Iridium Catalysts for Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Scope and Mechanism

We introduce iridium-based conditions for the conversion of primary alcohols to potassium carboxylates (or carboxylic acids) in the presence of potassium hydroxide and either [Ir­(2-PyCH2(C4H5N2))­(COD)]­OTf (1) or [Ir­(2-PyCH2PBu2t)­(COD)]­OTf (2). The method provides both aliphatic and benzylic carboxylates in high yield and with outstanding functional group tolerance. We illustrate the application of this method to a diverse variety of primary alcohols, including those involving heterocycles and even free amines. Complex 2 reacts with alcohols to form the crystallographically characterized catalytic intermediates [IrH­(η1,η3-C8H12)­(2-PyCH2PtBu2)] (2a) and [Ir2H3(CO)­(2-PyCH2PtBu2)­{μ-(C5H3N)­CH2PtBu2}] (2c). The unexpected similarities in reactivities of 1 and 2 in this reaction, along with synthetic studies on several of our iridium intermediates, enable us to form a general proposal of the mechanisms of catalyst activation that govern the disparate reactivities of 1 and 2, respectively, in glycerol and formic acid dehydrogenation. Moreover, careful analysis of the organic intermediates in the oxidation sequence enable new insights into the role of Tishchenko and Cannizzaro reactions in the overall oxidation.