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  • Theoretical, Physical and Computational Chemistry  (3)
  • Diastereoselectivity  (2)
  • Wiley-Blackwell  (5)
  • American Chemical Society
  • 1
    Electronic Resource
    Electronic Resource
    Diastereomerically Pure 3-(Silyloxy)oxetanes by a Selective Paternò-Büchi Reaction (1993)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 126 (1993), S. 2457-2466 
    Details
    ISSN: 0009-2940
    Keywords: Paternò-Büchi reaction ; Oxetanes ; Diastereoselectivity ; Enol silyl ethers ; Photocycloaddition ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Diastereomerically pure 3-[(trimethylsilyl)oxy]oxetanes 3 were prepared in moderate to good yields (45-72%) by the Paternò-Büchi reaction of silyl enol ethers 2 with benzaldehyde. The photocycloaddition exhibits a high degree of regio- and diastereoselectivity. The substituents R in the silyl enol ether have been varied [R = Me, Et, iPr, tBu, Ph, CH(OMe)2, CH(OCH2)2, C(OCH2)2Me], and it was found that steric bulk is mainly responsible for enhanced selectivity (diastereoselectivity from 70:30 up to 95:5). The regiochemical control is perfect (regioselectivity 〉95:5) except for silyl enol ether 2a (R = Me) in the case of which a 90:10 ratio of regioisomers was observed. Irradiation of the reaction mixture at lower temperature (-25°C) led to a further improvement of diastereoselectivity. The relative configuration of the products obtained was elucidated both by 1H-NMR spectroscopy and by chemical degradation. As a mechanistic hypothesis to explain the high observed diastereoselectivity we propose that the steric environment in the intermediate diradical 11 determines the selectivity according to two possible reaction pathways, i.e. bond formation and retrocleavage.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19931261120
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  • 2
    Electronic Resource
    Electronic Resource
    Silyl enol ethers in Paternò-Büchi reactions. Functional group tolerance and the effect of β-alkyl substitution (1995)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 855-865 
    Details
    ISSN: 0947-3440
    Keywords: Paternò-Büchi reaction ; Oxetanes ; Diastereoselectivity ; Silyl enol ethers ; Photocycloaddition ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The photochemically induced diastereoselective formation of 3-(silyloxy)oxetanes from silyl enol ethers and aromatic aldehydes was studied. It is shown that many heteroatom- and double bond-containing substituents (ether, ester, acetal, amide, alkene) withstand the reaction conditions and may therefore be attached either to the carbonyl compound (e.g. 9) or to the silyl enol ether (e.g. 1). The formation of several functionalized oxetanes such as 2 and 11 was achieved in decent yields (42-70%). Furthermore the effect of β-substituents at the silyl enol ether was investigated. A stereoconvergent Paternò-Büchi reaction of aromatic aldehydes afforded the corresponding oxetanes (13, 16-18) in fair to excellent yields (45-87%) and with diastereoselectivities (ds) of 65-95%. No detrimental influence of the steric bulk in the β-position (R1 = Me, Et, and iPr) on the regiochemistry of the reaction was observed. Large substituents in the α-position of the enol ether [R = iPr, tBu, C(OCH2)2Et, CMe2CH=CH2] proved beneficial, isolated yields of the major diastereoisomeric oxetane ranging between 60 and 87%. The configuration of the products was elucidated by 1H-NMR spectroscopy. In the major isomer all large vicinal substituents Ar (at C-2), R (at C-3), and R1 (at C-4) at the oxetane nucleus are oriented trans to each other. The stereoselective formation of three stereogenic centers at a time can be explained by a non-concerted mechanism which invokes the intermediacy of 1,4-diradicals as intermediates.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.1995199505124
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  • 3
    Electronic Resource
    Electronic Resource
    Many-body valence-bond theory (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 421-438 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The classical theory of chemical valence, first, is naturally formalized in mathematics in the area of graph theory and, second, finds an extension in quantum mechanics in terms of the Heitler-London-Pauling “valence-bond” (VB) theory. Thus, VB theory stands in a fairly unique position, although in quantum chemistry, there often has been a preference for the alternative (perhaps even “complementary”) molecular orbital (MO) theory, presumably in large part because of computational efficacy for general molecular structures. Indeed, as formulated by Pauling and others, VB theory was described as a configuration interaction (CI) problem when there were multiple relevant classical valence structures for the same molecular structure. Also, as now recognized, a direct assault on CI is computationally intensive, prone to size-inconsistency problems, and effectively limited to smaller systems - whereas indirect approaches, e.g., via wave-function cluster expansions or renormalization-group theory, often neatly avoid these problems. Thus, what is (and perhaps always has been) needed is “many-body” schemes for VB computations (as well as for higher-order MO-based approaches, too). Here, then, certain such many-body VB-amenable computational schemes are to be discussed, in the context of semiempirical (explicitly correlated) graphical models. The collection of models are described and interrelated in a fairly comprehensive systematic manner. A selection of many-body cluster expansion methods are then discussed with special reference to resonating VB wave functions and the fundamental graph-theoretic nature of the consequent problems (such as also are noted to arise in lattice-discretized statistical-mechanical problems, too). Some examples are described incorporating resonance among exponentially great numbers of VB structures as applied: for large icosahedral-symmetry fullerenic structures, for the (polyacetylenic) linear chain, and for ladderlike conjugated polymers. It is contended that practicable many-body VB-theoretic methods are now available, retaining clear links to classical chemical valence theory. Hopefully, too, these methods may soon find use beyond the semi-empirical framework.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 421-438, 1997
    Additional Material: 8 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    High-level computational study on the thermochemistry of saturated and unsaturated three- and four-membered nitrogen and phosphorus rings (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 62 (1997), S. 373-384 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The heats of formation and strain energies for saturated and unsaturated three- and four-membered nitrogen and phosphorus rings have been calculated using G2 theory. G2 heats of formation (ΔHf298) of triaziridine [(NH)3], triazirine (N3H), tetrazetidine [(NH)4], and tetrazetine (N4H2) are 405.0, 453.7, 522.5, and 514.1 kJ mol-1, respectively. Tetrazetidine is unstable (121.5 kJ mol-1 at 298 K) with respect to its dissociation into two trans-diazene (N2H2) molecules. The dissociation of tetrazetine into molecular nitrogen and trans-diazene is highly exothermic (ΔH298 = -308.3 kJ mol-1 calculated using G2 theory). G2 heats of formation (ΔHf298) of cyclotriphosphane [(PH)3], cyclotriphosphene (P3H), cyclotetraphosphane [(PH)4], and cyclotetraphosphene (P4H2) are 80.7, 167.2, 102.7, and 170.7 kJ mol-1, respectively. Cyclotetraphosphane and cyclotetraphosphene are stabilized by 145.8 and 101.2 kJ mol-1 relative to their dissociations into two diphosphene molecules or into diphosphene (HP(DOUBLE BOND)PH) and diphosphorus (P2), respectively. The strain energies of triaziridine [(NH)3], triazirine (N3H), tetrazetidine [(NH)4], and tetrazetine (N4H2) were calculated to be 115.0, 198.3, 135.8, and 162.0 kJ mol-1, respectively (at 298 K). While the strain energies of the nitrogen three-membered rings in triaziridine and triazirine are smaller than the strain energies of cyclopropane (117.4 kJ mol-1) and cyclopropene (232.2 kJ mol-1), the strain energies of the nitrogen four-membered rings in tetrazetidine and tetrazetine are larger than those of cyclobutane (110.2 kJ mol-1) and cyclobutene (132.0 kJ mol-1). In contrast to higher strain in cyclopropane as compared with cyclobutane, triaziridine is less strained than tetrazetidine. The strain energies of cyclotriphosphane [(PH)3, 21.8 kJ mol-1], cyclotriphosphene (P3H, 34.6 kJ mol-1), cyclotetraphosphane [(PH)4, 24.1 kJ mol-1], and cyclotetraphosphene (P4H2, 18.5 kJ mol-1), calculated at the G2 level are considerably smaller than those of their carbon and nitrogen analog. Cyclotetraphosphene containing the P(DOUBLE BOND)P double bond is less strained than cyclotetraphosphane, in sharp contrast to the ratio between the strain energies for the analogous unsaturated and saturated carbon and nitrogen rings. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62: 373-384, 1997
    Additional Material: 3 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    Computational study on nature of transition structure for oxygen transfer from dioxirane and carbonyloxide (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 19 (1998), S. 1353-1369 
    Details
    ISSN: 0192-8651
    Keywords: ab initio ; density functional theory ; oxidation reactions ; dioxiranes ; carbonyl oxides ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The relative reactivity of a series of nucleophiles that includes ethylene, sulfides, sulfoxides, amines, and phosphines toward dioxirane, dimethyldioxirane, carbonyloxide and dimethylcarbonyloxide has been examined at the MP4/6-31G*//MP2/6-31G*, QCISD(T)/6-31G*//MP2/6-31G*, and B3-LYP/6-31G* levels of theory. The barriers for the oxidations with dimethyldioxirane are higher (up to 2.5 kcal/mol for the oxidation of H2S) than those for the oxidations with the parent dioxirane. The oxidation barriers for dioxirane are larger than those for the oxidations with peroxyformic acid, except the barriers for the oxidation of sulfoxides. The reactivity of dimethylsulfide toward dimethyldioxirane was found to be comparable to that of dimethylsulfoxide both in the gas phase and in solution (chloroform). The classical gas phase barrier for the oxidation of trimethylamine to trimethylamine oxide was higher (6.3 kcal/mol at the MP4//MP2/6-31G* level) than that for oxygen atom transfer to trimethylphosphine. When the transition states were examined by self-consistent reaction field (SCRF) methods, the predicted barriers for the oxidation of amines and phosphines were found to be in good agreement with experiment. The general trend in reactivity for oxidation by dioxirane was R2S≈R2SO, R3P〉R3N in the gas phase, and R2S≈R2SO, R3N≈R3P (R=Me) in solution. The oxidation barriers calculated using the B3-LYP functional were lower than those computed at the MP4 and QCISD(T) levels.   © 1998 John Wiley & Sons, Inc.   J Comput Chem 19: 1353-1369, 1998
    Additional Material: 13 Ill.
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