Activation of Carbon-Carbon Bonds by Soluble Transition Metals

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Rhodium(I)-catalyzed synthetic transformations involving selective breaking of the C-C bond alpha to the carbonyl group of cyclobutanones was studied. Decarbonylation took place on treatment of a cyclobutanone with an equimolar amount of (Ph_3P)_3RhCl 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 alpha-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.Hydrogenolysis of cyclohexanone was next attempted. Various reaction conditions including ligands, hydrogen pressure, and solvent were examined, hydrogenolysis of cyclohexanone failed to proceed efficiently. Other metals like ruthenium and iridium were found to be ineffective. We will continue to investigate selective breaking of C-C bonds of ordinary ketones by a soluble transition metal complex.