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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 (ΔΔE = 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.