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 anti intramolecular E2 elimination with cis (Z)-preference over pure alumina and intermolecular E2 elimination with trans (E)-preference over doped alumina. The reactions of model compounds 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 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-hexanol 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 ratio 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 of Hofmann adducts. The ratio of Saytzeff to Hofmann elimination was found 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 surprisingly favored over Hofmann! The energy of activation (E,), entropy of activation (Delta S double dagger), selectivity, isotope effect (k(H)/k(D)), and semiempirical calculation (AM1) all agreed with concerted E2 elimination.