Polyhalogenoallenes. Part 11. The preparation of some new perfluoro-1,2-dienes and their reactions with nucleophiles

Abstract

Perfluoro(3,4-dimethylpenta-1,2-diene), perfluoro(3-methylhexa-1,2-diene), and perfluoro(3-methylpenta-1,2-diene) are readily prepared, almost quantitatively, by copper-induced de-iodofluorination. With methanol containing a small amount of methoxide ion, the dimethylpenta-1,2-diene gives almost exclusively 2-methoxyper-fluoro(3,4-dimethylpenta-1,3-diene), which undergoes spontaneous ring closure at room temperature. Under similar conditions the other two allenes give mixtures of the 1 : 1-adducts, formed by nucleophilic attack at C-2 and proton abstraction at C-3, and the cis- and trans-isomers of the 2-methoxy-1,3-dienes. The cis-dienes undergo more rapid ring closure than the trans-dienes. With caesium fluoride, the dimethylpenta-1,2-diene gives a low yield of 2H-perfluoro(3,4-dimethylpent-2-ene) by fluoride ion attack on the terminal :CF₂ group and abstraction of a proton, but the major products are formed by fluoride ion attack at C-2 and expulsion of the tertiary fluorine from C-4, followed by cyclisation or further rearrangement of the 1,3-diene.