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Current Microwave Chemistry, 2015, 2, 61-68 61
Solvent and Catalyst Free, Regioselective -Bromination of Aralkyl
Ketones Using N-Bromosuccinimide (NBS) Under Microwave Irradiation:
An Efficient Protocol
Rajesh H. Vekariya, Shyamali N. Panchal, Kinjal D. Patel and Hitesh D. Patel*
Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad, India
Abstract: An exceptionally efficient, environmentally safe and economic method has been developed for regioselective
-bromination of various aralkyl ketones in excellent yield with N-bromosuccinimide (NBS) under microwave irradiation
(CEM discover) without use of either a solvent or a catalyst. After completion of the reaction, the NBS can be regenerated
by using an environment friendly brominating agent NaBr/NaBrO
3
in acetic acid. The present procedure offers various
advantages such as a very short reaction time, simple work-up procedure, excellent yield of products and recovery of the
reagents by green protocol.
Keywords: -bromination, catalyst free, efficient protocol green bromination, microwave irradiation, N-bromosuccinimide,
regeneration, regioselectivity, solvent free.
INTRODUCTION
The -bromination of carbonyl compounds is an impor-
tant transformation in organic synthesis [1]. This important
class of compounds is a versatile building block in the syn-
thesis of a variety of effective pharmaceuticals and also is an
intermediate for a variety of biologically active compounds
such as quinoxalines [2-4], thiazoles [5-9], pyrroles [10, 11],
imidazoles [12], dihydropyrazines [13], triazolothiadiazines
[14-17], pyrazolines [18], thiophene and thiazolidin-4-one
[19], benzofuran [20], furans [21], selenazoles [22], indolizi-
nes [23], thiazoline, thiazinone and thiazolidinone [24] and
pyrazolo[1,5-a][1,3,5]triazin and thiadiazin [25]. Further-
more, they are foundation units for the retro-synthesis of a
huge range of natural products [26, 27]. In addition, -
bromoacetophenone derivatives actively participate in the
inhibition of protein tyrosine phosphatase such as SHP-1 and
PTP1B [28].
Development of efficient and green synthetic procedures
for the widely used organic compounds from readily avail-
able reagents is one of the major challenges in synthetic or-
ganic chemistry. The bromination of aralkyl ketones using
molecular bromine (Br
2
) may lead to side-chain -
bromination as well as ring bromination. Thus, recently
many scientists have developed various methods for the se-
lective synthesis of -bromo carbonyl compounds. We have
previously published a review article on the various routes
for synthesis of -bromo carbonyl compounds in recent years
[29]. The most commonly used reagents for this synthesis
includes molecular bromine [30-32], and N-bromosuccini-
mide (NBS) with different catalyst and solvents such as
*Address correspondence to this author at the Department of Chemistry,
School of Sciences, Gujarat University, Ahmedabad, India;
Tel: (O) +91-079-26300969; Fax: +91-079-26308545;
E-mail: drhiteshpatel1@gmail.com
silica-supported sodium bicarbonate [33], p-toluenesulfonic
acid (p-TSA) in the presence of ionic liquids [34], p-TSA
[35], tetrachlorosilane (SiCl
4
) [36], copper(II) triflate [37],
montmorillonite K-10 [38], p-TSA in ultrasound [39],
trifluoromethanesulfonate (TMSOTf) [40], UV-vis irradia-
tion [41], ureahydrogenperoxide (UHP) in the presence of
ionic liquid [42], [bmim]PF
6
[43], enol triethyl borates [44],
sulfonic acidfunctionalized silica [45], amberlyst-15 [46],
ammonium acetate [47] and Mg(ClO
4
)
2
[48]. Some other
brominating reagents have been also reported which include
dioxane dibromide and silica gel [49], tribromoisocyanuric
acid (TBCA) [50], bromotrimethylsilane (TMSBr) with po-
tassium nitrate (KNO
3
) [51], hexamethylenetetramineebro-
minecomplex (HMTAB) and basic alumina [52], ammonium
bromide (NH
4
Br) and oxone (2KHSO
5
. KHSO
4
. K
2
SO
4
)
[53], NH
4
Br and ammonium persulfate [(NH
4
)
2
S
2
O
8
] [54],
NH
4
Br, vanadium pentoxide (V
2
O
5
) and H
2
O
2
[55], cop-
per(II) bromide [Cu(Br)
2
] [56-59], HBr and H
2
O
2
[60-62],
HBr and H
2
O
2
or tert-butylhydroperoxide (TBHP) [63], KBr
and boric acid [64], [hydroxy(tosyloxy)iodo]benzene (HTIB)
(Koser’sreagent) and magnesium bromide (MgBr
2
) [65]. In
addition, microwave assisted -bromination in the presence
of ionic liquid was also carried out recently [66]. Moreover,
high microwave power (800 W) assisted -bromination of
carbonyl compounds using NBS (1.5 equiv.) was also re-
ported [67]. Some brominated ionic liquids were also used
for -bromination of carbonyl compounds [68, 69]. How-
ever, many of these methods still possess one or more
disadvantages such as longer reaction times, lower yield of
the products, poor selectivity, preparation of catalyst, use of
expensive reagents, harsh reaction conditions, formation of
dibrominated products and intricate work-up procedures.
Hence, the development of an efficient and eco-friendly
method for regioselective -monobromination of aralkyl
ketones remains a major challenge for synthetic organic
chemists. To resolve these problems, we have reported
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