An Iron Variant of the Noyori Hydrogenation Catalyst for the Asymmetric Transfer Hydrogenation of Ketones
We report the design of new iron catalysts for the asymmetric transfer hydrogenation of ketones. This type of iron catalyst combines the structural characteristics of the Noyori hydrogenation catalyst (an axially chiral 2,2’-bis(phosphino)-1,1’-binaphthyl fragment and the metal-ligand bifunctional motif) and an ene(amido) group that can activate the iron center. After activation by 8 equivalents of potassium tert-butoxide, (SA,RP,SS)-7a and (SA,RP,SS)-7b are active but nonenantioselective catalysts for the transfer hydrogenation of acetophenone and α,β-unsaturated aldehydes at room temperature in isopropanol. A maximum turnover number of 14480 were observed for (SA,RP,SS)-7a in the reduction of acetophenone. The right combination of stereochemistry of the axially chiral 2,2’-bis(phosphino)-1,1’-binaphthyl group and the carbon-centered chiral amine-imine moiety in (SA,RP,RR)-7b’ afforded a enantioselective catalyst for the preparation of chiral alcohols with moderate to good yields and broad functional group tolerance.