Stereochemistry of Wittig reactions

McKenna, E. G., Walker, B. J. The mechanism and stereochemistry of Wittig reactions of phosphonium ylide-anions. Phosphorus, Sulfur Silicon Relat. Elem. 1990, 49-50, 445-448. 

Table V compiles the ZIE product isomer ratios of reactions from Table I for which such information has been reported. With 0.5-2% cross-linked polystyrene supptxis, the isomer ratios are similar to those obtained from Wittig reactions in solution. The exceptions appear to be the reactions on more highly cross-linked supports shown in Table IV. There is a clear trend toward greater E selectivity as the degree of cross-linking of the polymer increases. This probably should be explained as an environmental effect, but comparison with solvent effects on stereochemistry of Wittig reactions in the literature reveals no tendency for aromatic solvents, structurally similar to polystyrene, to increase E selectivity.

Table VI. Stereochemistry of Wittig Reactions under -Selective and -Selective Conditions (42)—... 

The reactivity of ylides is influenced not only by the substituents R and R, but also by the character of the so-called stationary substituents on the phosphorus atom. If the phenyl groups normally used are replaced by electron donors such as alkyl groups, the electron density on the phosphorus and thus the nucleophilicity of the ylide are increased. The structure of the stationary substituents on the phosphorus has a considerable influence on the stereochemistry of Wittig reactions. These effects are discussed in Section C.2. 

The influence of experimental conditions on the yield and stereochemistry of Wittig reaction with electrogenerated bases has been investigated. ... 

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See also in sourсe #XX -- [ ]

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