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Aromatic Iodination with Iodine and Silver Sulfate

W.-W. Sy, B. A. Lodge & A. W. By
Synth. Commun. 20, 877-880 (1990)

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Abstract

Iodination of alkyl and alkoxybenzenes with iodine and silver sulfate at room temperature gives iodinated aryl compounds in good yield.

Direct iodination of aromatic compounds with molecular iodine is difficult because this is the least reactive halogen in electrophilic aromatic substitutions.1 Drastic reaction conditions such as nitric acid/sulfuric acid, sulfuric acid/acetic acid or a strong oxidizing agent with iodine are normally required for aromatic iodination.2 The lack of simple and reliable methods for the preparation of iodo-compounds has retarded a more extensive involvement of aryl iodides in practical organic syntheses3.

Table 1
Aromatic Iodination with
Iodine and Silver Sulfate
Substrate
Reaction
Time
Yield
Product
Ratioa
Benzene
74 h
  74%b
Toluene
50 h
  71%b
3:2c
p-Xylene
23 h
96%
m-Xylene
2.5 h
96%
10:1c
o-Xylene
23 h
97%
6:1c
Mesitylene
18 min
91%
10:1d
Durene
35 min
98%
Anisole
15 min
96%
Veratrole
9 min
76%
  1. All products characterized by
    GC/MS, NMR, IR, and comparison
    with authentic samples
  2. Reaction was not complete
  3. Ratio of para to ortho substitution
  4. Ratio of mono to di-iodo product
  5. Trace of di-iodo product detected

Recent work in our laboratory has shown that silver nitrite/iodine is a mild iodination reagent4. This has proven to be effective for iodinating alkylbenzenes; however, for alkoxybenzenes such as anisole, possessing a stronger electron donating group, a nitro-compound was found as a by-product.

We wish to report here the use of silver sulfate and iodine as a mild, effective aromatic iodination reagent.

In comparison with the silver nitrite/iodine mixture reported earlier4, the silver sulfate reagent has the advantages of, firstly, requiring a shorter reaction time, and, secondly, generating no nitro-compound from alkoxybenzenes. The results are summarized in Table 1.

Although iodine and silver sulfate, in the presence of sulfuric acid, has been used to iodinate deactivated benzenes such as nitrobenzene and benzaldehyde5, it was found that activated benzenes can be effectively iodinated with iodine and silver sulfate without the addition of any acid. For activated benzenes such as mesitylene, di- or tri-iodo derivatives can be prepared by using two or three equivalents respectively of this reagent.

Other silver salts such as the carbonate, nitrate and acetate were also tested, in combination with iodine, for aromatic iodination, but were found to be either too slow, or to produce undesirable by-products.

Experimental

p-Xylene (212 mg, 2 mmol) was added to a mixture of iodine (508mg, 2 mmol) and silver sulfate (622 mg, 2 mmol) in dichloromethane (20 mL) at room temperature. The mixture was stirred for 23h. After this time, the yellow precipitate was removed by filtration. The filtrate was washed with 10% aqueous sodium bisulfite solution, and subsequently with water. After drying over anhydrous sodium sulfate, the solvent was removed under reduced pressure to give a liquid. Column chromatography of the product on silica gel gave iodo-p-xylene in 96% yield.

References

  1. J. March, Advanced Organic Chemistry, 3rd. Ed., p. 478 (1985) Wiley-Interscience, New York.
  2. See: F. Radner, Lower nitrogen oxide species as catalysts in a convenient procedure for the iodination of aromatic compounds, J. Org. Chem., 53, 3548 (1988) and references cited therein.
  3. E. V. Merkushev, Russian Chem. Rev., 53, 343 (1984)
  4. W-W. Sy and B. A. Lodge, Iodination of alkylbenzenes with iodine and silver nitrite, Tetrahedron Lett. 30, 3769 (1989) [ Abstract ]
  5.  
    1. I. R. L. Barker and W. A. Waters, J. Chem. Soc., 150 (1952)
    2. D. H. Derbyshire and W. A. Waters, J. Chem. Soc., 3694 (1950)