Au(III) catalyst supported on a thermoresponsive hydrogel and its application to the A-3 coupling reaction in water

Article abstract of DOI:10.1016/j.jcat.2014.11.009

Poly(NIPAM-co-4-VP), synthesized from N-isopropylacrylamide and 4-vinylpyridine with a 1:1 monomer ratio, exhibits a temperature-dependent phase transition in water with a low critical solution temperature (LCST) of 47 °C. Coordination of Au(III) with the

Full text of DOI:10.1016/j.jcat.2014.11.009

Journal of Catalysis 322 (2015) 104–108
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Au(III) catalyst supported on a thermoresponsive hydrogel
and its application to the A-3 coupling reaction in water
Saira Shabbir a, Youngeun Lee b, Hakjune Rhee a,b,
⇑
a Department of Chemistry and Applied Chemistry, Hanyang University, Sangnok-gu Hanyang Daehak-ro 55, Ansan 426-791, Gyeonggi-do, South Korea
b Department of Bionanotechnology, Hanyang University, Sangnok-gu Hanyang Daehak-ro 55, Ansan 426-791, Gyeonggi-do, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Poly(NIPAM-co-4-VP), synthesized from N-isopropylacrylamide and 4-vinylpyridine with a 1:1 monomer
ratio, exhibits a temperature-dependent phase transition in water with a low critical solution temperature
(LCST) of 47 °C. Coordination of Au(III) with the polymer results in material with metal catalytic activity
combined with the inverse solubility phenomenon of the polymer, which catalyzes the three-component
coupling reaction of aldehydes, amines, and alkynes in water with excellent yields. The polymer not only
functions as a supporter for Au(III) but also accelerates the catalysis by undergoing a coil-to-globule phase
transition above the LCST. The catalyst is efficient, is stable in air, and tolerates a variety of substrates.
Moreover, the recyclability of the catalyst makes it eco-friendly and economical to use.
Ó 2014 Elsevier Inc. All rights reserved.
Received 15 October 2014
Revised 20 November 2014
Accepted 20 November 2014
Keywords:
Gold catalyst
Hydrogel
Low critical solution temperature
A-3 coupling reaction
Heterogeneous catalysis
Polymer
1. Introduction
by the Li group [22], who reported better activity with Au(III) salts
than with Au(I) catalysts. Furthermore, water gave the best results
Gold catalysts support a wide spectrum of transformations,
including a variety of organic reactions [1] and rearrangements
[2]. One such reaction is the three-component one-pot reaction
of an aldehyde, amine, and acetylene, designated as an A-3 cou-
pling, which has a substantial role in the synthesis of heterocycles
[3–5] as well as of biologically active compounds [6–8]. This trans-
formation has received good deal of attention by researchers
worldwide because of its high efficiency, excellent atom economy,
mild reaction conditions, tolerance to a variety of functionalities,
and the synthetic utility of the product propargylamines. Propar-
gylamines are of interest not only as useful synthetic building
blocks but also because they are an integral functionality found
in natural products [9] and drugs [10].
A-3 coupling has been found to be catalyzed by different transi-
tion metals including Cu [11], Ag [12], Au [13], Ni [14], Co [15], Fe
[16], and Zn [17], yet there is a need for improvements in the pro-
cess. Gold catalysis has recently been advancing through the use of
metallic nanoparticles [18,19] as well as in complexes or in salt
form [20,21]. Gold-catalyzed A-3 coupling was first investigated
among all the solvents investigated. While later reports utilizing
supported catalysts addressed many of the shortcomings of typical
homogeneous catalysis, such as difficult separation, poor recycla-
bility, thermal instability, and the high cost of the catalyst [23],
these procedures included harsh reaction conditions including
reflux or elevated temperatures [24].
In other studies, several equivalents of an oxidative additive
were sometimes required for efficient recyclability [25], or the
reaction time was prolonged [26]. Therefore, there is a need for a
heterogeneous catalyst that performs efficiently under mild condi-
tions and recyclability.
Hence, we focused on the development of a Au(III) catalyst sup-
ported on a thermosensitive polymer. Our group has previously
used poly(NIPAM-co-4-VP) as a support for palladium nanoparti-
cles and their subsequent application to the Suzuki–Miyaura cou-
pling reaction [27]. Thus, we focused on the development of a
Au(III) catalyst supported on thermoresponsive poly(NIPAM-
co-4-VP), as shown in Scheme 1. At temperatures higher than
47 °C, hydrogen bonds between the polymer and solvent molecules
become weak, and the polymer coils around itself, shrinking in size
as intramolecular hydrogen bonding of the polymer becomes
significant. This mechanism is proposed for the coil-to-globule
transition of polymer coils that takes place in hydrophilic solvents.
⇑
Corresponding author at: Department of Chemistry and Applied Chemistry,
Hanyang University, Sangnok-gu Hanyang Daehak-ro 55, Ansan 426-791, Gyeonggi-
do, South Korea.
E-mail address: hrhee@hanyang.ac.kr (H. Rhee).
http://dx.doi.org/10.1016/j.jcat.2014.11.009
0021-9517/Ó 2014 Elsevier Inc. All rights reserved.

S. Shabbir et al. / Journal of Catalysis 322 (2015) 104–108
105
the endothermic heat flow caused by the breakage of hydrogen
bonds between water and polymer molecules [28]. Infrared spec-
troscopic data were obtained using an Alpha-E FT-IR spectrometer
with a NaCl window for liquid samples. 1H and 13C NMR were
recorded on a Bruker 400 MHz instrument using CDCl3 solvent.
O
am
MBA
AIBN
,
/
N
N
H
o uene
T
e
M H
O
l
NIPAM
4-VP
)
(
Poly NIPAM-co-4-VP
3. Results and discussion
u
A Cl3
3.1. Polymer and catalyst behavior
rt
H
O
,
2
According to DSC data, the synthesized polymer shows inverse-
temperature solubility behavior at 47 °C, as shown by the curve in
Fig. 1. This analysis helped to make use of reaction temperatures
higher than 47 °C for the catalysis, as we predicted strong interac-
tions of the catalyst in the hydrophobic phase with organic
reactants in water. Typically, organic reactions in water are pre-
dicted to be slow; however, in our system, above this transition
temperature, known as the LCST, organic reactants are attracted
to the inner hydrophobic part of a small globule of heterogeneous
catalyst so that the reaction is accelerated.
-
Au(III) catalyst supported
Poly
(
)
NIPAM-co-4-VP
on Poly(
)
NIPAM-co-4-VP
Scheme 1. Schematics of polymerization to synthesize poly(NIPAM-co-4-VP) and
the subsequent coordination of Au (III) with poly(NIPAM-co-4-VP).
2. Experimental methods
2.1. Catalyst preparation
Coordination of Au(III) with the polymer was efficiently per-
formed at room temperature. The reaction was aided by the high
solubility (Ksp = 3.2 Â 10À25 at 25 °C) of gold trichloride in water
at room temperature. Gold has a high affinity to carbon as well
as other heteroatoms such as nitrogen, phosphorus, sulfur, and
even O-donor ligands, such that it forms stable complexes [29].
Pyridine is one such ligand, whose coordination with gold is so
strong that it can form complexes by ligand exchange [30]. There-
fore, the developed catalyst has Au(III) strongly anchored by the
pendent pyridine groups of the polymer support, which mini-
mizes the opportunity for gold to leach out into the reaction
mixture.
Poly(NIPAM-co-4-VP) copolymer hydrogels were prepared by
free radical solution polymerization using NIPAM and 4-VP (1:1)
in toluene according to the standard procedure [27].
To coordinate Au(III) with the polymer, 0.05 g of gold trichloride
trihydrate, AuCl3Á3H2O, was added to 25 mL of aqueous solution
containing 0.1 g poly(NIPAM-co-4-VP). The resulting mixture was
sonicated for 1 min, followed by vigorous mechanical shaking at
room temperature for 12 h. The catalyst was collected after filtra-
tion, washed with methanol, and dried in a vacuum oven at 60 °C.
2.2. General procedure for A-3 coupling reaction
However, autoreduction of Au(III) species to metallic gold is
possible in sonicated water [31]; there are some reports of the
use of oxidative additives in A-3 coupling reactions for the regen-
eration of Au(III) species [25]. However, to verify that the Au(III)
oxidation state was maintained, we analyzed the X-ray photoelec-
tron spectrum (XPS) for fresh and recycled catalyst (XPS spectra
provided in the Supplementary Information). Both fresh and recy-
cled catalyst XPS spectra showed peaks for the Au4f7/2 peaks with a
binding energy around 84 eV [32] with no noticeable changes in
the characteristic peaks, demonstrating the stability of our catalyst.
Further quantitative analysis of Au(III) was accomplished by ICP
analysis of the freshly prepared catalyst. The loading value of the
catalyst was found to be promising (2.7 mmol Au/g of polymer).
All the chemicals were purchased from commercial sources
(Aldrich & TCI) and used as received, among them aldehydes and
solvents were used after distillation.
For a typical experiment, benzaldehyde (1.0 equiv., 2.0 mmol,
0.20 mL), pyrrolidine (1.2 equiv., 2.4 mmol, 0.20 mL), phenylacety-
lene (1.5 equiv., 3.0 mmol, 0.34 mL), and the catalyst (6.75 mol%,
0.135 mmol, 0.05 g) were added to water (2.0 mL) in a 10-mL
round-bottom flask. The reaction mixture was subjected to vigor-
ous stirring at 60 °C under argon. The reaction was monitored by
thin layer chromatography (TLC). After completion, the reaction
mixture was cooled to room temperature and filtered to recover
the catalyst. The catalyst was washed with water and ethyl acetate
(10 mL each), and then the organic layer was separated by extrac-
tion with three 10-mL portions of ethyl acetate. The combined
organic layers were dried over MgSO4 and filtered using Celite-
545. The solvent was evaporated at reduced pressure, and the prod-
uct was separated by flash column chromatography. The product
was analyzed by 1H NMR. Further confirmation of the novel com-
pounds was carried out by analyses such as IR, 1H NMR, 13C NMR,
and HRMS (spectra provided in the Supplementary Information).
2.3. Characterization and analysis
Scanning electron microscopy (SEM) and energy dispersive
X-ray analysis (EDXA) measurements were performed on a JEOL
high-resolution transmission electron microscope (HRTEM) at an
acceleration of 300 kV. The loading value of Au(III) in the catalyst
was estimated by ICP analysis with a JY Ultima2C. ICP also helped
to check for leaching out after the recycle test. The lower critical
solution temperature (LCST) of the synthesized hydrogel was char-
acterized by differential scanning calorimetry (DSC) measurements
(TA Instrument, DSC 2010). The phase transition takes place with
Fig. 1. Differential scanning calorimetry curve for poly(NIPAM-co-4-VP).

106
S. Shabbir et al. / Journal of Catalysis 322 (2015) 104–108
3.2. A-3 coupling reaction
below the LCST did not give satisfactory results, in contrast to reac-
tions performed at temperatures above the LCST. The result can be
explained by the increased interactions of reactants with the
hydrophobic catalyst at higher temperatures.
The reaction was also successfully scaled up 10 times under the
mentioned conditions at 60 °C and 6.75 mol% of catalyst to obtain
around 5 g (92%) of the product after 5 h.
Moreover, the morphology of the catalyst was determined by
scanning electron microscopy (SEM) images that elaborated the
absence of any changes in morphology before and after catalysis.
The presence of Au in the catalyst was also confirmed by EDXA
before and after coupling experiments. See Fig. 2.
The catalyst was tested for the three-component–one-pot A-3
coupling reaction. The reaction proceeded smoothly in water, a sol-
vent that is in accordance with green chemistry. The reaction was
carried out with varying amounts of the catalyst at various temper-
atures, as described in Table 1. Initially, using less than 6.75 mol%
(0.05 g) of catalyst resulted in moderate yields of the product.
Later, an increase in the catalyst loading to 6.75 mol% gave excel-
lent results. Any further increase in the catalyst amount could only
decrease the reaction time, as the catalyst efficiency stay almost
the same, marked by comparable turnover frequencies (TOF). Thus,
we preferred to use 6.75 mol% of catalyst. However, in the absence
of catalyst, no product was observed. There are reports on gold cat-
alysts with high TOF values, too, but they are applicable to rela-
tively simpler reactions [33,34]. To thoroughly investigate the
effect of temperature on catalytic efficiency, reactions were per-
formed at temperatures below and above the LCST (RT, 40 °C,
and 60 °C). As predicted, reactions performed at temperatures
3.3. Substrate scope
The scope of the reaction was also determined using a general
protocol for the A-3 coupling reaction under the optimized condi-
tions for our catalyst in water. The catalyst performed efficiently
with both aryl and aliphatic aldehydes. Amines including pyrroli-
Table 1
The A-3 coupling reaction using Au(III) catalyst in water.a
Au Cat.
H2O
N
CHO
N
H
d
Entry
Catalyst (mol%) (supported catalyst (g))
Temp. (°C)
Conv. (%)b
Time (h)
TON (mol/mol)c
TOF (hÀ1
)
1
6.75 (0.050)
6.75 (0.050)
6.75 (0.050)
2.70 (0.020)
9.45 (0.070)
13.50 (0.100)
25
40
60
60
60
60
35
64
100
39
100
100
5
5
5
5
4
3
5
9
15
14
11
7
1.03
1.89
2.96
2.89
2.64
2.47
2
3e
4
5
6
a
b
c
Reaction conditions: benzaldehyde (1.0 equiv., 2.0 mmol), pyrrolidine (1.2 equiv., 2.4 mmol), phenylacetylene (1.5 equiv., 3.0 mmol).
Determined by GC analysis using dodecane as an internal standard.
Reactant moles converted/catalyst moles.
(Reactant moles converted/catalyst moles)/reaction time.
No reaction occurred in the absence of the catalyst.
d
e
Fig. 2. EDXA measurements and SEM images of (a) fresh and (b) recycled catalyst.

S. Shabbir et al. / Journal of Catalysis 322 (2015) 104–108
107
Table 2
the polymer was aided by the solubility of the Au(III) salt in water.
Moreover, we found the catalyst to be applicable to the A-3 cou-
pling reaction, demonstrating excellent yields under environmen-
tally friendly, safe, and mild reaction conditions.
We expect this catalyst to be equally suitable for other reactions
catalyzed by Au(III).
Substrate scope of A-3 coupling reaction using Au-catalyst supported on poly(NIPAM-
co-4-VP).a
R2
R1
R3
N
R2
R3
CHO
Au catalyst
H2O, 60°C
N
H
R1
R4
R4
Acknowledgments
Entry
R1
R2R3NH
R4
Time
Yield
This research was supported by the Basic Science Research Pro-
gram through the National Research Foundation of Korea (NRF),
funded by the Korean Ministry of Education (2012R1A1A2001005
and 2011-0023512). Y. Lee acknowledges financial support from
the Korean Ministry of Education through the BK21-Plus project
for the Hanyang University Graduate Program.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Ph
Ph
Ph
Ph
Pyrrolidine
Piperidine
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
6
5
4
3
7
3
8
8
7
93
95
78
96
80
89
93
83
80
85
91
81
100
96
97
91
96
Morpholine
Dibenzylamine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
2,4-diCl–C6H3
p-Br–C6H4
p-CH3O–C6H4
p-C2H5O–C6H4
p-iPr–C6H4
Appendix A. Supplementary material
o-HO–C6H4
5
C
10H7
15
10
5
8
1
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2014.11.009.
PhCH2CH2
CH3(CH2)4
tert-Bu
Ph
Ph
Ph
Ph
p-NH2–C6H4
3,4-diCl–C6H3
p-nBu–C6H4
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