Development of hydrogen transfer reactions catalyzed by iridium complexes and their applications to the synthesis of nitrogen heterocycles

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In recent years, much attention has been focused on the chemistry of iridium complexes bearing a pentamethylcyclopentadienyl (Cp^*) ligand. During the course of our investigation on the chemistry of Cp^* iridium complexes, we found a catalytic activity of [Cp^*IrCl_2]_2 toward hydrogen transfer reactions between organic molecules. In this research project, we have studied on the hydrogen transfer reactions catalyzed by Cp^* iridium complexes and their applications to the synthesis of nitrogen heterocycles.First, we have found a new catalytic oxidative cyclization of amino alcohols. Indole derivatives were synthesized in good to excellent yields from 2-aminophenethyl alcohols by means of a [Cp^*IrCl_2]_2 / K_2CO_3 catalytic system. This catalytic system was also effective for syntheses of 1,2,3,4-tetrahydroquinolines from 3-(2-aminophenyl)propanols and 2,3,4,5-tetrahydro-1-benzazepine from 4-(2-aminophenyl)butanol.Next, we have found an efficient catalytic system for N-alkylation of primary amines with alcohols. As an example, the reaction of aniline with benzyl alcohol in the presence of catalytic amounts of [Cp^*IrCl_2]_2 and K_2CO_3 gave N-benzylaniline in excellent yield. This catalytic system was applicable to a wide variety of amines and alcohols.On the basis of above results, we have found a new catalytic system for N-heterocyclization of primary amines with diols. Pyrrolidine, piperidine, azepane and morpholine derivatives could be synthesized in good yields by the reaction of benzylamine with 1,4-, 1,5-and 1,6-diols in the presence of Cp^*Ir catalyst and base.We also examined the transfer hydrogenation of quinolines using 2-propanol as a hydrogen source. 1,2,3,4-Tetrahydroquinoline derivatives were synthesized in good yields by regio-and chemoselective transfer hydrogenation catalyzed by Cp^*Ir catalyst.