C-C bond insertion of a complexed phosphinidene into 1,6-methano[10] annulene

Reaction of electrophilic phosphinidene complex [MePW(CO)₅] with 1,6-methano-[10]annulene results in the sole formation of the isomeric C-C insertion products 6c (main) and 6d (minor). The single-crystal X-ray structure of the complexed 1,7-methano-3-phospha[11]annulene (6c) shows a syn-W(CO) ₅ group at the exo bent phosphorus. The structure displays C-C bond alternation without bonding between the bridgehead carbon atoms. Density functional theory calculations indicate 6c to result from a concerted disrotatory ring opening of an undetected tricyclic exo-syn phosphirane intermediate. The endo-anti phosphirane cannot undergo ring expansion, due to the high barrier that is associated with an intramolecular antara-antara retro Diels-Alder reaction. The stabilizing effect of transition-metal coordination is discussed.