A Geometrically Constrained Tricyclic Phosphine: Coordination, Ring Expansion by Insertion of CO into a P–C Bond, and Lewis Acid Initiated Formation of an Oligocyclic Molecule with a P₂C₂₂ backbone

The geometrically constrained tricyclic phosphine 3b has two annulated five-membered PC₄ heterocycles with the P atom in a bridgehead position. Treatment with (THF)­M­(CO)₅ (M = Cr, Mo, and W) afforded M­(CO)₅ complexes 4–6, of which the Cr and Mo compounds showed in heat the unusual insertion of a carbonyl C atom into a P–C bond of one of the strained PC₄ rings. Ring expansion yielded a six-membered PC₅ heterocycle with an exocyclic CO group and afforded new phosphine ligand 8 that contains annulated five- (1×) and six-membered (2×) rings. Its Mo complex (7) was obtained by treatment of starting phosphine 3b with excess Mo­(CO)₆ in hot toluene. Phosphine 8 was removed from the metal atom by addition of the diphosphine Ph₂PC₂H₄PPh₂. Treatment of 8 with AlBr₃ resulted in enolization and an increased nucleophilicity of the β-C atom, which causes dimerization by P–C bond formation to yield a barrelane type molecule (9).