Cuprous chloride accelerated Stille reactions. A general and effective coupling system for sterically congested substrates and for enantioselective synthesis

A major limitation of Stille coupling reactions arises from steric screening, especially in the vinylstannane component. For example, with 1-substituted vinylstannanes and aryl perfluoroalkanesulfonates or halides negligible or low yields are generally observed, due to very slow reaction rates and competing cine substitution. This problem has been overcome in the present work through the discovery and application of cuprous chloride as an accelerant for coupling. Thus, using a protocol for coupling that involves the system Pd(PPh3)(4)/CuCl/LiCl under anaerobic conditions in dimethyl sulfoxide solution at 60 degrees C, a wide variety of Stille coupling reactions which otherwise fail can be effected in excellent yield (Table 1). The process has been applied to the enantioselective synthesis of the chiral 1-(arylethyl)alkylcarbinol 26, a model for the natural product nicandrenone. The acceleration of Stille coupling by cuprous chloride can be explained by the intervention of a vinylstannane --> vinylcopper(I) transmetalation step and a subsequent accelerated coupling of ArPdX with the vinylcopper(I) intermediate.