Global electrophilicity index

Quantitative relationships have been reported between the global electrophilicity index and the experimental rate coefficients for the reactions of thiolcarbonates and dithio-carbonates with piperidine. The validated scale of electrophilicity was then used to rationalize the reaction mechanisms of these systems. This scale also makes it possible to predict both rate coefficients and Hammett substituent constants for a series of systems that have not been experimentally evaluated to date.48... 

A global electrophilicity index of common benzylating and acylating agents has been established from MO calculations and it shows a quantitative linear correlation with the experimental substrate selectivity index from a series of benzylation and acylation reactions.21 The values of relative rate coefficients predicted from the index may be accurate to within 10%. The reaction of /-butyl chloride with anisole catalysed by /Moluenesulfonic acid in supercritical difluoromethane has been subject to kinetic analysis.22 The proportions of substitution at the ortho -position and disubstitution increase at lower pressures, attributed to the decrease in the hydrogen-bonding ability of the solvent. 

From the theoretical point of view, the electrophilicity concept has been recently discussed in terms of global reactivity indexes defined for the ground states of atoms and molecules by Roy et al.18 19. In the context of the conceptual density functional theory (DFT), a global electrophilicity index defined in terms of the electronic chemical potential and the global hardness was proposed by Maynard et al.20 in their study of reactivity of the HIV-1 nucleocapsid protein p7 zinc finger domains. Recently, Parr, Szentp ly and Liu proposed a formal derivation of the electrophilicity, co, from a second-order energy expression developed in terms of the variation in the number of electrons.21... 

The global electrophilicity index to, which measures the stabilization in energy when the system acquires an additional electronic charge AN from the environment, has been given the following simple expression 21... 

Most of the structural and electronic features induced by chemical substitution are often reflected as responses in the global reactivity indexes.47 We propose that the global electrophilicity index encompasses most of the relevant information that may roughly account for the global reactivity pattern observed in the DA reactions. For... 

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... 

A quantitative relationship between Hammett substituent constant (a) for substituted ethylene and the global electrophilicity index has been found.122 Therefore, it is not surprising to find a good correlation between In k for the addition of HOCH2CII2S to substituted a-nitrostilbenes reported by Bernasconi et a/.116 and the global electrophilicity index co, as shown in Figure 4. 

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... 

In this work, we have developed a model for the evaluation of proton affinity in terms of some akin conceptual reactivity descriptors which can be concepmally linked and associated with the physico-chemical process of protonation. The akin descriptors are the ionization energy (I), the global softness(S), the electronegativity (x), and the global electrophilicity index (co). 

A significant progress was made, when the global electrophilicity was introduced by Parr et al. [29]. Formerly proposed by Maynard [56], the global electrophilicity index was given the following working expression in terms of the fi and t] indices ... 

The global electrophilicity index (q), introduced by Parr (Parr, et al, 1999) is a useful descriptor of the reactivity that allows a quantitative classification of the global electrophilicity character of a molecule within a unique scale. Current studies based on the DFT and applied to DA reactions, have shown this classification of the diene/ dienophile pair is a powerful tool to predict the feasibility of the process and the type of mechanism involved. This index defined as Eq. 3... 

Normally the polarity of the normal electron demand DA process has been studied by means of global electrophilicity index difference between reactants and the regioselectivity of the normal electron demand DA reaction using the local electrophilidty index for dienophiles (electrophiles in the reaction) and the local nudeophilicity index for dienes (nucleophiles in the reaction). 

According to the global electrophilicity index co shown, the dienes will act as nucleophiles and the dienophiles as electrophiles. To study the regioselectivity we used the local... 

The reactions of naphthalenes properly mono- and disubstituted with an electron-withdrawing and a series of dienes in thermal conditions, were evaluated with the frontier molecular orbitals (FMO) theory which provide qualitative information about the feasibility of these DA reactions. Besides, the global electrophilicity index (co) is employed to estimate the electrophilic character of the dienophiles used in the cycloaddition reactions. 

Concerning the global electrophilicity index ro, Perez et al. [22] looked into the effect of solvent on this particular... 

Table 3 The global electrophilicity index co for the gas phase [1] and 5 different solvents (using lEFPCM and COSMO) in eV ...

Fig. 3 Correlation of the global electrophilicity index in water, calculated with lEF-PCM in Gaussian09 and with COSMO in MOLPRO 2010.1...

Fig. 4 Global electrophilicity index to for a total of 47 radicals in gas phase and different solvents. Green arrows radicals with the biggest increase in //. Red arrows radicals with the lowest increase in //, going fiom gas phase to water...

Parr et al. [6] defined one more global reactivity descriptor of atoms and molecules. The parameter was labeled as global electrophilicity index, (co). This has special reference to correlate the reactivity of the reference molecules, a drug, with a biomolecule when the reaction becomes either electrophilic or nucleophilic. The proposed ansatz electrophilicity index (co) is defined as... 

The calculated global reactivity parameters for twelve mesoionic derivatives have been presented in the Table 2. The DFT based local reactivity parameters of twelve instant heterocycles along their computed global electrophilicity index have been arranged in the Table 3. 

From the Table 2, it is clear that the unsubstituted mesoionic compounds (Compounds 1 and 7) are more reactive than their substituted derivatives. Global electrophilicity indexes of the instant molecules run hand-in-hand with their activities. From, the global reactivity parameters, it can be concluded that unsubstituted mesoionic compound containing oxygen atom at position 3 (Compound 7) is more reactive as compared to the unsubstituted mesoionic compound containing sulphur atom at position 3 (Compound 1). From the computed global DFT based reactivity parameters, the overall reactivity order of mesoionic compounds are nicely observed. 

---