A General Hydrotrifluoromethylation of Unactivated Olefins Enabled by Voltage-Gated Electrosynthesis.

Abstract

Here we present the first successful hydrotrifluoromethylation of unactivated olefins under electrochemical conditions. Commercially available trifluoromethyl thianthrenium salt (TT⁺-CF₃BF₄ ^-, E_p/2=-0.85 V vs Fc/Fc⁺) undergoes electrochemical reduction to generate CF₃ radicals which add to olefins with exclusive chemoselectivity. The resulting carbon centered radical undergoes a second cathodic reduction, instead of a classical HAT process, to generate a carbanion that can be terminated by protonation from solvent. The use of MgBr₂ (+0.20 V onset oxidation potential) plays a key role as an enabling sacrificial reductant for the reaction to operate in an undivided cell. Guided by cyclic voltammetry (CV) studies, fine-tuning the solvent system, trifluoromethylating reagent's counteranion and careful selection of redox processes, this work led to the development of a voltage-gated electrosynthesis by pairing two redox processes with a narrow potential difference (ΔE≈1.00 V) allowing the reaction to proceed with two important advances: (a) high reactivity and selectivity towards hydrotrifluoromethylation over undesired dibromination, and (b) an unprecedented functional group tolerance, including aniline, phenols, unprotected alcohol, epoxide, trialkyl amine, and several redox sensitive heterocycles.