The thermolysis of 2,2-dimethyl-1-vinylcyclobutane

Abstract

The kinetics of the thermolysis of 2,2-dimethyl-1-vinylcyclobutane have been investigated as a function of temperature from 263 to 301 °C. Primary products produced in the reaction include isobutene and butadiene, 4,4-dimethylcyclohexene, 2-methylhepta-1,6-diene, and cis-2-methylhepta-1,5-diene. trans-2-Methylhepta-1,5-diene and 2,4-dimethylhexa-1,5-diene are produced from cis-2-methylhepta-1,5-diene by way of a 3,3-sigmatropic rearrangement. The reaction obeys first-order kinetics and is unaffected by surface. Activation energies (kcal mol^–1) and (logA/s^–1) for the overall decomposition and for formation of the primary products are 45.73 ± 0.3 (14.427 ± 0.12), 47.71 ± 0.7 (15.087 ± 0.3), 44.35 ± 1.6 (12.53 ± 0.6), 45.0 ± 1.3 (12.24 ± 0.5), and 38.38 ± 1.7 (10.785 ± 0.7), respectively. The regiochemistry observed in fragmentation and in the 1,3-sigmatropic rearrangement of the starting material is discussed in terms of substituent effects found in other cyclobutane and vinylcyclobutane thermolyses. The fragmentation process and the isomerization to 4,4-dimethylcyclohexene and 2-methylhepta-1,6-diene is believed to proceed through the intervention of 6-methylhept-1-ene-3,6-diyl. cis-2-Methylhepta-1,5-diene is formed from a concerted 1,5-sigmatropic rearrangement of the starting material. The factors which affect the stereochemistry of the 1,5-hydrogen shift are discussed.