Applications of acceptorless dehydrogenation and related transformations in chemical synthesis

Abstract

Background: Acceptorless dehydrogenation (AD) reactions can result not only in simple removal of hydrogen gas from various substrates but also, importantly, in surprisingly effi cient and environ-mentally benign (“green”) synthetic methodologies when intermediates resulting from the initial dehydrogenation process undergo further reactions. Advances: Traditionally, dehydrogenation/oxidation reactions of organic compounds have been performed using stoichiometric amounts of inorganic oxidants, in addition to employing various additives, cocatalysts, and catalytic systems that result in generation of copious stoichiometric, often toxic, waste. Catalytic transfer hydrogenation methods, in which stoichiometric amounts of sacrifi cial organic acceptor compounds are used, also generate stoichiometric amounts of organic waste. Recent developments in catalysis by metal complexes have resulted in AD reactions that release hydrogen gas and in related reactions in which dehydrogenation is followed by in situ consumption of the generated hydrogen equivalents and no net hydrogen gas is liberated. These reactions circumvent the need for conventional oxidants or sacrifi cial acceptors and provide an assortment of applications in organic synthesis, including several methods based on further reactiv-ity of the dehydrogenated intermediate compounds. Moreover, the evolved hydrogen gas is valuable in itself.