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 ЊC 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 ∆H = Ϫ10.2 ± 1.0 kcal mol Ϫ1 . The corresponding reaction of the secondary alcohol 9-hydroxy-9,10-dihydroanthracene (1-OH) has a reaction enthalpy of ∆H = Ϫ12.3 ± 0.8 kcal mol Ϫ1 . Addition of azide ion (0.25 M) gives rise to a large fraction of azide adduct 2-N 3 , 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 × 10 8 s Ϫ1 and k e = 2.4 × 10 8 s Ϫ1 , respectively. The thermodynamic stability of the carbocation was estimated as pK R = Ϫ9.1. 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.

Stabilities and Partitioning of Arenonium Ions in Aqueous Media

“…In contrast to other carbocations, in water, arenonium ions, such as benzenonium or naphthalenonium, undergo loss of a proton rather than nucleophilic attack by solvent. This can be attributed to aromatic stabilization of the carbon−carbon double bond of the deprotonated product. ,, However, in the case of phenanthrene, this stabilization is reduced and products from both reaction paths can be detected. As a result, trapping by azide ions yields rate constants for both reactions, and thence equilibrium constants p K R and p K a .…”

“…This can be attributed to aromatic stabilization of the carbon-carbon double bond of the deprotonated product. 5,17,18 However, in the case of phenanthrene, this stabilization is reduced and products from both reaction paths can be detected. As a result, trapping by azide ions yields rate constants for both reactions, and thence equilibrium constants pK R and pK a .…”

6 Reaction mechanisms : Part (i) Polar reactions

“…Pirinccioglu et al examined the reactivity of the carbocation 16 produced by acid catalyzed solvolysis of 15 (R = Me) (Scheme 4), focusing on the factors that govern the competition between substitution and elimination. 9 In particular, the selectivity (elimination/substitution) of the carbocation was evaluated in terms of the thermodynamic stability of the products. The rate-ratio of substitution-to-elimination was measured as k S /k E = 0.93.…”

The Preparation of (1α,3β)-3-Hydroxycholestane-4,6-diene- 1,25-diol Diacetate from a 5,7-Diene Precursor: A New Method for the Synthesis of Heteroannular Dienes

“…The kinetics and aromatization of the acid-catalyzed elimination of anthranyl derivatives has recently been described, and the in the Roche 1α,25-dihydroxycholesterol-based synthesis of calcitriol, the separation of 1α,3β,25-triacetoxycholesta-5,7-diene ( 10 ) from the 4,6-diene triacetate 8 was achieved by the acid-catalyzed elimination of triacetate 8 to yield either triene 12 or the aromatized monoacetate 13 . In the presence of p -toluenesulfonic acid in dioxane at 70 °C, the triacetate 8 was consumed very rapidly to yield a component which was assigned as the 2,4,6-triene diacetate 12 .…”