Copper(I)-catalyzed enantioselective alkynylation of α-imino esters: ligand-to-metal ratio effects and mechanistic studies

An unprecedented effect of the ligand-to-metal ratio on the stereofacial selection in the copper-catalyzed enantioselective addition of terminal alkynes to N-PMP-alpha-imino esters was observed. An excess of ligand was found not to be beneficial, on the contrary, an excess of copper was found to be beneficial. Moreover, both enantiomers of the alkynylation product were obtained with almost the same enantiomeric excess with the same chiral ligand by simply adjusting the ligand-to-metal ratio. The investigation of the mechanism demonstrated the presence of a positive nonlinear effect [(+)-NLE]. (C) 2010 Elsevier Ltd. All rights reserved.