Polyfluoroallyl cations

Polyfluoropropenes (CF3·CFCFX) have been treated with antimony pentafluoride in sulphur dioxide at low temperatures. When X =p-MeO·C6H4, both cis- and trans-isomers gave the same long-lived allyl cation, which was observed by n.m.r. spectroscopy; quenching with methanol gave methyl 2,3-difluoro-3-(p-methoxyphenyl)-acrylate. In the case X = OMe, there is some C-1, C-3 interaction in the observed allyl cation; in the cases X = Ph or Me species were obtained which were not characterised. A long-lived ion was not observed with hexafluoropropene but dimerisation occurred. Cyclobutenium ions have been generated from 1,2-dimethyl- and 1,2-dimethoxy-3,3,4,4-tetrafluorocyclobutenes; these ions have charge concentrated at C-1 and C-3.