Nucleophilic aromatic INDEX

Turning to the good correlations of the actual scales, one may see that the highest order of CFD scheme, i.e., the spectral-like resolution chemical hardness-based aromaticity index is in best agreement with HOMA aromaticity, while the simpler scheme, i.e., the electrophi-licity-nucleophilicity chemical hardness gap (3.362), based aromaticity index A c correlating with REPE aromaticity. Both of these fits... 

The behavior of the different amines depends on at least four factors basicity, nucleophilicity, steric hindrance and solvation. In the literature (16), 126 aliphatic and aromatic amines have been classified by a statistical analysis of the data for the following parameters molar mass (mm), refractive index (nD), density (d), boiling point (bp), molar volume, and pKa. On such a premise, a Cartesian co-ordinate graph places the amines in four quadrants (16). In our preliminary tests, amines representative of each quadrant have been investigated, and chosen by consideration of their toxicity, commercial availability and price (Table 1). 

In addition to the above prescriptions, many other quantities such as solution phase ionization potentials (IPs) [15], nuclear magnetic resonance (NMR) chemical shifts and IR absorption frequencies [16-18], charge decompositions [19], lowest unoccupied molecular orbital (LUMO) energies [20-23], IPs [24], redox potentials [25], high-performance liquid chromatography (HPLC) [26], solid-state syntheses [27], Ke values [28], isoelectrophilic windows [29], and the harmonic oscillator models of the aromaticity (HOMA) index [30], have been proposed in the literature to understand the electrophilic and nucleophilic characteristics of chemical systems. 

Besides the applications of the electrophilicity index mentioned in the review article [40], following recent applications and developments have been observed, including relationship between basicity and nucleophilicity [64], 3D-quantitative structure activity analysis [65], Quantitative Structure-Toxicity Relationship (QSTR) [66], redox potential [67,68], Woodward-Hoffmann rules [69], Michael-type reactions [70], Sn2 reactions [71], multiphilic descriptions [72], etc. Molecular systems include silylenes [73], heterocyclohexanones [74], pyrido-di-indoles [65], bipyridine [75], aromatic and heterocyclic sulfonamides [76], substituted nitrenes and phosphi-nidenes [77], first-row transition metal ions [67], triruthenium ring core structures [78], benzhydryl derivatives [79], multivalent superatoms [80], nitrobenzodifuroxan [70], dialkylpyridinium ions [81], dioxins [82], arsenosugars and thioarsenicals [83], dynamic properties of clusters and nanostructures [84], porphyrin compounds [85-87], and so on. 

Orientation in electrophilic and nucleophilic reactions of aromatic compounds can be predicted with the aid of the reactivity index of MO theory. Electrophiles will attack positions of higher electron densities, larger superdelocalizability (electrophile), and the lower localization energy (electrophile). On the other hand, nucleophilic attack is preferred at positions of lower electron densities, larger superdelocalizability (nucleophile), and lower localization energy (nucleophile). Table XXIII shows reactivity indexes of some aromatic nitrogen cations. 

If only the electron density of the highest occupied molecular orbital (HOMO) is taken into account, an electrophilic attack is said to be regulated by the frontier electron density index (54JCP1433 79FCF1). In nucleophilic substitutions, the aromatic substrate tends to accept an electron pair in the transition state, and so the frontier orbital is taken as the lowest unoccupied molecular orbital (LUMO). In this case, the frontier electron density is assumed to be as the electron distribution that would be present in the LUMO if it were occupied by two electrons. In contrast to arguments based on the charge or 7c-electron densities, both nucleophilic and electrophilic substitution occur preferentially at the atom with the highest electron density within the appropriate frontier orbital, i.e., LUMO or HOMO, respectively. 

The first derivatives of the molecular valency index were also proposed as quantum descriptors [Balawender, Komorowski et al., 1998], where the left-hand-side derivative describes the effect of the electrophilic attack and the right-hand-side derivative measures reactivity toward a nucleophilic attack. This last one is also related to the aromatic character of a molecule, measured by the —> diamagnetic susceptibility exaltation. 

Balaban and Simon159 have argued that an index based on the relative electrophilicity and nucleophilicity of a ring compared with benzene is a useful means of expressing in numerical terms the aromaticity of a ring system. The authors derive an aromaticity constant K from a summation of k values for each ring position, where k is an expression of the tendency of the atom to release or attract -electrons from the delocalized 7r-cloud and is defined by... 

Zahradnlk et al.160 have combined a set of parameters which may be regarded as aromaticity indices into the stability index. The approach is more comprehensive than that of Balaban, but as the authors point out it is obviously difficult to assess the relative importance of the selected indices. Those included are the specific delocalization energy DE,P34 (Section II, A,3,a) related to the extent of -electron delocalization, Aq the difference between the maximum and minimum ir-electron density in the molecule (related to the reactivity toward nucleophilic and electrophilic reagents see the preceding section), the free valence F (related to radical reactivity) and the radical super-delocalizability ST (related to radical reactivity, oxidizability, and reducibility). 

Eisch et al. (24) performed a mechanistic study of the desulfurization of dibenzothiophene by a nickel(0)-bipyridyl complex and reported that a radical anion of the thiophene nucleus was formed and underwent C-S bond cleavage into S and an aromatic radical. In addition, they suggested that the oxidative reaction of the nickel(0)-bipyridyl complex toward dibenzothiophene had the characteristics of stepwise electron transfer rather than nucleophilic attack. However, no correlations occurred between the desulfurization rate and the reaction indexes of Fr(E), Fr(N), and Fr(R), as shown in Table II. The results suggested no evidence for either electron transfer or nucleophilic attack in this study. Moreover, the radical reaction was not... 

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