Efficient recycling of fluorous versions of highly enantioselective S,N-chelated zinc aminoarene thiolate catalysts for the 1,2-addition of diethylzinc to benzaldehydes

Abstract

New enantiomerically pure S,N-chelated (R)-Zn(Et)SR complexes, 3a–3c (R = 2-[CHMeN(CH2)4]-5-[SiMe3−n(CH2CH2CxF2x+1)n]C6H3; n = 1, x = 10; n = 2, 3 and x = 8), have been synthesized from the reaction of EtZnCl and the corresponding trimethylsilyl(aminoaryl) thioethers, or lithium aminoarene thiolate 2c. Complexes 3a–3c were successfully used as highly enantioselective catalysts for the 1,2-addition (e.e. up to 92%) of diethylzinc to benzaldehyde in perfluoromethylcyclohexane/n-octane. At 2.5 mol%, catalyst 3c, thanks to its three CH2CH2C8F17 tails, could be reused at least 10 times with retention of its initial high enantioselectivity, thereby significantly increasing the turnover number per unit amount of catalyst in comparison with earlier studies.