Water as a Sustainable Hydrogen Donor in Transfer Hydrogenation and Reduction Reactions

This personalized overview examines the growing use of water (H 2 O) and heavy water (D 2 O) as accessible, environmentally friendly, and cost‐effective hydrogen and deuterium donors in transfer hydrogenation and reduction processes. Structured by substrate type, the analysis begins with the stereoselective partial hydrogenation of alkynes, in which catalytic approaches (employing Pd, Ni, or Co) and catalyst‐free methodologies leverage water to direct alkene configuration and enable straightforward synthesis of deuterium‐labeled compounds. The review proceeds to examine the selective hydrogenation of alkenes, dienes, and enones based on functional group compatibility. Water's applicability extends to heteroarene reduction and the direct transformation of nitro compounds and azides into amines. Regarding carbonyl reduction, two distinct strategies are evaluated: approaches involving metal–hydride intermediates and an alternative metal‐free umpolung strategy using diboron species. Water further facilitates specialized transformations, including the cleavage of CC single bonds via hydrogenolysis and deoxygenation. This review underscores the fundamental mechanisms by which water is activated, including metal–hydride generation, hydrogen‐atom‐transfer processes, and diboron‐mediated routes, along with precise regulation of chemoselectivity, regioselectivity, and stereoselectivity, and extensive compatibility with various functional groups, as evidenced by more than 50 landmark investigations. The discussion emphasizes D 2 O's practical advantages for simple and economical deuterium incorporation. Taken together, this research demonstrates that water functions not simply as an environmentally benign solvent but as a multifunctional, safe, and essential reactant, providing an enhanced and sustainable alternative that competes with conventional hazardous hydride reagents and high‐pressure gaseous hydrogen in organic synthesis.