Electrochemical studies of six N-F electrophilic fluorinating reagents

The electrochemical reduction of six N-F electrophilic fluorinating reagent s has been studied in acetonitrile on platinum electrodes. The overall mech anism for reduction on platinum electrodes has been shown to be a le(-) pro cess on both long and short time scales via cyclic voltammetry and controll ed potential coulometry experiments. The voltammetry revealed that HF was a probable product of the reduction. Five of the compounds were shown to rea ct irreversibly with fluoride ion, a reaction that could deplete the amount of starting material leading to an overall process requiring le(-) per mol ecule. However, the rate constant for this reaction would need to be unreas onably large to account for the result. The products from the controlled po tential electrolysis of 1-fluoropyridinium tetrafluoroborate were determine d to be pyridine, 2-fluoropyridine, and 2-acetamidopyridine in yields (in t erms 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-fluoro pyridinium ion. The initial step in this mechanism is the formation of a ne utral radical by le(-) reduction of 1-fluoropyridinium, This radical dimeri zes quickly to N,N'-difluorotetrahydro-4,4'-bipyridine. The formation of th is dimer was postulated to be a reversible process. Loss of fluoride ion fr om 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 observ ed from the controlled potential coulometry of 1-fluoropyridinium tetrafluo roborate. (C) 1999 Elsevier Science S.A. All rights reserved.