Light-Driven Stepwise Reduction of Aliphatic Carboxylic Esters to Aldehydes and Alcohols

The reduction of carboxylic esters to aldehydes and alcohols is a fundamental functional group transformation in chemistry. However, the inertness of carbonyl group and the instability of ketyl radical anion intermediate impede the reduction of carboxylic esters via photochemical strategy. Herein, we described the reduction of aliphatic carboxylic esters with synergistic dual photocatalysis via phenolate-catalyzed single electron transfer process and thiol-catalyzed hydrogen atom transfer process. The competitive back electron transfer process was effectively inhibited by protonation of the ketyl-type radical anion. This protocol enabled the efficient reduction of carboxylic esters to alcohols under mild conditions. By interruption of the reduction with prolinol, the step-controlled reduction of carboxylic esters to aldehydes was accomplished. The developed process was also successfully applied to the preparation of deuterated alcohols and aldehydes from esters with D₂O as the deuterium source.