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 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.