Rate and equilibrium constants for the addition of N-heterocyclic carbenes into benzaldehydes: A remarkable 2-substituent effect

Abstract Rate and equilibrium constants for the reaction between N-aryl triazolium N-heterocyclic carbene (NHC) precatalysts and substituted benzaldehyde derivatives to form 3-(hydroxybenzyl)azolium adducts under both catalytic and stoichiometric conditions have been measured. Kinetic analysis and reaction profile fitting of both the forward and reverse reactions, plus onwards reaction to the Breslow intermediate, demonstrate the remarkable effect of the benzaldehyde 2-substituent in these reactions and provide insight into the chemoselectivity of cross-benzoin reactions. It takes 2-: Measurement of rate and equilibrium constants for the reaction between N-aryl triazolium NHC precatalysts and substituted benzaldehydes under catalytic and stoichiometric conditions demonstrate the remarkable kinetic and thermodynamic effect of the benzaldehyde 2-substituent in these reactions, potentially providing insight into the chemoselectivity of cross-benzoin reactions.