Photocatalytic monofluorination of benzene by fluoride via photoinduced electron transfer with 3-cyano-1-methylquinolinium

The photocatalytic fluorination of benzene occurs under photoirradiation of an oxygen-saturated acetonitrile (MeCN) of the 3-cyano-1-methylquinolinium ion (QuCN⁺) containing benzene and tetraethylammonium fluoride tetrahydrofluoride (TEAF·4HF) with a xenon lamp (500 W) attached to a colored-glass filter (λ < 290 nm) to yield fluorobenzene and hydrogen peroxide. The quantum yield of formation of fluorobenzene was 6%. Nanosecond laser flash photolysis measurements were performed to elucidate the mechanistic details for photocatalytic fluorination. Transient absorption spectra taken after the nanosecond laser excitation at 355 nm of a degassed MeCN solution of QuCN⁺ and benzene exhibited absorption bands due to QuCN ^• (λ_max = 500 nm) and the benzene dimer radical cation (λ_max = 900 nm), which were generated by photoinduced electron transfer from benzene to the singlet excited state of QuCN⁺. The decay rate of the transient absorption band due to the benzene dimer radical cation was accelerated by the addition of TEAF·4HF. The observed rate constant increased with increasing concentration of TEAF·4HF. The rate constant of the electrophilic addition of fluoride to the benzene radical cation was determined to be 9.4 × 10⁹ M^-1 s^-1. Thus, the photocatalytic reaction is initiated by intermolecular photoinduced electron transfer from benzene to the single excited state of QuCN⁺. The benzene radical cation formed by photoinduced electron transfer reacts with the fluoride anion to yield the F-adducted radical. However, QuCN^• can reduce O₂ to O₂^•-, and this is followed by the protonation of O₂^•- to afford HO₂^•. The hydrogen abstraction of HO₂^• from the F-adduct radical affords fluorobenzene and H₂O₂ as the final products.