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

Abstract

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.

Nucleophilicities and Carbon Basicities of Pyridines

“…Instrumentation and kinetic methods have been described previously. [6] Experimental details are given in the Supporting Information.…”

“…[6] We will now report on the kinetics of the reactions of various benzhydrylium ions (Table 1) with several pyridines 1 in different solvents (Scheme 1). The data will then be used to determine the nucleophilicity parameters N and s of the pyridines 1 a-g according to Equation (1), which has been used for characterizing a large variety of electrophiles and nucleophiles.…”

Theoretical substituent electrophilicities

“…Accordingly, a linear relationship would be anticipated here between the experimental electrophilicity value E and the electrophilic constants s w . A plot of the electrophilicity parameter E of diarylcarbenium ions 18 vs. the sum of the s w p values of the corresponding substituents is shown in Figure 6.…”

The Basicity of Unsaturated Hydrocarbons as Probed by Hydrogen‐Bond‐Acceptor Ability: Bifurcated NH+⋅⋅⋅π Hydrogen Bonding

“…Mayr has recently developed a nucleophilicity scale for π bases based on extensive kinetic data for reactions with electrophiles. [25,26] In favorable cases, distance information from crystallography might be utilized in a comparative manner [27] but, without the precise location of the H atom, typically achieved only in neutron diffraction studies, this method is limited.…”

“…We also anticipated that these rankings would not correlate with existing measures of the basicity of unsaturated hydrocarbons e.g., gas phase ionization energies, complexation enthalpies and proton affinities [29,30] or condensed phase equilibrium constants [3] and nucleophilicities. [25] These scales are based on different thermodynamic quantities or physical phenomena that do not readily lend themselves to quantitative connections between gas phase and condensed phase data. The X-H⋯π interaction under study is a weak, localized effect whereas the energetics of complete protonation (σ basicity) involves larger, more global molecular phenomena.…”