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Kinetics of the Friedel−Crafts Alkylations of Heterocyclic Arenes:  Comparison of the Nucleophilic Reactivities of Aromatic and Nonaromatic π-Systems

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journal contribution
posted on 10.12.1998, 00:00 by Matthias F. Gotta, Herbert Mayr
The kinetics of the reactions of benzhydryl cations with heteroarenes (N-methylpyrrole, furan, 2-methylfuran, and 2-methylthiophene) have been determined photometrically in dichloromethane, and the reaction products have been completely characterized by 1H and 13C NMR spectroscopy. The reactions follow second-order kinetics with rate-limiting formation of the σ-adducts. The second-order rate constants correlate linearly with the electrophilicity parameter E of the benzhydryl cations. This allows the determination of the reactivity parameters N and s for the heteroarenes according to the linear free enthalpy relationship log k = s(E + N) (Mayr, H.; Patz, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 938−957). The nucleophilicity parameters N thus defined allow a direct comparison of the nucleophilic reactivities of aromatic and nonaromatic π-systems. Since N correlates linearly with σ+arene, it becomes possible to derive N parameters for arenes and heteroarenes with known σ+ parameters and to calculate absolute rate constants for their reactions with carbenium ions and diazonium ions. Applications of the reactivity parameters thus determined for the quantitative and qualitative prediction of the rates of electrophilic aromatic substitutions are discussed.