Pd-MCM-48: A novel recyclable heterogeneous catalyst for chemo- and regioselective hydrogenation of olefins and coupling reactions

Article abstract of DOI:10.1039/c0ob00183j

A novel, heterogeneous Pd-MCM-48 catalyst has been developed by encapsulating palladium nanoparticles into the cubic phase of mesoporous MCM-48 matrix at room temperature. The catalyst demonstrated excellent chemo- and regioselectivity for the hydrogenation of olefins at room temperature within 30-80 min. The turnover frequency for the hydrogenation is very high (4400 h^-1). Interestingly, selectivity of the catalyst was significantly influenced by the mode of addition of palladium precursor. Moreover, the catalyst was also very effective for the coupling reactions with the formation of carbon-carbon and carbon-nitrogen bonds under ligand-free and aerobic conditions.

Full text of DOI:10.1039/c0ob00183j

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Pd-MCM-48: a novel recyclable heterogeneous catalyst for chemo- and
regioselective hydrogenation of olefins and coupling reactions†
Subhash Banerjee,* Vagulejan Balasanthiran, Ranjit T. Koodali* and Grigoriy A. Sereda*
Received 28th May 2010, Accepted 9th July 2010
DOI: 10.1039/c0ob00183j
A novel, heterogeneous Pd-MCM-48 catalyst has been developed by encapsulating palladium
nanoparticles into the cubic phase of mesoporous MCM-48 matrix at room temperature. The catalyst
demonstrated excellent chemo- and regioselectivity for the hydrogenation of olefins at room
temperature within 30–80 min. The turnover frequency for the hydrogenation is very high (4400 h-1).
Interestingly, selectivity of the catalyst was significantly influenced by the mode of addition of
palladium precursor. Moreover, the catalyst was also very effective for the coupling reactions with the
formation of carbon–carbon and carbon–nitrogen bonds under ligand-free and aerobic conditions.
Introduction
Result and Discussion
Silica supported heterogeneous palladium catalysts have been
referredas“smartcatalysts”inorganicsynthesis.1 Recentsynthetic
improvements of mesoporous siliceous materials, promoted devel-
opment of engineered silica based Pd-catalysts2 where active Pd-
species are encapsulated inside channels and/or pores.2g Among
the various classes of mesoporous materials, MCM-48 is highly
attractive due to the large surface area, pore volume and presence
of the interpenetrating network of three-dimensional pores that
is expected to facilitate molecular transport of reactants and
products more efficiently than the uni-dimensional network in
MCM-41.3 Literature studies revealed that a number of Pd-MCM-
41 catalysts have been reported for the efficient hydrogenation2b
and coupling2b,4 reactions. However, studies employing Pd-MCM-
48 are scarce due to the tedious hydrothermal preparative method,
requiring long reaction time (1–7 days) and a few Pd-MCM-
48 catalysts have been reported in the literature.2e–g Thus, rapid
and room temperature synthesis of Pd-MCM-48 has been long
awaited. In this study, we are motivated to explore facile syn-
thesis of Pd-MCM-48 and their possible applications. Chaudhari
et al.2e developed anchored Pd-complex in MCM-48 for the
hydrocarboxylation of olefins. Recently, Cheon et al.2f reported
a ball shaped Pd-MCM-48 catalyst, prepared by chemical vapor
infiltration of bis-1,1,1,5,5,5-hexafluoro-acetylacetonate Pd(II)
salt into the MCM-48 template for the hydrogenation and removal
of benzyl ether group. As a part of our research of catalysis5 and
MCM-48 material,6 we have developed a facile protocol for the
synthesis of MCM-48 at room temperature.6a Herein, we report
synthesis of a novel heterogeneous Pd-MCM-48 catalyst for rapid
and highly chemo- and regioselective hydrogenation of olefins and
coupling reactions under ligand-free and aerobic conditions.
At first, the Pd-MCM-48 catalyst (1) was prepared by encapsulat-
ing of Pd nanoparticles (Pd-NPs) into the cubic phase MCM-48
matrix followed by calcination and mild reduction by H2 gas.
Powder X-ray diffraction study revealed the formation of cubic
mesoporous MCM-48 (Fig. 1) while the transmission electron
microscopy (TEM) imageconfirmedthe presenceofPdNPs within
the sample (Fig. 2).
Fig. 1 Powder X-ray diffraction pattern of Pd-MCM-48 (1, 0.6 wt%)
showed the formation of mesoporous siliceous material.
The surface area of the Pd-MCM-48 was found to be much
higher (~1800 m2g-1) with a higher metal dispersion of ~ 22%
(CO pulse studies, see ESI) than earlier reported Pd-MCM-48
(3.3%).2f The Pd content (0.6 wt%) of the catalyst was determined
by atomic absorption spectroscopy study. Because of the employed
preparative conditions, most of the fine Pd particles (< 2 nm) are
located inside the MCM-48 pores (~2.5 nm) while some particles
(4–7 nm) are positioned outside the mesochannels yet highly
dispersed. This well-characterized catalyst have been tested for
the hydrogenation and coupling reactions.
414 E. Clark Street, Department of Chemistry, The University of South
Dakota, Vermillion, SD, 57069, USA. E-mail: ocsb2009@yahoo.com, ranjit.
koodali@usd.edu, gsereda@usd.edu; Fax: 605 677 6397
† Electronic supplementary information (ESI) available: Preparation of
Pd Nanoparticles, details characterization of catalyst 1 by N2 adsorption-
desorption isotherm, preparation of catalyst 2, reuse of the catalyst 1,
copies of 1H NMR spectra of all products listed in Tables 2–5 and Scheme
2. See DOI: 10.1039/c0ob00183j
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Table 1 Optimization of reaction conditions for the hydrogenation of cinnamic acid using catalyst 1
Entry
Catalyst
Amount of catalyst (mg)
Amount of Pd (mmol)
Solvent
Time (h)
Yield (%)a
1
2
3
4
5
Pd-MCM-48 (0.6 wt%)
Pd-MCM-48 (0.6 wt%)
Pd-MCM-48 (0.6 wt%)
Pd-MCM-48 (0.6 wt%)
Si-MCM-48
20
20
10
20
20
1.1 ¥ 10-3
1.1 ¥ 10-3
0.5 ¥ 10-3
1.1 ¥ 10-3
—
EtOH
H2O
EtOH
Toluene
EtOH
0.5
0.5
0.5
0.5
24
100
100
75
82
0
2.0 mmol of cinnamic acid was used in all cases.a Yields were determined by 1H NMR spectra.
drastic difference in the catalytic performance to the mode of Pd
deposition; the activity is highly dependent on the preparative
conditions employed. To explore further this structure–activity
relationship, we synthesized another catalyst (2) by depositing Pd
on synthesized MCM-48 (see ESI). The BET study reveals that
the surface area of the catalyst 2 (Pd wt ~0.8%) is 637 m2g-1, which
is much less than of the catalyst 1 with 12% metal dispersion and
average crystallite size of 12 nm.
The catalyst 1 exhibited superior selectivity in hydrogenation
of olefins (100%) over hydrogenolysis in contrast to the non-
selective catalyst 2, commercial Pd/C, ball shaped Pd-MCM-
482f and Pd-SiO2.2f A comparative results of hydrogenation were
presented in Table 3. Further, we established that the selectivity
significantly was influenced the mode and time of addition of
palladium precursors to the MCM-48 during its synthesis. Besides
excellent chemoselectivity, catalyst 1 also exhibited significant
regioselectivity. Therefore, terminal alkenes were hydrogenated
selectively in the presence of tri-substituted alkenes (Scheme 2).
Fig. 2 TEM image of Pd-MCM-48 catalyst (0.6%).
First, the catalyst 1 was tested for the hydrogenation of
olefins using H2 gas. It demonstrated excellent activity at room
temperature in ethanol or water (Scheme 1).
Scheme 1 Selective hydrogenation of olefins using catalyst 1.
The reaction conditions were optimized using hydrogenation of
cinnamic acid as a model reaction and the results were presented
in Table 1. Cinnamic acid (2 mmol) was hydrogenated completely
in presence of 20 mg of catalyst 1 in ethanol or water at room
temperature within 30 min.
Scheme 2 Regioselective hydrogenations olefins using catalyst 1.
The excellent reactivity and selectivity of the catalyst 1 for
hydrogenation has encouraged us to explore its catalytic perfor-
mance towards coupling reactions with the formation of carbon-
carbon bonds, which are of great synthetic importance in organic
chemistry.7 Over the past decades, countless studies report a
number of practical properties of heterogeneous Pd-catalysts,1c,2,4,7
including Pd-NPs,7f–h such as moisture stability, reusability, and
capability of performing the reaction under ligands-free condi-
tions. However, it is not uncommon for Heck and Suzuki coupling
to take long reaction times when performed without phosphine
and other ligands at the Pd-center.7c,i Here, for the first time,
we report application of mesoporous Pd-MCM-48 catalyst for
Heck and Suzuki coupling reactions under ligand-free and aerobic
conditions. The catalyst 1 demonstrated excellent activity towards
Heck coupling reactions. The results are summarized in Table 4.
We observed high chemoselective reduction of double bonds in
the the presence of a reducible carbonyl group (entry 4, Table 2)
using Pd-MCM-48 catalyst and H2 gas at room temperature.
The results were summarized in Table 2. The Pd-MCM-48
catalyzed hydrogenation of olefins was very fast (0.5–1 h) and
high yielding (97–100%). The excellent reactivity of the catalyst
is possibly due to the higher metal surface area (~95 m2g-1,
see ESI) and dispersion of Pd metal in MCM-48 matrix. The
double bonds were selectively hydrogenated in the presence of
ketone, OAc, CO2Bu etc. Interestingly, this selectivity was opposite
to the earlier reported Pd-MCM-48 (Pd was deposited on the
surface of synthesized MCM-48),2f which led to removal of the
benzyl group in the presence of double bonds. We attribute this
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Table 2 Pd-MCM-48 catalyzed hydrogenation of alkenes
Entry
1
Olefin
Time (min)
30
Product
Yield (%)a
100
2
3
4
45
60
80
99
98
97
5
45
98
6
7
50
45
100
100
a Yields refers to isolated products charecterized by 1H and 13C NMR. 20 mg (0.6 wt%) of catalyst was used for 2 mmol of alkene.
Table 3 Comparative results for the selectivity for hydrogenation using different Pd-catalysts
Yield (%)a
Alkene
Entry
Pd-Catalyst
Time (h)
1
2
1a
1b
1c
Catalyst 1b
Catalyst 2c
Pd/Cd
1
1
1
100
0
0
0
99
100
2a
2b
2c
Catalyst 1b
Catalyst 2c
Pd/Cd
24
4
2
0
0
0
0
100
100
3a
3b
3c
Catalyst 1b
Catalyst 2c
Pd/Cd
1
1
1
99
0
0
0
100
100
4a
4b
4c
Catalyst 1b
Catalyst 2c
Pd/Cd
1
4
2
100
0
0
0
100
100
a Yields and ratios were determined by 1H NMR spectroscopic data. b Pd precursor was added during the preparation of MCM-48. c Pd precursor was
diposited on synthesized MCM-48. d 10% Pd/C, purchased from Sigma-Aldrich.
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Table 4 Pd-MCM-48 catalyzed Heck coupling reactions
We believe that the excellent activity of catalyst 1 is mostly due
to its large surface area (~1800 m2g-1), higher metal dispersion
(22%) and interpenetrating network of three dimensional pores
of MCM-48. Finally, the recyclability of the catalyst has been
investigated for both hydrogenation and coupling reactions. The
catalyst can be reused at least ten times for hydrogenation with
minimal loss of activity. However, for Heck reaction the yield has
dropped to 70% in the fourth run. The results were summarized
in Table 6 in the ESI.†
Entry
R
X
R1
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
H
H
H
H
Br
I
Br
I
CO2Bu
CO2Bu
CN
CN
Ph
CO2Bu
CN
Ph
4-OMeC4H9
Ph
CO2Bu
7
5
6
5
6
4
4
4
5
7
12
90
98
92
96
90
97
92
90
95
85
0
H
I
Conclusion
NO2
NO2
NO2
NO2
OMe
NO2
Br
Br
Br
Br
I
In conclusion, we have developed a novel heterogeneous Pd-
MCM-48 catalyst by encapsulating palladium nanoparticles in
mesoporous siliceous MCM-48. The catalyst established remark-
able activity towards regio- and chemoselective hydrogenation
of olefins with very high turnover frequency (4400 h-1) at room
temperature. Thus, double bonds were selectively hydrogenated
in the presence of O-benzyl and O-THP protecting groups. In
addition, the catalyst was also very effective for the coupling
reactions with formation of C–C and C–N bonds under ligand-
free and aerobic conditions, including a usually inactive aryl
chloride for Suzuki coupling. Interestingly, the mode of addition
of palladium precursors to the MCM-48 greatly influences the
selectivity for hydrogenation reactions. Exploration of further
applications for the new catalyst in organic synthesis is under
way.
9
10
11
Cl
a Yields refer to those of pure isolated products. 1 mmol of aryl halide,
2 mmol of alkene, 2 mmol of NaOAc and 40 mg (0.6 wt %) of catalyst 1
was added to 2 ml of DMF and heated at 100 ◦C.
Table 5 Suzuki coupling using Pd-MCM-48 catalyst in water
Entry
R
X
Time (h)
Yield (%)a
1
2
3
4
5
6
H
H
NO2
NO2
Me
Me
Br
I
Cl
Br
Br
I
4
3
8
4
5
3
92
99
85
95
93
98
Experimental
Chemicals used
Hexadecyltrimethylammoniumbromide (CTAB, 99+%, Acros),
ethanol (absolute 200 Proof, AAPER), aqueous ammonia
(Fisher), tetraethoxysilane (99%, Alfa Aesar), palladium(II) 2,4-
pentanedionate (Alfa Aesar) and sodium terachloropalladate(II)
(Na2[PdCl4]) (Pressure Chemicals) were used for the synthesis
of Pd-MCM-48 materials. Dimethylaminopyridine (DMAP, 99%,
Acros), toluene (Acros), tetra-n-octylammoniumbromide (TOAB,
98+% Alfa Aesar), sodium borohydride (98% Acros) and sodium
sulfate anhydrous powder (Fisher) were used for the synthesis of
Pd(0) nanoparticles. All the chemicals used for hydrogenation and
coupling reactions were purchased form Sigma-Aldrich or Alfa-
Aesar and used as received.
a Yields refer to those of pure isolated products. 1 mmol of ArX, 1.5 mmol
of phenylboronic acid, 2 mmol of NaOAc and 40 (0.6 wt %) mg of catalyst
1 was added to 2 ml of H2O and heated at 80 ◦C.
A number of aryl halides with electron donating and electron
withdrawing groups were coupled with a variety of alkenes (e.g.
butyl acrylate, acrylonitrile, styrene, 1-methoxy-4-vinylbenzene
etc.) in the presence of 40 mg of catalyst 1 (0.6 wt%) and a mild
base (NaOAc) in DMF. The reactions were efficient at 100 ◦C.
After successful application of catalyst 1 in Heck coupling, we
investigate its efficiency for the Suzuki coupling. We observed that
Pd-MCM-48 efficiently catalyzed the reaction of bromobenzene
with phenylboronic acid in water at 80 ◦C in the presence of
NaOAc. A variety of aryl halides including relatively inactive
bromides and chlorides underwent coupling with phenylboronic
acid under the optimized reaction conditions in shorter reaction
times (3–8 h) yielding biaryl compounds in excellent yields (85–
99%). The results are presented in Table 5.
Experimental procedure for the synthesis of Pd-MCM-48
(catalyst 1)
Initially, Pd(0) nanoparticles were synthesized according to the
published procedure8 by the reduction of Na2PdCl4 with NaBH4
in the presence of tetra-n-octylammonium bromide (TOAB) as a
capping and stabilizing agent. Pd-MCM-48 material was synthe-
sized by the modified Sto¨ber method6a. In a typical procedure,
50 mL of the Pd(0) nanoparticles solution (the concentration was
adjusted to the required loading), 25 mL ethanol, 1.2 g (3.3 mmol)
of CTAB, 6 mL of aq. NH3 (0.09 mol) and 1.8 mL (8 mmol) of
TEOS were sequentially added to a 125 mL polypropylene bottle,
and stirred for 4 h at 300 rpm. The molar composition of the
formed siliceous gel is 0.41 CTAB: 11 aq. NH3: 1.0 TEOS: 53
EtOH: 344 H2O. After 4 h, the mesoporous material was washed
Moreover, the catalyst 1 was also very efficient for a coupling
reaction with formation of a carbon–nitrogen bond (Scheme 3).
Scheme 3 Pd-MCM-48 catalyzed C–N bond formation.
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extensively with deionized water and dried overnight at 100 ◦C.
The dried material was finely ground with a mortar and pestle
and calcined at 550 ◦C (ramp rate of 3 ◦C per min.) for 6 h in an
alumina crucible to remove the surfactant molecules. The calcined
Pd-MCM-48 may contain Pd in the +2 oxidation state. In order
to increase consistency in the catalyst’s composition, this material
was reduced in hydrogen-gas flow (20 mL/min) at 300 ◦C for 2 h
in a tubular furnace. The content of palladium in the synthesized
catalyst was determined by atomic absorption spectroscopy and
found to be 0.6 wt%.
2,6-Dimethyl-8-propoxyoct-2-ene(product C, Scheme 2)
Colorless liquid; IR (n[cm-1]) 1655, 1090, 720 cm-1; NMR: dH
(200 MHz, CDCl3) d = 0.82–0.99 (m, 6H), 1.09–1.52 (m, 5H),
1.54–1.78 (m, 8H), 1.94–2.17 (m, 2H), 3.28–3.51 (m, 4H), 5.02–
5.20 (m, 1H); NMR: dC (CDCl3) 10.8, 17.8, 19,8, 23.2, 25.7, 25.9,
36.9, 37.5, 69.3, 72.8,125.1, 131.3. Anal. calcd. for C13H26O: C
78.72, H 13.21. Found: C 78.66, H 13.174.
Representative experimental procedure for the Pd-MCM-48
catalyzed Heck coupling of butyl acrylate with iodobenzene
A mixture of iodobenzene (204 mg, 1 mmol), butyl acrylate
(256 mg, 2 mmol), NaOAc (272 mg, 2 mmol) and catalyst 1 (40 mg,
0.6 wt%) was heated in DMF (2 mL) for 5 h till completion of
reaction (TLC) at 100 ◦C. After the reaction mixture was cooled
to room temperature and extracted with ethyl acetate, washed
thoroughly with water (6 ¥ 1 mL) to remove DMF, dried over
Na2SO4, solvent was evaporated under vacuum to provide crude
product, which was passed short silica gel column leading to pure
3-phenylacrylic acid butyl ester (187 mg, 98%) as colorless liquid.
The product was characterized by FT-IR, 1H NMR and 13C NMR
spectroscopic data and these data were in good agreements with
literatures reported values.
Representative experimental procedure for the Pd-MCM-48
catalyzed hydrogenation of olefins
A mixture of trans-cinnamic acid (296 mg, 2 mmol) and catalyst
1 (20 mg) in ethanol (10 mL) was stirred under constant flow of
H2 (flow rate 20 mL/min) for 30 min untill completion of reaction
(TLC). The catalyst was filtered out on filter paper and washed
with ethanol (2 ¥ 5 mL). The combined filtrates were evaporated
to yield the pure hydrogenated product, 3-phenylpropanoic acid
◦
1
(300 mg, 99%) as white crystals (m.p. 48 C). The FT-IR, H
NMR and 13C NMR data were consistent with those reported in
literature. The hydrogenation was also successful under a steady
H2 atmosphere (a balloon filled with H2).
Representative experimental procedure for the Pd-MCM-48
catalyzed Suzuki coupling phenylboronic acid with iodobenzene
For hydrogenation in water, the reaction product was extracted
with ethyl acetate, dried over Na2SO4. Evaporation in vacuum gave
the pure hydrogenated product, 3-phenylpropanoic acid. For all
the reactions listed in Table 1, 2 and 3, ethanol was used as solvent
to avoid extraction from water. After the catalyst was filtered,
washed with ethanol, and dried in air, it was reused for subsequent
runs.
All the products listed in Table 1, 2 and 3 are known and their 1H
NMR and 13C NMR data were consistent with the reported values.
The products on Scheme 2 are not known and were additionally
characterized by elemental analysis. The characterization data for
the compounds A–C in Scheme 2 are given below:
A mixture of iodobenzene (204 mg, 1 mmol), phenylboronic acid
(183 mg, 1.5 mmol), NaOAc (272 mg, 2 mmol) and catalyst 1
(40 mg, 0.6 wt%) was heated in water (2 mL) for 3 h till completion
of reaction (TLC) at 80 ◦C. After the reaction mixture was cooled
to room temperature and extracted with ethyl acetate. The organic
layer was washed thoroughly with water (3 ¥ 2 mL), dried over
Na2SO4, solvent was evaporated under vacuum to provide crude
product, which was passed short silica gel column leading to
pure biphenyl (152 mg, 99%) as white crystal. The product was
characterized by IR, 1H NMR and 13C NMR spectroscopic data
and these data were in good agreements with those literatures
reported values.
(4-Propoxybenzylidene)propanedinitrile (product A, Scheme 2)
Acknowledgements
Light yellowish liquid; IR (n[cm-1]) 3320, 2250, 2223, 1112, 910;
NMR: dH (200 MHz, CDCl3) 1.06 (t, J = 7.4 Hz, 3H), 1.86 (q, J =
7.4 Hz, 2H), 4.03 (t, J = 7.4 Hz, 2H), 6.99 (d, J = 8.8 Hz, 2H), 7.65
(s, 1H), 7.90 (d, J = 8.8 Hz, 2H); NMR: dC (CDCl3) 10.7, 22.6,
70.4, 112.0, 114.1 (2C), 115.8 (2C), 124, 133.8 (2C), 159.2, 164.8.
Anal. calcd. for C14H14N2O: C 74.31, H 6.24; N 12.38. Found: C
74.25, H 6.29; N 12.31.
This work was supported by the Director, Office of Science, Office
of Biological & Environmental Research, Biological Systems
Science Division, of the U.S. Department of Energy under
Contract No. DE-FG02-08ER64624, NSF URC Grant 0532242
for the purchase of an Elemental Analyzer used in this research
and NSF-CHE-0722632.
Notes and references
(4-Propoxybenzyl)propanedinitrile (product B, Scheme 2)
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cm-1; dH (200 MHz, CDCl3) 1.03 (t, J = 8.0 Hz, 3H), 1.20 (q, J =
8.0 Hz, 2H), 1.80 (t, J = 8.0 Hz, 2H), 3.21 (d, J = 6.6 Hz, 1H), 3.70
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