Reactions of Substituted Phenylnitromethane Carbanions with Aromatic Nitro Compounds in Methanol: Carbanion Reactivity,Kinetic, and Equilibrium Studies

The feasibility of carrying out nucleophilic addition from electron‐deficient heteroaromatics has been addressed through a detailed investigation of the interaction of a two 7‐substituted‐nitrobenzofurazan (R = OMe 2a ; R = Cl 2b ) with a series of substituted‐nitroaryl anions (X = 4‐NO₂ 1a ; X = 3‐NO₂ 1b ; X = 4‐CN 1c ; X = 4‐Br 1d ), all reactions first lead to the quantitative formation of the σ‐adducts 3a–d and 4a–d arising from covalent addition of the nucleophile to the C‐5 carbon. The rate and equilibrium constants for the formation of σ‐adducts 3a–d and 4a–d (k₅, K ₅ ) together with the rate constants for their decomposition (k_?5) have been determined in methanol at 25°C, allowing a determination of intrinsic rate constants, k₀ = 0.03, the lower k₀ value reflects the very strong salvation by methanol of the negative charge on the nitro group. The discovery of a linear correlation between the E and pK_a^MeOH parameters allows a calibration of the electrophilicity power of 2a and 2b , E = ?11.67 and ?10.29, respectively. Applying the general approach to nucleophilicity/electrophilicity recently developed by Mayr et al. through the relationship log k = s(E + N), a successful ranking of our nitroaryl anions 1a–d on the general nucleophilicity scale (N) has been carried out. The N values of 1a–d are found to cover a range from 15.78 to 16.69. The results are compared with previously reported data in water and DMSO.