NEW TRANSITION-STATE MODELS AND KINETICS OF ELIMINATION-REACTIONS OF TERTIARY ALCOHOLS OVER ALUMINUM-OXIDE

A new transition-state model was developed in order to justify the ant i intramolecular E2 elimination with cis (Z)-preference over pure alum ina and intermolecular E2 elimination with trans (E)-preference over d oped alumina. The reactions of model compounds 1,2,3-triphenyl-2-propa nol (1), 1,2-diphenyl-2-propanol (2), and 3,3,3-trideuterio-1,2-diphen yl-2-propanol (3) with aluminum oxides with a pH range of 4.5-9.5 and thorium oxide in the temperature range of 200-350 degrees C in 2-hexan ol have been investigated. Over acidic alumina (pH = 4.5 +/- 0.5), the ratio of E-isomer to Z-isomer (E/Z congruent to 2) for 2 was found to remain unchanged in this temperature range. At 300 degrees C, however , Saytzeff elimination favored Hofmann. Over pure alumina the E/Z rati o was equal to 0.650 (2-alkene/1-alkene = 0.750). At equilibrium, the E/Z ratio for 2 was equal to 4.5 with the formation of trace amounts o f Hofmann adducts. The ratio of Saytzeff to Hofmann elimination was fo und to be pH independent. Any decrease in pH caused a slight increase in the E/Z ratio. The average primary kinetic isotope effect (k(H)/k(D )) for elimination at 230 degrees C was equal to 3.775 +/- 0.227. The ratio of E/Z over thorium oxide at 300 and 350 OC was similar to that of aluminum oxide at 300 degrees C, but the Saytzeff elimination was s urprisingly favored over Hofmann! The energy of activation (E,), entro py of activation (Delta S double dagger), selectivity, isotope effect (k(H)/k(D)), and semiempirical calculation (AM1) all agreed with conce rted E2 elimination.