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

Abstract

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 kN3/kHOH = 2.1 × 104 (ratio of second-order rate constants). The results indicate a relatively stable benzallylic carbocation toward trapping by nucleophiles (kw = 1 × 107 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, ke = 1.6 × 1010 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 ΔH = −23.7 and −18.4 kcal mol-1, respectively, as measured by microcalorimetry.