Chem. Pharm. Bull. 53(5) 457—470 (2005)

ganic chemistry. In particular, they play an important role in carbon–carbon bond formation. A metalacycle, i.e., a ring compound containing a transition metal, is a useful intermediate in synthetic organic chemistry. Oxidative cyclization of unsaturated hydrocarbons, such as alkene, alkyne, carbonyl compounds, or nitrile, using a low-valent transition metal is a useful reaction for synthesis of metalacycles. It is known that a metalacycle formed from a late transition metal and unsaturated hydrocarbons is not stable and is easily converted into the reductive elimination product or b-hydrogen elimination product (Eq. 1). Since the low-valent metal complex is regenerated in this reaction, the reaction proceeds with a catalytic amount of the transition metal. On the other hand, the metalacycle formed from an early transition metal and unsaturated hydrocarbons is fairly stable and can be converted into various compounds with functional groups by treatment with various agents (Eq. 2). Therefore a stoichiometric amount of the metal complex is required in this reaction.