N?Heterocyclic Carbene/Carboxylic Acid Co?Catalysis Enables Oxidative Esterification of Demanding Aldehydes/Enals, at Low Catalyst Loading

We report the discovery that simple carboxylic acids, such as benzoic acid, boost activity of N-heterocyclic carbene (NHC) catalysts in oxidative esterification aldehydes. A and efficient protocol for transformation a wide range sterically hindered ?- ?-substituted aliphatic aldehydes/enals, catalyzed by novel readily accessible N-Mes-/N-2,4,6-trichlorophenyl 1,2,4-triazolium salt, acid co-catalyst, was developed. whole series ?/?-substituted aldehydes/enals hitherto not amenable to NHC-catalyzed could be reacted at typical catalyst loadings 0.02–1.0 mol %. For benzaldehyde, even 0.005 % NHC proved sufficient: lowest value ever achieved catalysis. Preliminary studies point acid-induced acceleration acyl transfer from azolium enolate intermediates mechanistic basis observed effect.