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Mechanism and stereoselectivity in NHC-catalyzed β-functionalization of saturated carboxylic ester

机译：NHC催化饱和羧酸酯的β-官能化的机理和立体选择性

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摘要

To understand the mechanism and origin of the stereoselectivity of the [3 + 2] annulation reaction between a carboxylic ester and an isatin generating spirooxindole lactone catalyzed by N-heterocyclic carbene ( NHC ), density functional theory (DFT) calculations have been carried out. DFT results indicate that the catalytic cycle begins with the coupling of the catalyst with benzotriazole ester, followed by α-deprotonation to produce the enolate intermediate. The subsequent 1,4-proton transfer affords the homoenolate intermediate. The next crucial step is the stereoselective C–C bond formation. Then proton transfer takes place leading to the formation of the lactone intermediate. Finally, the elimination of the catalyst furnishes the final product. The presence of 1-hydroxybenzotriazole (HOBt) dramatically accelerates the proton transfer step. More importantly, HOBt has a non-negligible impact on stereoselective C–C bond formation, and the SR -configured product is the major stereoisomer of the annulation product, which is in good agreement with the experimental observations. The differential π?π stacking, C–H?π, lone pair (LP)?π and repulsion interactions are found to be responsible for the stereoselectivity. The obtained mechanistic insights should provide valuable information for understanding the important roles of the NHC catalyst and HOBt additive and be helpful for designing better NHC catalysts for this kind of reaction.
机译：为了了解N-杂环卡宾（NHC）催化的羧酸酯与生成Isatin的螺氧基吲哚内酯[3 + 2]环化反应的立体选择性的机理和起源，已进行了密度泛函理论（DFT）计算。 DFT结果表明，催化循环始于催化剂与苯并三唑酯的偶联，然后是α-去质子化以生成烯醇盐中间体。随后的1，4-质子转移得到均烯酸酯中间体。下一个关键步骤是形成立体选择性C–C键。然后发生质子转移，导致内酯中间体的形成。最后，消除催化剂提供了最终产物。 1-羟基苯并三唑（HOBt）的存在显着加速了质子转移步骤。更重要的是，HOBt对立体选择性C–C键的形成具有不可忽略的影响，并且SR构型的产物是环形产物的主要立体异构体，与实验观察结果非常吻合。差分πππ堆积，CH-ππ，孤对（LP）ππ和排斥相互作用被发现是立体选择性的原因。所获得的力学见解应为理解NHC催化剂和HOBt添加剂的重要作用提供有价值的信息，并有助于设计用于这种反应的更好的NHC催化剂。

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