Electrochemical studies of six N-F electrophilic fluorinating reagents

Abstract

The electrochemical reduction of six N-F electrophilic fluorinating reagents has been studied in acetonitrile on platinum electrodes. The overall mechanism for reduction on platinum electrodes has been shown to be a le- process on both long and short time scales via cyclic voltammetry and controlled potential coulometry experiments. The voltammetry revealed that HF was a probable product of the reduction. Five of the compounds were shown to react irreversibly with fluoride ion, a reaction that could deplete the amount of starting material leading to an overall process requiring le- per molecule. However, the rate constant for this reaction would need to be unreasonably large to account for the result. The products from the controlled potential electrolysis of 1-fluoropyridinium tetrafluoroborate were determined to be pyridine, 2-fluoropyridine, and 2-acetamidopyridine in yields (in terms of the pyridine moiety) of 36, 14, and 42%, respectively. This product distribution, combined with reactivity of these compounds toward fluoride ion, was used to propose an overall mechanism for the reduction of 1-fluoropyridinium ion. The initial step in this mechanism is the formation of a neutral radical by le- reduction of 1-fluoropyridinium. This radical dimerizes quickly to N,N′-difluorotetrahydro-4,4′-bipyridine. The formation of this dimer was postulated to be a reversible process. Loss of fluoride ion from the neutral radical in equilibrium with the dimer is believed to be the initial step in a multi-step mechanism which results in the products observed from the controlled potential coulometry of 1-fluoropyridinium tetrafluoroborate.