Selective Reduction of Carboxylic Acids to Alcohols in the Presence of Alcohols by a Dual Bulky Transition-Metal Complex/Lewis Acid Catalyst

Abstract

Here, we report a molecular method for the generally applicable reduction of mono- and dicarboxylic acids that selectively furnishes a diverse variety of alcohols, including mono- and diols. One of the inherent drawbacks of the direct hydrogenation of carboxylic acids to alcohols is the in situ formation of the corresponding esters via condensation of the carboxylic acids with the produced alcohols. Especially, the hydrogenation of polycarboxylic acids frequently suffers from the formation of a complex mixture of oligomeric esters. This issue was successfully overcome by the combined use of a dual catalyst that consists of a bulky (PNNP)iridium complex and a Lewis acid. Owing to the steric bulk and robustness of the iridium catalyst, the main role of the Lewis acid is to independently catalyze the esterification, albeit the cooperative activation of (a resting state of) the iridium catalyst by the Lewis acid also seems to be implied.