Synthesis of Enantiopure Iridium(I) and Iridium(III) Pybox Complexes and Their Application in the Asymmetric Transfer Hydrogenation of Ketones

Iridium(III) complexes have been obtained from the complexes [Ir(η2-C2H4)2(κ3-N,N,N-R-pybox)][PF6] or from [Ir(μ-Cl)(η2-C8H14)2]2 and R-pybox mixtures by oxidative addition reactions. The treatment of complexes [Ir(η2-C2H4)2(κ3-N,N,N-R-pybox)][PF6] (R-pybox = 2,6-bis[4′-(S)-isopropyloxazolin-2′-yl]pyridine (1), 2,6-bis[4′-(R)-phenyloxazolin-2′-yl]pyridine (2)) with HCI, I2, and allyl chloride under mild conditions results in most cases in the stereoselective formation of the iridium(III) complexes [IrClH(η2-C2H4)(κ3-N,N,N-R-pybox)][PF6] (R = iPr (4), Ph (5)), [IrI2(η2-C2H4)(κ3-N,N,N-R-pybox)][PF6] (R = iPr (8); R = Ph (9)), and [IrCl(η3-C3H5)(κ3-N,N,N-R-pybox)][PF6] (R = iPr (10), Ph (11)). The substitution of ethylene by acetonitrile in 4 and 5 affords the complexes [IrClH(MeCN)(κ3-N,N,N-R-pybox)][PF6] (R = iPr (6), Ph (7)). Neutral iridium(III) complexes [IrCl3(κ3-N,N,N-R-pybox)] (R-pybox = iPr (12), Ph (13)) have been prepared in high yield by reaction of the complex [Ir(μ-Cl)(η2-C8H14)2]2 with chlorine and R-pybox in dichloromethane at room temperature. The reaction of 12 or 13 in acetone with silver or thallium salts of poorly coordinating anions, e.g., AgBF4, AgSbF6, or TlPF6, at room temperature gives the complexes [IrCl2(κ3-N,N,N-R-pybox)(μ-Cl)M][Y] (R = iPr, M = Ag, Y = BF4 (14a), SbF6 (14b), M = Tl, X = PF6 (14c); R = Ph, M = Ag, Y = BF4 (15a), SbF6 (15b), M = Tl, Y = PF6 (15c)). The structures of derivatives 1, 8, 10, and 14c have been determined by single-crystal X-ray diffraction analysis. The complexes 1, 2, 4−6, 8, 10, 12, 13, and 14a−c and the previously reported [Ir(η2-C2H4)2(κ3-N,N,N-Ph-pybox)][PF6] (3) and [IrClH(CO)(κ3-N,N,N-Ph-pybox)][PF6] have been assayed as catalysts for the asymmetric transfer hydrogenation of ketones. The iPr-pybox-iridium(III) complexes 4, 6, 12, and 14a are shown to be more efficient catalysts, with high conversions and ee up to 77%.