Polydimethylsiloxane Sponge-Supported Nanometer Gold: Highly Efficient Recyclable Catalyst for Cross-Dehydrogenative Coupling in Water

Article abstract of DOI:10.1002/cssc.201801180

Polydimethylsiloxane (PDMS, a stable hydrophobic polymer material) sponge-supported nanometer-sized gold can be used as a highly efficient recyclable catalyst for cross-dehydrogenative coupling of tertiary amines with various nucleophiles in water. This P

Full text of DOI:10.1002/cssc.201801180

Accepted Article
Title: Polydimethylsiloxane Sponge Supported Nanometer Gold: Highly
Efficient Recyclable Catalyst for Cross-Dehydrogenative Coupling
in Water
Authors: Weiwei Liang, Teng Zhang, Yufei Liu, Yuxing Huang, Zhipeng
Liu, Yizhen Liu, Bo Yang, Xuechang Zhou, and Junmin Zhang
This manuscript has been accepted after peer review and appears as an
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To be cited as: ChemSusChem 10.1002/cssc.201801180
Link to VoR: http://dx.doi.org/10.1002/cssc.201801180
A Journal of
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10.1002/cssc.201801180
ChemSusChem
COMMUNICATION
Polydimethylsiloxane Sponge Supported Nanometer Gold: Highly
Efficient Recyclable Catalyst for Cross-Dehydrogenative
Coupling in Water
Weiwei Liang,‡ Teng Zhang,‡ Yufei Liu, Yuxing Huang, Zhipeng Liu, Yizhen Liu, Bo Yang, Xuechang
Zhou and Junmin Zhang*[a]
Abstract: The successfully developed polydimethylsiloxane (PDMS,
stable hydrophobic polymer material) sponge supported
water. With our continual interests on PDMS sponge supported
catalysts[9] and nanometal catalysts[10c], in this paper, we
demonstrate for the first time that a hydrophobic PDMS sponge
a
nanometer gold can be used as a highly efficient recyclable catalyst
for cross-dehydrogenative coupling of tertiary amines with various
nucleophiles in water. This PDMS sponge nanometer gold can
provide much better catalytic activity than free nanometer gold in
water. Through an easy-to-build continuous flow reactor, the sponge
nanometer gold catalyzed reaction can be facilely scaled up. It
should be believed that this porous hydrophobic PDMS sponge
material will be a promising carrier of highly efficient recyclable
catalysts in water.
supported nanometer gold can act as
a highly efficient
recyclable catalyst for cross-dehydrogenative coupling (CDC)[11]
of tertiary amines with various nucleophiles in water. We have
found out that PDMS sponge nano-gold can provide much better
catalytic activity than free nano gold in water. Therefore, it
should be believed that this porous hydrophobic PDMS sponge
material will be a promising carrier for the development of highly
efficient recyclable catalysts in water.
Here we adopt a facile approach to synthesize the supported
gold nanoparticles on PDMS sponge. The procedure involves
the functionalization of sponge with propylamine groups and
then reductive deposition of nanometer gold particles using the
readily available HAuCl4 precursor, as is represented
schematically in Fig. 1A. Due to long-chain functional groups are
helpful for immobilizing stable and robust metal nanoparticles on
polymer surface,[12] in the above process, propylamine groups
were covalently grafted on the surface of porous PDMS sponge,
which not only promote the interaction between sponge and
Gold is generally considered inactive, however, its form of nano-
size particles possesses a higher surface to volume ratio that
leads to its high catalytic activity.[1] In last few years, zero-valent
gold catalysis have received considerable attentions.[2] In this
regard, several types of gold (0), such as gold nanoparticles
(AuNPs),[3] gold nanopores (AuNPore)[4] and gold nanoclusters,[5]
have been well developed as catalysts for various organic
transformations. Despite these elegant pioneering studies,
conventional filtration or centrifugation is still required for the
recycle of these kinds of gold catalysts as they are generally
nano-size solid particles. The extra separation process greatly
limited their reusability and practical application in industry. The
recovery and reuse of catalysts is a very important and eternal
subject in green chemical synthesis,[6] water is the greenest and
cheapest reaction medium,[7] therefore, the development of
green, highly efficient and reusable catalyst in water is highly
anticipated. Recently we have found that
a hydrophobic
polydimethylsiloxane (PDMS) sponge[8] material can be
successfully applied to the immobilization of organic
photocatalyst and has achieved excellent results.[9] In addition to
its convenient separation and purification due to its sponge-type,
it is more interesting that this hydrophobic porous sponge
material supported catalyst has the distinct highly effective
catalytic activity in the water.[9b] Taking into consideration of the
remarkable catalytic activity of gold nanoparticles together with
the hydrophobicity of PDMS sponge, which should be beneficial
for organic reactions in water, we reasoned that the PDMS
sponge supported gold nanoparticles should be a promising
heterogeneous catalyst for efficient organic transformations in
[a]
W. Liang, Dr. T. Zhang, Y. Liu, Dr. Y. Huang, Z. Liu, Prof. Dr. Y. Liu,
Prof. Dr. B. Yang, Prof. Dr. X. Zhou, Prof. Dr. J. Zhang*
College of Chemistry and Environmental Engineering, Shenzhen
University, Shenzhen, 518060, P. R. China
Figure 1. A) Schematic representation of supported gold
nanoparticles on PDMS sponge; B) SEM image and EDX spectrum of
sponge nano-Au catalyst; C) TEM images for sponge nano-Au catalyst
dissolved by trifluoroacetic acid, Particle size distribution of sponge
nano-Au catalyst obtained by randomly counting 215 particles in
different areas.
E-mail: zhangjm@szu.edu.cn
Homepage: http://orcid.org/0000-0003-4061-8350
‡These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document. (Please delete this text if not appropriate)
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10.1002/cssc.201801180
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HAuCl4 precursor to enable gold particles enter into the sponge
pores, but also help stabilizing the formed particles to prevent
their mobility and aggregation. In this manner, the structural
properties of PDMS sponge were highly exploited. The
morphology of the obtained sponge nano-Au catalyst was
characterized by scanning electron microscopy (SEM), which
showed that PDMS sponge is a 3D-porous polymer material and
gold particles were evenly distribution on the sponge pores (Fig.
1B). The corresponding energy dispersive X-ray spectroscopy
(EDX) further proved the homogenous distribution of Si, C, N, O
and Au elements throughout the PDMS sponge (see the SI for
details). Gold particle sizes were analyzed by transmission
electron microscopy (TEM), the histograms showed that 7-14
nm gold particles were found on sponge surface (Fig. 1C).
Catalyst loading on PDMS sponge was determined by
inductively coupled plasma emission spectrometer (ICP) and the
analysis results was 1.34×10-5 mmol/g.
In order to investigate the catalytic activity of as-prepared
sponge nano-Au catalyst, the CDC reaction between N-phenyl-
tetrahydroisoquinoline (N-THIQ) and nitromethane was chosen
as the model reaction to study (Table 1). The sponge nano-Au
catalyst amount for the model reaction was set for 1 mol% and
molecular oxygen was used as oxidant in each reaction. Initially,
various solvents were screened to examine their effects in the
catalytic system. As our anticipated, a promising result was
found when the reaction was carried out in water (66% yield,
Table 1, entry 9). After prolong reaction time, the improved yield
were obtained (92%, Table 1, entry 10). In addition, by using
free gold nanoparticles as catalyst, only 26% yield was detected
even after 36 h (Table 1, entry 10). The above results indicated
that sponge nano-Au catalyst can provide much better catalytic
activity than free gold nanoparticles in water. In order to shorten
the reaction time, other oxidants were further examined (Table 1,
entries 11 and 12). Both H2O2 (30 wt.% in water) and TBHP (70
wt.% in water) were effective oxidants for this CDC reaction in
water, with providing 88% and 98% yields after 24 h,
respectively. It should be mentioned that the above two aqueous
oxidants were generally ineffective for this type of CDC
reactions.[11] The result is exciting due to its efficient construction
of carbon-carbon bonds in an environmental friendly and
efficient manner. In the viewpoint of green and sustainable
chemistry, both entries 10 and 12 were chosen as the optimized
conditions for CDC reactions catalyzed by sponge nano-Au
catalyst.
Having identified the optimal conditions, we next explored the
substrate scope of the sponge gold catalytic system in water. In
order to afford the best yield, the reactions were carried out
regardless of the optimized condition A and B. Generally, as
shown in Table 2, the different N-THIQ derivatives containing
electron-donating or electron-withdrawing groups reacted with
nitromethane very well in the condition A; While in the condition
B, compounds 3b-d and 3f-h were obtained with moderate
yields, this could be due to molecular oxygen less reactive than
TBHP in the catalytic system. More specifically, only low yield
was obtained for compounds 3h in the condition B, which may
contribute to its increasing electron-withdrawing ability of Cl-
group on the phenyl ring. In addition, the substrates bearing a
methoxyl group on the aryl ring of the THIQ also provided the
corresponding products with satisfied yield (Table 2, 3i and 3j).
The substrate scope was further investigated by a variety of
nucleophiles, as shown in Table 3. Both nitroalkanes and 1, 3-
diketones derivatives were active nucleophiles, which could give
the corresponding products in good yields (Table 3, 3k-n). It was
Table 2. Reaction scope with varying N-THIQs.a
Table 1. Optimization of reaction conditions.a
Time
Entry
1
2
3
4
5
6
7
8
Solvent
EtOH
MeOH
CH2Cl2
CH3CN
Toluene
DMF
DMSO
i-PrOH
H2O
Oxidant
O2
O2
O2
O2
O2
O2
O2
O2
Yieldb
10%
trace
44%
9%
trace
53%
18
18
18
18
18
18
18
18
18
36
24
24
24%
trace
66%
9
O2
10
11
12
H2O
H2O
H2O
O2
92%c, 26%d
88%
e
H2O2
TBHPf
98%c
a Reactions were performed using 1a (20.9 mg, 0.1 mmol), 2a (54 µL,
a
Method A: 1a-j (0.5 mmol), nitroalkane 2a (270 µL, 5.0 mmol),
1.0 mmol) and oxidant (1 atm for O2 or 2 equiv. for others) in 1.5 mL
o
sponge nano-Au catalyst (1 mol%), TBHP in water (70 wt.%, 2 equiv.),
80 °C, in 3 mL H2O for 24 h; Method B: 1a-j (0.5 mmol), nitroalkane 2a
(5.0 mmol), sponge nano-Au catalyst (1 mol%), O2 (1 atm), 80 °C, in 3
mL H2O for 36 h; The yields are isolated yields.
H2O, and were catalyzed by 1 mol% sponge nano-Au catalyst at 80 C
for 18-36 h; b Yield was determined by 1H NMR with 1,3,5-trimethoxy-
c
d
benzene as an internal standard; Isolated yield; Free nano-gold
particles as catalyst; e H2O2 (30 wt.% in water, 2 equiv.); f TBHP (70
wt.% in water, 2 equiv.).
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10.1002/cssc.201801180
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Table 3. Reaction scope with varying nucleophiles.a
Figure 2. Recycling experiments for sponge nano-Au catalyst.
reactions of N-aryl tertiary amines, such as 4-methly-N, N-
dimethylaniline and N, N,-dimethylaniline, reacted with excess
amounts of nitromethane efficiently to produce the
corresponding products in moderate yields (Table 4, 4a and 4b).
Cyclic tertiary amines, such as 1-phenylpiperidine and 1-
phenylpyrrolidine were examined to be suitable substrates, to
give the corresponding coupling products in good to excellent
yields (Table 4, 4c and 4d). Phenylpyrrolidine was also reacted
with dimethyl malonate to give the corresponding substituted
amine 4e in 64% yield using acetonitrile as solvent. The
mechanism of this sponge nano-Au catalyzed CDC reaction
should be consistent with the formation of the iminium
intermediate on gold particles, as proposed by Jin et al.[4h]
The reusability of sponge nano-Au catalyst was investigated
by the model reaction under the optimal condition (Table 1, entry
12). The yield was measured after 24 h in each cycle and the
separated sponge nano-Au catalyst was washed with alcohol
and dried for the next cycle directly. According to these
observations, the sponge nano-Au catalyst can be recycled
more than twenty times, giving consistently high isolated yields
and maintaining the size and distribution of gold particles on
PDMS sponge (Fig. 2, see the SI for details).
a Method A: 1a (104.5mg, 0.5 mmol), nucleophiles 2b-m (5.0 mmol),
sponge nano-Au catalyst (1 mol%), TBHP in water (70 wt.%, 2 equiv.),
80 °C, in 3 mL H2O for 24 h; Method B: 1a (0.5 mmol), nucleophiles
2b-m (5.0 mmol), sponge nano-Au catalyst (1 mol%), O2 (1 atm),
80 °C, in 3 mL H2O for 36 h; The yields are isolated yields.
noted that the compound 3m, there is no desired product
formed and only the oxidized by-product was detected under
method A. In addition, aliphatic ketone is another promising
nucleophiles, which generally resulted higher yields than
aromatic ketones (Table 3, 3o and 3p), because of the latter
conjugation effects between aromatic ring and carbonyl group.
To our satisfaction, better yields were also observed when
performing the coupling with cyclic ketones at the standard
conditions (Table 3, 3q and 3r). Furthermore, although indole
derivatives showed a lower reactivity under method B (Table 3,
3s-u, 37-48%), the improved yields of corresponding products
were obtained when conducted with TBHP as oxidant (Table 3,
3s-u, method A, 70-90%). In addition, the corresponding
cyanation product 3v was also obtained with satisfactory yield in
the condition A.
Due to the chemical inertness of ɑ C-H, it is hard for simple
tertiary amines to react with active nucleophiles. We therefore
extended the current gold catalytic system to the simple tertiary
amines using TBHP as an oxidant at 80oC (Table 4). The
In order to demonstrate the scalability and practicability of
sponge nano-Au catalysis in industrial application, a gram scale
of the model reaction was performed using an easy-to-build
continuous flow reactor, as described in our previous work.7b
The product was obtained with 90% of isolated yield after 24 h
(see the SI for details).
Table 4. Reaction scope of simple tertiary amines with nucleophiles.a
In conclusion, we have developed a novel PDMS sponge
supported nano-Au catalyst. This sponge nano-Au catalyst
combined the advantages of high catalytic activity of nano gold
and the hydrophobic function of PDMS sponge, which can
facilitate the efficient CDC reactions of tertiary amines with
various nucleophiles in water. In addition, through an easy-to-
build continuous flow reactor, the sponge nano-Au catalyzed
reaction can be facilely scaled up. All results indicated that the
porous hydrophobic PDMS sponge material will be a promising
carrier for the development of highly efficient recyclable catalysts
in water. Efforts toward immobilizing other nanometal catalysts
on the PDMS sponge and the related more interesting
transformations in water is currently underway in our lab and will
be reported in due course.
a
Reaction condition: 1k-o (0.4 mmol), TBHP in decane (5.5 M, 2
equiv.), sponge nano-Au catalyst (1 mol%), Nitromethane 2a (3 mL)
as solvent, 80 °C for 36h. Isolated yields are shown; Dimethyl
b
malonate 2d (4 mmol) as reagent and MeCN (3 mL) as solvent.
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10.1002/cssc.201801180
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COMMUNICATION
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Acknowledgements
We are grateful to the National Natural Science Foundation
of China (21402124 and 21777106), the Training
Foundation for Outstanding Young Teachers in Higher
Education Institutions of Guangdong (YQ2015145), the
Project of Featured Innovation in Higher Education
Institutions of Guangdong (2017KTSCX159), the Shenzhen
Science and Technology Foundation (JCYJ2017030214365
8923) and the Natural Science Foundation of SZU (827000
038). We also appreciate the thoughtful suggestions from
the referees.
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Keywords: polydimethylsiloxane sponge • supported gold
nanoparticles • recyclable catalyst • cross-dehydrogenative
coupling • in water
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10.1002/cssc.201801180
ChemSusChem
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
The PDMS sponge supported
nano-gold can be used as a
Weiwei Liang,‡ Teng Zhang,‡ Yufei
Liu, Yuxing Huang, Zhipeng Liu,
Yizhen Liu, Bo Yang, Xuechang
Zhou, Junmin Zhang*
highly
catalyst
efficient
for
recyclable
cross-
dehydrogenative coupling of
tertiary amines with various
nucleophiles in water. This
sponge nano-gold can provide
much better catalytic activity
than free nanometer gold in
water. Through an easy-to-
build continuous flow reactor,
the sponge nanometer gold
catalyzed reaction can be
facilely scaled up.
Page No. – Page No.
Polydimethylsiloxane Sponge
Supported Nanometer Gold:
Highly Efficient Recyclable
Catalyst for Cross-
Dehydrogenative Coupling in
Water
This article is protected by copyright. All rights reserved.