Breaking of the C−C Bond of Cyclobutanones by Rhodium(I) and Its Extension to Catalytic Synthetic Reactions

A study of rhodium(I)-catalyzed synthetic transformations involving selective breaking of the C−C bond α to the carbonyl group of cyclobutanones is described. Decarbonylation took place on treatment of a cyclobutanone with an equimolar amount of (Ph 3 P) 3 RhCl at reflux in toluene to afford the corresponding cyclopropane. The formation of the cyclopropane suggests that Rh(I) undergoes an insertion into the bond between the carbonyl carbon and the α-carbon in the initial step. Catalytic decarbonylation of cyclobutanone was also achieved. The mode and rate of the reaction depended greatly on the ligands of the rhodium(I) complex. When a cyclobutanone bearing a hydrogen atom at the 3-position was used, appropriate choice of the catalyst system led to the selective formation of either a cyclopropane or an alkene. Breaking of the carbon−carbon bond was next combined with hydrogenolysis. When cyclobutanone was treated under hydrogen pressure with a catalytic amount of a rhodium(I) complex having a bidentate diphosphine ligand like 1,2-bis(diphenylphosphino)ethane, a ring-opened alcohol was produced in good yield. Selective breaking of C−C bonds by a soluble transition metal complex is achieved in these practical synthetic processes.