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