Rapid and Total Bromination of Aromatic Compounds Using TsNBr2 Without Any Catalyst

Abstract N,N-Dibromo-p-toluenesulfonamide (TsNBr2) has been found to be a new reagent for bromination of aromatic compounds. The reaction is extremely fast and goes into completion instantaneously at ambient temperature to produce exclusively the corresponding polybrominated product. This procedure is applicable to various phenols, anisole, and anilines to give corresponding polybrominated compound as a single product in excellent yield. GRAPHICAL ABSTRACT


RESULTS AND DISCUSSION
We have recently reported few efficient protocols for various transformations using N,N-dibromo-p-toluenesulfonamide (TsNBr 2 ). [25] As a continuation of our work on TsNBr 2 , we report herein a rapid procedure for bromination of phenols and anilines (Scheme 1).
Initial experiments to study the feasibility of the reagent for bromination reaction were carried out by taking phenol as a model substrate. The reaction was carried Scheme 1. Bromination of aromatics using TsNBr 2 .
out by adding TsNBr 2 to the organic solution of phenol (1 mmol) in acetonitrile (2 mL) at room temperature. The reaction was found to complete instantaneously to produce the corresponding bromo products. After 10 min of exposure, the reaction mixture was treated with sodium thiosulfate and subjected to the usual workup procedure. When 0.5 mmol of TsNBr 2 was added, the reaction produced a mixture of 4-bromophenol (40%) and 2,4,6-tribrophenol (10%) along with unreacted phenol (Table 1). When 1 mmol of TsNBr 2 was used, the reaction produced a mixture of 4-bromo and tribromophenol. No unreacted phenol was found. Further increment of the amount of TsNBr 2 to 1.5 mmol led to the formation of 2,4,6-tribromo phenol exclusively with 87% isolated yield ( Table 1). The reaction was also carried out at low temperature (0 C). In both cases 2,4,6-tribromo phenol was obtained with similar yield in an instantaneous reaction. After obtaining an acceptable reaction condition from the initial experiments, we extended the procedure to varieties of substituted phenols (Table 2). However, total bromination of substituted phenols could be achieved using 1 mmol of TsNBr 2 per mole of the substrate. It can be seen from Table 2 that different kinds of phenolic substrates undergo total bromination reactions instantaneously at room temperature. 2-Naphthol produced the corresponding 1,3-dibromo product in 76% yield. However, in the case of 2-aminophenol, the yield of corresponding brominated product is relatively poor (Table 2, entry 9). When the reaction was extended to anisol, 77% of corresponding tribrmoanisole was obtained after 2 h of reaction.
The success of the method with phenols encouraged us to extend the reaction to anilines (Table 3). Initial reaction with aniline was encouraging. The reaction was completed instantaneously with 90% yield of the desired tribromo aniline in the presence of 1.5 equivalents of TsNBr 2 . In this case, however, the addition of TsNBr 2 should be done at 0 C for better yield. It is necessary to add the brominating agent at low temperature to subside the exothermicity. Then the process was extended to variety of anilines and the results are summarized in Table 3.
It can be seen from Table 3 that different kind of anilines could be converted to corresponding polybrominated product in excellent yield.

EXPERIMENTAL
TsNBr 2 (1 mmol) was added at 0 C to a solution of the aromatic compound (1 mmol) in acetonitrile (2 mL). After 10 min of stirring at room temperature, sodium

CONCLUSION
In conclusion, we have developed an efficient method for bromination of phenol and anilines. The reaction completes instantaneously to produce corresponding polybrominated product in excellent yield.

FUNDING
Financial support from the Department of Science and Technology (Grant Nos. SR=S1=RFPC-07=2006 and SR=S1=OC-43=2011) is gratefully acknowledged. I. S. and P. C. thank the Council for Scientific and Industrial Research for a senior research fellowship.

SUPPLEMENTAL MATERIAL
Supplemental data for this article can be accessed on the publisher's website.