Kinetic Studies on SNAr Reactions of Substituted Benzofurazan Derivatives: Quantification of the Electrophilic Reactivities and Effect of Amine Nature on Reaction Mechanism

Kinetics of the nucleophilic aromatic substitution reactions of 7-L-4-nitrobenzofurazans 1 (1a: L = Cl and 1b: L = OCH3) and secondary cyclic amines (morpholine, piperidine, and pyrolidine) 2a-c have been measured in acetonitrile solution at 20 degrees C. The derived values of second-order rate constants (k(1)) have been employed to determine the electrophilicity parameters E for both benzofurazans 1a and 1b according to the linear free enthalpy relationship: log k (20 degrees C) = s(N)(E + N) (Eq. ). The second-order rate constants for reactions of benzofurazans 1 with a series of 4-X-substituted anilines 3a-d (X = OH, OCH3, CH3, and H) have also been measured in MeCN and found to agree within a factor of 0.14-50 with those calculated by Eq. from the electrophilicity parameters E measured in this work and the known nucleophile-specific parameters N and s(N) of anilines 3. On the other hand, the reactions of these benzofurazans 1 with anilines 3 exhibit linear BrOnsted-type plots with (nuc) = 1.27 for 1a and 1.01 for 1b, which are considerably greater than those (0.57 for 1a and 0.62 for 1b) obtained with the secondary cyclic amines 2. These high values of (nuc) have been interpreted in terms of a single electron transfer mechanism. Secondary evidence for the validity of this mechanism is provided by the agreement between the rate constants, k(1), for substitution of benzofurazans 1 by the anilines 3 and their oxidation potentials E degrees.