Thermolabile Kohlenwasserstoffe, 32. Konkurrierende Cope‐;Umlagerung und Homolyse von meso‐ und DL ‐3,4‐Di(1‐cyclohexen‐1‐yl)‐2,2,5,5‐tetramethylhexan

Thermolabile Hydrocarbons, 32. — Competing Cope‐;Rearrangement and Homolytic Decomposition of meso‐ and DL‐3,4‐Di(1‐cyclohexen‐1‐yl)‐2,2,5,5‐tetramethylhexane meso‐ and DL‐3,4‐di(1‐cyclohexen‐1‐yl)‐2,2,5,5‐tetramethylhexane (3a) were prepared by an improved reduction procedure (Li/C2H5NH2) from meso‐ and DL‐2,2,5,5‐tetramethyl‐3,4‐diphenylhexane. Products of two competing thermal transformations, C — C cleavage vs. Cope rearrangement, were isolated and identified. From a kinetic analysis of this system of reactions, characteristic activation enthalpies and entropies were obtained for the competing transformations. The standard heat of formation of meso‐3a (ΔHf0(g) = –58.4 ± 1.2 kcal/mol) was determined by combustion calorimetry and by measurement of its heat of sublimation. A strain enthalpy Hs (meso‐ 3a) = 10.2 ± 1.2 kcal/mol was derived therefrom. Force‐field calculations of the diastereomers of 3a and the Cope products 8a were performed by using MM2, MM2ERW, and MM3 in order to analyse the conformational situation. The calculated ΔHf0(g) values for meso‐3a are higher than the experimental value by 4.4 (MM2), 5.2 (MM2ERW), and 14.5 (MM3) kcal/mol. Obviously, the three computational methods require reliable parameters for highly branched olefins like 3a and 5a. From the experimental heat of formation ΔHt0(g) and strain enthalpy Hs of meso‐3a and its ΔH* of homolysis the stabilization energy of the allylic radical 4 was determined to be 12.6 ± 1.5 kcal/mol.