Mechanistic insights into the triazolylidene-catalysed Stetter and benzoin reactions: role of the N-aryl substituent

The in situobservation, isolation and reversible formation of intermediate 3-(hydroxybenzyl)azolium salts derived from NHC addition to a range of substituted benzaldehydes is probed. Equilibrium constants for the formation of these 3-(hydroxybenzyl)azolium salts, as well as rate constants of hydrogen-deuterium exchange (k sub(ex)) at C( alpha ) of these intermediates for a range of N-aryl triazolinylidenes is reported. These combined studies give insight into the preference of N-pentafluorophenyl NHCs to participate in benzoin and Stetter reaction processes.