Theoretical study on the substituent effect of a Wittig reaction

The substituent effects of F, Hand methyl (Me) in replacement of phenyl (Ph) groups bonding with the ylide phosphorus in Wittig reactions have been examined theoretically by performing ab intio calculations. It is shown that the energy barrier for the Wittig reaction with F as the substituent is much higher than that with H, Me and Ph. The Wittig reaction is found to be more favorable with the substituent in the order F < H < Ph < Me. The reactions are found to proceed through two transition states: the formation and the decomposition of oxaphosphetane. We conclude that only the model of the Wittig reaction in which Ph is simplified to Me can reasonably describe the real Wittig reaction.