Thermal and Photochemistry of Perfluoro-1,6-heptadiene and the Perfluoro-1,3,6-heptatrienes

Article abstract of DOI:10.1021/jo00106a019

Pyrolysis transformed the title diene (1) into a 9:1 mixture of perfluorobicyclo<3.1.1>heptane (2) and cis-perfluorobicyclo<3.2.0>heptane (3) at 300 deg C and at >450 deg C principally into perfluorocyclopentene (11).Mercury-sensitized vapor phase photolysis of 1 also yielded 2 and 3, but the latter now dominated and its trans isomer 13 was formed as well.In the presence of nitrogen as a bath gas, the product was further enriched in the <3.2.0> isomers.Radical bromination of 1, a model reaction for the triplet photocyclization, gave cis- and trans-perfluoro-1,2-bis(bromomethyl)cyclopentane (16).Configurations were assigned to the cis and trans isomers of perfluoro-1,3,6-heptatriene (19 and 20), and they were equilibrated with iodine/visible light (K_t->c = 1.6(7) in CDCl3, 14 deg C).The cis triene underwent electrocyclization at 133 deg C to perfluoro-3-allylcyclobutene (27); thermal ring opening of this cyclobutene yielded exclusively the cis isomer.At 250 deg C both the cis triene and the allylcyclobutene were transformed quantitatively into perfluorobicyclo<3.1.1.>hept-2-ene (28).Trans triene gave no detectable cyclobutene over a range of temperatures, but yielded the <3.1.1> compound at 250 deg C.Ultraviolet irradiation of cis triene produced the allylcyclobutene, and mercury photosensitization of either compound (or the trans triene) proceeded a step farther to give tricycloheptane valence isomers (31 and 32).The internal <2 + 2> cycloadditions described here contribute to our knowledge of the ground rules for reactions of this type in unsaturated fluorocarbons.