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

Abstract

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 of Rh, the Rh-imido formation step is rate-limiting. These qualitative differences stem from a unique bonding feature of the dioxazole moiety and the relativistic contraction of the Ir(V), which affords much more favorable energetics for the reaction. For the first time, a full molecular orbital analysis is presented to rationalize and explain the electronic features that govern this behavior.