromatic substrates over a wide range of detergent concentrations. Measurements were made in solutions containing significant quantities of added hydroxyl ion and substrates which were themselves amphiphilic. Conventional methods cannot be applied with confidence to such systems. The effects of changing hydroxyl ion concentrations, added sodium bromide, changing the nature of the aromatic substrate (whether neutral or charged), the identity of the micellar counterion and the temperature were investigated. It was wound that added bromide or hydroxyl ions resulted in a lower CMC whereas increased temperature led to an increase in the CMC. The nature of the micellar counterion (Br, F, OH, SO4) had little effect on the CMC. The presence of a charged aromatic substrate led to a considerable lowering of the CMC, whereas the neutral aromatic substrate used showed very little effect.
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    Nucleophilic substitution reaction of benzyl bromide with N, N-Dimethylaniline: Significance of equilibrium cross-interaction constant (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 555-560 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Kinetic studies on the reversible reactions of benzyl bromides with N, N-dimethylanilines were carried out for both the forward (kf) and reverse (kr) directions. The equilibrium constants, K were calculated using the ratio kf/kr and the equilibrium cross-interaction constant, ρeXY was determined. The ρeXY value was shown to represent a maximum intensity of interactions between substituents X and Y through covalent bonds within a molecule. The normalized values of ρX (ρnuc) and ρXY indicate that in the transition state the fractional development or loss of polar and resonance interactions in the benzyl system are imbalanced or non-perfectly synchronized. In the forward reaction the fractional loss of resonance interaction becomes enhanced, whereas in the reverse reaction the fractional development of resonance interaction lags behind the corresponding changes of polar interactions by ca 45%.
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    Competing solvolytic substitution and elimination reactions via a common irreversibly formed ion-molecule pair intermediate (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 578-584 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The acid-catalysed solvolysis of 9-(2-phenoxy-2-propyl)fluorene in mixtures of water with acetonitrile or methanol at 25°C provides 9-(2-hydroxy-2-propyl)fluorene, 9-(2-propenyl)fluorene, and 9-(2-acetamido-2-propyl)fluorene or 9-(2-methoxy-2-propyl)fluorene, respectively. The overall kinetic deuterium isotope effects for the reactions of the hexadeuterated analogue 9-(1,1,1,3,3,3-2H6)-9-(2-phenoxy-2-propyl)fluorene in 90 vol.% acetonitrile in water were measured as (kEH + kSH)/(kED6 + kSD6) = 1·54 ± 0·05, which is composed of the isotope effect kSH/kSD6 = 1·4 ± 0·1 for formation of the substitution products and kEH/kED6 = 4·0 ± 0·2 for production of 9-(2-methoxy-2-propyl)fluorene. Similar isotope effects were measured in other solvent mixtures. The results strongly indicate a branched mechanism involving rate-limiting formation of a common carbocation-molecule pair (with a secondary isotope effect of 1·54), which either undergoes nucleophilic attack by a solvent molecule (with a secondary isotope effect of ca 1), or is dehydronated (isotope effect ca 2·8) by the leaving group or by the solvent. The ion-molecule pair shows very low selectivities. Thus, in 50 vol.% acetonitrile in water, an acetonitrile molecule is as efficient as a water molecule as a nucleophile towards the ion-molecule pair, kMeCN/kHOH ≥ 1 (ratio of second-order rate constants). The discrimination between methanol and water is anomalously small, kMeOH/kHOH = 0·7.
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    Directional hydrogen bonding in the MM3 force field. I (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 591-609 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The MM3 molecular mechanics program calculates a fair representation of hydrogen bonding interactions, but to improve the MM3 hydrogen bond potential, a directional term has been added to the hydrogen bonding function. The resulting total function was reoptimized. Comparisons of the hydrogen bonding potential functions from ab initio, the original MM3, the current MM3(92) force field and the reoptimized MM3 force field MM3(94) for a variety of C,N,O systems are described.
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    Gas phase thermolysis of N-(tert-butylthio)allylamine and N-(tert-butylthio)diethylamine (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 585-590 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The title sulphenamides were pyrolysed in a stirred-flow reactor at temperatures of 310-410°C, pressures of 8-15 Torr and residence times of 0·4-2 s using toluene as the carrier gas. N-(tert-Butylthio)allylamine formed 73 ± 4% isobutene, 23 ± 3% propene and N-allylthiohydroxylamine. The first-order rate coefficients for the formation of isobutene and propene, respectively, followed the Arrhenius equations kC4(s-1) = 1012·52 ± 0·36 exp(-163 ± 5 kJ mol-1 RT) and kC3(s-1) = 1010·99 ± 0·29 exp(-151 ± 4 kJ mol-1 RT) N-(tert-Butylthio)diethylamine gave 97 ± 1% isobutene, 1·9 ± 0·4% isobutane and N,N-diethylthiohydroxylamine. The first-order rate coefficients for isobutene elimination followed the Arrhenius equation k(s-1) = 1013·45 ± 0·24 exp(-164 ± 3 kJ mol-1 RT). The formation of the products is interpreted in terms of an elimination reaction with a unimolecular, four-centered, cyclic transition state. The reactivity of these sulphenamides was found to be much higher than that of previously studied alkyl or aryl tert-butyl sulphides and disulphides.
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    Reactions of 1,3-dihaloadamantanes with diphenylphosphide ions by the SRN1 mechanism. Competition between intermolecular and intramolecular electron transfer reactions (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 610-614 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The reactivity of 1,3-dihaloadamantanes with diphenylphosphide ions (Ph2P-) in liquid ammonia was studied. 1,3-Dichloroadamantane (1a), 1-bromo-3-chloroadamantane (1b) and 1,3-dibromoadamantane (1c) reacted with Ph2P- ions under photostimulation by the SRN1 mechanism. Irradiation of 1c without Ph2P- ions gave no reaction (〈5%). Three products were found: 1,3-bis(diphenyphosphinyl)adamantane (2), (3-X-1-adamantyl)diphenylphosphine (X = Cl, 3a; X = Br, 3b) and 1-adamantyldiphenylphosphine (4). Compounds 2 and 4 were formed by intramolecular electron transfer (ET) of the radical anion 3a-· (3b-·); whereas 3a (3b) were formed by intermolecular ET of this radical anion to the substrates. It was observed that the product distribution depends on the substrate and reaction conditions.
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    AB initio study of Diels-Alder reactions of 1,3,4-oxadiazole with ethylene, acrylonitrile, maleonitrile, fumaronitrile and 1,1-dicyanoethylene. Inverse order and ratio of endo/exo reactivity (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 634-640 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Transition structures for the Diels-Alder reactions of 1,3,4-oxadiazole with ethylene, acrylonitrile, maleonitrile, fumaronitrile, and 1,1-dicyanoethylene were located with ab initio molecular orbital calculations with the RHF/3-21G theoretical model. The activation energies were calculated by single-point calculations with the RHF/6-31G* and MP2/6-31G* theory levels on the RHF/3-21G geometries. Geometries of two transition structures and the corresponding reactants were generated at the MP2/6-31G* and the energies evaluated at the MP3/6-31G* level. The values obtained are comparable to those generated by MP2/6-31G*//3-21G. The asynchronicity of the transition structures follow the same trend as the corresponding all-carbon Diels-Alder reactions with buta-1,3-diene and cyclopentadiene, but the activation energies are almost in opposite correlation. This behavior is explained by the oxadiazole nitrogen lone pair repulsion with the π-orbitals of the nitrile groups.
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    Masthead (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994) 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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    URL: http://dx.doi.org/10.1002/poc.610071201
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    Effect of cyclic acetal-type cationic surfactants on the basic dehydrobromination reaction (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 646-651 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The reaction of 2-(p-nitrophenyl)ethyl bromide with hydroxide ion was studied at 50 and 25°C in the presence of cationic chemodegradable surfactants, i.e. [2-alkyl(1,3-dioxolan-4-yl)methyl] trimethylammonium bromides Ia-c (alkyl: a = n-C9H19, b = n-C11H23, c = n-C13H27). The kinetic data were interpreted with the pseudo-phase ion-exchange (PIE) formalizm at both temperatures. The results indicate that the major source of rate enhancement is the increased reactant concentration in the small micellar reaction volume. The surfactant stability in micellar conditions was probed by means of a hydrolysis reaction of the surfactant 1,3-dioxolane ring. The kinetics of acid hydrolysis of Ia-c micelles, as a result of specific hydronium ion concentration, is one order of magnitude smaller than that of unaggregated systems.
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    AB initio calculations of Diels-Alder transition structures for hetero-dienophile additions to cyclopentadiene (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 641-645 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Transition states for the hetero-Diels-Alder addition of formaldehyde, formaldimine, diazene and nitrosyl hydride to cyclopentadiene were located with ab initio molecular orbital calculations. Asynchronous transition structures were located for the asymmetric hetero-dienophiles, whereas the symmetrical hetero-dienophile, cis-diazene, produced a synchronous transition structure. The relative reactivities are in good agreement with the LUMO energies of the hetero-dienophiles, and the transition states with the exo oxygen or nitrogen lone pairs have lower energies than the corresponding endo lone electron pair transition states. Endo/exo energy differences are much higher than in the addition of ethylene derivatives to cyclopentadiene.
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    Professor Robert W. Taft (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 655-656 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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    URL: http://dx.doi.org/10.1002/poc.610071202
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    Micellar rate effects on reactions of hydroxide ion with phosphinate and thiophosphinate esters (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 618-628 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Cationic micelles of cetyltrimethylammonium chloride, bromide and mesylate (CTACl, CTABr, CTAOMs) speed reactions of OH- with phosphinate and thiophosphinate esters: Ph2PO.OPh (1a), Ph2PO.OC6H4NO2-p (1b), Ph(i-Pr)PO.OC6H4NO2-p (1c), Ph2PO.SEt (2a), Ph2PO.SPh (2b), (EtO)2PO.SPh (2c) and (EtO)2PS.OC6H4NO2-p (Parathion, 3). First-order rate constants go through maxima with increasing [surfactant]. The rate - surfactant profiles are fitted quantitatively in terms of a kinetic model that treats the distribution of OH- between aqueous and micellar pseudo-phases in terms of coulombic and non-coulombic ion-micelle interactions. Second-order rate constants at the micellar surface are lower than in water by factors that range from 0.035 for 1a to 0.7 for 1c. The thiophenyl derivative (2b) is more reactive than the corresponding phenoxy derivative (1a) and it is more reactive than the corresponding thioethyl derivative (2a). Parathion (3) is the least reactive substrate in both water and micelles.
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    Semiempirical SCF-MO calculations of kinetic isotope effects (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 635-638 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Carbon and deuterium kinetic isotope effects (KIEs) on the isomerization of methyl isocyanide were calculated by using the geometries and force fields obtained with MNDO, AM1 and PM3 Hamiltonians. These theoretical KIE values were compared with the corresponding experimental values. None of the hamiltonians used proved to be superior to any of the others. The best agreement for the carbon isotope effect was obtained by using the PM3 method; however, the deuterium isotope effect obtained by this method gave the poorest fit to the experimental data. The opposite is true when the MNDO Hamiltonian was used.
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    Relationships between electronic substituent parameters (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 629-634 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Core-electron energies for sulphur in a series of 2-substituted thiophenes have been used to investigate the relationship between electronic substituent parameters as developed by Taft and Topsom and by Charton. Initial- and final-state energies are analysed in terms of familiar substituent effects, such as field, delocalization and polarizability. The results show that the data correlate equally well with both sets of parameters and that there are many common features between the two methods. Both agree that resonance delocalization contributes significantly to the initial state, but has virtually no effect on the final state. In connection with this investigation we have also correlated the parameters for 31 substituents that are parameterized in the two approaches. The results of this more general study indicate some real differences between these and suggest that neither of the parameterizations incorporates all the electronic effects.
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    The effect of pressure on swern-moffatt oxidations (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 639-642 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rates of oxidation of cyclohexanol by dimethyl sulphoxide in conjunction both with acetic anhydride and with dicyclohexylcarbodiimide have been measured with variation of both temperature and pressure. Activation volumes were found to be -25 and -34 cm3 mol-1, respectively, and the absence of an α—d PKIE was noted. The reaction mechanism is discussed in the context of these values.
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    Masthead (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991) 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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    URL: http://dx.doi.org/10.1002/poc.610041101
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    Short communication. Synthesis and properties of 4-cyano-1-(7-carboxyheptadecyl)pyridinium bromide: A probe of the ionic composition near the surface of positively charged micelles (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 643-646 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 4-Cyano-1-(7-carboxyheptadecyl)pyridinium bromide (CCHP) was synthesized using standard procedures. The rates and product distribution of the reaction between OH- ion and CCHP are very similar to those previously obtained with alkyl-substituted 4-cyanopyridinium ions. The effect of hexadecyltrimethylammonium bromide micelles on the rate and product distribution of CCHP indicated that the cyanopyridinium ring of this compound is distal to the surface of the micelle. CCHP constitutes a probe for ionic composition in the vicinity of positively charged micelles.
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    Molecular mechanics (MM3) calculations on disulfides (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 659-666 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: An MM3 parameter set was developed for disulfides so as to permit the force field representation of these molecules with respect to geometries, including conformations, vibrational spectra and heats of formation.
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    Molecular Mechanics (MM3) calculations on sulfides (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 647-658 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The structures of eleven sulfide compounds including methanethiol, ethanethiol, dimethyl sulfide, ethyl methyl sulfide, di-tert-butyl sulfide, thiacyclopentane, thiacyclobutane, 5-thiabicyclo [2.1.1] hexane and 7-thiabicyclo [2.2.1] heptane have been calculated to agree with experimental data, along with the vibrational spectra of the first three of these. The heats of formation of 24 sulfides (including mercaptans) have also been calculated to agree with experimental data. In general, the force field for sulfides seems to be similar in accuracy with that for hydrocarbons.
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    Protolytic equilibria of several 4-acyl-substituted 1-phenyl-3-methylpyrazol-5-ones in dioxane-water mixtures (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 667-674 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Thermodynamic proton ionization constants, TpKa, of several 4-acyl-substituted pyrazol-5-ones [acyl = trifluoroacetyl (HPMTFP), acetyl (HPMAP), hexanoyl (HPMHP)] were determined in various dioxane-water mixtures at 25 and 35 ± 0.1°C. The TpKa values were determined by glass-electrode potentiometry and refined by using the extensive weighted least-squares FORTRAN program TPKA. Both extrapolation and leastsquares methods were used to obtain TpKa values in pure water (0%). All three acyl derivatives are weak monoprotic acids with TpKa values between 2.5 and 3.9, following the order HPMTFP 〈 HPMAP ≤ HPMHP. The TpKa values do not vary linearly with the reciprocal of the dielectic constant of the medium; however, a plot of TpKa versus the mole fraction of dioxane, n2, is linear at a given temperature. Values of standard free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) changes associated with their protolytic equilibria were also calculated. Temperature, medium and substituent effects are briefly discussed.
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    Molecular and electronic structure of substituted bithiophenes (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 675-680 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Several substituted bithiophenes were prepared and their electronic structure studied by photoelectron spectroscopy and semi-empirical MO calculations. Comparison of the spectra of related molecules and analysis of calculated molecular geometries gave an indication of molecular conformations in the gas phase.
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    Structural studies of 1,3-di(N,N-dimethylformamidyl)-2-cyanoguanidine. The case of a strongly lewis basic nitrogen atom in the cyano group VIII. Crystallographic studies of intra- and inter-molecular interactions (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 689-692 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610041107
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    Synthesis and NMR study of 5-furylmethylenehydantoins and 5-thienylmethylenehydantoins (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 681-688 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Two series of 5-furylmethylenehydantoins and 5-thienylmethylenehydantoins were prepared and their 1H and 13C NMR spectra studied in comparison with those of compounds in the analogous 5-aryl- and 5-pyridyl- series. Differences in the effects of the aromatic, six- or five-membered heteroaromatic rings are discussed. Spectral analysis enables Z/E configurations to be assigned with confidence and conformational preferences to be qualitatively deduced. Some interesting solvent effects were also observed.
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    Masthead (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991) 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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    URL: http://dx.doi.org/10.1002/poc.610041201
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    Topological aspects of chemical reactivity. Evans-dewar principle in terms of molecular similarity approach (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 701-705 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The Evans-Dewar concept of aromaticity of transition states is given new theoretical support in terms of the similarity approach to chemical reactivity. The principal goal of this approach consists in providing a simple, chemically lucid justification for the legitimacy of the original intuitively formulated concept.
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    Z/E, photoisomerization of 5-arylmethylenehydantoins and 5-pyridylmethylenehydantoins (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 707-713 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The Z/E photoisomerization of a series of 5-arylmethylenehydantoins and a series of 5-pyridylmethylenehydantoins by direct irradiation in methanol or ethanol solvent was investigated. The isomeric ratios at the photostationary state are dependent on the excitation wavelength and are related to the relative absorption coefficients of the two isomers. Measurements of quantum yields show the absence of quenching effects by oxygen or azulene, suggesting that triplet states either are not involved or are too short-lived for quenching to be observed. Preparative photoisomerization provides a route to the isomer which is not isolated or obtained only as a minor product by the usual method of synthesis.
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    Equilibrium acidities and homolytic bond dissociation enthalpies of the acidic C—H bonds in dialkyl malonates and related compounds (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 751-756 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The equilibrium acidities, pKHA, of 18 dialkyl malonates, five alkyl 2-cyanoacetates and nine malononitriles and the oxidation potentials of their conjugate anions, EOX(A-), were measured in dimethyl sulfoxide solution. The homolytic bond dissociation enthalpies (BDEs) of their acidic C-H were estimated by combining their pKHA and EOX(A-) values. The pKHA values of the dialkyl malonates were found to increase from 15·9 to 16·4 to 18·4 as the dialkyl groups were changed from dimethyl to diethyl to di-tert-butyl, but the BDEs of the acidic C-H bonds remained constant [95·3±0·3 kcal mol-1] (1 kcal = 4·184 kJ). Introduction of methyl, ethyl, isopropyl and tert-butyl groups into the 2-position of diethyl malonate caused the equilibrium acidities to increase by 2·0, 2·4, 3·8 and 8·0 pKHA units, respectively, and the BDE values to decrease by 4·4, 3·7, 2·5 and 0·8 kcal mol-1, respectively. Introduction of a phenyl group into the 2-position of diethyl malonate had no effect on the acidity, but weakened the acidic C-H bond by 10 kcal mol-1. The effects on acidity and BDE of introducing 3,4,5- (MeO)3C6H2CO, CF3, Me3N+, c-C5H10N, p-MeC6H4, p-NO2C6H4, PhO, F and c-C5H5N+ groups into the 2-position of diethyl malonate were also examined.
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    URL: http://dx.doi.org/10.1002/poc.610071214
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    Study of the hydrogen bond donor acidity of binary aqueous mixtures and their role in reversed-phase liquid chromatography (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 757-769 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The solvatochromic hydrogen bond donor (HBD) acidity parameter (αmix) of aqueous mixtures of methanol, acetonitrile, propan-2-ol and tetrahydrofuran were determined spectrophotometrically. The study was carried out at 25deg;C as a function of composition. The indicators used were 2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridino)phenolate, 2,6-dichloro-4-(2,4,6-triphenyl-N-pyridino)phenolate and Fe(LL)2(CN)2 (LL = N-(2-pyridylbenzylidene)-3,4-dimethylaniline). The HBD acidity of the aqueous organic mixtures was related to retention in reversed-phase liquid chromatography.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610071215
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  • 87
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    Reactivity and reaction patterns of alkyl alkoxybenzene radical cations. Mechanistic pathways of the reactions between 2,5-DI-tert-Butyl-1, 4-Dimethoxybenzene and perfluorodiacyl peroxides (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 1-6 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In the reactions of 2,5-di-tert-butyl-1,4-dimethoxybezene (1) with different oxidants, the radical cation 1+. is always detectable by EPR. However, the observed reactivity of 1+. depends greatly on the oxidation systems employed. In S2O82-—Cu2+—HOAc and Ce4+—HOAc systems (HOAc = acetic acid), 1+. appears to have long lifetimes and does not undergo fragmentation spontaneously. In contrast, in (RFCO2)2—CF2CICFCI2 (F113) systems, the readily formed 1+. is short-lived, and large amounts of de-tert-butylation products have been isolated. Experimental results imply that the C—C bond cleavage involved in de-tert-butylation could be a consequence of an attack by perfluoroacyloxy radical on 1+. in their original solvent cage. The fact that addition of methanol to the reaction mixture leads to the formation of a large amount of tBuOCH3 (46%) and other evidence suggest that the tert-butyl group leaves as a carbocation. On the basis of these results, we conclude that the reactions of 1 with (RFCO2)2 are initiated by electron transfer and followed by a fast coupling of various radical species, namely, 1+. with RFCO2 or with RF in the solvent cage, to form σ-complexes which collapse or react with nucleophiles to yield the final products.
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    URL: http://dx.doi.org/10.1002/poc.610040102
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    Interactions between tetracyanoethylene and rauwolfia alkaloids (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 25-30 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Interactions of the Rauwolfia alkaloids yohimbine, corynanthine, ajmalicine and reserpine with tetracyanoethylene were investigated by UV - visible spectroscopy. The results suggest the instantaneous formation of blue complexes whose thermodynamic and spectroscopic properties closely resemble those of 1 : 1 charge-transfer complexes of tetracyanoethylene with indole derivatives. The complexes are stable when the piperidinic nitrogen atom of the alkaloids is protonated or methylated. In contrast, those of the free bases rapidly decompose to give the corresponding 3,4-dehydro derivatives. The kinetics of these reactions have been studied and a mechanism is proposed.
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    URL: http://dx.doi.org/10.1002/poc.610040106
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    Acid hydrolysis of 2-substituted 3-methyl-tetrahydro-1,3-oxazines: Simple models for tertiary glycosylamines (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 53-57 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The hydrolytic decomposition of 2-phenyl- and 2-isopropyl-3-methyltetrahydro-1,3-oxazines to 3-methylaminopropan-1-ol and appropriate aldehydes was studied by 1H NMR and UV spectroscopy in acidic solutions. The time-dependent spectra confirmed that the formation of the final products was preceded by an equilibration of three components, the two Schiff base intermediates and the starting material, of which the former produced a carbinolamine which in turn underwent heterolysis to the final products.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610040109
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  • 90
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    Potentiometric and spectroscopic investigations of the aqueous phase acid-base chemistry of urazoles and substituted urazoles (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 67-69 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Aqueous-phase equilibrium acidity constants have been determined for urazole (1,2,4-triazolidine-3,5-dione), several substituted urazoles and other selected imides. Aqueous phase pKa values for urazole (5.8), 1-methylurazole (5.3), 4-methylurazole (5.7) and 1,4-dimethylurazole (5.3) indicate that (in water) the methyl group acidifies the urazole moiety when bonded to N-1 but has little effect on acidity when bonded to N-4. Aqueous phase pKa values for hydantoin (9.0) and 1-methylhydantoin (9.1) suggest that a 1-methyl substituent has little effect on the acidity of the imide proton present in hydantoin. These data, combined with aqueous phase pKa values for 1,2-dimethylurazole (7.5), a substituted urazole lacking amide protons, indicate that an amide proton in urazole (i.e. the proton bound to either N-1 or N-2 in urazole) is ca 1.5 pKa units more acidic than the imide proton (i.e. the proton bound to N-4). Changes in 13C NMR chemical shifts for the carbonyl carbon atoms present in variously substituted D2O phase urazoles, hydantoins, succinimide and their conjugate bases also confirm that amide N—H bonds found in urazole and 1-methylurazole are weaker, in a heterolytic acid - base sense, than imide N—H bonds.
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    URL: http://dx.doi.org/10.1002/poc.610040111
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    Activation and thermodynamic parameters of nucleophilic substitution. Reaction of alkyl halides with solid salts under phase-transfer catalysis conditions (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 341-345 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The paper is concerned with the effect of temperature and the nature of a solid ionophore (MCl, where M = Li, Na, K, Rb or Cs) on the rate and equilibrium constants in the process of substitution of chlorine for bromine in hexyl bromide. The non-linearity of the log k vs 1/T, dependence is shown to be linked with a change in the physical meaning of the constants with temperature variations. This is attributed to the occurrence of a strong exothermic process consisting in the formation of a kinetically independent intermediate, that precedes the limiting stage. A mechanism including the formation of two stable ternary complexes coordinated on the solid-phase surface, one of which is structurally close to the starting reagents and the other to the final reaction products, is proposed. The observed activation energies, enthalpies of formation of the ternary complex and activation energies of the limiting stages for the forward and reverse substitution processes were calculated. Thermal effects of the substitution in the presence of various solid MCl species were determined from the thermodynamic data and observed activation energies for both reaction types. The substitution is shown to involve a limiting stage represented by a transition between the ternary complexes. A linear relationship between the activation energy for the limiting stage and the solid ionophore crystalline lattice was established, suggesting incorporation of the solid salt molecule into the transition reaction state. Potential energy changes in the course of substitution in the presence of the various MCl solid salts are discussed. The Bell - Evans - Polyani principle is shown to hold for these studies.
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    URL: http://dx.doi.org/10.1002/poc.610040604
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    The nature of field effects and their fall-off with distance: The acidity of substituted quinuclidinium and bicyclooctylammonium ions (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 353-360 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Gas-phase acidities are reported for series of substituted quinuclidinium and bicyclo [2.2.2] octylammonium ions. Ab initio calculations are also reported for these equilibria at the STO-3G and 3-21G (for the quinuclidines) bases and can be used to extend the results. pKa values obtained for the bicyclooctylammonium ions are compared with previously reported results for the quinuclidinium ions. The results, as a whole, are considered together with similar values obtained for other saturated systems, to look at the varying transmission of field effects with distance. The falloff observed between the two series in the gas phase is considerably less than expected from simple electrostatics. In contradication with a recent claim, it is also shown that field effects, as measured by the parameter σF, give a good measure of the effect of the substituents in each series compared with a combination of resonance and electronegativity effects.
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    URL: http://dx.doi.org/10.1002/poc.610040606
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    Masthead (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991) 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040701
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    Applicability of excess acidity functions in low-acidity media (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 413-419 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: It is demonstrated that excess acidity functions do not correspond well to the pH scale in dilute solutions. Acidity constants, pKBH+, of seven moderately weak bases were determined; they were calculated by means of excess acidity functions, since the corresponding ionizations occur at acidity levels where the pH scale is partly used. The excess acidity method permitted extrapolation of measurements made in concentrated acids down to dilute solutions only up to about 5% (w/w) perchloric acid.
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    URL: http://dx.doi.org/10.1002/poc.610040704
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    Correlation of alkyl and polar substituents in the elimination kinetics of 2-substituted ethyl methanesulphonates in the gas phase (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 399-403 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The kinetics of the elimination of several polar 2-substituted ethyl methanesulphonates in the gas phase were determined in a static system, seasoned with allylbromide, and in the presence of at least equal amount of the freeradical suppressor propene and/toluene. The working temperature and pressure ranges were 290-360°C and 31°199 Torr (1 Torr = 133.3 Pa), respectively. The reactions are homogeneous, unimolecular, show a first-order rate law and take place according to the following equations: for 3-chloropropyl methanesulphonate, log k1 (s-1) = (12.01 ± 0.18) - (171.7 ± 2.1) kJ mol-1 (2.303RT)-1; for 4-chlorobutyl methanesulphonate, log k1 (S-1) = (11.78 ± 0.31) - (166.1 ± 3.5) kJ mol-1 (2.303RT)-1; for 3-methoxypropyl methanesulphonate, log k1 (s-1) = (11.50 ± 0.36) - (163.3 ± 4.0) kJ Mol-1 (2.303RT)-1; and for 2-ethoxyethyl methanesulphonate, log k1 (s-1) = (11.52 ± 0.37) - (167.3 ± 4.1) kJ mol-1 (2.303RT)-1. The present data together with those reported in the literature show that alkyl 2-substituted ethyl methanesulphonates give an approximate straight line when log k/ko vs σ* values (ρ* = -0.82 ± 0.088, intercept = 0.0084 ± 0.0209 and correlation coefficient r = 0.967 at 320°C) are plotted. However, polar substituents at the 2-position of ethyl methanesulphonates give rise to an inflection point at σ*(CH3) = 0.00 into another very good straight line (σ* = -0.29 ± 0.013, intercept = -0.0065 ± 0.0213 and r = 0.994 at 320°C). The mechanism of these reactions is considered to be heterolytic in nature, proceeding in terms of an intimate ion-pair intermediate.
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    URL: http://dx.doi.org/10.1002/poc.610040702
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    Correlations between empirical lewis acid-base solvent parameters and the thermodynamic parameters of ion solvation. Part II. Acidity parameters of cations and basicity parameters of anions (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 439-448 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The parameters α, β and γ of the equation WΔsGf(MX) = αBKT + βET + γ were calculated and analysed for different salts (MX) in pure solvents and in non-aqueous solvent-water mixtures (S). Among 109 different equations, only 64 were taken for the final analysis since in the case of 38 equations the parameters BKT and ET were highly correlated. After a statistical analysis and normalization, on the basis of the coefficients α and β obtained, it was found that the ions analysed may be divided into three classes characterized by the different dependences of α on β: class I, H+, Li+, Na+, Rb +, Cs+, Me4N+, Cl-, Br- and I-, where αN = 1.074βN + 1.639; class II, Ag+, Cl-, Br-, I-, OAc-, SCN- and N3- where αN = 2.057βN + 2.147; and class III, Ph4As+, BPh4-, Ph4C and Ph4Ge, where αN is variable and βN is almost constant. Two main conclusions can be drawn out from the results obtained: the behaviour of ions considered as Lewis acids and bases reflects different hard-soft properties of the ions, and the behaviour of the Ph4As+ ion is different from that of BPh4-.
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    AM1 studies on the intramolecular cyclization of ω-haloenolate anionsDetermination of reactivity by MO Theory, Part 72. For Part 71, see Ref. 1. (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 449-458 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The intramolecular cyclizations of ω-haloenolate anions, -CαH2—C(=O)—(CH2)n-3—CωH2—X with X = F, Cl and Br and n = 3-7, were investigated by the AM1 method. In most cases, cycloketone formation proceeds more favourably than cycloether formation, as predicted by the HSAB principle. The reactivity increases in the order X = F 〈 Br 〈 Cl for both processes, and for cycloether formation it is in the order n = 3 〈 4 〈 7 〈 6 〈 5 for all X but the relative order for n = 5 and 6 reverses in favour of n = 6 for cycloketone formation with X = Cl and Br. The softness of the acid centre, Cω, decreases for a harder X( = F) whereas it increases for a softer X( = Br). Thus the reactivity order with respect to X suggests that the softness of the base centres O and Cα belongs to the borderline class. The transition state (TS) structures indicate that the BEP principle is obeyed in all cases, and a less reactive process has a more product-like TS.
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    Masthead (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991) 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610040101
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    Geometry of 2-benzopyrylium and pyrylium cations calculated by semi-empirical methods. Crystal and molecular structure of 1,3-dimethyl-6,7-dimethoxy-2-benzopyrylium perchlorate (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 7-12 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: X-ray data on the crystal and molecular structure of 1,3-dimethyl-6,7- dimethoxy-2-benzopyrylium perchlorate show a good correlation with the geometry of the same cation calculated by the MINDO/3 and MNDO methods with total optimization of all geometrical parameters. The geometry of substituted pyrylium cations was determined by the MNDO and AM1 methods. On the basis of the data obtained, some effects of benzo [c] annelation are displayed for the 2-benzopyrylium cation in comparison with the pyrylium cation.
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    Micellar catalysis of the intramolecular aminolysis of the β-lactam antibiotic cephaclor (1991)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 4 (1991), S. 19-24 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rate of intramolecular degradation of cephaclor, involving the attack of the amino group at C-17 (N-18) on the β-lactam carbonyl, was increased up to 50-fold by neutral (polyoxyethylene-23 lauryl ether; Brij) and zwitterionic [3-(N-dodecyl-N, N-dimethylammonium) propane 1-sulphonate; (SDP)] micelles. The rate of OH- attack on cephaclor was increased 2-3-fold by Brij and SDP micelles. In the absence of micelles the rate of intramolecular degradation of cephaclor increased by up to 2-fold by addition of organic solvents. Distance calculations, based on the crystal structure of the antibiotic, showed that the intramolecular degration can only proceed in a conformation involving a cis-amide bond (N-14 - C-15). Micellar catalysis of the intramolecular degradation process was proposed to be due to the stabilization of the reactive conformation.
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    URL: http://dx.doi.org/10.1002/poc.610040105
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