Reference scales for the characterization of cationic electrophiles and neutral nucleophiles

Twenty-three diarylcarbenium ions and 38 pi -systems (arenes, alkenes, ally l silanes and stannanes, silyl enol ethers, silyl ketene acetals, and enami nes) have been defined as basis sets for establishing general reactivity sc ales for electrophiles and nucleophiles. The rate constants of 209 combinat ions of these benzhydrylium ions and pi -nucleophiles, 85 of which are firs t presented in this article, have been subjected to a correlation analysis to determine the electrophilicity parameters E and the nucleophilicity para meters N and s as defined by the equation log k(20 degreesC) = s(N + E) (Ma yr, H.; Patz, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 938-957). Though th e reactivity scales thus obtained cover more than 16 orders of magnitude, t he individual rate constants are reproduced with a standard deviation of a factor of 1.19 (Table 1). It is shown that the reactivity parameters thus d erived from the reactions of diarylearbenium ions with pi -nucleophiles (Fi gure 3) are also suitable for characterizing the nucleophilic reactivities of alkynes, metal-pi -complexes, and hydride donors (Table 2) and for chara cterizing the electrophilic reactivities of heterosubstituted and metal-coo rdinated carbenium ions (Table 3). The reactivity parameters in Figure 3 ar e, therefore, recommended for the characterization of any new electrophiles ; and nucleophiles in the reactivity range covered. The linear correlation between the electrophilicity parameters E of benzhydryl cations and the cor responding substituent constants sigma (+) provides Hammett sigma (+) const ants for 10 substituents from - 1.19 to - 2.11, i.e., in a range with only very few previous entries.