Electrophilicity scale

It is generally believed that it was Ingold [1] in the early 1930s who proposed the first global electrophilicity scale to describe electron-deficient (electrophile) and electron-rich (nucleophile) species based on the valence electron theory of Lewis. Much has been accomplished since then. One of the widely used electrophilicity scales derived from experimental data was proposed by Mayr et al. [5-12] ... 

Table 1 Global electrophilicity scale and global properties3 for some common dienes and dienophiles involved in Diels-Alder reactions...

The classification based on the global electrophilicity scale presented in Table 1, correctly describing activating and deactivating effects promoted by chemical substitution on the diene/dienophile pair, may be complemented with a static analysis of the charge transfer pattern expected for transition state structures involved in DA reactions. This aspect is useful to discuss since the difference in polar character of these structures... 

In Chart 7 we included a short series of benzylidenemalononitriles as well as a-nitrostilbenes.116 Note that the most common functionalities associated with the chemistry of the nucleophilic addition to the C=C bond, namely aldehydes, ketones, esters, anhydrides and nitriles, are EW substituents present in dienophiles and dipolarophiles used in NED DA and 1,3 dipolar cycloadditions, and they are included in Table l.121 In Table 14, we summarize the global properties and local electrophilicity values for the series of benzylidenemalononitriles and a-nitrostilbenes. Both series present ethylene derivatives with large electrophilicity values, to > 2.3 eV, they being classified as strong electrophiles within the theoretical electrophilicity scale.39... 

In this chapter, we have reviewed the usefulness of the global and local electrophilicity indexes to quantitatively account for the reactivity and selectivity patterns observed in a large series of classical organic reactions. The global electrophilicity index, w, categorizes within an unique absolute scale the propensity of the electron acceptors to acquire additional electronic charge from the environment. This classification allowed an impressive number of systems in DA reactions to be rationalized in terms of their reaction mechanisms in polar and nonpolar processes. The global electrophilicity scale provides a simple way to assess the more or less polar character of a process on the... 

ELECTROPHILICITY SCALE FOR ATOMS IN LONG-RANGE (VALENCE) STATES... 

Mayr has extended his electrophilicity scale to benzaldehyde-derived iminium ions through measurement of rate constants for their reactions with C-nucleophiles such as enamines, silylated ketene acetals and enol ethers." With an E value of -9.27 for Ph-CH=NMe2" (in a range from -8.34 to -10.69 forpara-C j, andpura-OMe, respectively), these iminium ions are 10 orders more reactive than the parent aldehydes. However, the values are restricted to C-nucleophiles the iminium ions react 10 -10 times faster with water and amines than these E values would predict. Such reactions benefit from the anomeric stabilization of 0,Af-acetals and Af,Af-aminals. 

The electrophilicity of the benzodithiolylium cation has been estimated to be between -4 and -3 on the Mayr electrophilicity scale. As such, this suggests a potential... 

Table 6. Global properties values and global electrophilicity scale for reagents of DA reaction. The calculated LUMO energy for 1,3-DNN and the calculated HOMO energies for...

In the context of our ongoing efforts in the field of conceptual DFT [27, 28], we will compute the electronic chemical potential, the chemical hardness and both the global and the local electrophilicity index for a set of uncharged radical systems in solvent. The resulting radical electrophilicity scales in solvent will be compared to the previously reported gas-phase scale. For water as a solvent, two different solvation methods (EF-PCM and COSMO) will be applied to exclude artificial effects inherent to one of the two approaches. 

For this study, we use our extended database [11] of 47 radical systems, so 12 more than our previously published gas-phase radical electrophilicity scale [1], including C-, N-, O- and S-centered radicals, as well as some halogens, thus comprising a representative set of radicals for applications in organic chemistry. The structures can be retrieved from the Supporting Information. In order to compute the electrophilicity index, Parr s definition was apphed to the solution phase as shown in Eq. 1, using lEF-PCM and—in the case of water—COSMO as the implicit solvation models. Five solvents were chosen, for which the static dielectric constant covers the entire range of nonpolar to polar solvents n-hexane = 1.8819), dichloromethane (Sr — 8.9300), 2-propanol = 19.2640), acetonitrile (Sr = 35.6880) and water = 78.3553). 

The nucleophilicity of 32 amine, amide, carbamate, amidine, and pyridine nucleophiles was calculated at the B3LYP/6-31G(d,p) level of theory using the gas-phase ionization potential based on the nucleophilicity index The correlation coefficient for the calculated pyridine nucleophilicity against Mayr s values was 0.947, and it was 0.987 against the inverse of the electrophilicity scale Mm. The site selectivity predicted for the nucleophiles with more than one nucleophilic centre correlates well with that predicted by the local nucleophilicity index and the philicity index m. ... 

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