摘要/Abstract

N-苯氧基乙酰胺是铑(III)催化无外加氧化剂条件下碳氢活化反应中的一类典型底物. 为了研究这类分子中O-NHAc部分作为氧化导向基团的起作用方式, 通过密度泛函理论(DFT)计算研究了铑(III)催化N-苯氧基乙酰胺与亚甲基氧杂环丁酮氧化还原中性的碳氢活化/环化反应的机理问题. 结果显示, 在形成铑(III)杂七元环中间体后, 直接的O—N键断裂形成铑(V)中间体的过程在能量上是不利的. 相反, 该中间体更容易发生β-氢消除/还原消除, 从而得到铑(I)中间体, 其再通过氢转移/O—N键断裂可再生活性的铑(III)催化剂. 形成烯基化中间体后, 通过分子内的亲核取代反应即可得到最终产物. 密度泛函理论(DFT)计算揭示的铑(III)/铑(I)/铑(III)催化循环过程为反应结果提供了深入理解.
关键词: 铑(III)催化, 碳氢活化, 反应机理, 氧化导向基团, 密度泛函理论计算, N-苯氧基乙酰胺
N-Phenoxyacetamides represent one category of typical substrates for Rh(III)-catalyzed C—H activation under external oxidant free conditions. To understand how the O-NHAc unit works as the oxidizing directing group, the mechanism for the Rh(III)-catalyzed C—H activation/annulation reactions of N-phenoxyacetamides with methyleneoxetanones was studied by density functional theory (DFT) calculations. It was uncovered that after the formation of the 7-membered rhodacycle from irreversible C—H activation and olefin insertion steps, the direct O—N bond cleavage of the internal oxidant unit to form a Rh(V)-nitrenoid species was energetically unfavorable. Instead, this intermediate underwent sequential β-H elimination/reductive elimination much more easily and formed a Rh(I) species. Once the hydrogen was transferred to the NAc moiety, the regeneration of Rh(III) occurred easily by O—N bond cleavage. From the olefination intermediate, the final product was formed by an intramolecular nucleophilic substitution reaction, in which the Cp*Rh(III) could be a catalyst. The experimental outcomes are well understood by the density functional theory (DFT)-suggested catalytic cycle of Rh(III)/Rh(I)/Rh(III).
Key words: Rh(III) catalysis, C—H activation, reaction mechanism, oxidizing directing group, density functional theory (DFT) calculation, N-phenoxyacetamide


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