Hydrogen donors in catalytic transfer hydrogenation: structure-reactivity relationships and mechanistic perspectives

Catalytic transfer hydrogenation (CTH) has emerged as a powerful and versatile alternative to high-pressure hydrogenation, providing enhanced safety, operational simplicity, and broad functional-group compatibility. This review presents a comprehensive analysis of hydrogen donors used in CTH, ranging from classical alcohols, formic acid, and organic hydride donors to emerging sustainable reductants such as glycerol, benzothiazoline, amine-boranes, and 1,4-butanediol. The structural, thermodynamic, and electronic features governing donor activation and hydrogen-release pathways are examined alongside the diverse mechanistic manifolds including MPV-type outer-sphere transfer, metal-hydride shuttling, metal-ligand cooperativity, base-promoted reductions, and concerted double-hydrogen transfer. Parallel advances in catalyst platforms are highlighted, encompassing noble and earth-abundant transition metals, pincer complexes, NHC-based systems, nanoparticles, main-group promoters, and metal-free bases. Representative applications across carbonyl, imine, nitrile, olefin, alkyne, and heteroarene reductions illustrate the expanding utility of CTH in modern synthesis. Emerging themes such as aqueous-phase processes, asymmetric transfer hydrogenation, biomass valorization, and sustainable donor-catalyst assemblies are also discussed. Together, these developments establish CTH as a central and rapidly advancing methodology, with significant potential for industrial implementation and future sustainable transformations.