Synthesis of new cationic Cp*Ir N-heterocyclic carbene complexes and their high catalytic activities in the Oppenauer-type oxidation of primary and secondary alcohols

Several new cationic Cp*Ir N-heterocyclic complexes have been synthesized and their catalytic activities in the Oppenauer-type oxidation have been investigated in order to improve the catalytic activity of [Cp*IrCl(mu-Cl)](2). The reactions of [Cp*IrCl(p-Cl)](2) (1) with N-heterocyclic carbene ligands afforded Cp*Ir(L)CL2 (3a-d; L = N-heterocyclic carbene ligands). The cationic complexes [Cp*Ir(L)(MeCN)(2)](2+) (5a-d) were obtained by the treatment of 3a-d with 2 equiv of AgOTf followed by addition of CH3CN. Structures of complexes 3a-d and 5a-d were determined by X-ray crystallographic studies. Complex 5a (L = 1,3,4,5-tetramethylimidazol-2-ylidene) catalyzed the Oppenauer-type oxidation of primary and secondary alcohols very selectively under mild conditions. In the oxidation of 1-phenylethanol and cyclopentanol using 5a as a catalyst, turnover numbers reached 3200 and 6640, respectively. These results demonstrate that, to the best of our knowledge, the cationic carbene complex 5a is the most effective catalyst in homogeneous oxidation of alcohols in terms of its high catalytic activity and wide applicability to the oxidation of primary and secondary alcohols. In this catalytic system, the stronger electron-donating ability of the N-heterocyclic carbene ligand than the phosphine ligand is more favorable for acceleration of the hydride transfer to acetone as a hydrogen acceptor. Additionally, dihydrido carbene complex Cp*Ir(L)(H)(2) (6) and dinuclear iridium carbene complex [Cp*Ir(L)(mu-H)](2)(2+) (7) were prepared to investigate the catalytically active species and fate of the catalyst. Thus, it is highly probable that an iridium-monohydride complex is the catalytically active species and that 7, which could be generated by dimerization of the iridium-monohydride complex in the catalytic system, is inactive.