Relationships between experimental and theoretical scales of electrophilicity of 7-L-4-nitrobenzofurazans

Kinetics of the reactions of 7-Imidazolium-4-nitrobenzofurazan 1a and 7-phenoxy-4-nitrobenzofurazan 1b with secondary amines 2a-c were studied spectrophotometrically in acetonitrile at 20 °C. The second-order rate constants have been used to evaluate the electrophilicity parameters E of 1a and 1b according to the linear free energy relationship log k (20 °C) = s (N + E). The theoretical electrophilic reactivity of the 7-phenoxy-4-nitrobenzofurazan 1a and 7-Imidazolium-4-nitrobenzofurazan 1b and two other nitrobenzofurazans 1c (L = OCH3) and 1d (L = Cl) have been studied by Density Functional Theory (DFT/B3LYP: Becke, 3-parameter, Lee- Yang-Parr) and the 6–311g(d,p) basis set implemented in Gaussian 09W Software. The derived electronic chemical potential (μ) and the corresponding chemical hardness (η) parameters have been used to calculate the global electrophilicity index (ω) of these series of electrophiles defined by the relationship ω = μ2/η. A good correlation between the calculated ω values and their experimentally observed electrophilic reactivities (i.e., Mayr’s electrophilicities E) is found and discussed.