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believed that the chlorination proceeds via the formation of hypochlorous
acid. The hypochlorous acid has higher instability due to pronounced ionic
nature and thus more reactivity towards the aromatic nucleus. The absence
of chlorination of the ring methyl group (Table 1, Entries 9 13) is indicative
of the electrophilic mechanism of the reaction rather than a radical pathway.
Furthermore, chlorination of alkylbenzenes gives large amounts of benzyl
chlorides as side products,21 such drawbacks could be avoided using this
system.
In conclusion, we have developed a novel system for the regioselective
oxychlorination of aromatic compounds by using KCl and Oxone� in aceto-
nitrile under ambient conditions. The results reported here demonstrate that
reagent system possess considerable practical advantages over traditional
reagents for electrophilic chlorination reactions. The commercial availability
of the reagents and reactions are clean, high yielding and work-up is simple.
The absence of side chain chlorination products in reaction conducted in
acetonitrile suggests a substantial increase in the rate of the ionic process.
We are currently extending this methodology to other halogenation
reactions.
General Procedure for the Chlorination of Aromatic Compounds:
Oxone� (2.2 mmol) was added to a well stirred solution of KCl
(2.2 mmol) and substrate (2 mmol) in acetonitrile (10 ml) and the reaction
mixture was allowed to stir at room temperature. The reaction was moni-
tered by thin layer chromatography (TLC). After the completion of the
reaction, the mixture was filtered and solvent evaporated under reduced
pressure. The products were purified by column chromatography over
1
silica gel and confirmed by H NMR and Mass spectra.
ACKNOWLEDGMENTS
We are thankful to Department of Science & Technology, New Delhi
for funding, DST project No.: SP/S1/H07/97.
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Received in the UK November 20, 2000 [ Pobierz całość w formacie PDF ]