Studies on the Wittig Reaction (V) Trans Stereoselectivity in the Reaction of Benzylidene Alkyl Diphenylphosphorane and Aldehydes

Abstract

In previous papers, we reported the E-selectivity in the Wittig reaction of phosphorus ylid of the general formula Ph₂R′CH₂P=CHR (R=alkenyl; R′=H, alkenyl) and aldehydes. This continuing research for the stereochemistry of diphenylphosphorane in the title reaction of Ph₂R′CH₂P=CHR and R″CHO, where R=Ph, R′=H,Ph and R″=Ph,p-substituted phenyl, has led to thosimilar results. However, the E-selectivity is dependent on the ylid structure, reaction condition and the nature of the substituents on the aromatic ring. Generally the effect is stronger with benzyl diphenylphosphorane(R=H), the proportion of E olefin is produced in a larger quantity in salt-free condition than in the salt-containing medium, and the aromatic aldehyde bearing an electronwithdrawing group on the ring gives a higher E/Z ratio. Moreover, we also found that the logarithms of the E/Z ratio of the substituted stilbene products are linear with the σ constant of the substituents of the aromatic aldehydes: log E/Z=aσ+ b