Electrochemically induced reactions of some halogenated organic compounds (Part I) ; Novel rearrangements of pyridazine derivatives (Part II)
The reduction of pentafluoropyridine gave, as the major products, octafluoro-4,4'-bipyridyl in aprotic systems and 4-H-tetrafluoropyridine with added proton donor. Garbanionic intermediates were not trapped by carbon dioxide to give acids. Voltammetric results for a series of polyfluoropyridines suggest that the negative charge in the radical anion is mainly in the 4-position. Reduotion of pentaohloropyridine in dime thylformamide gave 4-H-tetrachloropyridine, with some 4,4'-bistetraohloropyridylmeroury when using a mercury cathode. Intermediate carbanions were trapped by carbon dioxide, giving an acid mixture, which yielded 4-H-tetrachloropyridine and heptachlorobipyridyl on decarboxylation. Hexaohlorobenzene behaved similarly. Simple perfluoro-olefins eliminated fluoride ion on reduction and were subsequently oligomerised by it. Intermediate carbanions were trapped by water. The electrochemical products were high-boiling oils, resulting from reaction with the solvent. Oxidation of polyfluoroanisoles containing para hydrogen gave coupling to the corresponding 4,4'-dimethoxybiphenyl derivative. No coupling occurred when this position was blocked by fluorine. Trapping of intermediate radical cations with acetic acid was unproductive. PART II The photochemical and thermal rearrangements of a number of perfluoroalkylpyridazines in static systems was investigated. As had previously been reported, photolysis of perfluoro-4,5-dialkyl- pyridazines gave mainly perf1uoro-2,6-dialkylpyrazines, although in some cases perfluoro-2,6-dlal]sylpyrazines were also formed. The pyrolysis of perfluoro-4,5-clialkylpyridazines gave mainly perfluoro-4,5-dialkylpyrimidines and in some cases a small amount of the -2,5-pyrazine. Perfluoro-4-alkylpyridazines gave mixtures of perfluoro- 4-alkyl- and -5-alkylpyrimidines. These results can be explained by a mechanism involving diazabenzvalene Intermediates. The photolysis and pyrolysis of mixtures gave no exchange of substituent groups indicative of a bimolecular process. In the pyrolyses, the conversios of the less reactive pyridazlne was catalysed and that of the more reactive pyrldazlne Inhibited. The relative amounts rearranged ruled out a 1 : 1 Intermediate and a Slels-Alder mechanism, which would involve exchange of nitrogen, was further ruled out by (^15)N-labelling. The catalytic mechanism proposed Involves a transfer of vibrational excitation.