Nucleophilic Reactivities of Carbanions in Water: The Unique Behavior of the Malodinitrile Anion

The kinetics of the reactions of nine carbanions 1a−i, each stabilized by two acyl, ester, or cyano groups, with benzhydrylium ions in water were investigated photometrically at 20 °C. Because the competing reactions of the benzhydrylium ions with water and hydroxide ions are generally slower, the second-order rate constants of the reactions of the benzhydrylium ions with the carbanions can be determined with high precision. The rate constants thus obtained can be described by the Ritchie equation, log(k/k0) = N+ (eq 1), which allows us to calculate Ritchie N+ parameters for a series of stabilized carbanions, for example, malonate, acetoacetate, malodinitrile, etc., and compare them with those of other n-nucleophiles in water (hydroxide, amines, azide, thiolates, etc.). Because the Ritchie relationship (eq 1) is a special case of the more general relationship log k = s(N + E) (eq 4), the reactivity parameters N and s for the carbanions 1a−i can also be calculated and compared with the nucleophilic reactivities of a large variety of n-, π-, and σ-nucleophiles, including reactivities of carbanions in dimethyl sulfoxide. While the acyl and ester substituted carbanions are approximately 3 orders of magnitude less reactive in water than in dimethyl sulfoxide, the malodinitrile anion (1i) shows almost the same reactivity in both solvents. Correlations between the nucleophilic reactivities of carbanions with the pKa values of the corresponding CH acids reveal that the malodinitrile anion (1i) is considerably more nucleophilic than was expected on the basis of its pKa value. This deviation is assigned to the exceptionally low Marcus intrinsic barriers of the reactions of the malodinitrile anion (1i)

Nucleophilicities of Nitroalkyl Anions

The kinetics of the reactions of eight nitroalkyl anions (nitronate anions) with benzhydrylium ions and quinone methides in DMSO and water were investigated photometrically. The second-order rate constants were found to follow a Ritchie constant selectivity relationship with slightly smaller selectivities than those observed previously for other carbanions and O or N nucleophiles. Evaluation of the kinetic data by the correlation equation log k (20 °C) = s(N + E) yields the nucleophilicity parameters (N), which allow a comparison of the nucleophilicities of nitronates with those of other classes of compounds. Although the aliphatic nitronates 1a−c are more nucleophilic than the aromatic representatives 1d−h in DMSO, hydration reduces the nucleophilicities of aliphatic nitronates by a factor of 1 million, which is considerably greater than the reduction of the reactivities of the aromatic nitronates with the consequence that aromatic nitronates are more nucleophilic in water than aliphatic ones. The nucleophilic reactivities of nitronates are only slightly affected by substituent variation in DMSO and even less so in aqueous solution, which is considered to be the reason for the unusual rate equilibrium relationships, the so-called nitroalkane anomaly. Outer-sphere electron transfer does not occur in any of the reactions that were investigated

Reference Scales for the Characterization of Cationic Electrophiles and Neutral Nucleophiles^†^,^‡

Twenty-three diarylcarbenium ions and 38 π-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 π-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 °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 ). It is shown that the reactivity parameters thus derived from the reactions of diarylcarbenium ions with π-nucleophiles (Figure ) are also suitable for characterizing the nucleophilic reactivities of alkynes, metal-π-complexes, and hydride donors (Table ) and for characterizing the electrophilic reactivities of heterosubstituted and metal-coordinated carbenium ions (Table ). The reactivity parameters in Figure 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 σ+ provides Hammett σ+ constants for 10 substituents from −1.19 to −2.11, i.e., in a range with only very few previous entries

Reference Scales for the Characterization of Cationic Electrophiles and Neutral Nucleophiles^†^,^‡

Twenty-three diarylcarbenium ions and 38 π-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 π-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 °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 ). It is shown that the reactivity parameters thus derived from the reactions of diarylcarbenium ions with π-nucleophiles (Figure ) are also suitable for characterizing the nucleophilic reactivities of alkynes, metal-π-complexes, and hydride donors (Table ) and for characterizing the electrophilic reactivities of heterosubstituted and metal-coordinated carbenium ions (Table ). The reactivity parameters in Figure 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 σ+ provides Hammett σ+ constants for 10 substituents from −1.19 to −2.11, i.e., in a range with only very few previous entries