Alkanes are an abundant and inexpensive
source of hydrocarbons;
thus, development of new methods to convert the hydrocarbon feedstocks
to value-added chemicals is of high interest. However, it is challenging
to achieve such transformation in a direct and selective manner mainly
due to the intrinsic inertness of their C–H bonds. We herein
report a tailored Cp*Co(III)(LX)-catalyzed efficient and site-selective
intermolecular amidation of unactivated hydrocarbons including light
alkanes. Electronic modulation of the cobalt complexes led to the
enhanced amidation efficiency, and these effects were theoretically
rationalized by the FMO analysis of presupposed cobalt nitrenoid species.
Under the current cobalt protocol, a secondary C–H bond selectivity
was observed in various nonactivated alkanes to reverse the intrinsic
tertiary preference, which is attributed to the steric demands of
the cobalt system that imposes difficulties in accessing tertiary
C–H bonds. Experimental and computational studies suggested
that the putative triplet Co nitrenoids are transferred to the C–H
bonds of alkanes via a radical-like hydrogen abstraction pathway.
Herein, we report the development
of a tailored cobalt catalyst
system of Cp*Co(III)(LX) toward intramolecular C–H nitrene
insertion of azidoformates to afford cyclic carbamates. The cobalt
complexes were easy to prepare and bench-stable, thus offering a convenient
reaction protocol. The catalytic reactivity was significantly improved
by the electronic tuning of the bidentate LX ligands, and the observed
regioselectivity was rationalized by the conformational analysis and
DFT calculations of the transition states. The superior performance
of the newly developed cobalt catalyst system could be broadly applied
to both C(sp2)–H and C(sp3)–H
carbamation reactions under mild conditions.
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