文摘
Using RuII complex as a mediator, Hong and co-workers recently developed a redox-neutral synthetic strategy to produce amide from primary alcohol and nitrile with complete atom economy. Intrigued by the novel strategy, we performed DFT computations to unravel the catalytic mechanism of the system. The transformation is catalyzed by RuIIH2(CO)(PPh3)(IiPr) (IiPr = 1,2-diisopropylimidazol-2-ylidene) via four stages including nitrile reduction, alcohol dehydrogenation, C鈥揘 coupling, and amide production. Generally, alcohol dehydrogenation in dehydrogenative coupling (DHC) or borrowing hydrogen methodology (BHM) takes place separately, transferring the H伪 and hydroxyl HOH atoms of alcohol to the catalyst to form the catalyst-H2 hydride. Differently, the alcohol dehydrogenation in the present system couples with nitrile hydrogenation; alcohol plays a reductant role to aid nitrile reduction by transferring its HOH to nitrile N atom directly and H伪 to the catalyst and meanwhile becomes partially oxidized. In our proposed preferred mechanism-B, the RuII state of the catalyst is retained in the whole catalytic cycle. Mechanism-A, postulated by experimentalists, involves RuII 鈫?Ru0 鈫?RuII oxidation state alternation, and the Ru0 intermediate is used to dehydrogenate alcohol separately via oxidative addition, followed by 尾-hydride elimination. As a result, mechanism-B is energetically more favorable than mechanism-A. In mechanism-B, the (N-)H atom of the amide bond exclusively originates from the hydroxyl HOH of alcohol. In comparison, the (N-)H atom in mechanism-A stems from either HOH or H伪 of alcohol. The way of borrowing hydrogen that is used by nitrile is via participating in alcohol dehydrogenation, which is different from that in the conventional DHC/BHM reactions and may help expand the strategy and develop new routes for utilizing DHC and BHM strategies.
Keywords:
DFT computations; dehydrogenative coupling; borrowing hydrogen methodology; acceptorless dehydrogenation; amide synthesis; alcohol dehydrogenation