Mechanistic Understanding of the Aryl-Dependent Ring Formations in Rh(III)-Catalyzed C–H Activation/Cycloaddition of Benzamides and Methylenecyclopropanes by DFT Calculations

The divergence between Rh­(III)-catalyzed C–H activation/cycloaddition of phenyl- and 2-furanyl-containing benzamides with methylenecyclopropanes (MCP) was studied by DFT calculations. Calculations found that the C–H activation via a CMD mechanism is the most difficult step of the reaction involving phenyl. In contrast, the C–H activation of the 2-furanyl-containing substrate is kinetically easier but the formed five-membered rhodacycle is relatively unstable, making the following MCP insertion more difficult. Thus, different KIE data was obtained in experiments. The MCP insertion forms a seven-membered-ring rhodacycle intermediate, from which the chemoselectivity of the whole reaction is determined by the competitive pivalate migration (path I) and β-C elimination (path II). While the β-C elimination is lower in energy when a furanylene is contained in the intermediate, a reversed preference of pivalate migration was predicted for the phenylene counterpart. Structural analysis suggested that the unfavorable β-C elimination in the phenylene case should be attributed to the obviously increased ring strain in the corresponding transition state, instead of the difference in electronic properties between the aryl groups. This accounts for why aryl-dependent chemoselectivity was observed. In addition, the results indicated that for both paths I and II the generation of a Rh­(V)–nitrenoid intermediate from pivalate migration is crucial for the final C–N bond formation. This explains why no reaction occurred when the N–OPiv moiety was replaced with an N–OMe group, as no Rh­(V) intermediate could be formed in this system.