Isoindole stability

Unlike quinoline and isoquinoline which are of comparable stability the compounds indole and isoindole are quite different from each other Which one is more stable Explain the reason for your choice... 

It is of interest to compare the stability of the protonated forms of benzo[u], benzo[Z>] and benzo[c] fused pyrroles, i.e. the cations derived from indolizines, indoles and isoindoles. Indolizine gives a stable pyridinium ion and does not polymerize in the presence of acid. 

In a formal sense, isoindole can be regarde,d as a IOtt- electron system and, as such, complies vith the Hiickel (4w- -2) rule for aromatic stabilization, with the usual implicit assumption that the crossing bond (8, 9 in 1) represents a relatively small perturbation of the monocyclic, conjugated system. The question in more explicit terms is whether isoindole possesses aromatic stabilization in excess of that exhibited by pyrrole. 

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-... 

A route based on a benzyne intermediate can afford the isoindole ring via a category lb process. iV-Cyanomethyl-Af-methyl-0-chlorobenzylamine cyclizes to 1-methylisoindole under the influence of potassium amide in liquid ammonia (equation 36) (77T581). The cyano group plays two important roles. First, it provides stabilization of the carbanion... 

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)). 

Indole is more stable than isoindole. Although the bonding patterns in both five-membered rings are the same, the six-membered ring in indole has a pattern of bonds identical to benzene and so is highly stabilized. The six-membered ring in isoindole is not of the benzene type. 

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