Catalytic C–H Trifluoromethylation of Arenes and Heteroarenes via Visible Light Photoexcitation of a Co(III)–CF₃ Complex

Abstract

High Resolution Image Download MS PowerPoint Slide A cobalt photocatalyst for direct trifluoromethylation of (hetero)arene C(sp 2 )–H bonds is described and shown to operate via visible light activation of a Co–CF 3 intermediate, which functions as a combined chromophore and organometallic reaction center. Chemical oxidations of previously reported (OCO)Co complexes containing a redox-active [OCO] pincer ligand afford a Co–CF 3 complex two oxidation states above Co(II). Computational and spectroscopic studies are consistent with formulation of the product as [(OCO • )Co III (CF 3 )(THF)(OTf)] ( II ) containing an open-shell [OCO • ] 1– radical ligand bound to a S = 0 Co(III) center. II is thermodynamically stable, but exposure to blue (440 nm) light induces Co–CF 3 bond homolysis and release of • CF 3, which is trapped by radical acceptors including TEMPO •, (hetero)arenes, or the radical [OCO • ] ligand in II . The latter comprises a competitive degradation pathway, which is overcome under catalytic conditions by using excess substrate. Accordingly, generation of II from the reaction of [(OCO)Co II L] ( III ) (L = THF, MeCN) with Umemoto’s dibenzothiophenium trifluoromethylating reagent ( 1 ) followed by photolytic Co–CF 3 bond activation completes a photoredox catalytic cycle for C–H (hetero)arene trifluoromethylation utilizing visible light. Electronic structure and photophysical studies, including time-dependent density functional theory (TDDFT) calculations, suggest that Co–CF 3 bond homolysis at II occurs via an ligand-to-metal charge-transfer (LMCT) (OCO 0 )Co II (CF 3 ) state, revealing ligand redox activity as a critical design feature and establishing design principles for the use of base metal chromophores for selectivity in photoredox bond activations occurring via free radical intermediates.

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