Cp*Ir Complex-Catalyzed Hydrogen Transfer Reactions Directed toward Environmentally Benign Organic Synthesis

Abstract

Catalytic activity of Cp*Ir complexes toward hydrogen transfer reactions are discussed. Three different types of reactions have been developed. The first is Oppenauer-type oxidation of alcohols. This reaction proceeds under quite mild conditions (room temperature in acetone) catalyzed by [Cp*IrCl2]2/K2CO3, and both primary and secondary alcohols can be used as substrates. Introduction of a N-heterocyclic carbene ligand to the catalyst considerably enhances the catalytic activity, and a very high turnover number of 950 has been obtained in the oxidation of 1-phenylethanol. The ­second is the N-alkylation of amines with alcohols. A new effective catalytic system consisting of [Cp*IrCl2]2/K2CO3 for the N-alkylation of primary amines with alcohols has been developed. Synthesis of indoles and 1,2,3,4-tetrahydroquinolines via intramolecular N-alkylation of amino alcohols and synthesis of nitrogen heterocycles via intermolecular N-alkylation of primary amines with diols catalyzed by a Cp*Ir complex have been also achieved. The third is the regio- and chemoselective transfer hydrogenation of quinolines. An efficient method for the transfer hydrogenation of quinolines catalyzed by [Cp*IrCl2]2/HClO4 using 2-propanol as a hydrogen source has been realized. A variety of 1,2,3,4-tetrahydroquinoline derivatives can be synthesized by this method. These results show that Cp*Ir complexes can be useful catalysts for hydrogen transfer ­reactions from the viewpoint of developing environmentally benign organic synthesis.