DFT Mechanistic Study of Ru-II-Catalyzed Amide Synthesis from Alcohol and Nitrile Unveils a Different Mechanism for Borrowing Hydrogen
DFT Mechanistic Study of Ru-II-Catalyzed Amide Synthesis from Alcohol and Nitrile Unveils a Different Mechanism for Borrowing Hydrogen
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Ru-II 催化醇和腈合成酰胺的 DFT 机理研究揭示了一种不同的借氢机制
DOI：
10.1021/cs5008156
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发表时间：
2014
期刊：
影响因子：
12.9
通讯作者：
Wang Zhi-Xiang
中科院分区：
文献类型：
--
作者：
Song Chunyu;Qu Shuanglin;Tao Yuan;Dang Yanfeng;Wang Zhi-Xiang
以RuIIcomplex为介体，Hong及其同事最近开发了一种氧化还原中性的合成策略，以完全原子经济性从伯醇和腈制备酰胺。被这种新的策略所吸引，我们进行了DFT计算来解开系统的催化机制。RuIIH_2（CO）（PPh_3）（IiPr）（IiPr = 1，2-二异丙基咪唑-2-亚基）催化的转化过程包括腈还原、醇脱氢、C-N偶联和酰胺生成四个阶段。醇脱氢反应一般采用双偶联法（DHC）或借氢法（BHM），将醇的Hα和羟基HOH原子转移到催化剂上，生成催化剂-H2氢化物。同时，醇脱氢与腈加氢耦合，醇作为还原剂直接将OH转移到腈N原子上，Hα转移到催化剂上，同时部分氧化。在我们提出的优选机理-B中，催化剂的Ru II态在整个催化循环中保持不变。机理A是RuII→ Ru 0 → RuII氧化态的交替，Ru 0中间体通过氧化加成和β-氢化物消除分别与醇反应。因此，机制B在能量上比机制A更有利。在机理B中，酰胺键的（N-）H原子仅来自醇的羟基HOH。相比之下，机制A中的（N-）H原子来自醇的HOH或Hα。腈的借氢方式是通过参与醇的脱氢反应，这与传统的DHC/BHM反应不同，有助于拓展DHC和BHM策略的应用范围，开辟新的途径。
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.