A Close Look to the Oxaphosphetane Formation along the Wittig Reaction: A [2+2] Cycloaddition?

The Wittig reaction between triphenylphosphine methylide and benzaldehyde has been studied both from conceptual and computational approaches. The supernucleophilic character of ylide accounts for the feasibility of the initial nucleophilic attack. The nature of bonding driving the formation of the first oxaphosphetane (OPA) intermediate in such a domino reaction is examined within a topological-based bonding evolution theory perspective. The sequence of the electronic flow associated to the changes in electron density supports a rationalization via two main electronic stages characterizing the single kinetic step: first, the C-C bond formation, which takes place via donation of electron density of the ylide carbon to the carbonyl carbon of benzaldehyde at a C-C distance of 2.02 Å, is formally associated to the transition state region; then, the P-O bond formation via the donation of electron density of the nonbonding region of the carbonyl oxygen to phosphorus at a P-O distance of 2.06 Å is located at the end of the reaction path. The detailed picture of bonding patterns suggests that the OPA formation in the Wittig mechanism can be better understood in terms of a two-stage one-step mechanism beyond molecular orbital considerations behind the traditionally accepted [2+2] cycloaddition proposal.