Imidazolium-functionalized β-cyclodextrin as a highly recyclable multifunctional ligand in water

Article abstract of DOI:10.1039/c4gc00365a

We describe here the synthesis and the catalytic properties of a novel dodecyl-imidazolium modified β-cyclodextrin as a self-assembled catalytic system (Fig. 1) in neat water for an effective Suzuki-Miyaura reaction. The introduction of the dodecyl-imidazolium motif on the primary face of the β-cyclodextrin allowed the development of a green highly recyclable catalytic system for reactions in an aqueous environment. We present the application of this system to the Suzuki-Miyaura coupling without the use of a co-solvent or stabilizing phosphine ligands in aqueous media. The catalytic system is highly recyclable, allowing the reuse of the palladium catalyst in subsequent catalytic runs without loss of activity. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00365a

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Imidazolium-functionalized β-cyclodextrin as a
highly recyclable multifunctional ligand in water†
Cite this: DOI: 10.1039/c4gc00365a
Vanessa Kairouz and Andreea R. Schmitzer*
We describe here the synthesis and the catalytic properties of a novel dodecyl-imidazolium modified
β-cyclodextrin as a self-assembled catalytic system (Fig. 1) in neat water for an effective Suzuki–Miyaura
reaction. The introduction of the dodecyl-imidazolium motif on the primary face of the β-cyclodextrin
allowed the development of a green highly recyclable catalytic system for reactions in an aqueous
environment. We present the application of this system to the Suzuki–Miyaura coupling without the use
of a co-solvent or stabilizing phosphine ligands in aqueous media. The catalytic system is highly recycl-
able, allowing the reuse of the palladium catalyst in subsequent catalytic runs without loss of activity.
Received 28th February 2014,
Accepted 18th March 2014
DOI: 10.1039/c4gc00365a
www.rsc.org/greenchem
stability toward oxygen and moisture. Their strong σ-donor
character and robust bonds to metals make NHC metal com-
Introduction
For the past few years, chemists have been facing many chal- plexes excellent catalysts for a wide range of organometallic
lenges in their metamorphosis to become green chemists. reactions. Although several examples of NHC water soluble
Today, the development of environmentally friendly processes complexes have been reported in the literature, transformation
to convert molecules into products of interest, using clean of hydrophobic substrates into desired products without the
transformations and technologies in accordance with the 12 use of toxic co-solvents and exhaustive procedures remains a
2
0–27
principles of green chemistry, is a major goal. Great effort was challenge.
made to replace traditional organic solvents, often incriminated Among various approaches developed to counter the poor
due to pollution problems, by environmentally benign reaction substrate solubility, mass transfer strategies employing
1
–5
media.
Neat water remains a less harmful solvent for the hydrophilic carriers such as cyclodextrins (CDs) have shown
4
,6
28–33
synthesis of many target molecules. With the expansion in promising potential in aqueous catalysis.
CDs are cyclic
finding new routes for reactions in water combined with the oligosaccharides composed of 6 to 8 units of α-1,4 linked
emergence of metal catalyzed reactions, there is an urge to D-glucopyranose in a rigid truncate conic structure, displaying
7
8
develop mass transfer strategies and water soluble ligands.
a hydrophilic external rim and a hydrophobic cavity. The dis-
In this context, 2010 Nobel Prize awarded palladium cross parity between both rims leads to solubilization of hydro-
9
,10
coupling reactions,
powerful tools for C–C bond formation phobic molecules in aqueous media via reversible
1
1–16
widely used in both industry and academia,
are an impor- complexation in the hydrophobic cavity of the CD. Although
tant target for catalysis in water. Commonly, when performed the use of CDs as a mass transfer strategy was previously
in water, these reactions require the use of amphiphile phos- described in catalysis by improving the catalytic activity of the
9
,10,14,17
phine ligands.
upon storage, require handling under nitrogen and generate the reaction conditions.
hazardous byproducts. Furthermore, the lack of commercial Our strategy was the construction of a single amphiphile
sources for the generation of new phosphine ligands, as well bimodal ligand by the combination of a mass transfer unit, co-
These ligands suffer from poor stability coupling reaction in water, recycling was not possible under
7
,34
2
5,31,35
as their extensive and often expensive synthetic routes, makes valently bound to a ligand moiety for aqueous catalysis.
them not so suitable targets for green chemistry.
1
1
A major advantage of combining both cyclodextrins and imid-
In this context, water soluble N-heterocyclic carbene (NHC) azolium salts, precursors of the NHC ligand, is the versatility
ligands emerged as potential candidates for palladium cross of the hybrid properties of the resulting system, combining
1
8,19
coupling reactions in water.
These ligands feature good those of hydrophilic carriers and NHCs. To the best of our
3
5
knowledge, Monflier et al. were the first to report the syn-
thesis of an NHC-appended methylated CD as a catalyst in
dioxane. Nonetheless, a major drawback of their system was
the inefficient catalysis in neat water and the absence of recy-
cling possibility, due to palladium degradation.
Département de Chimie, Université de Montréal, 2900 Edouard Montpetit CP 6128
succ. Centre ville, Montréal, Qc, Canada. E-mail: ar.schmitzer@umontreal.ca
Electronic supplementary information (ESI) available: Materials and character-
†
isation, synthesis, and recycling procedure. See DOI: 10.1039/c4gc00365a
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Fig. 1 Schematic representation of the self-assembly of 4 in water.
Herein, we report the synthesis, characterization and cata-
lytic properties of a novel alkyl imidazolium modified β-CD 4.
Results and discussion
We demonstrate that the introduction of a dodecyl chain on The synthetic pathway to obtain the alkyl imidazolium modi-
the imidazolium moiety attached to the primary face of a fied β-CD 4 is shown in Scheme 1. The mono-6-tosyl-β-cyclo-
native β-CD allows the formation of a micellar self-assembled dextrin 2 can be easily obtained via tosylation of the
3
6,37
38
catalytic system
(Fig. 1) in neat water with highly recyclable commercially available β-CD with the tosyl precursor 1. Sub-
properties in the Suzuki–Miyaura reaction.
sequent reaction of 2 with 1-dodecyl-imidazole 3 followed by
Scheme 1 Synthesis of 4.
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anion-exchange on Amberlite Cl− resin afforded the desired were observed (see ESI†). No self-inclusion or intramolecular
amphiphile imidazolium-functionalized β-CD 4 as a pale inclusion of the hydrophobic alkyl chain into the hydrophobic
yellow solid, stable towards oxygen and moisture. The syn- cavity of the β-CD was observed (see ESI†).
thesis of this NHC-precursor was achieved in a 4 step straight-
Having in hand a self-assembled bimodal ligand precursor,
forward synthesis with 12% overall yield.
the catalytic activity of 4 was evaluated in the Suzuki–Miyaura
To gain insights into the self-assembly properties of 4 in cross coupling reaction in neat water. In all the experiments,
aqueous media, the evolution of its surface tension (∏) versus the catalyst was formed in situ. Initial studies were carried out
the concentration in water is shown in Fig. 2. At 25 °C, upon in water using phenyl boronic acid (1.2 equiv.) and 4-bromo-
increasing the concentration of 4, the curve exhibits a first acetophenone (1 equiv.) as model substrates in the presence of
inflexion point at 100 μM, suggesting the presence of a pre- 1 mol% (1 mM) of 4 and 0.5 mol% of Pd(OAc)
2
with 2 equiv. of
aggregation state, followed by micelle formation at 500 μM different bases at 100 °C. A 4 : Pd 2 : 1 ratio was chosen as the
(
critical micellar concentration, CMC). These two states are starting point in order to obtain a better stabilization of the
3
5
also present at 75 °C, where micelle formation occurs at catalytic species, as previously proposed.
As shown in
2
50 μM. As the temperature increases, compound 4 takes more Table 1, the presence of the base is required for the reaction to
space at the air–water interface due to the fluidity and mobility occur (entry 1). All tested bases gave the desired coupling
of the alkyl chain, and the CMC is reached at lower concen- product in high yields (entries 2–8). The next step consisted of
trations. The CMC values are low, which indicates that fewer examining different Pd derivatives. Both Pd(II) and Pd(0)
molecules are required to form a micelle and hence allows sig- sources presented in Table 2 promoted the formation of the
nificant atom economy when micellar catalysis is performed in desired biaryl products in good yields. Nonetheless, only
the presence of this compound. Furthermore, the hydrophobic Pd(OAc) in the presence of Cs CO inhibits the degradation
2
2
3
core of the micelle, along with the mass transfer units (β-CDs), and precipitation of the palladium species in the form of black
forms several hydrophobic clusters among a single micelle Pd. Even if good results in terms of conversion were observed
(
Fig. 1), enhancing the solubilization of hydrophobic sub- in the presence of less toxic K CO , palladium precipitation
2 3
strates. The formation of micelles is also supported by 2D was observed at the end of the reaction. Considering the
ROESY NMR experiments where only head to head (β-CD/ recycling perspectives, Cs CO and Pd(OAc) were used for
2
3
2
β-CD) and tail to tail (alkyl chain/alkyl chain) interactions of 4 subsequent optimizations. Afterwards, catalyst loading was
investigated (Table 3). In the absence of 4, no reaction was
Table 1 Optimisation of the nature of the base
Entry
Base
Isolated yield (%)
1
2
3
4
5
6
7
8
—
7
95
93
87
83
85
93
84
Cs
CsF
CO
2
CO
3
K
2
3
NaOH
KOH
K PO
3 4
3
Et N
Table 2 Optimisation of the nature of the Pd source
Entry
Pd
Isolated yield (%)
1
2
3
4
Pd(OAc)
2
95
90
92
84
PdCl
Pd(ACN)
Pd (dba)
3
2
2
Cl
2
Fig. 2 Surface tension variation of 4 in water at (a) 25 °C and (b) 75 °C.
2
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Table 3 Variation of the catalyst loading
mation of a metal–ligand (M–L) complex is also supported by
1
3
the C NMR spectrum, where an upfield shift of the C-2 from
34.8 ppm for 4 to 138.2 and 147 ppm for the M–L complex
Fig. 4) can be observed. The existence of two different signals
1
(
for C-2 of the M–L complex could be related to the presence of
two catalytic species in solution: 4-Pd-4 (bis-(imidazol-2-
ylidene) Pd) at 138.2 ppm and Pd-4 (mono imidazol-2-ylidene
Entry
a
b
Isolated yield (%)
3
5,39,40
Pd) at 147 ppm.
Since the Suzuki–Miyaura cross coup-
1
2
3
4
5
6
7
0
1
1
1
0.5
0.2
0.1
0.5
0.5
0.3
0.1
0.5
0.1
0.05
0
95
95
95
95
95
95
ling reactions are performed in a micellar state, the formation
of both mono- and bis-imidazol-2-ylidene Pd structures is
possible (Fig. 5). Also, the C-4 and C-5 upfield shifts respecti-
vely from 123.1 and 125.1 ppm (Fig. 4a) to 126.0 and
1
28.6 ppm (Fig. 4b) are consistent with the formation of an
3
5,39,40
M–L complex.
Unfortunately, further attempts to
confirm the exact structure of the complex were unsuccessful
observed (entry 1). In contrast, C–C bond formation leading to due to the poor stability of the complex under ESI† conditions
the desired product is efficient at a loading as low as 0.05 mol% and the difficulty to obtain suitable crystals for X-ray analysis.
of Pd(OAc)
the surface tension curve (Fig. 2), a loading of 1 mol% of 4 imidazolium-β-CD 5, which inhibits the formation of an NHC
entries 2 and 3) correlates to the presence of micelles in the and prevents the formation of an M–L complex, afforded the
reaction media, while a loading of 0.1 to 0.5 mol% of 4 desired biaryl in comparable yields to those obtained in
entries 4–6) corresponds respectively to a transitory state the presence of 4. However, major palladium degradation in
2
and 0.1 mol% of 4 (entry 6). In accordance with An experiment conducted with the C-2 methylated dodecyl
(
(
between the monomer, the pre-aggregate and the micelles. the form of black Pd was observed and efficient recycling was
The efficient catalysis at low catalyst loading suggests that the not possible in this case. These results suggest that the hydro-
monomeric complex formed by 4 and Pd(OAc) is the active phobic clusters (Fig. 1) contribute to the formation of the
2
catalytic species. However, a slight palladium leak was coupling product via a ligandless catalytic process but are
observed at lower catalyst loadings (entries 5 and 6), indicating enable to anchor the metal to the micelle in order to allow
8
,34
that the self-assembly of 4 plays a major role in terms of stabi- recycling of the catalyst.
lity of this catalytic system in water. Again, with recycling goals observed in the presence of 4 (Table 5, entry 1), suggesting
Highly efficient recycling was
in mind, 1 mol% of 4 and 0.5 mol% of Pd(OAc)
2
(entry 2, 2 : 1 that, along with the cooperative catalytic processes combining
ratio) were retained for further optimizations.
the hydrophobic clusters and NHC formation, the Pd adhere
In order to gain a better understanding of the interactions to the self-assembled system via the formation of an M–L
1
13
between 4 and Pd(OAc)
2
, H and C NMR spectroscopy experi- complex.
1
ments were conducted in deuterium oxide (Fig. 3). The
H
With the optimized conditions in hand, the scope of the
NMR spectrum shows quantitative disappearance of the H-2 Suzuki–Miyaura cross coupling reaction with aryl halides and
resonance at 8.8 ppm jointly with a slight shielding of the H-4 boronic acid derivatives catalyzed by the self-assembled
and H-5 protons (Fig. 3) after addition of Pd(OAc) . The for- bimodal ligand precursor 4 and Pd(OAc) in water was investi-
2
2
1
Fig. 3 Partial H NMR spectra (25 °C, 700 MHz, D
2
2 3
O) of (a) 4 and (b) 2 : 1 ratio of 4 : Pd in the presence of Cs CO .
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1
3
2 2 3
Fig. 4 Partial C NMR spectra (25 °C, 700 MHz, D O) of (a) 4 and (b) 2 : 1 ratio of 4 : Pd in the presence of Cs CO .
Fig. 5 Simplified representation of the proposed mono- and bis-4-Pd structures in water.
gated. A first set of aryl halides with the phenylboronic acid azolium-β-cyclodextrin 4 : Pd catalytic species is possible.
was used in order to test the activity and tolerance of the cata- Herein we developed a simple procedure to recycle the phase
lytic system to various functional groups. As shown in Table 4, containing catalyst (Fig. 6). The recyclability of the catalytic
all reactions gave the corresponding hetero coupling products system is directly related to the self-assembled state of 4
in moderate to high yields despite the nature of the halide or (Fig. 2). As shown in Table 5, more than 10 catalytic cycles
the presence of electron donating groups and electron with- (entry 1) were performed without significant loss of activity in
drawing groups as substrates (Table 4, entries 1–8). Moreover, the micellar state. The number of possible catalytic cycles
traditionally challenging reactions in neat water involving decreases to 6 (entry 2) when 4 was used at a concentration
naphthalene substrates which have increased hydrophobicity corresponding to the pre-aggregation state and to 2 (entry 3)
and potential steric hindrance were achieved in good yields when used at a monomeric concentration. The presence of the
(Table 4, entries 9–11). To the best of our knowledge, this is same size of aggregates was confirmed by DLS measurements
the first example reporting a Suzuki–Miyaura cross coupling of the 4 : Pd micellar solution before the reaction and in the
reaction in neat water with sterically hindered and strong recycled aqueous solution (see ESI†). In fact, the self-assembly,
hydrophobic character substrates without the use of degassed resulting from the combination of the β-CD covalently bound
water, nor a co-solvent or a N
2
atmosphere.
to the dodecyl imidazolium moiety, adds robustness to the cat-
From the green chemistry point of view, recycling is a major alytic system and enhances its recyclability.
concern, especially in metal catalysis. In contrast to traditional
To further investigate the importance of the combination of
water soluble M–L complexes, recycling of the alkyl imid- the mass transfer unit covalently bound to a ligand moiety, the
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Table 4 Scope of the alkyl imidazolium β-CD (4)-Pd catalysed Suzuki–
Table 5 Recyclability of the 4 : Pd catalyst
Miyaura cross coupling reactions in neat water
Entry
1
Reactant
Product
Isolated yield (%)
83
2
91
Entry
a
b
Isolated yields (%)
No. of cycles
1
2
3
1
0.2
0.1
0.5
0.1
0.05
95–80
90–77
95–79
10
6
2
3
4
5
6
76
60
79
87
75
59
66
catalytic activity in neat water of the self-assembled ligand pre-
cursor 6 in the presence and absence of free native β-CD was
investigated in the Suzuki–Miyaura cross coupling reaction
(
Table 6). As expected, the presence of 6 at a concentration
a
7
above its CMC, with or without the unbound β-CD, allowed the
solubilisation of the substrates and the Pd species, as well as
the formation of the desired coupling products with compar-
able yields to the 4 : Pd catalytic system. However, in this
system, a poor recyclability was observed (entries 1 and 4). Yet,
adding free β-CD does not improve the catalytic activity of this
system and completely inhibits the recycling of the catalyst
8
a
9
(
entries 2 and 5). These results are consistent with similar
a
1
0
79
69
8,30,37
micellar systems reported in the literature.
Therefore, the
use of β-CD covalently bound to the dodecyl chain imid-
azolium salt greatly enhances the stability of the catalytic
system, allowing both efficient catalysis and recycling in neat
water. This result supports the hypothesis of protection of the
NHC moiety by the hydrophilic CD close to the metal centre.
The presence of the covalent CD may also modify the shape
and the space around the metal, resulting in a stable micellar
complex.
11
a
t = 2 h.
Fig. 6 Recycling procedure of the 4 : Pd catalytic solution: (1) in situ formation of the catalytic species. (2) Addition of boronic acid and aryl
bromide. (3) Stirring for 30 min at 100 °C. (4) Cooling down to 23 °C. (5) Extraction with Et
2
O. (6) Evaporation of the solvent. (7) Reuse of the
aqueous phase containing 4 : Pd in subsequent catalytic runs.
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Table 6 Contribution of each component to the recyclability of the ditions, the self-assembled bimodal system tolerated various
catalytic system
electronics and bulky substrates. Hence, the combination of
the β-CD with the dodecyl chain imidazolium motif, a mass
transfer unit with a self-assembled ligand precursor, cherishes
the hybrid properties of green catalysis along with the robust-
ness of a self-assembled system. Future work is devoted to
determine the precise structure of the M–L complex and to the
study of the versatility of this self-assembled bimodal system
Entry
1
Substrate
β-CD
Isolated yield (%)
No. of cycles
3
toward other important C–C bond forming reactions as novel
greener alternatives.
—
95–82
2
3
95
79
0
0
a
—
General procedure for the Suzuki–
Miyaura cross coupling reaction
a
4
77
0
In a typical experiment, Pd(OAc)2 (0.005 mmol) and Cs CO
2
3
(
2 mmol) were added to a 1 mM solution of 4 (0.01 mmol) in
1
2
0 mL of water. The catalytic solution is stirred at 100 °C for
h. Phenylboronic acid (1.2 mmol) and arylhalide (1 mmol)
a
t = 2 h.
were then added to the catalytic solution. The reaction
Having shown the higher recyclability of our catalytic medium was stirred at 100 °C for 30 min. Afterwards, the reac-
tion mixture was cooled to room temperature and the coupling
in the context of already described green processes for the product was extracted with 3 × 5 mL of Et O. After drying over
Suzuki–Miyaura coupling. Among already described similar MgSO , the solvent was evaporated in vacuo and the desired
product was purified by flash column chromatography on
process in water, we were interested in positioning our system
2
4
processes, in terms of efficiency, are the ones recently
4
1
42
described by Zhang and Liu. Zhang’s water-soluble cyclo- silica gel to obtain the pure product.
dextrin–palladium complex is similar to ours in terms of syn-
thetic chemistry employed for the synthesis of the catalyst and
the use of small amounts of organic solvents and reactants.
However, in our case, the recyclability of the system increases
the mass intensity, and the synthetic steps can be neglected process
compared to the benefits of the recycling. In the case of Liu’s
General procedure for the recycling
After performing a typical Suzuki–Miyaura cross coupling reac-
tion (Fig. 6, 1–6) as described in the previous section, Cs CO
ligand free protocol, 6 grams of polyethylene glycol (PEG) are
required as a co-solvent for the synthesis of 1 mmol of coup-
2
3
(
1 mmol), boronic acid (1.2 mmol) and aryl halide (1 mmol)
ling product while using 2 mol% of Pd(OAc) and 2 mmol of
2
were added to the remaining aqueous phase (Fig. 6, 7). The
reaction medium was then stirred for 30 min at 100 °C (Fig. 6,
2–3). After cooling to room temperature, the coupling product
K CO (mass intensity of the process was estimated to be
2
3
0.74 kg kg−1). Compared to this process, our system is more
3
−
1
efficient, as 1.36 kg kg mass intensity was calculated for the
−1
2
was extracted with 3 × 5 mL of Et O (Fig. 6, 4–5). The organic
synthesis of 4 : Pd and 2.61 kg kg
mmol of the coupling product.
for the synthesis of
phase was then dried over MgSO , the solvent was evaporated
4
1
in vacuo and the desired product was purified by flash column
chromatography on silica gel to obtain the pure product
(
Fig. 6, 6). The aqueous phase is once again available for sub-
Conclusion
sequent cycles (Fig. 6, 7).
As a contribution towards a greener chemistry, we character-
ized and studied the catalytic properties of a novel dodecyl
imidazolium modified β-CD 4 in neat water as a catalyst for
the Suzuki–Miyaura coupling reaction. The introduction of a
Acknowledgements
dodecyl chain on the imidazolium moiety attached to the This work was supported by the Natural Sciences and Engin-
primary face of a native β-cyclodextrin allows the formation of eering Research Council of Canada, the Fonds Quebécois de la
a highly stable self-assembled catalytic reactor for the Suzuki– Recherche sur la Nature et les Technologies, the Centre of
Miyaura cross coupling reaction in neat water. High catalytic Green Chemistry and Catalysis, the Canada Foundation for
activity and multiple time recyclability were achieved as a Innovation, and the Université de Montreal. We thank col-
result of the cooperative catalytic processes combining hydro- leagues for
philic carriers and NHCs. Under the optimal reaction con- manuscript.
a
careful reading and discussion of this
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2
3 Z. Y. Wang, G. Q. Chen and L. X. Shao, J. Org. Chem., 2012,
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