Silica-supported metal acetylacetonate catalysts with a robust and flexible linker constructed by using 2-butoxy-3,4-dihydropyrans as dual anchoring reagents and ligand donors

Article abstract of DOI:10.1039/c5cy01012h

Anchoring of a metal complex catalyst onto a solid support with a covalent bond has been widely used in designing a recyclable catalyst at molecular level. However, the performance of the obtained solid catalyst relies heavily on the stability and flexibility of the linker between the metal complex and the solid support. A ring-opening reaction of 2-butoxy-3,4-dihydropyrans with mercaptan was known to produce 2-alkylated 1,3-dicarbonyl compounds, which featured good atom economy, excellent yield and mild conditions. With the aid of this reaction, we used, in this work, 2-butoxy-3,4-dihydropyrans as dual anchoring reagents and ligand donors to modify a ready-made SH-functionalized HMS. This opened an easy way to construct a robust and flexible linker for anchoring a metal acetylacetonate complex catalyst onto the HMS support. The thereby obtained HMS can be used to immobilize Cu(acac)₂, Zn(acac)₂ and Ru(acac)₃ complexes. The obtained solid catalysts were fully characterized by many physicochemical methods. In the selected reactions, these catalysts not only displayed better or comparable activity in various organic reactions as compared with their homogeneous counterparts but also were proved to be quite robust and can be recycled several times without significant loss of their activities.

Full text of DOI:10.1039/c5cy01012h

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Silica-supported metal acetylacetonate catalysts with a robust
and flexible linker constructed by using 2-butoxy-3,4-
dihydropyrans as dual anchoring reagents and ligand donors
Received 00th January 20xx,
Accepted 00th January 20xx
Bingbing Lai,a Zhipeng Huang, aZhifang Jia,a Rongxian Baia,* and Yanlong Gua,b,
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Anchoring of a metal complex catalyst onto a solid support with covalent bond has been widely used in designing a
recyclable catalyst at molecular level. However, performance of the obtained solid catalyst relies heavily on the stability and
flexibility of the linker between metal complex and solid support. A ringꢀopening reaction of 2ꢀbutoxyꢀ3,4ꢀdihydropyrans
with mercaptan was known to produce 2ꢀalkylated 1,3ꢀdicarbonyl compounds, which featured also by good atomꢀeconomy,
excellent yield and mild conditions. With the aid of this reaction, we used, in this work, 2ꢀbutoxyꢀ3,4ꢀdihydropyrans as dual
anchoring reagents and ligand donors to modify a readyꢀmade SHꢀfunctionalized HMS. This opened an easy way to
construct a robust and flexible linker for anchoring a metal acetylacetonate complex catalyst onto HMS support. The thereby
obtained HMS can be used to immobilize Cu(acac)2, Zn(acac)2 and Ru(acac)3 complexes. The obtained solid catalysts were
fully characterized by many physicochemical methods. In the selected reactions, these catalysts not only displayed better or
comparable activity in various organic reactions as compared with heir homogeneous counterparts, but also were proved to
be quite robust and can be recycled several times without significant loss of their activities.
modified. As a result, the prices of these ligands are generally very
high, so that their recovery and reuse are in general imperative.6
Out of these considerations, many immobilization methods of 1,3-
Introduction
Heterogenizing homogeneous catalysts by immobilization is a trend
toward the development of chemically homogeneous but physically
heterogeneous catalysts.1 Anchoring of a metal complex catalyst
onto a solid support with covalent bond has been widely used in
diketones have been developed. For example, by using a NH2-
containing solid support, an acetylacetone ligand can be anchored
via formation of an enamine.7 Because of the nucleophilicity of C2
position of acetylacetone, this ligand can also be anchored onto a
solid support through a nucleophilic substitution reaction.8 An
acetylacetone fragment has also been covalently bonded to an
imidazolium-based ionic liquid.9 These approaches offered in theory
possible way to solve the problems of recycling metal complexes of
1,3-diketones. However, as a matter of fact, their practical uses
often encountered several obstacles. First, the enamine formation
strategy changed the core structure of 1,3-diketone, diminishing
thus the catalytic activity of the catalyst. Second, the commonly
used anchoring reagents employed to establish a linker between
1,3-diketone ligand and the solid support, such as 3-
chloropropyltrimethoxysilane and aminopropyltrimethoxysilane,
contain only three carbons. It is therefore conceivable that the
immobilized metal complexes are less flexible. As a result, despite
designing
a
recyclable catalyst at molecular level.2 However,
performance of the obtained solid catalyst relies heavily on the
stability and flexibility of the linker between metal complex and
solid support.3
Metal complexes of 1,3-diketone have been widely used as
catalysts in organic reactions.4 Because these compounds are
readily soluble in organic solvents, the catalytic reactions have long
been plagued by the following difficulties: (i) recycling metal
complex catalysts; and (ii) removing metal species from organic
product. 1,3-Diketones have also been used as ligands in selective
extraction of metal ions.5 However, to increase the efficiency of
extraction, the diketone structure has to be finely manipulated and
the fact that this strategy reached
a compromise between
accessibility and efficiency, but failed to give a highly active catalyst.
The ionic liquid with a 1,3-diketone tail associated however with the
use of some reagents that are not easily available and expensive.
Their preparations are also hard tasks. Therefore there is a definite
need to develop a simple and an efficient method to anchor 1,3-
diketone fragment onto solid support with a structurally flexible
reagent.
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We have recently developed a ring-opening reaction of 2-alkoxy-
Journal Name
3,4-dihydropyrans with a nucleophile, which is able to produce 2-
substituted 1,3-dicarbonyl compounds under mild conditions.10
Particularly, when mercaptans were used as nucleophiles, the ring-
opening reactions proceeded very smoothly, and almost
quantitative yields could be obtained.11 Taking a cue from these
observations, we envisaged that, by using 2-alkoxy-3,4-
dihydropyrans as dual anchoring reagents and ligand donors, an
easily accessible SH-functionalized silica can be derivatized to a
hitherto unreported organic/inorganic hybrid material featured by a
robust and flexible 1,3-diketone tail. We have therefore initiated an
investigation into the preparation of this novel hybrid material.
Herein, we disclose that this method is indeed feasible, and the
obtained 1,3-diketone-functionalized material can be used in
immobilizing Cu, Zn and Ru complexes. Benefiting from the good
flexibility of the ligand linker, the obtained catalysts displayed
remarkable catalytic activity in several organic reactions. Taking into
account the fact that many 1,3-diketones can be used to prepare
the substituted 3,4-dihydropyrans through a simple procedure,12
this approach should be applicable for implementing the
immobilization of many 1,3-diketone ligands.
Figure 1. Silica-supported catalysts with different metals in this
work.
(Figure S1). By means of elemental analysis, the loading of SH group
in HMS-SH was confirmed to be 2.21 mmol/g. To facilitate the
reaction, excess amount of 1a was used (SH/1a = 1/1). Amount of
ZnCl2 catalyst was also doubled. After 10 hours of reaction in
o
nitromethane at 80 C, an acetylacetone-modified HMS, HMS-acac,
was obtained and submitted to elemental analysis. The carbon and
hydrogen contents increased significantly, indicating occurrence of
the grafting. Amount of the grafted acac fragment on the HMS is
0.67 mmol/g approximately (calculated based on the carbon
context). IR spectroscopy was also used to elucidate the occurrence
of the expected grafting reaction. As shown in Figure 2, all the
hybrid materials showed characteristic peaks of –CH2 stretching
band at 2930 cm-1. The disappearance of the S–H stretching band at
2450 cm-1, accompanied by the appearance of a characteristic band
of carbonyl group in 1700 cm-1, constitutes strong evidence for the
nondestructive tethering of an acac fragment. Characterization with
13C MAS NMR spectra revealed also successful anchoring of acac
onto HMS-SH because of the apprearance of two peaks around 200
ppm, which can be assigned to the keto-carbonyl group (Figure 3).
These spectra showed also that the latter contained more aliphatic
carbons than the former. Two characteristic carbons of the ring-
opening product of 1a with mercaptan, including (i) a carbon
between two sulfur atoms and (ii) C3 carbon of the diketone
Results and Discussion
Because a SH-functionalized HMS material that contains a high
loading of mercapto group can be easily synthesized through a well-
known procedure,13 our study commenced from verifying the
feasibility using a thiol as a nucleophile in the ring-opening reaction
of 2-butoxy-3,4-dihydropyran 1a. Thus, 1a was treated with benzyl
mercaptan in the presence of catalytic amount of ZnCl2. After 10
hours of reaction at 80 oC, the expected 2-alkylated 1,3-diketone 2a
was obtained in 99 % yield. This result implies that (i) this type of
3,4-dihydropyran can easily react with thiol; and (ii) the generated
1,3-diketone is quite stable under the reaction conditions. In view
of the fact that the experimental procedure is quite simple and
convenient, and the reaction conditions are amenable to scale-up,
we therefore deduced that the ring-opening reaction of 1a with
thiol should be applicable in a solid/liquid interface reaction, with
which
a 1,3-diketone-functionalized organic/inorganic hybrid
material can be prepared.
1.0
a
b
c
d
Scheme 1. Ring-opening reaction of dihydropyran 1a with benzyl
mercaptan.
0.8
e
We then started to investigate the possibility of preparing a
silica-supported metal acetylacetonate (acac) complex with the aid
of this ring-opening reaction. The aim of the first series of
experiments was to evaluate whether or not the ring-opening
reaction of dihydropyran 1a could be used to anchor an acac
fragment onto the surface of HMS-SH. Thus, a SH-functionalized
HMS, HMS-SH, was prepared and then treated by 1a. The XRD
patterns show the amorphous texture of these silica materials
3000
2800
2600
1800 1750 1700 1650 1600 1550 1500
Wavenumber cm-1
Figure 2. IR spectra of a) HMS-SH, b) HMS-acac, c) HMS-DP-Cu II, d)
HMS-DP-Ru, e) HMS-DP-Zn.
2 | J. Name., 2012, 00, 1-3
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Table 1. Physicochemical data of the obtained organic/inorganic
hybrid materials.
loading amount
BET surface
(m2/g)
1316
entry
catalyst
(mmol/g)
0
1
2
3
4
5
6
HMS-HS
HMS-acac
HMS-PD-Cu
0
1203
I
0.15
0.26
0.20
0.16
1120
HMS-DP-Cu II
HMS-DP-Zn
HMS-DP-Ru
1038
1123
1116
Figure 3. 13C MAS NMR spectra of HMS-SH and HMS-acac.
4
, the particle size of HMS-SH and HMS-acac is not very uniform,
and generally in the range of 0.5 to 1.5 m. TEM images of HMS-SH
µ
fragment, were clearly observed, which appear at 65 to 70 ppm.
These results implied that the grafting of acac onto the surface of
HMS-SH is indeed successful. Due perhaps to occurrence of over-
reactions, some aromatic peaks were also observed in the range of
110 to 140 ppm. These materials were also submitted to TEM, SEM
and BET analysis. No significant change in terms of morphology (see
Figure 4) and surface area (see Table 1) was observed before and
after implementing the grafting with 1a. From SEM images of Figure
and HMS-acac indicate that these hybrid materials have abundant
nanoporous structures (Figure 5). Grafting of HMS-SH with 1a
occurred probably in the inside of the pore. The incorporation of
the organic entities within the silica network can be verified by a
color addition of TEM images.
We then tried to use the obtained HMS-acac to immobilize some
homogeneous metal acetylacetonate complexes. As we expected,
HMS-acac is indeed amenable to heterogenizing copper (II)
acetylacetonate. Simple treatment of HMS-acac with Cu(acac)2 in
ethanol solvent under reflux conditions provided two catalysts,
which were denoted as HMS-DP-Cu
I and HMS-DP-Cu II,
respectively. Likewise, HMS-DP-Zn and HMS-DP-Ru can also be
prepared through a similar procedure. The metal loadings in these
materials are in a range from 0.15 mmol/g to 0.26 mmol/g (Figure
1
). The install of acac fragment onto HMS surface is the key to make
the immobilization possible as a very low metal loading, 0.01
mmol/g, was obtained by treating HMS-SH with Cu(acac)2 under the
identical conditions. The incorporation of the copper complex with
silica support could also be confirmed by energy-dispersive X-ray
analysis (EDX) (Figure 4, picture
c), which indicates also the
presence of all the other expected elements. SEM image of HMS-
DP-Cu II indicated that irregular agglomeration of the particles
occurred during the immobilization of metal complex (Figure 4
,
picture ). The IR spectra revealed that the coordination of metal
c
with the tethered acac ligand induced a weakened C=O bond,
resulting in a slight red-shift on the stretch vibration of the carbonyl
group. In addition, new peaks appear at 1550 cm-1 after loading
metal acetylacetonate, which could be attributed to the stretch
vibration of C=C band in the enol form of the acac fragment. It is in
a good accordance with the IR data of the metal acetylacetone
complex (see Figure S2-S4).
Figure 4. SEM images of functionalized silica materials. a) HMS-SH,
b) HMS-acac, c) fresh HMS-DP-Cu II, d) recycled HMS-DP-Cu II
.
The hybrid materials were also submitted to XPS analysis, and the
results are shown in Figure 6. In Cu 2p XPS spectrum of HMS-DP-Cu
II, no characteristic peak of Cu2+ was observed (around 942 eV),
indicating that the copper species was reduced in the course of
immobilization. Meanwhile, a sharp peak appeared in 932.5 eV,
which is a strong evidence to proof the occurrence of a reduction of
Cu2+. A strong interaction between the anchored thiol and Cu(acac)2
should be responsible to the reduction,14 which is able to convert
thiol to disulfide. Indeed, the XPS pattern of S 2p region showed a
component located at around 163.8 eV, which is attributed to the
presence of disulfide on the surface of HMS-DP-Cu II. In the XPS
pattern of C 1s region, contributions of C-O (286.2 eV), C=O (287.6
Figure 5. TEM images of functionalized silica materials. a) HMS-SH,
b) HMS-acac, c) fresh HMS-DP-Cu II, d) recycled HMS-DP-Cu II
.
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Cu is not very stable and it starts to decompose at 130 oC.
These results imply that the organic/inorganic hybrid materials
synthesized based on the ring-opening reaction of
dihydropyran 1a and thiol have a decent thermal stability, and
thus are qualified to be used as catalysts under mild
conditions.
Having the expected materials in hand, we then started to
examine their catalytic activities. In order to compare the
efficiency, three referential supported metal acetylacetonate
catalysts were also prepared according to a reported enamine
formation method
supported copper catalysts in
reaction of NaN3, phenylacetylene 3a and benzyl bromide 4a
(Figure
1
). Firstly, we examined the
a
three-component click
.
This reaction has been extensively investigated and widely
used in chemistry and biology.15 Generally, homogenous
copper salts, such as Cu(OAc)2.H2O16 and CuI17, were used as
catalysts. To facilitate the catalyst recovery, various solid
copper catalysts, such as ammonium salt-tagged NHC-Cu(I)
complexes,18 cross-linked polymeric ionic liquid material-
supported copper,19 and glutathione bearing nano-ferrites,20
have been developed in this type of reaction, sometimes with
remarkable results.21 However, these catalyst systems are
associated with one or more disadvantages, such as the use of
sodium ascorbate as additive,22 tedious procedure for
preparation, hazardous and carcinogenic organic solvents like
nitrobenzene, acetonitrile, dioxane for reactions. Hence, a
simple, green, and efficient procedure avoiding these
drawbacks will be of much use. HMS-DP-Cu II was proved to be
highly active for catalyzing this reaction, and the desired
cycloaddition product 5a was obtained in almost quantitative
yield with exclusive 1,4-regioselectivity (Table 2, entry 7). The
Figure 6. XPS spectra of HMS-DP-Cu II in the regions of Cu 2p, C 1s
and S 2p, XPS spectrum of HMS-DP-Zn in the region of Zn 2p, and
XPS spectrum of HMS-DP-Ru in the region of Ru 3d
.
eV), C-C (285.1 eV), C=C (284.2 eV) and C-S (286.5 eV) bonds can all
be found. With HMS-DP-Zn, characteristic peaks of Zn2+ (1022.3 eV)
were clearly observed in the XPS pattern of Zn 2p region. However,
in case of HMS-DP-Ru, three ruthenium species could be found in
the XPS pattern of Ru 3d region, which involves RuO4 (283.5 eV),
Ru4+ (282.2 eV) and Ru3+ (281.6 eV).
The obtained organic/inorganic hybrid materials were also
submitted thermal gravimetric analysis, and the obtained
results are shown in Figure 7. HMS-SH was found to be quite
Table 2. Optimization of the conditions of the three-component
click reaction of NaN3, phenylacetylenes and benzyl halides.a
stable in temperature below 250 oC. After treatment of 1a
,
due to a significant increase of organic component in the
obtained hybrid material, a drastic weight loss comes about
100 oC earlier. Loading of Zn and Ru did not change the
thermal stability of the hybrid material. But, the congener with
entry
catalyst
solvent
yield(%)b)
1
2
HMS–DP- Cu II
HMS–DP- Cu II
HMS–DP- Cu II
HMS–DP- Cu II
HMS–DP- Cu II
HMS–DP- Cu II
HMS–DP- Cu II
—
1,4-Dioxane
CH3NO2
DCE
19
trace
20
100
3
4
trace
trace
70
99 (98f))
70
90
5
toluene
H2O
a
6
7
EtOH
b
8
HMS–DP- Cu
Cu(acac)2
I
EtOH
c
80
9
EtOH
57
d
e
10
11c)
12d)
13e)
SiO2-Pr-Cu
EtOH
64
HMS–DP- Cu II
HMS–DP- Cu II
HMS–DP- Cu II
EtOH
63
100
200
300
400
500
Temperature (oC)
EtOH
75
EtOH
98
a 3a: 0.2 mmol, 4a: 0.2 mmol, NaN3: 0.24 mmol, solvent: 1.0 ml. b
Isolated yield. c 40 oC. d 6 h. e The catalyst was reused in the third
time. f 10.0 mmol scale reaction.
Figure 7. Thermogravimetric weight loss of the obtained hybrid
materials. a) HMS-SH, b) HMS-acac, c) HMS-DP-Cu II, d) HMS-DP-Ru,
e) HMS-DP-Zn
.
4 | J. Name., 2012, 00, 1-3
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reaction was performed in the absence of sodium ascorbate.
Taking this together with the results obtained from the XPS
pattern of Cu 2p region, we deduced that, during the
preparation of HMS-DP-Cu II, Cu2+ was reduced to Cu+, which
was considered to have higher catalytic activity in this type of
reaction that the former one.17 Among the various solvents
that were screened, ethanol was recognized to be the best
Table
3
.
The substrate scope of HMS-DP-Cu-catalyzed three-
component click reaction of NaN3, phenylacetylenes and benzyl
halides.a
medium for this reaction (entries 1 to 7). HMS-DP-Cu I can also
catalyze this reaction, but the efficiency is much lower as
compared with HMS-DP-Cu II (entry 8). Interestingly, with a
homogeneous counterpart of these catalysts, Cu(acac)2, only
57 % yield was obtained under the identical conditions (entry
9). When a referential copper catalyst, SiO2-Pr-Cu, which was
prepared through a reported enamination reaction, was used,
the yield of 5a is rather poor (entry 10). This result
demonstrates that this immobilization method is able to offer
us a better catalyst than the conventional route. The reaction
was also affected by temperature and reaction time, and the
optimal conditions were finally confirmed to be the followings:
entry
1
R1
H
X
R2
Ph
product
5a
yield(%)b
99
Br
Cl
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
I
2
H
Ph
5a
99
3
F
Ph
5b
5c
99
4
OMe
t-Bu
Et
Ph
93
5
Ph
5d
5e
5f
80
6
Ph
85
7
n-Pr
n-Bu
n-amyl
H
Ph
90
8
Ph
5g
99
9
Ph
5h
5i
90
10
11
12
13
14
4-NO2-C6H4
n-heptyl
n-amyl
Bn
95
o
ethanol solvent, 5 mol % of HMS-DP-Cu II, 80 C and 8 hours
H
5j
99
(entries 11 and 12). It is worth noting that the reaction can
also be effectively scaled up with the similar efficiency. For
example, the reaction of 3a (10 mmol) with 4a (10 mmol) and
NaN3 (12 mmol) gave the corresponding cycloaddition product
5a in 98% yield (2.3 g).
H
5k
99
H
5l
99
H
Me
5m
70
b
a Reaction conditions: 3a
/
4a/NaN3 = 1:1:1.2, solvent: EtOH: l.0 ml.
Isolated yield.
The aim of this study is to develop a new catalyst system that not
only is capable of catalyzing three-component click reactions
efficiently, but also could be recycled easily and steadily. To test the
recyclability of HMS-DP-Cu II catalyst, after reaction, the recovered
solid catalyst was washed with ethanol and dried under vacuum
condition at 60 oC, and then, subjected to the next run. After four
consecutive runs, HMS-DP-Cu II catalyst is still capable of catalyzing
the model reaction in 98 % of yield (Table 2, entry 13), indicating
the fact that the present HMS-DP-Cu catalyst is quite robust under
the reaction conditions. SEM and TEM images of the recycled HMS-
DP-Cu II showed that the particle size does not changed significantly,
and the integrity of the catalyst remained very well (Figure 4 and
Figure 5). We also investigated Cu leaching during the reaction. The
heterogeneous catalyst was isolated by centrifugation after 4 hours
of reaction (ca. 65 % yield) and the solution phase was allowed to
react for a further 4 hours under the same conditions. The yield of
5a was only 66 % after that time, which confirmed that there was
hardly any leaching of Cu into the reaction mixture. ICP-MS showed
that the Cu content of the catalyst did not change appreciably after
the reaction. The slight decrease in catalytic activity in subsequent
runs may be due to the unavoidable loss of solid catalyst during
recovery and washing.
efficiency by using benzyl chloride as substrate instead of benzyl
bromide (entry 2). Ethyl iodide participated readily in this reaction
as well, and the expected product, 5m, was obtained in 70 % yield
(entry 14).
In order to extend the utility of this immobilized metal
acetylacetonate catalyst, we then examined some other reactions
with HMS-DP-Cu. The second reaction we investigated is Glaser
oxidative homo-coupling of phenylacetylene, which is an important
organic transformation that can produce, in a direct way, diyne
derivatives.23 Recently, a plethora of methods have flourished
around Glaser coupling reactions by using Cu species as catalyst.24 It
is pertinent to note here that, in this type of reaction, solid catalysts
showed not only a good performance but also, in some cases, a
robust activity. Unfortunately, the costs of many solid catalysts are
generally high, and thus restrict its application in practical synthesis.
On the contrary, homogeneous Cu catalysts are very cheap, but
often suffer from inefficiency in recycling the catalysts. Therefore, a
new system that can combine both advantages of homogeneous
and heterogeneous systems is appealingly needed. Although
Cu(acac)2 has been used in this reaction as catalyst, but no
successful result was obtained.25 HMS-DP-Cu II was found again to
With the optimized conditions in hand, we probed the scope of
the reaction with respect to both the phenylacetylene and the
be an active catalyst for the homo-coupling of phenylacetylene 6a
.
o
alkylhalide components. As evidenced by the results in Table 3
,
The reaction was performed at 80 C with a balloon of oxygen. The
solvent effect indicated that better result was obtained with
ethanol (Table 4, entries 1 to 6). Various organic bases could
promote the reaction, to some extent, and the best result was
obtained when using pyridine as a base (entries 7 to 9). While good
to excellent yields was obtained with HMS-DP-Cu II catalysts, the
reaction over its homogeneous counterpart, Cu(acac)2, proceeded
sluggishly (entry 10). The use of a referential catalyst, SiO2-Pr-Cu,
phenylacetylenes with different substituents smoothly reacted with
4a, producing the corresponding cycloaddition products in generally
excellent yields (entries 1 to 9). Various benzyl bromides were then
utilized in this reaction (entries 10 to 13). In all cases, excellent
yields were obtained, whatever the nature of the substituent
present on the phenyl ring (electron-donating or electron-
withdrawing). Compound 5a can also be synthesized with the same
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resulted in the recovery of the most unreacted phenylacetylene then turned to a concept of supported catalyst with ionic liquid.27
(entry 11). We then investigated the recyclability of this catalyst, The HMS-DP-Cu catalyst was therefore coated by 30 wt % of
and it was found that HMS-DP-Cu II could be recycled in the model [OMIm]NTf2, and the obtained fine powder was denoted as HMS-
Glaser coupling reaction at least three times without significant loss DP-Cu II@IL. Gratifyingly, this IL-decorated catalyst showed also an
of its activity (entry 12). Some other terminal alkynes can also be excellent catalytic activity in the oxidation of benzhydrol, and
converted to the corresponding dikynes in excellent yields with the benzophenone could be obtained in 99 % yield after 7 hours of
o
aid of HMS-DP-Cu II catalyst (entries 13 to 19). 2-Ethynyl aniline can reaction at 40 C. Direct use of HMS-DP-Cu II as catalyst under the
also be converted smoothly to the corresponding product albeit the identical conditions resulted in an insufficient conversion of
yield is slightly inferior as compared with 6a (entry 20). These benzhydrol. Therefore, coating of [OMIm]NTf2 onto this ready-
results demonstrated that HMS-DP-Cu is indeed a robust and an made solid catalyst is critical to achieve a good efficiency. The
efficient catalyst for Glaser coupling reactions.
reaction over
a referential catalyst, SiO2-Pr-Cu, sluggishly
proceeded, and benzophenone was obtained only in 30 % of yield.
Because the reaction was performed in water, considering also the
fact that [OMIm]NTf2 is a hydrophobic fluid, we therefore deduced
Table 4. HMS-DP-Cu II catalyzed Glaser coupling reaction of alkyne.a
that the reaction might be accelerated by
a hydrophobic
microenvironment created by loading this ionic liquid onto the
solid.28 HMS-DP-Cu II@IL can be easily recovered and reused. As
shown in Scheme 2, its catalytic activity persisted very well, and the
desired benzophenone could be obtained in 97 % yield in the third
run.
entry
1
2
3
4
alkyne
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6b
6c
solvent
—
EtOH
H2O
product
7a
7a
7a
7a
7a
7a
7a
7a
7a
7a
7a
7a
7b
7c
yield (%) b)
60
99
38
13
56
43
60
50
trace
29
N.D
97
99
94
90
90
90
Scheme 2. HMS-DP-Cu II@IL catalyzed alcohol oxidation reaction.
DCE
5
6
Toluene
CH3NO2
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
The above-mentioned results demonstrated that the use of
dihydropyran 1a as dual anchoring reagent and ligand donor is
indeed feasible for the immobilization of Cu(acac)2. Good results
obtained by using HMS-DP-Cu as catalyst encouraged us to
scrutinize the performance of other support metal acetylacetonate
7c)
8d)
9e)
10f)
11g)
12h)
13
14
15i)
16
17
18
19
20j)
catalysts. Gratifyingly,
a hitherto unreported three-component
reaction of 2-carboxybenzaldehyde, aniline and nitromethane was
established by using HMS-DP-Zn as catalyst, which produced a 2,3-
dihydroisoindol-1-one derivative 10a (Scheme 3). The reaction
6d
6e
6f
6g
6h
6i
7d
7e
7f
7g
7h
7i
might proceed through a tandem nitro-Mannich and amidation
reaction.29 HMS-DP-Zn works very well for promoting this reaction,
and the expected product, 10a, was obtained in 80 % yield. The
homogeneous counterpart, Zn(acac)2, and a commonly used Lewis
acid, ZnCl2, can also be used as decent catalysts, but the yields
obtained are inferior. It should be noted that, by increasing either
the reaction time to 24 hours or the catalyst loading to 10 mmol %,
good yields can be obtained with these homogeneous catalysts.
When a referential catalyst, SiO2-Pr-Zn, the yield of 10a reached 65
%. We thus believe that the support in HMS-DP-Zn catalyst played
to some extent the role of concentration amplifier of the metal
catalyst. The support tethered all the metal species onto its surface,
thanks to the adequate banding manner, a good flexibility of the
metal species was achieved, ensuring thus a good activity of the
immobilized catalyst. In this three-component reaction, HMS-DP-Zn
catalyst can be recycled at least two times without significant loss
of its activity.
85
80
60
a alkyne: 0.4 mmol, pyridine: 0.4 mmol, solvent: 1.0 ml. Isolated
b
c
d
e
yield. TMEDA was used as base. Morpholine was used as base.
f
g
Triethylamine was used as base. Cu(acac)2 was used as catalyst.
SiO2-Pr-Cu was used as catalyst. h The catalyst was reused in the
third time. i 12 h, catalyst loading: 2 mol %. j 10 h, catalyst loading: 2
mol %.
Cu(acac)2 has been used as catalyst in oxidation of secondary
alcohols in an ionic liquid [BMIm]BF4.26 Taking a cue from this
report, we then used HMS-DP-Cu catalyst in the oxidation of
benzhydrol by using TBHP as oxidant. As expected, in an ionic liquid
[OMIm]NTf2, benzhydrol can be converted to benzophenone in
almost quantitative yield over HMS-DP-Cu II catalyst. Because the
solvent, [OMIm]NTf2, is rather expensive, to decrease the cost, we
Anilines with electron-donating groups, such as methyl, methoxy
and fluoro, participated readily in the condensation reactions,
6 | J. Name., 2012, 00, 1-3
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providing the corresponding 2,3-dihydroisoindol-1-ones in good to
(10m and 10n). However, due perhaps to steric hindrance,
excellent yields (10b, 10d, 10i, and 10j). Anilines with electron- cyclohexylamine participated in this reaction reluctantly (10o).
withdrawing groups, such as methoxycarbonyl, acetyl and nitro, can Tryptamine and furfurylamine can be converted to the expected
also be used in the reaction (10f to 10h). But, in some cases, the product without damage of the heterocyclic fragments (10p to
10q). Double and triple bonds in the starting substrates can all be
delivered uneventfully into the skeletons of the products (10r to
10v). 2-Carboxy-3,4-dimethoxybenzaldehyde can also be used as a
viable substrate of this three-component reaction (10w).
HMS-DP-Zn can also be used as a recyclable catalyst in a three-
component reaction of phenylacetylene, benzaldehyde and
piperidine (Scheme 4).31 While good yield of 14a (90 %) was
obtained with this catalyst, only 45 % of yield was obtained by using
SiO2-Pr-Zn as catalyst. Likewise, 14b and 14c can also be synthesized
Scheme 4. HMS-DP-Zn catalyzed A3 reaction.
in excellent yields under the identical conditions.
reaction time has to be increased to 12 hours. Even though, the
yields of 10f and 10h are rather inferior. Heterocycle-containing
anilines, N-(Boc)-6-aminoindole and 4-morpholinoaniline can also
be smoothly converted without affecting the stability of these
moieties (10j and 10k). It should be noted that although a three-
component reaction of 2-carboxybenzaldehyde, aniline and indole
has been reported,30 the reaction of N-(Boc)-6-aminoindole
selectively occurred in the site of NH2 group under the present
conditions. 4-Aminophenylboronic acid pinacol ester reacted readily
also with 2-carboxybenzaldehyde and nitromethane, producing the
corresponding product 10l in 50 % of yield. Aliphatic amines, such
Finally, HMS-DP-Ru was used in the oxidation of isoeugenol
methyl ether, which can be used to produce veratraldehyde.
Previously, this type reaction has been investigated by many
systems,
such
as
methyltrioxorhenium/H2O2,32
H2O2/n-
VCl2(salen)/TBHP,34
Bu4NVO3/pyrazine-2-carboxylic
acid,33
.
KMnO4/CuSO4 5H2O,35 NaIO4/OsO4,36 PhI(OAc)2-NaY,37 and so on. A
ruthenium salt, RuCl3, has been used as catalyst in this type of
reaction in conjunction with NaIO4 oxidant, but the yield obtained is
rather low.38 Cleavage of a C=C double bond adjacent to an
aromatic ring to yield the corresponding carbonyl compounds can
also use molecular oxygen as the oxidant, which however,
associated with a biocatalytic method.39 In view of the increasing
synthetic importance of the target aldehydes as synthons in various
organic transformations, the development of simple, convenient,
and environmentally friendly oxidation systems are desirable. We
found unexpectedly that Ru(acac)3 is
a potent catalyst for
promoting the oxidation of isoeugenol methyl etherto
veratraldehyde. In the presence of 1 mol% of Ru(acac)3, after 4
hours reaction at 120 oC in a sealed tube, veratraldehyde could be
obtained in 85 % yield (Table 5, entry 1). Among the various
solvents that were screened, DMSO was recognized to be the best
one for this reaction (entries 2 to 8). In the absence of Ru(acac)3,
the reaction proceeded sluggishly, and only 34 % yield was obtained
under the identical conditions (entry 9). With an increase of
reaction time from 4 hours to 6 hours, yield of 16a could be
increased from 85 % to 95 % (entry 10). Gratifyingly, in the
oxidation of isoeugenol methyl ether, catalyst load as low as 0.4
mol % can be used to lead to high yield (entry 11). To facilitate the
recycling of catalyst, HMS-DP-Ru and SiO2-Pr-Ru were used instead
of the homogeneous catalyst, Ru(acac)3. While almost the same
yield was obtained with the former, only 60 % yield was obtained
with the latter (entries 12 and 13). It is conceivable that the
immobilization allows all the metal species stay steadily on the silica
surface, increasing thus the catalyst concentration of local reaction
domain. Considering the good flexibility conferred by the unique
linker structure, it is quite reasonable to have a good catalytic
activity with HMS-DP-Ru. Under optimal conditions, TON of this
catalyst in the model oxidation reaction reached 234, which is quite
remarkable. To test the recyclability of HMS-DP-Ru, this catalyst
was recycled 6 times, as shown in Figure 8. The high activities were
preserved throughout recycling. It is noteworthy that, after 6 runs,
the TOF decreased by only about 10% with respect to that of the
fresh catalyst (29 vs 26 h-1), giving a total TON of 1338.
Scheme 3. HMS-DP-Zn catalyzed three-component reaction of 2-
carboxybenzaldehyde, amine and nitromethane.
as n-butylamine and benzylamine, have also been examined, and
the desired products were obtained in almost quantitative yields
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Table 5. HMS-DP-Ru catalysed oxidation of isoeugenol methyl ether
to veratraldehyde.a
Experimental Section
General
MeO
MeO
MeO
catalyst, O2 (1 atm)
MeO
Melting points were determined on a microscopic melting point
apparatus and are uncorrected. IR spectra were recorded on a
Bruker FT-IR (EQUINOX 55) using KBr pellets or neat liquid
technology. Thermogravimetric analysis (TGA) was performed
under a flow of nitrogen by heating from room temperature to 500
oC at a rate of 10 oC/min. The catalysts surface areas, N2 adsorption
isotherm (77.3 K) were measured using Micromeritics ASAP 2020 M
surface area. Before analysis, the samples were degassed at 110 oC
for 8 h under vacuum (10-5 bar). Elemental analysis (EA) was
CHO
15a
16a
temp. time
entry
catalyst
solvent
yildb
(oC)
120
60
(h)
4
4
4
4
4
4
4
4
4
6
8
8
8
8
1
2
Ru(acac)3 (1 %)
Ru(acac)3 (1 %)
Ru(acac)3 (1 %)
Ru(acac)3 (1 %)
Ru(acac)3 (1 %)
Ru(acac)3 (1 %)
Ru(acac)3 (1 %)
Ru(acac)3 (1 %)
—
DMSO
EtOH
85
NR
10
3
CH3NO2
DCE
100
80
4
20
5
Toluene
CH3CN
1,4-Dioxane
THF
100
80
Trace
NR
50
determined using
a Vario Micro cube Elemental Analyser
6
(Elementar, Germany). AES data were recorded on an Agilent
Technologies 4100 MP-AES. Field emission scanning electron
microscope (FSEM) and energy-dispersive X-ray spectroscopy (EDX)
analyses were carried out on a Holland FEI Instruments Sirion 200
operating at 30 kV. FTEM samples were prepared by placing drops
of the catalysts suspension dispersed in ethanol on a carbon-coated
copper grid. X-ray diffraction (XRD) patterns were recorded on a
PANalytical B.V. X'Pert PRO diffractometer. 13C CP MAS NMR
7
100
60
8
40
9
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
120
120
120
120
120
120
34
10
11
12
13
Ru(acac)3 (1 %)
Ru(acac)3 (0.4 %)
HMS-DP-Ru (0.4 %)
SiO2-Pr-Ru (0.4 %)
HMS-DP-Ru (0.4 %)
95
94
93
60
14c
93
a
b
The reaction was performed with 0.4 mmol, solvent: 1.0 ml.
Isolated Yield. c 10.0 mmol scale reaction.
experiments were performed on
a Varian Infinity Plus-400
1
spectrometer working at 9.39T, corresponding to a H frequency of
399.5 MHz and a 13C frequency of 100.5 MHz. A 5 mm double
resonance MAS probe was used. For CP, a 1H 90o pulse of 8 μs
length was followed by a contact time of 2.0 ms. Field strengths for
high-power decoupling during acquisition were in the range 70-80
kHz. The spinning frequencies were 5.0 kHz during the
measurements of all samples, the -CH3 of the standard sample with
the external standard is 17.35 ppm. X-ray photoelectron spectra
(XPS) were recorded on a SHIMADZU -Kratos AXIS-ULTRA DLD-
600W X-ray photoelectron spectrometer at a base pressure of 2 ×
-9
10 Pa in the analysis chamber using Al Kα radiation. 1H and 13C
Figure 8. Reuse of HMS-DP-Ru catalyst.
NMR spectra were recorded on a Bruker AV-400. Chemical shifts
are expressed in ppm relative to Me4Si in solvent. All chemicals
used were of reagent grade and were used as received without
further purification. All reactions were conducted in a 10-mL V-type
flask equipped with triangle magnetic stirring.
Conclusions
A new method for grafting acetylacetone ligand onto HMS support
is described, which stems upon a ring-opening reaction of 2-alkoxy-
3,4-dihydropyran with thiol. The use of easily accessible
dihydropyran as dual anchoring reagent and ligand donor not only
enables successful tethering of acetylacetone fragment onto the
surface of a ready-made SH-functionalized HMS, but also ensures a
good flexibility, maximizing thus catalytic activity of the immobilized
metal acetylacetonate complex. Three supported metal catalysts
were prepared and characterized by many physicochemical
methods including FT-IR, elemental analysis, 13C MAS NMR, SEM,
TEM, EDX, XPS, and TG. Their catalytic applications in the selected
Typical procedure for the preparation of HMS-DP-M (M = Cu, Zn,
Ru) catalysts, for example (HMS-DP-Cu II). To a 50 ml round bottle,
3.2 g (0.013mol) of n-hexadecylamine was dissolved at room
temperature in aqueous ethanol (ethanol/water = 7/9 v/v). Then,
8.3 g of tetraethoxysilane (TEOS, 0.039mol) and 1.97 g of 3-
mercaptopropyltrimethoxysilane (MPTMS, 0.01 mol) was
simultaneously but separately added to the template mixture. The
resulting solution was stirred for 20 hours at room temperature and
the white solid was recovered by filtration. Removal of n-
hexadecylamine was carried out by soxhlet extraction over boiling
ethanol for 18 hours affording an organic/inorganic hybrid material,
organic reactions demonstrated
a
great success of this
immobilization strategy. This method is also characterized by some
other advantages, such as simple operational procedure, the use of
easily available reagents, and mild conditions. All these alluring
properties conferred the present protocol a great potential to be
applied in practical uses.
o
HMS-SH. Then, HMS-SH was dried in an oven at 60 C (10-1 mmHg)
for 18 hours. After that, 2 g of the obtained HMS-SH was treated
with excess amount of 1a (0.85 g, SH/1a = 1/1). After 10 hours of
reaction in nitromethane at 80 oC, the acetylacetone-modified HMS,
HMS-acac, was obtained by filtration and then washed with ethanol
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(50 ml ×3). Then the HMS-acac was dried in an oven at 60 oC for 18 was obtained by preparative TLC using a mixture of ethyl acetate
hours. After that, 2 g of acetylacetone-modified HMS and excess and petroleum ether as eluent (EA/PE = 1/3 v/v).
amount of Cu(acac)2 (0.5 mmol, 0.13g, HMS-acac/Cu(acac)2 = 2/1 General procedure for A3 reaction. A stirred solution of alkyne
(n/n)) were added to a 100 ml round bottom flask in 30 ml ethanol, (0.75 mmol), HMS-DP-Zn (3 mol %, 0.015 mmol), aldehyde (0.5
the solution was reflux for 48 hours, then the HMS-DP-Cu solid was mmol) and amine (0.65 mmol) in toluene (2.0 ml) was taken into a
obtained by filtration and then washed with ethanol (50 ml × 5), preheated oil bath (120 °C). After 12 hours of refluxing, the solution
then catalyst was dried in an oven at 60 oC for 24 hours. The loading was centrifuged, and the upper organic phase was isolated. The
of copper was measured by AES, and the value was 0.26 mmol/g-1.
bottom solid was washed with ethyl acetate (0.5 ml × 3). the
General procedure for the ring-opening reaction of a dihydropyran organic layer was concentrated in vacuo. The pure product was
1a with benzyl mercaptan for the synthesis 2a: A solution of 1a (10 obtained by preparative TLC using a mixture of ethyl acetate and
mmol, 2.1 g) in nitromethane (25 ml) was mixed with benzyl petroleum ether as eluent (EA/PE = 1/5 v/v).
mercaptan (25 mmol, 3.1 g), then ZnCl2 (20 mol %, 2.0 mmol, 273 General procedure for oxidation of isoeugenol methyl ether to
mg) was added. The mixture was stirred at 80 °C for 10 h. After the veratraldehyde. Reaction conditions: Isoeugenol methyl ether (0.4
reaction, the product 2a was obtained in 99% yield (9.9 mmol, 3.82 mmol), DMSO, 1ml, HMS-DP-Ru (0.1 mol %, 0.0002 mmol). The
g) by column chromatography (elutent: petroleum ether/ethyl reaction was performed at 120 oC under O2 atmosphere for 8 hours.
acetate = 10/1 (v/v)).
After completion of the reaction, water (1 ml) was added. The
General procedure for the click reaction of sodium azides, alkynes solution was centrifuged and extracted with EA (3 ml × 2). The
and benzyl halides. To a reactor containing alkyne (0.2 mmol), alkyl acquired organic layer was concentrated in vacuo. The pure product
halide (0.2 mmol), sodium azide (0.24 mmol) and EtOH (1.0 ml), was obtained by preparative TLC using a mixture of ethyl acetate
HMS-DP-Cu (5 mol %, 0.01 mmol) was added. The mixture was and petroleum ether as eluent (EA/PE = 1/5 v/v).
stirred for 8 hours at 80 oC. After the reaction completed, the
solution was centrifuged, and the upper organic phase was isolated.
Acknowledgements
The bottom solid was washed with ethyl acetate (0.5 ml × 3). The
acquired organic layer was concentrated in vacuo. The pure product
was obtained by preparative TLC using a mixture of ethyl acetate
and petroleum ether as eluent (EA/PE=1/3 v/v).
We thank the National Natural Science Foundation of China for
financial support (21173089 and 21373093). We are also
grateful to the Analytical and Testing Centre of HUST. The
General procedure for the Glaser coupling reaction. Reaction Chutian Scholar Program of the Hubei Provincial Government
and the Cooperative Innovation Center of Hubei Province are
also acknowledged. This work is also supported by
fundamental research funds for the central universities in
China (2014ZZGH019).
conditions: alkyne (0.4 mmol), EtOH, 1.0 ml, HMS-DP-Cu (1 mol %,
0.002 mmol) and piperidine (1.0 equivalent, 0.2 mmol). The mixture
was stirred for 8 hours at 80 oC under O2 atomosphere. After
completion of the reaction, the solution was centrifuged, and the
upper organic phase was isolated. The bottom solid was washed
with ethyl acetate (0.5 ml × 3). The acquired organic layer was
concentrated in vacuo. The pure product was obtained by
preparative TLC using petroleum ether.
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amine and nitromethane. A solution of 2-carboxybenzaldehyde (0.3
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Catalysis Science & Technology
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DOI: 10.1039/C5CY01012H
Silica-supported metal acetylacetonate catalysts with a robust and
flexible linker constructed by using 2-butoxy-3,4-dihydropyrans as
dual anchoring reagents and ligand donors
Bingbing Lai,a Zhipeng Huang, aZhifang Jia,a Rongxian Baia,* and Yanlong Gua,b,
*
2-Butoxy-3,4-dihydropyrans were used, for the first time, as dual anchoring reagents and
ligand donors to modify a ready-made SH-functionalized HMS. The obtained
acac-functionalized silica was demonstrated to be an excellent support to immobilize
homogeneous metal acetylacetonate catalysts.