Sterically crowded allylic tributylphosphorus ylides such as β,α-disubstituted allylic ylides react with various aldehydes to afford E olefins with high stereoselectivity (E > 92%). 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.
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