Mild regioselective monobromination of activated aromatics and heteroaromatics with N-bromosuccinimide in tetrabutylammonium bromide

Article abstract of DOI:10.1055/s-2005-861866

Highly regioselective nuclear bromination of activated aromatic and heteroaromatic compounds has been accomplished using N-bromosuccinimide in tetrabutylammonium bromide. Pre-dominant para-selective monobromination of activated aromatics such as phenols and anilines, rate acceleration of bromination for moderately activated and less reactive substrates on addition of acidic montmorillonite K-10 clay, with or without microwave assistance, are the notable features of this protocol.

Full text of DOI:10.1055/s-2005-861866

PAPER
1103
Mild Regioselective Monobromination of Activated Aromatics and Hetero-
aromatics with N-Bromosuccinimide in Tetrabutylammonium Bromide
B
N
romination with
N
e
-Bromosu
m
ccinimide in Tetrab
a
utylammon
i
ium
B
romideC. Ganguly,* Prithwiraj De, Sanjoy Dutta
Department of Chemistry, University of Kalyani, Kalyani- 741 235, India
Fax +91(33)25828282; E-mail: nemai_g@yahoo.co.in
Received 19 November 2004; revised 11 January 2005
the extent of activation of N–Br bond in NBS,6
e,f,7a
actual
Abstract: Highly regioselective nuclear bromination of activated
aromatic and heteroaromatic compounds has been accomplished us-
ing N-bromosuccinimide in tetrabutylammonium bromide. Pre-
brominating species involved6d and control exhibited by
the heterocyclic ring and oxygenated function, if
dominant para-selective monobromination of activated aromatics present.7 The increasing use of room temperature ionic
b,c
8
such as phenols and anilines, rate acceleration of bromination for liquids as alternative green solvents and catalysts for or-
moderately activated and less reactive substrates on addition of
ganic transformations prompted us to evaluate the effica-
acidic montmorillonite K-10 clay, with or without microwave assis-
cy of NBS for nuclear brominations of arenes and
tance, are the notable features of this protocol.
heteroarenes
(
in
tetrabutylammonium
bromide
TBAB).8 Herein we reveal our results in Table 1.
a,9
Key words: bromination, ionic liquid , tetrabutylammonium bro-
mide, N-bromosuccinimide, aromatics, heteroaromatics, regiose-
lectivity
Activated aromatic substrates represented by phenols and
anilines undergo monobromination at 100 °C with para-
selectivity in a clean fast process. Gratifyingly, this meth-
od gives monobrominated products uncontaminated with
di- or tribromo derivatives with these substrates, which
are otherwise difficult to obtain. It is also significant that
exclusive para-substitution is observed without any spe-
Bromination of aromatics and heteroaromatics is an elec-
trophilic substitution reaction of immense synthetic and
industrial importance. Brominated arenes and hetero-
arenes are useful as pharmaceuticals, flame retardants,
7
d
1
cial para-directing additives, such as zeolites or
cyclodextrins7e and protection-deprotection sequence.
Bromination by this method was also much faster than
agrochemicals, specialty chemicals and synthetic inter-
mediates capable of undergoing carbon–carbon bond for-
mation via transmetalation reactions such as Heck, Stille,
6a
2
similar bromination in DMF, which took 24 hours even
for activated aromatics (entries 1,3, and 5). The free bro-
mide ion of TBAB acts as a strong hydrogen bond base10
with phenolic hydroxy group thereby enhancing its nu-
cleophilicity. Coupled with the activation of NBS by way
of promoting nucleophilic cleavage of N–Br bond by un-
solvated bromide ion (Scheme 1) this factor enhances the
rate of bromination substantially.
Suzuki, Sonogashira and Tamao–Kumada reactions. In
general, ring brominated aromatics and heteroaromatics
3
exhibit interesting biological activities. Halogen substi-
tuted pyrimidines and purines are particularly important
for their chemotherapeutic, biochemical and biophysical
4
properties. Apart from their usefulness as fluorophores
and potential intermediates for photophysical probes,5a
bromocoumarins are also important as synthetic precur-
sors of furocoumarins and dihydrofurocoumarins that are
widely used as photosensitizers and chemotherapeutic
5
b–d
agents to combat skin diseases,
and naturally abundant
5
e
linear coumarins. Despite their usefulness, scanty work
is documented for regiospecific monobromination of cou-
5
f,g
marins. Bromination of activated arenes and hetero-
arenes is often an unselective reaction resulting in a
mixture of mono- and polybrominated derivatives with
consequent tedious separation problems and poor atom
economy. Therefore, the search for new regioselective
methods of bromination has evoked great contemporary
interest. Use of NBS for nuclear bromination in polar me-
Scheme 1
Addition of 10 mol% water to the reaction mixture dra-
matically slowed down the bromination in the case of 1-
naphthol (entry 4, ii). This observation suggests that the
presence of unsolvated bromide ion is crucial to brominat-
ing ability of this reagent system and solvation of bromide
ion in the presence of water makes it a poorer nucleophile
impeding its function. Exclusive para-bromination of ar-
omatics is consistent with generation of molecular bro-
mine at low concentration in a slow rate-determining step
6
a
6b,c
6d
dia such as DMF, MeCN,
aqueous NaOH, in the
6
e,f,g
6h
6i
6j
presence of acids,
silica gel, zeolite, HZSM-5 and
7
a
isopropylamine is well-documented. These protocols of-
fer different levels of selectivity depending primarily on
SYNTHESIS 2005, No. 7, pp 1103–1108
x
x
.
x
x
.2
0
0
5
Advanced online publication: 10.03.2005
DOI: 10.1055/s-2005-861866; Art ID: Z22004SS
Georg Thieme Verlag Stuttgart · New York
©

1
104
N. C. Ganguly et al.
PAPER
and its fast consumption by the substrate in a kinetically
controlled reaction. This obviously suggests short lifetime
of the ionic intermediate for bromination and similar sug-
gestion has been made to explain the stereochemical out-
come of bromine addition to alkenes and alkynes in ionic
1
1
liquid. The possibility of molecular bromine forming ad-
Scheme 2
duct of tetrabutylammonium tribromide (TBATB) with
1
2
TBAB cannot be ruled out at this stage. Significantly,
No decarboxylation was observed when cinnamic acid
was allowed to react with TBATB (1 mol equiv) in TBAB
or NBS in dichloromethane suggesting the special catalyt-
ic role of TBAB in the Hunsdiecker reaction. Absence of
side-chain bromination at the methyl groups of 4,6- and
para-monobromination of phenols and anilines employ-
1
2a–c
has been reported. The presence of Br3–
ing TBATB
can be ascertained from its characteristic UV absorption
14
1
3
band in the vicinity of 267 nm (ethylene dichloride). In
fact, TBATB, separately prepared, displayed l at 263
max
4
,7-dimethylcoumarins (entries 17, 18) despite prolonged
nm in TBAB-CHCl mixture. On the other hand, a mix-
3
exposure (6 h, 100 °C) is another notable feature of this
protocol. For moderately activated substrates (entries
ture of NBS in TBAB exhibited a quite different l at
max
2
78 nm. The intermediacy of TBATB seems unlikely in
7
,8), addition of boron trifluoride etherate (1 mol equiv)
view of this observation. It seems plausible that attain-
ment of a reasonable concentration of TBATB in the vis-
cous medium by a diffusion controlled process is slow or
the liberated bromine is rapidly consumed by the substrate
by an ionic mechanism before it engages itself in an equi-
librium with TBAB. Attempted bromination of cinnamic
acid with NBS in TBAB led to facile decarboxylation re-
sulting in formation of b-bromostyrene (entry 19) by
(
entry 6) or solid acid montmorillonite K-10 clay to the re-
action mixture substantially accelerates bromination,
which can be further expedited with microwave assis-
tance. This is an obvious synthetic advantage over bromi-
nations based on TBATB, which does not work with
acetanilides. Exclusive bromination at C-3 of 7-hydroxy-
coumarin in preference to nucleophilic sites at C-6 and
C-8 ortho to hydroxy group further demonstrates the ab-
sence of directive effect of hydroxy group through its as-
sociation with NBS promoting delivery of bromine at
ortho-positions. This protocol is efficient for bromination
of nucleoside bases uracil and cytosine in terms of reac-
tion time and yield. However, purine bases adenine and
guanine are resistant to this reagent system (36 h, r.t./8
min, MW, 300 W) due to the strong deactivating nature of
purine bases.
13
Hunsdiecker reaction. In contrast, TBATB is reported to
yield the 2,3-dibromo-3-phenylpropionic acid thereby re-
vealing further the different nature of the brominating spe-
cies with NBS in TBAB. The formation of a mixture of E-
and Z-bromoalkene is consistent with the intermediacy of
benzylic b-bromocarbonium ion supported by the ionic
nature TBAB rather than rigid cyclic bromonium ion
(
Scheme 2).
Table 1 Bromination of Aromatics and Heteroaromatics with NBS in TBAB
Entry
1
Substrate
Product
Reaction Time
2 h
Yield (%)a
90
2
3
4
2 h
84
86
75
2.5 h
i. 4 h
ii. 8 h
b
73
Synthesis 2005, No. 7, 1103–1108 © Thieme Stuttgart · New York

PAPER
Bromination with N-Bromosuccinimide in Tetrabutylammonium Bromide
1105
Table 1 Bromination of Aromatics and Heteroaromatics with NBS in TBAB (continued)
Entry
5
Substrate
Product
Reaction Time
4 h
Yield (%)a
88
6
7
8
9
i 10 h
ii. 6 h
84
87
c
i. 10 h
ii. 2 h
iii. 5 min
80
85
92
d
e
i. 48 h
ii. 4 h
iii. 8 min
78
82
90
d
e
i. 3 h
ii. 6 min
78
82
e
1
1
0
1
4 h
3 h
60
76
1
1
2
3
4 h
4 h
68
75
1
1
4
5
i. 45 min
ii. 2.5 min
85
92
e
i. 1 h
ii. 4 min
82
90
e
1
6
i. 2 h
ii. 4 min
78
96
e
Synthesis 2005, No. 7, 1103–1108 © Thieme Stuttgart · New York

1
106
N. C. Ganguly et al.
PAPER
Table 1 Bromination of Aromatics and Heteroaromatics with NBS in TBAB (continued)
Entry
Substrate
Product
Reaction Time
6 h
Yield (%)a
–
1
7
no reaction
1
8
no reaction
6 h
–
1
9
40 min
96
(
E:Z = 2:1)
a
b
c
d
e
Yields refer to those of pure isolated products characterized by spectroscopic data (IR, H and 13C NMR and EIMS).
1
Reaction conditions: substrate (1 mmol), NBS (1 mol equiv), TBAB (0.2 g), 10 mol% H O, 100 °C.
2
Reaction conditions: substrate, NBS and BF ·Et O (each 1 mol equiv), TBAB (0.2 g).
3
2
Reaction conditions: substrate (1 mmol), NBS (1 mol equiv), TBAB (0.2 g), K-10 clay (0.5 g/mol equiv of substrate), 100 °C.
Reaction conditions: substrate (1 mmol), NBS (1 mol equiv), TBAB (0.2 g), substrate separately adsorbed on K-10 clay (0.5/mmol each),
MW, 300 W.
In conclusion, a mild efficient protocol for monobromina- The reaction mixture was cooled to r.t., diluted with EtOAc (4 mL)
and directly charged into a silica gel column for chromatographic
purification. 4-Bromoacetanilide was obtained as a crystalline solid
tion of activated aromatics and heteroaromatics has been
developed employing NBS in TBAB. Para selectivity for
aromatics, use of non-excess stoichiometry leading to
monobromination and consequent less demanding purifi-
cation procedures are the advantageous features of this
method.
(
400 mg, 91%) using CH Cl as eluent; mp 166–167 °C. TBAB was
2 2
also recovered as before from later fractions of CH Cl eluates and
2
2
was reused.
Bromination of Aromatics with NBS-TBAB in the Presence of
Boron Trifluoride Etherate; 2-Bromo-4-nitroaniline; Typical
Procedure (Table 1, Entry 6, i)
TBAB and NBS were procured from Spectrochem, India and E.
Merck, Germany respectively. Solvents used for extraction and
chromatography were distilled prior to use. A domestic microwave
oven (BPL India, 2450 MHz) was used for microwave experiments.
An intimate mixture of 4-nitroaniline (148 mg, 1.08 mmol) and
TBAB (0.22 g) was heated at 100 °C. The mixture was cooled to
r.t., and to it was added NBS (195 mg, 1.09 mmol) and finally
BF ·OEt (155 mg, 1.07 mmol) in a glass vial. The closed glass vial
3
2
was kept at r.t. for 6 h and it was then quenched with sat. aq
Bromination with NBS in TBAB; 5-Bromo-6-aminocoumarin;
Typical Procedure (Table 1, Entry 13)
NaHCO (3 mL). This was followed by extraction with EtOAc (2 ×
3
4
mL) and chromatographic filtration over silica gel to afford 2-bro-
6
(
-Aminocoumarin (342 mg, 2.12 mmol) was added to solid TBAB
0.4 g) with stirring and when the resulting mixture became a clean
melt, NBS (380 mg, 2.13 mmol) was added. This was heated to
00 °C in a closed glass vial and then kept at that temperature for 4
15
mo-4-nitroaniline (200 mg, 87%); mp 103–104 °C (Lit. mp
04.5 °C) and TBAB.
1
1
Characterization Data of Some Selected Products
3-Bromo-7-hydroxycoumarin
Mp 214–216 °C.
h (TLC monitoring). H O (4 mL) was then added to remove succin-
2
imide followed by extraction with a minimum amount of EtOAc.
The organic extract, after washing with brine, drying (Na SO ) and
2
4
FTIR (KBr): 3257, 3048, 1695, 1621, 1591, 1441, 1362, 1260,
careful removal of solvent, gave one product exclusively along with
TBAB. The crude product was purified by filtration through a short
pad of silica gel using CH Cl –light petroleum ether mixture (1:1)
–
1
1
232, 1148, 1120, 1007, 838, 751 cm .
1
2
2
H NMR (300 MHz, DMSO-d ): d = 6.74 (1 H, d, J = 2.1 Hz, H-8),
6
as the eluent. Shining bright yellow crystals of 5-bromo-6-ami-
nocoumarin (380 mg, 75%) were obtained; mp 168–169 °C. TBAB
was recovered almost quantitatively from later fractions as shining
white crystals, which was reused without any decrease of yield of
bromo products.
6.82 (1 H, dd, J = 2.1, 8.7 Hz, H-6), 7.53 (1 H, d, J = 8.7 Hz, H-5),
8.48 (1 H, s, H-4), 10.76 (1 H, s, 7-OH).
+
EIMS (70 eV): m/z (%) = 242, 240 (89.0, 84.3, M ), 214, 212 (19.3,
2
4.2), 186, 184 (7.5, 8.0), 161 (23.2).
Anal. Calcd for C H O Br: C, 44.83; H, 2.07. Found: C, 44.89; H,
9
5
3
Microwave-Assisted Bromination of Acetanilide with NBS-
TBAB in the Presence of Montmorillonite K-10 Clay; 4-Bro-
moacetanilide; Typical Procedure (Table 1, Entry 7, ii)
To an intimate mixture of acetanilide (280 mg, 2.07 mmol) and
TBAB (0.4 g) in an Erlenmeyer flask was added NBS (370 mg, 2.08
mmol) followed by montmorillonite K-10 clay (1 g). The mixture,
after thorough stirring, was irradiated with microwave at 300 W for
2.01.
Note: This compound has been reported5f as an oil with H NMR
spectrum (90 MHz, DMSO-d ) exhibiting H-4 at d = 7.8 and 7-OH
at d = 5.5; both values seem unusual in view of the fact that 3-Br will
certainly cause substantial low-field shift of H-4 which usually ap-
pears at d = 7.8. Hydroxy proton of 7-hydroxycoumarins usually ap-
pears in the region d = 10–11 in DMSO-d . However, no supportive
mass spectral and elemental analysis data were reported for the oil.
1
6
6
2
× 2.5 min cycles with 30 s in between to allow TLC monitoring.
Synthesis 2005, No. 7, 1103–1108 © Thieme Stuttgart · New York

PAPER
-Bromo-4-methyl-7-methoxycoumarin
Bromination with N-Bromosuccinimide in Tetrabutylammonium Bromide
1107
3
(4) (a) Asakura, J.-I.; Robins, M. J. J. Org. Chem. 1990, 55,
4928. (b) Irani, R.-J.; Santa Lucia, J. Tetrahedron Lett. 1999,
Mp 130–133 °C.
4
0, 8961. (c) Robins, M. J. In Nucleoside Analogues:
IR (KBr): 2940, 1700, 1590, 1530, 1500, 1370, 1350, 1280, 1240,
–
1
Chemistry, Biology and Medical Applications, NATO
Advanced Study Institute Series, Vol. 26A; Walker, R. T.;
De Clercq, E.; Eckstine, F., Eds.; Plenum Press: New York,
1979, 165–192.
1
200, 1060, 1010, 990, 820, 740 cm .
1
H NMR (300 MHz, CDCl ): d = 2.58 (3 H, s, 4-CH ), 3.87 (3 H, s,
3
3
7
-OCH ), 6.81 (1 H, d, J = 2.4 Hz, H-8), 6.88 (1 H, dd, J = 9.0, 2.4
3
Hz, H-6), 7.55 (1 H, d, J = 9.0 Hz, H-5).
(
5) (a) Coleman, R. S.; Madaras, M. L. J. Org. Chem. 1998, 63,
5700. (b) Knobler, R. M.; Hönigmann, H.; Edelson, R. L. In
Psoralen DNA Photobiology, Vol. II; CRC Press: Boco
Raton, FL, 1988, 117–134. (c) Parrish, J. A.; Stern, R. S.;
Pathak, M. A.; Fitzpatric, T. B. In The Science of
Photomedicine; Plenum Press: New York, 1982, 595–624.
1
3
C NMR (75.5 MHz, CDCl ): d = 162.83 (C-7), 153.70 (C-2),
3
1
52.60 (C-8a), 150.96 (C-4), 126.00 (C-5), 113.48 (C-6), 112.93
(
C-3), 109.78 (C-4a), 100.83 (C-8), 55.82 (7-OCH ), 19.40 (4-
3
CH3).
DEPT-135: d = 126.06 (C-5), 113.00 (C-6), 100.78 (C-8), 55.86 (7-
(
d) Pescitelli, G.; Berova, N.; Xiao, T. L.; Rozhkov, V. R.;
OCH ), 19.49 (4-CH ).
3
3
Larock, R. C.; Armstrong, D. W. Org. Biomol. Chem. 2003,
186; and references cited therein. (e) Pardanani, N. H.;
Trivedi, K. N. Aust. J. Chem. 1972, 25, 1537. (f) Thapliyal,
P. C.; Singh, P. K.; Khanna, R. N. Synth. Commun. 1993, 23,
+
EIMS (70 eV): m/z (%) = 270, 268 (74.6, 100.0, M ), 242, 240
29.4, 38.6), 227, 225 (34.7, 45.1), 214, 212 (12.7, 10.1), 189 (27.7),
(
1
62 (30.1), 133 (50.6).
2
821. (g) Bansal, V.; Kanodia, S.; Thapliyal, P. C.; Khanna,
5
-Bromo-6-hydroxycoumarin
R. N. Synth. Commun. 1996, 26, 887.
Mp 210–211 °C.
(6) (a) Mitchell, R. H.; Lai, Y.-H.; Williams, R. V. J. Org.
Chem. 1979, 44, 4733. (b) Carreño, M. C.; Garćia Ruano, J.
L.; Sanz, G.; Toledo, M. A.; Urbano, A. J. Org. Chem. 1995,
IR (KBr): 3180, 3120, 1660, 1600, 1540, 1455, 1380, 1290, 1270,
1
–
1
240, 1220, 1120, 990, 940, 890 cm .
6
0, 5228. (c) Meana, A.; Rodríguez, J. F.; Sanz-Tezdedor,
M. A.; Garćia Ruano, J. L. Synlett 2003, 1678.
d) Auerbach, J.; Weissmann, S. A.; Blacklock, T. J.;
1
H NMR (300 MHz, DMSO-d ): d = 6.51 (1 H, d, J = 9.6 Hz, H-3),
6
7
.16 (1 H, d, J = 9.0 Hz, H-7), 7.26 (1 H, d, J = 9.0 Hz, H-8), 8.55
(
(
1 H, d, J = 9.6 Hz, H-4), 10.57 (1 H, s, 6-OH).
Angeles, M. R.; Hoogstein, K. Tetrahedron Lett. 1993, 34,
931. (e) Oberhauser, T. J. Org. Chem. 1997, 62, 4504.
(f) Diwu, Z. J.; Lown, J. W. Tetrahedron 1992, 48, 45.
(g) Eguchi, H.; Kawaguchi, H.; Yoshinaga, S.; Nishida, A.;
Nishiguchi, T.; Fujisaki, S. Bull. Chem. Soc. Jpn. 1994, 67,
1
3
C NMR (75.5 MHz, DMSO-d ): d = 162.20 (C-2), 151.36 (C-6),
47.47 (C-8a), 142.41 (C-4), 119.76 (C-8), 118.91 (C-4a), 118.54
6
1
(
C-7), 117.81 (C-3), 107.39 (C-5).
EIMS (70 eV): m/z (%) = 242, 240 (68.0, 100.0, M ), 214, 212
74.7, 86.8), 161 (40.0), 133 (44.1).
Anal. Calcd for C H BrO : C, 44.83, H, 2.07. Found: C, 44.73; H,
+
1918. (h) Konishi, H.; Aritomi, K.; Okano, T.; Kiji, J. Bull.
(
Chem. Soc. Jpn. 1989, 62, 591. (i) Smith, K.; James, D. M.;
Mistry, A.-G.; Bye, M. R.; Faulkner, D. J. Tetrahedron
1992, 48, 7479. (j) Paul, V.; Sudalai, A.; Daniel, J.;
Srinivasan, K. V. Tetrahedron Lett. 1994, 35, 7055.
(7) (a) Fujisaki, S.; Eguchi, H.; Omura, A.; Okamoto, A.;
Nishida, A. Bull. Chem. Soc. Jpn. 1993, 66, 1576.
(b) Cañibano, V.; Rodríguez, J. F.; Santos, M.; Sanz-
Tededor, M. A.; Carreño, M. C.; Gonzáles, G.; García
Ruano, J. L. Synthesis 2001, 2175. (c) Carreño, M. C.;
García Ruano, J. L.; Sanz, G.; Toledo, M. A.; Urbano, A.
Synlett 1997, 1241. (d) Smith, K.; El-Hiti, G. A.; Hammond,
M. E. W.; Bahzad, D.; Li, Z.; Siquet, C. J. Chem. Soc.,
Perkin Trans. 1 2000, 2745. (e) Breslow, R.; Campbell, P.
Bioorg. Chem. 1971, 1, 140.
9
5
3
2
.10.
5
-Bromo-6-aminocoumarin
Mp 168–169 °C.
–
1
FTIR (KBr): 3456, 3331, 1708, 1623,1559, 813 cm .
1
H NMR (300 MHz, CDCl ): d = 3.97 (2 H, br s, 6-NH ), 6.46 (1 H,
3
2
d, J = 9.6 Hz, H-3), 6.97 (1 H, d, J = 8.7 Hz, H-7), 7.14 (1 H, d,
J = 8.7 Hz, H-8), 8.03 (1 H, d, J = 9.6 Hz, H-4).
Anal. Calcd for C H BrNO : C, 45.18; H, 2.51; N, 5.85. Found: C,
4
9
7
2
5.21; H, 2.49; N, 5.86.
(
8) (a) The boiling point of H O (100 °C) is usually accepted as
2
Acknowledgment
the cut-off point for melting point of room temperature ionic
liquids. The melting point of TBAB is slightly above this
One of the authors (PD) sincerely thanks University of Kalyani for
financial assistance by way of UGC Minor Research Grant. The fa-
cility provided by DST-FIST Grant to the Department of Chemistry
is also acknowledged.
(104 °C) but addition of substrate and reagent causes
lowering of its melting point below the cut-off temperature.
Therefore, it can be considerd as an in situ ionic liquid
(
(
b) Welton, T. Chem. Rev. 1999, 99, 2071.
c) Wasserschield, P.; Keim, W. Angew. Chem. Int. Ed.
2
000, 39, 3772. (d) Du Pont, J.; De Souza, R. F.; Suarez, P.
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(
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(9) For some recent applications of TBAB as ionic liquid, see:
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(
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N. C. Ganguly et al.
PAPER
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