Highly Efficient Transfer Hydrogenation of Alkenes With Ammonia Borane Mediated by a Simple Ni(II) Catalyst System

A practical nickel platform for transfer hydrogenation (TH) of alkenes using ammonia borane (AB) is described. With NiCl 2 ·6H 2 O/ethylenediamine ( L2 ) at 25–50 °C, aromatic and aliphatic alkenes reduce rapidly and α,β‐unsaturated esters undergo chemoselective C═C hydrogenation; at low loading the system achieves turnover number up to 3960. Discrete Ni(II) complexes, C1–C4 , mirror the behavior of in situ Ni/ L systems, indicating common active species. Mechanistic studies reveal that AB initiates catalysis by converting Ni(II) to Ni(0), while AB dehydrogenation supplies H 2 required for hydrogenation. Ni–H signatures appear only under specific ligand environments ( L1 ) and are transient on the NMR timescale; L2 promotes efficient initiation without detectable hydride resting states. Hg‐drop tests, observation of ligand dissociation/nickel black, AB‐purity effects, DLS, and TEM analyses confirm catalyst evolution to nickel‐containing particulate material after catalysis, while the collective mechanistic data support AB‐mediated activation of Ni(II) precatalysts and hydrogenation in the presence of H 2 generated from AB. The method uses earth‐abundant nickel, bench‐stable AB, and simple ligands to deliver fast, scalable TH under mild conditions.