Stereospecific stilbene formation from β-hydroxy-α,β-diphenylethylphosphoranes. Mechanistic proposals based upon stereochemistry

All four diastereomers of β-hydroxy-α,β-diphenylethylspirophosphoranes bearing two Martin ligands were prepared from a P-H (equatorial) spirophosphorane. Treatment of these phosphoranes with t-BuOK or t-BuONa at low temperature (ca. −40°C) led to the formation of stilbene with complete stereospecificity. In half of the cases the reaction proceeded quantitatively, whereas the other half gave rise to retro-aldol reaction with the formation of varying amounts of benzaldehyde and benzylphosphorane. A kinetic examination revealed that of the four the quantitatively proceeding reaction that gives cis-stilbene was the fastest. Based on the experimental observations, a mechanistic rationale for the previously reported Wittig-type reaction of exceptionally high Z-selectivity using an ethoxycarbonylmethylspirophosphorane is presented.