Why is the Ir(III)-Mediated Amido Transfer Much Faster Than the Rh(III)-Mediated Reaction? A Combined Experimental and Computational Study

The mechanism of the Ir(III)- and Rh(III)-mediated C–N coupling reaction, which is the key step for catalytic C–H amidation, was investigated in an integrated experimental and computational study. Novel amidating agents containing a 1,4,2-dioxazole moiety allowed for designing a stoichiometric version of the catalytic C–N coupling reaction and giving access to reaction intermediates that reveal details about each step of the reaction. Both DFT and kinetic studies strongly point to a mechanism where the M(III)-complex engages the amidating agent via oxidative coupling to form a M(V)–imido intermediate, which then undergoes migratory insertion to afford the final C–N coupled product. For the first time, the stoichiometric versions of the Ir- and Rh-mediated amidation reaction were compared systematically to each other. Iridium reacts much faster than rhodium (∼1100 times at 6.7 °C) with the oxidative coupling being so fast that the activation of the initial Ir(III)-complex becomes rate-limiting. In the case...