Reactivity in SNAr reactions of 2-(4-chloro-3-nitrophenyl)-1,3-diphenyl-1,3,4-triazol-1-ium-5-thiolate with some anionic and neutral nucleophiles

Abstract

The S_NAr reactions in methanol of the title compound with a selected range of anionic and neutral nucleophiles, viz., azide, benzenethiolate, and methoxide ions, and with the amines N,N-diethylethane-1,2-diamine, guanidine, morpholine, pyrrolidine and piperidine have been studied. We have also studied the reaction with azide ion in N,N-dimethylformamide (DMF). As far as possible we have compared these reactions with those of the nucleophiles with the convenient reference substrate, 1-chloro-2,4-dinitrobenzene.

With all the nucleophiles we verified large differences in the Arrhenius parameters by comparing the mesoionic with reference substrates. For the former, values of log B and of ΔE^‡ were always much higher. This has been ascribed to the betaine character of the mesoionic substrate with wellseparated positive and negative regions and the marked effects of this on solvation energy and solvation entropy.

We have also verified significant changes in relative reactivities of azide and benzenethiolate ions, the former being better and the latter poorer than that of 1-chloro-2,4-dinitrobenzene. The rationale involves the high polarity of the covalently bound azido group and its distance from the main region of negative charge in the σ-complex (and rate-limiting transition state), and adverse steric interactions in the reactions of benzenethiolate ion.

Comparing DMF and MeOH as solvents in the azide-ion reactions, we found an increase in reactivity in DMF similar to the well known solvent effects with typical S_NAr substrates.