Direct enantioselective alkynylation of α-ketoesters and α-ketiminoesters catalyzed by [bis(oxazoline)phenyl]rhodium(III) complexes

This review summarizes our studies of the direct enantioselective alkynylation of α-ketoesters and α-ketiminoesters catalyzed by [bis(oxazoline)phenyl]rhodium(III) ((phebox)Rh(III)) complexes. The reactions provide chiral α-tetrasubstituted propargyl alcohols and propargylamines under proton-transfer conditions in high yield and with high enantioselectivity. The unique nature of (phebox)Rh(III) complexes allows the reactions to occur in the presence of various functional groups, including an electrophilic aldehyde functionality. Mechanistic studies revealed that the generation of (alkynyl)Rh(III) complexes limited the overall reactivity, which led us to use (trimethylsilylethynyl)(phebox)Rh(III) complexes as efficient pre-catalysts. The use of (trimethylsilylethynyl)(phebox)Rh(III) complexes reduced catalyst loading to as low as 0.5 mol%, and expanded the substrate scope to unprecedented α-ketiminophosphonate and cyclic N-sulfonyl α-ketiminoesters.