Lithium diphenylphosphide, elimination

In 1955 Wittig et al. found that triphenylphosphine could induce the deoxygenation of epoxides at 200 Mechanistically, this process probably involves anti opening of the epoxide followed by syn elimination of triphenylphosphine oxide from a betaine intermediate. Accordingly, the reaction proceeds with inversion of stereochemistry, which means that franj-ejjoxides give ci5-alkenes. The systems of bis(di-methylamino)phosphorous acid/butyllithium, o and lithium diphenylphosphide have been examined... 

Olefin inversion.2 Lithium diphenylphosphide in THF opens epoxides stereo-specifically quatemization of the crude product with methyl iodide leads to betaines, which fragment under mild conditions (25°) to give an olefin and methyldiphenyl-phosphine oxide. The product olefins are formed with inversion of stereochemistry relative to the starting epoxide owing to SN2 epoxide opening followed by cis elimination of the phosphine oxide. Thus the oxide of fraus-stilbene (1) is converted into the betaine (2), which at 25° gives cis-stilbene (3) in 95% yield overall. The conversion is stereospecific (> 98%). Similarly cis-stilbene can be converted into franx-stilbene. 

Nucleophilic substitution of a,/3-epoxysilanes followed by the Peterson elimination is valuable for the stereoselective synthesis of alkenes.3 The reactions with lithium phenylsulfide and diphenylphosphide form alkenyl sulfides and alkenylphosphines, respectively, in a stereospecific manner. 7-Metallo-a,/ -epoxysilanes are isomerized to a-siloxyallylmetals by anionic ring opening and subsequent Brook rearrangement (Equation... 

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