Mechanistic Insights into Light-Driven Acceptorless Dehydrogenation Reactions

Dehydrogenation reactions are thermodynamically constrained by their inherent endothermic nature. The concomitant thermodynamic barriers can be overcome via photochemical strategies that harness light to activate intrinsically strong C═O, C─H, and O─H bonds. This review surveys recent progress and challenges in acceptorless light-driven dehydrogenation reactions, focusing on dehydrogenation of linear sp³-hybridized bonds, dehydrogenative coupling reactions, dehydrogenative cyclizations, and dehydrogenation of cyclic hydrocarbons. We identified distinct trends in the catalysts used for light-driven dehydrogenations, including homogeneous unary photocatalysts in which a single molecule absorbs light and catalyzes both oxidation and hydrogen evolution, cooperative homogeneous systems in which two separate catalysts fulfill these roles, as well as heterogeneous systems including nanostructured semiconductors and hybrid materials. In particular, this work uniquely synthesizes mechanistic knowledge across these classes and introduces a unifying classification framework that clarifies how distinct photochemical mechanisms achieve bond activation and hydrogen evolution without external acceptors. First, homogeneous unary photoactive Rh(I) catalysts promote dehydrogenation of both linear and cyclic sp³-hybridized C-C bonds in hydrocarbon substrates via oxidative C-H addition with subsequent β-hydride elimination. Second, binary homogeneous photocatalytic systems, consisting of a photosensitizer and a transition-metal-based proton reduction catalyst, enable all four types of dehydrogenation reactions via SET. Third, heterogeneous catalysts employed in light-driven dehydrogenation reactions often comprise a semiconductive support material integrated with a transition-metal-based active site, functioning via Mott-Schottky type photoinduced charge separation.