1152 Bull. Chem. Soc. Jpn., 74, No. 6 (2001)
© 2001 The Chemical Society of Japan
additive, toluene reacts with bromine instantly. But the inter-
esting result was that we separated benzyl bromide as the only
product. The reaction was applied to a range of aromatic hy-
drocarbons (scheme 1). Benzene does not react significantly
with this reagent. By placing the toluene on the borderline,
one finds a great difference between the electron-withdrawing
and the electron-donating groups. In compounds containing
electron-withdrawing substituents, the corresponding benzyl
derivative was obtained in good yield (entries 5 and 6) (Table
1).
Experimental
All melting points are uncorrected. 1H NMR spectra were re-
corded on an EM-390 and 13C NMR spectra were recorded on a
Bruker Advance 500. Mass spectra were recorded on a Solid
Probe Mass Spectrometer Fisons model Trio 1000.
p-Chlorobenzyl Bromide; Typical Procedure:
2
g
(15.8
mmol, 1.87 mL) p-chlorotoluene in tetrachloromethane (20 mL)
was mixed with 4 g of dried silica gel (250 ˚C, 1 h). While keep-
ing the mixture at 25 ˚C, 2.52 g (15.8 mmol, 0.87 mL) of bromine
was added. Mixing was continued for 8 h, then the catalyst was
removed by filtration and washed well with chloroform. The fil-
trate and washings were treated with aqueous thiosulfate solution
(10 mL). The organic phase was separated and dried over MgSO4,
and concentrated under reduced pressure to give 2.27 g, (70%) of
p-cholrobenzyl bromide as colorless crystals, mp 49–50 ˚C (Ref.
16 mp. 51 ˚C).
We have realized that acetophenone under this condition
was dibrominated12 adjacent to the carbonyl group in accept-
able yield (entry 14) (69%). 1-p-Tolylethanone was also di-
brominated adjacent to the carbonyl group with a trace of the
corresponding benzyl derivative. Phenyl p-tolyl sulfone and p-
nitrotoluene were unreactive under these conditions, and were
recovered virtually unchanged after treating with bromine.
Compounds containing electron-donating substituents show
mainly bromination on the rings. The behaviour of o-, m- and
p-xylene show predominant substitution on the ring (entries 2,
3 and 4). p-Bromination of phenol has previously been
achieved by the use of Amberlyst-A26,13 and other re-
agents.14,15 In this work we have brominated phenol to p-bro-
mophenol in good yield (81%) (entry 22). Mesitylene, with its
three activating groups, is so reactive that it undergoes bromi-
nation instantly (entry 10). These simple observations demon-
strate that SiO2 has the potential to alter reaction selectivity
markedly. It may be able to switch a mechanism from radical
to polar, or to influence the regioselectivity of the products
formed. In this respect, one can compare the product distribu-
tion of bromination of aromatic hydrocarbon in the presence of
silica gel and in the absence of it. In the absence of silica gel,
we lose the selectivity and obtain a mixture of products by sub-
stitution of bromine atom on the ring and on the side chain
without any preferability.
Evidently, the Lewis and Brønsted acid sites on silica gel
help the bromine to produce Brꢀ and Br•, here Brꢀ can be sta-
bilized more by the Lewis bases of the silica gel surface. Then,
for an aromatic hydrocarbon with a withdrawing substituent,
the side chain bromination would occur. But in the case of ar-
omatic hydrocarbons with more than one donating substitu-
ents, the substitution on the ring would be preferential.
In order to explore the scope of the reaction, similar condi-
tions were applied to naphthalene and anthracene. Here, naph-
thalene was monobrominated to 1-bromonaphthalene in good
yield (84%) in less than 3 min (entry 17), and anthracene was
dibrominated to 9,10-dibromoanthracene in 86% yield in less
than 5 min (entry 18). Fluorene is dibrominated to 2,6-dibro-
mofluorene in 85% yield in 20 min (entry 19).
We are grateful to Isfahan University of Technology for the
support of this work.
References
1
a) A. Mckillop and D.W. Young, Synthesis, 1979, 401. b) J.
H. Clark, A. P. Kybett, and D. J. Macquarrie, “Supported Re-
agents: Preparation, Analysis, and Application,” VCH, New York
(1992). c) “Solid Supports and Catalysts in Organic Synthesis,”
ed by K. Smith, Ellis Harwood, Chichester (1992). d) J. H. Clark,
“Catalysis of Organic Reactions by Supported Inorganic Re-
agents,” VCH, New York (1994).
2
K. Smith and M. R. Bye, Tetrahedron Lett., 27, 1051,
(1986).
3
G. A. Olah and W. John, Synth. Commun., 1974, 653.
A. Mckillop and D. Bromley, J. Org. Chem., 37, 88,
4
(1972).
5
P. R. Brian and C. Wilson, J. Chem. Soc., Chem. Commun.,
1978, 752.
6
S. D. Saraf and Z. A. Malik, Chem. Abstr., 80, 36784z,
(1974).
7
8
9
S. Lee Irvin, Org. Syn., Coll. Vol. 2, 95.
V. P. Kravets, Zh. Org. Khim., 2, 1244, (1966).
G. A. Olah, Synthesis, 12, 895, (1974).
10 F. A. Vingiello, T. J. Delia, P. Polss, and D. Farrier, J.
Chem. Educ., 40, 544, (1963).
11 I. M. Herlborn and J. S. Heaton, Org. Syn., Coll. Vol. 1,
207.
12 S. Kajigaeshi, M. Noguchi, and S. Fujisaki, Bull. Chem.
Soc. Jpn., 60, 2667, (1987).
13 K. Smith, D. M. James, I. Mattews, and M. R. Bye, J.
Chem. Soc., Perkin. Trans. 1, 1992, 1877.
14 J. Berthelot, C. Gutte, M. Ouchefoune, P. L. Desbene, and
J. J. Basselier, J. Chem. Res. Synop., 1986, 381.
15 C. Venkatachalapathy and K. Pitchumani, Tetrahedron, 53,
7, 2581, (1997).
16 M. S. Kharasch, E. Margolis, P. C. White, and F. R. Mayo,
J. Am. Chem. Soc., 59, 1405, (1937).
The results reported here demonstrate that Br2/SiO2 possess-
es considerable practical advantages over traditional reagents
for bromination of aromatic hydrocarbons. The reactions are
clean and good-yielding and work-up is simple.