Kinetics and mechanism of organocatalytic aza-Michael additions: direct observation of enamine intermediates.

The imidazoles 1a-g add to the CC-double bond of the iminium ion 2 with rate constants as predicted by the equation log k = s(N)(N + E). Unfavourable proton shifts from the imidazolium unit to the enamine fragment in the adduct 3 account for the failure of imidazoles to take part in iminium-activated aza-Michael additions to enals. This journal is © The Royal Society of Chemistry 2012