Cation–anion combination reactions. 26. A review

Abstract

Rate and equilibrium constants for reactions of carbocations, and a few other electrophiles, with anionic and neutral nucleophiles in several solvents are summarized. Equilibrium constants for such reactions are a measure of the carbon basicities of the nucleophiles, and are discussed in the context of differences in proton and carbon basicities. The factors affecting the kinetic reactivities of nucleophiles are not well understood, even though the reactivity patterns are frequently the same toward widely different electrophiles.

Limitations of the s(E+N) and Related Equations: Solvent Dependence of Electrophilicity

“…In general, caution is required when applying multiparameter correlations. [13] The complex data processing (Scheme 1) has obscured the 1:1 relationship [8] between log k for 1 (Z = NMe 2 ) and 3 ( Figure 2). Inconsistent results for 3 show that Equations (1)- (3) are not robust.…”

“…Furthermore, it has recently been proposed [7] that Equation (2) is the basis of a general scale of nucleophilicity from which various other scales can be derived: for example, Equation (1) is derived from Equation (2) if s E = 1, as observed for many cation-anion recombinations. [8] When an equation fits a huge amount of data, it is tempting to conclude that a fundamental "general relationship" [9] has been established. The situation is reminiscent of claims made in the early 1980s for solvent effects; after lively exchanges of strongly opposing views, [10][11][12] the consensus [13,14] is that linear free-energy relationships are local empirical rules or quantitative similarity models.…”

“…[32] For cation-nucleophile recombinations in protic solvents, anion desolvation is a dominant process for both anions [33] and amines; [34] for cations R 1 R 2 CH + , k H /k D % 1.00, [35] thus suggesting that there is little change in hybridization at the carbenium center in the transition state. [8] In contrast, reactions of alkenes in dichloromethane show k H /k D % 0.80, thus suggesting that "rehybridization of the carbenium ion center is far advanced in the transition state." [36] It is reasonable in principle to correlate these effects by a nucleophilicity parameter (e.g.…”

“…[8,27] The corresponding terms (E f for electrofugality of the cation or electrophile and N f for Figure 2. Plot of log k for 3 for amines in water at 25 8C vs. log k for 1 (Z = NMe 2 ) at 208C: slope = 1.03 AE 0.04; intercept = À4.74 AE 0.14 (n = 35; R 2 = 0.954; std error = 0.21); data for 3 from Ref.…”

A DFT Study of the Reactivity Indexes of Ionic [4 + 2+] Diels-Alder Cycloaddition to Nitrilium and Immonium Ions

“…In a formal framework it has allowed introducing in a many empirical chemical concepts like electronegativity [10], hardness [11], so-called reaction force [12], Fukui function [13], electrophilicity [14], electrofugality and nucleofugality [1,2,9,[15][16][17][18], nucleophilicity and solution phase ionization potentials, homofugality [19,20], nucleophilicity [21], among others. In particular, the introduction of concepts like electrophilicity and nucleophilicity to define electron deficient (electrophile) and electron rich (nucleophile) species has gained a continuous interest to construct empirical scales classifying atoms, molecules and charged species [22][23][24][25][26][27][28][29]. The availability of empirical scales of electrophilicity and nucleophilicity, usually based on kinetic parameters, has been very useful to rationalize the chemical reactivity, in terms of selectivity, reaction mechanisms, solvent, substituent effects, etc.…”