78. Ueber den Anteil sigmatroper 1,5-Wanderung von Kohlenwasserstoffgruppen bei der thermolytischen Skelettisomerisierung 5,5-disubstituierter 1,3-Cyclohexadiene

Article abstract of DOI:10.1002/hlca.19810640319

The uncatalyzed skeletal isomerization of 5,5-disubstituted 1,3-cyclohexadienes was investigated with the aim to establish the extent to which sigmatropic 1,5-shifts of hydrocarbon groups are participating in these reactions.Gas phase pyrolysis of 5,5-diethyl-1,3-cyclohexadiene (7) at 460 deg C followed by chloranil aromatization yields only 4percent of 1,3-diethylbenzene resulting from 7 through a 1,5-ethyl migration in the primary reaction step 2,3-Dimethylethylbenzene (56percent) and 1,4-diethylbentene (4percent) are obtained as other C₁₀-compounds.This shows that isomerization proceeds mainly through a sequence of electrocyclic and 1,7-shift reactions.Ethylbenzene (24percent) and other aromatic C₈- and C₉-hydrocarbons are formed to a considerable extent, indicating that C,C-bond cleavage is a major competing process and that the 1,3-diethylbenzene found is the result of a radical recombination reaction and not of a concerted sigmatropic shift of the ethyl group. 5-Methyl-5-phenyl-1,3-cyclohexadiene (12) yields 3-methylbiphenyl (14) and biphenyl upon thermolysis and aromatization.Through 13C-substitution of the methyl group in 12 it is shown that in solution at 300 deg skeletal isomerization proceeds through electrocyclic and 1,7-H-shift reactions exclusively.In the gas phase at 500 deg 4percent of the isomerization product is formed by a 1,5-shift of a substituent, presumably of the methyl group, through a dissociative mechanism.Thermolysis of 5,5-diphenyl-1,3-cyclohexadiene (22) at 560 deg in the gas phase leads to 1,1-diphenyl-1,3,5-hexatriene (23) and 1-vinyl-4-phenyl-1,2 dihydronaphthalene (24) through electrocyclic reaction steps.In addition a small amount of m-terphenyl is obtained at high conversion of 22.This indicates that sigmatropic 1,5-phenyl migration can participate in product formation only at high temperature and in the absence of other irreversible pathways to stable products.