N-Atom Transfer Mechanisms

We are particularly interested in determining if metal nitrenoids are reactive intermediates in these reactions or if the transition metal functions as a
Lewis acid. While a mechanism involving a nitrenoid intermediate is possible, other pathways could account for product formation (Scheme 1). Nitrenoid formation could be avoided through a Schmidt-like mechanistic shunt. Alternatively, rhodium could function as a π-Lewis acid to mediate a vicarious nucleophilic substitution reaction.


Scheme 1. Possible mechanisms.


We completed a
study that examined the mechanism of carbazole formation from triaryl azides. Our intramolecular competition experiments suggested that nitrenoid formation was assisted by electron-donating groups. This assistance enabled C–N bond formation to occur via a 5-atom-4π-electron electrocyclization (Scheme 2).


Scheme 2. Mechanism for carbazole formation.


In contrast to the reactivity trends that we observed using rhodium carboxylate complexes, we found that
iridium(I)-catalyzed benzylic C–H bond amination was accelerated when the aryl azide was rendered electron deficient. We also observed a significant intramolecular kinetic isotope effect of 5 for this transformation. We have interpreted these results as evidence to suggest that C–N bond formation occurs through either a concerted or stepwise reaction of the iridium nitrenoid with the benzylic C–H bond (Scheme 3).


Scheme 3. Mechanism for indoline formation.