A number of olefins prepared by the phosphonate carbanion modification of the Wittig reaction were shown to have trans configurations. Vapor phase chromatography of the intact reaction mixtures showed, at most, only trace amounts of cis isomers. To determine the degree of stereospecificity of the reaction under adverse circumstances, several experiments were devised with systems which, based on the generally accepted reaction mechanism,1•2 34567should be more favorable for the preparation of cis olefins. In only one case, however, was a significant amount of a cis isomer formed [6.4% cis-l,2-{ 1,1 z-dinaphthyl)ethylene in a 1:10 cis-trans mixture], demonstrating the great stereospecific tendency of the reaction. Possible interpretations of the observed differences in stereospecificity of olefin formation from phosphonate carbanions and phosphorus ylids are discussed.
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