A3-Coupling catalyzed by robust Au nanoparticles covalently bonded to HS-functionalized cellulose nanocrystalline films

Article abstract of DOI:10.3762/bjoc.9.155

We decorated HS-functionalized cellulose nanocrystallite (CNC) films with monodisperse Au nanoparticles (AuNPs) to form a novel nanocomposite catalyst AuNPs@HS-CNC. The uniform, fine AuNPs were made by the reduction of HAuCl4 solution with thiol (HS-) group-functionalized CNC films. The AuNPs@HS-CNC nanocomposites were examined by X-ray photoelectron spectroscopy (XPS), TEM, ATR-IR and solid-state NMR. Characterizations suggested that the size of the AuNPs was about 2-3 nm and they were evenly distributed onto the surface of CNC films. Furthermore, the unique nanocomposite Au@HS-CNC catalyst displayed high catalytic efficiency in promoting three-component coupling of an aldehyde, an alkyne, and an amine (A3-coupling) either in water or without solvent. Most importantly, the catalyst could be used repetitively more than 11 times without significant deactivation. Our strategy also promotes the use of naturally renewable cellulose to prepare reusable nanocomposite catalysts for organic synthesis.

Full text of DOI:10.3762/bjoc.9.155

A3-Coupling catalyzed by robust Au nanoparticles
covalently bonded to HS-functionalized
cellulose nanocrystalline films
Jian-Lin Huang, Derek G. Gray* and Chao-Jun Li*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 1388–1396.
Department of Chemistry, McGill University, 801 Sherbrooke Street
West, Montreal, Quebec H3A0B8, Canada
doi:10.3762/bjoc.9.155
Received: 15 April 2013
Accepted: 19 June 2013
Published: 10 July 2013
Email:
Derek G. Gray* - derek.gray@mcgill.ca; Chao-Jun Li* - cj.li@mcgill.ca
* Corresponding author
This article is part of the Thematic Series "Gold catalysis for organic
synthesis II".
Keywords:
A3-coupling reaction; cellulose nanocrystallites (CNCs) films; gold
catalysis; water or without solvent
Guest Editor: F. D. Toste
© 2013 Huang et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
We decorated HS-functionalized cellulose nanocrystallite (CNC) films with monodisperse Au nanoparticles (AuNPs) to form a
novel nanocomposite catalyst AuNPs@HS-CNC. The uniform, fine AuNPs were made by the reduction of HAuCl4 solution with
thiol (HS-) group-functionalized CNC films. The AuNPs@HS-CNC nanocomposites were examined by X-ray photoelectron spec-
troscopy (XPS), TEM, ATR-IR and solid-state NMR. Characterizations suggested that the size of the AuNPs was about 2–3 nm and
they were evenly distributed onto the surface of CNC films. Furthermore, the unique nanocomposite Au@HS-CNC catalyst
displayed high catalytic efficiency in promoting three-component coupling of an aldehyde, an alkyne, and an amine (A3-coupling)
either in water or without solvent. Most importantly, the catalyst could be used repetitively more than 11 times without significant
deactivation. Our strategy also promotes the use of naturally renewable cellulose to prepare reusable nanocomposite catalysts for
organic synthesis.
Introduction
Organic synthesis is usually performed in organic solvents; reactions [6,7], transition-metal catalyzed C–C bond forma-
however, from a green chemistry perspective, evaporation and tions [8,9] and cross-dehydrogenative coupling (CDC)
discharge of organic solvents not only generates chemical waste reactions [10-13]. In addition, the three-component
but also causes environmental pollution [1,2]. In the past few aldehyde–alkyne–amine (A3) coupling and asymmetric alde-
decades, aqueous-phase organic reactions have achieved great hyde–alkyne–amine (AA3) coupling reactions have received
success [3-5]. The classic examples include the Grignard-type increasing attention due to the easy formation of high-value
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product propargylamines [14-16]. Notably, the A3 coupling catalyst displays high catalytic efficiency in A3-coupling reac-
reaction has also been achieved in aqueous media or without tions performed in either aqueous media or without solvent.
solvent by gold catalysis [17-19]. However, up until now, most More importantly, it can be used repetitively up to 11 times
of the reactions are conducted by using homogenous without significant loss of catalytic efficiency.
organometallic catalysts. They usually show the high catalytic
Results and Discussion
activity and selectivity [20]; however, homogeneous catalysts
Preparation and characterizations of
nanocomposite Au@HS-CNC catalyst
are difficult to adopt in large-scale industrial settings because of
challenges associated with recovery and reuse of the catalysts
from the reaction system, which may also increase the cost and
cause environmental pollution by metallic ions. Heterogeneous
catalysts could overcome the above problems [21]; however,
they usually show lower catalytic activities compared with
homogeneous catalysts, which may be caused by blocking the
diffusion and adsorption of organic reactant molecules and
products or the poor dispersion of active sites [22,23].
Scheme 1 briefly illustrates the preparation of the Au@HS-
CNC catalyst. First, the HS-CNC composite was prepared by
using a modified procedure reported by MacLachlan et al [30].
In a typical procedure, 30 mL of a 2.1% aqueous CNC suspen-
sion was sonicated for 10 min (see Methods for details of CNC
preparation in Supporting Information File 1) and pH adjusted
to 2.9 with AcOH. 3-Mercaptopropyltrimethoxysilane (1.0 mL,
4.5 mmol) was added to the CNC suspension and the mixture
was stirred at 25 °C until a homogeneous mixture was obtained
(typically about 4 h). This solution was cooled to room
temperature, and then dried on a polystyrene Petri dish. After
slow evaporation at room temperature, the nanocomposite films
of the HS-CNC materials were dried at 120 °C for 2 h. Then,
the films were successively soxhlet extracted with EtOH for 6 h
and filtered. Finally, the HS-CNC films were added into 0.12 M
HAuCl4 ethanol solution and kept under stirring at room
temperature for 24 h (during this step, the Au3+ was reduced to
Au0 by the HS-groups attached on CNC), then filtered and dried
at 40 °C overnight. Alternatively, the Au@HS-CNC was also
synthesized by using a modified procedure reported by Tingaut
et al. Only the method of thiol functionalized CNC support
(HS-CNC) is different from that reported by MacLachlan et al.
(see Methods for the details about the catalyst preparation).
Cellulose nanocrystals (CNCs) have emerged as a new class of
nanomaterials owing to their renewable, environmentally
benign, naturally abundant, biodegradable and biocompatible
nature, as well as their excellent mechanical properties and
anticipated low cost [24-26]. CNCs are obtained from semicrys-
talline cellulose derived from wood fibers and plants. Potential
applications for CNCs include nanocomposite formulation,
polymer reinforcement, drug delivery [27], enzyme immobiliz-
ation [28], biomedical applications [29] and as templates for the
synthesis of nanomaterials [30]. The deposition of metal
nanoparticles onto the surface of CNCs can lead to new nano-
heterogeneous catalysts for organic synthesis. Recently, CNCs
have been used as an effective support for Pd nanoparticles
[31], AuNPs [32], SeNPs [33], NiNPs [34] and Au–Ag alloy
NPs [35] for greener organic synthesis. However, to date,
studies on combining the surface chemistry of CNCs with metal
nanoparticles in catalysis are still very limited.
Structure characterizations
The XPS spectra (Figure 1) demonstrated that the binding
This paper describes the use of HS-functionalized CNCs deco- energy of the Au species in the Au@HS-CNC (4.4 mol %)
rated with gold nanoparticles as a novel class of heterogeneous sample was 84.8 eV for the Au4f7/2 level corresponding to zero-
catalysts for greener organic reactions. AuNPs were formed and valent Au, according to reference data reported by Li et al. [17],
deposited on the surface of HS-functionalized CNCs by coordi- and no other peak was observed. This indicated that metal ions
nation with the free HS-ligands. The as-prepared Au@HS-CNC (Au3+) have been reduced to their metallic states (Au0). TEM
Scheme 1: Sketch illustrating preparation of the Au@HS-CNC catalyst.
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Beilstein J. Org. Chem. 2013, 9, 1388–1396.
Figure 1: Au4f and S2p XPS spectra of the Au@HS-CNC (4.4 mol %) catalyst.
pictures in Figure 2 further confirm this result. The HRTEM
images in Figure S1 clearly show the size (2–3 nm) and lattice
of the Au nanoparticles on the surface of the Au@HS-CNC
(4.4 mol %) catalyst. The S species were mainly present in −2
states, corresponding to HS-groups with the binding energy
around 163.5 in the S2p level. Thermogravimetric analysis
(Figure 3) showed that the deposition of AuNPs onto CNC
apparently enhanced the thermal stability of the Au@HS-CNC
(4.4 mol %) films, which might be due to a composite of the
saline reagent (3-mercaptopropyltrimethoxysilane). The
Au@HS-CNC (4.4 mol %) decomposed at above 250 °C under
an inert atmosphere, making them an attractive catalyst for
catalytic reactions. The FT-IR spectra of CNC, HS-CNC and
Au@HS-CNC (4.4 mol %) (Figure 4) showed absorbance bands
around 2920 cm−1 due to the stretching vibration of the C–H
bond in the HS–CH2–CH2–CH2-group. The peaks at
600–1180 cm−1 were designated to the νSi-O-Si and νSi-C-Si
Figure 3: Thermogravimetric behavior of the Au@HS-CNC
(4.4 mol %) catalyst (A) and CNC (B).
Figure 4: FT-IR spectra of CNC, HS-CNC, and Au@HS-CNC
Figure 2: TEM pictures of the HS-NCC and Au@HS-CNC (4.4 mol %)
(4.4 mol %) catalyst.
catalyst (scale bar: 5 nm).
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vibrations [36]. In comparison with the pure CNC, the HS-CNC effects of reaction solvents, temperature and reaction time on
and the Au@HS-CNC (4.4 mol %) catalysts exhibited an ad- the catalytic efficiency. As shown in Table 1, one could
ditional peak at 2546 cm−1 corresponding to the vibration of the conclude from the influence of the reaction time on the activity
HS-group [37]. However, the Au@HS-CNC (4.4 mol %) cata- that the A3-coupling reaction reaches completion after 24 h
lyst showed a weaker signal of the HS-group than the HS-CNC under the present conditions. At a lower reaction temperature
sample due to the coordination of the HS-ligand with the (25 °C), the Au@HS-CNC(4.4 mol %) showed lower conver-
AuNPs. The solid-state 13C NMR spectra (Figure 5) further sion due to an incomplete reaction. We obtained the best
confirmed the presence of SH-groups in the Au@HS-CNC conversion at a higher reaction temperature (above 80 °C).
(4.4 mol %). In comparison with the pure CNCs, the Au@HS- Solvent-free conditions proved to be the most effective for the
CNC (4.4 mol %) catalyst clearly displayed two strong peaks at A3-coupling reaction (Table 1, entry 23) and the conversion was
around 17 and 25 ppm owing to the C atoms connected with the comparable to that of the homogeneous catalyst (Table 1, entry
S atoms in the HS–CH2–CH2–CH2 group [38]. The other peaks 3). We obtained slightly lower conversions when using water or
at around 65–80, and 105 ppm could be assigned to carbon toluene as the solvent (Table 1, entries 14 and 15). Ethanol,
atoms in the cellulose framework.
acetonitrile, dichloromethane, tetrahydrofuran (THF), ethyl
acetate (EA), dimethyl sulfoxide (DMSO), and N,N-dimethyl-
formamide (DMF) afforded the products in moderate or low
conversions (Table 1, entries 16–22). The optimized reaction
conditions include 1.0 equiv of aldehyde, 1.2 equiv of amine,
1.5 equiv of alkyne, and 4.4 mol % of Au nanoparticles at
80 °C, solvent-free in air.
To expand the scope of this A3-coupling, we used various alde-
hydes and amines as substrates under the optimized reaction
conditions, and the results are summarized in Table 2. Both
aromatic and aliphatic aldehydes provided the desired products
in good to moderate yields (Table 2, entries 1–8). However,
long chain aldehydes had a lower activity, giving lower yields
(Table 1, entries 9, 10). We also observed good to moderate
yields when the cyclic dialkylamines such as pyrrolidine,
morpholine and azepane were used (Table 2, entries 11–19).
Figure 5: Solid-state 13C NMR spectra of the CNC and Au@HS-CNC
(4.4 mol %) catalyst.
Catalyst recycling
In order to determine the recycling ability of the catalysts, the
following experiments were conducted. After completion of the
reaction, the mixture was diluted with 0.5 mL deuterated chlo-
Catalytic performances
The A3-coupling reaction of benzaldehyde, piperidine, and roform (CDCl3) and filtered, and then the solid Au@HS-
phenylacetylene was selected as the probe reaction to examine CNC(4.4 mol %) catalyst was washed 3 times with CDCl3,
the catalytic activity of the Au@HS-CNC catalyst. Table 1 dried in vacuum, and then reused with a fresh charge of reac-
summarizes the catalytic performances of the catalyst with tants for a subsequent run of reactions under identical condi-
different Au-loadings, which were measured by inductively tions. Figure 6 demonstrates that the catalyst could be used
coupled plasma (ICP) analytical techniques. Both the HS-CNC repetitively more than 11 times without significant deactivation,
and the Au sponge were inactive, implying that the Au is the suggesting its good reusability in solvent-free A3-coupling of
active site and that the controlling of Au nanoparticle size is formaldehyde, piperidine, and phenylacetylene. It is important
essential for the present reactions. The catalytic activity first to verify that the actual catalytic process is heterogeneous and
increased with the increase of the Au loading up to 4.4 mol %. not homogeneous [39]. For this reason, we did the following
However, the activity slightly decreased with further increases experiment: the solid catalyst was removed by filtering when
in Au-loading up to 5.2 and 6.3 mol %. This decrease might be the conversion was up to 45% in A3-coupling reactions, and
due to both the poor distribution of the Au active sites and the then the solution reaction was continued under the same condi-
aggregation of the nanoparticles (See Figure S2, Supporting tions. The conversion of the formaldehyde did not significantly
Information File 1). We determined the optimal Au-loading to increase, which strongly suggested that this catalytic process
be 4.4 mol %. Besides the Au-loading, we also investigated the was a heterogeneous process.
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Table 1: Three-component coupling of benzaldehyde, piperidine, and phenylacetylene catalyzed by Au-based catalysts.a
entry
catalyst (mol %)
solvent/temp (°C)/time (h)
conversion (%)b
1
HS-CNC (0)
H2O/80/24
H2O/80/24
H2O/80/24
H2O/80/24
H2O/80/24
H2O/80/24
H2O/80/24
H2O/80/24
H2O/80/48
H2O/80/12
H2O/80/6
0
2
Au0 sponge (4.0)
HAuCl4 (1.0)
0
3
>99
61
87
82
81
67
86
73
65
85
78
32
92
76
56
54
54
10
trace
trace
100
4
Au@SH-CNC (2.9)
Au@SH-CNC (4.4)
Au@SH-CNC (5.2)
Au@SH-CNC (6.3)
Au@SH-CNCc(4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
Au@SH-CNC (4.4)
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
H2O/120/24
H2O/60/24
H2O/rt/24
toluene /80/24
CH2Cl2/80/24
ethanol/80/24
MeCN/80/24
DMSO/80/24
THF/80/24
DMF/80/24
EA/80/24
neat/80/24
aAll reactions were carried out with benzaldehyde (0.2 mmol), piperidine (0.24 mmol), phenylacetylene (0.3 mmol), 0.2 mL solvent in a sealed well
tube. bConversions were determined by 1H NMR of the crude reaction mixture. cCatalyst was prepared by using a modified procedure reported by
Tingaut et al.
Table 2: Three-component coupling of aldehyde, amine, and phenylacetylene catalyzed by Au@SH-CNC catalysts in solvent-free conditions.a
entry
aldehyde
amine
product
yield (%)b
1
formaldehyde
piperidine
100
2
benzaldehyde
piperidine
93
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Table 2: Three-component coupling of aldehyde, amine, and phenylacetylene catalyzed by Au@SH-CNC catalysts in solvent-free conditions.a
(continued)
3
4
5
6
7
ethylbutyraldehyde
piperidine
piperidine
piperidine
piperidine
piperidine
93
83
82
81
81
isobutyraldehyde
1-naphthaldehyde
2-methylbutyraldehyde
cyclohexanecarboxaldehyde
8
hydrocinnamaldehyde
piperidine
51
9
decanal
piperidine
piperidine
42
36
10
valeraldehyde
11
benzaldehyde
morpholine
60
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Table 2: Three-component coupling of aldehyde, amine, and phenylacetylene catalyzed by Au@SH-CNC catalysts in solvent-free conditions.a
(continued)
12
benzaldehyde
pyrrolidine
34
13
benzaldehyde
azepane
30
14
15
16
formaldehyde
formaldehyde
formaldehyde
morpholine
pyrrolidine
azepane
99
84
61
17
ethylbutyraldehyde
morpholine
56
18
19
ethylbutyraldehyde
ethylbutyraldehyde
pyrrolidine
50
25
azepane
aAll reactions were carried out with aldehyde (0.2 mmol), amine (0.24 mmol), phenylacetylene (0.3 mmol), and catalyst containing Au (4.4 mol %) in a
sealed well tube, at 80 °C (oil bath) for 24 h. bYields were determined by 1H NMR of the crude reaction mixture.
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Acknowledgements
The work was supported by a team grant from Fonds de
recherche sur la nature et les technologies Québec. We thank
Dr. X. D. Liu, of the Facility for Electron Microscopy (FEMR)
for TEM imaging.
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