Catalytic activity of aqueous cationic polyurethane dispersions: A novel feature of polyurethanes

Article abstract of DOI:10.1016/j.apcata.2013.08.023

Polyurethane ionomers are well-known and user-friendly polymers. Here, we introduce their catalytic activity for organic reactions as a novel aspect of these polymers. We selected an isocyanide-based multicomponent reaction for proving the catalytic activity of polyurethane ionomers. Therefore, a convenient and very mild methodology is described for the preparation of 3-aminoimidazo[1,2-a]pyridines via a three component reaction between 2-aminopyridines, aldehydes and isocyanides in the presence of a cationic polyurethane dispersion at low reaction temperatures, in short reaction times and excellent yields.

Full text of DOI:10.1016/j.apcata.2013.08.023

Accepted Manuscript
Title: Catalytic activity of aqueous cationic polyurethane
dispersions: A novel feature of polyurethanes
Author: Hamed Daemi Reza Rezaieyeh Rad Mehdi Barikani
Mehdi Adib
PII:
S0926-860X(13)00509-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2013.08.023
APCATA 14405
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
23-5-2013
10-8-2013
13-8-2013
Please cite this article as: H. Daemi, R.R. Rad, M. Barikani, M. Adib, Catalytic activity
of aqueous cationic polyurethane dispersions: A novel feature of polyurethanes, Applied
Catalysis A, General (2013), http://dx.doi.org/10.1016/j.apcata.2013.08.023
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Catalytic activity of aqueous cationic polyurethane dispersions:
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A novel feature of polyurethanes
Hamed Daemia,*, Reza Rezaieyeh Radb, Mehdi Barikanic, Mehdi Adibb
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a Young Researchers and Elites Club, Tehran North Branch, Islamic Azad University, P.O. Box
15655-461, Tehran, Iran
b School of Chemistry, University College of Science, University of Tehran, P.O. Box 14155-
6455, Tehran, Iran
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c Polyurethane Department, Iran Polymers and Petrochemicals Institute, P.O. Box 14965-115,
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Tehran, Iran
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Abstract
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Polyurethane ionomers are well-known and user-friendly polymers. Here, we introduce their
catalytic activity for organic reactions as a novel aspect of these polymers. We selected an
isocyanide-based multicomponent reaction for proving the catalytic activity of polyurethane
ionomers. Therefore, a convenient and very mild methodology is described for the preparation of
3-aminoimidazo[1,2-a]pyridines via a three component reaction between 2-aminopyridines,
aldehydes and isocyanides in the presence of a cationic polyurethane dispersion at low reaction
temperatures, in short reaction times and excellent yields.
Keywords
Imidazo[1,2-a]pyridines, Cationic polyurethane, Aqueous dispersion, Thermal properties,
Catalyst
*
Corresponding author. Tel.: +98 21 44580000; fax: +98 21 44580032.
E-mail address: H.Daemi@ippi.ac.ir (H. Daemi)
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1. Introduction
Polyurethanes (PUs) have been gaining importance in a wide range of applications such as
paints, textile, adhesives, gloves and wood finishing because of their good physical and
mechanical properties; or for use in heart valves, aortic grafts, dialysis membranes and intra-
aortic balloons due to their biocompatibility characteristics [15]. Unfortunately, the
environmental regulations regarding the use of volatile organic chemicals (VOCs) has limited
traditional applications of the solvent-based PUs [68]. Aqueous polyurethane dispersions
(PUDs), however, have been more widely accepted because they are more environmentally
friendly with regards to solvent release into the atmosphere [9,10]. Conventional polyurethanes
are insoluble in water and should contain hydrophilic groups in their structure to be dispersed or
dissolved in water. The hydrophilic groups, or emulsifiers, are either ionic or nonionic types
based on their structures. Nonionic types have hydrophilic soft segment pendant groups such as
polyethylene oxide [11], while ionic types contain pendant carboxylate or sulphonate groups, or
quaternary ammonium salts. Cationic aqueous polyurethane dispersions (CAPUs) are often used
in coagulants, adhesives and membranes [12,13]. N-Methyl diethanolamine (N-MEDA) has been
reported to be used as an ionic dispersing agent in aqueous cationic polyurethane dispersions
[1416].
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Multi-component reactions (MCRs) have attracted much attention during recent years. These
one-pot high-yielding procedures are interesting synthetic methods for attaining new and useful
products. Imidazo[1,2-a]pyridines are an important class of fused heterocyclic compounds, as
many compounds of this class possess antibacterial, antifungal, anthelmintic, antiviral,
antiprotozoal and anticonvulsant properties. They have also been shown to act as glycosidase
inhibitors and display gastrointestinal, anti-ulcer and immunomodulatory activities [17–19].
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Several synthetic methods have been reported for the preparation of 2- or 3-substituted
imidazo[1,2-a]pyridines with the most common procedure being the condensation of 2-
aminopyridine with -halocarbonyl compounds to form the five-membered ring [20, 21]. The
preparation of imidazo[1,2-a]pyridines has been reported by different approaches including a) c
opper-catalyzed tandem coupling of 1,4-dihalo-1,3-dienes with azoles, b) the condensation of
ethyl bromopyruvate with 2-aminopyridine, c) the condensation of an 2- aminopyridine and
acetophenone, d) condensation of 2-aminopyridine with glyoxal trimer dihydrate, e) an
intermolecular oxidative diamination of alkynes, f) the condensation of aldehydes, 2-
aminopyridines and isonitriles, g) the condensation of aldehydes, 2-aminopyridines and alkynes,
h) the reaction of 2-chloropyridine with 1,2,3-triazoles, i) the reaction of 2-aminopyridine and
ketones, j) the condensation of 2-aminopyridine and diketones, k) the reaction of 2-
aminopyridine with 1-nitro-1-alkenes, l) the reaction of a pyridine, phenacyl bromide and
substituted N-hydroxyarylimidoyl chloride, m) direct intramolecular amination of N-aryl-2
aminopyridines, n) the reaction of β-lactam carbenes with 2-pyridyl isonitriles, o) the reaction of
(arylacetyl)imidazoles with acetylenedicarboxylic esters and p) the cyclization of 1-(2-alkynyl)-
2-aminomethylimidazoles (Scheme 1) [22]. However, most of these methods involve three or
more sequential synthetic steps or the use of harsh reaction conditions that afford low yield of the
products [2327]. A useful approach to imidazo[1,2-a]pyridines is a one-pot MCR between 2-
aminopyridine, an aldehyde and an isocyanide [28]. We have reported a few alternative synthetic
methods for the preparation of 3-imidazo[1,2-a]pyridines [29, 30].
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As part of our continuing efforts on the development of new applications of PUDs [3137], we
focused on the novel aspects of these ionomers, and examined their potential application as a
polymer catalyst in organic reactions. In particular, we have selected an isocyanide-based three-
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component reaction between 2-aminopyridines, aldehydes and isocyanides to assess the catalytic
activity of these polymers (Scheme 2). To the best of our knowledge, there is no report in the
open literature concerning the catalytic activity of polyurethane ionomers.
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2. Experimental
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2.1. Materials
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Polytetramethylene glycol (PTMG) with a molecular weight 1000 was supplied by Arak
Petrochemical Company (Iran) and dried at 50 °C under vacuum for 24 h before use to ensure
the removal of all air bubbles and water vapors that may interfere with the isocyanate reactions.
N-Methyldiethanolamine (MDEA) (Merck) was dried at 100 °C for 4 h before use. N-
Methylpyrrolidone (NMP), isophoronediisocyanate (IPDI) and acetic acid (HOAc) were
purchased from Merck, Germany. All other chemical reagents were obtained from Merck and
Aldrich chemical companies and were used without further purification.
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2.2. Preparation of the aqueous cationic polyurethane dispersions
The CPUDs were synthesized via the following procedure [38]: Polytetramethylene glycol and
isophorone diisocyanate were charged into the reaction vessel and the temperature of the oil bath
was increased to 90 °C. After 3 h, a solution of N-methyldiethanolamine (MDEA) in N-
methylpyrrolidone (NMP) and 1,4-butanediol (1,4-BD) were added into the reactor. After that,
acetic acid (HOAc) was added to the system in order to neutralize the amine groups at 65 °C.
Finally, required amount of deionized water was added dropwise into the system at room
temperature to obtain CPUDs with 30 wt % solid content (Schemes 3 and 4). The polyurethane
sample with mol ratio 1:3:1:1 showed the best physical and mechanical properties from the
different formulations of the CPUDs (Table 1), therefore, all of the catalysts were synthesized
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and characterized based on this block ratio. In addition, all of the reactions were tested in the
presence of the CPUD with block ratio 1:3:1:1.
2.3. Preparation of 3-Imidazo[1,2-a]pyridines
The procedure for the preparation of 3-cyclohexylamino-2-phenylimidazo[1,2-a]pyridine 4a
(entry 1, Table 2) is described as an example: A mixture of benzaldehyde (0.106 g, 1 mmol), 2-
aminopyridine (0.094 g, 1 mmol) and cyclohexyl isocyanide (0.109 g, 1 mmol) in the presence
of cationic polyurethane dispersion (1 mL) was stirred at 20 C for 3 h. Then, n-hexane as a
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nonsolvent was added (3  3 mL) to the reaction mixture and the product and initial materials
were separated from CPUD and dried over MgSO4. The solvent was removed and the crude
product was purified by crystallization from n-hexane-EtOAc (3:1) to give 4a. In addition,
following experiments were considered to assess the definite role of the polyurethane as catalyst
and/or solvent of the reaction. In the first experiment, the reaction was tested in ethanol (as a
standard solvent system) and the 0.05 ml CPUD (i.e. 0.015 mmol solid CPU) was added to the
reaction (entry 1, Table 3). The low conversion of the raw materials clarified that the catalytic
activity of a CPUD is determined by its concentration. Then, the varying amounts of the CPUD
was tested in the reaction for the preparation of 3-cyclohexylamino-2-phenylimidazo[1,2-
a]pyridine 4a at 25 °C and 3 h. The results of this experiment also confirmed the previous data
and showed that the CPUD probably acts as both catalyst and solvent of the reaction (Table 3).
3. Results and discussion
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The chemical structure of the prepared cationic polyurethane with mol ratio 1:3:1:1 was
characterized by Fourier transform infrared (FTIR) spectroscopy (Fig. 1) [37]. The synthesized
polyurethane was characterized with the disappearance of the NCO vibration at 2265 cm-1 and
the presence of stretching vibrations of N–H at 3319 cm-1, C=O at 1714 cm-1 and N–H for
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Amide II band at 1538 cm-1. The asymmetric and symmetric stretching vibrations of aliphatic
groups appeared at 2945, 2859 and 2795 cm-1. The absorption bands at 1463, 1365, and 1305 cm-
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were assigned to CH2 bending vibration, C–H bending symmetric vibration and CH2 wagging,
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respectively. C–O–C stretching vibrations of the polyol were observed over a range at 1000–
1150 cm-1.
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In addition, some of the thermal, mechanical and physical properties of the synthesized
polyurethane are shown in Table 4. Additional data is provided as supplementary information.
Differential scanning calorimetry (DSC) measurement was used to characterize the thermal
properties of the CPU. The DSC thermogram of the cationic PU extended with 1,4-butanediol is
shown in Fig. 2. The transition temperature of the cationic polyurethane appeared at 25 °C based
on the results of the DSC data.
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Thermal mechanical properties of the synthesized polyurethane were studied by DMTA.
Different types of transitions which are related to the morphology and structure of polymer
sample can be detected by using DMTA measurements. The storage modulus (E') versus
temperature curve of CPU catalyst is shown in Fig. 3. Based on this curve, the prepared
polyurethane showed a transition at 26 C and therefore, it is expected that dried films of
polyurethanes could be used as hot-melt polymer catalysts for organic reactions carried out at
elevated temperatures.
We assessed the catalytic activity of our CPUD in a multi-component coupling reaction used for
the preparation of 3-aminoimidazo[1,2-a]pyridines. Thus, a mixture of an aldehyde 1, a 2-
aminopyridine 2 and an isocyanide 3 were treated with our prepared CPUD and the reaction
progress monitored by TLC. Initially, it was observed that the CPUD gradually coagulated in the
presence of the 2-aminopyridine. This may be attributed to the competition of the 2-
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aminopyridine and surfactant of the polyurethane for bonding with protons of the catalyst. This
problem was eliminated by adding a trace amount of ethanol to the mixture to obtain a CPUD
with a 20 wt % solid content. As indicated in Table 2, all the reactions proceeded at 15-25 C
and went to completion within 1.5–2.5 h. TLC and NMR spectroscopic analysis of the reaction
mixtures confirmed the formation of the corresponding 3-aminoimidazo[1,2-a]pyridines 4a–l in
excellent yields of 90-97%. Any product other than 4 could not be detected by NMR
spectroscopy. All the products were characterized by comparison of their melting points and
their spectral data with those of authentic samples.
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The reusability of the CPUD as the catalyst was also investigated for the synthesis of compound
4h (entry 7, Table 2). After each run, n-hexane (3  3 mL) was added to the reaction mixture to
separate the products and the unreacted starting materials. After washing, the recycled CPU was
used for further runs. No decrease of activity was observed even after four runs (Fig. 4). This
shows that the CPUD is an effective and recyclable catalyst for the synthesis of 3-alkylamino-2-
arylimidazo[1,2-a]pyridines.
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The excellent yields and significantly lower temperatures of the reaction in the presence of
CPUD compared to previous reports were related to the specific structure of the polyurethane.
The chemical structure of the polyurethane is formed from two polar and non-polar segments:
the urethane and polyol moieties, respectively. On the other hand, the reactants in this 3CR have
two polar functional groups and non-polar hydrocarbon parts. It seems the reactants escape from
the polar solvent (water) to the surface of polyurethane because of their non-polar parts. The
presence of active protons in the structure of the polyurethane and their interactions with the
polar functional groups of the starting materials and reaction intermediates is another reason for
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completion of the reaction under very mild conditions. Both quaternary ammonium carboxylate
and urethane moieties may act as acid catalyst in this 3CR.
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A mechanistic rationalization for this reaction is provided, which involves a multistep sequence
of events including: condensation of the 2-aminopyridine 2 with the aldehyde 1 and formation of
the aminoalcohol intermediate 5, dehydration of aminoalcohol and formation of imine 6,
nucleophilic addition of the isocyanide 3 on imine 5 to give the isonitrilium intermediate 7
followed by cyclization into the 3-imino-2H-imidazo[1,2-a]pyridine intermediate 8, and finally
tautomerization of this imino intermediate under the reaction conditions to afford the 3-
alkylamino-2-arylimidazo[1,2-a]pyridine 4. All these events may be facilitated by protonation-
deprotonation sequences by the ammonium carboxylate and urethane moieties of the CPUD, as
shown in scheme 5.
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We are currently investigating the catalytic activity of the prepared CPUD system for other well-
known organic reactions for example, Cannizarro reaction, Biginelli reaction, Ugi reaction,
Passerini reaction and Mannich reaction. As another model, we compared the catalytic efficiency
of the CPUD to prepare the benzoic acid and benzyl alcohol through a redox disproportionation
of benzaldehyde (Cannizarro reaction) with the selected previously known catalysts (Table 5).
The significant roles of the CPUD as the catalyst and solvent for the Groebke reaction were also
confirmed by the results of Cannizarro reaction. These results will be reported in future
publications.
4. Conclusions
In summary, we introduced a new and straightforward polymer catalyst and solvent for the
synthesis of 3-imidazo[1,2-a]pyridines via a one-pot, three-component condensation reaction
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between 2-amino-pyridines, benzaldehydes and isocyanides. The very mild conditions of the
reaction and the reusability of the catalyst are the main advantages of this reaction.
Acknowledgment
The authors would like to thank the Research Council of the University of Tehran and Iran
Polymer and Petrochemical Institute, Tehran, I.R. Iran for funding this project.
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Lett. 40 (1999) 1179–1180; e) M.S. Abaee, R. Sharifi, M.M. Mojtahedi, Org. Lett. 7,
(2005) 5893–5895; f) M.M. Mojtahedi, E. Akbarzadeh, R. Sharifi, M.S. Abaee, Org.
Lett. 9 (2007) 2791–2793.
9
10
11
12
13
14
13
Page 13 of 31

1
2
3
4
5
6
Figure captions
Figure 1. FTIR spectrum of the aqueous cationic polyurethanes
Figure 2. DSC thermogram of the CPU catalyst extended with 1,4-butanediamine
Figure 3. DMTA measurements of CPU catalyst
Figure 4. The effect of reusability of CPUD catalyst on the product 4h yield
14
Page 14 of 31

1
2
3
4
5
6
7
8
Scheme captions
Scheme 1. The methods for the synthesis of imidazo[1,2-a]pyridines
Scheme 2. Schematic illustration of the one-pot synthesis of imidazo[1,2-a]pyridines
Scheme 3. Chemical procedure for synthesis of the aqueous cationic polyurethane dispersions
Scheme 4. Schematic procedure for micro-structure formation of CPUDs
Scheme 5. Plausible mechanism for preparation of imidazo[1,2-a]pyridines in the presence of
polymeric catalyst
15
Page 15 of 31

1
2
3
4
5
6
7
8
Table captions
Table 1. Feed composition of cationic polyurethanes with variable block ratios.
Table 2. One-pot synthesis of imidazo[1,2-a]pyridines 4.
Table 3. The effect of CPUD content on the yield of 3-cyclohexylamino-2-phenylimidazo[1,2-
a]pyridine.
Table 4. Thermal, mechanical and physical properties of the synthesized polyurethanes.
Table 5. CPUD-catalyzed Cannizzaro reaction.
16
Page 16 of 31

1
Table 1
IPDI MDEA HOAc
Sample no. Block ratio PTMG (g)
Film property
(g)
(g)
(g)
1
2
3
1:3:1:1
1:4:1:2
1:4:2:1
20
20
20
13.34
17.78
17.78
2.38
2.38
4.77
1.20
1.20
2.40
Excellent
Semi-brittle
Sticky
2
3
17
Page 17 of 31

1
Table 2
X
O
X
_
C
CPUD
+
N
+
+
R
N
Ar
H
N
N
NH2
R
N
Ar
H
2
3
1
4
Entry
1
ArCHO 1
2-Aminopyridine 2
R
Time (h) t (C)
4, %Yielda
mp (C)
199–200 39
CHO
2.5
1.5
2.5
2
20
15
20
25
25
15
15
15
20
20
25
25
4a, 94
N
N
N
NH2
NH2
NH2
168169 21
149–150 26
216–217 40
H3C
Cl
CHO
2
3
4f, 96
CHO
4i, 95
H3C
CHO
4
4c, 95
N
NH2
Br
205–206 40
201–203 40
218–220 40
212–213 40
208–211 40
200–202 41
175–176 41
208–210 41
CHO
CHO
5
2.5
1.5
1.5
2
4e, 91
N
NH2
NH2
NH2
NH2
NH2
H3C
H3C
H3C
6
4b, 96
N
N
N
H3C
H3C
CHO
7
4h, 97,96,95,94
4g, 95
CHO
8
Br
CHO
9
2
4d, 92
N
Cl
Cl
Cl
CHO
CHO
CHO
10
11
12
2
4j, 92
N
NH2
Br
2.5
2
4l, 92
N
NH2
NH2
H3C
4k, 95
N
a Isolated yield.
2
3
18
Page 18 of 31

1
Table 3
Entry CPUD amount (ml) 4a, %Yielda
1
2
3
4
0.05
0.1
0.2
0.5
1
15
24
38
70
94
5
2
3
a Isolated yield.
19
Page 19 of 31

1
Table 4
Tensile
Strength3
(MPa)
Thermal
Glass Transition
temperature2 (Tg)
Elongation at
Break3 (%)
Contact
decomposition1 (T10)
angle4 (°)
260
26
1300
23.3
75
2
3
1 T10 = temperature of 10% weight loss (from TGA measurements)
2 The central point of the maximum of the tan δ peak (from DMTA measurements)
3
4
5
6
The date were obtained based on stress–strain measurements by Instron Mechanical Testing
instrument (MTS System Corporation, Eden Prairie, MN) tensile tester model 10/M at a strain
rate of 50 mm/min.
4
7
8
9
Contact angle measurements were performed at room temperature by a G10 (Kruss, Hamburg,
Germany) instrument via the sessile drop method.
20
Page 20 of 31

1
Table 5
O
O
OH
+
Ar
H
Ar
OH
Ar
1
3
2
Entry
1
Catalyst
NaOH
mol % Solvent Temp (°C) Time (min) Yield (%) Reference
100
unknown
10
1200
40
96
42a

H2O
toluene

2
3
4
5
6
7
TMG
NaOH
100
100
2
42b
100
40
63a
97b
90c
98d
96
42c
NaOH:Al2O3
600:3 g
unknown
0.25
180
42d
MgBr2.OEt2:TEA 50:200 CH2Cl2
rt
rt
rt
42e
LiBr:TEA
50:150
50:1.00e
2 days
150
42f

NaOH:CPUD
H2O
This work
2
3
4
5
6
7
a Cannizzaro reaction with sonication. The frequency of ultrasound: 20 kHz.
b Microwave-assisted Cannizzaro reaction
c Based on the alcohol
d Based on the alcohol
e The MW of synthesized CPUD is approximately 30000 g.mol-1
21
Page 21 of 31

1
2
3
4
Highlights
 Catalytic activity of a polyurethane dispersion was examined for the first time.
 CPUD (cationic polyurethane dispersion) was considered as a green catalyst.
 CPUD was shown to be an effective and reusable catalyst in the Groebke reaction.
 3-Imidazo[1,2-a]pyridines were synthesized under mild condition by using PUD catalyst.
5
6
22
Page 22 of 31

Figure(1)
Page 23 of 31

Figure(2)
Page 24 of 31

Figure(3)
Page 25 of 31

Figure(4)
Page 26 of 31

Scheme(1)
Page 27 of 31

Scheme(2)
Page 28 of 31

Scheme(3)
Page 29 of 31