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 efficient and environmentally 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 sacrificial organic acceptor compounds are used, also generate stoichiometric amounts of organic waste. Recent developments in catalysis by metal complexes have resulted in acceptorless dehydrogenation 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 sacrificial acceptors and provide an assortment of applications in organic synthesis, including several methods based on further reactivity of the dehydrogenated intermediate compounds. Moreover, the evolved hydrogen gas is valuable in itself. Outlook: Further development of new ADs for green, efficient chemical synthesis is expected to be greatly influenced by fundamental organometallic chemistry as a basis for catalyst design. Such processes are highly desirable and are expected to gradually displace elaborate conventional laboratory and industrial synthetic methods. They may also provide opportunities for hydrogen storage cycles, because the dehydrogenation reactions can be reversed under hydrogen pressure using the same catalyst. In general, AD and related dehydrogenative coupling reactions have the potential for redirecting synthetic strategies to the use of sustainable resources, devoid of toxic reagents and deleterious side reactions, with no waste generation.