Twenty-three diarylcarbenium ions and 38 pi-systems (arenes, alkenes, allyl silanes and stannanes, silyl enol ethers, silyl ketene acetals, and enamines) have been defined as basis sets for establishing general reactivity scales for electrophiles and nucleophiles. The rate constants of 209 combinations of these benzhydrylium ions and pi-nucleophiles, 85 of which are first presented in this article, have been subjected to a correlation analysis to determine the electrophilicity parameters E and the nucleophilicity parameters N and s as defined by the equation log k(20 degrees C) = s(N + E) (Mayr, H.; Patz, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 938-957). Though the reactivity scales thus obtained cover more than 16 orders of magnitude, the 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 derived from the reactions of diarylcarbenium ions with pi-nucleophiles (Figure 3) are also suitable for characterizing the nucleophilic reactivities of alkynes, metal-pi-complexes, and hydride donors (Table 2) and for characterizing the electrophilic reactivities of heterosubstituted and metal-coordinated carbenium ions (Table 3). The reactivity parameters in Figure 3 are, 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 corresponding substituent constants sigma(+) provides Hammett sigma(+) constants for 10 substituents from -1.19 to -2.11, i.e., in a range with only very few previous entries.

Which electrophiles react with which nucleophiles? The correlation log k(20 degrees Celsius) = s(E + N), in which electrophiles (carbocations, metal-pi-complexes, diazonium ions) are characterized by one (E) and nucleophiles are characterized by two parameters (N, s), proved to be applicable for a wide variety of electrophile-nucleophile combinations. Since the introduction of this correlation in 1994 (Angew. Chem., Int. Ed. Engl. 1994, 33, 938-957), numerous new reagents have been characterized, and in 2001 (J. Am. Chem. Soc. 2001, 123, 9500-9512), a new method of parametrization was proposed that facilitates a continuous extension of the data sets without the need for reparametrization of existing data. This Account adjusts the N and s parameters of all presently characterized pi-nucleophiles (arenes, alkenes, organometallics) to the new parametrization and illustrates how to employ the resulting reactivity scales for analyzing synthetic and mechanistic problems in organic and macromolecular chemistry. Predictions of absolute rate constants, inter- and intramolecular selectivities, and analyses of reaction mechanisms are discussed. We outline how new compounds can be added to the scales and present our view on the scope and limitations of this approach to polar organic reactivity.

The acetylation of tert-butanol with acetic anhydride catalyzed by 4-(dimethylamino)pyridine (DMAP) has been studied at the Becke3 LYP/6-311 + G(d,p)//Becke3 LYP/6-31G(d) level of theory. Solvent effects have been estimated through single-point calculations with the PCM/UAHF solvation model. The energetically most favorable pathway proceeds through nucleophilic attack of DMAP at the anhydride carbonyl group and subsequent formation of the corresponding acetylpyridinium/acetate ion pair. Reaction of this ion pair with the alcohol substrate yields the final product, tert-butylacetate. The competing base-catalyzed reaction pathway can either proceed in a concerted or in a stepwise manner. In both cases the reaction barrier far exceeds that of the nucleophilic catalysis mechanism. The reaction mechanism has also been studied experimentally in dichloromethane through analysis of the reaction kinetics for the acetylation of cyclohexanol with acetic anhydride, in the presence of DMAP as catalyst and triethylamine as the auxiliary base. The reaction is found to be first-order with respect to acetic anhydride, cyclohexanol, and DMAP, and zero-order with respect to triethyl amine. Both the theoretical as well as the experimental studies strongly support the nucleophilic catalysis pathway.

The kinetics of the reactions of benzhydryl cations with 22 enamines, three pyrroles, and three indoles were investigated photometrically in dichloromethane. The nucleophilicity parameters N and slope parameters s of these electron-rich pi-systems were derived from equation log k (20 degrees C)=s(E+N) and compared with the nucleophilicities of other pi-systems (silyl enol ethers, silyl ketene acetals) and carbanions. It is shown that the nucleophilic reactivities of enamines cover more than ten orders of magnitude, comparable to enol ethers on the low reactivity end and to carbanions on the high reactivity end. Since the products of N-attack are thermodynamically less stable than the reactants, the observed rate constants refer to the formation of the carbon bond;carbon bonds. In some cases, equilibrium constants for the formation of iminium ions were measured, which allow one to determine the intrinsic rate constants of these reactions.

Rate and equilibrium constants for the reactions of pyridines with donor‐substituted benzhydrylium ions have been determined spectrophotometrically. The correlation equation log k(20 °C)=s(N+E), in which s and N are nucleophile‐specific parameters and E is an electrophile‐specific parameter, has been used to determine the nucleophilicity parameters of various pyridines in CH2Cl2 and aqueous solution and to compare them with N of other nucleophiles. It is found that the nucleophilic organocatalyst 4‐(dimethylamino)pyridine (DMAP) and tertiary phosphanes have comparable nucleophilicities and carbon basicities despite widely differing Brønsted basicities. For that reason, these reactivity parameters are suggested as guidelines for the development of novel organocatalysts. The Marcus equation is employed for the determination of the intrinsic barriers of these reactions.