Isoindole

Isoindole in heterocyclic chemistry is a benzo-fused pyrrole.[1] The compound is an isomer of indole. Its reduced form is isoindoline. The parent isoindole is a rarely encountered in the technical literature, but substituted derivatives are useful commercially and occur naturally. Isoindoles units occur in phthalocyanines, an important family of dyes. Some alkaloids containing isoindole have been isolated and characterized.[2][3]

Isoindole

Names
IUPAC name 2H-Isoindole
Identifiers
CAS Number	270-68-8
3D model (JSmol)	Interactive image
ChEBI	CHEBI:33179
ChemSpider	2282425
PubChem CID	3013853
CompTox Dashboard (EPA)	DTXSID40181488
InChI InChI=1S/C8H7N/c1-2-4-8-6-9-5-7(8)3-1/h1-6,9H Key: VHMICKWLTGFITH-UHFFFAOYSA-N InChI=1/C8H7N/c1-2-4-8-6-9-5-7(8)3-1/h1-6,9H Key: VHMICKWLTGFITH-UHFFFAOYAW
SMILES c1cccc2c1c[nH]c2
Properties
Chemical formula	C8H7N
Molar mass	117.15 g/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Synthesis

The parent isoindole was prepared by flash vacuum pyrolysis of an N-substituted isoindoline.[4] N-Substituted isoindoles, which are easier to handle, can be prepared by dehydration of isoindoline-N-oxides. They also arise by myriad other methods, e.g., starting from xylylene dibromide (C₆H₄(CH₂Br)₂).

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 solution, the 2H-isoindole tautomer predominates. It resembles a pyrrole more than a simple imine.[5] The degree to which the 2H predominates depends on the solvent, and can vary with the substituent in substituted isoindoles.[6]

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

N-Substituted isoindoles do not engage is tautomerism and are therefore simpler to study.

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.

• Illustrative Isoindoline Derivatives
• 
  Pigment yellow 139, a common high performance pigment.
• 
  Pigment yellow 185, a common high performance pigment.
• 
  Copper phthalocyanine, one of the most pervasive synthetic pigments.

See also

• 1,3-Disubstituted Isoindolines.
• Isoindene with nitrogen replaced by a methylene group.

References

1. Gilchrist, T. L. (1987). Heterocyclic Chemistry. Longman. ISBN 0-582-01422-0.
2. Heugebaert, Thomas S. A.; Roman, Bart I.; Stevens, Christian V. "Synthesis of isoindoles and related iso-condensed heteroaromatic pyrroles" Chemical Society Reviews 2012, volume 41, pp. 5626-5640. doi:10.1039/c2cs35093a
3. See for example: Zhang, X.; Ye, W.; Zhao, S.; Che, C. T. (2004). "Isoquinoline and isoindole alkaloids from Menispermum dauricum". Phytochemistry. 65 (7): 929–932. doi:10.1016/j.phytochem.2003.12.004. PMID 15081297.
4. R. Bonnett and R. F. C. Brown "Isoindole" J. Chem. Soc., Chem. Commun., 1972, 393-395. doi:10.1039/C39720000393
5. Alan R. Katritzky; Christopher A. Ramsden; J. Joule; Viktor V. Zhdankin (2010). Handbook of Heterocyclic Chemistry. Elsevier. p. 133.
6. John A. Joule; Keith Mills (2010). Heterocyclic Chemistry. John Wiley & Sons. p. 447.