Immobilization of grubbs catalyst as supported ionic liquid catalyst (Ru-SILC)

Article abstract of DOI:10.1055/s-2008-1078570

Grubbs olefin metathesis catalyst was immobilized as a ruthenium-supported ionic liquid catalyst (Ru-SILC) in pores of amorphous alumina with the aid of ionic liquid [hmim]PF₆. This Ru-SILC was effective for various olefin metathesis reactions

Full text of DOI:10.1055/s-2008-1078570

LETTER
1813
Immobilization of Grubbs Catalyst as Supported Ionic Liquid Catalyst
(Ru-SILC)
I
mmobilization of
i
G
rubbs
s
C
atalyst
a
as Supporte
h
d Ionic
L
iqu
i
id Cata
r
lyst o Hagiwara,*a Naotaro Okunaka,a Takashi Hoshi,b Toshio Suzukib
a
Graduate 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
b
Faculty of Engineering, Niigata University, 8050, 2-Nocho, Ikarashi, Nishi-ku, Niigata 950-2181, Japan
Received 16 April 2008
ruthenium carbene complexes even in nonpolar organic
solvents such as n-hexane.
Abstract: Grubbs olefin metathesis catalyst was immobilized as a
ruthenium-supported ionic liquid catalyst (Ru-SILC) in pores of
amorphous alumina with the aid of ionic liquid [hmim]PF6. This
Ru-SILC was effective for various olefin metathesis reactions such
as intra- or intermolecular macrocyclization and dimerization, and
used up to six times after simple decantation.
To solve these issues, immobilization of the Grubbs ruthe-
nium catalyst has been investigated intensively not only
on solid support but also on liquid support. Ionic liquid
was used as a liquid support4,5 due to its nonlipophilicity,
nonhydrophilicity and nonvolatility, although the higher
solubility of Grubbs catalysts in organic solvents could
not prevent loss of the catalyst from the ionic liquid layer
during extraction of the product.6 Immobilization of the
Grubbs ruthenium catalyst as a task-specific ionic liquid7
was successful to realize efficient recycle use by precipi-
tation with a poor solvent. Additional efforts to immobi-
lize the catalyst on organic supports include
immobilization on polymers,8 dendrimers,9 polyethylene
glycol,10 and fluorous polymers,11 and encapsulation in
polymer beads.12 On the other hand, immobilization on in-
organic supports is rare except on silica,13 in which the
Grubbs ruthenium catalyst was anchored with covalent
bond on the silica surface. Toward this end, an easier and
more economical protocol is desired to avoid tedious im-
mobilization procedures.
Key words: heterogeneous catalysis, ionic liquids, methathesis, ru-
thenium, supported catalysis
Since the development of Ru and Mo metallocarbene
complexes by Grubbs and Schrock (Figure 1), the olefin
metathesis reaction has been extensively utilized for vari-
ous synthetic transformations, and this innovated the con-
cept of not only contemporary target-oriented synthesis
but also various aspects of organic synthesis including
polymer synthesis.1 These metallocarbene complexes re-
act only with olefins without affecting or being affected
by other functional groups. In a substrate having two ole-
fins in the same molecule, a cyclic compound is synthe-
sized, particularly large carbocycles difficult to synthesize
thus far. Therefore, the reaction has been utilized as a key
process for syntheses of multifunctional compounds, es-
pecially complex natural products.2
In the present study, the Grubbs Ru catalyst in an ionic liq-
uid was immobilized in pores of an inorganic support. The
heterogeneous catalyst thus prepared [ruthenium-support-
ed ionic liquid catalyst (Ru-SILC)] exhibited higher cata-
lytic activity and a recyclable nature in olefin metathesis
reactions (Figure 2).
N
N
N
N
PCy3
Mes
Mes
Cl
Mes
Mes
Cl
Cl
Ru
Ru
O
Ru
Cl
Cl
Cl
Ph
Ph
PCy3
PCy3
1
2
3
Figure 1 Representative Grubbs olefin metathesis catalysts
However, these excellent catalysts have some drawbacks.
Since the catalysts are homogeneous and easily deterio-
rate on workup, recycle use is difficult. Also, catalyst
turnover number (TON) is not high. In addition, high
price and large molecular weight circumvent a catalytic
reaction in large scale. Another issue is ruthenium con-
tamination of the product3 due to higher lipophilicity of
Figure 2 Ru-SILC and metathesis reaction
The Ru-SILC was prepared according to the same proce-
dure used for Pd-SILC.14 A suspension of amorphous in-
organic solid in a solution of the Grubbs ruthenium
catalyst and ionic liquid in tetrahydrofuran (THF) was
SYNLETT 2008, No. 12, pp 1813–1816
Advanced online publication: 02.07.2008
DOI: 10.1055/s-2008-1078570; Art ID: U03508ST
x
x
.x
x
.2
0
0
8
© Georg Thieme Verlag Stuttgart · New York

1814
H. Hagiwara et al.
LETTER
stirred until pale blue, then the catalyst was transferred to Employing the ring-closing metathesis (RCM) reaction of
a solid support. Subsequently, THF was evaporated to diethyl diallylmalonate (4) as a probe to tune reaction pa-
dryness to give pale blue powder. The free-flowing nature rameters (Equation 1), various Ru-SILC based on normal-
of Ru-SILC along with the amount of the ionic liquid used phase alumina were prepared by changing ionic liquid
(10 wt% to support) suggest that the ionic liquid layer ex- support, and then their catalytic activities were evaluated.
ists in the pores of amorphous solid in the same manner as Among them, Ru-SILC immobilized with [bmim]PF6 or
Pd-SILC.14a
[hmim]PF6 provided the best result in the RCM reaction
(Table 2, entries 4 and 5). Employing the catalyst
(Table 1, entry 12) prepared without an ionic liquid, a
large amount of starting material was recovered in entry 6.
This result suggests that the ionic liquid layer of SILC
plays an important role to provide medium for the RCM
reaction, as illustrated in Figure 2.
Among various inorganic solid supports including hy-
droxyapatite, molecular sieves and their surface-modified
support, normal-phase amorphous alumina powder en-
abled immobilization of Grubbs I catalyst (1) (Table 1).
Immobilization of the Grubbs catalyst with reversed-
phase silica or alumina was not successful (Table 1, en-
tries 4, 6, and 7).3a Grubbs II (2) and Hoveyda–Grubbs II
(3) catalysts were also immobilized, although at slightly
low loading (Table 1, entries 10 and 11). The loading of
Grubbs I catalyst (1) was 0.03 mmol/g of the alumina at
the maximum. Although amount of loading was evaluated
roughly by weight gain, more than 99% of Grubbs I cata-
lyst (1) was immobilized according to inductively coupled
plasma atomic emission spectroscopy (ICP-AES) analysis
of the ether rinse in entry 9.20
Table 2 Investigation of Optimum Ionic Liquid for Support in
RCM Reaction of 4
Entrya Ionic liquid
Time (h) Yield (%)b
c
1
[bmim]Br
4
4
–
–
c
2
[bmim]NTf2
3
[bmim]CF2H(CF2)3CH2OSO3
18
58
77
79
4
4
[bmim]PF6
[hmim]PF6
–
2.5
Table 1 Investigation of Optimum Solid Support for Immobiliza-
tion of 1 with the Aid of [bmim]PF6
5
1
2
6d
Entry
1
Support material
hydroxyapatite
3Al2O3·2SiO2
MS 4 Å pellets
Loading of 1 (mmol/g)a
a Reaction was catalyzed by 0.1 equiv of Grubbs I Ru-SILC (1) in re-
–
fluxing toluene.
b Isolated pure product.
2
–
c Starting material was recovered.
d Reaction was carried out in refluxing benzene with 0.05 equiv of Ru-
SILC.
3
–
b
4
NDEAP- SiO2
–
5
SiO2
0.006
0.0007
0.009
0.0057
0.025–0.03
0.01
0.01
0.02
Then, the effect of solvents was investigated, in which ar-
omatic hydrocarbon solvents provided better results
(Table 3, entries 6–8). It is worthy of note that the leach-
ing of Ru from Ru-SILC was 0 ppm by ICP-AES analysis.
6
NDEAP-Al2O3
c
7
NAP-Al2O3
8d
9
Al2O3
Al2O3
Al2O3
Al2O3
Al2O3
Table 3 Effect of Solvents on RCM Reaction of 4
Entrya Solvent
Conditions
r.t. to 120 °C, 10 h
reflux, 10 h
reflux, 10 h
reflux, 4 h
Yield (%)b
10e
11f
12g
c
1
2
3
4
5
6
7d
8
perfluoropolyether
–
c
tert-butylmethyl ether
CH2Cl2
–
c
–
a Obtained by weight gain.
c
b N,N-Diethylaminopropylated silica.
c Aminopropylated silica.
EtOH
–
c
d Immobilization in aluminum pellet without ionic liquid.
e Grubbs II catalyst (2) was immobilized.
H2O
r.t., 10 h
–
benzene
reflux, 1 h
82
77
76
f Hoveyda–Grubbs II catalyst (3) was immobilized.
g Immobilization in aluminum powder without ionic liquid.
benzene
reflux, 18 h
reflux, 2.5 h
toluene
EtO2C
CO2Et
EtO2C
CO2Et
a Reaction was catalyzed by 0.1 equiv of Grubbs I Ru-SILC (1) immo-
bilized on normal-phase alumina with the aid of [hmim]PF6.
b Isolated pure product.
Grubbs I Ru-SILC
solvent
c Starting material was recovered.
4
5
d Grubbs I Ru-SILC (0.005 equiv) was used. TON was calculated to
be 154.
Equation 1 RCM reaction of diethyl diallylmalonate (4)
Synlett 2008, No. 12, 1813–1816 © Thieme Stuttgart · New York

LETTER
Immobilization of Grubbs Catalyst as Supported Ionic Liquid Catalyst
1815
EtO2C
CO2Et
EtO2C
CO2Et
This result means that the RCM reaction proceeds inside
of the Ru-SILC. A TON of 154 is reasonable (Table 3, en-
try 7) compared with literature precedents.13
(1)b,c
1 h, 82%
4
516
By employing the optimized reaction conditions (Table 3,
entry 6), Ru-SILC was successfully recycled in the RCM
reaction of diethyl diallylmalonate (4) employing 0.05
equivalents of Ru-SILC in refluxing benzene up to five
times in 78% average yield (Table 4). During recycle ex-
periments, the decrease in catalytic activity was moderate.
EtO2C
CO2Et
EtO2C
CO2Et
(2)d
3 h, 60% (92%, BRSM)
6
716
Ts
N
Ts
N
(3)c
Table 4 Recycle Use of Ru-SILC in RCM Reaction of 421
1 h, 96%
8
916
Entrya
Time (h)
Yield (%)b
EtO2C
CO2Et
CO2Et
EtO2C
1
2
3
4
5
6
1
1
1
2
2
2
82
81
76
81
78
70
(4)c
1.5 h, 94%
10
1116
CO2Et
EtO2C
CO2Et
EtO2C
(5)e,f
12 h, 75%
a Reaction was carried out in refluxing benzene catalyzed by 0.05
equiv of Grubbs I Ru-SILC (1) immobilized on alumina with the aid
of [hmim]PF6.
12
1322
O
O
b Isolated pure product.
(6)c,g
12 h, 76%
The present RCM reaction conditions catalyzed by Ru-
SILC were successful in closing 7-, 14- and 15-membered
rings (Scheme 1, equations 4, 5, and 6). Unfortunately,
dimeric compounds were obtained even in very dilute so-
lutions due to inherent difficulty in cyclizing 9- and 11-
membered carbocycles (Scheme 1, equations 7 and 8). In
equations 5, 7–9 (Scheme 1), single stereoisomers were
obtained. By employing Ru-SILC, which immobilized
Hoveyda–Grubbs II catalyst (3), trisubstituted olefin 7
was synthesized (Scheme 1, equation 2).
1517
14
EtO2C
CO2Et
EtO2C
CO2Et
(7)c,h
14 h, 68%
CO2Et
EtO2C
1618
1723
CO2Et
In summary, Grubbs ruthenium catalysts were immobi-
lized as Ru-SILC in alumina pores with the aid of the ionic
liquid [hmim]PF6.15 Immobilization was simple and cost
effective, since there was no need to synthesize a polymer
or to employ a large amount of ionic liquid. The present
protocol offers an effective and simple method of immo-
bilizing sophisticated but unstable homogeneous organo-
metallic catalysts with the aid of ionic liquid in pores of
amorphous inorganic supports.
O
O
CO2Et
(8)c,h
12 h, 43%
1924
O
EtO2C
18
EtO2C
CO2Et
CO2Et
CO2Et
EtO2C
(9)c,h
14 h, 78%
Acknowledgment
2019
2125
This work was partially supported by a Grant-in-Aid for Scientific
Research on Priority Areas (20031011 for H. H.) from The Ministry
of Education, Culture, Sports, Science and Technology (MEXT).
We thank Prof. T. Ito, Tottori University, for providing the ionic
liquid [bmim]CF2H(CF2)3CH2OSO3.
a Reaction was carried out in toluene at 100 °C.
b Reaction was carried out in refluxing benzene.
c Grubbs I Ru-SILC 1 (0.05 equiv) was used.
d Hoveyda–Grubbs II Ru-SILC 3 (0.1 equiv) was used.
e Grubbs I Ru-SILC 1 (0.1 equiv) was used.
CO2Et
f
Inseparable mixture of cis/trans (1:2.3) isomers.
g Separable mixture of cis/trans (1:2.3) isomers.
h A single isomer was obtained.
Scheme 1 Catalytic activity of Ru-SILC16–19,22–25
Synlett 2008, No. 12, 1813–1816 © Thieme Stuttgart · New York

1816
H. Hagiwara et al.
LETTER
(16) Clavier, H.; Nolan, S. P. Chem. Eur. J. 2007, 13, 8029.
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39, 3772. (b) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z.
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Organometallics 2003, 22, 2426; and earlier references cited
therein. (b) Gallivan, J. P.; Jordan, J. P.; Grubbs, R. H.
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(11) Yao, Q.; Zhang, Y. J. Am. Chem. Soc. 2004, 126, 74.
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(15) Recent advances on SILC: (a) Mehnert, C. P.; Mozeleski, E.
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(20) Preparation of Ru-SILC
To activated Al2O3 (powder for column chromatography
purchased from Wako Chemical Co., 300 mesh, 1.545 g) and
Grubbs I catalyst (1, 38 mg, 0.046 mmol, 0.03 mmol/g of
Al2O3) was added a solution of [hmim]PF6 (150 mg, 10
wt%) in THF. The resulting slurry was stirred at r.t. for 4 h,
when the pale blue color of THF solution was transferred to
Al2O3. After evaporation of THF in vacuo, the powder was
rinsed with anhyd Et2O twice. Evacuation in vacuo provided
Ru-SILC (1.732 g) as a pale blue powder. Since leaching of
Ru into the ether rinse was 0.05 ppm (0.105% as the catalyst
1) by ICP-AES analysis, more than 99% of Grubbs I
catalyst(1) was immobilized on Al2O3.
(21) RCM Reaction of Diethyl Diallylmalonate 4
A stirred suspension of diethyl 2,2-diprop-2-enylpropane-
1,3-dioate (4, 31 mg, 0.13 mmol) and Ru-SILC [308 mg,
0.0065 mmol of Grubbs I catalyst(1)] in benzene (1.5 mL)
was heated under reflux for 1 h. The organic layer was
separated by filtration, and the flask was rinsed with Et2O.
The combined organic layer was evaporated to dryness in
vacuo. The residue was purified by medium pressure LC
(eluent: n-hexane–EtOAc = 9:1) to give ethyl 1-(ethoxy-
carbonyl)cyclopent-3-enecarboxylate (5, 23 mg, 82%).
Recovered Ru-SILC was used intact for further recycle
experiments.
(22) Compound 13: 1H NMR (500 MHz, CDCl3): d = 1.19–1.47
(m, 20 H), 1.82–2.12 (m, 8 H), 4.13–4.19 (m, 4 H), 5.24–
5.33 (m, 1 H), 5.49–5.55 (m, 0.3 H), 5.57–5.63 (m, 0.7 H).
13C NMR (67.5 MHz, CDCl3): d = 171.9, 131.5, 130.7, 61.0,
56.5, 31.2, 30.4, 30.3, 28.4, 27.3, 27.0, 26.0, 24.5, 24.2, 20.7,
14.2. MS: m/z (%) = 338 (67) [M+], 293 (90), 265 (27), 246
(100), 218 (100), 191 (100), 173 (100), 168 (91). IR: 1723,
1464, 1448, 1368, 1260, 1231, 1190 cm–1. HRMS: m/z calcd
for C20H34O4 [M]+: 338.2457; found: 338.2458.
(23) Compound 17: 1H NMR (270 MHz, CDCl3): d = 1.21–1.28
(m, 20 H), 1.84–2.00 (m, 16 H), 4.14–4.22 (m, 8 H), 5.39–
5.42 (m, 4 H). 13C NMR (67.5 MHz, CDCl3): d = 171.7,
130.1, 61.1, 57.0, 32.1, 30.2, 23.5, 14.2. MS: m/z (%) = 536
(7) [M+], 491 (13), 444 (8), 399 (15), 336 (19), 279 (8), 265
(16), 251 (12), 173 (100). IR: 1719, 1463, 1445, 1369, 1299,
1254, 1095, 1028 cm–1. HRMS: m/z calcd for C28H43O7 [M
– OCH2CH3]+: 491.3009; found: 491.3017.
(24) Compound 19: 1H NMR (500 MHz, CDCl3): d = 1.21–1.34
(m, 14 H), 1.55–1.67 (m, 4 H), 1.73–2.05 (m, 12 H), 2.43–
2.60 (m, 4 H), 3.39–3.45 (m, 2 H), 4.14–4.19 (m, 4 H), 5.34–
5.42 (m, 4 H). 13C NMR (67.5 MHz, CDCl3): d = 205.1,
169.7, 130.3, 129.5, 61.2, 58.3, 41.4, 32.3, 31.8, 28.9, 27.9,
27.4, 23.0, 14.2. MS: m/z (%) = 476 (3) [M+], 431 (7), 412
(16), 402 (4), 384 (15), 366 (11), 175 (19), 108 (45), 91 (51),
79 (73), 67 (95), 55 (100). IR: 1738, 1710, 1445, 1370, 1268,
1188, 909 cm–1. HRMS: m/z calcd for C26H39O5 [M –
OCH2CH3]+: 431.2797; found: 431.2802.
(25) Compound 21: 1H NMR (500 MHz, CDCl3): d = 1.25–1.28
(m, 12 H), 1.34–1.41 (m, 4 H), 1.86–1.92 (m, 4 H), 1.99–
2.07 (m, 4 H), 3.29–3.33 (m, 2 H), 4.16–4.23 (m, 8 H), 5.34–
5.36 (m, 0.6 H), 5.37–5.39 (m, 1.4 H). 13C NMR (67.5 MHz,
CDCl3): d = 169.3, 129.9, 61.3, 51.9, 32.2, 28.3, 27.3, 14.2.
IR: 1726, 1462, 1446, 1371, 1178, 1157 cm-1. MS: m/z (%)
= 428 (3) [M+], 383 (5), 337 (100), 269 (70), 227 (10), 173
(72), 160 (86), 130 (50). HRMS: m/z calcd for C22H36O8
[M]+: 428.2410; found: 428.2394.
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