High Brønsted βnuc values in SNAr displacement. An indicator of the SET pathway?

Abstract

Nucleophilic substitutions of 4-chloro-7-nitrobenzofurazan (NBD-Cl) and 3-methyl-1-(4-nitrobenzofurazanyl)imidazolium ions (NBD-Im⁺) with a series of 4-X-substituted anilines have been kinetically investigated in 70–30 (v/v) and 20–80 (v/v) H₂O–Me₂SO mixtures. The rate-limiting step in these reactions is nucleophilic addition with formation of Meisenheimer-type σ-adducts followed by fast expulsion of the leaving group (Cl⁻ or Im). The reactions are characterized by a notable sensitivity to basicity of the aniline nucleophiles, with Hammett ρ values of −2.68 and −3.82 in 30% and 80% Me₂SO, respectively, for NBD-Cl and even more negative values, −3.43 and −5.27, respectively, for NBD-Im⁺. This is consistent with significant development of positive charge at the nitrogen atom of the zwitterionic σ-adduct. Unexpectedly, the Brønsted-type plots reveal abnormally high β_nuc values, ca. 1.0 and 1.3–1.4, respectively. Satisfactory correlations between the rates of the reactions and the oxidation potentials of the respective anilines support a SET mechanism for this process, i.e. initial (fast) electron-transfer from the aniline donor to the nitrobenzofurazan acceptor moiety and subsequent (slow) coupling of the resulting cation and anion radicals within the solvent cage with formation of the σ-adduct. An alternative possible explanation of the high β_nuc values being related to the strong −I effect exerted by the negatively charged 4-nitrobenzofurazanyl structure, which would induce a greater positive charge at the developing anilinium nitrogen atom in the σ-adduct-like transition state as compared with the situation in the reference protonation equilibria of anilines, is considered less probable. It is thus proposed that obtention of abnormal β_nuc values may be an indicator of electron-transfer in nucleophilic aromatic substitution and highlights the transition from the polar (S_NAr) to the single electron-transfer (SET) mechanism.