Efficient bromination of polyaniline base to poly(2-bromoaniline)-bromide salt and its application as a recyclable catalyst for the synthesis of 2-methyl-4-anilino-1,2,3,4-tetrahydroquinolines

Article abstract of DOI:10.1016/j.catcom.2014.08.011

Efficient bromination of polyaniline base to poly(2-bromoaniline)-bromide is successfully carried out using bromodimethylsulfonium bromide. Poly(2-bromoaniline)-bromide salt was obtained in a dry amorphous nature, having high conductivity (2.5 S/cm) with

Full text of DOI:10.1016/j.catcom.2014.08.011

Efficient bromination of polyaniline base to poly(2-bromoaniline)-bromide
salt and its application as a recyclable catalyst for the synthesis of 2-methyl-4-
anilino-1,2,3,4-tetrahydroquinolines
Boddula Rajender, Srinivasan Palaniappan
PII:
DOI:
Reference:
S1566-7367(14)00315-X
doi: 10.1016/j.catcom.2014.08.011
CATCOM 4010
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
13 June 2014
1 August 2014
5 August 2014
Please cite this article as: Boddula Rajender, Srinivasan Palaniappan, Efficient bromi-
nation of polyaniline base to poly(2-bromoaniline)-bromide salt and its application as
a recyclable catalyst for the synthesis of 2-methyl-4-anilino-1,2,3,4-tetrahydroquinolines,
Catalysis Communications (2014), doi: 10.1016/j.catcom.2014.08.011
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ACCEPTED MANUSCRIPT
Efficient bromination of polyaniline base to poly(2-bromoaniline)-
bromide salt and its application as a recyclable catalyst for the synthesis
of 2-methyl-4-anilino-1,2,3,4-tetrahydroquinolines
Boddula Rajender, Srinivasan Palaniappan*
Polymers & Functional Materials Division,
CSIR-Indian Institute of Chemical Technology,
Hyderabad, 500 007.
INDIA.
Correspondence To
Email : palani74@rediffmail.com
palaniappan@iict.res.in
S. Palaniappan
Polymers & Functional Materials Division
Indian Institute of Chemical Technology
Hyderabad 500 007
Phone : +91-40-27191474
Fax
:
+91-40-27193991
India
www.iictindia.org
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ABSTRACT
________________________________________________________________________
Efficient bromination of polyaniline base to poly(2-bromoaniline)-bromide is
successfully carried out using bromodimethylsulfonium bromide. Poly(2-bromoaniline)-
bromide salt was obtained in a dry amorphous nature, having high conductivity (2.5
S/cm) with agglomerated spherical morphology. Poly(2-bromoaniline)-bromide salt was
proved to be a novel, efficient, reusable, cheaper and eco-friendly polymer-based solid
acid catalyst in the reaction between aromatic amines and N-vinyl lactam under solvent-
free conditions to provide 2-methyl-4-anilino-1,2,3,4-tetrahydroquinolines. These 2,4-
disubstituted tetrahydroquinolines were synthesized with trans selectivity.
________________________________________________________________________
Keywords: Polyaniline salt; Reusable catalyst; Green chemistry; Tetrahydroquinoline;
Stereoselectivity.
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1. Introduction
Polyaniline (PANI) is one of the most interesting conducting polymers due to its
good environmental stability, interesting redox properties, optoelectronic features and
controllable electrical and electrochemical properties [1]. As a result, many technological
applications have been demonstrated with polyaniline in the development of sensors,
rechargeable batteries, super capacitors, electrochromic materials, electromagnetic
shielding, light emitting devices, photovoltaic materials, ESD, and corrosion-protecting
coatings [2-5]. Despite the vast bibliography devoted to PANI, new studies focusing on
the polymerization process are reported each year [6-9]. Recently, PANI has been
receiving considerable attention for catalytic reactions as a catalyst and support owing to
its highly conducting and redox properties [10]. Furthermore, it's easy preparative
protocol from non-expensive starting material (aniline), controllable doping levels
through an acid doping/ base dedoping process, non-solubility in most organic solvents
and water make it useful in heterogeneous catalysts. By exploiting these properties,
polyaniline-based solid acid catalysts have been reported earlier by our group for various
organic transformations [11].
The tetrahydroquinoline scaffold is a core structure in many biologically-active
natural products and synthetic pharmaceutical agents. 2,4-Disubstituted
tetrahydroquinolines have been synthesized by Batey and other groups’ research, via 1:2
coupling of substituted anilines with vinyl ethers [12-16]. Our group very recently
reported the synthesis of cis-2-methyl-4-substituted tetrahydroquinoline derivatives from
the reaction of substituted aniline and N-vinyl lactam [17] using polyaniline-based solid
acid catalysts. In 2010, Jia et al. reported for the first time for the synthesis of 2-methyl-
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4-anilino-1,2,3,4-tetrahydroquinoline derivatives [18] via a tandem cyclization of imine
with its enamine isomer induced by the radical cation salt tris(4-bromophenyl)aminium
.
hexachloroantimonate (TBPA+ ). However, the drawbacks of this method include the
non-reusable, toxic, and expensive nature of the catalyst. In order to improve the
synthetic methodology, a polyaniline salt is used as a cheaper, reusable and
environmentally-friendly catalyst in the stereoselective synthesis of trans-2-methyl-4-
anilino tetrahydroquinoline derivatives.
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2. Experimental
2.1. Preparation of poly(2-bromoaniline)-bromide salt (P2BrABr-I) using BDMS
Bromodimethylsulfonium bromide (BDMS) [19] and polyaniline base [20] were
prepared by following a procedure reported earlier [19, 20]. Polyaniline base (0.5 g) was
added to 50 mL of dichloromethane solution containing 0.5 g BDMS and the mixture was
stirred at ambient temperature for 4 h. The resultant mixture was filtered, washed with
dichloromethane and then acetone until colorless filtrate and dried at 50 C until a
constant weight.
2.2. Preparation of poly(2-bromo aniline)-bromide salt using Br2 (P2BrABr-II)
Polyaniline base (0.5 g) was added to 50 mL of dichloromethane solution
containing 2.57 mL of bromine and the mixture was stirred at ambient temperature for 4
h. The product was isolated by using the above procedure.
2.3. General procedure for P2BrABr salt-catalyzed domino tetrahydroquinoline synthesis
A mixture of aromatic amine (2 mmol), N-vinyl lactam (1 mmol) and P2BrABr
catalyst (14 wt.% with respect to aromatic amine) under solvent-free conditions was
o
stirred at 80 C for a particular period of time. Ethyl acetate was added to the reaction
mixture. The catalyst was separated by simple filtration. Ethyl acetate solution was
washed with water, followed by brine, dried over anhydrous Na2SO4, and concentrated.
The products were isolated by column chromatography using silica gel (60–120 mesh)
and eluted with petroleum ether. All of the products were authenticated by 1H NMR and
mass spectroscopy data [18,21,22]. The recyclability experiment was carried out using
the recovered catalyst, wherein, the recovered catalyst was washed with acetone and
dried in an oven for 50 oC for 10 min.
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3. Results and discussion
P2BrABr salt was prepared by oxidizing aniline using sodium persulfate oxidant
in the presence of hydrochloric acid, followed by dedoping of polyaniline-hydrochloride
salt (PANI-HCl) to polyaniline base (PANI) and subsequent doping with
bromodimethylsulfonium bromide (BDMS) or bromine (Br2) to 2-bromopolyaniline-
bromide salts, P2BrABr-I and P2BrABr-II respectively.
3.1. Physical properties of P2BrABr
Bromination of polyaniline base using BDMS and Br2 yielded 0.7 g and 0.85 g
(with respect to 0.5 g of PANI base used) respectively. Conductivity of the P2BrABr-I
prepared using BDMS (2.5 S/cm) was found to be higher than that of P2BrABr-II sample
prepared using Br2 (0.2 S/cm). This result indicates that the efficiency of bromination of
polyaniline base is higher with the use of BDMS. Conductivity of P2BrABr is 12 orders
of magnitude higher than that of PANI base (insulator level, <10-12 S/cm). Increase in
conductivity result confirms that insulating PANI base is converted to doped PANI salt.
The density of P2BrABr-I and P2BrABr-II were found to be 1.5 and 1.7 g/cm3,
respectively, which are higher than that of PANI base (1.2 to 1.3 g/cm3). This result is
due to the presence of higher density bromine on polyaniline system. In order to
determine the amount of moisture present in the brominated polyaniline salt, the sample
was heated at 100 oC for 48 h and weighed. The weight of the brominated polyaniline salt
before and after heating was found to be almost the same. This result indicates that the
brominated polyaniline salt does not have moisture compared to conventional polyaniline
salt, which contains 5 - 8 % moisture [23,24].
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3.2. Infrared spectra of P2BrABr
Infrared spectra of P2BrABr-I, P2BrABr-II and dedoped P2BrABr-I are shown in
(Fig. 1). The major characteristic peaks obtained from 2000 to 750 cm−1 for the three
polymer samples are reported in Table 1.
Table 1
Infrared spectral results of P2BrABr.
Reported
PANI base
[17]
Reported
PANI salt
[25]
P2BrABr-I
1560
P2BrABr-II Dedoped
P2BrABr-I
C=C str.,
quinonoid
ring
C=C str.,
benzenoid
ring
1585-1590
1558-1570
1575
1490
1585
1490
1490-1500
1470-1485
1470
1375-1380
1312
NIL
NIL
1355
1270
1380
1295
C-N str.,
quinonoid
ring)
1298-1302 1300
C-N str.,
benzenoid
ring
N=Q=N
vibration,
where Q
represents
the quionoid
ring)
1217-1213
1158-1162
1230 - 1243 1240
1220
1160
1110-1130
792-800
1125
800
1110
1,4-
825-832
825
disubstituted
benzene
Infrared spectral peaks for P2BrABr-I are very similar to the infrared peaks reported in
polyaniline salt [25]. Infrared spectral peaks of its corresponding base are nearly the same
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with that of the polyaniline base peaks reported in the literature [17] except the shifting of
C-N str. of quinoid ring peak from 1310 to 1295 cm-1. This is due to the presence of
bromine in the ortho-position of the polyaniline base. The IR spectral peaks of
P2BrABr-II consisted of polyaniline base peaks in addition to salt peaks (Table 1). This
result indicates the lower efficiency of doping of bromine on polyaniline system by
bromination of polyaniline base with bromine.
Fig. 1.
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3.3. XRD pattern of P2BrABr
The X-ray diffraction pattern of conventionally synthesized semicrystalline
polyaniline salts showed four clear peaks around 2θ = 9.3, 15, 20-22, 25 and 27 o [26-28].
In this work, the X-ray diffraction profile registered for P2BrABr-I (Fig. 2b) showed
broad peaks around 2θ = 7, 14 and 24o with corresponding d-spacing 12.1, 6.3 and 4.3
respectively, indicating less crystallinity. X-ray diffraction pattern of P2BrABr-II (Fig.
2a) showed a broad peak around 2θ = 25o, which indicates an amorphous nature for the
polyaniline salt. X-ray pattern of dedoped P2BrABr-I sample showed a broad band
around 2θ = 25o, similar to that of P2BrABr-II. Recycled P2BrABr-I showed a broad
peak around 2θ = 21o representing amorphous nature.
Fig. 2.
3.4. FE-SEM and EDAX of P2BrABr
Morphologies of polyaniline samples are found out from field emission scanning
electron microscopy. The SEM picture of brominated polyaniline base by BDMS
(P2BrABr-I) showed aggregated nanofibers with spheres (Fig. 3a) and by bromination
with bromine, morphology changed to agglomerated spheres (Fig. 3b). The SEM picture
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of dedoped P2BrABr-I showed nanospheres with a diameter of 55-105 nm size (Fig. 3c).
The SEM picture of recycled P2BrABr-I also showed nanospheres with a diameter of 55-
110 nm size (Fig. 3d). EDAX of polyaniline base brominated using BDMS, P2BrABr-I,
showed 30 wt % of bromine and its corresponding dedoped polyaniline base showed 14
wt.% of bromine. This result indicates that dedoped polyaniline base contains bromine
due to the formation of poly(2-bromoaniline). Higher bromine weight percentage on
polyaniline salt indicated that poly(2-bromoaniline) contains bromide dopant, i.e.,
formation of poly(2-bromoaniline)-bromide salt.
Fig. 3.
3.5. Synthesis of 2-methyl-4-anilino tetrahydroquinoline using P2BrABr catalyst
In 2010, for the first time, Jia et al. carried out the above reaction by changing the
stoichiometric amount of reactants as 2 mmol aniline and 1 mmol N-vinyl pyrrolidone,
which gave the product, cis and trans isomers of 2-methyl-4-anilino-1,2,3,4-
tetrahydroquinoline [18]. In order to improve the methodology in terms of toxicity,
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cheaper and reusable catalyst, we used polyaniline-based solid acid material as a catalyst
in this work.
We attempted out the reaction of aniline (2 mmol) with N-vinyl pyrrolidone (1
mmol) using iodine-doped polyaniline catalyst. However, this reaction gave two
products, cis isomer of 2-methyl-4-pyrrolidin-2-one-1,2,3,4-tetrahydroquinoline (20 %)
and mixtures of (trans & cis) isomers of 2-methyl-4-anilino tetrahydroquinoline (50 %).
A similar result was obtained with the use of polyaniline-hydrochloride or polyaniline-
sulfate salt catalyst. However, the use of poly(2-bromoaniline) salt gave a single product
with major trans selectivity.
Reaction of aniline (1 mmol) with various amounts of N-vinyl pyrrolidone (2, 1.5,
0.7 and 0.5 mmol) were carried out using P2BrABr-I catalyst (14 wt % with respect to
the amount of aniline) under solvent-free conditions at 80 °C. Use of 2 mmol of N-vinyl
pyrrolidone gave 94% yield of the cis isomer of 2-methyl-4-substituted-1,2,3,4-
tetrahydroquinoline. However, decreasing the amount of N-vinyl pyrrolidone from 2
mmol to 1.5, the reaction gave a mixture of products, i.e., 66% of cis 2-methyl-4-
substituted-1,2,3,4-tetrahydroquinoline and 34% of trans and cis isomers of 2-methyl-4-
anilino tetrahydroquinoline (Scheme 1). On further decreasing the amount of N-vinyl
pyrrolidone to 0.7 or 0.5 mmol, the reaction gave only trans and cis isomers of 2-methyl-
4-anilino tetrahydroquinoline. In a typical procedure, the reaction of aniline (2 mmol)
o
with N-vinyl pyrrolidone (1 mmol) using P2BrABr-I catalyst at 80 C under solvent-free
conditions for 20 min., gave the product of 2-methyl-4-anilino-1,2,3,4-
tetrahydroquinoline in 90% yield with 72% trans selectivity.
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Scheme 1.
Catalytic activity of P2BrABr-I is checked in different solvents. Use of solvents such as
ethanol, acetonitrile, tetrahydrofuran, and chloroform all allowed formation of cis and
trans isomers of 2-methyl-4-anilino-1,2,3,4-tetrahydroquinoline. Tetrahydrofuran solvent
gave higher yield of the product (70%) with 75% trans selectivity, acetonitrile (66%)
with 67%, and ethanol (45%) with equal amounts of isomers. However, reaction did not
proceed with the use of solvents such as toluene or water. In this reaction, the N-vinyl
lactams such as N-vinyl pyrrolidone, N-vinyl lactam or N-vinyl carbazole are not only the
dienophiles but also serve as precursors of acetaldehyde [17,29].
In order to determine the effect of catalyst, P2BrABr-I, the reaction was extended
0
with various aromatic amines (2 mmol) and N-vinyl lactams (1 mmol) at 80 C under
solvent-free condition and the results are reported in Table 2. The reaction went smoothly
in all of the cases with excellent yields (75-93%) and with major trans selectivity.
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Table 2
Reaction of anilines (2 mmol) with N-vinyl lactams (1 mmol) using P2BrABr catalyst (14
wt % w.r.t. the amount of anilines used) under solvent-free conditions at 80 °C.
Entry
R
N-vinyl
Time (m.)
Isolated
trans:cis
lactam
Yield (%)
1
H
N-Vinyl
Pyrrolidone
N-Vinyl
Caprolactam
N-Vinyl
Carbazole
N-Vinyl
Pyrrolidone
N-Vinyl
Caprolactam
N-Vinyl
Carbazole
N-Vinyl
Pyrrolidone
N-Vinyl
Caprolactam
N-Vinyl
Carbazole
N-Vinyl
Pyrrolidone
N-Vinyl
Caprolactam
N-Vinyl
Carbazole
N-Vinyl
Pyrrolidone
N-Vinyl
Caprolactam
N-Vinyl
Carbazole
N-Vinyl
20
30
5
90
92
93
85
87
90
88
90
89
84
82
84
87
81
90
75
80
83
72:28
66:34
73:27
62:38
62:38
70:30
55:45
58:42
64:36
64:36
67:33
80:20
76:24
77:23
76:24
66:34
70:30
67:33
2
3
4
p-F
30
30
10
30
30
30
20
30
20
30
20
10
60
60
30
5
6
7
p-Cl
p-Br
p-CH3
o- Cl
8
9
10
11
12
13
14
15
16
17
18
Pyrrolidone
N-Vinyl
Caprolactam
N-Vinyl
Carbazole
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3.6. Recyclability
Reusability of the P2BrABr-I catalyst was verified for the model reaction of
o
aniline with N-vinyl pyrrolidone under solvent-free condition at 80 C for 20 min. The
reaction gave 72 % selectivity of trans isomer with 90 % yield. Reusability was checked
by separating the catalyst from the reaction mixture by filtration followed by purification
using acetone and used in the reaction again. This procedure was carried out for six
cycles and provided the product in 90 (72), 89 (70), 90 (69), 91 (68), 87 (57) and 87
(52)%, respectively, with decreasing amounts of the trans isomer. After the recyclability
experiment, the catalyst is analyzed by spectral analyses (Fig. 1-3) and similar results
were obtained for the catalysts before and after the reaction. The above results indicate
that P2BrABr catalyst shows recyclability.
4. Conclusions
Polyaniline base was successfully brominated using bromodimethylsulfonium bromide as
bromine source. Poly(2-bromoaniline)-bromide salt was proved to be a cheap, efficient
and recyclable catalyst for synthesis of trans-2-methyl-4-anilino-1,2,3,4-
tetrahydroquinoline derivatives.
Acknowledgements
The authors are thankful to Dr. Lakshmi Kantam, Director, IICT for her support and
encouragement. B.R. thanks the University Grants Commission, New Delhi for senior
research fellowship
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FIGURE CAPTIONS
Fig. 1. Infrared spectra of (a) P2BrABr-II, (b) P2BrABr-I and (c) dedoped P2BrABr-I.
Fig. 2. X-ray diffraction patterns of (a) P2BrABr-II, (b) P2BrABr-I and (c) recycled
P2BrABr-I.
Fig. 3. FE-SEM pictures of (a) P2BrABr-I, (b) P2BrABr-II, (c) dedoped P2BrABr-I and
(d) recycled P2BrABr-I.
Scheme 1. Synthesis of 2-methyl-4-anilino-1,2,3,4-tetrahydroquinolines using P2BrABr
catalyst.
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GRAPHICAL ABSTRACT
Efficient bromination of polyaniline base to poly(2-bromoaniline)-bromide salt and
its application as a recyclable catalyst for the synthesis of 2-methyl-4-anilino-1,2,3,4-
tetrahydroquinolines
Boddula Rajender, Srinivasan Palaniappan*
Synthesized brominated polyaniline and acts as novel reusable polymeric catalyst in
organic synthesis. This methodology is economic, very simple, green benign and energy
efficient.
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Research Highlights
 Polyaniline base is effectively brominated by BDMS
 Highly conducting poly(2-bromoaniline) salt – (2.5 S/cm)
 Poly(2-bromoaniline) salt with non-hygroscopic nature
 Heterogeneous catalyst for tetrahydroquinolines
 Reusable, easily handled and environmentally friendly catalyst
19