β-cyclodextrin-capped palladium nanoparticle-catalyzed ligand-free Suzuki and Heck couplings in low-melting β-cyclodextrin/NMU mixtures

Article abstract of DOI:10.1002/aoc.3173

Low-melting β-cyclodextrin/N-methylurea (NMU) mixture, an efficient catalytic system for ligand-free Suzuki and Heck couplings in the presence of fresh native β-CD-capped Pd⁰ nanoparticles, has been successfully reported. This natural and convenient system can be performed in air and could afford the corresponding cross-coupled products in good to excellent isolated yields after a simple workup under every low Pd loading (0.05 mol%). Remarkably, the catalytic system can be recycled and reused without loss of catalytic activity. Copyright

Full text of DOI:10.1002/aoc.3173

Full Paper
Received: 21 March 2014
Revised: 4 May 2014
Accepted: 5 May 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3173
β-cyclodextrin-capped palladium nanoparticle-
catalyzed ligand-free Suzuki and Heck couplings
in low-melting β-cyclodextrin/NMU mixtures
Xiaohua Zhaoa*, Xiang Liub and Ming Lub
Low-melting β-cyclodextrin/N-methylurea (NMU) mixture, an efficient catalytic system for ligand-free Suzuki and Heck
couplings in the presence of fresh native β-CD-capped Pd0 nanoparticles, has been successfully reported. This natural and
convenient system can be performed in air and could afford the corresponding cross-coupled products in good to excellent
isolated yields after a simple workup under every low Pd loading (0.05 mol%). Remarkably, the catalytic system can be
recycled and reused without loss of catalytic activity. Copyright © 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: β-CD-capped Pd0 nanoparticles; β-cyclodextrin/N-methylurea mixture; ligand-free
have been used successfully as solvents in various different
Introduction
reactions. B. König and co-workers[30] reported organic transfor-
Since the discovery of Suzuki À Miyaura and Heck reactions, both
mations in low-melting mixtures consisting of sugars, urea and
cross-couplings have attracted widespread attention and find
important industrial application as one of the most important
methods of forming C–C bonds.[1–6] It is well known that the
use of phosphine ligands in homogeneous systems has proven
to be efficient for these kinds of cross-couplings, but they are
highly sensitive to air and moisture owing to the physical charac-
teristics of the ligands. Moreover, they are comparatively difficult
to prepare and tend to be expensive.[7–11] Consequently, the de-
velopment of cheap and facile ligand-free palladium catalyst to
overcome these difficulties is considered to be one of the most
challenging fields in organic chemistry.[12–15]
inorganic salts as a solvent. In 2011, a paper about the synthesis
of dihydropyrimidinones in low melting tartaric acid–urea
mixtures was reported by B. Koenig and co-workers.[31] In this
communication, we have contributed to this field and present
the use of low-melting mixtures of native β-cyclodextrin/
N-methylurea for Suzuki À Miyaura and Heck reactions. The novel
medium consists only of non-toxic compounds from readily
available commercial resources and has, like ionic liquids, a low
vapor pressure. To the best of our knowledge, there is no report
on the utility of this novel system for metal palladium-catalyzed
coupling reaction. Moreover, no toxic and air-sensitive ligands
need to be utilized during the entire cross-coupling process. To
our delight, this system exhibited outstanding catalytic activity,
stability and excellent recyclability in the presence of fresh native
β-cyclodextrin-supported Pd0 nanoparticles.
Another major concern is the uses of organic solvents because
of their associated environmental hazards. The main disadvan-
tages are their pyrophoric nature, volatility and poor recovery.
To circumvent some of these issues, attempts have been made
to develop solvent-green chemistry.[16] Among them, solvent-
free reactions have been successful for a few transforma-
tions.[17,18] However, in performing most organic reactions,
solvents play a critical role in making the reaction homogeneous
and allowing molecular interactions to be more efficient. In
principle, water is the ideal solvent, being non-toxic, cheap and
readily available, but its use is limited because most organic
compounds do not dissolve in pure water and many reactive
substrates or reagents decompose in water.[19,20] In recent years,
ionic liquids have received a lot of attention as green solvents for
their excellent properties: no measurable vapor pressure, stability
over a wide temperature range and recyclability.[21] In addition,
various publications and reviews have already appeared, includ-
ing their application in metal-catalyzed cross-coupling.[22–25]
Even though ionic liquids offer some advantages, the tedious
preparation of ionic liquids (and raw materials for ionic liquids)
and their environmental safety are still debated. To date, avail-
able liquid polymers or low-melting polymers, e.g. PEGs,[26–29]
Experimental
General Information
All chemicals were reagent grade. Both β-cyclodextrin and
N-methylurea (NMU) were purchased from Aladdin Chemistry Co.
Ltd. Field emission scanning electron microscopy (SEM) analyses
*
Correspondence to: Xiaohua Zhao, School of Materials Science and Engineering,
Jiangsu University, Zhenjiang 212013, China. E-mail: zhao12_19@163.com
a School of Materials Science and Engineering, Jiangsu University, Zhenjiang
212013, China
b School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing 210094, China
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.

X. Zhao et al.
were performed with a JEOL JSM-7001F instrument. IR spectra
were recorded in KBr disks with a Shimadzu IR Prestige-21 FT-IR
spectrometer. TG was performed on a DSC 204 differential
scanning calorimeter. 1H NMR spectra were measured with a
Bruker Avance III 500 analyzer. GC-MS analyses were performed
on a Saturn 2000 GC-MS instrument.
et al. showed that cavity-shaped ligands could bring an added
value to the efficiency of Suzuki–Miyaura coupling.[36] In this
report, we discovered that β-CD/NMU could provide a clear
viscose melt at 80°C, while the addition of N-methylurea was
necessary to achieve such low melting temperature (Fig. 1). To
evaluate the thermal stability of the mixture it was analyzed
through three heating–cooling cycles, which showed similar
melting points (78, 81 and 80°C, respectively). In addition,
the mixture was heated for 4 h to 90°C without any evident
decomposition. The novel medium was prepared easily and had
a small vapor pressure, just like ionic liquid. It was hence
tempting and logical to study its potential for Pd-catalyzed
coupling reactions.
At the outset, application of the novel system was first
investigated using palladium(II) in the Suzuki reaction, which
could also give the desired product in 45% yield (Table 1, entry
11). According to the widely accepted mechanism of Suzuki
coupling, the real catalyst is palladium(0) species. Ligands are
used to support the active palladium(0) species, thus preventing
it from deactivation. Native CDs prove to be insufficiently reactive
to reduce Pd(II) to Pd(0) under mild conditions.[37] It is worth
mentioning that previously Yao et al. proposed a mechanism in
which the acetate anion acted as a ligand in the rate-determining
oxidative addition step.[38] Compared to this mechanism, it
appears that the dramatic effect of NMU is due to the formation
of N,O-bidentate complex (Fig. 2). To some extent, NMU plays a
very important part in the coupling reaction, just like the ligand.
Moreover, urea compounds were reported to be effective
alternatives for phosphine ligands in Pd-catalyzed processes.[39,40]
Metal nanoparticles (NPs) have appeared as a very promising
approach to efficient catalysis under mild, environmentally
benign conditions because of their large surface-to-volume ratio.
Ligand-free Heck reactions, Suzuki reactions and Sonogashira
reactions catalyzed by modified cyclodextrin-capped Pd
nanoparticles in water have been reported.[41–45] However, in
contrast to a wide variety of traditional organic solvents or green
media such as low-melting polymers, the availability of water is
rather limited. The reactants and products of these reactions tend
to be rather insoluble in aqueous media. The CD-capped PdNPs
also slowly decomposed to Pd black in water at high
temperature, which is not catalytically active. To the best of our
knowledge, there are few examples employing cheaper native
β-CD-capped metal nanoparticles as catalyst for C–C coupling in
a low-melting mixture system – in particular, the Miyaura–Suzuki
reactions.
Preparation of β-Cyclodextrin-Capped Palladium Nanoparticles
(β-CD-supported Pd nanoparticles)
The procedure used in this work to prepare β-CD-supported Pd
nanoparticles was reported previously.[32] In a typical preparation,
146.8 mg Na2Cl4Pd and 100 mg native β-cyclodextrin were
dissolved in 80 ml DMF. This solution was then divided into four
parts. 300 mg NaBH4 was dissolved in 80 ml DMF under N2
atmosphere. The reaction was continued overnight. DMF was
then removed by centrifugation. The residue was washed four
times with 100 ml DMF and subsequently with 100 ml ethanol/
water (90/10) mixed solvent. The final product was redissolved
in 5 ml pure water in a 100 ml round-bottom flask and freeze-
dried to yield a deep-brown powder.
General Procedure for Suzuki Cross-Coupling Reactions
Under air atmosphere, a round-bottomed flask was charged
with aryl halide (1.0 mmol), phenylboronic acid (1.2 mmol),
K2CO3 (2.0 mmol), β-CD/NMU (3:7) (1 g), a catalytic amount of
water (no more than 3 drops: approximately 0.06 ml) and β-CD-
supported Pd nanoparticles (0.05 mol%). The mixture was heated
to 85°C for the indicated time and reaction progress was
monitored by thin-layer chromatography (TLC). After the reaction
completed, water (10 ml) and hexane (20 ml) were added while
the mixture was still hot. The organic layer was separated, dried
over Na2SO4 and evaporated. The crude product obtained was
purified by flash chromatography with PE/EtOAc as eluent to
afford the corresponding products. The remainder was dried
under vacuum for the next cycle.
General Procedure for Heck Cross-Coupling Reactions
Under air atmosphere, a round-bottom flask was charged with
aryl halides (1.0 mmol), terminal olefins (2.0 mmol), Et3N (2 mmol),
β-CD/NMU (3:7) (1 g) and β-CD-supported Pd nanoparticles (0.05
mol%). The mixture was heated to 85°C for the indicated time
and reaction progress was monitored by TLC. After the reaction
completed, water (10 ml) and hexane (20 ml) were added while
the mixture was still hot. The organic layer was separated, dried
over Na2SO4 and evaporated. The crude product obtained was
purified by flash chromatography with PE/EtOAc as eluent to
afford the corresponding products. The remainder was dried
under vacuum for the next cycle.
The procedure used in this work to prepare β-CD/Pd nanoparti-
cles was reported previously.[32] Native β-CD-supported Pd
nanoparticles were prepared by NaBH4 reduction of Na2Cl4Pd in
Results and Discussion
Cyclodextrins (CDs) are a class of naturally occurring receptors
which are cyclic oligosaccharides constituted of six (in α), seven
(in β) or eight (in γ) D-glucopyranose residues joined by α-1,4
linkages. Indeed, CD derivatives have commonly been applied
successfully in various organic reactions, especial involving green
chemistry. Among these, CDs have attracted a wide interest in
the synthesis and stabilization of transition metal nanoparticles
for carbon–carbon bond formation.[33–35] Furthermore, Matt
Figure 1. Mixture of β-cyclodextrin/N-methylurea (3/7) at room temperature
(left) and at 80°C (right).
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Appl. Organometal. Chem. (2014)

Suzuki and Heck couplings in low-melting CD/NMU mixtures
Table 1. Suzuki reaction of benzeneboronic acid with bromobenzene in low-melting β-cyclodextrin/NMU mixturesa
Entry
1
Base
KF
Pd (mol%)
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.01
0.05
0.1
Timeb (h)
Yieldc (%)
77
2
2
KOH
2
80
3
NaOAc
Et3N
4
55
4
2
82
5
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
2
76
6
2
90
7
2
95e
95f
68
8
2
9
6
10
11
12
2
86
1.5
6
95
45d
0.05
aReaction conditions: bromobenzene (1.0 mmol), phenylboronic acid (1.2 mmol), base (2.0 mmol), β-CD/NMU (3:7) (1 g).
bThe reaction was monitored by TLC.
cIsolated yield.
dPd(OAc)2 was used as catalyst.
eA catalytic amount of water was added (no more than 3 drops, approximately 0.06 ml).
fα-CD/NMU (3:7, melting point 75°C).
DMF solution, leading to the isolation of dark precipitates. Field
further characterization was conducted. Interestingly, the FT-IR
spectra of capped palladium nanoparticles and β-CD were very
similar. It seemed plausible that the primary structure of β-CD
is not involved in the reduction process but does play a role in
particle stabilization (Fig. 4). These conclusions agree with
previous observations.[37,46] The concentration of Pd loaded
onto the catalyst was calculated based on elemental analysis
and TG data, which indicated approximately 6% (w/w) of mean
palladium content.
emission SEM images of the isolated palladium nanoparticles
(Fig. 3), obtained as depicted in the Experimental section, show
spherical nanoparticles (with a size range of 20–30 nm ). In an
attempt to probe the native β-CD-supported Pd nanoparticles,
Having gained insight into the nature of the Pd nanoclusters
stabilized by native β-CD, we proceeded to evaluate the catalytic
performance of these nanoparticles in C–C cross-coupling
reactions. Our investigation of the activity of Suzuki–Miyaura
cross-coupling reaction was employed in this novel media using
native β-CD-supported Pd nanoparticles as catalyst and
Figure 2. Proposed N,O-bidentate intermediates using Pd(0) species as
catalyst in reported novel medium.
Figure 3. A typical SEM image of β-CD-supported Pd nanoparticles.
Appl. Organometal. Chem. (2014)
Figure 4. (a) Native β-CD; (b) β-CD-supported Pd nanoparticles.
Copyright © 2014 John Wiley & Sons, Ltd.
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X. Zhao et al.
optimizing the conditions in terms of bases, temperature (Table 1)
and the amount of catalyst. The coupling between bromo-
benzene and benzeneboronic acid was chosen as a model
reaction, and the results are summarized in Table 1. As demon-
strated in the table, the best yield was not obtained until the
Suzuki reaction was carried at 85°C in low-melting β-CD/NMU
mixture using K2CO3 as base and 0.05 mol% Pd. It is worth noting
that a catalytic amount of water is necessary to improve the
reaction (Table 1, entry 7). It is possible that in neat low-melting
β-CD/NMU mixture Pd nanoparticles were overly stabilized and
that the substrates were unable to compete effectively with the
catalyst. The addition of water may lead to opening up of active
sites on the catalyst surface. Under the same conditions as
described above, the result showed there was not a significant
beneficial effect on the performance of other cyclodextrins
during the model reaction with lower-melting mixture as solvent
(α-CD/NMU 3/7, melting point 75°C; Table 1, entry 8).
With the optimized reaction conditions in hand, we next
examined the application of the cross-coupling of a variety of aryl
halides with several arylboronic acids in the novel media. As
expected, aryl iodides as the substrate under the same conditions
could afford excellent yields and the conversion rates were faster
than with other aryl halides, which were reflected in the shorter
reaction time (Table 2, entries 1–3). It was found that both
electron-rich and electron-deficient aryl bromides delivered the
desired products with high yields in approximately 2 h.
Furthermore, the catalytic system could tolerate a broad range
of functional groups, such as NO2, NH2, OMe and CN (Table 2,
entries 4–7). It was worth noting that the coupling of
4-bromochlorobenzene with phenylboronic acid gave exclusively
4-chlorobiphenyl in 93% yield, showing good chemoselectivity
(Table 2, entry 10). To our delight, aryl chlorides gave the
corresponding product in moderate yields – in particular,
activated electron-deficient aryl chlorides – although a prolonged
reaction time and increased usage of β-CD-supported Pd
nanoparticles were required (Table 2, entries 11–14).
Encouraged by the success for Suzuki coupling, we further
extended the application of the novel media to Heck coupling,
which is another most important palladium-catalyzed C–C
bond-forming reaction. Initially, we explored the conditions for
the Heck reaction using iodobenzene and methyl acrylate
(Table 3, entry 9) as the reactants. When no base was used in this
reaction, only a trace amount of the desired product was
detected. The results in Table 3 also demonstrated that Et3N as
the base was the best choice for the cross-coupling reaction,
other bases such as KOH, Na2CO3 and K2CO3 being less effective.
Many aryl iodides including electron-poor and electron-rich
substrates could react with terminal olefins within a short time
(approximately 2 h) and the yields of corresponding coupling
products were very satisfactory (Table 3, entries 1–9). Compared
to aryl iodides, poorer yields of coupling products were obtained
when aryl bromides were used as substrates, even if longer
reaction time and higher amounts of catalyst were required.
(Table 3, entries 10–13).
The reusability of the catalyst or catalyst system is a very
important theme, especially for commercial application. The
advantage of the novel catalyst system (β-CD-supported Pd
nanoparticles/β-CD/NMU) lies not only in the high catalytic
activity attributed to the formation of C–C bond, but also in the
ease of separation and recyclability. Recycling experiments were
carried out for the cross-coupling of bromobenzene and
benzeneboronic acid under the conditions described in Table 1.
After the reaction was finished, water was added to the reaction
mixture while still hot and the desired product was isolated
with hexane. Water was removed from the aqueous phase by
applying reduced pressure and the residue containing the Pd
Table 2. Suzuki reaction of aryl halides with phenylboronic acid in low-melting β-cyclodextrin/NMU mixturesa
Entry
X
R1
4-NO2
Time b (h)
Yield c (%)
98
1
I
1
2
I
H
1
1
98
3
I
4-OCH3
4-NO2
4-NH2
4-OCH3
4-CN
H
96
4
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
2
97
5
2
96
6
2
91
7
2
90
8
2
95
9
4-F
2
92
10
11
12
13
14
Cl
2
93
4-NO2
4-CHO
H
18
18
30
40
82d
76d
74d
46d
4-CH3
aReaction conditions: bromobenzene (1.0 mmol), phenylboronic acid (1.2 mmol), base (2.0 mmol), β-CD/NMU
(3:7) (1 g), catalytic amount of water (no more than 3 drops, approximately 0.06 ml).
bThe reaction was monitored by TLC.
cIsolated yield.
dPd was 0.5 mol%.
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Appl. Organometal. Chem. (2014)

Suzuki and Heck couplings in low-melting CD/NMU mixtures
Table 3. Heck reaction of various aryl halides with terminal olefins in low-melting β-cyclodextrin/NMU mixturesa
Entry
1
R1
3-F
R2
X
Timeb (h)
Yieldc (%)
95
Ph(4-F)
Ph(4-F)
Ph(4-F)
Ph(4-F)
Ph(4-F)
Ph(4-Cl)
Ph(4-Cl)
Ph(4-Cl)
COOMe
COOMe
COOEt
COOMe
COOEt
I
2
2
2
4-CH3
4-OCH3
4-Cl
H
I
96
3
I
2
93
4
I
2
95
5
I
2
92
6
3-F
I
2
90
7
4-OCH3
4-Cl
H
I
2
94
8
I
2
95
9
I
2
94
10
11
12
13
H
Br
Br
Br
Br
10
10
10
10
66d
71d
55d
76d
H
4-NO2
4-NO2
aReaction conditions: ArX (1.0 mmol), terminal olefins (2.0 equiv.), Et3N (2.0 equiv.), β-CD/NMU (3:7) (1 g), Pd ( 0.05 mol%), 85°C (external).
bThe reaction was monitored by TLC.
cIsolated yield.
dPd (0.5 mol%); the temperature was 110°C.
nanoparticles was loaded with reactants and base. It is noted
that a little water in the residue would not adversely affect the
cross-coupling reaction, as previous researchers discovered that
water could promote this typical reaction effectively. As shown
in Fig. 5(a), the catalytic activity of the recovered β-CD-supported
Pd nanoparticles/β-CD/NMU was consistent with efficient stability
after four runs. More agglomeration of the real catalyst Pd
nanoparticles was observed in the used catalyst (Fig. 5b). As a
result, a decrease in catalytic activity was observed in the fifth
and sixth recycling experiments, which indicated that particles
size had a clear correlation with catalytic activity during the
cycles. The conclusion is also consistent with the well-established
finding.[37]
Conclusion
In summary, we have presented the use of a low-melting mixture
of β-cyclodextrin/N-methylurea (NMU) as reaction media for
Suzuki–Miyaura and Heck reaction in the presence of β-CD-
supported Pd nanoparticles without any ligand. The advantages
of our new method include mild reaction conditions, ease of
workup, high yields, stability and easy isolation of the
compounds. In addition, the non-toxic reaction media could be
applied to organic transformations as a substitute for ionic
liquids. Currently, further studies are underway in our laboratory,
addressing extension of the system to other palladium-catalyzed
transformations.
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Figure 5. (a) Catalytic activity of the recycled β-CD-supported Pd
nanoparticles/β-CD/NMU mixture; (b) SEM image of the sixth reused
native β-CD-supported Pd nanoparticles.
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Appl. Organometal. Chem. (2014)