The perfluoro-1,3,5-hexatrienes

Abstract

trans-Perfluoro-1,3,5-hexatriene was obtained by catalytic isomerization of the cis isomer, and their configurations were confirmed by a photochemical method. Equilibration of these isomers with iodine and light at 13 o C revealed that the cis isomer is slightly more stable than the trans (K c→t =0.83). The trienes undergo reactions under the influence of heat and light which contrast with those of their hydrocarbon counterparts. Cis triene cyclizes reversibly to perfluoro-3-vinylcyclobutene at 160 o C (K eq =12.2), and at higher temperatures perfluoro-1,3-cyclohexadiene is formed irreversibly. Cyclization of trans triene to the vinylcyclobutene is much slower than that of the cis isomer, as required by the finding that thermal ring opening of the vinylcyclobutene gives cis triene cleanly. Ultraviolet irradiation of the trienes with and without mercury sensitization yields the same two cyclization products, but the cyclohexadiene undergoes a further rapid photocyclization to give perfluorobicyclo[2.2.0]hex-2-ene, as reported recently by Dedek's group. Mercury photosensitization of the vinylcyclobutene proceeds smoothly to yield this same bicyclic olefin