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?

摘要

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 degrees C from 167.7 to 201.6 degrees 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 Konig. 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 Konig: 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 degrees 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.