Oxidative bromination reaction using Cu2+- perfluorophthalocyanine-immobilized silica gel catalyst under mild reaction conditions

Article abstract of DOI:10.1016/j.tetlet.2010.06.067

A simplified, facile route has been applied for the grafting of copper(II) perfluorophthalocyanine complex onto functionalized silica gel. The resulting organic-inorganic hybrid material is used as an efficient and recyclable catalyst for the regioselective oxidative bromination of various aromatic substrates using KBr/H₂O₂ as the reagents, affording high yields under mild conditions. High catalytic activity efficiency could be attributed to the heterogenization of soluble metal complexes on the high surface area host.

Full text of DOI:10.1016/j.tetlet.2010.06.067

Tetrahedron Letters 51 (2010) 4415–4418
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
2+
Oxidative bromination reaction using Cu -perfluorophthalocyanine-immobi-
lized silica gel catalyst under mild reaction conditions
*
R. K. Sharma , Chetna Sharma
Department of Chemistry, University of Delhi, New Delhi 110 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A simplified, facile route has been applied for the grafting of copper(II) perfluorophthalocyanine complex
onto functionalized silica gel. The resulting organic–inorganic hybrid material is used as an efficient and
recyclable catalyst for the regioselective oxidative bromination of various aromatic substrates using KBr/
H O as the reagents, affording high yields under mild conditions. High catalytic activity efficiency could
2 2
be attributed to the heterogenization of soluble metal complexes on the high surface area host.
Received 16 February 2010
Revised 11 June 2010
Accepted 15 June 2010
Available online 18 June 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Copper(II) perfluorophthalocyanine
complex
Silica gel
Oxidative bromination
Regioselective
In recent years, considerable emphasis has been placed on
improvement in the environmental impact of industrial chemical
processes. It is well recognized that solid catalysts can play a sig-
and greener, are the use of low-cost and easy-to-handle metal ha-
lides as halogenating agents and atom economy, that is, a full uti-
lization of halogen atoms instead of using only half of the available
amount. Only a few examples of the use of bromide or chloride
salts as halogen source have appeared, all reporting low substrate
conversions and/or low selectivities for individual products.2
We have designed a novel heterogeneous catalytic system to
generate electrophilic bromine in situ from easily available KBr
1
nificant role in the development of cleaner technologies through
their abilities to act as catalysts, support reagents, entrain by-prod-
ucts, avoid aqueous work-ups, and influence product selectivities,
and several books on the applications of solids in organic synthesis
0–22
2
have appeared. Advances are particularly needed in the area of
3
electrophilic aromatic substitutions, where traditional Lewis acid
2 2
as a bromine source and H O as an oxidant for the oxidative bro-
catalysts are a cause of considerable concern and where reactions
are frequently unselective. Bromoaromatics are particularly versa-
tile synthetic starting materials due to the possibility of their use in
carbon–carbon bond formation reactions via Heck, Stille, and Suzu-
ki transmetallation processes.4 Aromatic haloamines are used for
the manufacture of polyurethanes, rubber chemicals, agricultural
mination as a possible alternative to solve the disadvantages de-
scribed in the earlier methods. Inorganic solids such as silica,
particularly known for their mechanical and thermal stability
and also for their chemical inertia, represent interesting supports
23–25
for heterogenizing catalysts.
In this context, the formation of
2
6
organic–inorganic hybrids by the grafting method is a convenient
route to solid materials with catalytic properties. Present work
deals with the preparation of hybrid organic–inorganic materials
and the preliminary results of the activity and recyclability of this
material in the oxidative bromination of aromatic substrates. Liter-
ature survey reveals that the oxidative bromination of aromatic
substrates using covalently grafted silica gel as catalyst in the
5
,6
products, and drugs. The reported methods involve an electro-
philic bromination of aromatic rings using brominating agents
7
,8
other than molecular bromine, such as N-bromosuccinimide,
a
9
hexamethylenetetraminebromine complex, and an alkyl bro-
mide/sodium hydride/DMSO combination; whereas the others
10
are based on oxidative bromination.1
1–19
In oxidative halogenation, halide ions can be used as a halogen
source together with a suitable oxidant; thus, this method
represents more ecologically benign and economically attractive
pathway to haloaromatics than the classical electrophilic substitu-
tions. Main advantages of oxyhalogenation, which makes it safer
liquid phase and using H
reported so far.
The synthesis of the organic–inorganic hybrid materials-immo-
bilized copper catalyst is illustrated in Scheme 1. It was readily
prepared through a two-step procedure. The activated silica gel
2 2
O as the oxidizing agent has not been
(
60–120 mesh, from Aldrich) was reacted with 3-aminopropyltri-
ethoxy silane in dry toluene at 110 °C for 12 h to afford the 3-ami-
*
Corresponding author. Tel./fax: +91 11 27666250.
E-mail address: rksharmagreenchem@hotmail.com (R.K. Sharma).
nopropyl functionalized silica gel (APSG). The organic–inorganic
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.06.067

4
416
R. K. Sharma, C. Sharma / Tetrahedron Letters 51 (2010) 4415–4418
OH
O
toluene, reflux
OH
OH
+
(CH
CH
O)
Si(CH
)
3
NH
O
Si
(CH ) NH
2 3
3
2
3
2
2
2
O
F
F
F
F
diglyme/N2
O
H
CuII
F
CuII
140 oC, 4h
O
O
Si
(CH ) N
2
3
F
F
F
F
Scheme 1. Preparation of silica-supported copper catalyst.
Table 2
hybrid materials then reacted with copper(II) perfluorophthalocy-
anine complex in diglyme under nitrogen at 140 °C for 4 h to gen-
erate the silica-supported copper(II) catalyst. The catalyst
characterized by surface area (BET), elemental analysis, FT-IR, dif-
fuse reflectance UV–vis, and atomic absorption spectroscopy con-
firms the immobilization of CuPcF16 onto APSG.
Effect of different bromine source on the oxidative bromination of phenol and
resorcinol using CuPcF16-APSG as catalyst
Entry Salt
Phenola
Resorcinola
Conv.b Selectivityb,c
(%)
Conv.b Selectivityb,c
(%)
Table 1 presents the literature precedents of copper-mediated
oxidative bromination of various substrates under different reac-
tion conditions and the results have been compared with those ob-
tained by using CuPcF16-APSG as catalyst (Table 3).27 It is evident
from the comparison of the two tables that conversion and selec-
tivity of substrates obtained from the hybrid technique (Table 3)
are far better than those obtained by copper phthalocyanine
1
2
3
LiBr
95
4-Bromophenol
(76) + 4-
bromophenyl
acetate (24)
2-Bromophenol
92
92
97
4-Bromo-1,3-
dihydroxybenzene
(100)
NaBr 85
2-Bromo-1,3-
(
35) + 4-
bromophenol
65)
4-Bromophenol
100)
dihydroxybenzene
(3) + 4-bromo-1,3-
dihydroxybenzene (97)
4-Bromo-1,3-
(
20
KBr
>99
embedded in zeolite cavities (Table 2). However, the use of
(
dihydroxybenzene
homogeneous copper catalyst2
8–31
gave higher conversions of sub-
(
100)
strates but as expected, turnover numbers were low. Moreover, the
catalyst could not be reutilized.
The salt effect was also studied for the given oxidative bromin-
ation reaction. The catalyst, CuPcF16-APSG, was employed using
a
Conditions: substrate (2 mmol); MBr (2.2 mmol); glacial acetic acid (4 mL); 30%
aq H (2.2 mmol), 50 mg catalyst at 60 °C.
2 2
O
b
Conversion and selectivity were determined by GC.
Products were characterized by GC–MS.
c
Table 1
2 2
H O as an oxidant and MBr (M = Li, Na, K) as a bromine source (Ta-
Literature precedents of copper-catalyzed oxidative bromination reaction
ble 2) (Scheme 2). Among the three salts, KBr has been found to be
the most efficient bromine source and a monoselective product is
obtained. This could be well explained by the proposed mechanism
where the generation of Br is much easier in this case. The use of
2
sodium bromide did not produce regioselective products. LiBr,
though efficient, is less selective than KBr.
Entry Substrate
Catalytic
conditions
Conv.
(%)
Selectivity (wt %)
Mono- Di- Tri-
33.5 26.5
Phenol20
1
2
3
4
5
6
7
8
9
0
1
2
3
4
CuCl16Pc-NaX;
24.1
90
40.0
—
H
2
O
2
/KBr
/K Cr
Phenol28
CuBr
HOAc
Cu(OAc)
HOAc; O
CuCl16Pc-NaX;
/KBr
/K Cr
O
/
—
—
100.0
—
2
2
2
7
Substrates like phenol, aniline, and acetanilide showed excel-
lent para-selectivity. The activated aromatics, such as phenol,
resorcinol, naphthol, cresol, and aniline, have shown the regiose-
lective conversion into their respective monobrominated products
in high turnover numbers, while inactive ones such as benzene,
naphthalene, and anthracene showed very low conversion, though
monoselective. The deactivated substrates did not show any con-
version at all. In case of styrene, the oxidative bromination of aro-
matic nucleus does not take place, though side chain leads to
oxidation as well as oxidative bromination products. We also
examined the behavior of aniline under both oxidative bromina-
tion as well as oxychlorination conditions that is, using KBr and
KCl, respectively, as a halogen source. Surprisingly, the conversion
of aniline with KCl was slower than in the presence of KBr and the
corresponding chlorinated derivative, that is, 2-chloroaniline ap-
peared only as a minor product as identified by GC/MS.
Phenol29
/HBr/
(1 atm)
75
97.0
63.0
—
2
2
Aniline20
10.3
85
15.5 21.5
2 2
H O
Aniline28
CuBr
HOAc
Cu(OAc)
HOAc; O
CuCl16Pc-NaX;
/KBr
/K Cr
O
—
100.0
—
2
2
2
7
/
Aniline30
/HBr/
(1 atm)
90
70.0
58.0
100.0
54.1
100.0
100.0
100.0
100.0
100.0
7.0
2
2
Benzene20
11.5
65
34.0 8.0
2 2
H O
Benzene28
CuBr
HOAc
CuCl16Pc-NaX;
/KBr
O
—
—
2
2
2
7
/
Resorcinol20
Acetanilide28
31.6
70
26.5 19.0
2 2
H O
1
1
1
1
1
CuBr
HOAc
CuBr
HOAc
CuBr
HOAc
Naphthalene31 CuBr
HOAc
CuBr
HOAc
2
/K
/K
/K
2
2
2
Cr
2
Cr
2
Cr
2
O
O
O
7
/
7
/
7
/
—
—
—
—
—
—
2-Naphthol31
80
2
Naphthalene28
2
88
Further, GC/MS identification of the reaction mixture, experi-
ment 7 (see Table 3), has shown that a main transformation of
aniline, which competes with the oxidative bromination, is N-acet-
ylation giving N-phenylacetamide. At the end of the reaction, mon-
obrominated acetamide (major) and monobrominated aniline
2
/NaBiO
3
/
72
Anthracene28
O
90
—
—
2
/K Cr
2
2
7
/

R. K. Sharma, C. Sharma / Tetrahedron Letters 51 (2010) 4415–4418
4417
Table 3
Silica-supported copper-catalyzed oxidative bromination of various aromatic substrates
3
Product selectivityb,c
Entry
Substratea
Conv.b (%)
TONd  10 (time/h)
1e
2
3
Phenol
Resorcinol
>99, 99, 98, 98, 98, 95
440 (1.5)
431 (1.5)
333 (2.0)
431 (1.5)
418 (3.0)
377 (2.5)
444 (2.0)
386 (2.2)
444 (1.0)
133 (6.0)
80 (6.0)
53 (6.0)
—
4-Bromophenol (100%)
4-Bromo-1,3-dihydroxybenzene (100)
1-Bromo-2-naphthol (100%)
2-Bromo-4-methylphenol (100%)
5-Bromo-2-hydroxy benzaldehyde
1-Bromo-2-methoxynaphthalene (100%)
Styrene dibromide + styrene glycol + 1-phenylethanol (50:25:25)
4-Bromoacetanilide (100%)
N-Phenylacetamide: 4-bromoaniline: 4-bromo-N-phenylacetamide (4:6:90)
9-Bromoanthracene:anthra-9,10-quinone (85:15)
1-Bromonaphthalene (100%)
Bromobenzene (100%)
97
75
97
94
85
100
87
100
30
18
12
—
2-Naphthol
4-Methylphenol
Salicylaldehyde
2-Methoxynaphthalene
Styrene
Acetanilide
Aniline
Anthracene
Naphthalene
Benzene
4
5
6
7
8
9
1
1
1
1
1
.
0
1
2
3
4
Benzoic acid
Nitrobenzene
—
—
—
—
a
b
c
Conditions: substrate (2 mmol); KBr (2.2 mmol); glacial acetic acid (4 mL); 30% aq H
Conversion and selectivity were determined by GC.
Products were characterized by GC–MS.
Turnover number (TON) = mole of product formed/mole of metal present in the catalyst.
Catalytic runs to test recyclability.
2
O
2
(2.2 mmol), 50 mg catalyst at 60 °C.
d
e
o
Br
2KBr + H2O2 + 2 CH3COOH
Br2
Br3
+
2H2O + 2CH3COOK
CH COOH/ 60 C
3
-
-
Br2
Br -
+
Br
CuPcF16-APSG/ KBr/H O
CuII
2
2
-
+
2 H O2
3 HOBr + OH
3
2
R
R
H
slow
+
XH + Br
Scheme 2. Oxidative bromination of aromatics.
XH CuII
CuII
Br
(
minor) were also detected. This major product seems to be a re-
sult of N-acetylation of the primarily formed 4-bromoaniline
rather than bromination of N-phenylacetamide, as at lower conver-
sions, higher selectivities for 4-bromoaniline were observed as
identified by GC/MS. Whereas, at longer reaction times 4-bromo-
aniline decreased and 4-bromo-N-phenylacetamide increased. This
product, 4-bromo-N-phenylacetamine, could be easily hydrolyzed
by aqueous base back to 4-bromoaniline, if desired. The only by-
product of this reaction is N-phenylacetamide, also a valuable
chemical, making the method synthetically useful.
fast - H+
CuII
Br
XH
Scheme 3. Plausible rationale for oxidative bromination of aromatics.
Oxidative bromination of the substrates by dissolved copper
complex leached out from the solid catalyst is negligible since no
copper was detected in the filtrate (atomic absorption spectros-
respectively. After being dried, it was reused directly without fur-
ther purification. The silica-supported copper was recovered, recy-
cled, and used for 6 consecutive trials without loss of activity
(Table 3).
2 2
copy). Also, in the absence of the catalyst, H O alone is unable
to oxyhalogenate the substrate to any significant extent. It is worth
mentioning here that unsubstituted copper phthalocyanine has a
low activity; only oxidation reactions by H O were observed. No
2 2
The catalyst has been prepared by following the slightly modi-
3
2,33
fied method of Mansuy et al.
The decrease in the surface area of
34
catalyst is indicative of the grafting of the complex onto silica gel.
halogenated products were obtained.20 The reaction could also be
carried out with homogeneous catalyst, that is, CuPcF16, but the
catalyst cannot be recovered or reused and the turnover numbers
for the conversions were also low. The probable mechanism has
been illustrated in Scheme 3. Apparently the nucleophilic halide
ions, when present, coordinate with the copper ions and suppress
the formation of intermediates (like dioxygen complexes) that lead
Chemical analyses of CuPcF16-APSG (Anal. Found: C: 7.139, H:
1.273, N: 1.739) revealed the presence of organic matter with a
C/N ratio roughly similar to that of phthalocyanine. The IR spec-
À1
trum of catalyst shows band at 1637 cm that is due to C@N
stretching vibration of the imine bond. The diffuse reflectance
(DR)-UV–vis spectra of silica gel do not have any absorption band
in the region 250–850 nm. The curve of CuPcF16 exhibits two
to nuclear hydroxylation. Since the liberation of Br
2
from KBr by
shoulders around 300 nm and a broad absorption band in the re-
*
gion of 550–800 nm due to ligand p–p electronic transitions
2
H O
2
in the presence of homogeneous and enzyme catalysts is well
known, it was expected that the same situation may prevail in the
present case also. Probably, the catalyst enhances the oxidation of
Br (Br3 ) to Br (HOBr), which reacts in the presence of acidic cen-
ters of catalyst including lewis acid metal site with the substrate to
give brominated compounds.
The recyclability of the silica-supported copper was also sur-
veyed. After reaction, the solution was vacuum filtered using a sin-
tered glass funnel and the catalyst (50 mg) was washed with
(Fig. 1). The two shoulders in CuPcF16-APSG and the shifting of
band maximum from ꢀ730 to ꢀ715 nm (blue shift) is indicative
À
À
+
of an increased
plex molecule.
p
overlap on immobilization/grafting of the com-
3
5
In conclusion, the application of an organic–inorganic hybrid
copper catalyst has been demonstrated for the first time for the
oxidative bromination of aromatics. The method has been found
to offer additional advantages such as commercial availability of
the reagents, simple reaction conditions, no evolution of hydrogen
2 2 2 2 5
CH Cl (5 mL), Et O (5 mL), C H OH (5 mL), and hexane (5 mL),

4
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R. K. Sharma, C. Sharma / Tetrahedron Letters 51 (2010) 4415–4418
1
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the substrate (2 mmol), and KBr (2.2 mmol) in acetic acid (4 ml). Thirty percent
2
Acknowledgment
2 2
H O (2.2 mmol) was then added dropwise to the reaction mixture and the
contents allowed to stir at 60 °C (Scheme 2). The reaction mixture was
monitored by thin layer chromatography (TLC). After the completion of the
reaction, the catalyst was filtered and the reaction contents were subjected to
multiple ether extractions. The combined filtrates were washed with saturated
sodium bicarbonate solution. The organic extract was dried over anhydrous
sodium sulfate and the solvent was evaporated under reduced pressure. The
products obtained were quantified and confirmed by GC/MS (Agilent).
One of the authors, C. Sharma, thanks the Council of Scientific &
Industrial Research (CSIR), New Delhi, India, for the award of Junior
and Senior research fellowships.
Supplementary data
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2
5b
was prepared according to the literature method.
IR (KBr): mmax = 3434,
À1 13
2924, 2851, 1083 cm
.
C CP-MAS NMR analysis: 9.3, 22.3, and 42.9 ppm.
1
.
.
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2
À1
Surface area analysis: 151.91 m
g . Anal. Found: C, 5.281; H, 1.269; N, 2.046,
1
corresponding to 1.46 mmol/g of 3-aminopropyl groups based on the nitrogen
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.
.
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3
3
ambient temperature to obtain the catalyst. IR (KBr):
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À1
2848, 1637, 1390, 1055 cm
.
Diffuse reflectance (DR)-UV analysis: Two
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g
À1
.
Atomic
À1
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