8696
B. Das et al. / Tetrahedron Letters 47 (2006) 8693–8697
Table 3 (continued)
Entry
Substrate 3
Time (h)
5.0
Product 4
No reaction
OH
Isolated yield (%)
CH3
O
H3C
o
p
O
OH
O
Br
2.0
78
72
O
O
O
OH
O
O
OH
O
O
HO
OCH
HO
OCH
3
3
q
3.0b
OH
OH
H CO
3
H CO
3
Br
oxyanin-B
OCH
OCH
3
3
N
N
N
Br
r
s
0.5
3.0
91
77
N
Br
N
N
N
H
N
H
a The structures of the products were determined from spectral (1H and 13C NMR and MS) and elemental analysis data.
b CH3OH–CH3CN (1:3) was used as a solvent.
with coumarins, the bromination occurred mainly at
C-3. Nitrogen heterocycles, DMAP (Table 3, entry r)
and benzimidazole (entry s) also afforded monobromo
products in high yields. The structures of products were
settled from their spectral (1H and 13C NMR and MS)
and elemental analysis data.8
References and notes
1. Larock, J. R. C. Comprehensive Organic Transformations:
A Guide to Functional Group Protection; Wiley-VCH: New
York, 1997.
2. Taylor, R. Electrophilic Aromatic Substitution; Wiley: New
York, 1990.
3. Butler, A.; Walker, J. V. Chem. Rev. 1993, 93, 1937–1944.
4. Metal-catalyzed Cross-coupling Reactions; Diederich, F.,
Stang, P. J., Eds.; Wiley-VCH: Weinheim, 1998.
5. De la Mare, P. B. Electrophilic Halogenation; Cambridge
University Press: Cambridge, 1997; Chapter 5.
Sulfonic-acid-functionalized silica10 acts as an organic–
inorganic hybrid (interphase) catalyst wherein
a
Brønsted acid site has been selectively created. It works
under heterogeneous conditions but its reaction centres
are highly mobile, as in a homogeneous catalyst. The
catalyst was prepared10b by immobilization of propyl
thiol on silica using 3-mercaptopropyltrimethoxysilane
followed by the selective oxidation of the thiol groups
by aqueous H2O2 to the sulfonic acid groups. It was recy-
cled consecutively three times without the loss of its
activity.
6. (a) Cram, D. J.; Dicker, I. B.; Lauer, M.; Knobler, C. B.;
Trueblood, K. N. J. Am. Chem. Soc. 1984, 106, 7150–
7167; (b) Kajigaeshi, S.; Kakinami, T.; Okamoto, T.;
Nakamura, H.; Fujikawa, M. Bull. Chem. Soc. Jpn. 1987,
60, 4187–4189; (c) Konishi, H.; Aritomi, K.; Okano, T.;
Kiji, J. Bull. Chem. Soc. Jpn. 1989, 62, 591–593;
(d) Ganguly, N. C.; De, P.; Dutta, S. Synthesis 2005,
1103–1105; (e) Paul, V.; Sudalai, A.; Daniel, T.; Srini-
vasan, K. V. Tetrahedron Lett. 1994, 35, 7055–7056;
(f) Oberhauser, T. J. Org. Chem. 1997, 62, 4504–4506; (g)
Muathen, H. A. J. Org. Chem. 1992, 57, 2740–2741;
(h) Bisarya, S. C.; Rao, R. Synth. Commun. 1993, 23, 779–
788; (i) Majetich, G.; Hicks, R.; Reister, S. J. Org. Chem.
1997, 62, 4321–4326; (j) Roche, D.; Prasad, K.; Repic, O.;
Blacklock, T. J. Tetrahedron Lett. 2000, 41, 2083–2085;
(k) Vyas, P. V.; Bhatt, A. K.; Ramachandraiah, G.;
Bedekar, A. V. Tetrahedron Lett. 2003, 44, 4085–4088; (l)
Zhang, G.; Liu, R.; Xu, Q.; Ma, L.; Liang, X. Adv. Synth.
Catal. 2006, 348, 862–866; (m) Guo, M.; Varady, L.;
Fokas, D.; Baldino, C.; Yu, L. Tetrahedron Lett. 2006, 47,
3889–3892.
In conclusion, we have developed an efficient and versa-
tile method for the nuclear monobromination of aro-
matics and some heteroaromatics using NBS in the
presence of sulfonic-acid-functionalized silica as a hetero-
geneous catalyst. The method is highly regioselective
offering potential in various synthetic applications. The
mild reaction conditions, simple experimental proce-
dure, rapid conversion, excellent yields and reusability
of the catalyst are notable advantages of the method.
Acknowledgements
7. (a) Das, B.; Ramu, R.; Ravikanth, B.; Reddy, K. R.
Tetrahedron Lett. 2006, 47, 779–782; (b) Das, B.; Thiru-
pathi, P.; Mahender, I.; Reddy, V. S.; Rao, Y. K. J. Mol.
Catal. A: Chem. 2006, 247, 233–239; (c) Das, B.; Ravi-
The authors thank CSIR and UGC, New Delhi, for
financial assistance.