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

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 beta-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).

Automated summary

Density functional theory (DFT) calculations revealed the mechanism of Rh(III)-catalyzed C-H activation/annulation reactions of N-phenoxyacetamides with methyleneoxetanones. Direct O-N bond cleavage to form a Rh(V)-nitrenoid species was energetically unfavorable, while sequential beta-H elimination/reductive elimination was more facile. The experimental outcomes were explained by the catalytic cycle of Rh(III)/Rh(I)/Rh(III) suggested by DFT.