384
J. Ju et al. / Chinese Chemical Letters 22 (2011) 382–384
36:1 at 0 8C under 60 W incandescent lamp. At last, the ratio of substrate was also studied. It showed that only
monobrominated 2a was found under 1a:H2O2:HBr as 1:1.25:1.25 at 0 8C with 60 W incandescent lamp.
With the optimized condition, bromide 2a was yielded in 61% with 3-chloro toluene:HBr:H2O2 in 1:1.25:1.25 at
0 8C under illumination with 60 W incandescent lamp [14].
Then the different substituted toluenes were studied under this optimal condition. High selectivity of
monobrominated product was obtained in most reactions (Table 2).
As shown in Table 2, bromination of aromatic side-chain all gave the corresponding high selectively
monobrominated products in good yields (Table 2, entries 1, 3, 4, 5 and 6, 7). However, the 3NO2 toluene gave the
mono- and di-bromo compounds 2b and 3b in yield 72.5% and 14.5% (Table 2, entry 2).
In summary, an efficient and high selective bromination process for aromatic side-chain was researched in the
aqueous system of H2O2–HBr which can be improved by irradiation with the incandescent light. This method had
good selectivity for monobromination of aromatic side chain. The bromination use HBr accompanied by H2O2 as
oxidant to improve the bromine atom efficiency, which is the great advantage of this method. This method is proved to
be efficient, economic, environmentally friendly, and therefore practical. It was a green route for effective synthesis of
benzyl bromide derivatives.
Acknowledgments
This project was supported by the National Natural Science Foundation of Zhejiang Province (No. Y407306) and
the National Natural Science Foundation of China (No. 20876148).
References
[1] R.C. Larock, Comprehensive Organic Transformations, 2nd ed., Wiley-VCH, New York, 1999.
[2] M. Kuroboshi, Y. Waki, H. Tanaka, J. Org. Chem. 68 (2003) 3938.
[3] C. Gao, X. Tao, Y. Qian, et al. Chem. Commun. (2003) 1444.
[4] A. Buttler, J.V. Walker, Chem. Rev. 93 (1993) 1937.
[5] G.W. Gribble, Chem. Soc. Rev. (1999) 335.
[6] J. Clark, Green Chem. 1 (1999) 1.
[7] E. Mu¨ller, Methoden der Organischen Chemie (Houben-Weyl), Band V/4, Georg Thieme Verlag, Stuttgart, 1960, p. 13.
[8] A.R. Katritzky, O. Meth-Cohn, C.W. Rees, Comprehensive Organic Functional Group Transformations, vol. 2, Pergamon, Oxford, 1995, p. 3.
[9] B.M. Trost, I. Fleming, Comprehensive Organic Synthesis, vol. 7, Pergamon, Oxford, 1991, p. 15.
[10] P.V. Vyas, A.K. Bhatt, G. Ramachandraiah, et al. Tetrahedron Lett. 44 (2003) 4085.
[11] D. Ikuchi, S. Sakaguchi, Y. Ishii, J. Org. Chem. 63 (1998) 6023.
[12] R. Mestres, J. Palenzuela, Green Chem. 4 (2002) 314.
[13] A. Podgorsek, S. Stavber, M. Zupan, J. Iskra, Tetrahedron Lett. 47 (2006) 7245.
[14] A mixture of 3-chloro toluene (2 g) and 40% aqueous solution of HBr (4.0 g, 19.8 mmol) was stirred at 0 8C, followed by the addition of 30%
H2O2 (2.2 g, 19.8 mmol) with a peristaltic pump in slow speed at about 1 mL/h for 2–3 h under illumination with 60 W incandescent lamp.
After the disappearance of the bromine color at about 7 h, the mixture was filtered and the filtrate was washed with aqueous NaHSO3 to remove
the unreacted Br2 and H2O2 and dried over anhydrous Na2SO4. The product was isolated by silica gel column chromatography with petroleum
ether as the eluent. The yield of 2a was 61%. 1H NMR (500 MHz, CDCl3): d 4.40–4.50 (m, 2H, CH2Br), 7.25–7.60 (m, 4H, Ph).