Isoindole is a heterocyclic compound consisting of a benzene ring fused with pyrrole along the carbon-carbon bond furthest from its nitrogen atom. The compound is an isomer of indole. Isoindoline is a hydrogenated form of isoindole. The parent compound is rarely encountered in the literature, but its derivatives (notably phthalocyanine dyes) are of significant utility and commercial value. Several alkaloids containing this ring system have been isolated from plants.

Synthesis

The parent isoindole was prepared by flash vacuum pyrolysis of N-(methoxycarbonyloxy)isoindoline.

N-Substituted isoindoles, which are more stable than the parent compound, can be prepared by dehydration of isoindoline-N-oxides. Other starting materials, including o-xylylene dibromide (o-C6H4(CH2Br)2), may be used in their synthesis.

Structure and tautomerism of 2-H-isoindoles

Unlike indole, isoindoles exhibit noticeable alternation in the C-C bond lengths, which is consistent with their description as pyrrole derivatives fused to a butadiene.

In solutions, the 2H-isoindole tautomer predominates. It resembles a pyrrole more than a simple imine. The degree to which the 2H predominates depends on the solvent and on the arrangement of substituents (for isoindole derivatives).

2H-Isoindole (right) is the predominant tautomer relative to 1H-isoindole (left)

N-Substituted isoindoles do not tautomerize.

Isoindole-1,3-diones and related derivatives

The commercially important phthalimide is an isoindole-1,3-dione with two carbonyl groups attached to the heterocyclic ring.

See also