Electrophilicity values

The wide scope of application of the electrophilicity index of Parr, Szentpaly, and Liu has been reviewed.1 Applications to electrophilic aromatic substitutions discussed are few. However, some alkylation and acylation reactions do correlate well with electrophilicity values. In the case of the nitration of toluene and chlorobenzene, correlation is not very good and it is suggested2 that electrophilicity is a kinetic quantity with inherent thermodynamic information. 

An attempt has been made to analyse whether the electrophilicity index is a reliable descriptor of the kinetic behaviour. Relative experimental rates of Friedel-Crafts benzylation, acetylation, and benzoylation reactions were found to correlate well with the corresponding calculated electrophilicity values. In the case of chlorination of various substituted ethylenes and nitration of toluene and chlorobenzene, the correlation was generally poor but somewhat better in the case of the experimental and the calculated activation energies for selected Markovnikov and anti-Markovnikov addition reactions. Reaction electrophilicity, local electrophilicity, and activation hardness were used together to provide a transparent picture of reaction rates and also the orientation of aromatic electrophilic substitution reactions. Ambiguity in the definition of the electrophilicity was highlighted.15... 

Since most of the dienophiles considered are substituted ethylenes, we can take ethylene as a reference to discuss the variations in local reactivity induced by chemical substitution. Ethylene (29) presents a local electrophilicity value tok = 0.37eV at the equivalent carbon atoms Cl and C2. Note that acetylene (32), having equivalent Fukui functions for both electrophilic and nucleophilic attacks, presents a lower electrophilicity pattern as compared with that of ethylene (tok = 0.27 eV at the equivalent carbon centres of structure 29). 

In Section 3.3.1, we have shown the reliability of the Ao> index to predict the reactivity of polar DA reactions.39 The model based on Ao>, used to characterize the DA cycloadditions, may be extensible to the 1,3-DC reactions.92,93 Thus, depending on the electrophilicity potential displayed by dipole/dipolarophile pairs, the mechanisms for these 1,3-DC reactions will have a more or less marked polar character. The global electrophilicity values of the simplest dipole reagents (see Chart 4) used in the 1,3-DC reactions quoted in Table 8 were evaluated using (2). 

A -mcl.hyI-phenylnitrone (67d) has an electrophilicity value of w = 1.35eV. Therefore, it can react also with strong electrophiles like methyl acrylate (Scheme 11, entry i), or with marginal electrophiles (nucleophiles) like butyl vinyl ether (Scheme 11 entry ii), in NED or IED 1,3-DC reactions, respectively. 

The electronic chemical potential /x, chemical hardness 17, and global electrophilicity 10 for the dipoles 83-86 are displayed in Table 11. Also included in Table 11 are the values of local electrophilicity and the values of the Fukui function for an electrophilic attack and for a nucleophilic attack fk at sites k for these dipoles. The two dipo-larophiles present similar electrophilicity values, 1.52 eV (14) and 1.49 eV (15) (see Table 1). According to the absolute scale of electrophilicity based on the co index,39 these compounds may be classified as strong electrophiles. 

In order to further validate the theoretical scale of electrophilicity based on the global electrophilicity index,105 we first compared the theoretical scale with the experimental electrophilicity determined from kinetic data by Mayr et al. for a series of 28 carbenium ions.106,107 They are displayed in Chart 6. Included in these series are the tritylium, benzhydrylium and benzylium ions. These charged electrophiles are expected to show enhanced electrophilicity patterns as compared with the neutral electron donors we will discuss later. The electrophilicity values are in the range [9.0-14.0] eV (see Table 13, second column). The local electrophilicity pattern at the carbocation site is displayed in Table 13, fourth column. They are obtained by projecting the global electrophilicity w with the electrophilic Fukui function ff, using (5). These values have been used to... 

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... 

The regioselectivity is well represented by a local reactivity picture in asymmetrically substituted ethylenes.121 The local electrophilicity values, tok, for the two carbon atom belonging to the C=C double bond, named as Cl and C2 (C2=C1(CN)2 and C2=C1N02), of the series of benzylidenemalononitriles and the a-nitrostilbenes 124-137 are given in Table 14, while local electrophilicity values of the electron-poor... 

Table 17 Global and local electrophilicity3, electrophilic Fukui function at C atom of the carbene and relative global electrophilicity values of singlet carbenes...

In Table 18 are displayed the global electrophilicity values for a series of alkyloxonium and carboxonium dications and diprotonated carboxylic acids. The values are presented in the order of increasing electrophilicity for each group, with reference to the neutral parent compounds (see last column in Table 18). 

The obtained energies show that the Ixndo isomer is more stable than the l.exo one. Moreover, the more stable l.endo isomer has lower electrophilicity value than the Lexo isomer in all cases. Regarding the hardness, we found that only in the presence of solvent the maximum hardness principle is obeyed. 

The presence of a PILs in the reaction media improves significantly the electrophilic character of the dienophile. However, the differences between the exp>erimental results using PILs or molecular solvents are not so bigger how the calculated electrophilic values indicated. 

Combining the solvent effects on fi and ri, we can determine the effect on the radical electrophilicity index to. The values for co in gas phase and solvent (lEF-PCM and COSMO) can be found in Table 3. In Fig. 3 the correlation between the lEF-PCM and COSMO electrophilicity values... 

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