Isotope Effect Studies on Elimination Reactions. VIII. The Mechanism of the 1,3-Elimination Reaction of 3-Phenylpropyltrimethylammonium Iodide with Amide Ion in Liquid Ammonia

Abstract

Reaction of 3-phenylpropyltrimethylammonium iodide with potassium amide in liquid ammonia at −55 °C was found to give concurrent 1,3-elimination forming phenylcyclopropane and 1,2-elimination forming 3-phenylpropene and cis- and trans-1-phenylpropene. Deuterium tracer studies on the salt labelled at C-3 established that neither a carbene nor an ylide are intermediates in the 1,3-elimination process. Isotopic exchange at C-3 was shown to accompany the reaction of the deuterated salt in ordinary ammonia, but it was not detected in the reaction of unlabelled salt in ammonia-d3. A nitrogen isotope effect, k14/k15, of 1.022 ± 0.001 was found for the 1,3-elimination, while the corresponding hydrogen isotope effect, estimated from the effect of isotopic substitution on the 1,3- and 1,2-elimination ratio, was shown to exceed 20. The hydrogen isotope effect for the disappearance of undeuterated and deuterated salts (elimination and exchange) was approximately 7.4. These observations, as well as the influence of deuterium substitution in both the quaternary salt and the solvent on the relative rates of 1,3- and 1,2-elimination, have been shown to be in accord with an Elcb mechanism in which the rate constants for the conversion of the intermediate 3-carbanion to 1,3-elimination product and for return to reactant by abstraction of a proton from ammonia are of the same order of magnitude.