| Abstract
| - Prompted by extensive theoretical interest in the role of tunneling in the intramolecular 1,5-hydrogen shift in 1,3(Z)-pentadienes and the large uncertainty in the published values of the theoreticallyrelevant kinetic deuterium-isotope effect and its dependence on temperature, we have examined adegenerate bicyclic version, 2-methyl-10-methylenebicyclo[4.4.0]dec-1-ene, which is locked into therearrangement-competent cisoid conformation, in the hope of obtaining more precise and accurate values.From rate constants determined over a range of 33 °C from 167.7 to 201.6 °C, Arrhenius parameters, Ea= 32.8 ± 0.4 kcal mol-1 and log A = 11.1 ± 0.2, were obtained. An average kinetic isotope effect of 4.2± 0.5 obtained from all values for kH/kD and k-H/k-D may be compared with a value of 5.0 ± 0.3, recalculatedfrom data in the pioneering publication of Roth and König. From a highly problematic extrapolation of thetemperature dependence, a value of kH/kD of 16.6 (standard error between 6.5 and 43) is calculated for thekinetic isotope effect at 25 °C (Roth and König: 12.2). With curvature in Arrhenius plots being one of thethree types of experimental evidence considered indicative of tunneling, the kinetic study of the previouslypublished rearrangement of 1-phenyl-5-p-tolyl-1,3(Z)-pentadiene has been extended over a period of 339days to a range of 108 °C (77−185 °C) without discerning any deviation from a straight-line Arrhenius plot: Ea = 28.7 ± 0.5 (kcal mol-1) and log A = 9.41 ± 0.30.
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