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 triphenylphosphine–carbon tetrachloride in the temperature range of 25–78°C in several solvents are investigated. In a non-polar solvent (CCl4), the reaction of (2) proceeds via intermolecular anti E2 elimination and/or intermolecular SN2 nucleophilic substitution (28% substitution, ratio of 2-alkene/1-alkene=1.06, E/Z≥49). In a polar solvent (CH3CN) reaction proceeds via E1 and/or SN1 (24% substitution, 2-alkene/1-alkene=1.9, E/Z≥6. At equilibrium, the ratio of 2-alkene/1-alkene is equal to 99 with E/Z≥4.21. The primary kinetic isotope effect (kH/kD) for the elimination pathway in the non-polar solvent is equal to 4.90 and 3.90 at temperatures of 25 and 60°C, respectively. A small secondary β-isotope effect of 1.10 was observed for substitution reaction at both temperatures. Direction of substitution (SN2 vs. E2) depends on temperature and polarity of the solvent. The energetics (ΔS‡, ΔG‡, ΔH‡), the rate orders, and optimization of molecular geometry of intermediates by semiempirical methods (AM1 and CNDO) all agree with intermolecular E2 and SN2 mechanisms. New rules for stereoselectivity and Hofmann–Saytzeff eliminations are considered.