N‐Heterocyclic Carbene/Carboxylic Acid Co‐Catalysis Enables Oxidative Esterification of Demanding Aldehydes/Enals, at Low Catalyst Loading
Wacharee Harnying, Panyapon Sudkaow, Animesh Biswas, Albrecht Berkessel
Abstract
Abstract We report the discovery that simple carboxylic acids, such as benzoic acid, boost the activity of N‐heterocyclic carbene (NHC) catalysts in the oxidative esterification of aldehydes. A simple and efficient protocol for the transformation of a wide range of sterically hindered α‐ and β‐substituted aliphatic aldehydes/enals, catalyzed by a novel and readily accessible N‐Mes‐/N‐2,4,6‐trichlorophenyl 1,2,4‐triazolium salt, and benzoic acid as co‐catalyst, was developed. A whole series of α/β‐substituted aliphatic aldehydes/enals hitherto not amenable to NHC‐catalyzed esterification could be reacted at typical catalyst loadings of 0.02–1.0 mol %. For benzaldehyde, even 0.005 mol % of NHC catalyst proved sufficient: the lowest value ever achieved in NHC catalysis. Preliminary studies point to carboxylic acid‐induced acceleration of acyl transfer from azolium enolate intermediates as the mechanistic basis of the observed effect.