Chiral phosphoric-acid-catalyzed transfer hydrogenation of ethyl ketimine derivatives by using benzothiazoline

Chiral phosphoric acid catalyzed transfer hydrogenation of ketimines derived from propiophenone derivatives and reductive amination of alkyl ethyl ketone derivatives were extensively examined in the presence of two representative hydrogen donors. The excellent enantioselective transfer hydrogenation was achieved by use of benzothiazoline as a hydrogen donor. The theoretical studies elucidated that the unsymmetrical structure of benzothiazoline plays an important role in high enantioselective hydrogenation. Asymmetric transfer hydrogenation of aromatic and aliphatic ketimines derived from ethyl ketone derivatives was examined by the combined use of a chiral phosphoric-acid catalyst and a hydrogen donor. The hydrogen donor had a remarkable effect on the enantioselectivity: excellent enantioselectivity was achieved when benzothiazoline was used as the hydrogen donor (see scheme, TRIP=3,3'-bis(2,4,6-triisopropylphenyl)-1,1'-binaphthyl-2,2'- diylhydrogenphosphate).