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Transfer of hydrogen from diimide (HN=NH) to a carbon–carbon multiple bond constitutes diimide reduction. The reagent is generated in situ, typically by oxidation of hydrazine or by thermal decomposition of an arylsulfonylhydrazide, and delivers both hydrogen atoms in a concerted, syn fashion to an alkene or alkyne through a six-membered transition state. Carbonyl groups, nitro groups, and most aromatic rings are not reduced under standard conditions, so the process is a chemoselective saturation of C=C and C≡C bonds rather than a general hydride reduction of carbonyls to alcohols. The method applies to solution-phase and solid-supported alkenes ^[1]. Organocatalytic generation of diimide enables reduction of enamides in water while preserving the same HN=NH delivery step ^[2]. Selective diimide reduction has been used to saturate a strained alkene in the presence of a peroxide, underscoring discrimination between a C=C bond and other reducible functions ^[3]. The concept is bounded as stoichiometric or catalytic use of diimide as the hydrogen donor, distinct from catalytic hydrogenation on a metal surface and from dissolving-metal reduction.