Nucleophilic displacement in polyhalogenoaromatic compounds. Part 14. Chloroazulenes

Abstract

Methoxydechlorination of 5,6-dichloroazulene in NaOMe–MeOH shows second-order kinetics, apparently proceeding by the S_NAr mechanism in which the attack of the nucleophile, and not the loss of chloride ion, is rate-determining. No evidence of a Meisenheimer-type intermediate is found. The rate of the reaction is not affected by light, nor by the presence of nitrobenzene; radical contributions to the process are therefore unlikely. The corresponding reactions of 4,5,6,7- and 1,3,5,6-tetrachloroazulene, and of 1- and 3-nitro-5,6-dichloroazulene all show second-order kinetics and are all accelerated compared to the parent compound; 1,3,4,5,6,7-hexachloroazulene reacted too rapidly for measurement. As expected, the extent of such acceleration is conditioned not only by the nature of the substituent but also by its position. Unexpectedly, chlorine substituent effects are clearly greater from the five-membered ring than from the ring undergoing attack.

Semi-empirical all-valence-electron SCF–MO calculations are presented which rationalise the orientation of attack and describe the observed substituent effects.