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

Christopher S Kuehner; Andrew G Hill; Caleb F Harris; Christian A Owens; John Bacsa; Jake D Soper

doi:10.1021/acscatal.3c03832

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

ACS Catal. 2023 Oct 9;13(20):13607-13617. doi: 10.1021/acscatal.3c03832. eCollection 2023 Oct 20.

Authors

Christopher S Kuehner¹, Andrew G Hill¹, Caleb F Harris¹, Christian A Owens¹, John Bacsa^{1

2}, Jake D Soper¹

Affiliations

¹ School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States.
² X-ray Crystallography Center, Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, United States.

Abstract

A cobalt photocatalyst for direct trifluoromethylation of (hetero)arene C(sp²)-H bonds is described and shown to operate via visible light activation of a Co-CF₃ 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-CF₃ complex two oxidation states above Co(II). Computational and spectroscopic studies are consistent with formulation of the product as [(OCO^•)Co^III(CF₃)(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-CF₃ bond homolysis and release of ^•CF₃, 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^IIL] (III) (L = THF, MeCN) with Umemoto's dibenzothiophenium trifluoromethylating reagent (1) followed by photolytic Co-CF₃ 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-CF₃ bond homolysis at II occurs via an ligand-to-metal charge-transfer (LMCT) (OCO⁰)Co^II(CF₃) 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.