Abstract
The kinetics of the coupling of 2-bromo-3,5-dinitrothiophene 1 with various anilines 2a-f were studied in CH3CN at 20 degrees C. The derived second-order rate constants of anilines possessing an electron-withdrawing group (e.g., 2d-f) were employed to determine the electrophilicity parameter E of the thiophene 1 according to the correlation equation log k(20 degrees C )= s(N) (E + N), where N and s(N) are nucleophile-specific parameters. Nonlinear Bronsted and Hammett relationships are discussed, and it is shown that the reactions for donor anilines 2a-d proceed through a single-electron transfer mechanism. Further support for this assumption was given by the observation that the nucleophilicity parameters N and E degrees values of oxidation potential constants are linearly related in a direct relationship with a correlation parameter of R-2 = 0.9914. The effect of aniline nucleophilicity on reactivity was examined quantitatively on the basis of kinetic measurements, leading to a nonlinear relationship of log (k(20 degrees C)) with nucleophilicity parameters (N). It is suggested that the observed nonlinear Mayr correlation can be effectively explored to evaluate the nucleophilicity parameters N of four-substituted aniline radical cations XC6H4NH2+center dot (X = OH, OMe, Me) and studying their reactivity patterns. On the other hand, it is confirmed that the N values of HOC6H4NH2+center dot (15.60), MeOC6H4NH2+center dot (15.23) and MeC6H4NH2+center dot (14.19) thus obtained can be used to predict the second-order rate constants for their reactions with N1-methyl-4-nitro-2,1,3 benzothiadiazolium tetrafluoroborate of known electrophilicity E. This study was one of the first that employed the Mayr approach for determining nucleophilicity parameters of radical cations.