Influence of Ion Pairing, Steric Effects, and Other Specific Interactions on the Reactivity of Thioanions with Chloronitrobenzenes. Nucleophilic Aromatic Substitution vs Reduction

Article abstract of DOI:10.1021/jo00013a037

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_NAr mechanism also when O2 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_NAr reactions, the difference in reactivity being, however, considerably greater in the latter process.The MeS^- > i-PrS^- > PhS^- > t-BuS^- reactivity order observed in the S_NAr reactions is opposite, as far as the aliphatic thiolates are concerned, to the order of basicity.Notably, reactivity drops with increasing bulkiness of the attacking nucleophile.However, kinetic results obtained under conditions of ion paired and of "free" anions and the effects of ion pairing on the k_ortho/k_para ratios suggest that steric effects in the transition states are scarcely dependent on the bulkiness of the substituent R in the nucleophile RS^- and that nucleophilic reactivity is largely determined by the extent of charge concentration on the attacking atom, which, in turn, affects the strength of ion-pairing interactions.