Electrochemical method for the preparation of dibromomethyl, bis(bromomethyl), and bis(dibromomethyl) arenes

Article abstract of DOI:10.1080/00397910802654757

Electrochemical bromination of alkyl aromatic compounds by two-phase electrolysis yields the corresponding α,α-dibrominated products. The reaction has been carried out in a single-compartment electrochemical cell using aqueous sodium bromide (40-50%), containing a catalytic amount of HBr as electrolyte, and chloroform, containing an alkyl aromatic compound, as the organic phase with a Pt plate as anode at 10-15C. Two-phase electrolysis results in high yields (70-90%) of dibromomethyl, bis(bromomethyl), and bis(dibromomethyl) arenes, depending upon the charge passed.

Full text of DOI:10.1080/00397910802654757

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Electrochemical Method for the
Preparation of Dibromomethyl,
Bis(bromomethyl), and
Bis(dibromomethyl) Arenes
K. Kulangiappar a , G. Karthik a & M. Anbu
Kulandainathan a
a Electro-organic Department, Central
Electrochemical Research Institute, Karaikudi, India
Version of record first published: 01 Jun 2009.
To cite this article: K. Kulangiappar , G. Karthik & M. Anbu Kulandainathan (2009):
Electrochemical Method for the Preparation of Dibromomethyl, Bis(bromomethyl),
and Bis(dibromomethyl) Arenes, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 39:13, 2304-2309
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Synthetic Communications1, 39: 2304–2309, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802654757
Electrochemical Method for the Preparation
of Dibromomethyl, Bis(bromomethyl),
and Bis(dibromomethyl) Arenes
K. Kulangiappar, G. Karthik, and M. Anbu Kulandainathan
Electro-organic Department, Central Electrochemical Research Institute,
Karaikudi, India
Abstract: Electrochemical bromination of alkyl aromatic compounds by two-
phase electrolysis yields the corresponding a,a-dibrominated products. The
reaction has been carried out in a single-compartment electrochemical cell using
aqueous sodium bromide (40–50%), containing a catalytic amount of HBr as elec-
trolyte, and chloroform, containing an alkyl aromatic compound, as the organic
phase with a Pt plate as anode at 10–15ꢀC. Two-phase electrolysis results in high
yields (70–90%) of dibromomethyl, bis(bromomethyl), and bis(dibromomethyl)
arenes, depending upon the charge passed.
Keywords: Alkyl aromatic compound, dibromomethyl arene, electrochemical
bromination, two-phase electrolysis
The conventional benzylic bromination process for producing benzyl or
benzal bromide consists of contacting gaseous bromine with a reaction
mixture having an organic liquid phase with an (un)substituted aromatic
ring–containing compound bearing one or more benzylic carbon atoms.
The temperature of the liquid phase is maintained at 100–170ꢀC with
light irradiation to effect benzylic bromination of the benzylic methyl
group.[1–8] Alternatively, N-bromosuccinimide can be used along with
azobisisobutyronitrile (AIBN) or benzoyl peroxide as a catalyst at high
Received September 11, 2008.
Address correspondence to M. Anbu Kulandainathan, Electro-organic
Department, Central Electrochemical Research Institute, Karaikudi 630006,
India. E-mail: manbu123@yahoo.com
2304

Electrochemical Bis(bromomethyl)
2305
temperatures.[9–11] However, these methods have the drawbacks of pro-
longed heating, light irradiation, and the use of bromine as vapor (a ser-
ious safety problem).
Herein, we report a simple method for the preparation of dibromo-
methyl, bis(bromomethyl), and bis(dibromomethyl) arenes from methy-
larenes by two-phase electrolysis at 10–15ꢀC in a single-compartment
cell as shown in Scheme 1. We have previously described the electro-
chemical method of side-chain bromination and nuclear bromination
using an NaBr–HBr system.[12,13]
A number of alkyl aromatic and substituted alkyl aromatic
compounds were conveniently subjected to side-chain bromination by
two-phase electrolysis using a 40–50% NaBr solution as an electrolyte,
containing a catalytic amount of HBr (5%) in the aqueous phase, along
with an organic phase of chloroform, containing an alkyl aromatic
compound. The product distributions from the reactions are listed in
Table 1.
This process allows the preparation of dibromomethyl, bis(bromo-
methyl), and bis(dibromomethyl) arenes, depending upon the total
charge passed, in reasonable purities. These can be used for further trans-
formations and as building blocks in material chemistry. To avoid the
formation of a,a,a-tribromomethyl arenes, the electrolysis temperature
was reduced to 5ꢀC from 15ꢀC after passing 75% of the charge
(mentioned in Table 1) to control the reactive species Br2O.
When the reaction was not controlled at the final stage, a mixture
of a,a,a-tribromomethyl and nuclear brominated arenes was formed
along with a,a-dibromomethyl arene. Initial attempts to obtain pure
bis(dibromomethyl) arene were not successful when the reaction was
conducted at 15ꢀC throughout the experiment and resulted in the for-
mation of a,a,a-tribromometyl arene in 30–40%. The reaction pro-
ceeded under mild conditions in the presence of a less hazardous
brominating agent than Br2 (N-bromosuccinimide and pyridinium tri-
bromide) and at a lower temperature than the conventional methods.
Although dibromine monoxide is exceptionally reactive in the side-
chain bromination of nonactivated (entry 1) and deactivated alkyl
aromatic arenes (entries 2 and 3), activated compound 3,5-dimethyl
phenol (entry 9) gives nuclear brominated compound as a sole
product.
Scheme 1. Electrochemical dibromination of arenes.

2306
K. Kulangiappar, G. Karthik, and M. A. Kulandainathan
Table 1. Ratio of reactants and products of the electrochemical bromination of
alkyl aromatic compounds by two-phase electrolysis
Charge passed
(F=mole)
Current
efficiency (%)
Entry
1
Reactant
Producta
Yieldb (%)
72
20
17
20
24
2
3
4
72
70
70
25
17
16
24
5
6
80
20
33
20
12
50c
7
8
85
19
16
21
51d
25
(Continued )

Electrochemical Bis(bromomethyl)
2307
Table 1. Continued
Charge passed
(F=mole)
Current
efficiency (%)
Entry
9
Reactant
Producta
Yieldb (%)
90
6
81
10
11
90
76
12
30
15
24
aStructures were confirmed from their spectra (NMR, IR) data.
bIsolated yield.
cp-Nitro bromomethyl benzene (50%) is present along with 50% p-nitro dibro-
momethyl benzene.
dIsolated as a solid after trituration with n-hexane.
However, if the phenolic group is protected as methyl ether, the
product obtained is a-bromomethyl arene at the first stage. On further
bromination, nuclear-brominated a-bromomethyl arene was obtained
as a sole product instead of a,a-dibromomethyl arene (entry 5). Because
of the high deactivating nature of the nitro group in the p-nitrotoluene,
the current efficiency of product formation is comparatively less. Side-
chain bromination of alkyl aromatic compound follows a radical
mechanism, and the brominating species is dibromine monoxide
(Br2O), which is formed in aqueous phase as shown in Eqs. (1) and (2)
and extracted by the organic phase, where the selective bromination
occurs:
Br2 þ H2O ꢁꢁꢁ! HOBr þ HBr
ð1Þ
2HOBr ꢁꢁꢁ! Br2O þ H2O
ð2Þ
Hþ
ꢂ
As the reactive Br2O cleaves homolytically into Br and
ꢂOBr,
ꢂOBr abstracts benzylic hydrogen to form HOBr.
the more reactive

2308
K. Kulangiappar, G. Karthik, and M. A. Kulandainathan
Subsequently, the bromine radical attacks the benzyl radical, giving the
products benzyl bromide and subsequently a,a-dibromomethyl arene.
In conclusion, the electrochemical method for the side-chain bromi-
nation of alkyl aromatic compounds to yield a,a-dibromomethyl arenes is
a convenient and efficient procedure.
EXPERIMENTAL
Representative Procedure for Electrochemical Bromination
1,4-Bis(bromomethyl)benzene
A solution of 1,4-dimethyl benzene (entry 10) (1.06 g, 10 mmol) in 30 ml
chloroform was taken in a beaker-type undivided cell. A 40% aqueous
sodium bromide solution (50 ml) containing 5 ml of HBr (46% solution)
was added to the solution. Platinum electrodes (each of 10 cm2 area) were
placed in the upper layer of the aqueous phase. The organic phase alone
was stirred with a magnetic stirrer at a rate of 40 rpm in such a way that
the organic layer did not touch the electrodes. The electrolysis was
conducted galvanostatically at a current density of 30 mA=cm2 until the
quantity of charge indicated in Table 1 was passed at a temperature of
15ꢀC. The electrolysis was monitored by high-performance liquid chro-
matography (HPLC) using a Shimpack ODS column (120 ꢃ 4.5 mm) as
the stationary phase. The eluent consisted of methanol–water (70:30) at
a flow rate of 1 ml=min. Samples were analyzed using an ultraviolet
(UV) detector at a wavelength of 254 nm. After completion of electroly-
sis, the lower organic phase was separated, washed with 10% sodium
thiosulphate solution followed by water (2 ꢃ 25 ml), and dried with anhy-
drous Na2SO4. The solvent was removed under reduced pressure. The
residue obtained was crystallized with ethanol to give 1,4-bis(bromo-
1
methyl)benzene (2.37 g, 90%); mp 145ꢀC; H NMR (400 MHz,CDCl3):
d 4.5 (s, 4H), 7.4 (s, 4H).
1,4-Bis(dibromomethyl)benzene
1,4-Bis(bromomethyl)benzene (1.32 g, 5 mmol) in 35 ml of chloroform
was electrolyzed in a similar manner. After passing 75% of the charge
mentioned in Table 1, the temperature of the electrolyte was reduced to
5ꢀC and the electrolysis was continued. The electrolysis was monitored
by HPLC. After completion of electrolysis, the organic phase was sepa-
rated and worked up as mentioned previously. The solvent volume was

Electrochemical Bis(bromomethyl)
2309
reduced by one third under reduced pressure. After cooling the solution,
1,4-bis(dibromomethyl) benzene was separated out as a solid product and
crystallized with chloroform (1.6 g, 76%); mp 169ꢀC; 1H NMR (400 MHz,
CDCl3): d 6.7 (s, 2H), 7.6 (s, 4H).
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