On the Extent of Sigmatropic 1, 5‐Migration of Hydrocarbon Groups in the Thermolytic Skeletal Rearrangement of 5,5‐Disubstituted 1,3‐Cyclohexadienes
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° followed by chloranil aromatization yields only 4% of 1,3‐diethylbenzene resulting from 7 through a 1, 5‐ethyl migration in the primary reaction step. 2, 3‐Dimethylethylbenzene (56%) and 1, 4‐diethylbenzene (4%) are obtained as other C~10~‐compounds. This shows that isomerization proceeds mainly through a sequence of electrocyclic and 1, 7‐shift reactions. Ethylbenzene (24%) and other aromatic C~8~‐ and C~9~‐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 ^13^C‐substitution of the methyl group in 12 it is shown that in solution at 300° skeletal isomerization proceeds through electrocyclic and 1, 7‐H‐shift reactions exclusively. In the gas phase at 500° 4% of the isomerization product is formed by a 1, 5‐shift of a substitutent, presumably of the methyl group, through a dissociative mechanism.
Thermolysis of 5, 5‐diphenyl‐1, 3‐cyclohexadiene (22) at 560° 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.