Homogeneous Catalysis of Cycloaddition Reactions by Transition Metal Complexes
On going efforts in our laboratories have culminated in the development of novel procedures for effecting the catalysis of representative intramolecular [4+2] and [2+2+2] cycloaddition reactions on Rh(I) templates (J. Am. Chem. Soc.,112, 4965, 1990). Significantly, we have discovered that highly enantioselective variations of these transforma-tions can be achieved by the use of rhodium catalysts modified by specifically designed chiral bisphosphine ligands which differentiate the metal center both spatially and electronically (Tetrahedron,50, 6155, 1994). These new chiral rhodium complexes are currently being exploited in asymmetric variations of the Rh(I) catalyzed [4+2] cycloaddition reaction (Tetrahedron,50, 6145, 1994) and the hydroboration of alkenes. Recently, we have discovered a new protocol for effecting [2 + 2 + 1] cyclopentenone annulations that are catalyzed by cobalt complexes (J. Am. Chem. Soc.,118, 2285, 1996; Tetrahedron Lett.,39, 7637 and 7641, 1998). This remarkable catalytic reaction is currently being exploited in an efficient synthetic approach to the alkaloid magellanenone as well as several other natural products and bioactive molecules.