Fluoro-olefin chemistry. Part 14. Preparation and photochemical rearrangement of some acyclic perfluoroalkyl and polyfluoroalkyl olefins

Abstract

Photochemical addition of iodopolyfluoroalkanes [R_FI; R_F= n-C₃F₇, (CF₃)₂CF, CF₂ClCF₂, CF₂ClCFCl] to perfluorobut-2-yne gives mixtures of the (E)- and (Z)-1 : 1-adducts [R_F(CF₃)CClCF₃]. With potassium fluoride at 220 °C, the vinylic iodine is replaced by fluorine and partial or complete rearrangement of the double bond takes place. The allylic chlorine of 4,5-dichloro-2-iodo-3-trifluoromethylhexafluoropent-2-ene is also replaced by fluorine to give a mixture of (E)- and (Z)-1-chloro- and 5-chloro-perfluoro(3-methylpent-2-enes), identical with that prepared from 5-chloro-2-iodoperfluoro(3-methylpent-2-ene).

Ultraviolet irradiation of perfluoro-2-methylhex-2-ene or -3-ene results initially in cis-trans-isomerisation, and under more vigorous conditions mixtures of two terminal olefins are formed by migration of fluorine or a perfluoroalkyl group to theCFR_F group; no migration from the CFCF₃ group to RR′C or interconversion of the methylhexenes (6) and (7) takes place. Perfluoro(2,3-dimethylpent-2-ene) showed 1,3-fluorine shifts from (CF₃)₂C CF₃C, and CF₃CF₂C, and also CF₃ group migration from the CF₃CF₂C moiety, but migration of F and CF₃ from the (C₂F₅)₂C group were the only detectable reaction of perfluoro(3-ethyl-2-methylpent-2-ene); no product corresponding to migration from the (CF₃)₂C group was observable. Interconversion of the trialkyl chlorodecafluoro-(10) and undecafluoro-olefins (11) takes place under mild conditions, but the products formed under more vigorous conditions were not positively identified. The migrations appear to involve symmetry-allowed 1,3-sigmatropic shifts of fluorine or fluoroalkyl groups.