Chemoselective bromination in a two-step substitution under the influence of tetrachlorosilane and N-bromosuccinimide

Article abstract of DOI:10.1080/10426500600574838

The synthesis of gem dibromide carbonyl compounds via a cheep and readily available combined reagent from tetrachlorosilane and N-bromosuccinimide (TCS-NBS). Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500600574838

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Phosphorus, Sulfur, and Silicon
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Chemoselective Bromination
in a Two-Step Substitution
Under the Influence of
Tetrachlorosilane and N-
Bromosuccinimide
Saad S. Elmorsy a , Doria S. Badawy a & Tamer K.
Khatab a
a Chemistry Department , Mansoura University ,
Mansoura, Egypt
Published online: 21 Sep 2006.
To cite this article: Saad S. Elmorsy , Doria S. Badawy & Tamer K. Khatab (2006)
Chemoselective Bromination in a Two-Step Substitution Under the Influence of
Tetrachlorosilane and N-Bromosuccinimide, Phosphorus, Sulfur, and Silicon and the
Related Elements, 181:9, 2005-2012, DOI: 10.1080/10426500600574838
To link to this article: http://dx.doi.org/10.1080/10426500600574838
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Phosphorus, Sulfur, and Silicon, 181:2005–2012, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500600574838
Chemoselective Bromination in a Two-Step Substitution
Under the Influence of Tetrachlorosilane and
N-Bromosuccinimide
Saad S. Elmorsy
Doria S. Badawy
Tamer K. Khatab
Chemistry Department, Mansoura University, Mansoura, Egypt
The synthesis of gem dibromide carbonyl compounds via a cheep and read-
ily available combined reagent from tetrachlorosilane and N-bromosuccinimide
(TCS-NBS).
N-bromosuccinimide has been used as a source of bromine atoms for the
monosubstitution of an allylic position, aromatic side chains,1,2 or an
addition to a double bond.3 Also, it has been used for the oxidation4–7
and halogenation of 9-methylanthracene, which occurs slowly in the
absence of iodine to afford solely 9-(bromomethyl)anthracene. The
bromo product reacted more slowly with NBS in the presence of
iodine to give 9-bromo-10-methylanthracene (67%) together with 9-
(bromomethyl)anthracene8 (31%). Iodine is known to retard free rad-
ical brominations.9 Also, an aromatic nuclei reacted with NBS in
the presence of stoichiometric amounts or catalytic amounts of Lewis
acids or concentrated H2SO4,10–12 TsOH,13 silica gel,14 and the zeolite
HZSM-5.15
Recently, gem-dibromoalkenes have been prepared either via the re-
action of appropriate carbonyl compounds with diethyl dibromomethyl
phosphonate16 or by the dibromomethylenation reaction17 of the corre-
sponding ketones in the presence of CBr4 and Ph3P.
We set for ourselves the goal of inventing a new type of bromination
reaction that involves the ordered reassembly of ketone and a reagent
for a maximum influence. Then we examined the reaction of various
Received September 5, 2005; accepted December 2, 2005.
Address correspondence to Tamer. K. Khatab, Mansoura University, Chemistry
Department, Faculty of Science, Mansoura, Egypt. E-mail: moemn20032000@yahoo.com
2005

2006
S. S. Elmorsy et al.
methylene ketones with TCS-NBS in the presence of CH2Cl2 as a sol-
vent at ambient temperature; the corresponding 2,2-dibromoethanones
were obtained in good yields. In the case of the reaction of TCS-NBS
with acetophenone, the bromination took place selectively at the methyl
group of the ethanone chain giving a gem dibromide. The highest yield
(95%) of 2,2-dibromo-1-phenylethanone (1) was obtained with a two-
fold equimolecular amount of TCS-NBS reagent as on mechanism. The
use of three or four moles of a brominating reagent led to a more
complex mixture with a diminished yield of the required compound
(1) (Scheme 1).
SCHEME 1
This procedure was applied to a number of useful selected substi-
tuted aromatic ethanones and aliphatic methyl carbonyl compounds.
The bromination of some ethanones with TCS-NBS took place to give
gem-dibromoethanone derivatives, as can be judged from the results
compiled in Table I.
The use of a halogenated solvent, such as CH2Cl2, enhanced the
reactivity of TCS-NBS for bromination. The halogenation media, which
favored the propose mechanism, improved the use of TCS-NBS as a
halogenated system. In accordance with this work, the use of TCS-NBS
TABLE I Using TCS/ZnCl2. as a Binary Reagent in a Gem
Dibromination Reaction
Exp.
no.
Time
(h)
Yield
(%)
Substrate
Product
1
2
3
4
5
6
7
8
Acetophenone
5
4
6
3
2,2-dibromo-1-phenylethanone (1)
95
4-Chloroacetophenone
4-Nitroacetophenone
4-Methylacetophenone
4-Acetylbiphenyl
2-Acetylnaphthalene
4-Acetylpyridine
Acetanilide
2,2-dibromo-1-(4ꢀ-chlorophenyl)ethanone (2) 91
2,2-dibromo-1-(4ꢀ-nitrophenyl)ethanone (3)
90
96
92
92
82
95
2,2-dibromo-1-(p-tolyl)ethanone (4)
3.5 2,2-dibromo-1-(4ꢀ-biphenyl)ethanone (5)
4.5 2,2-dibromo-1-(2ꢀ-naphthyl)ethanone (6)
5
2,2-dibromo-1-(4ꢀ-pyridyl)ethanone (7)
1.5 2ꢀ,4ꢀ-dibromo acetanilide (8)

Chemoselective Bromination
2007
for bromination, an addition mechanism to vinyl silyl ether, may be
operated in these processes.
SCHEME 2
In the proposed mechanism, mixture of tetrachlorosilane and N-
bromosuccinimide affords a siloxy imine (I) and bromonium chloride,
which is considered a source of positive bromine. The addition of a pos-
itive bromine to an olefinic bond is believed to proceed via a bridged
bromonium ion18 (intermediate III) if the corresponding open β-bromo-
carbocation is less stable. The bromonium ion (III) may be intercon-
verted by way of a strong nucleophile. The opening of the bromonium
ion by a nucleophile (Cl−) followed by the elimination of HCl would lead
to intermediate (V) as illustrated in Scheme 2. The favored reaction of
the monobromo-ethanone derivative by a second bromination, rather
than the ring bromination, probably is due to the conformation of the
monobromo-ethanone substituents, which preclude alignment of the
methylene hydrogens with the formation of vinyl silyl enols. The sec-
ond bromination proceeds the same way, and the product was obtained
after hydrolysis.
As indicated in Scheme 2, the electron density around the nitrogen
atom in NBS is decreased on the coordination of SiCl4, and the N Br
bond becomes weaker.19 Subsequently, the electrophilic character, and
therefore the hydride ion abstracting capacity of NBS, is increased after

2008
S. S. Elmorsy et al.
complexation with TCS, resulting in an interaction with the protons of
methylene groups.
The combination of stoichiometric quantities of NBS with TCS is
quite efficient for a reaction of acetanilide. The isolated product was
identified as 2ꢀ,4ꢀ-dibromoacetanilide (8). In this reaction, a mixture
of tetrachlorosilane and acetanilide may be afforded the correspond-
ing trichlorosiloxy imine (i) and not the trichlorosilyl vinyl ether (ii),
which was not so sufficiently nucleophilic to react with the brominat-
ing reagent. The positive bromine may be interconverted by way of the
activated aromatic nucleus of acetanilide as a strong nucleophile to
form the corresponding 2ꢀ,4ꢀ-dibromo compound.
SCHEME 3
EXPERIMENTAL
All reactions were carried out under atmospheric conditions at r.t. in
the closed system with a condenser to abstract any HCl produced and
avoid moisture. TCS and NBS were obtained in pure from a fluka bottle.
The solvents were distilled and dried over CaCl2 before use. Reac-
tions were monitored by TLC on 0.25 mm Merk Silica gel sheets (60 GF
354) (4 × 2 cm) developed with U.V. light or I2 vapor.
General Procedure
A mixture of tetrachlorosilane (4.8 mL, 40 mmole) N-bromosuccinimide
(3.6 g, 20 mmole), in CH2Cl2(20 ml) as solvent, was allowed to stir 5
min with an exclusion of moisture at an ambient temperature, 25◦C.
Ketones with α-two hydrogen atoms (10 mmole) were, added and the
reaction mixtures were allowed to stir again for the time specified in
Table I. It was poured into ice-cold (100 mL) water neutralized with a
Na2CO3 solution and extracted with CHCl3 (2 × 50 mL). The extract
was dried over anhydrous Na2SO4, and the solvent was removed by

Chemoselective Bromination
2009
distillation. The residue was purified by column chromatography us-
ing pet. ether (60:80):ethyl acetate (6:1) as an eluant for isolated pure
products.
2,2-Dibromo-1-phenylethanone (1)
The general procedure yielded the title compound (1)(2.94 g, 95%) from
acetophenone (1.17 mL. 10 mmole), which was isolated using pet. ether
(60:80): ethyl acetate (6:1) as an eluent system for column chromatog-
raphy, Rf = 0.65 using an eluent (10:1), m.p. = 73–75◦C.(Lit.20) IR ν =
3034–3000 (CH, Ar); 2935 (CH); 1698 (CO); 1600 (C C); 600 (C Br)
–1
cm
.
1H NMR(CDCl3) δ = 7.86–7.80 (d, 2H, ArH), 7.37, 7.32 (m,
3H, Ar); 6.75 (s, 1H, CH); ppm. M.S.: ( m/z, % ) 278 (M+, 1.16); 149
(M+ - CHBr2, 12.46); 105 (M+ - CHBr2, 100).
2,2-Dibromo-1-(4’-chlorophenyl)ethanone (2)
The general procedure yielded the title compound (2) (2.83 g, 91%) from
p-chloroacetophenone (1.29 mL, 10 mmole), which was isolated using
pet. ether (60:80):ethyl acetate (6:1) as an eluent system for column
chromatography, m.p. = 99◦C (Lit.1 m.p. = 101◦C), Rf = 0.6 using an
eluent (10:1).
IR (KBr plate): ν = 3024–2953 (CH, Ar); 2890 (CH); 1693 (CO); 1600,
–1
1575 (C C); 630 (C Br)cm
.
1H NMR(CDCl3) δ = 8.11–8.08 (d, 2H, ArH), 7.58, 7.48 (d, 2H, Ar);
6.63 (s, 1H, CH); ppm.
M.S.: (m/z,%) 312 (M+, 0.7); 232 (M+ - Br, 0.2); 139 (M+- CHBr2, 100);
105 (M+,-[CHBr2, Cl], 0.2).
2,2-Dibromo-1-(4’-nitrophenyl)ethanone (3)
A mixture of tetrachlorosilane (8.2 mL, 70 mmole) N-bromosuccinimide
(3.6 gm, 20 mmole) and p-nitroacetophenone (1.65 mL, 10 mmole)
yielded the title compound (3) (2.90 g, 90%), which was isolated us-
ing pet. ether (60:80):ethyl acetate (6:1) as an eluent system for column
chromatography, m.p. = 110◦C, Rf = 0.52 using an eluent (10:1) (Lit.2
m.p. = 111◦C).
IR ( KBr plate ): ν = 3074–3047 (CH, Ar); 2934 (CH); 1704 (CO); 1599
–1
(C C); 600 (C Br) cm
.
1H NMR(CDCl3) δ = 8.3–8.1 (d, 2H, ArH), 8.12–8.10 (d, 2H, Ar); 6.91
(s, 1H, CH); ppm.

2010
S. S. Elmorsy et al.
2,2-Dibromo-1-(p-tolyl)ethanone (4)
The general procedure yielded the title compound (4) (2.80 gm, 96%)
from p-methylacetophenone (1.34 mL, 10 mmole), which was isolated
using pet. ether (60:80):ethyl acetate (6:1) as an eluent system for col-
umn chromatography, m.p. = 89◦C (Lit.1 m.p. = 89◦C), Rf = 0.62 using
an eluent (10:1).
IR ( KBr plate ): ν = 3023–3007 (CH, Ar); 2955–2939 (CH); 1695 (CO);
–1
1607, 1572 (C C); 600 (C Br) cm
.
1H NMR(CDCl3) δ = 7.74–7.7 (d, 2H, ArH), 7.17 (d, 2H, Ar); 2.2(s,
3H, Ar); 6.61(s, 1H, CH); ppm.
2,2-Dibromo-1-(4‘-biphenyl)ethanone (5)
The general procedure] yielded the title compound (5) (3.15 gm, 92%)
from 4-acetylbiphenyl (1.85 mL, 10 mmole), which was isolated using
pet. ether (60:80):ethyl acetate (6:1) as an eluent system for column
chromatography, m.p. = 126◦C (Lit.3 m.p. = 128◦C), Rf = 0.7 using an
eluent (10:1).
IR ( KBr plate ): ν = 3049–2957-(CH, Ar); 2851(CH); 1675 (CO); 1620,
–1
1595 (C C); 630 (C Br) cm
.
1HNMR(CDCl3) δ = 7.92–7.88 (d, 2H, ArH), 7.59 (d, 2H, ArH), 7.48–
7.42 (d, 2H, ArH), 7.37 7.22 (m, 3H, Ar); 6.88 (s,1H,CH); ppm.
2,2 -Dibromo-1-(2’-naphthyl)ethanone (6)
The general procedure yielded the title compound (6) (3.00 gm, 92%)
from 2ꢀ-acetyl aphthalene (1.28 mL, 10 mmole), which was isolated us-
ing pet. ether (60:80):ethyl acetate (6:1) as an eluent system for column
chromatography, m.p. = 108◦C (Lit.4 m.p. = 109◦C), Rf = 0.63 using an
eluent (10:1).
IR (KBr plate): ν = 3058–2992-(CH, Ar); 2851(CH); 1672 (CO); 1620,
–1
1598 (C C); 620 (C Br) cm
.
1H NMR(CDCl3) δ = 8.67 (s, 1H, ArH), 8.15–8.14 (d, 1H, ArH), 8.00–
8.09 (d, 1H, ArH), 7.99–7.92 (d, d, 2H, Ar); 7.79–7.59 (m, 2H, Ar) 6.90
(s, 1H, CH); ppm.
M.S.: (m/z, %) 328 (M+, 1.4); 155 (M+-CH Br2 , 100); 174 (CHBr2, 8.1)
139 (M+-[CHBr2, O], 11.9); 127 (naphthylion, 56).
2,2 -Dibromo-1-(4‘-pyridyl)ethanone (7)
The general procedure yielded the title compound (7) (2.37 g, 85%) from
4ꢀ-acetyl pyridine (1.1 mL, 10 mmole), which was isolated using pet.

Chemoselective Bromination
2011
ether (60:80):ethyl acetate (6:1) as an eluent system for column chro-
matography, m.p. = 166◦C, Rf = 0.48 using an eluent (10:1).
IR (KBr plate): ν = 3016, 2986 (CH, Ar); 2914 (CH); 1693 (CO); 1620,
–1
1591 (C N, C C); 680 (C Br) cm
.
1H NMR(CDCl3) δ = 7.99 (d, 2H, ArH), 7.96 (d, 2H, ArH); 6.24
(s, 1H, CH); ppm.
M.S.: (m/z,%) 279 (M+, 2.1); 106 (M+-CH Br2 , 100); 78 (pyridylion,
36).
2‘,4‘-Dibromo acetanilide (8)
The general procedure yielded the title compound (8) (2.78 gm, 95%)
from acetanilide (1.35 g, 10 mmole), which was isolated using pet. ether
(60:80): ethyl acetate (6:1) as an eluent system for column chromatogra-
phy, m.p. = 149◦C, (Lit.5 m.p. = 150◦C). Rf = 0.2 using an eluent (10:1).
IR (KBr plate): ν = 3294 (NH); 3194–3026 (CH, Ar); 2850 (CH); 1661
–1
(CO); 1600 (C C); 550 (C Br) cm
.
1H NMR(DMSO) δ = 9.53 (s, 1H, NH); 7.90 (s, 1H, ArH); 7.58 (s, 2H,
ArH); 2.09 (s, 3H, CH3); ppm.
M.S.: (m/z,%) 293 (M+, 15.22); 251 (M+-1-acetyl, 95.39); 223 (dibromo
cyclopentadienyl cation, 11.25); 214 (M+-1-acetyl-Br, 28.31); 171 (M+-
acetyl-2Br, 22.17); 63 (C5H3 cation, 100).
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