Mechanisms of Elimination Reactions. 32. Tritium Isotope Effects and Tunnel Effects in the Reaction of 2,2-Diphenylethyl-2-t Derivatives with Various Bases

Article abstract of DOI:10.1021/ja00540a020

Primary tritium isotope effects as a function of temperature have been determined for the E2 reaction of 2,2-diphenylethyl-2-t tosylate with ethoxide in ethanol and tert-butoxide as well as the anions derived from imidazole, 2-methylimidazole, and 2,4,5-trimethylimidazole in tert-butyl alcohol.The temperature dependence of the tritium isotope effect has also been determined for the E2 reaction of 2,2-diphenylethyl-2-t bromide with ethoxide in ethanol and tert-butoxide in tert-butyl alcohol.Observed k_H/k_T values at 50 deg C ran from 17.6 to 31.3, and the ratios of Arrhenius preexponential factors, A_aH/A_aT, from 0.15 to 0.74, indicating moderate tunneling.A temperature dependence calculated from the Bell theory of tunneling was fitted to the observed temperature dependence to obtain tunnel corrections, Q_tH/Q_tT, ranging from 1.3 to 2.5 at 50 deg C.Observed isotope effects are larger with tert-butoxide in tert-butyl alcohol than with ethoxide in ethyl alcohol for both the tosylate and the bromide, but the difference arises primary from tunneling.Isotope effects are lager with the imidazole anions than with tert-butoxide.There is no marked effect of methyl substitution in the imidazole anion on either the observed isotope effect or the tunnel effect, even with the sterically hindered 2,4,5-trimehtylimidazole anion.The semiclassical (without tunneling) isotope effects run approximately 80-100percent of the maximum values predicted by the treatment of Melander and Westheimer.The proton thus seems to be nearlly symmetrically located in the transition states, and the contribution of heavy-atom motion to the reaction coordinates cannot be large.