Acid-Catalyzed Solvolysis of Allylic Ethers and Alcohols. Competing Elimination and Substitution via a Thermodynamically “Stable” Carbocation

Specific acid-catalyzed solvolysis of 1-methoxy-1,4-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 N 3 / k HOH = 2.1 × 10 4 (ratio of second-order rate constants). The results indicate a relatively stable benzallylic carbocation toward trapping by nucleophiles ( k w = 1 × 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 × 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 Δ H = −23.7 and −18.4 kcal mol - 1, respectively, as measured by microcalorimetry.