Light-Driven N-Heterocyclic Carbene Catalysis Using Alkylborates

Abstract

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.

Enantioselective functionalization at the C4 position of pyridinium salts through NHC catalysis

“…Since these pioneering studies, important contributions that enable the direct assembly of functional molecules with broad utility have been made to this field. Recently, Ohmiya and coworkers reported that the NHC-catalyzed SET process has important potential for carbon–carbon bond formation through radical–radical coupling 42 – 46 . Moreover, asymmetric radical reactions under chiral NHC catalysis offer unique opportunities for unconventional approaches to the synthesis of chiral molecules.…”

Research Progress on N-Heterocyclic Carbene Catalyzed Reactions for Synthesizing Ketones through Radical Mechanism

“…Scheme 30 NHC-catalyzed light-induced direct and conjugative cross-coupling of acyl Imidazoles with alkylborates 42 Ohmiya reported NHC-catalyzed direct and conjugative cross-coupling of acyl Imidazoles with alkylborates under the illumination of blue light (Scheme 30). 42 Compared with the Template for SYNTHESIS Thieme previous method using NHPI ester as alkyl radical precursor, 12 the alkene alkylacylation demonstrates a much broader substrate applicability. The redox photocatalyst is unnecessary in the protocol, because the photoexcitable borates possess sufficient reduction potential (E1/2 = ca.…”

“…Scheme 30 NHC-catalyzed light-induced direct and conjugative crosscoupling of acylimidazoles with alkylborates 42 Potassium trifluoroborate salts are convenient and stable reagents for alkyl transfer, and can also yield an alkyl radical under photoredox conditions. The synthesis of aryloxy ketones via NHC/photoredox catalyzed aryloxylacylation of olefins with trifluoroborate salts was described by Scheidt and co-workers (Scheme 31).…”

The Impact of Boron Hybridisation on Photocatalytic Processes

“…The applicability of this activation mode has been demonstrated in many reactions including addition to heteroarenes, Giese‐type addition, and cross‐coupling [53] . Since these seminal studies, the group have made further advances in ligand design improving synthesis and absorption properties, [54] while the boronate has also been strategically aligned in NHC‐catalysed methods [55] …”