Quantification of the Electrophilic Reactivities of Benzotriazoles and Structure-Reactivity Relationships

The kinetics of the coupling of 2-N-(4'-X-phenyl)-4,6-dinitrobenzotriazole 1-oxides 1 a-d (X =OMe, H, Br and CN) with nitroalkyl anions 2a-c have been investigated in aqueous solution at 20 C. Using the derived second-order rate constants and this series of nucleophiles 2a-c as reference, the electrophilicity parameters E of the four electrophiles 1a-d have been determined according to the linear free enthalpy relationship log k (20 degrees C) =s (N) (E + N). With the electrophilicity parameters E range from 9.72 to -11.78, the substituent effects on the electrophilic reactivity was examined quantitatively, leading to linear correlation of E with Hammett substituent constants (sigma). Based on this correlation, the unknown electrophilicity parameters E of other 2-N-(4'-X-phenyl)-4,6-dinitrobenzotriazole 1-oxides (X = CF3, Me and OH) were evaluated. Excellent linear correlation (r(2) = 0.9962) between the electrophilicity parameters E of benzotriazoles 1a-d and their pK(a) values for sigma-complexation has also been observed and discussed.