Direct Enantioselective alkynylation of ketimines is an environmentally benign approach for synthesizing optically active propargylamines with tetrasubstituted carbon stereocenter at α-position. However, its development has had limited success and few mechanistic studies have been performed. Here we present development and mechanistic studies of Rh(III)-catalyzed direct enantioselective alkynylation of α-ketiminoesters. The mechanistic studies revealed that less acidic alkyne rather than more acidic acetic acid acted as a proton source in the catalytc cycle, and the generation of (acetato-κ2O,O')(alkynyl)(phebox)Rh(III) complex (2), kinetically competent species, from the (aqua)(diacetato)(phebox)Rh(III) (1) limited the overall reactivity. We synthesized (acetato-κ2O,O')(trimethylsilylethynyl)(phebox)Rh(III) (3) as a general precatalyst. By using the new precatalyst, we achieved reducing the catalyst loading for the alkynylation with α-ketiminoesters and the expansion of the substrate scope to unprecended α-ketiminophosphate and cyclic N-sulfonyl α-ketiminoesters.

Development and mechanistic studies of Rh-catalyzed direct enantioselective alkynylation of α-ketiminoesters

252nd National Meeting (2016)

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