Ambident electrophilicity of 4-nitrobenzochalcogenadiazoles: Kinetic studies and structure-reactivity relationships

The kinetics of the reactions of 4-nitrobenzofurazane 1a, 4-nitrobenzothiadiazole 1b, and 4-nitrobenzoselenadiazole 1c with a series of 4-Y-substituted phenoxide anions 2a-e (Y = OMe, Me, H, Cl, and CN) in aqueous solution at 20 degrees C were investigated photometrically. The derived second-order rate constants (k(2)) have been combined with the nucleophilicity parameters values of these series of anions 2a-e to determine the electrophilicity parameters E of electrophiles 1a-c according to the linear free-energy relationship (log k(2))/s versus N. General reactivity of these electrophiles 1a-c is found to be fairly similar with E values ranging in -10.77 +/- 0.61 E 1a-c are more reactive than 1,3,5-trinitrobenzene, as benchmark aromatic electrophile used in nucleophilic addition or substitution processes. The rate constants for the reactions of 4-nitrobenzochalcogenadiazoles 1a-c with some other nucleophiles were measured and found to agree with those calculated from Mayr's equation. Finally, analysis of the rate data in terms of the Bronsted approach reveals that 1a-c exhibits especially low intrinsic reactivity in sigma-adducts 3 forming reactions.