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  • Theoretical, Physical and Computational Chemistry  (1,576)
  • Wiley-Blackwell  (1,576)
  • American Association for the Advancement of Science
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  • Wiley-Blackwell  (1,576)
  • American Association for the Advancement of Science
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  • 1
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 216-222 
    ISSN: 0894-3230
    Keywords: o-carboxybenzohydroxamic acid ; phthalic anhydride ; phthalic acid ; hydrolysis ; intramolecular acid catalysis ; kinetics ; activation parameters ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Phthalic anhydride was detected spectrophotometrically in the hydrolysis of o-carboxybenzohydroxamic acid (OCBA) in CH3CN-H2O solvent containing 0.03 mol dm-3 HCl. Pseudo-first-order rate constants (k1) for hydrolysis of OCBA are almost independent of the change in CH3CN content from 10 to 80% (v/v) in mixed aqueous solvents. The rate constants k1 are more than 10-fold larger than the corresponding rate constants for hydrolysis of phthalamic acid. These observations are explained in terms of a mechanism slightly different from the mechanism for hydrolysis of phthalamic acid. The activation parameters, ΔH* and ΔS*, are not affected appreciably by an increase in CH3CN content from 10 to 80% in mixed aqueous solvents. © 1998 John Wiley & Sons, Ltd.
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  • 2
    ISSN: 0894-3230
    Keywords: solute-solvent interactions ; solvent-solvent interactions ; binary solvents ; alcohol-water mixtures ; water structure ; solvatochromic indicators ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A preferential solvation model that takes into account the enhancement of the structure of water when small amounts of alcohol are added was applied to solvatochromic data for binary mixtures of water with 2-methylpropan-2-ol, propan-2-ol, ethanol and methanol. Application of the model allows the calculation of the effect of the enhancement of the water structure on solvatochromic solvent properties. It is demonstrated that the enhancement of water structure increases the solvent dipolarity/polarizability and hydrogen-bond donor acidity and decreases the solvent hydrogen-bond acceptor basicity. The effect decreases in the order 2-methylpropan-2-ol-water, propan-2-ol-water, ethanol-water and methanol-water. © 1998 John Wiley & Sons, Ltd.
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  • 3
    Electronic Resource
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 157-170 
    ISSN: 0894-3230
    Keywords: heterolytic bond dissociation energy ; carbon-carbon σ bonds ; hydrocarbons ; structural dependence ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The coordination of a resonance-stabilized hydrocarbon cation and anion yielded a series of unprecedented hydrocarbons, which are susceptible to thermal heterolytic cleavage of carbon-carbon σ bonds in polar media, generating the original ions under reversible conditions. When the component ions were sufficiently stabilized, some ion pairs were even isolated as solids, thus providing the first examples of hydrocarbon salts. The direct observation of the heterolysis by means of spectroscopy permitted reliable thermodynamic treatments of the observed degree of ionic dissociation. Correlation of the free energy of heterolysis with solvent dielectric constants and parameters of ion stabilities such as pKHA, pKR+ and redox potentials revealed the importance of the thermodynamic stabilities of ions, the degree of solvation and steric congestion in the starting molecule as major controlling factors in the heterolysis. © 1998 John Wiley & Sons, Ltd.
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  • 4
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 273-276 
    ISSN: 0894-3230
    Keywords: N,N-diphenylcarbamoylpyridinium ion ; solvolysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Solvolyses of the N,N-diphenylcarbamoylpyridinium ion are subject to specific and/or general base catalysis, which can be eliminated by addition of perchloric acid or increased, especially in fluoroalcohol-containing solvents, by addition of pyridine. The uncatalyzed solvolyses in aqueous methanol and aqueous ethanol involve a weakly nucleophilically assisted (l  =  0.22) heterolysis and the solvolyses in the pure alcohols are anomalously slow. © 1998 John Wiley & Sons, Ltd.
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  • 5
    Electronic Resource
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 209-215 
    ISSN: 0894-3230
    Keywords: securinine ; cationic micelles ; anionic micelle ; non-ionic micelle ; alkaline hydrolysis ; kinetics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The effects of micelles of cetyltrimethylammonium bromide (CTABr), tetradecyltrimethylammonium bromide (TTABr), sodium dodecyl sulfate (SDS) and polyoxyethylene 10 lauryl ether (C12E10) on the rates of alkaline hydrolysis of securinine were studied at a constant [-OH] (0.05 M). An increase in the total concentrations of CTABr, TTABr, SDS and C12E10 from 0.0 to 0.2 M causes a decrease in the observed pseudo-first-order rate constants (kobs) by factors of ca 2.5, 3, 7 and 4, respectively. The observed data are explained in terms of pseudophase and pseudophase ion-exchange (PIE) models of micelle. The binding constants, KS, of securinine with SDS, C12E10, CTABr and TTABr micelles are 32.4, 14.8, 22.1, and 9.1 M-1, respectively. The magnitudes of the second-order rate constants, kM, for the reactions in the micellar pseudophase are negligible compared with the corresponding rate constant, kW, for the reaction occurring in the aqueous pseudophase for CTABr, TTABr, SDS and C12E10. © 1998 John Wiley & Sons, Ltd.
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  • 6
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 277-282 
    ISSN: 0894-3230
    Keywords: colloidal CdS-induced photochemical reactions ; photocatalytic reactions ; photooxidation ; 2-methylindole ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 2-Methylindole (2-MI) is adsorbed on the surface of colloidal CdS particles with an adsorption intensity of 0.6 × 103 dm3 mol-1. A new emission band at 530 nm is produced by forming an exciplex between excited CdS and 2-MI and the red emission due to CdS is simultaneously quenched. The emission maxima of green bands for different indoles increase in the order indole 〈 tryptophan 〈 2-MI 〈 3-MI and are observed at 508, 520, 530 and 540 nm, respectively. The shift in emission maxima is related to the oxidation potential of these substrates. The irradiation of an aerated reaction mixture containing CdS and 2-MI with visible light induces the oxidation of adsorbed 2-MI by photogenerated holes to produce 2-methyl-3-indolinone and 2-acetamidobenzaldehyde. The latter product is formed due to oxidative C - C bond cleavage of the pyrrole ring. The reactivity of trapped holes towards the adsorbed 2-MI is evidenced by a decrease in the lifetime of the red emission of CdS in the presence of 2-MI. In this reaction the possibility of the participation of singlet oxygen is ruled out. A general mechanism of CdS-induced oxidation of indoles is discussed. © 1998 John Wiley & Sons, Ltd.
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  • 7
    ISSN: 0894-3230
    Keywords: Menschutkin reaction ; duality of SN1, SN2 mechanisms ; benzyl tosylates ; Yukawa-Tsuno equation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rate data for the Menschutkin reaction between strongly activated Z-substituted benzyl p-toluenesulfonates and Y-substituted N,N-dimethylanilines in acetonitrile at 35 °C fit the equation, kobs  =  k1 + k2 [DMA], which is consistent with concurrent first- and second-order processes. The k1 and k2 values for each substrate were separated based on the above equation. The SN1 rate constant, k1, is unaffected by the nature of the nucleophile, whereas the SN2 rate constant, k2, increased with the electron-donating substituent of the N,N-dimethylaniline. The substituent effect on the k1 values is linearly correlated by the Yukawa-Tsuno equation with ρ  =  -5.2 and r  =  1.3. The unimolecular reaction can be regarded as a classical SN1 mechanism. In contrast, that on the k2 values shows an upward curvature when analyzed by the Brown σ+ treatment. These results are ascribed to the simultaneous and independent occurrence of SN1 and SN2 mechanisms in the present Menschutkin reaction. © 1998 John Wiley & Sons, Ltd.
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  • 8
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 231-231 
    ISSN: 0894-3230
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: No Abstract
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  • 9
    ISSN: 0894-3230
    Keywords: calorimetry ; IR spectra ; ketones ; solvation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Solution enthalpies of n-hexane, acetone, butan-2-one and octan-2-one in a series of tetrachloromethane-acetonitrile mixtures and the carbonyl stretching absorption frequencies in the IR spectra of these ketones were determined. It was found that over the whole range of concentrations (varying from neat tetrachloromethane up to neat acetonitrile) the solvation enthalpy of these compounds can be obtained additively from the contributions of the alkyl and carbonyl fragments. The solvent effect on the solvation enthalpy of the carbonyl group was found to be satisfactorily correlated with the corresponding IR frequency shifts of the C=O group. It was also found that the sensitivities of the carbonyl IR frequencies to the solvent composition are different for various ketones. From both IR and calorimetric data, the preferential solvation parameters were evaluated. The differences between the IR spectroscopic and calorimetric data are discussed. © 1998 John Wiley & Sons, Ltd.
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  • 10
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    Journal of Physical Organic Chemistry 11 (1998), S. 261-272 
    ISSN: 0894-3230
    Keywords: 1-substituted 2-pyrrolidinones ; C=O stretching frequencies ; solvent effects ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In an effort to model solute-solvent interactions, the C=O stretching frequencies of five 1-substituted 2-pyrrolidinones and four other carbonyl-containing compounds were measured for 30 common solvents. These were then correlated with four empirical parameter sets and one theoretical (computational) parameter set. While an empirical parameter set gave the best correlation equations, the theoretical parameter equations are physically and statistically significant. Solvent volume, polarizability and hydrogen bond donor acidity (capacity) terms are significant in the correlation equations. © 1998 John Wiley & Sons, Ltd.
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  • 11
    Electronic Resource
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 232-240 
    ISSN: 0894-3230
    Keywords: P - S bond cleavage ; hydroperoxidolysis ; nerve agent VX ; molecular orbital calculations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The P - S bond cleavage process in the hydroperoxidolysis of a model system for the nerve agent VX was studied using ab initio and semiempirical molecular orbital methods. Aqueous solvation effects were included through single-point calculations using the semiempirical SM5.2PD/A continuum solvation model and geometries optimized at the HF/MIDI! level of theory. The predominant pathway for P - S bond cleavage involves pseudorotation of a low-energy trigonal bipyramidal intermediate followed by apical ligand ejection. In aqueous solution, the free energy barriers for these processes are found to be 14.3 and 4.6 kcal mol-1, respectively, with electronic energies calculated at the MP2/cc-pVDZ//HF/MIDI! level of theory. By comparison with another continuum model of solvation (PCM), it is concluded that the SM5.2PD/A model performs well even for hypervalent phosphorus species, in spite of not having included any such molecules in the model's parameterization set. © 1998 John Wiley & Sons, Ltd.
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  • 12
    ISSN: 0894-3230
    Keywords: crown dithioether ; conformational analysis ; multicomponent equilibrium ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The solid-state molecular structure and the conformational behaviour in solution of the 12-membered crown dithioether 8-methyl-1,4-dioxa-7,10-dithiacyclododecane-5,12-dione were studied by x-ray crystallography, 1H and 13C NMR spectroscopy and molecular mechanics. The conformational rigidity of some constituent structural fragments allowed a detailed analysis of the structure and distribution of the conformers. A protocol for studies of multiconformational equilibrium was developed by means of the combined use of structure calculations and dynamic NMR measurements. © 1998 John Wiley & Sons, Ltd.
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  • 13
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    Journal of Physical Organic Chemistry 11 (1998), S. 321-333 
    ISSN: 0894-3230
    Keywords: metal-catalyzed carbenoid reactions ; iodonium ylides ; sulfonium ylides ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Transition metal-catalyzed decomposition of phenyliodonium and diphenylsulfonium ylides was investigated with regard to application in asymmetric carbenoid reactions. Phenyliodonium ylides react in the presence of Rh(II) catalysts with the same selectivity in inter- and intramolecular cyclopropanations as the corresponding diazo compounds, and intramolecular CH insertions proceed with identical enantioselectivities. With diphenylsulfonium ethoxycarbonylmethylide the Cu(I)-catalyzed cyclopropanation of olefins affords trans/cis ratios and asymmetric inductions identical with those of diazo compounds, but with Rh(II) catalysts some small, although significant, selectivity variations occur, which are ascribed to coordination of diphenyl sulfide to one of the coordination sites of the catalyst. © 1998 John Wiley & Sons, Ltd.
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  • 14
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    Journal of Physical Organic Chemistry 11 (1998), S. 529-535 
    ISSN: 0894-3230
    Keywords: biotin ; in vivo C - S bond formation ; radical chemistry ; reducing conditions ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The last step in the biosynthesis of biotin involves the formation of carbon-sulfur bonds at non-activated carbons catalyzed by biotin synthase. S-Adenosylmethionine (AdoMet) and an electron source are essential for activity in cell-free systems. This important finding connects biotin synthase to a family of enzymes, namely pyruvate-formate lyase, anaerobic ribonucleotide reductase and lysine 2,3-aminomutase, which use the same cofactors. Additional experimental data led to the proposition of the following general mechanism. The carbons to be functionalized are first activated by homolytic cleavage of the C - H bonds, initiated by the deoxyadenosyl radical produced by a monoelectronic reductive cleavage of AdoMet with NADPH as electron source. The electron transfer system involves flavoproteins and very likely the [Fe - S] center of biotin synthase. NADPH and the flavoproteins can be replaced by photoreduced deazaflavin. By using a deuterated substrate, a deuterium transfer into deoxyadenosine has been observed, indicating that biotin synthase should be closely related to lysine 2,3-aminomutase, which uses AdoMet as a surrogate of vitamin B12. The source of sulfur, the nature of the immediate sulfur donor and hence the mechanism of trapping of the intermediate radicals are still unknown. © 1998 John Wiley & Sons, Ltd.
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  • 15
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    Journal of Physical Organic Chemistry 10 (1997), S. 687-696 
    ISSN: 0894-3230
    Keywords: bsorption ; fluorescence ; solvatochromism ; hydrogen bonding ; dipole moment ; naphthalenones ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---Electronic absorption and steady-state fluorescence emission of seven 3,4-dihydro-1-(2-p-substituted benzylidene)naphthalenones (1-7) show sizable solvent dependence. The charge-transfer (CT) absorption maxima of these compounds in various solvents show a red shift for the electron-donating substituted compounds (1-5), whereas a blue shift is observed for compounds possessing electron-withdrawing substituents (6, 7). Excitation into the lowest energy absorption gives emission from the locally excited state, which relaxes to the emitting intramolecular charge-transfer state for compounds with strong electron-donating substituents (1-3) in hydroxylic solvents. However, in moderately polar solvents, dual emissions are observed for these compounds. No CT emission is observed for the compounds with moderate electron-donating substituents (3 and 4) or those with electron-withdrawing substituents (6 and 7). Compound 1 with an -N(CH3)2 substituent shows an excited-state dipole moment of 17 D. © 1997 John Wiley & Sons, Ltd.
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  • 16
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    Journal of Physical Organic Chemistry 11 (1998), S. 622-631 
    ISSN: 0894-3230
    Keywords: secologanin ; vincoside derivatives ; acidic deglycosylation ; graphical analysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Acidic hydrolysis and cyclization were studied in vincoside glycosides (‘natural’ series) and their dihydro derivatives (‘dihydro’ series) in which either one or both N atoms were free or blocked by an alkyl group. For interpretation of the results, a graph was constructed in which 25 points (actually circles) represent a maximum of 81 aglycone types and 40 arrows indicate 131 possible cyclizations. The reaction matrix of the graph was under thermodynamic control and in most cases afforded the thermodynamically most stable product aglycones. In addition to the deglycosylation, two types of cyclization were observed. In azacyclizations, the preferred nucleophilic site is N-4 over N-1, and the preferred electrophilic site is C-22 in the glycosides, C-21 over C-19 and C-17 in aglycones. In oxacyclizations, the preferred nucleophilic site is O-17 over C-18 and C-21, and the preferred electrophilic site is C-19 over C-21 and C-17 in the ‘natural’ series, C-21 over C-17 in the ‘dihydro’ series. In one case, the kinetically favoured aglycone types which had been generated in the reaction mixture were trapped in a subsequent reaction (outside the graph) before thermodynamic equilibrium was attained. With the help of graphical analysis it was possible to justify the formation of the most favourable and actually isolated products and pathways out of a large number of possibilities. © 1998 John Wiley & Sons, Ltd.
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  • 17
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    Journal of Physical Organic Chemistry 11 (1998), S. 841-846 
    ISSN: 0894-3230
    Keywords: reactant-solute encounters ; aqueous solutions ; water ; kinetic methods ; hydration cosphere destructive overlap ; camouflage effects ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rates of chemical reactions in aqueous solutions are often sensitive to low concentrations of added solutes such as ureas, alcohols, α-amino acids and carbohydrates. In this work, several simple chemical reactions were used to probe this sensitivity, which arises from interactions between added solute and the reacting solute. It was found that derived interaction parameters provide a novel insight into interactions between solute molecules in aqueous solution. For example, it was possible to identify two interesting effects, which are termed (i) ‘destructive overlap’ of hydration cospheres and (ii) ‘camouflage effects’ by water-solute interactions. Copyright © 1998 John Wiley & Sons, Ltd.
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  • 18
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    Journal of Physical Organic Chemistry 11 (1998), S. 663-669 
    ISSN: 0894-3230
    Keywords: ipso proton affinities ; additivity ; disubstituted toluenes ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The problem of the ipso protonation of toluene and its predominantly disubstituted derivatives was considered by the MP2(fc)/6-31G**//HF/6-31G*+ZPE(HF/6-31G*) theoretical model. The substituents involved covered a wide range of different donor-acceptor capabilities. It is shown that the calculated MP2 ipso proton affinities of substituted toluenes follow mutatis mutandis the same additivity rule which was found earlier to be operative in polysubstituted benzenes, naphthalenes and biphenylenes. The additivity equation is both intuitively appealing and useful, being able to offer quantitative estimates of the proton affinity by very simple calculation. It is based on the concept of the increment, which in turn describes the influence of a single substituent on the proton affinity. Any substituent behaves as a rule as if the other were non-existent, thus giving rise to the independent substituent approximation (ISA). The performance of the additivity rule of thumb is very good, as evidenced by the average absolute deviation of 1 kcal mol-1. Larger deviations are possible, but they rarely occur, being indicative of a difference in interactions between substituents in the initial neutral base and in the final cationic conjugate acid. Finally, it follows as a corollary of the present analysis that protonation ipso to the CH3 group is never thermodynamically the most favourable site of proton attack in the benzene ring, provided that there is a single unsubstituted carbon atom within the aromatic moiety. The relevance of ipso protonation in persubstituted benzenes is briefly discussed. © 1998 John Wiley & Sons, Ltd.
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  • 19
    ISSN: 0894-3230
    Keywords: pyrenyl-DNA complexes and adducts ; photoinduced electron transfer ; strand cleavage ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The fluorescence of pyrenyl residues in complexes with the nucleic acid bases G, C and T, but not A, is strongly quenched by photoinduced electron transfer mechanisms. Site-specifically modified 11-mer oligonucleotide duplexes containing a single modified guanosyl base G* bearing a covalently attached pyrenyl residue were prepared in order to probe for photochemical damage associated with these photoinduced electron transfer reactions. When the pyrenyl residue positioned at G* is photoexcited with 355 nm light, direct strand cleavage is observed at that site with low quantum yield. Frank strand breaks are also observed up to five base pairs away from G*, suggesting that intrastrand migration of a reactive intermediate from base to base is occurring. © 1998 John Wiley & Sons, Ltd.
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  • 20
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    Journal of Physical Organic Chemistry 11 (1998), S. 602-609 
    ISSN: 0894-3230
    Keywords: olefin metathesis ; olefin cyclopropanation ; catalyst engineering ; fine organic synthesis ; polymer chemistry ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: An overview of the recent synthetic breakthroughs brought about by the discovery of new, functional group tolerant late transition metal (Rh and Ru)-based catalysts is proposed. Whereas dirhodium(II)-based complexes promote only carbene transfer reactions to olefins (i.e. olefin cyclopropanation), a few ruthenium-based catalysts can be engineered and fine tuned so as to mediate either carbene-transfer reactions or olefin metathesis. The different outcome of the reactions can be rationalized by the capability of the metal center to coordinate or not both the carbene and the olefin. This quite simple-minded approach indicates that several available coordination sites at the metal center favors metathesis reactions to the prejudice of olefin cyclopropanation. Examples of recent applications in ring opening metathesis polymerizations and copolymerizations include the formation of postpolymers of polydienes, of carbohydrate-substituted polymers and of telechelic oligomers. Application of the same ruthenium-based catalysts in ring-closing metathesis is illustrated by the formation of crown ether analogs, of unsaturated peptides and amino acids, of β-lactams, and of different bicyclic systems.
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  • 21
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    Journal of Physical Organic Chemistry 11 (1998), S. 512-518 
    ISSN: 0894-3230
    Keywords: sugar isomerization ; sugar phosphate isomerization ; enzymatic catalysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The relative yields of the products of isomerization of D,L-glyceraldehyde in D2O with intramolecular transfer of a hydride ([1-1H]DHA) and with proton transfer ([1-2H]DHA) were determined by high-resolution 1H NMR analyses. A study of the catalysis of this isomerization by deuteroxide ion, buffer anions and Zn2+ established the following: (1) isomerization with proton and hydride transfer occurs at approximately equal rates in dilute solutions of sodium deuteroxide; (2) Brønsted bases catalyze isomerization with proton transfer in a bimolecular reaction; (3) Zn2+ catalyzes isomerization with hydride transfer in a bimolecular reaction; and (4) Zn2+ and acetate ion react in concert to catalyze isomerization with proton transfer in a termolecular reaction. These results show that the various pathways for isomerization with proton and hydride transfer proceed via transition states of similar energies, so that there is no strong imperative for enzymatic catalysis by any particular reaction mechanism. The relevance of these results to the mechanism of action of xylose isomerase is discussed briefly. © 1998 John Wiley & Sons, Ltd.
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  • 22
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    Journal of Physical Organic Chemistry 10 (1997), S. 717-724 
    ISSN: 0894-3230
    Keywords: SN1 solvolysis ; carbocation ; Grunwald-Winstein relationship ; solvent effect ; solvation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---The rates of solvolysis in various solvents at 25 °C were determined for five tertiary alkyl chlorides: 2-chloro-2,4,4-trimethylpentane (4), 2-chloro-2,4-dimethylpentane, 2-chloro-2-methylpentane, 1-chloro-1,3,3-trimethylcyclopentane (7) and 1-chloro-1-methylcyclopentane. The rate data were analysed on the basis of the original and extended Grunwald-Winstein-type equation [log(k/k0)=mYCl+c and log(k/k0)=lNT+mYCl+c] and the results were compared with those reported for 2-chloro-2-methylpropane (1) and 2-chloro-2,3,3-trimethylbutane (3). The rate data for 4 in 18 solvents give an excellent correlation with l=0·00±0·02 and m=0·74±0·01. The neopentyl group in 4 more effectively shields the rear-side of the reaction center than the tert-butyl group in 3 that is correlated by l=0·10±0·04 and m=0·81±0·04. The rate ratio between 4 and 1 at 25 °C is 275 in TFE and predicted to increase to 950 in TFA. The previous 4/1 rate ratio of 21 in 80% ethanol evidently underestimates the B-strain effect on the solvolysis rate of 4 by a factor of at least 40. The remote methyl groups in 7 works to increase rear-side shielding without increasing B-strain. The marked difference in the effect of the remote methyl groups between 4 and 7 suggests that the leaving chloride ion in 4 takes a locus that is nearly antiperiplanar to the tert-butyl group. © 1997 John Wiley & Sons, Ltd.
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  • 23
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    Journal of Physical Organic Chemistry 10 (1997), S. 725-730 
    ISSN: 0894-3230
    Keywords: p-nitrophenyl N-phenylcarbamates ; stepwise mechanism ; rate-limiting breakdown of T± ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---Kinetic studies of the reactions of p-nitrophenyl N-phenylcarbamates with benzylamines were carried out in acetonitrile at 25·0 °C. Second-order (k2) and third-order (k3) rate constants were observed for all the Y-substituted carbamates except for Y=m-Cl. The relatively large magnitude of ρX (for X-substituted benzylamines) and ρY together with a positive cross-interaction constant ρXY supports a stepwise mechanism involving rate-limiting breakdown of the zwitterionic tetrahedral intermediate T±. Kinetic isotope effect studies with deuterated benzylamine (XC6H4CH2ND2) indicate that in the base-catalyzed path, k3, rate-limiting deprotonation occurs at the amino group of benzylamine within the T± intermediate. The low δH≠ and δS≠ values for the k3 process are in accord with the proposed mechanism © 1997 John Wiley & Sons, Ltd.
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  • 24
    ISSN: 0894-3230
    Keywords: ureido sugars ; amino acids ; 1H NMR ; IR ; H-D exchange ; intramolecular hydrogen bonding ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---Ureido 2-deoxy-β-D-glucopyranosides with seven different amino acid ester residues were studied by means of IR and 1H NMR spectroscopy. The H-D exchange rates increase in the order L-Val〈L-Leu〈L-Ala〈Gly for both NH protons; however, the exchange rate at N-1-H (linked to the glucopyranoside) is significantly faster than that at N-3-H (at the amino acid residue). The analysis of IR spectra in the region of the NH stretching vibrations shows, in agreement with other investigations, that the signals at 3454, 3423 and 3355 cm-1 are due to the free and the intramolecular associated NH groups forming a five- and seven-membered ring, respectively. It was found that the C7 associated ring is formed by hydrogen bonding between the N-1-H function and the C=O of the acetyl group at the C-3 position in the glucopyranoside. The N-3-H group is involved in a hydrogen bond with the C=O; function of the ester group protecting the amino acid residue. Furthermore, it was found that the ureido sugar with L-Val exhibits a stronger C7 hydrogen bond than the other amino acid residues. This result conforms with the lowest H-D exchange rate at the N-1 position of this compound. Steric effects resulting in the shielding of this hydrogen bond against OD attack are considered as a reason for this peculiarity. © 1997 John Wiley & Sons, Ltd.
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  • 25
    ISSN: 0894-3230
    Keywords: solvent effects ; aromatic nucleophilic substitutions ; empirical polarity parameter ; halonitrobenzenes ; aliphaticamines ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Empirical solvent polarity parameters ET(30) were determined by UV/VIS spectroscopy, using Dimroth-Reichardt's betaine dye, as a function of composition, for several binary solvent mixtures [i.e. polar hydrogen bond acceptor (PHBA) solvents+chloroform or dichloromethane]. Each solvent system was analyzed according to its deviations from additivity due to preferential solvation of the chemical probe and also from complicated intermolecular interactions of the mixed solvents. The ET(30) parameter of many of these mixtures has presented synergism. The synergetic effects were more significant for those binary solvent systems in which chloroform is the co-solvent. These results were related to the solvent effects on some aromatic nucleophilic substitution reactions. The kinetics of the reactions between 1-halo-2,4-dinitrobenzenes and primary or secondary aliphatic amines were studied in three solvent systems (PHBA+chloroform) where the synergism for the ET(30) polarity parameter is the rule. In all the aminodehalogenation reactions discussed the formation of the intermediate is the rate-determining step. The kinetic data show a tendency to decrease with decrease in the overall solvation capability of the binary mixture. In general, the reaction rates presented a gradual decrease in the PHBA solvent-rich zone and a large decrease at high co-solvent concentrations. The ET(30) values corresponding to binary dipolar hydrogen bond acceptor-hydrogen bond donor mixtures may be not generally valid for interpreting solvation effects on the reactions under consideration. © 1997 John Wiley & Sons, Ltd.
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  • 26
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    Journal of Physical Organic Chemistry 10 (1997), S. 835-840 
    ISSN: 0894-3230
    Keywords: N-methylaminopyridines ; polar properties ; spectroscopic properties ; solvent effect ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Dipole moments of 2-N-methylamino-5-nitro-6-methylpyridine (I) and 2-N-methylamino-3-nitro-6-methylpyridine (II) were determined in solvents of different polarity and basicity. The solvent effect is discussed in terms of the two-parameter (π*-β) Kamlet-Taft expression. The solvent dependences of the energy of intramolecular charge-transfer transition and the frequency of the ν(NH) stretching vibration are in keeping with the dielectric results. It was found that the solvent-induced disturbance in the electronic structure of I is brought about by hydrogen bond interaction with the NH group. The strong intramolecular hydrogen bond in II is stable in weakly basic solvents and the electronic structure is insensitive to the environment. However, in more basic solvents the intramolecular hydrogen bond is partly broken and an N - H· · ·solvent hydrogen bond is formed, bringing about distinct changes in polarity and spectroscopic properties. © 1997 John Wiley & Sons, Ltd.
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  • 27
    ISSN: 0894-3230
    Keywords: thiapyrylium salt ; photoinduced electron transfer ; laser flash photolysis ; trans-stilbene cation radical ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Photoinduced electron transfer reactions of trans-stilbene sensitized by 2,4,6-triphenylthiapyrylium tetrafluoroborate (STPP) were carried out by steady-state and laser flash photolysis techniques in the presence and absence of oxygen in dichloromethane. Rapid dimerization of the trans-stilbene cation radical with its neutral species was observed, as previously observed in 2,4,6-triphenylpyrylium tetrafluoroborate (TPP)-sensitized reactions in dichloromethane. Electrochemical and photophysical properties such as fluorescence quantum yield and T-T absorption spectrum of STPP were also studied, and the results were compared with those for TPP. The properties of STPP as an electron transfer sensitizer are discussed. © 1997 John Wiley & Sons, Ltd.
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  • 28
    ISSN: 0894-3230
    Keywords: Selective arylation ; exocyclic N-position ; 2-pyrimidylnitrenium ion ; tetrazolo[1,5a]pyrimidine ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Photo-reactions of tetrazolo[1,5-a]pyrimidine (1) with benzene (2a) and substituted benzenes (2b-f) in the presence of trifluoroacetic acid (TFA) gave 2-(2-, 3- and 4-substituted anilino)pyrimidines (3-5) together with 2-aminopyridine (6) and biphenyl (7) or diarylmethanes (8b, 8c and 8e). From the effect of heavy-atom solvent on the reactions, it is reasonable to assume that 3-5 are formed via a singlet species, but 6-8 via a triplet species. The intermediacy of 2-pyrimidylnitrenium ion is consistent with the evidence derived by the above effect, by a Hammett plot with ρ = -2.9 and by effects of solvent nucleophilicity and counter-anions. The selective exocyclic N-arylation giving 3-5 is discussed. © 1998 John Wiley & Sons, Ltd.
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  • 29
    ISSN: 0894-3230
    Keywords: Cyclic N-substituted sulfonamides ; 1,2,5-thiadiazole 1,1-dioxide derivatives ; hydrolysis ; kinetics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The hydrolysis reactions of 4-amino-2-phenethyl- and 4-amino-2-cyclohexyl-2,3-dihydro-3-oxo-1,2,5-thiadiazole 1,1-dioxide (Ia and Ib) were investigated in the range 24-73 °C in buffered aqueous solutions. The observed rate constants (kobs) are independent of pH in the range ca 1-4 pH, but increase with increase in pH above ca 4. A linear log kobs vs pH profile with unit slope is observed from pH ca 4 up to the highest experimental pH (ca 10). The products are the corresponding new compounds: 2-amino-2-[(N-substituted-sulfamoyl)imino]acetic acid salts. The C=N bond of these compounds hydrolyses further, in a slow reaction, to the sulfamide and oxalic acid derivatives. The substrates decompose to the final products without accumulation of the acetic acid derivatives under these experimental conditions. A mechanism is proposed. Rate constants and activation parameters are given for the first reaction step. Owing to steric effects, the reaction rate is higher for the N-phenethyl-substituted derivative than for the 2-cyclohexyl-substituted derivative. © 1998 John Wiley & Sons, Ltd.
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  • 30
    ISSN: 0894-3230
    Keywords: 1,3-dipolar cycloaddition ; N-methylindole ; Grignard reagents ; rate enhancement ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The reactions of 2-N-methylindolyl and 2- and 3-benzo[b]thiophenyl anions with nitrile oxides and nitrilimines were performed in order to investigate whether the use of anionic dipolarophiles modifies a classical 1,3-dipolar cycloaddition. When lithium compounds were used as bases, the heterocyclic anions invariably acted as nucleophilic species; in contrast, when a Grignard reagent was employed, the N-methylindole gave cycloaddition products with an extraordinary rate enhancement. The hypothesis that N-methylindole and ethylmagnesium bromide give an adduct much more reactive than N-methylindole itself was supported by the results of a theoretical investigation. The structure and electron distribution of the adduct were determined by ab initio calculations and compared with those of known Grignard complexes with nitrogen ligands. The performance of different basis sets was tested. The quantum theory of atoms in molecules was used to determine atomic charges and to describe the nature of bonds in terms of the properties of the electron density at the bond critical points. © 1998 John Wiley & Sons, Ltd.
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  • 31
    ISSN: 0894-3230
    Keywords: cycloheptatriene ; silacycloheptatriene ; ring inversion ; aromaticity ; ab initio calculation ; nucleus-independent chemical shift ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Nucleus-independent chemical shifts (NICS) of boat and planar structure of silepins (1) and cycloheptatriene (2) were calculated at the HF/6-31+G* level for the Becke3LYP/6-31G* optimized geometries in order to investigate the correlation between the inversion barrier and aromatic nature in the boat and planar forms of 1 and 2. The NICS of unsubstituted silepin 1a indicated that there is almost no stabilization in the boat form (NICS, -0.5) and that the planar form (NICS, 3.5) is relatively destabilized in comparison with the boat form. On the other hand, the NICS of 2 showed that there is considerable homoconjugative stabilization in the boat form (NICS, -4.2) and destabilization in the planar form (NICS, 8.2). In addition to the geometrical effects resulting from angle strain, these electronic effects are considered to affect the energy barriers for ring inversion, which have been calculated to be 0.9 kcal mol-1 for 1a and 5.2 kcal mol-1 for 2. © 1998 John Wiley & Sons, Ltd.
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  • 32
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    Journal of Physical Organic Chemistry 11 (1998), S. 737-742 
    ISSN: 0894-3230
    Keywords: thermochromism ; proton transfer ; hydrogen bond ; phenol ; amine ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Variable-temperature UV-vis, 13C NMR and IR studies showed that proton-transferred complexes were formed between phenols and amines in apolar solvents at low temperature. Upon cooling a solution of p-nitrophenol and diisopropylamine in toluene, the colour of the solution changed from colourless to yellow. This thermochromism was ascribed to the proton transfer in the hydrogen-bonding complex. Under UV-vis conditions, butylamine and imidazole also caused similar thermochromism upon complexation with p-nitrophenol, while triethylamine, quinuclidine and pyridine did not. The thermochromic behaviour was particularly dependent on the stoichiometry of the amine and the phenol: a solution of 3,3′-dibromo-5,5′-dinitro-2,2′-biphenyldiol and diisopropylamine with a molar ratio of 1:1 showed no thermochromism, while solutions with 1:2 or higher ratios showed thermochromism, indicating that excess amine is required to obtain the proton-transferred species. These results revealed that the proton-transferred species forms in apolar solvents at low temperature if an appropriate hydrogen-bonding network between the phenol and the amine can stabilize it. © 1998 John Wiley & Sons, Ltd.
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  • 33
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    Journal of Physical Organic Chemistry 11 (1998), S. 774-780 
    ISSN: 0894-3230
    Keywords: double fragmentation ; cation radicals ; NADH analogues ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The cation radical of 9-tert-butyl-N-methylacridan, generated electrochemically or photochemically, offers, in the presence of strong bases, a remarkable example of a double fragmentation. Whereas in acidic or weakly basic media the tert-butyl radical is cleaved with concomitant formation of the methylacridinium cation, the presence of a strong base triggers the cleavage of both the methyl group borne by the nitrogen atom and the tert-butyl group on C-9 leading to acridine, formaldehyde and the tert-butyl anion, even though methylacridinium cation is stable under these conditions. The origin of this unprecedented behavior resides in the prior deprotonation of the methyl group borne by the nitrogen atom which outruns the usual deprotonation at the 9-carbon because this is slowed by the steric hindrance due to the presence of the tert-butyl group. © 1998 John Wiley & Sons, Ltd.
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  • 34
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    Journal of Physical Organic Chemistry 11 (1998), S. 803-808 
    ISSN: 0894-3230
    Keywords: peroxymonosulfate ion ; oxidation ; sulfuric acid ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Peroxymonosulfate ion, HSO5-, as Oxone in aqueous H2SO4, oxidizes sulfides [MeSC6H4X(p), X = Me, H, NO2] to sulfoxides and converts aryl thiobenzoates [PhCOSC6H4X(p), X = Me, H] and thiol phosphorus(V) esters [Ph2POSPh, (EtO)2POSPh, Ph2POSEt, Ph(EtO)POSEt] into the acids and sulfonate ions. Second-order rate constants increase with increasing concentration of H2SO4 (10-53 wt%), owing to the high polarity of the medium rather than to acid catalysis. The rate increases fit the Grunwald-Winstein equation based on YOTs solvent parameters derived from rate effects on SN1 solvolyses. © 1998 John Wiley & Sons, Ltd.
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  • 35
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    Journal of Physical Organic Chemistry 11 (1998), S. 825-830 
    ISSN: 0894-3230
    Keywords: cation radical cycloaddition polymerization ; Diels-Alder copolymerization ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The Diels-Alder cycloaddition copolymerization of a bis(diene) with ionizable bis(dienophiles) via a cation radical mechanism has been accomplished using tris(4-bromophenyl)aminium hexachloroantimonate as a catalyst in dichloromethane solvent. The reactions occur at 0°C and yield Diels-Alder polymers of MW up to ca. 10,000 and a polydispersity index ca. 2. © 1998 John Wiley & Sons, Ltd.
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  • 36
    ISSN: 0894-3230
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The kinetics of the reaction between 1-chloro-2,4-dinitrobenzene and piperidine was studied in several completely non-aqueous binary solvent mixtures where the preferential solvation is the rule at 15, 25 and 40°C. The reaction was chosen as the simplest example of aromatic nucleophilic substitutions (ANS). For (aprotic solvent + aprotic co-solvent) binary systems the co-solvent was toluene, and the rest of the solvents used were selected with different structural characteristics and an extensive range of polarity. In this kind of mixture a property of mixed binary solvents would be defined by means of ET(30) values and the solvent effects on this simple model of ANS reactions are similar to those of aprotic pure solvents, especially if hydrogen-bond donor solvent mixtures are excluded from the analysis. For (aprotic solvent + protic co-solvent) binary systems the co-solvent used was methanol. The presence of a protic solvent in the mixture strongly determines the solvent effects on the reaction. In this type of binary mixture, the chemical probe under consideration may not be generally valid to interpret solvation effects. Additionally, empirical solvent polarity parameters ET(30) were determined UV-VIS spectrophotometrically for some pure aprotic solvents and, as a function of the composition, for (dimethylformamide + toluene), (toluene + methanol) and (1,1,1-trichloroethane + methanol) at 15 and 40°C, with the purpose of extending the studies on the empirical polarity indices in binary solvent mixtures to the thermo-solvatochromic area.
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  • 37
    ISSN: 0894-3230
    Keywords: 1,1,1-trihalo-2,2-bis(dimethoxyphenyl)ethanes ; base-promoted elimination ; reaction mechanism ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The base-promoted elimination reactions of 1,1,1-trichloro-2,2-bis(dimethoxyphenyl)ethanes were investigated. The bis(3,4-dimethoxyphenyl)ethane was found to be more reactive than the bis(2,5-dimethoxyphenyl)ethane and the latter more reactive than the bis(2,4-dimethoxyphenyl)ethane. Kinetic data relative to 1,1,1-trihalo(chloro or bromo)-2,2-bis(3,4-dimethoxyphenyl)ethanes show that the tribromo reacts faster than trichloro derivative and that the reactions are general-base promoted with Brønsted β values of about 0.6. A kinetic isotope effect, with kH/kD ratio ranging from 3.5 to 5.7, for the base-promoted elimination reaction of 1,1,1-trichloro-2,2-bis(3,4-dimethoxyphenyl)ethane was found. Tunneling occurs for methoxide and ethoxide ion-promoted eliminations. Activation parameters for alkoxy-promoted elimination show a similar trend for chloride and bromide derivatives. The data collected seem to confirm that there is contiguity between E1cBirr and E2 mechanisms. © 1998 John Wiley & Sons, Ltd.
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  • 38
    ISSN: 0894-3230
    Keywords: haloalkanes ; diols ; kinetics ; thermodynamics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rate constants, k, were determined for the solvolytic reactions of 2-bromo-2-methylbutane, 2-chloro-2-methylbutane and 3-chloro-3-methylpentane in 10 pure diols at 298.15 K. The Gibbs energies of activation were calculated in order to study the influence of the solvent and to provide mechanistic information about the activation process. By combining these values with a set of infinite dilution activity coefficients, γ∞, the transfer Gibbs energies of the reactants (initial state) and of the activated complex (transition state) were obtained, which allowed the solvent effects on both states to be quantitatively analysed. The γ∞ values were calculated using the UNIFAC group contribution method. The modified Flory-Huggins equation was used in the combinatorial term. The interaction parameters were taken from the specific UNIFAC γ∞ interaction parameter table, except for the γ∞ values of 2-chloro-2-methylbutane and 3-chloro-3-methylpentane in diethylene glycol and triethylene glycol, which were obtained using the γ∞ UNIFAC-VLE interaction parameter tables. For comparative purposes all the other infinite dilution activity coefficients of the three solutes in the diols were also calculated using the UNIFAC-VLE parameter tables. © 1998 John Wiley & Sons, Ltd.
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  • 39
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    Journal of Physical Organic Chemistry 11 (1998), S. 71-76 
    ISSN: 0894-3230
    Keywords: guanosine ; nitrenium ; DNA structure ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The N-acetyl-N-(2-fluorenyl)nitrenium ion (2a) reacts very efficiently with monomeric 2′-deoxyguanosine (d-G) to form a C-8 adduct, N-(2′-deoxyguanosin-8-yl)-2-acetylaminofluorene (6), in an aqueous environment, with a selectivity ratio, kd-G/ks, of 13.1 × 103 M-1 at 0°C and 4.8 × 103 M-1 at 30°C. The reactivity of the self-complementary oligomer d-ATGCAT with 2a can be separated into components due to the single-stranded (SS) and double-stranded (DS) forms. Within the error limits of the measurements kSS/kd-G ≈ 0.27 and kDS/kd-G ≈ 0. Another measure of the reactivity of d-G moieties in the DNA double helix can be obtained from measurements with the circular super-coiled plasmid pUC19. This plasmid provides an upper limit for kpUC19/kd-G of 0.02, where kpUC19 is the average trapping rate constant per d-G moiety in pUC19. The strong inhibition of the trapping reaction caused by the tertiary structure of the DNA double helix may be responsible for the change in product distribution of 2a-d-G adducts found from reaction with d-G, and denatured DNA (exclusive C-8 adduct, 6) and native DNA [5-20% N-2 adduct,3-(2′-deoxyguanosin-N2-yl)-2-acetylaminofluorene, 7]. © 1998 John Wiley & Sons, Ltd.
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  • 40
    ISSN: 0894-3230
    Keywords: phenyl salicylate ; ethane-1,2-diol ; urea ; sodium salt ; lithium salt ; transesterification ; kinetics ; intramolecular general base catalysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Pseudo-first-order rate constants (k1) for the reaction of ethane-1,2-diol (DOL) with ionized phenyl salicylate (PS-), obtained in mixed DOL-CH3CN solvent at constant [H2O] and [NaOH], obey the relationship k1  =  α[DOL]T/(1 + 2KA[DOL]T), where α is the apparent second-order rate constant, KA is the association constant for the dimerization of DOL and [DOL]T is the total concentration of DOL. The values of KA, in the presence of Na+ ions, decrease with increase in [H2O]. Lithium ions cause almost complete depolymerization of polymeric DOL (i.e. KA ≈ 0) under the experimental conditions imposed. The effect of 0.5 M urea on the structural behavior of the mixed solvent is kinetically insignificant. © 1998 John Wiley & Sons, Ltd.
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  • 41
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    Journal of Physical Organic Chemistry 11 (1998), S. 79-83 
    ISSN: 0894-3230
    Keywords: [2 + 2] ; photocycloaddition ; quantum yield ; oligooxyehylene linkage ; cyclophane ; styrene derivatives ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: There are two possible explanations for the high yields in the intramolecular [2 + 2] photocycloaddition of compounds 3a-d in which two styrene moieties are bridged by an oligooxyethylene linkage: one is the electronic effects of phenoxy oxygen atoms at the para-position of the vinyl group and the other is the steric effects due to the flexibility of oligooxyethylene linkages. In order to clarify the contributions of the two effects, a study was made of the photoreaction of similar compounds in which oxygen atoms are present only at the ends of the chain, and therefore only the electronic effects are involved. In these compounds, the quantum yields of the photocycloaddition were found to be low. Hence the high yields in the photocycloaddition of 3a-d are concluded to be ascribable to the flexibility of oligooxyethylene linkages. © 1998 John Wiley & Sons, Ltd.
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  • 42
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    Journal of Physical Organic Chemistry 11 (1998), S. 819-824 
    ISSN: 0894-3230
    Keywords: zero field splitting parameter, D ; organic biradicals ; triplets ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: An ‘average’ approximation for calculating the zero-field splitting parameter, D, gives reasonable results when used to calculate D values for non-disjoint delocalized organic biradicals. When used to calculate disjoint localized organic biradicals the D values are approximately half the experimental values. © 1998 John Wiley & Sons, Ltd.
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  • 43
    ISSN: 0894-3230
    Keywords: UV spectra ; dual-parameter equation ; correlation analysis ; polar effect ; spin-delocalization effect ; p-Y-substituted benzaldehyde phenylhydrazones ; p-Y-substituted benzaldehyde 2,4-dinitrophenylhydrazones ; p-Y-substituted benzaldehyde 4-nitrophenylhydrazones ; p-Y-substituted benzaldehyde N-phenylsemicarbazones ; p-Y-substituted benzaldehyde semicarbazones ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The K-band λmax values of the UV spectra of five types of p-Y-benzaldehyde derivatives, namely phenylhydrazones (1-Ys), 2,4-dinitrophenylhydrazones (2-Ys), 4-nitrophenylhydrazones (3-Ys), semicarbazones (4-Ys) and N-phenylsemicarbazones (5-Ys), were measured and the corresponding wavenumbers (νmax) calculated. Correlation analyses of the νmax values by the dual-parameter equation show that the νmax values of these compounds are affected, albeit to different degrees, by both spin-delocalization effects (σ·) and polar effects (σx) of the substituents. The spin-delocalization effects of substituents always facilitate bathochromic shifts of the λmax values (ρ· negative). However, polar effects of the substituents on these λmax data depend on the mode of polarization of these hydrazone or semicarbazone molecules, i.e. electron-pair acceptor substituents (Ys) facilitate bathochromic shifts for 1-Ys (ρx negative), but hypsochromic shifts for 2-Ys and 3-Ys (ρx positive), and induce very small shifts for 4-Ys and 5-Ys. As expected, donor Ys facilitate hypsochromic shifts for 1-Ys and bathochromic shifts for 2-Ys and 3-Ys and induce very small shifts for 4-Ys and 5-Ys. Possibly, a higher degree of polarization of the substrate molecule would demand a higher degree of polar assistance from the substituents at the transition states. © 1998 John Wiley & Sons, Ltd.
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  • 44
    ISSN: 0894-3230
    Keywords: merocyanines ; solvatochromism ; solvent polarity ; VBHB model ; SA-SAB-SB model ; binary solvent mixtures ; solvation equilibria ; spectra simulation ; pyridinium betaines ; 7H-indolo[1,2-a]quinolinium betaines ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---A quantitative model of solvatochromism in a binary solvent system is presented. Although it is derived for merocyanine and betaine dyes, it explains a majority of known examples of solvatochromism in binary solvents. The model provides an estimation of equilibrium constants between particular types of solvates present therein. UV-VIS absorption spectra of solvated species can be simulated. They perfectly fit the experimental data. The model proposed describes the internal solvent picture from the solute point of view, which differs from other known models and may be useful for studying the structure of liquids © 1997 John Wiley & Sons, Ltd.
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  • 45
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 614-617 
    ISSN: 0894-3230
    Keywords: ΔG°Acid (gas phase) ; kinetic acidities ; methanolic sodium methoxide ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Hydron exchange rates, kexc (M-1s-1), using methanolic sodium methoxide were compared with ΔG°Acid, (kcal mol-1) (gas phase) for 9-phenylfluorene, C6H5CH(CF3)2, m-CF3C6H4CH(CF3)2, p-CF3C6H4CHClCF3, m-CF3C6H4CHClCF3, 3,5-(CF3)2C6H3CHClCF3, fluorene and C6F5H. There is a good linear correlation for p-CF3C6H4CHClCF3, m-CF3C6H4CHClCF3 and 3,5-(CF3)2C6H3CHClCF3, with the others falling off the line. The fluorinated benzyl compounds and pentafluorobenzene have near-unity isotope effects and therefore differ from the fluorenyl compounds. Although the acidity and the exchange rates for three of the compounds [9-phenylfluorene, C6H5CH(CF3)2 and p-CF3C6H4CHClCF3] are similar, the important proton-transfer step to form a hydrogen-bonded carbanion intermediate and the subsequent breaking of that weak bond to form a free carbanion in methanol differ significantly for the fluoernyl compound compared with the two fluorinated benzylic compounds. © 1998 John Wiley & Sons, Ltd.
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  • 46
    ISSN: 0894-3230
    Keywords: intramolecular thermal (4σ + 2π) dyotropy ; kinetic isotope effects ; limiting barrier parameters ; quantum tunnelling ; 2H transfer processes ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Primary deuterium kinetic isotope effects (PDKIE) in parazoline-annelated syn-sesquinorbornenes exhibiting irreversible intramolecular (4σ + 2π) thermal dyotropy reveal unambiguous evidence for a tunnelling contribution to the kinetics in one instance but not for a close analogue. For analogous dyotropy of a cyclohexadiene-annelated syn-sesquinorbornene, the tunnelling components of the kinetic behaviour is small by comparison. The H atom traverse between alternative loci for the pyrazolines, deduced from x-ray and neutron diffraction data, is in agreement with approximate barrier parameters obtained by fitting of the PDKIE data to the Bell equation; barrier penetration is 3.22 kcal below the computed barrier corrected for the tunnelling contribution. The relative kinetic effect of systematic variation of the π-donor/acceptor groups on aryl ring substituents at C and N in the pyrazoline ring is consistent with a pericyclic process for dyotropy of these compounds, but not with rearrangement mediated by biradicals resulting from single H atom transfer in the rate-limiting step. Computer modelling of the transition state for dyotropy of these compounds is also consistent with a thermal, orbital symmetry conserved pericyclic reaction. Copyright © 1998 John Wiley & Sons, Ltd.
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  • 47
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 362-376 
    ISSN: 0894-3230
    Keywords: highly strained cyclophanes ; reactivity ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: An essential feature of the concept of aromaticity has been the stability and lack of reactivity of aromatics relative to that of other unsaturated compounds. Contrary to this general experience, high and unusual reactivity is encountered when simple, monocyclic benzene rings are bent by short bridges into a boat-shaped conformation, as is the case in small [n] paracyclophanes (n ≤ 8) and [n]metacyclophanes (n ≤ 7). This is illustrated, mostly with examples taken from the authors' own work, for thermal and photochemical behavior and reactions with electrophiles, nucleophiles and dienophiles. © 1998 John Wiley & Sons, Ltd.
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  • 48
    Electronic Resource
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 11 (1998), S. 356-361 
    ISSN: 0894-3230
    Keywords: heterogeneous catalysis ; organic reactions ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A decade's worth of work is reviewed: building on the demonstrated prowess of heterogeneous catalysis in industrial organic chemistry, the author's laboratory devised efficient catalysts for a number of organic reactions, based on aluminosilicates such as clays and zeolites. This review also spells out, at a time when creative research is overshadowed by imitative research, some of the methodological musts that also happen to characterize physical organic chemistry: the devising of experiments so that accurate numerical data can be obtained; the importance of remote, interdisciplinary connections; the need for estrangement from stereotypic preconceptions that may obscure the true explanations for the phenomena; and the over-riding need for concerning oneself only, or at least predominantly, with the important facts that nature tells, if one cares to listen. © 1998 John Wiley & Sons, Ltd.
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  • 49
    ISSN: 0894-3230
    Keywords: complex stabilities ; alkali metals ; alkaline earth metals ; dibenzo crown ethers ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---An experimental method was developed to determine the stability constants with nearly insoluble ligands in homogeneous solution. This method was tested using dibenzo crown ethers, which have a very low solubility in aqueous solution. The stability constants for the complexation of alkali and alkaline earth metal cations with different dibenzo crown ethers were determined in aqueous solution. Owing to the complex formation the total concentration of the ligand in solution increases. Dibenzo crown ethers absorb in the ultraviolet spectral range, hence the increase in the ligand concentration can be easily detected. Without the knowledge of the molar absorptivities of the ligands and of the corresponding complexes and of the solubilities, the stabilities of the complexes formed can be calculated under certain assumptions. To verify these assumptions, the solubilities of the dibenzo crown ethers and the molar absorptivities were determined. The kinetics of the solubilization process of the ligands was followed by spectrophotometric measurements. © 1997 John Wiley & Sons, Ltd.
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  • 50
    Electronic Resource
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 777-780 
    ISSN: 0894-3230
    Keywords: tandem energy transfer-electron transfer ; photosensitized alkylation ; α,β-Unsaturated ketones ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: -α,β-Unsaturated ketones are not conveniently alkylated by radicals generated from tetraalkylstannanes via photoinduced electron transfer (PET), either by direct irradiation or when a singlet sensitizer (an aromatic nitrile, a pyrilium salt) is used. However, the procedure is successful with tetramethyl pyromellitate (TMPM) as the sensitizer. TMPM is promoted to the triplet state by energy transfer from the unsaturated ketones and then sensitizes the cleavage of alkylstannanes by electron transfer. The alkyl radicals thus formed finally add to the unsaturated ketones, giving the β-alkyl derivatives. © 1997 John Wiley & Sons, Ltd.
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