THE WITTIG REACTION: COMMENTS ON THE MECHANISM AND APPLICATION AS A TOOL IN THE SYNTHESIS OF CONJUGATED DIENES

Article abstract of DOI:10.1016/S0040-4020(01)81031-0

For construction, by the Wittig reaction, of conjugated dienes with specific stereochemistry (either Z or E) at the newly formed double bond, without isomerization of the existing ("old") double bond, it is better to react a reactive (nonstabilized) saturated ylide with an α,β-unsaturated aldehyde.The opposite approach, namely, the reaction of a moderate (semi-stabilized) allylic ylide with a saturated aldehyde produces a mixture of geometric isomers, as a result of increased production of the E-configuration at the new double bond, and significant isomerization of the existing double bond.The proposed betaine structure for the intermediate could account only for the two equivalents of Li reaction.In other reactions, the less stable erythro-oxaphosphetane is probably the intermediate, produced by an early anti- or gauche C-C bond formation from the ylide and the carbonyl, from which the Z-double bond is then formed.