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 exocydic C=O group and afforded new phosphine ligand 8 that contains annulated five- (1X) and six-membered (2X) 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 beta-C atom, which causes dimerization by P-C bond formation to yield a barrelane type molecule (9).