Recent Developments in the Stereochemistry of Cyclopropane Ring Opening

This review article deals with the stereochemistry of the following ring opening reactions: 1. Homoallylic rearrangement of secondary and tertiary cyclopropyl alcohols in the presence of electrophilic reagents, 2. The addition of carboxylic acids to vinylcyclopropanes, 3. The thermal rearrangement of vinylcyclopropanes into cyclopentenes. An account is also given of the NMR spectroscopic determination of the conformations of substituted vinylcyclopropanes.     

Stereochemistry of Asymmetric Kabachnik-Fields and Pudovik Reactions

Abstract

Kabachnik-Fields and Pudovik reactions were studied with the use of chiral initial reagents. In all cases the products of these reactions represent a mixture of diastereomers.     

Highly Stereoselective Reactions of γ-Sulfinyl Carbanions with Achiral Imines

Abstract

Lithium 2-p-tolylsulfinylbenzyl carbanions react with different N-substituted imines affording 1,2-diaryl ethyl (and propyl) amines with a high stereoselectivity control at both benzyllic (only dependent of the sulfur configuration) and iminic carbons. The anti:syn ratio, ranging between > 96: < 4 and < 2: > 98, dependent on the electronic density at nitrogen.     

Effect of Microwave Irradiation on the Direction and Stereochemistry of Rodionov and Michael Reactions

The reactions of 2-phenylpropanal with ethyl hydrogen malonate and benzylamine (or benzylammonium acetate) and of ethyl 4-phenyl-2-pentenoate with benzylamine take different pathways, depending on the conditions.     

Control of Absolute Stereochemistry in Transition-Metal-Catalysed Hydrogen-Borrowing Reactions

Hydrogen-borrowing catalysis represents a powerful method for the alkylation of amine or enolate nucleophiles with non-activated alcohols. This approach relies upon a catalyst that can mediate a strategic series of redox events, enabling the formation of C−C and C−N bonds and producing water as the sole by-product. In the majority of cases these reactions have been employed to target achiral or racemic products. In contrast, the focus of this Minireview is upon hydrogen-borrowing-catalysed reactions in which the absolute stereochemical outcome of the process can be controlled. Asymmetric hydrogen-borrowing catalysis is rapidly emerging as a powerful approach for the synthesis of enantioenriched amine and carbonyl containing products and examples involving both C−N and C−C bond formation are presented. A variety of different approaches are discussed including use of chiral auxiliaries, asymmetric catalysis and enantiospecific processes.     

The Effects of Homoconjugated Cyclopropane and Oxirane Rings on the Diels-Alder Reactivity of 2,3-bis (methylidene)norbornane

Preparation of 6exo, 7exo-bis (methylidene)-tricyclo[3.2.1.0^2,4]octane ( 6 ) is described. The reactivity of 6 towards tetracyanoethylene is evaluated and compared with that of exo-2,3-epoxy-5,6-bis (methylidene)norbornane ( 5 ), 2,3-bis-(methylidene)norbornane ( 1 ) and other related dienes. The cyclopropane group in 6 exerts an unsignificant rate retardation effect on the Diels-Alder reactivity of this diene relative to that of 1 , whereas a relatively important rate retardation effect is caused by the exo-oxirane ring in 5 . The observed effects are probably due to electronic factors (through-space and through-bond interactions).     

Sigmatropic Reactions in Carbanions. II. The Rearrangement of Allylic Ammonium Ylides

Two representative cases of the [2, 3] sigmatropic rearrangement are described, and the mechanism of this reaction is discussed.     

The Mechanism and Stereochemistry of Wittig Reactions of Phosphonium Ylide-Anions

Abstract

Schmidbaur's group have extensively investigated¹ the coordination properties of phosphonium 1,3-ylide-anions (1) in organometallic chemistry and more recently Cristau² has reported the potential usefulness of these compounds in synthetic organic chemistry. There have been a few studies³ of specific phosphonium 1,1-ylide-anions (2), however their only use in the Wittig reaction appears to be that of (2a).⁴ We have set out to study the stereochemistry and mechanism of Wittig reactions of (1) and (2) and to carry out some preliminary investigations on the arsenic analogues (3).     

Stereochemistry of the Reactions of Cyclic Ketones with Lithium Aluminum Hydride and Methyllithium. IV. The Stereochemistry of the Reactions in 2,2-Dimethyl-4-t-Butylcyclohexanone Systems

Previous publication showed that the stereochemistry of lithium aluminum hydride reduction of cyclic ketones can be evaluated by the use of an empirical equation, . This paper reports further test of this relationship in the new system of 2,2-dimethyl-4-t-butyl-cyclohexanone. Lithium aluminum hydride reduction of this ketone yields 95.6% trans-alcohol (lit., 95.9%) corresponding to D?(D?G*)_H of 1.67 Kcal/mol. With methyllithium, the ketone formed 77.5% of trans-alcohol (D?(D?G*)_D?fs = 0.67 Kcal/mol). These data give a calculated D?G* = 0.74 Kcal/mol which agrees well with the literature value of 0.87 or 0.28 Kcal/mol. Structural assignments of 2,2-dimethyl-4-t-butylyclohexanone, 1,2,2-trimethyl-cis- and trans-4-t-butylcyclohexanols by pmr spectroscopic method are also given.     

Gas-Phase Reactions of Dimethyl Disulfide with Aliphatic Carbanions - A Mass Spectrometry and Computational Study

Ion-molecule reactions of Me₂S₂ with a wide range of aliphatic carbanions differing by structure and proton affinity values have been studied in the gas phase using mass spectrometry techniques and DFT calculations. The analysis of the spectra shows a variety of product ions formed via different reaction mechanisms, depending on the structure and proton affinity of the carbanion. Product ions of thiophilic reaction (m/z 47), S_N2 (m/z 79), and E2 elimination – addition sequence of reactions (m/z 93) can be observed. Primary products of thiophilic reaction can undergo subsequent S_N2 and proton transfer reactions. Gibbs free energy profiles calculated for experimentally observed reactions using PBE0/6-311+G(2d,p) method show good agreement with experimental results.

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Aldol缩合反应的立体化学——Zimmerman-Traxler过渡态

介绍Aldol缩合反应中Zimmerman-Traxler模型,讨论烯醇盐构型与产物立体化学之间的对应关系：E型烯醇盐倾向生成反式产物,而Z型烯醇盐倾向生成顺式产物。阐述了遵循Zimmerman-Traxler模型需要满足的条件以及影响烯醇盐构型的因素。