Acid-catalyzed solvolytic elimination (aromatization) of allylic ethers and alcohols

Abstract

Acid-catalyzed solvolysis of 1-methoxy-1-methyl-1,4-dihydronaphthalene (1-OMe) in 50 vol% acetonitrile in water yields exclusively the elimination product 1-methylnaphthalene (3). Neither intramolecular rearrangement giving 2-methoxy-4-methyl-1,2-dihydronaphthalene (2-OMe), nor nucleophilic substitution with water giving the corresponding alcohols (1-OH) or (2-OH), was observed. However, addition of the strongly nucleophilic azide ion gives rise to competing substitution. The 2-N₃3 adduct rapidly looses HN₃, affording 3 as the final product. These results are consistent with a carbocation relatively stable towards trapping with nucleophiles, but which easily loses a hydron. This process has a rate constant of about k_e = 6 × 10⁸ s⁻¹, which is more than an order of magnitude slower than the dehydronation of the corresponding unsubstituted benzallylic carbocation (J. Am. Chem. Soc., 1998, 129, 6512). The heat of reaction of 1-OH in 25 vol% glycerol in water, measured by microcalorimetry, is ΔH = −21.7 ± 0.9 kcal mol⁻¹.