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

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 sigma-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 rho 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 sigma-adduct. Unexpectedly, the Brønsted-type plots reveal abnormally high beta 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 sigma-adduct. An alternative possible explanation of the high beta 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 sigma-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 beta nuc values may be an indicator of electron-transfer in nucleophilic aromatic substitution and highlights the transition from the polar (SNAr) to the single electron-transfer (SET) mechanism.