無極性溶媒中、ジニトロ系基質の示す高次S_NAr反応における詳細解析

抄録

Detailed analyses were made of the kinetic parameters for the S_NAr reactions of 2,4-dinitrohalobenzenes (DNXB) and of picryl halides with aniline in non-polar solvent based on the specific base-general acid catalyzed reaction scheme (SB-GA mechanism). This mechanism involves a nucleophilic addition equilibrium to give zwitterionic Meisenheimer complex MH (K₁ step), followed by SB catalyzed proton transfer equilibrium between the MH and anionic Meisenheimer complex (M^-) (K₂ step), the latter produces "substitution" product in GA catalyzed elimination of halide from MH and M^-. (k₁ and k₂ step).In the k₂ step, a better nucleofuge Cl shows a much less reactivity compared with F thus indicating the elimination reaction should have GA nature that should represent a preference on the higher exothermicity of the elementary process. In addition, the large difference on k₂ between picryl substrate and the DNXB should reflect the difference of the acidity of the GA catalysts, anilinium ion for the former and sandwiched bis aniline conjugate acid for the latter. Proton flanked by two molecules of aniline should show highly reduced acidity.Difference in k₁'s in the chloride substrates is reminiscent that this rate process is also of GA catalyzed by acidic proton among the MH themselves.