An efficient synthesis of benzyl bromides from aromatic aldehydes using polymethylhydrosiloxane and (bromodimethyl)sulfonium bromide or N-bromosuccinimide

Article abstract of DOI:10.1016/j.tetlet.2007.07.125

Polymethylhydrosiloxane (PMHS) in combination with (bromodimethyl)sulfonium bromide or NBS has been utilized for the first time for reductive bromination of aromatic aldehydes at room temperature to afford the corresponding benzyl bromides in excellent yields.

Full text of DOI:10.1016/j.tetlet.2007.07.125

Tetrahedron Letters 48 (2007) 6681–6683
An efficient synthesis of benzyl bromides from aromatic
aldehydes using polymethylhydrosiloxane and
bromodimethyl)sulfonium bromide or N-bromosuccinimide
I
(
*
Biswanath Das, Yallamalla Srinivas, Harish Holla, Keetha Laxminarayana and
Ravirala Narender
Organic Chemistry Division-I, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 29 May 2007; revised 9 July 2007; accepted 19 July 2007
Available online 27 July 2007
Abstract—Polymethylhydrosiloxane (PMHS) in combination with (bromodimethyl)sulfonium bromide or NBS has been utilized for
the first time for reductive bromination of aromatic aldehydes at room temperature to afford the corresponding benzyl bromides in
excellent yields.
ꢀ
2007 Elsevier Ltd. All rights reserved.
+
À
1. Introduction
(Me SBr Br ) or NBS at room temperature to form
2
the corresponding benzyl bromides in excellent yields
(Scheme 1).
Polymethylhydrosiloxane (PMHS) (1) is an important
reducing agent for eco-friendly reductive processes as
1
it is non-toxic and stable to air and moisture. It is a
A series of benzyl bromides were prepared from various
aromatic aldehydes (Table 1). No disubstituted product
was obtained. Aromatic aldehydes containing both elec-
tron-donating and electron-withdrawing groups under-
went the conversion smoothly. However, aldehydes
possessing electron-withdrawing groups required longer
reaction times (Table 1, entries 4 and 5) though the yields
were very high. Alkyl, halogen, ether and nitro groups re-
mained uneffected. Aliphatic aldehydes and ketones
co-product of the silicon industry and is inexpensive.
It has recently been utilized to carry out various organic
syntheses. However, this reagent was found unsuitable
1
for the synthesis of benzyl bromides starting from
2
benzaldehydes.
Me
Me SiO
Si
H
O
SiMe3
3
n
(both aliphatic and aromatic) did not show any change
under the present reaction conditions. The combination
+
À
of PMHS and Me S BrBr was found to be better for
2
1
reductive bromination of aromatic aldehydes compared
to the combination of PMHS and NBS. For example,
the conversion of p-chlorobenzaldehyde into p-chloro-
2
. Results and discussion
+
À
In continuation of our work3 on the application of
PMHS for the development of novel synthetic method-
ologies, we have observed that aromatic aldehydes can
easily undergo reductive bromination with PMHS in
combination with (bromodimethyl)sulfonium bromide
benzyl bromide (entry 2) using PMHS/Me S BrBr
2
took 4 h while PMHS/NBS required 11 h. Both
O
+
-
PMHS/ Me S BrBr
2
H
Br
CHCl , 4-8 h, r. t.
3
Keywords: Polymethylhydrosiloxane (PMHS); (Bromodimethyl)sulfo-
nium bromide; NBS; Aromatic aldehydes; Benzylic bromide.
q
Part 147 in the series ‘Studies on novel synthetic methodologies’.
or
R
R
PMHS/ NBS
CHCl , 10-12 h, r. t.
92-98%
3
*
Corresponding author. Tel./fax: +91 40 27160512; e-mail:
biswanathdas@yahoo.com
Scheme 1.
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.07.125

6682
B. Das et al. / Tetrahedron Letters 48 (2007) 6681–6683
+
À
Table 1. Preparation of benzyl bromides from aromatic aldehydes using PMHS/Me
2
S BrBr
a
Entry
Substrate
Product
Time (h)
5
Isolated yield (%)
96
O
Br
1
H
O
Br
Br
2
3
H
4
4
98
98
Cl
Cl
Cl
Cl
O
H
Cl
Cl
O
Br
H
4
5
7
8
94
92
NO2
NO2
O
Br
H
O N
2
O N
2
O
Br
Br
H
6
7
94
CF3
CF3
O
7
8
H
4
4
95
96
Br
Br
O
Br
H
H C
3
H C
3
O
Br
9
H
4
4
96
95
Et
Et
O
Br
H
H C
1
1
0
1
3
H C
3
CH3
CH3
O
O
O
H
Br
4
96
H C
H C
3
3
O
O

B. Das et al. / Tetrahedron Letters 48 (2007) 6681–6683
6683
Table 1 (continued)
a
Entry
Substrate
Product
Time (h)
Isolated yield (%)
94
O
Br
1
1
2
3
6
5
H
O
MeO
MeO
MeO
MeO
Br
H
98
OMe
OMe
a
1
The structures of the products were determined from spectral (IR, H NMR and MS) data.
O-SMe Br
2
at room temperature. (Bromodimethyl)sulfonium
bromide or NBS (1 mmol) was added and stirring con-
tinued. After a few hours, effervescence appeared. The
progress of the reaction was followed by TLC. After
Ar-CH-Br
PMHS
+
-
Me S BrBr
A
2
completion, the CHCl was evaporated under vacuum
3
Ar-CHO
Ar-CH Br
2
and water (10 mL) was added. The mixture was
extracted with EtOAc (3 · 10 mL) and the extract was
dried and concentrated. The crude mass was subjected
to column chromatography (silica gel, hexane–EtOAc)
to afford the pure benzyl bromide. (CAUTION! The
products are hazardous and potent lachrymators.)
PMHS
Br-
+
-
Me S BrBr
2
Ar-CH -O-SMe Br
2
2
B
Scheme 2.
combinations converted the aromatic aldehydes into the
corresponding benzyl bromides in similar yields.
Acknowledgement
(
Bromodimethyl)sulfonium bromide, an inexpensive re-
The authors thank CSIR and UGC, New Delhi, for
financial assistance.
agent, has been used mainly as a catalyst but the scope
of its reaction has not yet been fully explored.
4
The formation of benzyl bromides from aromatic alde-
References and notes
+
À
hydes and PMHS/Me S BrBr may take place by
2
reduction of the intermediate bromohydrin A or of the
aldehyde into the corresponding alcohol derivative B
followed by subsequent attack by bromide (Scheme 2).
1. Lawrence, N. J.; Drew, M. D.; Bushell, S. M. J. Chem.
Soc., Perkin Trans. 1 1999, 3381–3391.
2. Aizpurua, J. M.; Lecea, B.; Palomo, C. Can. J. Chem. 1986,
6
4, 2342–2348.
3
4
. Das, B.; Holla, H.; Srinivas, Y.; Chowdhury, N.; Bandgar,
B. P. Tetrahedron Lett. 2007, 48, 3201–3204.
. (a) Furukawa, N.; Inoue, T.; Aida, T.; Oae, S. J. Chem.
Soc., Chem. Commun. 1973, 212; (b) Olah, G. A.; Yash-
want, D. V.; Massoud, A.; Suryaprakash, G. A. Synthesis
In conclusion, the combination of PMHS and (bromo-
dimethyl)sulfonium bromide or NBS has been utilized
here for the first time for an efficient synthesis of benzyl
bromides from aromatic aldehydes under mild reaction
conditions.
1
979, 720–721; (c) Khan, A. T.; Mondal, E.; Ghosh, S.;
Islam, S. Eur. J. Org. Chem. 2004, 2002–2009.
3
. General experimental procedure
An aromatic aldehyde (1 mmol) and PMHS (1.5 mmol)
were added to CHCl (5 mL) and the mixture was stirred
3