Palladium-supported ionic liquid catalyst (Pd-SH-SILC) immobilized on mercaptopropyl silica gel as a chemoselective, reusable and heterogeneous catalyst for catalytic hydrogenation

Article abstract of DOI:10.1039/c1gc15217c

Palladium acetate was immobilized as Pd-SH-SILC (Pd-supported ionic liquid catalyst) in the pores of amorphous mercaptopropyl silica gel with the aid of an ionic liquid [bmim]BF₄. The heterogeneous Pd-SH-SILC was effective for catalytic hydrogenation of a variety of olefins at atmospheric pressure and room temperature. It withstood recycling up to 10 times by decantation.

Full text of DOI:10.1039/c1gc15217c

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Palladium-supported ionic liquid catalyst (Pd-SH-SILC) immobilized on
mercaptopropyl silica gel as a chemoselective, reusable and heterogeneous
catalyst for catalytic hydrogenation†
Hisahiro Hagiwara,*a Tomomi Nakamura,a Takashi Hoshib and Toshio Suzukib
Received 28th February 2011, Accepted 15th March 2011
DOI: 10.1039/c1gc15217c
Palladium acetate was immobilized as Pd-SH-SILC (Pd-
supported ionic liquid catalyst) in the pores of amorphous
mercaptopropyl silica gel with the aid of an ionic liquid
[bmim]BF4. The heterogeneous Pd-SH-SILC was effective
for catalytic hydrogenation of a variety of olefins at at-
mospheric pressure and room temperature. It withstood
recycling up to 10 times by decantation.
vessels, allowing straightforward separation of the catalyst and
product by filtration and recycling of the catalyst.
A
variety of SILCs for catalytic hydrogenation have
been reported in combination with various catalysts, ionic
liquids and solid supports: Rh/[bmim]PF6/silica gel,4a
Pd/guanidium lactate/molecular sieves,4c porous Ni/[bmim][n-
C8H17OSO3],4h Rh/[bmim]PF6/carbon nanotubes,4i and
Pd/[bmim]PF6/carbon cloth.4k However, in most cases, the
hydrogenations were carried out under high pressure, the
substrates were simple olefins with no functional groups, and
some SILCs lacked recyclability.
As a part of our ongoing efforts to immobilize homogeneous
catalysts as SILCs,5 we immobilized palladium acetate as a
pre-catalyst with an ionic liquid as the supporting liquid layer
in the pores of an amorphous reversed-phase silica gel. The
heterogeneous catalyst thus prepared (Pd-SH-SILC) exhibited
higher general and chemoselective catalytic activity, and was
also recyclable (Fig. 1).
Catalytic hydrogenation is one of the most fundamentally
important synthetic transformations both in the laboratory and
in industry. More active, mild, selective and environmentally
benign protocols are continuously being sought. Although
homogeneous catalysts exhibit higher, more selective catalytic
activity, they lack stability and recyclability. One way to improve
recyclability is to immobilize a homogeneous catalyst in a
liquid support; this can be done by simply dissolving a known
homogeneous catalysts in the liquid.
Ionic liquids have attracted considerable interest as liquid
supports for homogeneous catalysts, because of they are non-
lipophilic, non-hydrophilic, non-volatile and non-coordinating
and are capable of dissolving highly ionic substances.1 Catalytic
hydrogenations in ionic liquids have been extensively inves-
tigated with a variety of homogeneous catalysts,2 and have
reaction rates greater than those of conventional solvent.3
However, catalytic reactions in ionic liquids suffer from high
cost and the laborious extraction of the product from the
viscous ionic liquid. To solve these issues, SILC (supported
ionic liquid catalyst) immobilization was designed to confine
the homogeneous catalyst in an ionic liquid to the pores
of an amorphous solid.1,4 SILC immobilization transforms
the homogeneous catalyst into a heterogeneous catalyst, and
converts the pores of the amorphous solid into micro reaction
Fig. 1 Catalytic hydrogenation by Pd-SH-SILC.
Immobilization of palladium acetate was carried out accord-
ing to the procedure described in our previous papers (Fig. 1).5a–c
A suspension of silica gel in a solution of palladium acetate and
the ionic liquid was stirred until the orange color of the solution
was transferred to the silica gel. After evaporation of the solvent,
an orange free-flowing powder was obtained. Of the silica gels,
mercaptopropyl silica gel (sulfur content: 0.67 mmol g-1) was
aGraduate School of Science and Technology, Niigata University, 8050,
2-Nocho, Ikarashi, Nishi-ku, Niigata, 950-2181, Japan.
E-mail: hagiwara@gs.niigata-u.ac.jp; Fax: +81-252627368;
Tel: +81-252627368
bFaculty of Engineering, Niigata University, 8050, 2-Nocho, Ikarashi,
Nishi-ku, Niigata, 950-2181, Japan
† Electronic supplementary information (ESI) available: SEM images of
Pd-SH-SILC and GC charts. See DOI: 10.1039/c1gc15217c
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Table 1 Optimization of solid support and ionic liquid for the immobilization of palladium acetate
Entry
Support material
Ionic liquid
Abbreviation
Pd-SH-SILC
Loading of Pd (mmol g-1)
1
2
3
4a
5
6
7
Mercaptopropyl SiO2
[bmim]PF6
[bmim]BF4
—
[bmim]NTf2
[bmim]PF6
0.50
0.51
0.53
0.43
0.32
0.30
0.25
Aminopropyl SiO2
Diethylaminopropyl SiO2
SiO2
Pd-NH2-SILC
Pd-NEt2-SILC
Pd-SILC
a The ionic liquid was not confined in the pores of the silica gel.
found to immobilize a sufficient amount of palladium acetate
in the presence or absence of an ionic liquid (Table 1, entries
1~3). The higher uptake of palladium acetate in mercaptopropyl
silica gel than normal silica gel might be partly because the
mercapto group has a better affinity for palladium.6 Whereas
palladium acetate was absorbed into the mercaptopropyl silica
gel, [bmim]NTf2 was not confined to the pores (Table 1,
entry 4). Mercaptopropyl silica gel has already been used as
a scavenger of metal ions7 and as a solid support for Cu-SILC.5e
Palladium immobilized in mercaptopropyl mesoporous silica gel
was also found to be effective as a heterogeneous catalyst for
Mizoroki–Heck and Suzuki–Miyaura reactions by Shimizu7a
and Crudden,7b respectively. Mercaptopropyl silica gel can be
prepared by grafting mercaptopropyl trimethoxysilane on to
normal silica gel.
was maintained for more than 3 months when kept at room
temperature under nitrogen. Despite its higher catalytic activity,
the reduced Pd-SH-SILC did not ignite even after prolonged
exposure to air.
Pd-SH-SILC could be recycled up to 10 times by decantation,
while maintaining a 100% conversion with virtually no loss of
reactivity (Table 3, entries 1 and 2). The activity of Pd-SH-
SILC immobilized with the aid of [bmim]BF4 was found to best
withstand recycling (Table 3, entry 1).9 It is interesting to note
that all subsequent recycle experiments carried out in Table 3,
entry 1, terminated in 40~50 min as the result of activation of
Pd-SH-SILC in the initial hydrogenation. Although palladium
acetate could also be immobilized on mercaptopropyl silica
without an ionic liquid, its catalytic activity was reduced
(Table 3, entry 3).
The hydrogenation of cyclohexene was used as a benchmark
reaction to investigate the catalytic performance of a variety of
Pd-SILCs (Table 2). The Pd-SILC immobilized in the pores of
mercaptopropyl silica (Pd-SH-SILC) using [bmim]PF6 exhibited
comparable catalytic performance to Pd/C (Table 2, entry 2).
Moreover, pre-treatment of the Pd-SH-SILC under a hydrogen
atmosphere resulted in black powder due to the formation of
Pd(0), which was more active (Table 2, entries 6 and 7) than
the other pre-treatments (Table 2, entries 8 and 9). Pd-SH-
SILC immobilized with [bmim]BF4 exhibited the best catalytic
performance (Table 2, entry 7). This result might be because the
molecular hydrogen is four times more soluble in [bmim]BF4
than in [bmim]PF6.8 The activity of the reduced Pd-SH-SILC
The leaching of palladium from the Pd-SH-SILC was below
the limit of detection for ICP-AES analysis in the reduction
of cyclohexene using 0.05 eq. of Pd-SH-SILC. The turnover
number (TON) for Pd-SH-SILC was evaluated as 40,000 and
the turnover frequency (TOF) was found to be 555 h-1.
The catalytic performance of Pd-SH-SILC immobilized with
[bmim]BF4 was general to a variety of olefinic substrates
(Table 4).10 Allylic alcohol was reduced without accompanying
hydrogenolysis11 (Table 4, entry 1), and acid or base sensitive
protecting groups were not de-protected (Table 4, entries 2
and 3). Furthermore, the benzyl ether protecting group was
left intact under these reaction conditions, even for prolonged
reaction times (Table 4, entry 5). Similarly, the Cbz group
remained intact (Table 4, entry 6), when ethyl acetate was
used as solvent.12 Aryl bromide was not reduced (Table 4,
entry 7),13 and a single stereoisomer with a trans-ring juncture
was obtained (Table 4, entry 8).14 a,b-Unsaturated aldehydes
were reduced in quantitative yield without further reduction
to primary alcohols or formation of acetals (Table 4, entries
9 and 10). Moderate yields are because of the volatile nature
Table 2 Optimization of Pd-SILC for the catalytic hydrogenation of
cyclohexene
Entrya
Pd-SILCb
Time (min)
120
Conversion (%)c
1
2
3
4
Pd/C
100
100
88
Pd-SH-SILC
Pd-NEt2-SILC
Pd-SILC
63
5
Pd-NH2-SILC
Pd-SH-SILC
46
Table 3 Effect of ionic liquid on catalytic performance of Pd-SH-SILC
6d
7d,e
8f
9g
70
70
100
150
100
100
100
100
in hydrogenation of cyclohexene in 10 cycles
Entrya
Ionic liquid
Average reaction time (min)
a Reaction carried out in ethanol at atmospheric pressure at 25 ◦C with
0.05 eq. of Pd-SILC. b Immobilized with [bmim]PF6. c Determined by
GC with toluene as an internal standard. d Pd-SILC was reduced with
hydrogen in ethanol for 5 h before use. e Immobilized with [bmim]BF4.
f Pd-SILC was placed in ethanol for 18 h before use. g Pd-SILC was
heated at reflux in n-hexane for 21 h before use.
1
2
3
[bmim]BF4
[bmim]PF6
—
45
80
60
a Reaction was carried out in ethanol at atmospheric pressure and room
temperature with 0.05 equiv of Pd-SH-SILC until 100% conversion.
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Table 4 Catalytic performance of Pd-SH-SILC for hydrogenation
Entrya
1
Olefin 1
Product 2
Time (h)
2
Yield (%)a
100
2
3
4
R = Ac 1b
R = TBS 1c
R = Bn 1d
R = Bn 1d
R = Ac 2b
R = TBS 2c
R = Bn 2d
R = Bn 2d
0.5
0.5
0.5
0.5
24
97
96
97
97
99
5
6c
7
8
1
5
94
72
9
5
3
78
86
10
11
5
100
12
13
5
86
99
0.5
14
15
2
5
100
84
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Table 4 (Contd.)
Entrya
16
Olefin 1
Product 2
Time (h)
1
Yield (%)a
98
17
18
24
82
94
0.5
19
2
82
a Reaction was carried out in ethanol at atmospheric pressure and room temperature with 0.05 eq. of Pd-SH-SILC, which was immobilized with
[bmim]BF4, reduced with hydrogen in ethanol for 5 h before use and stored at room temperature under nitrogen. b Isolated pure product after MPLC
purification. c Ethyl acetate was used as the solvent.
of products during purification. These results show Pd-SH-
SILC is a mild and selective catalyst, which could be applied
to the hydrogenation of a variety of olefinic substrates with a
wide range of functional groups. Moreover, tetrasubstituted a,b-
Notes and references
1 Ionic Liquids in Synthesis, ed. P. Wasserscheid, T. Welton, Wiley-VCH
Verlags, GmbH&Co, KGaA, Weinheim, 2nd edn 2008.
2 Some representative reviews on hydrogenation in an ionic liquid:
unsaturated carbonyl compounds were reduced at atmospheric
pressure (Table 4, entries 11, 13 and 14), which verifies the
superior catalytic activity of Pd-SH-SILC. In contrast to the
hydrogenation of pulegone catalyzed by Pd/C, thymol was not
a by-product when Pd-SH-SILC was used (Table 4, entry 12).15
The acetylenic bond was also smoothly hydrogenated (Table 4,
entry 18), and the carbonyl group of chalcone remained intact
(Table 4, entry 19).13
(a) H. Zhao and S. V. Malhotra, Aldrichimica Acta, 2002, 35, 75;
(b) P. J. Dyson, Appl. Organomet. Chem., 2002, 16, 495; (c) V. I.
Parvulescu and C. Hardacre, Chem. Rev., 2007, 107, 2615.
3 Y. Chauvin, L. Mussman and H. Olivier, Angew. Chem., Int. Ed.
Engl., 1995, 34, 2698.
4 Recent advances on SILC (a) C. P. Mehnert, E. J. Mozeleski and R. A.
Cook, Chem. Commun., 2002, 3010; (b) A. Riisager, P. Wasserscheid,
R. van Hal and R. Fehrmann, J. Catal., 2003, 219, 452; (c) J. Huang,
T. Jiang, H. Gao, B. Han, Z. Liu, W. Wu, Y. Chang and G. Zhao,
Angew. Chem., Int. Ed., 2004, 43, 1397; (d) S. Breitenlechner, M.
Fleck, T. E. Mu¨ller and A. Suppan, J. Mol. Catal. A: Chem., 2004,
214, 175; (e) A. Riisager, R. Fehrmann, S. Flicker, R. van Hal, M.
Hanmann and P. Wasserscheid, Angew. Chem., Int. Ed., 2005, 44,
815; (f) C. P. Mehnert, Chem.–Eur. J., 2005, 11, 50; (g) L.-L. Lou, K.
Yu, F. Ding, W. Thou, X. Peng and S. Liu, Tetrahedron Lett., 2006,
47, 6513; (h) U. Kernchen, B. Etzol, W. Korth and A. Jess, Chem.
Eng. Technol., 2007, 30, 985; (i) L. Rodriguez-Perez, E. Teuma, A.
Falqui, M. Goomez and P. Serp, Chem. Commun., 2008, 4201; (j) Y.
Gu and G. Li, Adv. Synth. Catal., 2009, 351, 817; (k) J.-P. Mikkola, P.
Virtanen, H. Karhu, T. Salmi and D. U. Murzin, Green Chem., 2006,
8, 197.
There are two published examples of hydrogenation by het-
erogeneous palladium catalysts poisoned by sulfur. Sajiki et al.
used Pd/C poisoned with diphenylsulfide to give chemoselective
hydrogenation at room temperature and atmospheric pressure,
in which a variety of olefins were reduced leaving benzyl ether,
benzyl ester or N-Cbz protecting groups intact.13 Rossi et al.
reported the catalytic hydrogenation of cyclohexene by palla-
dium on iron nanoparticles grafted with mercaptopropionic acid
◦
at 75 C under 10 atm of hydrogen.16 However, Pd-SH-SILC
5 (a) H. Hagiwara, Y. Sugawara, K. Isobe, T. Hoshi and T. Suzuki,
Org. Lett., 2004, 6, 2325; (b) H. Hagiwara, Y. Sugawara, T. Hoshi
and T. Suzuki, Chem. Commun., 2005, 2942; (c) H. Hagiwara, K. H.
Ko, T. Hoshi and T. Suzuki, Chem. Commun., 2007, 2838; (d) H.
Hagiwara, N. Okunaka, T. Hoshi and T. Suzuki, Synlett, 2008, 1813;
(e) H. Hagiwara, H. Sasaki, T. Hoshi and T. Suzuki, Synlett, 2009,
643; (f) H. Hagiwara, T. Nakamura, N. Okunaka, T. Hoshi and
T. Suzuki, Helv. Chim. Acta, 2010, 93, 175; (g) H. Hagiwara, T.
Kuroda, T. Hoshi and T. Suzuki, Adv. Synth. Catal., 2010, 352,
909.
6 T.-L. Ho, Hard and Soft Acids and Bases Principle in Organic
Chemistry, Academic Press, New York, 1977.
7 (a) K.-I. Shimizu, S. Koizumi, T. Hatamachi, H. Yoshida, S. Komai,
T. Kodama and Y. Kitayama, J. Catal., 2004, 228, 141; (b) C. M.
Crudden, M. Sateesh and R. Lewis, J. Am. Chem. Soc., 2005, 127,
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exhibited improved catalytic performance along with better
recyclability.
In summary, palladium acetate was immobilized as Pd-SH-
SILC in the pores of amorphous mercaptopropyl silica gel with
the ionic liquid [bmim]BF4. The immobilization procedure was
simple and cost effective without employing synthetic transfor-
mations or a large amount of costly ionic liquid. The reduced
Pd-SH-SILC exhibited higher, more selective catalytic activity
at room temperature under atmospheric pressure, and could be
recycled up to 10 times by simple decantation, while maintaining
100% conversions. The protocol we report herein may be suitable
for liquid-phase hydrogenation of substrates, especially those of
higher molecular weight, those with multifunctional groups, or
for process or industrial-scale applications.
8 A. Berger, R. F. de Souza, M. R. Delgado and J. Dupont, Tetrahedron:
Asymmetry, 2001, 12, 1825.
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9 Preparation of Pd-SH-SILC: A solution of [bmim]BF4 (30 mg,
10 wt%) in THF (2 mL) was added to mercaptopropyl silica gel
(powder supplied by Fuji Silysia Chemical LTD., 75~150 mm, SH
content 0.67 mmol g-1, 0.300 g) and Pd(OAc)2 (47 mg, 0.21 mmol).
The resulting slurry was stirred at room temperature for 4 h
under nitrogen, until the orange solution became transparent. After
evaporation of THF in vacuo, the orange powder was rinsed with
anhydrous ether five times. Removal of the solvent provided Pd-SH-
SILC (0.373 g, 0.51 mmol g-1 of silica gel). A suspension of Pd-SH-
SILC (0.300 g) in EtOH (5 mL) was stirred at room temperature
under hydrogen for 5 h. After evaporation of EtOH in vacuo, the
black catalyst was stored at room temperature under nitrogen.
10 Catalytic hydrogenation of cinnamyl alcohol: A suspension of
cinnamyl alcohol (54 mg, 0.4 mmol) and Pd-SH-SILC (40 mg,
0.02 mmol) in EtOH (4 mL) was stirred under hydrogen at room
temperature for 2 h. After centrifugation, the organic layer was
separated by decantation and the flask was rinsed with ether. The
combined organic layers were evaporated to dryness in vacuo. The
residue was purified by column chromatography (eluent: n-hexane–
ethyl acetate = 3 : 1) and subsequently by medium pressure LC (eluent:
n-hexane–ethyl acetate = 6 : 1) to give 3-phenylpropanol (55 mg,
quant.).
11 L. Zhang, J. M. Minterbottom, A. P. Boyes and S. Raymahasay,
J. Chem. Technol. Biotechnol., 1998, 72, 264.
12 T. Ikawa, K. Hattori, H. Sajiki and K. Hirota, Tetrahedron, 2004, 60,
6901.
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Maegawa, Y. Monguchi and H. Sajiki, Tetrahedron, 2006, 62, 11925;
(b) A. Mori, T. Mizusaki, M. Kawase, T. Maegawa, Y. Monguchi,
S. Takao, Y. Takagi and H. Sajiki, Adv. Synth. Catal., 2008, 350,
406.
14 S. Nishimura, I. Takahashi, M. Shiota and M. Ishige, Chem. Lett.,
1981, 877.
15 Reduction of pulegone with Pd/C accompanied 5% of thymol.
16 L. M. Rossi, L. L. R. Vono, F. P. Silva, P. K. Kiyohara, E. L. Duarte
and J. R. Matos, Appl. Catal., A, 2007, 330, 139.
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