Polyfluroheterocyclic compounds. Part XXIII. Monoenes and dienes derived by the fluorination of hexafluorobenzene and of perfluoro- and chlorofluoro-heteroaromatic compounds. A mechanism for fluorination by cobalt fluorides

Abstract

Hexafluorobenzene, pentafluoro-, 3- and 4-chlorotetrafluoro-, and 3,4- and 3,5-dichlorotrifluoro-pyridines have been fluorinated with a mixture of cobalt(III) and calcium fluorides. Reaction conditions can be adjusted so that hexafluorobenzene gives only octafluorocyclohexa-1,4-diene and decafluorocyclohexene. Pentafluoro-, 3-chlorotetrafluoro-, and 3,5-dichlorotrifluoro-pyridines give only perhalogeno-1-azacyclohex-1-enes together with small amounts of acyclic aza-alkenes when fresh fluorinating agent is used. When there is a chlorine atom on position 4 of the ring system, cyclic dienes are also produced and may be the main products. If a much depleted fluorinating agent is used, pentafluoropyridine gives a trace of heptafluoro-1-azacyclohexa-1,3-diene. The latter can also be prepared from the 4-chloro- and 3,4-dichloro-cyclic aza-monoenes by dechlorination or by the action of fluoride ion on the 4-chloro-diene.

When tetrafluoropyrazine is fluorinated, the only product is hexafluoro-1,4-diazacyclohexa-1,3-diene.

Calculations of the charge and spin densities on the atoms in the ring system at various stages in the fluorinaton have been made and a mechanism for the process involving, at each stage, the quenching of the first formed radical cation by fluoride ion and then by a fluorine atom, is suggested. The mechanism is supported by the isolation of a dimer from the fluorination of tetrafluoropyrimidine.