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

Christopher J Collett; Richard S Massey; James E Taylor; Oliver R Maguire; AnnMarie C O'Donoghue; Andrew D Smith

doi:10.1002/anie.201501840

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

Angew Chem Int Ed Engl. 2015 Jun 1;54(23):6887-92. doi: 10.1002/anie.201501840. Epub 2015 Apr 23.

Authors

Christopher J Collett¹, Richard S Massey², James E Taylor¹, Oliver R Maguire², AnnMarie C O'Donoghue³, Andrew D Smith⁴

Affiliations

¹ EaStCHEM, School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9ST (UK) http://ch-www.st-andrews.ac.uk/staff/ads/group/
² Department of Chemistry, Durham University, South Road, Durham, DH1 3LE (UK).
³ Department of Chemistry, Durham University, South Road, Durham, DH1 3LE (UK). annmarie.odonoghue@durham.ac.uk.
⁴ EaStCHEM, School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9ST (UK) http://ch-www.st-andrews.ac.uk/staff/ads/group/. ads10@st-andrews.ac.uk.

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.

Keywords: 2-substituent effect; N-heterocyclic carbenes; kinetics; mechanistic studies; organocatalysis.

© 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.