AbstractIn the borrowing hydrogen catalysis, hydrogenation of an in situ generated imine or olefinic bond is a crucial step. There is a growing body of literature in olefinic hydrogenation promoted by metal hydride of Earth‐abundant metals, where radical mechanism is followed. This report presents a thorough study of the mechanistic details of a nickel catalyzed α‐alkylation of ketones with secondary alcohols and showcases that the olefinic hydrogenation of an enone happens, completely bypassing the involvement of a metal hydride. This pathway is radical promoted, where a single electron reduction of the substrate olefin and a subsequent hydrogen atom transfer step are most critical. A series of control reactions, detection of critical reaction intermediates, and radical probe experiments provide compelling proofs for such radical‐promoted olefinic hydrogenation. The experimental clues, further aided by DFT calculations altogether suggest the precise one‐electron chemistry where the involvement of metal‐hydride is not required. Notably, the redox non‐innocence of the azophenolate backbone, as well as imposed noninnocence of the substrate olefin, when bound to the catalyst molecule makes such mechanism feasible.
摘要 在借氢催化中,原位生成的亚胺或烯烃键的氢化是一个关键步骤。有关地球富集金属氢化物促进烯烃氢化的文献越来越多,其中遵循的是自由基机制。本报告对镍催化α-烷基化酮与仲醇的机理细节进行了深入研究,并展示了烯酮的烯烃氢化完全绕过了金属氢化物的参与。这一途径由自由基促进,其中底物烯烃的单电子还原和随后的氢原子转移步骤最为关键。一系列控制反应、关键反应中间产物的检测以及自由基探针实验为这种自由基促进的烯烃氢化提供了有力的证明。在 DFT 计算的进一步帮助下,实验线索完全表明了精确的单电子化学反应,其中不需要金属酸酐的参与。值得注意的是,偶氮苯酚骨架的氧化还原不失效以及底物烯烃与催化剂分子结合时的强加不失效使得这种机制变得可行。