Iridium(I) complexes bearing the (S,S)-iPr-pybox ligand in the asymmetric transfer hydrogenation of acetophenone

Abstract

The complex [Ir(η2-EtO2CCCCO2Et)2{κ3-N,N,N-(S,S)-iPr-pybox}][PF6] ((S,S)-iPr-pybox=2,6-bis[4′-(S)-isopropyloxazolin-2′-yl]pyridine) (2) has been synthesized from the complex [Ir(η2-C2H4)2{κ3-N,N,N-(S,S)-iPr-pybox}][PF6] (1) by substitution of both ethylene ligands by two diethyl acetylenedicarboxylate ligands. Iridium(III) complexes have been obtained from the complexes [Ir(η2-C2H4)2{κ3-N,N,N-(S,S)-iPr-pybox}][PF6] (1) and [Ir(η2-RCCR)2{κ3-N,N,N-(S,S)-iPr-pybox}][PF6] (R=CO2Et (2), CO2Me (3)) by oxidative addition reactions. Thus, the addition of PhICl2 and Br2 to the iridium(I) complex 1 provides the iridium(III) complexes [IrCl2(η2-C2H4){κ3-N,N,N-(S,S)-iPr-pybox}][PF6] (4) and [IrBr(η1-CH2CH2Br)(η2-C2H4){κ3-N,N,N-(S,S)-iPr-pybox}][PF6] (5), respectively. The structure of the complex 5 has been determined by single-crystal X-ray diffraction analysis. The treatment of complexes 3 and 2 with I2 and allyl chloride results in the stereoselective formation of the iridium(III) complexes [IrI2(η2-MeO2CCCCO2Me){κ3-N,N,N-(S,S)-iPr-pybox}][PF6] (6) and [IrCl(η1-CH2CHCH2)(η2-EtO2CCCCO2Et){κ3-N,N,N-(S,S)-iPr-pybox}][PF6] (7), respectively. The catalytic activity of the iridium(I) complexes 2 and 3 in transfer hydrogenation of acetophenone has been examined.