Molecular-orbital calculations electrophilic aromatic

Considerable advances have been made in recent years in the understanding of the aromatic substitution reactions of oxazoles. Molecular orbital calculations (Section III, B) predict that electrophilic attack should occur preferentially at position 5, and indeed this is observed. The relative order of reactivity calculated theoretically is not in complete accord with the experimentally observed order (5 > 4 > 2) therefore it is evident that the electrophilic substitution reactions are rather more complex than the present theoretical calculations would predict. 

Electrophilic Aromatic Substitution. The Tt-excessive character of the pyrrole ring makes the indole ring susceptible to electrophilic attack. The reactivity is greater at the 3-position than at the 2-position. This reactivity pattern is suggested both by electron density distributions calculated by molecular orbital methods and by the relative energies of the intermediates for electrophilic substitution, as represented by the protonated stmctures (7a) and (7b). Stmcture (7b) is more favorable than (7a) because it retains the ben2enoid character of the carbocycHc ring (12). 

The modeling of monomeric (,R,S)-2 [B3LYP/6-31+G(d)] indicates that the highest occupied molecular orbital (HOMO) is chiefly located at the metalated carbon center and the aromatic ring system (Fig. 5). It can be deduced from the calculated orbital coefficients that both inversion and retention of configuration are almost equally likely to result from electrophilic attack. Only the fact that the site opposite the lithium center is sterically accessible to attack by electrophiles (the coordination polymer of the solid-state structure should be broken up in solution) makes it possible for (R,S)-2 to react selectively with inversion of configuration at C(3) under kinetic control in nonpolar solvents. 

Phosphorus.—Oxoanions. Evidence for the formation of the metaphosphate anion [POs] as an intermediate in acyl phosphate reactions has been reported, and ah initio calculations on this species suggest that its high electrophilicity may be due to contributions from both n and o molecular orbitals. Its methyl ester, MeOPOa, can substitute electrophilically in the aromatic ring for activated aro-... 

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