Activation of E-H (E=hetero atom) and C-H bonds by iridium complexes and their application to homogeneous catalysis

Abstract

Preparations and reactivity of iridium complexes bearing several different kinds of ligands such as peraryldiphosphines, PN hybrid ligands, and an optically active chelated diamine and their application to catalysis are described. The catalyst system, iridium(I)/BINAP{2,2′-bis(diphenylphosphino)-1,1′-binaphthyl}/ benzylamine, serves as an efficient asymmetric hydrogenation catalyst of imines in methanol. As the catalyst precursor and the related complex, complexes [IrCl(diphosphine)]2 (diphosphine=peraryldiphosphine such as BINAP, BPBP{2,2′-bis(diphenylphosphino)-1,1′-biphenyl}), which have been known as neutral complexes so far and used as catalyst precursors in many catalytic reactions, could be isolated and fully characterized including X-ray analysis of the BINAP complex. These complexes activated easily E-H(E=heteroatom) bonds and gave the corresponding hydride complexes, [{Ir(H)(diphosphine)}2(μ-Cl)(μ-ER)2]Cl, (ER=OMe, OH, OC(=O)Me, OC(=O)Ph, and SPh). The structure of [{Ir(H)(binap)}2(μ-Cl)(μ-OMe)2]Cl (4a) was determined by X-ray analysis. The complex 4a was found to catalyze hydrogenation of alkyne to trans-olefin selectively and the possible reaction pathways are shown on the basis of the isolated complexes related to the reaction intermediates. By employing the PN ligand which shows flexible coordination ability, we have succeeded in direct observation of the reversible intramolecular C(sp3)-H bond activation and estimated thermodynamic and kinetic parameters regarding the fundamental C-H activation reaction. We could isolate both complexes before and after the C-H activation and characterize them completely including the X-ray structures. By applying the concept of the isolobal analogy, we have shown that the rhodium and the iridium complexes bearing an optically active chelated diamine ligand become efficient asymmetric catalysts for asymmetric transfer hydrogenation of ketone.