Valence-bond method, aromatic reactivity

The discussion of aromatic reactivity has proceeded in terms of molecular orbital theory. The valence-bond method has also been used to evaluate quantities which are parallel to those discussed above. The methods have in common the charge densities, and the free valency and bond order of molecular orbital theory have counterparts of similar but not identical significance in the valence-bond method. The latter has not been used widely in dealing with heterocyclic compounds, and so will not be much referred to here. In Table 3J are included data obtained by the valence-bond method for pyridine and quinoline. The use of molecular diagrams originated with studies in the valence-bond method fr,... 

During my early years as an assistant professor at the University of Kentucky, I demonstrated the synthesis of a simple quinone methide as the product of the nucleophilic aromatic substitution reaction of water at a highly destabilized 4-methoxybenzyl carbocation. I was struck by the notion that the distinctive chemical reactivity of quinone methides is related to the striking combination of neutral nonaromatic and zwitterionic aromatic valence bond resonance structures that contribute to their hybrid resonance structures. This served as the starting point for the interpretation of the results of our studies on nucleophile addition to quinone methides. At the same time, many other talented chemists have worked to develop methods for the generation of quinone methides and applications for these compounds in organic syntheses and chemical biology. The chapter coauthored with Maria Toteva presents an overview of this work. 

Theoretical methods to predict chemical reactivity properties of polycyclic benzenoid aromatic hydrocarbons are reviewed. These methods include the usual molecular orbital (MO) quantum chemical calculations, as well as pencil-and-paper MO and valence-bond procedures to derive indexes related to the rates of chemical reactions. Justification for the pencil-and-paper procedure termed the pertur-bational molecular orbitahfree-electron method (PMO F) is presented, and the modifications (PMO.Fw) of this procedure necessary to handle the differing reactivity patterns with neutral and ionic intermediates are also given. Examples of correlations of experimental results are used to illustrate these modifications. 

Aromatic substitution reactions are often complicated and multistep processes. A correlation, however, in many cases can be found between the charged attacking species and the electron density distribution in the molecule attacked during electrophilic and nucleoph c substitution. No such correlation is expected in radical substitution where the attacking particles are neutral, rather a correlation between the reactivities of separate bonds and a free valency index of the bond order. This allows the prediction of the most reactive bonds. Such an approach has been used by researchers who applied quantum calculations to estimate the reactivities of the isomeric thienothiophenes and to compare them with thiophene or naphthalene. " Until recently quantum methods for studying reactivities of aromatics and heteroaromatics were developed mainly in the r-electron approximation (see, for example, Streitwieser and Zahradnik ). The M orbitals of a sulfur atom were shown not to contribute substantially to calculations of dipole moments, polarographic reduction potentials, spin-density distribution, ... 

Semiempirical methods of calculation with consideration of all valence electrons have been used only recently but already have given results on the reactivities of some aromatic and heteroaromatic com-pounds. " Thus, to analyze the reactivities of thiophene and the isomeric thienothiophenes 1-3 to electrophilic substitution, the semiempirical SCF LCAO MO method CNDO/2 was used, taking into account all valence electrons.The 3s, 3p, and 3d orbitals have been taken into account for the sulfur atom. Tlie reactivities were estimated from the difference between bond energies of the initial and the protonated molecule (in a complex). ... 

Chia and Simmons388 calculated the resonance energies (ER)20 of four mono- and dibenzotetraazapentalenes (Scheme 24). Values are comparable with those of o-condensed aromatic systems (naphthacene, ER = 110 kcal mol-1 chrysene, ER = 116.5 kcal mol-1), and, like these carbocyclic systems, angularly-shaped molecules are more stable than linear ones. HMO calculations of delocalization energies (DE) show that the tetraazapentalene structure 15 is more stable than the tetra-azacyclooctatetraene valence isomer 324 (Scheme 14, Section IV,B,2) whether 324 is planar or tub-shaped. Calculations of electrophilic reactivity (Section IV,C,4,d), electronic spectra (by the PPP method employing all singly excited configurations), and bond orders have been carried out, and they confirm the aromatic nature of these systems. 

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