Mechanistic Studies on the Rh(III)-Mediated Amido Transfer Process Leading to Robust C–H Amination with a New Type of Amidating Reagent

Mechanistic investigations on the
Cp*Rh­(III)-catalyzed direct C–H
amination reaction led us to reveal the new utility of 1,4,2-dioxazol-5-one
and its derivatives as highly efficient amino sources. Stepwise analysis
on the C–N bond-forming process showed that competitive binding
of rhodium metal center to amidating reagent or substrate is closely
related to the reaction efficiency. In this line, 1,4,2-dioxazol-5-ones
were observed to have a strong affinity to the cationic Rh­(III) giving
rise to dramatically improved amidation efficiency when compared to
azides. Kinetics and computational studies suggested that the high
amidating reactivity of 1,4,2-dioxazol-5-one can also be attributed
to the low activation energy of an imido-insertion process in addition
to the high coordination ability. While the characterization of a
cationic Cp*Rh­(III) complex bearing an amidating reagent was achieved,
its facile conversion to an amido-inserted rhodacycle allowed for
a clear picture on the C–H amidation process. The newly developed
amidating reagent of 1,4,2-dioxazol-5-ones was applicable to a broad
range of substrates with high functional group tolerance, releasing
carbon dioxide as a single byproduct. Additional attractive features
of this amino source, such as they are more convenient to prepare,
store, and use when compared to the corresponding azides, take a step
closer toward an ideal C–H amination protocol.