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/环化反应的机理研究

Mechanistic Understanding of Rh(Ⅲ)-Catalyzed Redox-Neutral C-H Activation/Annulation Reactions of N-Phenoxyacetamides and Methyleneoxetanones

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.

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).