Study On Frustrated Lewis Pairs Catalyzed Asymmetric Transfer Hydrogenation

The chemistry of Frustrated Lewis Pairs(FLPs)has provided a powerful new way for metal-free catalytic hydrogenation.Great progress has been made in this area and a broad range of unsaturated compounds have been hydrogenated using FLP catalysis in recent years.However,asymmetric metal-free hydrogenations are far less developed,which attribute to a severe shortage of readily available chiral frustrated Lewis pair catalysts.The current chiral FLP catalysts mainly focus on the chiral boron Lewis acids which are moisture sensitive,there are few reports on chiral Lewis base.In need for developing new catalytic systems,we put forward a novel frustrated Lewis pair by the combination of H-and H+ incorporated Lewis acid and base together.Piers' borane and chiral tert-butylsulfinamide were chosen as the FLP,and metal-free asymmetric transfer hydrogenation of imines and quinolines was realized with ammonia borane as hydrogen source.Based on the good performance of this chiral FLP catalyst,some challenging asymmetric transfer hydrogenation has been realized and high activity and enantioselectivity were achieved.The details are as follows:1.Asymmetric transfer hydrogenation of ?-enamines:Chiral ?-amino acid esters are important intermediate for the synthesis of biologically and pharmacologically active ?-peptides and ?-lactams.Asymmetric transfer hydrogenation of ?-enamines was successfully realized using a combination of HB(C6FS)2 and(S)-tert-butylsulfinamide,and ammonia borane as hydrogen donor,and to give up to 93%ee.The substituent on the nitrogen atom in the substrate has great influence on the enantioselectivity,and ?-methoxyphenyl was the optimal substituent.The resulting ?-amino acid esters can be conveniently converted to biologically active ?-lactams with high enantioselectivity.2.Asymmetric transfer hydrogenation of 1H-indoles:Chiral indolines are important structural unit and exist widely in natural products and drug molecules,the hydrogenation and transfer hydrogenation of 1H-indoles is the most convenient and straight method to achieve them.By using a FLPs catalyst,an asymmetric transfer hydrogenation of 2-substituted indoles was performed under mild conditions to afford corresponding indolines with up to 90%ee.