Combined kinetic, and theoretical approaches for the study of the S_EAr reactions of 2-(2′,4′,6′-trinitrophenyl)-4,6-dinitrobenzotriazole 1-oxide with 5-R-substituted indoles in acetonitrile

We present a kinetic and theoretical study of electrophilic aromatic substitution (S_EAr) involving 2-(2′,4′,6′-trinitrophenyl)-4,6-dinitrobenzotriazole 1-oxide 1 with a series of 5-R-substituted indoles 2a-e (R = CN, Cl, H, Me and NH₂) in acetonitrile at 20 °C. Single electron transfer (SET) mechanism was proposed and confirmed by the agreement between the rate constants (k) and the oxidation potentials (E_p^ox) of these series of indoles. Using Mayr's equation, the electrophilicity parameter (E) of 1 at C-7 position is derived and compared with the same parameter estimated using empirical equation E versus. pK_a. Density Functional Theory (DFT) calculations were performed to confirm the suggested reaction mechanisms and elucidate the origin of the electrophilic reactivity of 1. Notably, a linear correlation (R² = 0.9957) between the experimental nucleophilicity (N) and the theoretical model of nucleophilicity (ω⁻¹) determined in this work of various 5-R-substituted indoles has been obtained and discussed.

Mayr and Parr's approaches were combined with the correlation N vs ω⁻¹ established in the present work, to evaluate the empirical electrophilicity parameter (E) of 2-(2′,4′,6′-trinitrophenyl)-4,6-dinitrobenzotriazole 1-oxide and the unknown nucleophile specific parameters (N and s_N) of four new 5-R-substituted indoles (R = NO₂, F, H and N(CH₃)₂) in acetonitrile.