Electrophilic reactions electrochemical conditions

Pyridine is converted into perfluoropiperidine (82) in low yield by reaction with fluorine in the presence of cobalt trifluoride (50JCS1966) quinoline affords (83) under similar conditions (56JCS783). Perfluoropiperidine can be obtained electrochemically. This is useful, as it may be readily aromatized to perfluoropyridine by passing it over iron or nickel at ca. 600 °C (74HC(14-S2)407). Recently, pyridine has been treated with xenon difluoride to yield 2-fluoropyridine (35%), 3-fluoropyridine (20%) and 2,6-difluoropyridine (11%), but it is not likely that this is simply an electrophilic substitution reaction (76MI20500). 

The general reaction pattern for the reduction of group 14 organometal halides is shown in Eq. (45) [166]. The one-electron reduction of metal halides leads to the formation of the metal centered radicals. The metal radicals may undergo radical coupling to produce metal-metal bonds or addition to unsaturated compounds. The metal radical may also be reduced electrochemically under some conditions to give the metal anions, which react with electrophiles. 

A closer look at the optimum reaction conditions and at the byproduct spectrum reveals that the observed reactivity can most probably be attributed to a chain reaction involving electrochemically generated and regenerated perfluoroalkyl radicals [6] (Scheme 2.97). In contrast to alkyl radicals, perfluoroalkyl radicals are rather electrophilic in nature. Therefore, the radical pathway sometimes mimics the outcome of a nucleophilic substitution on a perfluoroalkyl bromide or iodide. 

The regiochemistry of the coupling of Ar with different bidentate nitrogen heteroaryl anions has been investigated. For example, diverse arylpyrroles, arylindoles, and arylimidazoles are synthesized electrochemically by reaction of pyrrolyl [52], indolyl [53], and imidazolyl [52] anions in liquid ammonia with ArX with a C—C bond formation. The most reactive positions of nitrogen heteroaryl carbanions under conditions are the same as those observed in electrophilic substitution. For pyrrolyl ion, substitution at C-2 on the heterocycle is the main product [52]. For indolyl ions, substitution at C-3 is the major product observed [53]. The imidazolyl anion is the least reactive and leads to a mixture of arylation products at the C-2 and CA(5), where the 4-position was estimated to be about four times more reactive (Eq. 10.21) [53] ... 

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