METAL-ION EFFECTS IN WITTIG REACTIONS - A GENERAL HYPOTHESIS FOR THE MECHANISM OF THE WITTIG REACTION

To evaluate better the effects of N2p-P(IV) through-space interactions in Wittig reactions, a thorough study of metal ion effects in Wittig reactions lacking such interactions has been completed. As in the case of Wittig reactions with stabilized ylides, it has been determined that, with a moderated ylide, the stereochemistry (E:Z ratio) of alkene formation with an aldehyde is determined at the point that a new carbon—carbon bond has been formed to give a betaine or an oxaphosphetane intermediate. The observations that in most cases where lithium ion is present, the product mixture is enriched with the Z alkene, while when sodium or potassium ions are present, the E isomer predominates, are deemed to support the concept that a spin-paired diradical is formed as an unstable intermediate when sodium or potassium ions are present but that an ionic reaction occurs when lithium ions are present. The evidence that has been accumulated suggests that the reactions studied here (involving aromatic, heterocyclic, and aliphatic aldehydes and benzylidenediphenylmethylphosphorane or the preformed betaines, in THF at -78 °C) occur under kinetic control and without any significant degree of equilibration or Wittig reversal. © 1990, American Chemical Society. All rights reserved.