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.