Uncatalyzed Transfer Hydrogenation and Transfer Hydrogenolysis: Two Novel Types of Hydrogen-Transfer Reactions

Uncatalyzed transfer hydrogenations are H2‐transfer reactions in which donors that contain weakly bound hydrogen atoms undergo an H‐atom transfer (retrodisproportionation) onto an acceptor containing unsaturated bonds such as CC, CO, CN, NN, NO. Transfer hydrogenolyses are reactions in which σ bonds are cleaved upon additon of H2. These hydrogenations are terminated by H, transfer; they do not follow a radical‐chain mechanism. The initial steps of both types of reactions, H‐atom transfer, complement the bimolecular formation of 1,4‐diradicals from alkenes or heteroalkenes within the scope of bimolecular radical formation (Molecule‐Induced Radical Formation, MIRF). The title reactions play an important role in coal liquefaction, aromatization reactions with nitroarenes or quinones, and possibly biochemical dehydrogenations. This review focuses on mechanistic studies, structure–reactivity relationships, and current applications of these reactions. Retrodisproportionation—H‐atom transfer onto unsaturated systems such as aryl‐substituted alkenes, azo, nitro, and nitroso compounds as well as quinones—represents an important, though previously disregarded H‐trapping reaction. The mechanism of the retrodisproportionation, the structures of the transition states, and the first examples of transfer hydrogenation are discussed.