Acid-catalyzed solvolysis of allylic ethers and alcohols. Competing elimination and substitution via a thermodynamically "stable" carbocation

Specific acid-catalyzed solvolysis of l-methoxy-l,4-dihydronaphthalene (1-OMe) or 2-methoxy-1,2-dihydronaphthalene (2-OMe) in 25 vol % acetonitrile in water yields mainly the elimination product naphthalene, which is accompanied by a trace of 2-hydroxy-1,2-dihydronaphthalene (2-OH). No intramolecular rearrangement or formation of the alcohol 1-OH from 1-OMe was found. The nucleophilic selectivity between added azide ion and solvent water was measured as k(N3)/k(HOH) = 2.1 x 10(4) (ratio of second-order rate constants). The results indicate a relatively stable benzallylic carbocation toward trapping by nucleophiles (k(w) = 1 x 10(7) s(-1)). However, the elimination-to-substitution ratio with solvent water as base/nucleophile is large. Thus, in contrast to other carbocations of similar reactivity toward nucleophiles, the barrier to dehydronation is very low, k(e)= 1.6 x 10(10) s(-1), and accordingly, this step does not show any catalysis from added general bases. The heats of reaction of the solvolytic eliminations of 1-OH and 2-OH are Delta H = -23.7 and -18.4 kcal mol(-1), respectively, as measured by microcalorimetry.