DFT mechanistic study of RuII-catalyzed amide synthesis from alcohol and nitrile unveils a different mechanism for borrowing hydrogen

Chunyu Song, Shuanglin Qu, Yuan Tao, Yanfeng Dang, Zhi Xiang Wang

Research output: Contribution to journalArticlepeer-review

24 Scopus citations

Abstract

Using RuIIcomplex 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-N coupling, and amide production. Generally, alcohol dehydrogenation in dehydrogenative coupling (DHC) or borrowing hydrogen methodology (BHM) takes place separately, transferring the Hαand hydroxyl HOHatoms of alcohol to the catalyst to form the catalyst-H2hydride. Differently, the alcohol dehydrogenation in the present system couples with nitrile hydrogenation; alcohol plays a reductant role to aid nitrile reduction by transferring its HOHto nitrile N atom directly and Hαto the catalyst and meanwhile becomes partially oxidized. In our proposed preferred mechanism-B, the RuIIstate of the catalyst is retained in the whole catalytic cycle. Mechanism-A, postulated by experimentalists, involves RuII→ Ru0→ RuIIoxidation state alternation, and the Ru0intermediate is used to dehydrogenate alcohol separately via oxidative addition, followed by β-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 HOHof alcohol. In comparison, the (N-)H atom in mechanism-A stems from either HOHor Hαof 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. (Chemical Equation Presented)

Original languageEnglish (US)
Pages (from-to)2854-2865
Number of pages12
JournalACS Catalysis
Volume4
Issue number9
DOIs
StatePublished - Sep 5 2014

Bibliographical note

Publisher Copyright:
© 2014 American Chemical Society.

Keywords

  • Acceptorless dehydrogenation
  • Alcohol dehydrogenation
  • Amide synthesis
  • Borrowing hydrogen methodology
  • DFT computations
  • Dehydrogenative coupling

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