Heavy atom isotope effect studies of elimination reaction mechanisms. 1. A kinetic and carbon-14 kinetic isotope effect study of the base-promoted dehydrochlorination of substituted 1-phenylethyl-2-14C chlorides

Upon treatment with the solvent conjugate base, the primary reaction of substituted 1-phenylethyl chlorides is elimination to substituted styrenes in tert-butanol (t-BuOH)-10% v/v dimethyl sulfoxide (Me2SO) at 600C and in bis(2-hydroxyethyl) ether-10% v/v Me2SO at 450C. Kinetic studies using eight substituted chlorides show that these reactions are strongly accelerated by both electron-donating and electron-withdrawing substituents, probably indicating a fairly reactant-like E2 transition state with the mechanism changing from El-like E2 to ElcB-like E2 as the substituents change from electron donating group (EDG) to electron withdrawing (EWG). In this first carbon isotope effect study of an elimination reaction of an alkyl chloride, carbon-14 kinetic isotope effects have been measured in the alkoxide/bis(2-hydroxyethyl) ether-Me2SO system for 1-(4-methylphenyl)ethyl-2-14C chloride, k/sup β/k = 1.038, 1-phenylethyl-2-14C chloride, k/sup β/k = 1.058, and 1-(4-chlorophenyl)ethyl-2-14C chloride, k/sup β/k = 1.068. Clearly none of these compounds reacts by an irreversible El mechanism, for which no β-carbon isotope effect should be observed. The trend in the results is what would be expected from increased C/sub β/-H bond rupture as the E2 transition state shifts from El-like to ElcB-like. Theoretical calculations involving El-like E2 transition-state models show that in this region only for a reaction coordinate motion tht strongly couples C/sub β/-H bond rupture, C/sub α/-C/sub β/ double bond formation, and C/sub α/-Cl bond rupture does the calculated value for k/sup β/k come into the experimentally observed range, and then only for reactant-like models. 2 figures, 5 tables