Influence of ion pairing, steric effects, and other specific interactions on the reactivity of thioanions with chloronitrobenzenes. Nucleophilic aromatic substitution vs. reduction

Abstract

The reactions in 2-propanol of the isomeric chloronitrobenzenes with thiolate nucleophiles, RS − (R=Me, i-Pr, t-Bu, Ph), have been studied to test for the ability of these representative thioanions of inducing chloride displacement and/or nitro reduction. m-Chloronitrobenzene gives a complex mixture of products, all still retaining the chlorine substituent, via redox processes involving nitro reduction and ring alkylthiolation. In contrast, the ortho and para isomers undergo substitution of chloride according to the addition/elimination S N Ar mechanism also when O 2 is removed from the reaction environment. Notably, treatment of o- and p-chloronitrobenzene with the oxanion 2-propoxide in oxygen-free i-PrOH results, instead, in nitro reduction. Kinetic and product studies indicate that i-PrS − is more reactive than i-PrO − in both redox and S N Ar reactions, the difference in reactivity being, however, considerably greater in the latter process