Cobalt Photoredox Catalysts for Selective Radical Trifluoromethylations


Direct trifluoromethylation of unactivated C–H bonds offers a powerful, atom-economical route to CF3-containing molecules of interest in pharma and agrochemicals. Photoredox methods to generate electrophilic •CF3 are robust and powerful, but outer-sphere methods suffer from the lack of selectivity inherent to •CF3 radicals. Organometallic approaches tantalize, but metal-catalyzed trifluoromethylations are frequently challenged by the intrinsic properties of the M−CF3 intermediates. Whereas early transition metal M−CF3 bonds readily undergo α-fluoride abstraction to generate difluoromethyl carbene complexes, M−CF3 bonds to low-valent later 3d metals are often thermodynamically robust and kinetically inert. We have developed a photoinduced method to activate thermodynamically stable CoIII–CF3 bonds for selective arene C–H trifluoromethylations. Central to this strategy is the capacity of pincer (OCO)Co complexes to function as combined chromophores and organometallic reaction centers. Low energy visible light capture leads to intramolecular LMCT from a redox-active [OCO] pincer ligand to Co, which destabilizes the Co–CF3 bond toward homolysis and release of a persistent •CF3 radical. The capacity of the Co(II) byproduct to catalyze H2 production permits direct arene C–H trifluoromethylation without a sacrificial or substrate-derived oxidant. This seminar will describe our recent extensions to photocatalysis and ongoing efforts to elaborate regiospecific radical substitution via intramolecular photolytic arene C–H trifluoromethylations and alkylations. Structure-property relationships will be presented, which are guiding new catalyst designs and establishing design principles for next generation base metal photocatalysis.