The 3‐Acetyloxaphosphirane/1,3,2‐Dioxaphosphol‐4‐ene Rearrangement

An Li/Cl phosphinidenoid complex, obtained by chlorine/lithium exchange from a [dichloro(organo)phosphane]tungsten(0) complex, reacted with aliphatic dicarbonyl derivatives to provide oxaphosphirane complexes, a 1,3,2‐dioxaphosphol‐4‐ene complex, and a P ‐alkoxyphosphane complex; the latter is formally derived from the enol form of the β‐diketone. DFT calculations on the ring‐expansion rearrangement support a preferred mechanism involving a pericyclic [1,3] shift of the phosphorus fragment in an oxaphosphirane complex rather than a stepwise diradical or ionic mechanism. The latter is slightly unfavored (ΔΔ E ‡ = 2.2 kcal/mol) and involves heterolytic P–C bond cleavage to give a methylene oxonium phosphanide intermediate and cyclization through a low‐lying transition state featuring an unusual linear C–O–P geometry.