An efficient and fast method for the preparation of benzylic bromides

Article abstract of DOI:10.1081/SCC-200026648

A mixture of triphenylphosphine and N-bromosuccinimide system is found to promote efficient conversion of benzylic alcohols into benzylic bromides under microwave irradiation conditions.

Full text of DOI:10.1081/SCC-200026648

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An Efficient and Fast Method
for the Preparation of
Benzylic Bromides
Jong Chan Lee a & Eui Yong Hwang a
a Department of Chemistry, Chung‐Ang University,
Seoul, 156‐756, South Korea
Version of record first published: 06 Jan 2011.
To cite this article: Jong Chan Lee & Eui Yong Hwang (2004): An Efficient and Fast
Method for the Preparation of Benzylic Bromides, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry,
34:16, 2959-2963
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 16, pp. 2959–2963, 2004
An Efficient and Fast Method for the
Preparation of Benzylic Bromides
*
Jong Chan Lee and Eui Yong Hwang
Department of Chemistry, Chung-Ang University, Seoul, South Korea
ABSTRACT
A mixture of triphenylphosphine and N-bromosuccinimide system is
found to promote efficient conversion of benzylic alcohols into benzylic
bromides under microwave irradiation conditions.
Key Words: Benzyl alcohol; Bromide; Microwave; Phosphine.
INTRODUCTION
The conversion of benzylic alcohols into the corresponding benzylic
bromides is a fundamental reaction in the synthesis of a variety of organic
compounds.[1,2] Generally, the reagent systems of bromotrimethylsilanes,[3]
Ph3P/Br2,[4] and dimethylsulfide/N-bromosuccinimide[5] were commonly
utilized to effect this transformation in various organic solvents.
*
Correspondence: Jong Chan Lee, Department of Chemistry, Chung-Ang University,
Seoul 156-756, South Korea; Fax: þ82-2-825-4736; E-mail: jclee@cau.ac.kr.
2959
DOI: 10.1081/SCC-200026648
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Lee and Hwang
In recent years, microwave promoted organic transformations in solvent-
free conditions received much attention due to its environmentally very
friendly conditions.[6] In connection with our interests on microwave acceler-
ated organic reactions, we wanted to develop an efficient method for conver-
sion of benzylic alcohols into the corresponding benzylic bromide under
solvent-free conditions. To our best knowledge, the only previous example
of microwave promoted bromination of benzylic alcohols into benzyl bro-
mides under microwave irradiation conditions was reported by using
sodium bromide doped on K10 clay for the 6–9 min reaction time.[7]
However, in view of very fast heating nature of microwave irradiation con-
ditions, the required reaction time of 9 min under strong acidic conditions in
this method might be considered somewhat drastic in certain reactions. It is
often preferable to conduct microwave induced reactions under neutral con-
ditions at shorter reaction time due to the prolonged reaction time under exces-
sive heating often could potentially lead to product decomposition. Thus, there
is a need to develop a more fast and improved microwave promoted synthetic
method under neutral reaction conditions for the preparation of benzylic
bromides. After extensive search for suitable reagent system for the bromina-
tion of benzylic alcohols, we found the combination of triphenylphosphine/
N-bromosuccinimide to be most suitable for the solvent-free microwave
irradiation conditions. Thus, microwave irradiation to the neat mixture of
benzyl alcohol, triphenylphosphine (1.5 equiv.), and N-bromosuccinimide
(1.5 equiv.) afforded benzyl bromide in just 10 sec (Sch. 1). A series of
benzylic alcohols was reacted under the same reaction conditions and
results are summarized in Table 1. All reactions studied were completed in
less than 30 sec in very high yields. As can be seen in Table 1, primary
benzyl alcohols generally afforded much higher yields than secondary alco-
hols (entries 5–7), which probably results from steric hindrance between
phosphonium alkoxide intermediate and nucleophilic bromide ion.
In summary, the microwave promoted reaction of various benzylic
alcohols with triphenylphosphine/N-bromosuccinimide under solvent-free
conditions gave benzylic alcohols with high yields in very short reaction
time. The neutral conditions and rapid reaction conditions together with
eco-friendly solvent-free conditions should make present protocol an import-
ant alternative to other classical methods.
Scheme 1.

Preparation of Benzylic Bromides
Table 1. Microwave promoted preparation of benzylic bromides.
2961
Time
(sec)
Yielda
(%)
Entry
1
Substrate
Product
10
10
20
20
30
30
30
20
88
98
96
98
76
75
71
90
2
3
4
5
6
7
8
aIsolated yield.
EXPERIMENTAL
All reactions were performed in a commercial domestic microwave oven
1
(Samsung RE-21C). H NMR spectra were obtained on a Varian Gemini
2000-300 MHz spectrometer.
General Procedure
A mixture of benzylic alcohol (1.0 mmol), N-bromosuccinimide (0.26 g,
1.5 mmol), and triphenylphosphine (0.39 g, 1.5 mmol) was placed in a
50 mL of glass tube. The reaction mixture was inserted in an alumina bath
inside a domestic microwave oven and irradiated (700 W) indicated time in
Table 1. On completion of the reaction, the product was extracted with
dichloromethane and washed with water (40 mL). The organic layer was
separated and dried over MgSO4. After evaporation of solvent, the product

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Lee and Hwang
was purified by flash column chromatography on silica gel using n-pentane as
eluent to furnished pure benzylic bromide.
1a. 1H-NMR (300 MHz, CDCl3): d 4.50 (s, 2H), 7.26–7.39 (m, 5H).
2a. 1H-NMR (300 MHz, CDCl3): d 4.45 (s, 2H), 7.26–7.32 (m, 5H).
3a. 1H-NMR (300 MHz, CDCl3): d 2.34 (t, 3H), 4.49 (s, 2H), 7.13–7.16
(m, 2H), 7.26–7.30 (m, 2H).
4a. 1H-NMR (300 MHz, CDCl3): d 4.52 (s, 2H), 7.54–7.57 (m, 2H),
8.19–8.20 (m, 2H).
5a. 1H-NMR (300 MHz, CDCl3): d 2.06 (d, 3H), 5.24 (q, 1H), 7.31–7.49
(m, 5H).
6a. 1H-NMR (300 MHz, CDCl3): d 2.02 (d, 3H), 2.33 (s, 3H), 5.18
(q, 1H), 7.13–7.16 (m, 2H), 7.32–7.34 (m, 2H).
7a. 1H-NMR (300 MHz, CDCl3): d 2.02 (d, 3H), 5.16 (q, 1H), 7.27–7.39
(m, 4H).
8a. 1H-NMR (300 MHz, CDCl3): d 4.48 (s, 4H), 7.26–7.42 (m, 3H).
ACKNOWLEDGMENT
This research was supported by the Chung-Ang University Research
Grants in 2004.
REFERENCES
1. Chen, Y.-C.; Wu, T.-F.; Deng, J.-G.; Liu, F.; Jiang, Y.-Z.; Chai, M.C.K.;
Chan, A.S.C. Dendritic catalysts for asymmetric transfer hydrogenation.
Chem. Comm. 2001, 1488–1489.
2. Otsuka, H.; Araki, K.; Matsumoto, H.; Harada, T.; Shinkai, S. Synthesis
and NMR spectroscopic studies of bridged and capped calix[6]arenes:
High-yield syntheses of unimolecular caged compounds from calix[6]-
arene. J. Org. Chem. 1995, 60, 4862–4867.
3. Jung, M.E.; Hatfield, G.C. Preparation of bromides from alcohols via treat-
ment with trimethylsilyl bromide. Tetrahedron Lett. 1978, 4483–4486.
4. Wiley, G.A.; Hershkowitz, R.L.; Rein, B.M.; Chung, B.C. Organo-
phosphorus chemistry. I. Conversion of alcohol and phenols to halides
by tertiary phophine dihalides. J. Am. Chem. Soc. 1964, 86, 964.
5. Corey, E.J.; Kim, C.U.; Takeda, M. A method for selective conversion of
allylic and benzylic alcohols to halides under neutral conditions. Tetra-
hedron Lett. 1972, 13, 4332–4339.
6. Varma, R.S. Solvent-free organic synthesis. Green Chem. 1999, 43–55.

Preparation of Benzylic Bromides
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7. Kad, G.L.; Singh, V.; Kaur, K.P.; Singh, J. Selective preparation of
benzylic bromides in dry media coupled with microwave irradiation.
Tetrahedron Lett. 1997, 38, 1079–1080.
Received in Japan February 23, 2004