The mechanism of thermal eliminations. Part 21. Rate data for pyrolysis of 2-ethoxyquinoline, 1- and 3-ethoxyisoquinoline, and 1-ethoxythiazole: correlation of reactivities with π-bond order of the CN bond

Abstract

We have measured the rates of thermal elimination of ethylene from the title compounds between 587.3 and 722.9 K. The reactivities relative to 2-ethoxypyridine at 650 K are: 3-ethoxyisoquinoline (0.21), 2-ethoxyquinoline (3.13), 1-ethoxyisoquinoline (6.47), 2-ethoxythiazole (63.1). These reactivities parallel the π-bond order of the CN bond, though the exceptional reactivity of 2-ethoxythiazole is attributed to additional acceleration through +M electron release from sulphur to nitrogen. This emphasizes the greater relative importance of nucleophilic attack by the nitrogen upon the β-hydrogen atom as compared with the analogous mechanism for pyrolysis of esters. Because of the semi-concerted nature of the reaction, interruption of aromaticity is much less significant than in, for example, electrophilic aromatic substitution. Thus retention of the benzenoid character of the ring not involved in the elimination is not an important rate-determining feature, as shown by the lower reactivity of 3-ethoxyisoquinoline relative to 2-ethoxypyridine. The unimportance of the interruption of aromaticity of the benzenoid ring means that conjugative effects are better relayed to nitrogen in the β-naphthalene-like position (isoquinoline) than in the α-naphthalene-like position (quinoline). This is the reverse of the familiar pattern for reactions of naphthalene-like systems where full charges are involved, and may be an additional factor contributing to the higher reactivity of 1-ethoxyisoquinoline than of 2-ethoxyquinoline, as may also be the –I effect of the benzo substituent. The conclusions are used to predict elimination rates for alkoxyheterocycles not yet studied.