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 -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 (E-double dagger = 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.