Mannich reactions of alkynes: mechanistic insights and the role of sub-stoichiometric amounts of alkynylcopper(I) compounds in the catalytic cycle

Yellow polymeric alkynylcopper(I) compounds observed in the three-component Mannich reactions of alkynes, secondary amines and aldehydes are shown to be pre-catalysts that give rise to catalytic copper(I) alkyne complexes by interaction with secondary amines (see scheme). Interaction of these complexes with iminium ions (formed from aldehydes or their equivalents, including CH2Cl2), results in the formation of Mannich bases in high yields.