High Brønsted β_nucvalues in S_NAr 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 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.

Structural factors influencing the reaction rates of 4-aryloxy-7-nitrobenzofurazans with amino acids

“…In order to explain the above observation about reaction rates, one must take into account that (i) the benzofurazan ring has a stronger inductive effect and a lower resonance effect than a nitro group [50]; (ii) the combined result for the nitrobenzofurazan is a strong electron-withdrawing effect facilitating the S N Ar substitution; (iii) methoxy and aryloxy substituents appear to be better leaving groups than chloro; and (iv) there is a transition state leading to the immediate formation of a red-colored Meisenheimer complex 4 [51][52][53][54][55] and then, more slowly, to the substitution product 3 that is detectably fluorescent. One may assume that the basic AAs, which are more nucleophilic than all other AAs, prevail whenever the aryloxy group is not activated (2a) or deactivated by an electronwithdrawing formyl group (2d), even when there are methoxy groups (2e).…”

Single Electron Transfer (SET) Pathway: Nucleophilic Substitution Reaction of 4-Chloro-7-nitrobenzofurazan with Anilines in MeOH-MeCN Mixtures

“…2(a), 6,7,9,24 This indicates that deprotonation of complex 5 to give conjugate base 6 as well as subsequent re-aromatization of these species is a fast process relative to the nucleophilic addition step. 9,24 The reactions of NBF-Cl with 4-X-subsiututed anilines have also been performed at 25.0, 35.0, and 45 o C to determine activation parameters.…”

“…9,24 The reactions of NBF-Cl with 4-X-subsiututed anilines have also been performed at 25.0, 35.0, and 45 o C to determine activation parameters. The ∆H ‡ and ∆S ‡ values determined in the present system are summarized in Table 3.…”

“…12 The normal range of βnuc values would be between 0 and 1. 9 However, in previous works βnuc values outside the normal range have often been observed. 9,[13][14][15] In particular, a nucleophilic aromatic substitution reaction with high β nuc values proceeds through the SET pathway rather than the normal SNAr-Ad.E mechanism.…”

“…[3][4][5] The significance of structure-reactivity relationships such as Hammett ρ, Brønsted β coefficients, and crossinteraction constants ρij as a mechanistic criterion for nucleophilic substitution or addition reactions as well as in the proton transfer process has been discussed by Jenks, 8 Terrier, 9 and Lee. 9,10 The βnuc values are commonly accepted as measures of the degree of charge transfer from a base to an acid partner in the transition state (TS).…”