Theoretical study on the transition state of oxaphosphetane formation between ethylidenetriphenylphosphorane and acetaldehyde

The MNDO MO calculations were carried out for the formation of oxaphosphetanes from substituted ylides and aldehydes (Equations 2–4). It was found that the reactions proceed through a nearly planar cyclic four-membered ring transition state (TS) in all cases studied and that phenyl substitution on phosphorus has little effect on the TS geometry. This finding is not in line with the TS models that have been proposed to explain the experimentally observed cis-selectivity.