Isotope effect and kinetic studies of the reaction of tertiary alcohols with triphenylphosphine-carbon tetrachloride: Ion pair or concerted?

Kinetics of the reactions of 1,2,3-triphenyl-2-propanol (1), 1,2-diphenyl-2 -propanol (2) and 3,3,3-trideuterio-1,2-diphenyl-2-propanol (3) with triphe nylphosphine-carbon tetrachloride in the temperature range of 25-78 degrees C in several solvents are investigated. In a non-polar solvent (CCl4), the reaction of (2) proceeds via intermolecular anti E2 elimination and/or int ermolecular S(N)2 nucleophilic substitution (28% substitution, ratio of 2-a lkene/1-alkene=1.06, E/Z greater than or equal to 49). In a polar solvent ( CH3CN) reaction proceeds via E1 and/or S(N)1 (24% substitution, 2-alkene/1- alkene=1.9, E/Z greater than or equal to 6. At equilibrium, the ratio of 2- alkene/1-alkene is equal to 99 with E/Z greater than or equal to 4.21. The primary kinetic isotope effect (k(H)/k(D)) for the elimination pathway in t he non-polar solvent is equal to 4.90 and 3.90 at temperatures of 25 and 60 degrees C, respectively. A small secondary beta-isotope effect of 1.10 was observed for substitution reaction at both temperatures. Direction of subs titution (S(N)2 vs. E2) depends on temperature and polarity of the solvent. The energetics (Delta S double dagger Delta G double dagger, Delta H doubl e dagger), the rate orders, and optimization of molecular geometry of inter mediates by semiempirical methods (AM1 and CNDO) all agree with intermolecu lar E2 and S(N)2 mechanisms. New rules for stereoselectivity and Hofmann-Sa ytzeff eliminations are considered. (C) 2000 Published by Elsevier Science Ltd.