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- Title
Über den Anteil sigmatroper 1, 5-Wanderung von Kohlenwasserstoffgruppen bei der thermolytischen Skelettisomerisierung 5,5-disubstituierter 1, 3-Cyclohexadiene.
- Authors
Schiess, Peter; Dinkel, Rolf
- Abstract
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 C10-compounds. This shows that isomerization proceeds mainly through a sequence of electrocyclic and 1, 7-shift reactions. Ethylbenzene (24%) 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° 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.
- Publication
Helvetica Chimica Acta, 1981, Vol 64, Issue 3, p801
- ISSN
0018-019X
- Publication type
Article
- DOI
10.1002/hlca.19810640319