Secondary deuterium isotope effects in the thermolysis of 2,2-dimethyl-1-vinyl-cyclobutane

Abstract

Secondary deuterium isotope effects for the thermolysis of 2,2-dimethyl-1-vinyl-3,3,4,4-tetradeuteriocyclobutane are reported. A value of k_H/k_D(262.2 °C) for the overall decomposition of the starting material of 1.018 ± 0.008 was determined and values of 1.06 ± 0.04, 0.95 ± 0.04, 0.95 ± 0.08 and 0.98 ± 0.1 are reported for fragmentation to butadiene and 2-methylpropene, and for rearrangement to 4,4-dimethylcyclohexane, cis-2-methylhepta-1,5-diene, and 2-methylhepta-1,6-diene, respectively. The isotope effects are discussed in terms of the potential energy surfaces proposed for 1,4-diradicals. It is concluded from these results and the isotope effect observed in thermolysis of [²H₈]cyclobutane that most of the effect in [²H₈]cyclobutane results from cleavage of the first bond, and that the subsequent surface for fragmentation is relatively flat.