Mechanistic Studies of the Rhodium-Catalyzed Direct C–H Amination Reaction Using Azides as the Nitrogen Source

Direct
C–H amination of arenes offers a straightforward
route to aniline compounds without necessitating aryl (pseudo)­halides
as the starting materials. The recent development in this area, in
particular in the metal-mediated transformations, is significant with
regard to substrate scope and reaction conditions. Described herein
are the mechanistic details on the Rh-catalyzed direct C–H
amination reaction using organic azides as the amino source. The most
important two stages were investigated especially in detail: (i) the
formation of metal nitrenoid species and its subsequent insertion
into a rhodacycle intermediate, and (ii) the regeneration of catalyst
with concomitant release of products. It was revealed that a stepwise
pathway involving a key Rh­(V)–nitrenoid species that subsequently
undergoes amido insertion is favored over a concerted C–N bond
formation pathway. DFT calculations and kinetic studies suggest that
the rate-limiting step in the current C–H amination reaction
is more closely related to the formation of Rh–nitrenoid intermediate
rather than the presupposed C–H activation process. The present
study provides mechanistic details of the direct C–H amination
reaction, which bears both aspects of the inner- and outer-sphere
paths within a catalytic cycle.