500
Silica-Supported Quinolinium Tribromide: a Recoverable Solid Brominating Reagent
J. Braz. Chem. Soc.
groups. If the substituents are electron-rich groups, the
reaction time is shorter than the electron-withdrawing
groups (Table 2, entries 2-9). When the para position of
the substituted anilines is occupied, the reactions lead to
selective monobromination in the ortho position (Table 2,
entries 5-9).
In order to explore the generality of the method
developed for the monobromination of activated aromatic
substrates, experiments for reactions of secondary or
tertiary anilines or amides with SQTB were also conducted.
They were also effective and gave the corresponding
monobromination products in excellent yield (Table 2,
entries 10-14). Furthermore, in a similar fashion, the
reactions of 1-aminonaphthalene, 2-aminobenzothiazole,
2-amino-4-methyl-benzothiazole, carbazole and
9-ethylcarbazole with SQTB were also attempted.
Corresponding monobromination products were also
obtained in high yield (Table 2, entries 15-19).
In comparison with other brominating reagents
according to Eissen’s method,42 SQTB used in this protocol
has similar property to sol-gel entrapped pyridinium
hydrobromide perbromide reported by Levin,30 both of
which could be easily recovered from the reaction systems,
regenerated by further reactions with hydrobromic acid/
sodium bromate or bromine, and reused many runs
without obvious loss of activities. Employing SQTB as a
transport medium for bromine could reduce the hazardous
potential of using molecular bromine.43 The cost and
environmental impact of SQTB could be minimized by
its recyclability. However, producing by-product (sodium
chloride) in the preparation of SQTB and using volatile
solvent (acetonitrile) in the brominating reactions are still
the insufficiency of this protocol.
In conclusion, the silica-supported quinolinium
tribromide was found to be an efficient, regioselective,
stable and recoverable solid brominating reagent for the
monobromination of aromatic amines including primary,
secondary and tertiary amines, as well as amides and
heterocycles. The ease of work up, high yield and high
selectivity are the features of this reagent.
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