Interplay of arene radical cations with anions and fluorinated alcohols in hole catalysis

Chemical reactions via radical cation intermediates are of great interest in photoredox catalysis and electrosynthesis, while their reactivities are not clearly understood. For example, how the counter anions correlate with the reactivity of radical cations is still ambiguous. Here we report the effect of anions and fluorinated alcohols on the reactivity of organic radical cations in hole catalysis. The addition of salts in a radical cation Diels–Alder reaction under photoredox catalysis demonstrates that common anions significantly decease the efficiency of hole catalysis. The use of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) restores the reaction efficiency in the presence of salts, presumably due to solvation of the anions by HFIP to reduce their nucleophilicity. These findings enable hole catalysis under electrolytic conditions with greatly improved efficiency. The effect of anions and fluorinated alcohol described in this paper gives important insights on the fundamental understanding for the reactivity of arene radical cations.