Effect on Kinetics by Deuterium in the 1,5-Hydrogen Shift of a Cisoid-Locked 1,3(Z)-Pentadiene, 2-Methyl-10-methylenebicyclo[4.4.0]dec-1-ene:  Evidence for Tunneling?

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 theoretically relevant kinetic deuterium-isotope effect and its dependence on temperature, we have examined a degenerate bicyclic version, 2-methyl-10-methylenebicyclo[4.4.0]dec-1-ene, which is locked into the rearrangement-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, E_a = 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 k_H/k_D and k_-H/k_-D may be compared with a value of 5.0 ± 0.3, recalculated from data in the pioneering publication of Roth and König. From a highly problematic extrapolation of the temperature dependence, a value of k_H/k_D of 16.6 (standard error between 6.5 and 43) is calculated for the kinetic isotope effect at 25 °C (Roth and König:  12.2). With curvature in Arrhenius plots being one of the three types of experimental evidence considered indicative of tunneling, the kinetic study of the previously published rearrangement of 1-phenyl-5-p-tolyl-1,3(Z)-pentadiene has been extended over a period of 339 days to a range of 108 °C (77−185 °C) without discerning any deviation from a straight-line Arrhenius plot:  E_a = 28.7 ± 0.5 (kcal mol^-1) and log A = 9.41 ± 0.30.