Stereoselective E and Z Olefin Formation by Wittig Olefination of Aldehydes with Allylic Phosphorus Ylides. Stereochemistry

Article abstract of DOI:10.1021/jo00247a012

Sterically crowded allylic tributylphosphorus ylides such as β-γ-disubstituted allylic ylides react with various aldehydes to afford E olefins with high stereoselectivity (E>92percent).As the steric demand of the ylides was decreased, bulky aldehydes were required to achieve high E selectivity.On the other hand, predominant or exclusive formation of Z olefins was achieved by using allylic triphenylphosphorus ylides and tertiary aldehydes like pivaldehyde, while the combination of allylic triphenylphosphorus ylides and such large secondary aldehydes as cyclohexanecarboxaldehyde led to E olefin formation under the lithium salt free conditions.The distinct lithium salt effect was observed in the reaction effected with triphenylphosphorus ylides.The origin of the observed E or Z selectivity can be reasonably explained according to Vedejs' rationale on the Wittig reaction stereochemistry.