Acid-catalysed aromatization of anthranyl derivatives. A kinetic and thermodynamic study

Abstract

The acid-catalysed solvolysis reaction of 9-methoxy-9-methyl-9,10-dihydroanthracene (2-OMe) in 50 vol% acetonitrile in water at 25 °C provides the substitution product 9-hydroxy-9-methyl-9,10-dihydroanthracene (2-OH) and the elimination product 9-methylanthracene (3). The rate-ratio of substitution-to-elimination was measured as k_S/k_E = 0.93. The alcohol also undergoes acid-catalysed aromatization to give the thermodynamically favoured product 3. The reaction enthalpy of this dehydration reaction was measured as ΔH = −10.2 ± 1.0 kcal mol⁻¹. The corresponding reaction of the secondary alcohol 9-hydroxy-9,10-dihydroanthracene (1-OH) has a reaction enthalpy of ΔH = −12.3 ± 0.8 kcal mol⁻¹. Addition of azide ion (0.25 M) gives rise to a large fraction of azide adduct 2-N₃3, which rapidly undergoes solvolysis. The “azide-clock” method yields rate constants for reaction of the carbocation to give alcohol and elimination product of k_w = 2.2 × 10⁸ s⁻¹ and k_e = 2.4 × 10⁸ s⁻¹, respectively. The thermodynamic stability of the carbocation was estimated as pK_R = −9.1. The alkene 9-methylene-9,10-dihydroanthracene (4) undergoes a slow acid-catalysed aromatization to give 3; no competing formation of 2-OH was observed.