The
generation of tertiary, secondary, and primary alkyl radicals
has been achieved by the direct visible-light excitation of a boracene-based
alkylborate. This system is based on the photophysical properties
of the organoboron molecule. The protocol is applicable to decyanoalkylation,
Giese addition, and nickel-catalyzed carbon–carbon bond formations
such as alkyl–aryl cross-coupling or vicinal alkylarylation
of alkenes, enabling the introduction of various C(sp3)
fragments to organic molecules.
Radical–radical
coupling, the selective reaction between
two different radical species, has contributed to the methodology
for connecting bulky units. Light-driven N-heterocyclic
carbene (NHC) organocatalysis is recognized as a state-of-the-art
methodology enabling radical–radical coupling. The catalytic
process involves forming an acyl azolium intermediate from the NHC
catalyst and an acyl donor, followed by single electron reduction
of this key intermediate, which is largely dependent on the photoredox
catalyst. We designed a radical NHC catalysis in which the direct
photoexcitation of a borate to form a high reducing agent facilitated
the single electron reduction event. The borate produces an alkyl
radical for the single electron transfer process to accomplish the
radical–radical coupling. This protocol enables cross-coupling
between alkylborates and acyl imidazoles in addition to radical relay-type
alkylacylations of alkenes with alkylborates and acyl imidazoles,
affording ketones with a broad scope.
The boracene-based alkylborate enabled visible light-mediated metallaphotoredox catalysis. The directly excited borate was easily oxidatively quenched by the excited Ir photoredox catalyst. The Ni/Ir hybrid catalysis afforded the products under...
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