Bifunctional mesoporous silica catalyst for C-C bond forming tandem reactions

Article abstract of DOI:10.1002/ejic.201001332

We report on the synthesis and catalytic properties of a class of hybrid organic-inorganic mesoporous bifunctional heterogeneous catalysts for efficient catalysis of two-step tandem reactions in one pot. With a solvent-assisted grafting method, two differ

Full text of DOI:10.1002/ejic.201001332

FULL PAPER
DOI: 10.1002/ejic.201001332
Bifunctional Mesoporous Silica Catalyst for C–C Bond Forming Tandem
Reactions
Krishna K. Sharma,[a][‡] Ankush V. Biradar,[b,c] Sayantani Das,[b] and Tewodros Asefa*[b,c]
Keywords: Heterogeneous catalysis / C–C coupling / Palladium / Mesoporous materials / Bifunctional catalysts
We report on the synthesis and catalytic properties of a class
of hybrid organic–inorganic mesoporous bifunctional hetero-
geneous catalysts for efficient catalysis of two-step tandem
reactions in one pot. With a solvent-assisted grafting method,
two different catalytic groups, that is, an organoamine and a
palladium–organodiamine complex, were immobilized onto
high-surface-area mesoporous silica sequentially, by using
their corresponding organosilanes in 2-propanol or toluene
as the solvent in the first step and toluene as the solvent in
the second step. Both MCM-41 and SBA-15 mesoporous sil-
icas were used as support materials for the two catalytic
groups, and the effect of different sequential grafting of the
two organosilanes in 2-propanol or toluene on the structures
and the catalytic properties of the resulting bifunctional cata-
lysts were investigated. By using the resulting amine/PdII–
diamine bifunctional mesoporous material as catalyst, the oc-
currence of two very important C–C bond forming reactions,
that is, the Sonogashira and the Henry reactions, was demon-
strated in one pot for the first time. Yields of approximately
100% in 2.5 h for the Sonogashira reaction and approxi-
mately 100% in 45 min for the Henry reaction were obtained
in the presence of the bifunctional catalyst when the reac-
tions were run individually. When the bifunctional catalyst
was used to catalyze the Sonogashira–Henry reactions in tan-
dem in one pot, a yield of up to approximately 60% of the
Sonogashira–Henry product in 5 h was obtained. The synthe-
sis and use of such bifunctional catalysts for efficient catalysis
of the tandem reaction is of importance in three significant
ways: it prevents the unnecessary use of solvents and other
chemicals required for the purification of the intermediate
products of either individual reaction, eliminates some of the
work-up procedures, and lowers the cost of synthesis of the
final product.
tions.[6–8] For instance, organopalladium complexes, which
have been widely used to catalyze various C–C bond cou-
pling reactions such as the Sonogashira, Heck, and Suzuki
reactions,[9–18] can be tethered onto the surface amine
groups of primary amine-functionalized mesoporous silica.
These organopalladium complexes can also be supported
on mesoporous materials by means of other ligands such as
phosphanes and diamines.[11–16,20] The resulting materials
can serve as active heterogeneous catalysts for various or-
ganic reactions. The use of such materials as heterogeneous
catalysts in industrial processes results in more cost-effec-
tive catalysis, as they allow the rapid recovery and multiple
uses of the relatively “expensive” active catalysts such as
palladium complexes. Consequently, mesoporous materials
functionalized with palladium or other metal complexes
hold great promise for use as efficient catalysts for various
organic reactions.[9,11–16,22–27]
Similarly, numerous types of homogeneous and hetero-
geneous catalysts for the Henry reaction, which is another
very important C–C bond forming reaction, have been re-
ported.[28] Different types of amine groups that are tethered
onto mesoporous silica support material have also been suc-
cessfully used to catalyze the Henry reaction.[29–36] In our
recent work, we showed that a solvent-assisted grafting syn-
thetic method, involving the use of simple polar-protic sol-
vents, can place organoamines in spatial isolation within
Introduction
Since the first report on mesoporous silica in the early
1990s, the synthesis of various hybrid organic–inorganic
mesoporous materials and their potential applications in
chemistry, catalysis, biology, and engineering have been
widely investigated.[1–5] Mesoporous materials have drawn
considerable attention because of their high surface areas,
uniform nanometer pores, and monodisperse pore-size dis-
tribution, and because they can easily be functionalized
with various organic and organometallic species to tune
their surface properties as well as their potential applica-
[a] Department of Chemistry, 111 College Place Syracuse
University,
Syracuse, New York 13244, USA
[b] Department of Chemistry and Chemical Biology, Rutgers, The
State University of New Jersey
610 Taylor Road, Piscataway, New Jersey 08854, USA
Fax: +1-732-445-5312
E-mail: tasefa@rci.rutgers.edu
[c] Department of Chemical and Biochemical Engineering,
Rutgers, The State University of New Jersey
98 Brett Road, Piscataway, New Jersey 08854, USA
Fax: +1-732-445-5312
[‡] Current Address: Department of Chemical Engineering,
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001332 or from
the author.
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Bifunctional Mesoporous Silica Catalyst for C–C Bond Forming Reactions
the pores of mesoporous silica material and can result in each other. Furthermore, their catalytic activities in the
efficient catalytic activities in the Henry and aldol conden- two-step tandem reactions in one pot involving the Sonoga-
sation reactions.[29–31] In fact, these materials superseded shira and Henry reactions is demonstrated (Scheme 2).
any previously reported mesoporous catalysts in terms of
catalytic activity in the C–C bond coupling Henry and aldol
condensation reactions.
The Henry and Sonogashira catalytic reactions follow
two different reaction mechanisms. While the Sonogashira
reaction is an oxidative coupling between an aryl halide and
an alkyne, the amine-catalyzed Henry reaction takes place
between benzaldehyde or substituted benzaldehydes and a
nitroalkane (or a nucleophile) by an ion-pair or imine
mechanism.[34] For the Sonogashira coupling reaction, pal-
ladium is the most widely used catalyst, whereas for the
Henry reaction, organoamines are often employed. As men-
tioned above, numerous methods have been used to immo-
bilize palladium as well as amine groups on solid supports
including mesoporous silicas.[11–14,16,20,27] Most importantly
and in relevance to the work here, these two reactions, that
is, the Sonogashira and the Henry reactions, are sometimes
carried out one after another in order to generate various
pharmaceutically and industrially valuable com-
pounds.[37–39] However, catalyzing both reactions in one pot
without isolating their intermediate products and at the
same time controlling the selectivity of the reactions
Scheme 1. Schematic description of the synthesis of bifunctional
towards a particular product needs considerable effort. Al-
though a mixture of two different catalysts in the homogen-
rous MCM-41 silica and the corresponding control catalyst con-
amine/PdII–diamine catalyst (AP–Pd–DT) supported on mesopo-
taining only PdII–diamine catalyst (Pd–DT). In the synthesis of the
materials, first 3-aminopropyltrimethoxysilane (APTS) in 2-prop-
anol and then PdII–[N-(2-aminoethyl)-3-aminopropyltrimethoxy-
silane] in toluene were grafted onto MCM-41, giving monofunc-
tional catalysts AP-I and Pd–DT, respectively. AP-I was sub-
sequently grafted with PdII–[N-(2-aminoethyl)-3-aminopropyltri-
methoxysilane] complex in toluene to afford the bifunctional cata-
lyst AP–Pd–DT.
eous phase can be used to catalyze the two reactions in
one pot, this method is rather complex; specifically, it is
accompanied by multiple products, involves costly separa-
tion methods, and results in the loss of expensive catalysts
such as palladium complexes.[15] Recently a successful
“domino” type halogen exchange (HALEX)-Sonogashira
reaction was successfully demonstrated by Thathagar and
Rothenberg.[40] To the best of our knowledge, however, the
catalytic transformation of two of the most important
C–C bond-forming reactions, the Sonogashira and the
Henry reactions, in one pot with a single bifunctional
heterogeneous catalyst has not been demonstrated pre-
viously.
Here we report the synthesis of bifunctional mesoporous
catalysts by immobilization of two catalytic groups, a PdII–
diamine complex and a primary amine (-NH2), onto meso-
porous MCM-41 and SBA-15 silicas, and demonstrate their
use as heterogeneous catalysts for the transformation of
two-step Sonogashira and Henry reactions in one pot suc-
cessfully without isolating the intermediate product.
Scheme 2. The two-step-in-one-pot Sonogashira and Henry reac-
tions catalyzed by a bifunctional amine/PdII–diamine catalyst, AP–
Pd–DT.
To synthesize the bifunctional catalyst, AP–Pd–DT, first,
primary amine groups were immobilized onto mesoporous
silica (MCM-41) by using 2-propanol as the solvent. This
produced primary-amine-functionalized mesoporous silica
Results and Discussion
The method for anchoring two catalytic groups, that is, (AP-I). 2-Propanol was chosen here as the solvent to graft
a PdII–diamine complex and an amine (NH2), onto the the primary amine groups onto mesoporous silica, because,
walls of mesoporous silicas (MCM-41 and SBA-15) in or- according to our previous studies,[29,30] 2-propanol pro-
der to catalyze the Sonogashira and Henry reactions in tan- duces site-isolated organoamine catalytic groups, which are
dem in a one-pot system is described in Scheme 1. The two more efficient catalysts in the Henry reaction. Furthermore
active catalytic groups are conveniently placed next to one and most importantly, the grafting of mesoporous silica
other without affecting the individual catalytic properties of with organic groups in 2-propanol leaves the mesoporous
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K. K. Sharma, A. V. Biradar, S. Das, T. Asefa
FULL PAPER
silica with ample amount of “ungrafted” surface, onto
which secondary functional groups or catalytic sites can be
anchored.[29,30] In the second step, the PdII–diaminoor-
ganosilane complex was grafted on the leftover space on
AP-I by using toluene as the solvent (Scheme 1). Toluene
was used as a solvent in the second step in order to help the
second organosilane graft well enough within the remaining
relatively smaller leftover space on the mesoporous channel
walls of AP-I. Unlike 2-propanol, toluene strongly favors
the grafting of organosilanes onto mesoporous silica,[29,30]
and this leads to enough density of the secondary func-
tional group (in this case, the PdII–diamine complexes) even
in the smaller available space.
Figure 1. Nitrogen gas adsorption isotherms for MCM-41, AP-I,
and AP–Pd–DT.
This sequential grafting of the two different organosil-
anes by using two different solvents resulted in an optimum
bifunctional mesoporous silica catalyst containing two dif-
ferent catalytic sites, that is, the primary amine (NH2) and
the PdII–diamine complex. Consequently, the catalyst was
expected to catalyze the Sonogashira and the Henry reac-
tions individually as well as in tandem in one pot.
Nitrogen gas adsorption/desorption measurements were
used to characterize the mesoporous structure, the surface
areas, pore diameters, and pore volumes of the materials,
before and after each step of organosilane functionalization
during catalyst synthesis. Figures 1 and 2 show the nitrogen
gas adsorption isotherms and pore size distributions,
respectively, of the parent material (MCM-41), the amine
grafted sample (AP-I), the PdII–diamine-functionalized ma-
terial (Pd–DT), and the bifunctional amine/PdII–diamine
mesoporous catalyst (AP–Pd–DT). As expected, the meso-
porous structures and other physical attributes of the mate-
rials such as surface areas, pore sizes, and mesopore vol-
umes decreased slightly as more and more organosilanes
were grafted within the pores of the mesoporous silica. The
Brunauer–Emmett–Teller (BET) surface area, pore volume,
and Barrett–Joyner–Halenda (BJH) pore diameter of
MCM-41 were found to be 1161 m2/g, 0.74 cm3/g, and
24.2 Å, respectively. However, these values decreased suc-
cessively as 3-aminopropyl groups and/or PdII–diamine
complexes were placed within the mesopores. For instance,
the BET surface area, pore volume, and BJH pore diameter
of AP-I were 1065 m2/g, 0.45 cm3/g, and 21.9 Å, respec-
tively. The BET surface area, pore volume, and BJH pore
diameter of the amine/PdII–diamine functionalized sample
(AP–Pd–DT) were even significantly lower with values of
509 m2/g, 0.17 cm3/g, and 21.7 Å, respectively. These results
indicate that the latter samples have reduced BET surface
area, pore volume, and BJH pore diameter with respect to
the parent mesoporous material (MCM-41), owing to the
presence of more grafted organic and organometallic
groups in its mesopores.
Figure 2. Pore-size distribution for MCM-41, AP-I, and AP–Pd–
DT.
Figure 3. Transmission electron microscope (TEM) image of AP–
Pd–DT. Scale bar: 20 nm.
water adsorbed on the materials. The weight loss from AP-
I in the range 175 °C to 600 °C was due to the loss of 3-
aminopropyl groups, residual surfactants, and some surface
The mesoporous structure in the catalysts was further
confirmed by transmission electron microscopy (TEM).
Representative TEM images of the bifunctional catalyst
AP–Pd–DT, showing highly ordered mesoporous structure,
are presented in Figure 3.
The thermogravimetric traces (Figure 4) showed a weight
Figure 4. Thermogravimetric traces of MCM-41, AP-I, Pd–DT,
loss below 100 °C for all the samples due to the loss of and AP–Pd–DT.
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Bifunctional Mesoporous Silica Catalyst for C–C Bond Forming Reactions
silanol groups. In AP–Pd–DT and Pd–DT samples, signifi- the Sonogashira reaction had to be determined from a
cant weight losses in the range 175 °C to 210 °C and in the series of experiments. Furthermore, since K2CO3 is “toxic”
range 250 °C to 600 °C were observed. The first weight loss and may be hard to recover from the reaction, its use in the
was most likely due to the loss of the acetate/water ligands minimum possible amount in heterogeneous catalysis would
anchored on the PdII ions, and the second was probably have an added benefit. To the best of our knowledge, the
due to the loss of organodiamine (and propylamine) groups. optimum mol-% of K2CO3 needed to perform the Sonoga-
The exact amount of aminopropyl groups and palladium in shira reaction in reasonable time without leaving K2CO3
the materials was determined by elemental analyses. Ele- behind in the reaction mixture has not been reported be-
mental analysis of AP-I gave C 8.43, H 2.99, and N 2.57%. fore. All previous studies of Sonogashira reactions em-
Pd–DT was found to contain C 15.13, H 4.31, N 4.78%, ployed excess amounts of base or K2CO3, often much more
while AP–Pd–DT was found to possess C 19.86, H 3.99, N than the stoichiometric ratios.[15,23,25]
6.06%. ICP-AES analysis of AP–Pd–DT gave Pd 16.7%,
Thus, a series of Sonogashira reactions using different
which confirmed the presence of palladium in the bifunc- mol ratios of K2CO3 with respect to 0.5 mmol phenylacetyl-
tional catalyst (AP–Pd–DT); the amount of palladium was ene and 0.5 mmol iodobenzene were performed. Various
calculated to be 1.6 mmol Pd/g sample.
mol ratios of K2CO3 with respect to one of the reactants,
Performing the Sonogashira and Henry reactions to- that is, phenylacetylene or p-iodobenzaldehyde, as both of
gether in a one-pot system can be a fairly complicated pro- them are limiting reactants, were used. This included
cess. The Sonogashira reaction requires a homogeneous K2CO3/phenylacetylene ratios of 2:1, 1.6:1, 1.4:1, and 1:1.
base such as K2CO3 for abstracting alkyne protons of the Of these, the 1.6:1 of was found to be the optimum ratio
phenylacetylene reactant and to make the reaction go to for the Sonogashira reaction as it gave approximately 100%
completion. However, homogeneous bases such as K2CO3 reactant conversion in a reasonably short time (about 3 h)
can also catalyze or participate in the base-catalyzed Henry in the presence of Pd–DT and AP–Pd–DT catalysts, and it
reaction and give the nitroalcohol product. This means that barely left any more K2CO3 behind to possibly affect the
K2CO3 could potentially interfere with any intended Sono- successive Henry reaction. The 1:1 mol ratio of K2CO3 to
gashira–Henry (dehydrated) tandem product.
phenylacetylene was found to be insufficient in order to
Thus, as a control experiment, we conducted the Henry convert all of the phenylacetylene reactant in the Sonoga-
reaction between p-hydroxybenzaldehyde and nitromethane shira reaction in reasonable times of 3–5 h. In contrast, the
in the presence of K2CO3 (homogeneous catalyst). The re- 2:1 mol ratio was found to be slightly in excess, which
action gave approximately 90% conversion of p-hydroxy- means that, although it resulted in the completion of the
benzaldehyde in 30 min; however, it predominantly yielded Sonogashira reaction, it also gave the Henry reaction prod-
1-(4-hydroxyphenyl)-2-nitroethanol (a nitroalcohol) as uct, immediately after the addition of both nitromethane
product. In addition to p-hydroxybenzaldehyde, the cata- and p-hydroxybenzaldehyde into the reaction mixture. Since
lytic activity of K2CO3 on p-iodobenzaldehyde, that is, the the 1.6:1 mol ratio of K2CO3/phenylacetylene was able to
reactant involved in the tandem reaction with its iodo and completely drive the Sonogashira reaction to completion in
aldehyde groups, was investigated. This reaction also gave 3 h without catalyzing the Henry reaction upon addition of
1-(4-iodophenyl)-2-nitroethanol (also a nitroalcohol) as the nitromethane and p-hydroxybenzaldehyde afterwards, it
product with a low reactant conversion of 38% in 2 h. Thus, was used as the optimum mol ratio in the subsequent stud-
this possibility of nitroalcohol formation by K2CO3 could ies of tandem reactions.
further complicate the outcome of the Sonogashira–Henry
However, because the Henry reaction can be catalyzed by
reaction if the two reactions were to be performed in tan- K2CO3, this reaction would still compete for the optimum
dem in a one-pot system in the presence of excess K2CO3, K2CO3 available for the Sonogashira reaction. Thus, per-
which is typically used in the Sonogashira reaction, and if forming the two reactions in sequence in the one-pot system
the Sonogashira–Henry dehydrated product were the in- was necessary in order to obtain the intended tandem prod-
tended target. Besides giving the 1-(4-iodophenyl)-2-nitro- ucts in higher yield. Indeed, when the Henry reaction took
ethanol (nitroalcohol) product, K2CO3 catalyzes the Henry place before the Sonogashira reaction, it deprived the Sono-
reaction more slowly[41] relative to our amine-function- gashira reaction of the K2CO3 and led to lower yields in
alized-mesoporous-silica-catalyzed Henry reaction, which the tandem process (see Supporting Information). Further-
gives approximately 100% conversion into the nitrostyrene more, the Henry reaction with K2CO3 yielded predomi-
product in 15–20 min. This means that K2CO3 would po- nantly the nitroaldol product and also left a significant
tentially leave some unreacted the p-iodobenzaldehyde, amount of unreacted p-iodobenzaldehyde. An when com-
which would further undergo the Sonogashira reaction to bined with the Sonogashira reaction, the reaction mixture
yield multiple unwanted products in the tandem reactions of the Henry reaction with K2CO3 further led to multiple
(see below).
types of Sonogashira–Henry tandem products and other
Thus, to circumvent these problems associated with ex- side products (see Supporting Information). Attempts to
cess K2CO3 and to avoid the possibility of its participation obtain even a tandem Sonogashira–nitroalcohol product by
as a homogeneous catalyst in the Henry reaction during the using excess K2CO3, which is large enough to catalyze both
one-pot tandem Sonogashira–Henry reactions, the opti- the Henry and the Sonogashira reactions, also failed to give
mum or minimum amount of K2CO3 required to complete reasonable yields (Figures S1 and S2). However, these issues
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K. K. Sharma, A. V. Biradar, S. Das, T. Asefa
FULL PAPER
were easily avoided by simply changing the order of ad- which lacks free primary amine groups but has diamine
dition of the reactants in the one-pot reaction. By per- groups that are complexed with PdII, failed to give any
forming the Sonogashira reaction to completion with the Henry reaction product up to 12 h. It, however, catalyzed
bifunctional AP–Pd–DT catalyst in the presence of the op- the Sonogashira reaction efficiently, giving approximately
timized amount of K2CO3, followed by the addition of ni- 100% Sonogashira product in 2.5 h for the reaction be-
tromethane into the same reaction mixture, the formation tween aryl halide and phenylacetylene. These results indi-
of such multiple products was prevented, and the Sonoga- cate that both the amine (AP-I) and the palladium–diamine
shira–Henry dehydrated tandem product was predomi- grafted catalyst (Pd–DT) alone can catalyze the respective
nantly formed.
individual reactions efficiently, but not both.
For the first part of the study, various reference mono-
Thus, the AP-I catalyst was prepared and further grafted
functional mesoporous catalysts, that is, AP-I and Pd–DT, with PdII–diamine complex in toluene yielding the bifunc-
were synthesized by using MCM-41 as a support material. tional mesoporous catalyst, AP–Pd–DT, which contained
The catalytic properties of these materials as well as MCM- both the primary amine and the PdII–diamine complex
41 were then tested and the results were used as reference (Scheme 1). To demonstrate the catalytic activity of AP–
for demonstrating the importance and the ability of our bi- Pd–DT in the tandem reactions, p-iodobenzaldehyde was
functional mesoporous catalyst (AP–Pd–DT) to catalyze purposely chosen as substrate, because it has both func-
the two reactions efficiently in one pot (Table 1). The meso- tional groups necessary to perform the Sonogashira and the
porous silica (MCM-41), which lacks primary amines or Henry reactions in a one-pot system. AP–Pd–DT gave ap-
palladium complexes, catalyzed neither the Sonogashira re- proximately 100% conversion for the Sonogashira reaction
action nor the Henry reaction, as expected. The amine- between p-iodobenzaldehyde and phenylacetylene in 2.5 h.
grafted catalyst (AP-I) gave 90% yield in 15 min for the After confirming the completion of the Sonogashira reac-
Henry reaction, which is in accordance with our previously tion (ca. 100 conversion of phenylacetylene) by GC and
reported results.[29,30] However, the AP-I catalyst did not GC–MS, nitromethane was added into the reaction mix-
catalyze the Sonogashira reaction, also as expected ture. Nitromethane served both as a solvent and as a nu-
(Table 1). The catalytic activity of the PdII–diamine func- cleophile for the Henry reaction. This reaction gave approx-
tionalized sample (Pd–DT) was then tested in both the imately 100% conversion of the p-substituted benzaldehyde,
Henry and the Sonogashira reactions individually. Pd–DT, with a yield of 60% for the Sonogashira–Henry dehydrated
tandem product (see Scheme 2). Thus, the bifunctional hy-
brid heterogeneous catalyst catalyzed not only the oxidative
C–C coupling, or Sonogashira reaction, between p-iodo-
benzaldehyde and phenylacetylene but also the Henry reac-
tion between the p-substituted benzaldehyde group and ni-
tromethane to give p-substituted nitrostyrene. Furthermore,
the catalyst transformed these two reactions in one pot,
without isolation of the intermediate product.
To determine the effects of the pore size of the bifunc-
tional mesoporous materials and the relative density of
their catalytic groups, additional bifunctional catalysts were
prepared. By using mesoporous SBA-15, which has higher
pore diameter than MCM-41, as a support material, three
more catalysts were synthesized for this investigation. The
first sample was prepared by following the same procedure
as that for the synthesis of AP–Pd–DT. The resulting mate-
rial was labeled as SBA–AP–Pd–DT. The other two samples
were prepared by following slightly different grafting se-
quences in order to produce materials with different struc-
tures (see Experimental Section for details of the syntheses).
The second material was prepared by grafting the primary
Table 1. Relative catalytic efficiencies for the individual and tandem
Sonogashira and Henry reactions in the presence of K2CO3 or vari-
ous mesoporous catalysts synthesized from MCM-41 by grafting
various organosilanes on it.[a]
Catalyst
Reaction
Time
% Conversion[d]
MCM-41
Sonogashira[b]
Henry[b]
24 h
24 h
24 h
ca. 0
ca. 0
ca. 0
API
Sonogashira[b]
Henry[b]
15 min 95[d]
K2CO3
Pd–DT
AP–Pd–DT
Sonogashira[b]
Henry[b]
24 h
2 h
ca. 0
38[e]
Sonogashira[b]
Henry[b]
2.5 h
12 h
2.5 h
ca. 100
ca. 0
ca. 100
Sonogashira[b]
(Bifunctional catalyst) Henry[b]
45 min ca. 100[d]
Sonogashira–Henry 5 h
60[f]
tandem[c]
[a] The reactions were performed with 30 mg of catalyst. For the
Sonogashira reaction, phenylacetylene (0.5 mmol), p-iodobenzene
(0.5 mmol), and K2CO3 (0.8 mmol) were used. For the Henry reac-
tion, p-substituted benzaldehyde (0.5 mmol) in CH3NO2 (5 mL)
was used. The Sonogashira reaction was performed at 80 °C, while
the Henry reaction was performed at 90 °C. In case of the Sonoga- amine groups by using APTS in toluene instead of APTS
shira–Henry tandem reaction, the Sonogashira reaction was per-
in 2-propanol. (Please note that both AP–Pd–DT and
formed at 80 °C until the reaction reached completion, and the
SBA–AP–Pd–DT involved grafting of APTS in 2-propanol
temperature was then raised from 80 to 90 °C, before adding nitro-
in the first step.) This was followed by grafting PdII–diami-
methane. [b] When the reaction was performed individually. [c] The
nosilane with toluene as a solvent. This generated a catalyst
labeled as SBA–AP–Pd–1. The third catalyst, denoted as
SBA–AP–Pd–2, was prepared from SBA-15 support by first
grafting the PdII–diaminosilane in toluene, followed by the
grafting of the primary amine in toluene. As shown below,
these materials had slight differences in their structures/
two reactions were performed in tandem in one pot. [d] This result
reflects also the % yield of the p-iodo-β-nitrostyrene product. [e]
This result reflects also the % yield of the nitroalcohol product. [f]
The yield was lower than approximately 100.0%, because of the
formation of some tandem Sonogashira–nitroaldol product
(ca. 15%) and the loss of some of the p-iodobenzaldehyde reactant
as benzaldehyde (ca. 25%).
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Bifunctional Mesoporous Silica Catalyst for C–C Bond Forming Reactions
composition as characterized by powder X-ray diffraction (100) Bragg reflection corresponded to 121, 114, and 110 Å
(XRD) (Figure 5), UV/Vis spectroscopy (Figure 6), X-ray for bifunctional catalysts SBA–AP–Pd–DT, SBA–AP–Pd–
photoemission spectroscopy (XPS) (Supporting Infor- 1, and SBA–AP–Pd–2, respectively.
mation), and thermogravimetric analysis (Figure 7).
These SBA-15-based bifunctional catalysts were further
characterized by UV/Vis absorption spectroscopy (Fig-
ure 6). The UV/Vis spectrum of the bifunctional catalyst
from MCM-41, that is, AP–Pd–DT, was included for com-
parison. All the samples showed strong absorption bands
at approximately 350–370 nm and a second band at approx-
imately 225 nm. These absorption bands can be assigned to
typical metal–ligand charge transfer (MLCT) transitions in
PdII–diamine complexes, which seem to exist in different
forms as Pd(N)2(OAc)2 and [Pd(N)2(OH2)(OAc)]OAc,
respectively, in our samples.[42,43] The presence of a slight
difference in the intensity of the two bands in the MCM-
41-based catalyst (AP–Pd–DT) relative to that of the bands
in the three SBA-15-based bifunctional catalysts can also
be observed.
Figure 5. Powder X-ray diffraction (XRD) patterns of bifunctional
catalysts SBA–AP–Pd–DT, SBA–AP–Pd–1, and SBA–AP–Pd–2.
The inset contains the d-spacing and unit cell values of the materi-
als.
In addition, the presence of the catalytic groups (Pd and
amine) was characterized by X-ray photoemission spec-
troscopy (XPS). Representative XPS spectra are shown in
the Supporting Information (Figures S3 and S4). The XPS
of AP–Pd–DT shows a peak at 399.8 eV, which is the char-
acteristic binding energy for N1s. Furthermore, the spec-
trum contains a strong peak at 532.1 eV and a weaker peak
at 535.0 eV. The first peak corresponds to siloxane (Si–O–
Si) oxygen atoms of SBA-15 in the sample, while the second
peak corresponds to the oxygen atoms of residual silanol
groups (Si–OH).[44,45] Furthermore, the spectrum has two
peaks at 337.8 and 342.9 eV. These binding energies corre-
spond to Pd 3d5/2 and 3d3/2, respectively, of PdII complexes
having two N ligands and two O ligands. These peak assign-
ments were consistent with those reported for other similar
materials previously.[46,47] Similarly, the XPS spectrum of
catalyst SBA–AP–Pd–DT showed a peak at 399.4 eV corre-
sponding to N1s, and a peak at 531.7 eV and a shoulder at
534.7 eV, which correspond to O1s. In addition, the spec-
trum showed peaks at 337.4 and 342.8 eV corresponding to
3d5/2 and 3d3/2, respectively. No significant difference was
observed between the XPS spectra of AP–Pd–DT and
SBA–AP–Pd–DT.
Figure 6. UV/Vis absorption spectra of SBA–AP–Pd–DT, SBA–
AP–Pd–1, and SBA–AP–Pd–2 bifunctional catalysts. The spectrum
for AP–Pd–DT (from MCM-41) is also included for comparison.
The TGA traces for SBA–AP–Pd–DT, SBA–AP–Pd–1,
and SBA–AP–Pd–2 are shown in Figure 7. The results
showed a loss of approximately 5–6 wt.-% water in the tem-
perature range 30 to approximately 100 °C in all the sam-
ples. Furthermore, a weight loss in the temperature range
of approximately 175–220 °C was observed, which corre-
sponds to acetate ions bound to PdII–diamine complex. An
additional weight loss in the temperature range of approxi-
mately 240–630 °C, mainly associated with the loss of or-
ganoamine groups, is also exhibited. Interestingly, the TGA
traces of the samples prepared by different grafting se-
quences, that is, SBA–AP–Pd–DT and SBA–AP–Pd–1 as
opposed to SBA–AP–Pd–2, revealed slight differences in
Figure 7. Thermogravimetric traces for bifunctional catalysts SBA–
Pd–DT, SBA–AP–Pd–1, and SBA–AP–Pd–2.
The XRD patterns of these new catalysts (Figure 5) their weight loss patterns (Figure 7). The sample that was
showed the typical (100) Bragg reflection of SBA-15-type functionalized with the PdII–diaminosilane complex in the
materials, which is indicative of the presence of ordered me- first grafting step (i.e. SBA–AP–Pd–2) showed a weight loss
sostructures in the materials. The d-spacing values of the after approximately 240 °C, more quickly than the ones
Eur. J. Inorg. Chem. 2011, 3174–3182
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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K. K. Sharma, A. V. Biradar, S. Das, T. Asefa
FULL PAPER
grafted with PdII–diamine complex in the second grafting Table 3. Relative catalytic efficiencies for the individual and tandem
Sonogashira and Henry reactions in the presence of K2CO3 or vari-
step (i.e. SBA–AP–Pd–DT and SBA–AP–Pd–1). This is
ous mesoporous catalysts synthesized from SBA-15 by grafting
possibly because of the fact that the PdII–diamine com-
various organosilanes on it under different conditions (see Experi-
plexes can be grafted in more accessible locations in the
mesoporous channels when they are grafted in the first
grafting step than in the second. The PdII–diamine com-
plexes in more accessible locations will in turn have a
greater chance to come off or decompose at higher tempera-
tures. On the other hand, the organosilanes grafted in the
second step may end up in the microporous or corrugated
leftover porous structures that SBA-15 type materials typi-
cally have,[48,49] making them decompose rather slowly.
The presence of the organoamine and palladium catalytic
groups in the bifunctional catalysts was further corrobo-
rated by elemental analyses and ICP-AES, respectively
(Table 2). Furthermore, the ICP-AES results indicated that
the SBA-15-based bifunctional catalysts contained 1.1–
1.3 mmol Pd/g of sample, which was slightly lower than the
1.6 mmol Pd/g obtained for AP–Pd–DT.
mental Section for details).[a]
Catalyst
Reaction
Time
% Conversion[d]
SBA–AP–Pd–DT Sonogashira[b]
3 h
2 h
3.5 h
3 h
2 h
3.5 h
3 h
2 h
ca. 100
Henry[b]
ca. 100[d]
ca. 59.6[e]
ca. 100
Tandem[c]
SBA–AP–Pd–1
SBA–AP–Pd–2
Sonogashira[b]
Henry[b]
ca. 100[d]
ca. 58.2[e]
ca. 100
Tandem[c]
Sonogashira[b]
Henry[b]
ca. 100[d]
ca. 52.1[e]
Tandem[c]
3.5 h
[a] The reactions were performed with 30 mg of catalyst. For the
Sonogashira reaction, phenylacetylene (0.5 mmol), p-iodobenzene
(0.5 mmol), and K2CO3 (0.8 mmol) were used. For the Henry reac-
tion, p-substituted benzaldehyde (0.5 mmol) in CH3NO2 (5 mL)
was used. The Sonogashira reaction was performed at 80 °C, while
the Henry reaction was performed at 90 °C. In case of the Sonoga-
shira–Henry tandem reaction, the Sonogashira reaction was per-
formed at 80 °C until the reaction reached completion, and the
temperature was then raised from 80 to 90 °C, before the addition
of nitromethane. [b] When the reaction was performed individually.
[c] The two reactions were performed in tandem in one pot. [d]
This result reflects also the % yield of the p-iodo-β-nitrostyrene
product. [e] This result reflects also the % yield of the nitroalcohol
product. The yield was lower than approximately 100.0%, because
of the formation of some tandem Sonogashira–nitroaldol product
(ca. 15%) and the loss of some of the iodobenzene reactant as
benzaldehyde (ca. 25%).
Table 2. Chemical compositions of SBA-15-based amine/PdII–di-
amine bifunctional mesoporous catalysts SBA–Pd–DT, SBA–AP–
Pd–1, and SBA–AP–Pd–2.[a]
Sample
Wt.-% N (mmol/g)
Wt.-% Pd (mmol/g)
SBA–AP–Pd–DT 5.3 (3.8)
13.4 (1.3)
11.7 (1.1)
11.2 (1.1)
SBA–AP–Pd–1
SBA–AP–Pd–2
5.4 (3.9)
5.4 (3.9)
[a] The wt.-% N and wt.-% Pd were obtained by elemental and
ICP-AES analysis, respectively.
The catalytic activities of the SBA-15-based bifunctional
catalysts in the Sonogashira and Henry reactions individu-
ally and in tandem were then investigated in the same way
as for the AP–Pd–DT catalyst. All the three SBA-15-based
bifunctional catalysts, that is, SBA–AP–Pd–DT, SBA–AP–
Pd–1, and SBA–AP–Pd–2, were found to catalyze the the
Sonogashira reaction effectively, giving approximately
100% conversion in 2.5 h (Table 3). In addition, they were
also found to catalyze the Sonogashira and Henry reactions
in tandem, affording the tandem products in 5 h with slight
differences. Catalysts SBA–AP–Pd–DT and SBA–AP–Pd–1
gave slightly higher yields of approximately 60%, whereas
catalyst SBA–AP–Pd–2 gave a yield of approximately 52%
in 3.5 h. Further analysis of this product revealed that 59%
of it was the Sonogashira–Henry dehydrated tandem prod-
uct, while the rest was the Sonogashira–nitroalcohol tan-
dem product. It can also be concluded that the SBA-15-
based bifunctional catalysts did not show significant differ-
ences in catalytic activity among each other as well as when
compared with the MCM-41-based bifunctional catalysts.
gashira product from the reaction mixture and thus avoid
unnecessary use of solvents and other chemical reagents to
isolate the intermediate product. Thus, the use of the bi-
functional catalyst can potentially cut the cost of prepara-
tion of the tandem product. Furthermore, unlike in many
other studies in which K2CO3 was used in excess, here the
optimum amount of K2CO3 was determined for the first
time and employed without compromising the results and
without leaving residual K2CO3 in the reaction mixture at
the end of the reaction. In addition, the bifunctional cata-
lyst was proven to catalyze the Sonogashira reaction with-
out the presence of any toxic or expensive phosphane li-
gands or copper co-catalysts that are commonly used in
Sonogashira reactions.[15,23,25]
Conclusions
We have synthesized, by using a solvent-assisted post-
grafting method, a bifunctional organic–inorganic hybrid
In addition, leaching tests for palladium was performed mesoporous catalyst containing two different catalytic sites,
by taking the reaction mixture and analyzing it by ICP- that is, a PdII–diamine complex and a primary amine, next
AES. The result indicated that there was less than 0.19 ppm to each other within the channels of mesoporous silica. The
Pd in the reaction mixture after 5 h of catalytic reaction. bifunctional catalyst and the two catalytic groups are
This suggests that the PdII was barely leached into the reac- proven to be active catalysts for catalyzing two different C–
tion mixture and that the Sonogashira reaction was essen- C bond-forming reactions, that is, the Sonogashira and
tially catalyzed by the bifunctional solid catalyst.
Henry reactions, in one pot. This is the first time that these
The bifunctional catalyst has enabled us to omit the sep- two widely used C–C bond-forming chemical reactions
aration of the intermediate of either the Henry or the Sono- were performed in tandem in one pot by using a single bi-
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Eur. J. Inorg. Chem. 2011, 3174–3182

Bifunctional Mesoporous Silica Catalyst for C–C Bond Forming Reactions
multiple times and dried in an oven at 60 °C for 4 h. The resulting
material was labeled as bifunctional catalyst AP–PD–DT.
Synthesis of SBA-15 and Different SBA-15-Based Amine/PdII Bi-
functional catalyst and without separating or purifying the
intermediate products. The bifunctional catalyst gave 60%
yield for the Sonogashira–Henry tandem product in 5 h by
the two-step tandem reaction in one pot. The bifunctional functional Catalysts
catalyst also catalyzed both reactions, individually resulting
in approximately 100% reactant conversion in 2.5 h in the
Sonogashira reaction and approximately 100% reactant
To determine the effect of pore size of the mesoporous materials,
additional catalysts were prepared by using mesoporous SBA-15
parent material as support for the two catalytic sites. In addition,
conversion in 45 min in the Henry reaction. Furthermore, the effect of the relative density of the catalytic groups in the mate-
rials was investigated by varying the relative density of the catalytic
groups through the synthesis.
the optimized or minimum amount of K2CO3 required to
catalyze the Sonogashira reaction effectively without com-
promising the Henry reaction during the tandem reaction
SBA-15 mesoporous silica (SBA-15) was synthesized by the method
in the one-pot system was determined and utilized. The de- reported previously.[31] Three different catalysts were prepared from
SBA-15. The first sample was prepared in the same way as AP–
Pd–DT above by grafting APTS (3.68 mml) in 2-propanol
(250 mL) onto SBA-15 (500 mg) for 6 h at 80 °C to produce spa-
tially isolated primary amine groups on the mesoporous channel
walls of SBA-15. This sample or catalyst was labeled as SBA–AP–
I. SBA–AP–I was then grafted with PdII–diaminopropyltriethoxys-
ilane prepared as described above. Briefly, SBA–AP–I (200 mg) was
stirred with the PdII–diaminopropyltriethoxysilane solution in tolu-
ene (200 mL) at 80 °C for 5 h. The solution was filtered, and the
solid product was washed with ethanol multiple times and dried in
an oven at 60 °C for 4 h. The resulting material was labeled as
bifunctional catalyst SBA–AP–Pd–DT.
velopment of such bifunctional catalysts for tandem reac-
tions would have implications such as reducing the cost of
purification and separation of intermediate products in
multi-step synthesis. Furthermore, soluble toxic copper
compounds and phosphane-based ligands, which are typi-
cally used in the Sonogashira reaction, were not required
here to successfully perform the two-step, one-pot tandem
reaction.
Experimental Section
A second sample was prepared by grafting APTS (3.68 mmol) in
anhydrous toluene (250 mL) onto SBA-15 (500 mg) for 6 h to pro-
duce more densely populated primary amine groups on the meso-
porous channel walls of SBA-15 or a sample denoted as SBA–AP–
T. This sample was then grafted with PdII–diaminopropyltriethoxy-
silane prepared as described above. Briefly, SBA–AP–T sample
(200 mg) was stirred with the PdII–diaminopropyltriethoxysilane
solution in toluene (200 mL) at 80 °C for 5 h. The solution was
filtered, and the solid product was washed with ethanol multiple
times and dried in an oven at 60 °C for 4 h. The resulting material
was expected to contain less PdII and more primary amine groups
and is labeled as bifunctional catalyst SBA–AP–Pd–1.
Materials: All materials and reagents were purchased and used as
received. Toluene, ethanol, 2-propanol, dichloromethane, and N,N-
dimethylformamide (DMF) were obtained from Fischer Scientific.
PdII acetate, 4-hydroxybenzaldehyde, phenylacetylene, p-iodoben-
zaldehyde, K2CO3, tetraethyl orthosilicate (TEOS), and nitrometh-
ane were purchased from Sigma–Aldrich. 3-Aminopropyltrimeth-
oxysilane (APTS) and N-(2-aminoethyl)-3-aminopropyltrimeth-
oxysilane (AAPT) were obtained from Gelest, Inc. Poly(ethylene
glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)
block copolymer (Pluronic® 123, average molecular mass ca. 5800)
was obtained from BASF.
Synthesis of MCM-41 and Amine-Functionalized MCM-41 (AP-I):
Mesoporous silica (MCM-41) was synthesized by the method re-
ported previously.[29,30] Then, MCM-41 (500 mg) was grafted with
APTS (3.68 mmol) in 2-propanol (250 mL) to give spatially isolated
primary amine groups tethered on its mesoporous channel walls.
This sample or catalyst was labeled as AP-I.[29,30]
The third and final SBA-15-based catalyst was prepared by grafting
PdII–diaminopropyltriethoxysilane solution in toluene (200 mL) on
SBA-15 (200 mg) at 80 °C for 5 h. Then, APTS (3.68 mmol) in an-
hydrous toluene (250 mL) was grafted on the resulting material at
80 °C for 5 h. The solution was filtered, and the solid product was
washed with ethanol multiple times and dried in an oven at 60 °C
for 4 h. The resulting material was expected to contain more PdII
and less primary amine groups and is labeled as bifunctional cata-
lyst SBA–AP–Pd–2.
Synthesis of Palladium–Diamine-Immobilized MCM-41 (Pd–DT):
MCM-41 was grafted with PdII–diaminopropyltriethoxysilane
complex in toluene. The PdII–diaminopropyltriethoxysilane was
prepared in toluene by mixing a 1:1 mol ratio of PdII acetate
(0.819 mmol) and [N-(2-aminoethyl)-3-aminopropyltrimethoxysil-
ane] (0.819 mmol) in toluene (5 mL) and stirring the solution for
2 h at room temperature. The PdII–aminoorganosilane solution was
mixed with additional toluene (195 mL). The resultuing 200 mL of
PdII–aminoorganosilane solution in toluene was then added into a
round-bottomed flask containing MCM-41 (200 mg). The mixture
was stirred at 80 °C for 5 h. The solution was filtered, and the solid
material that contained the PdII–diamine complex was washed mul-
tiple times with ethanol and dried in an oven at 60 °C for 4 h. The
resulting sample was labeled as Pd–DT.
The Henry and Sonogashira Reactions Individually Performed: The
Henry reaction was performed by using AP-I, Pd–DT, and AP–
Pd–DT under the same conditions as reported before.[29,30] The
progress of the reaction was monitored with TLC and 1H NMR
spectroscopy. For the Sonogashira reaction, iodobenzene
(0.5 mmol) and phenylacetylene (0.5 mmol) were added into a
100 mL round-bottomed flask containing the catalyst (30 mg),
K2CO3/phenylacetylene (1.6:1 mol ratio), and a mixture of ethanol/
DMF (4:2 v/v) solvent. The mixture was stirred at 80 °C, and the
progress of the reaction was monitored by GC and GC–MS.
Synthesis of Amine and Palladium–Diamine Complex (AP–Pd–DT) The Tandem Sonogashira–Henry Reactions in One Pot: For the tan-
Containing Bifunctional Catalyst: Here AP-I was grafted with the
PdII–diaminopropyltriethoxysilane prepared as described above.
Briefly, AP-I (200 mg) was stirred with the PdII–diaminopropyltrie-
thoxysilane solution in toluene (200 mL) at 80 °C for 5 h. The solu-
tion was filtered, and the solid product was washed with ethanol
dem Sonogashira and Henry reactions, phenylacetylene (0.5 mmol)
and p-iodobenzaldehyde (0.5 mmol) were added into a solution
containing ethanol/DMF (4:2 v/v), K2CO3/phenylacetylene
(1.6:1 mol ratio), and AP–Pd–DT catalyst (30 mg). The solution
was stirred at 80 °C while the reaction product was continually
Eur. J. Inorg. Chem. 2011, 3174–3182
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K. K. Sharma, A. V. Biradar, S. Das, T. Asefa
FULL PAPER
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Acknowledgments
We gratefully acknowledge the financial assistance by the US
National Science Foundation (NSF), CAREER Grant No. CHE-
1004218 and NSF DMR-0968937, for this work. T. A. also thanks
the NSF for nominating him as one of its ten NSF American Com-
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Published Online: June 17, 2011
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