Theoretical Calculations on the Wittig Reaction Revisited

Abstract

A series of Wittig reactions was calculated at the HF/3-21G* and B3LYP/6-31G* levels to understand the origin of the different product selectivities for different classes of ylides. Both alkylidenetriphenylphosphorane (nonstabilized ylide) and benzylidenetriphenylphosphorane (semistabilized ylide) yielded two types of transition states (TS) with a nearly planar and a puckered structure. The planar TS gave trans oxaphosphetane (OP), whereas the puckered TS led to cis OP. In contrast to previous semiempirical calculations, the present calculations showed that while a planar trans TS is more stable than a puckered cis TS for the semistabilized ylide, a puckered cis TS is more stable for the reaction of the nonstabilized ylide with benzaldehyde. These calculated selectivities agree with experiment. The carbonyl carbon kinetic isotope effects (KIEs) were computed at HF/3-21G* for the reactions of benzaldehyde with butylidenetriphenylphosphorane and with benzylidenetriphenylphosphorane. The reaction of the semi...