Chiral FLPs Catalyze Asymmetric Hydrogenation Of Oxime Ethers And Hydrazones

Catalytic hydrogenation is one of the most useful tools in organic synthesis,for which transition-metal complexes have always been the predominated catalysts,while metal-free catalyzed hydrogenation emerged relatively late.Since Stephan first reported the reversible activation of H₂with frustrated Lewis pairs,metal-free catalytic hydrogenation,especially asymmetric hydrogenation has been rapidly developed and a variety of unsaturated substrates has been realized.The strategy for accessing chiral boranes by the in situ hydroboration of chiral dienes or diynes with Piers'borane HB(C₆F₅)₂was developed.With the above catalysts,the asymmetric hydrogenation of a series of unsaturated compounds such as imines,quinolines,quinoxalines,enol silyl ethers,and so on,have been achieved with high activity and enantioselectivity.Although the asymmetric hydrogenation with FLPs has become more and more mature,there are still some unsolved unsaturated compounds.To develop novel FLP catalytic systems and expand the substrate scope is important direction in this field.This paper focuses on the asymmetric catalytic hydrogenation of oxime ethers and preliminary exploration of the reaction conditions for the asymmetric hydrogenation of hydrazones.1.The asymmetric hydrogenation of oxime ethers could be achieved in high yields and moderate enantioselectivities using a chiral borane catalyst generated from chiral diene and HB(C₆F₅)₂.The chiral hydroxylamine compounds were afforded in 89-99%yields and with45-68%ee's.The reaction has good compatibility with the substrates containing both electron-donating and electron-withdrawing groups.The triisopropylsilyl group of the resulting products can be easily removed to give the corresponding hydroxylamines in high yields with the enantioselectivity maintained.2.The reaction conditions for the asymmetric hydrogenation of hydrazones with chiral FLPs were screened and the optimal conditons were determined.Using 5 mol%chiral diene and 10 mol%HB(C₆F₅)₂as catalyst,the asymmetric hydrogenation of hydrazones can be achieved with high yields and high enantioselectivity at-10?.The chiral hydrazine compound was obtained with 96%yields and 93%ee.