Isoindole, resonance energy

In summary, all estimates of resonance energies indicate a decrease in aromaticity in the sequence benzene > thiophene > pyrrole > furan. Similar sequences are also found for the benzo[6] and dibenzo analogues. A somewhat different sequence is found for the benzo[c] fused heterocycles with isoindole > benzo[c]thiophene > benzo[c]furan. As would be anticipated, the resonance energies for the benzo[c] heterocycles are substantially lower than those for their benzo[6] isomers. 

Although isoindole itself exists preferentially as such (73JCS(Pl)1432, 72TL4295, 77TL1095), in accord with predictions of MO calculations (77JA8248 and refs, therein), the low resonance energy of isoindoles often results in the isoindolenine tautomer being favored in substituted derivatives. 1H NMR spectroscopy has been especially useful in the study of this tautomeric... 

It is easily seen that the 7r-stabilization brought about by the introduction of a nitrogen atom is of the order indole > isoindole > indolizine. A confirmation of this consideration may be taken from a comparison of the resonance energies of indole,-isoindole and indolizine (REPE values (72T3657)). 

A linear relationship between the ratio of the vicinal coupling constants and the resonance energy has been found, indicating that the electron delocalization of the indole 7r-system is more pronounced than that of isoindole and indolizine respectively (77TL535) (see Table 5 and Section 3.08.2.1 for theoretical considerations). 

In our view, the explanation for this high reactivity is not to be ascribed to a lack of aromaticity these compounds have resonance energies in excess of that of benzene (Section IV,A). Rather, the reactivity is thought to be due to the ease with which this rc-excessive heteroaromatic system can undergo one-electron oxidation or 1,3-addition to generate another aromatic (benzenoid) system, as shown at structures 67 and 68, respectively. In terms of more familiar systems, the reactivity of the 1,3-positions of isoindole may be thought of as a compounding of the reactivity at the a-positions of pyrrole with that at the meso positions of anthracene. 

Two independent molecular orbital calculations (HMO method) of delocalization energies for isoindole and isoindolenine tautomers agree that the isoindole form should possess the more resonance stabilization. The actual difference calculated for isoindole-isoindolenine is about 8 kcal/mole, but increases in favor of the isoindole with phenyl substitution at position 1 (Table VI).Since isoindole and isoindolenine tautomers have roughly comparable thermodynamic stabilities, the tautomeric proce.ss is readily obser-... 

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