Esterification of Carboxylic Acids utilizing Eosin Y as a Photoacid Catalyst

Esters represent a vital functional group in organic chemistry and are prominently featured in valuable molecules. Traditional approaches for ester synthesis from acids typically entail the use of strong acids and toxic activating agents. Photoacid-catalyzed organic synthesis has recently come to the fore in several important transformations accruing the benefits of acid-based reactions under mild conditions. In this context, a green and sustainable method for esterification is proposed wherein, Eosin Y is introduced as a multifunctional photoacid catalyst that can be activated with visible light and catalyze the esterification of carboxylic acids. The present strategy is fairly broad and applies to the esterification of cinnamic acids, α-keto acids, and phenyl propiolic acids. The scope of carboxylic acids and alkyl alcohols includes several examples and therefore provides various carboxylic ester building blocks of pharmaceutical interest. Elaborated mechanistic studies revealed that proton-coupled electron transfer (PCET) between Eosin Y and carboxylic acid might facilitate the formation of a transient ketyl radical. This ketyl radical would subsequently undergo coupling with alkyl alcohols, yielding carboxylic esters in good to excellent yields.