Carboxylic Acid Co‐Catalysts Facilitate N‐Heterocyclic Carbene Organocatalysis Through Proton Relay and Polarization Effects

ABSTRACT

While oxidative esterification of aldehydes with alcohols can be catalyzed by triazolium salts under basic conditions, contradicting behavior of a reaction in acidic media has been observed as well. In the hereby suggested reaction pathway, the hitherto surmised strongly basic, reactive intermediates of the reaction—N‐heterocyclic carbenes and azolium enolates—are bypassed, owing to proton relaying effects of the carboxyl group of the acid. The formation of the carbene is circumvented by a direct, concerted reaction between the triazolium cation and the aldehyde substrate to initiate the reaction. Similarly, the acyl triazolium cation can react directly with the alcohol in the presence of benzoic acid, without the occurrence of the azolium enolate species. Interestingly, through hydrogen bonding, benzoic acid makes the aldehyde and the acyl azolium intermediate more electrophilic, whereas it makes the alcohol reactant more nucleophilic, activating them for the key reaction steps, giving the reaction a further boost. The emerging mechanistic picture is fully consistent with earlier findings, enabling the further improvement of this catalytic system.