Thermal isomerization of 1,1-diethyl-2-vinylcyclopropane

Abstract

At 195–250°, 1,1-diethyl-2-vinylcyclopropane (I) isomerizes to a mixture of 3-ethyl-trans,cis-hepta-2,5-diene (II) and 3-ethyl-cis,cis-hepta-2,5-diene (III). In an ‘aged’ Pyrex vessel the isomerization is first order and homogeneous. For 0·8–2 torr of reactant the rate constants are independent of pressure and are unaffected by the addition of up to 200 torr of nitrogen. The reaction is almost certainly unimolecular. The rates of formation of (II) and (III) are given by the Arrhenius equations, log k_II= 11·31 ± 0·06 – 33,715 ± 140/2·303 RT sec.^–1, log k_III= 11·26 ± 0·06 – 33,621 ± 150/2·303 RT sec.^–1(where R= 1·987 cal. deg.^–1 mole^–1). The ratio of (II) to (III) is markedly different to the ratios of cis- and trans-isomers produced in related 1,5-hydrogen migration in diene systems. This difference is rationalized in terms of the direction of the p-orbitals in the six-centre transition complex and the treatment is extended to cover the cis : trans product ratios for isomerizations proceeding through similar cyclic complexes in which an oxygen atom forms part of the ring.