Ionic CNT-supported Ru-carbene complex for recyclable ring-closing metathesis

Article abstract of DOI:10.1016/j.tetlet.2012.12.002

The ionic multi-walled carbon nanotubes (IM-f-MWCNTs), which were covalently functionalized with imidazolium salt, have been utilized as support materials for the immobilization of Hoveyda-type Ru-carbene complex. The catalytic activity and recyclability

Full text of DOI:10.1016/j.tetlet.2012.12.002

Tetrahedron Letters 54 (2013) 684–687
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Ionic CNT-supported Ru–carbene complex for recyclable
ring-closing metathesis
⇑
Sujin Lee, Ju Yeon Shin, Sang-gi Lee
Department of Chemistry and Nano Science (BK21), Ewha Womans University, 120-750 Seoul, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The ionic multi-walled carbon nanotubes (IM-f-MWCNTs), which were covalently functionalized with
imidazolium salt, have been utilized as support materials for the immobilization of Hoveyda-type Ru–
carbene complex. The catalytic activity and recyclability of the ionic CNT-supported Ru–carbene com-
plexes have been investigated for ring-closing metathesis of various dienes, and the recovered catalyst
can be reused up to 10 times without any significant loss of catalytic activity.
Received 29 October 2012
Revised 1 December 2012
Accepted 3 December 2012
Available online 7 December 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Ruthenium–carbene complex
Metathesis
Carbon nanotubes
Ionic liquid
Immobilization
Development of cost-effective and environmentally benign cat-
alytic system is of primary importance in meeting the stringent
and compelling demands for greener process.1 In this regard,
immobilization of homogeneous catalysts on support materials is
very attractive to facilitate the recycling and reuse of catalysts. In
the field of supported catalysts, the development of new support
materials is also one of the most challenging issues because the
stability as well as compatibility of support with reaction medium
influence to catalytic activity and recyclability.
Due to the unique properties of carbon nanotubes (CNTs) such
as high mechanical strength, chemical stability, and a large surface
area-to-volume ratio, they have recently received a great deal of
attention as new support materials for the immobilization of cata-
lysts. However, most of current researches on CNT-supported cat-
alysts are focused on immobilization of nanoparticle catalysts,2
and only limited structurally-defined organometallic catalysts
were immobilized onto the CNTs.3 During our continuing studies
on ionic liquids (ILs) in catalysis4 and material science,5 we found
that the use of imidazolium salt-functionalized multi-walled ionic
CNTs (IM-f-MWCNTs) facilitated immobilization of nanoparticle
catalysts with increased stability.5f In the present work, we at-
tempted anchoring of Grubbs–Hoveyda’s Ru–carbene complex
onto IM-f-MWCNTs to demonstrate the utility of ionic CNTs as sup-
port materials for organometallic catalysts. Ruthenium(Ru)-
catalyzed metathesis has become an indispensable method to
construct carbon–carbon double bonds, and is widely employed
in a variety of chemistry fields, including natural products, phar-
maceuticals, and polymer chemistry.6 Despite their great potential
in metathesis, the well-defined Ru-pre-catalysts such as Grubbs
and Hoveyda-type complexes suffer from drawbacks associated
with high cost and leaching problems. To address these challenges,
significant efforts have been devoted toward the development of
recyclable catalysts.7,8 Although there are still arguments on the
‘release–return’ mechanism of the Hoveyda-type catalysts,9
anchoring the chelating isopropoxybenzylidene moiety to a sup-
port material is one of the most common approaches, and a wide
range of materials such as silica, insoluble or soluble polymers
have been investigated. However, very limited Ru-metathesis
catalysts are immobilized onto CNTs.10 Moreover, only one exam-
ple of Hoveyda-type Ru–carbene complex, which was tagged with
pyrene to immobilize via noncovalent p–p stacking, has been so far
reported as a recyclable CNT-supported Ru-metathesis catalyst.10c
Herein we report immobilized Hoveyda-type Ru-catalysts onto
ionic IM-f-MWCNTs and their applications in ring-closing metath-
esis showing high catalytic activity and recyclability.
Scheme 1 showed the synthesis of the ionic CNT-supported Ru–
carbene complexes 4a and 4b having bromide and PF6 anions,
respectively. The imidazole-functionalized MWCNTs 1 were pre-
pared according to our reported procedure,5d and elemental analy-
sis indicated 0.4305 mmol/g of imidazole moiety was incorporated.
The
1-[2-(6-bromohexyloxy)-1-methylethoxy]-2-propenylben-
zene 2 can be synthesized straightforwardly starting from commer-
cially available E/Z-mixture (5:1) of 2-propenylphenol in three-step
with 65% overall yield. Reaction of the imidazole-functionalized
MWCNTs 1 with bromide 2 in acetonitrile reflux for 48 h afforded
⇑
Corresponding author. Tel.: +82 2 3277 4505; fax: +82 2 3277 3419.
E-mail address: sanggi@ewha.ac.kr (S. Lee).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.12.002

S. Lee et al. / Tetrahedron Letters 54 (2013) 684–687
685
N
N
(CH2)6-O
N
N
O
X
O
NH
CH3CN, reflux, 48 h
O
NH
Mes
N
N Mes
Cl
2
CH2Cl2
40 oC, 48 h
Ru
0.4305 mmol/g of imidazole
Ph
Cl
3a: X = Br
3b: X = PF6
PCy3
1
NaPF6
Mes
(1) CH3CH(Br)CO2Me
Cl
Cl
N
N
(CH2)6-O
K2CO3/cat.CsCO3
DMF, rt, 10 h, 96%
Br-(CH2)6-O
N
O
Ru
O
OH
N
X
Mes
(2) LiAlH4, THF
0 oC, 1 h, 94%
(3) Br(CH2)6Br/NaH
DMF, rt, 24 h, 70%.
O
NH
2
Mes = 2,4,6-trimethylphenyl
Ru-Carbene@ IM(Br)-
f-MWCNT 4a: X = Br
Ru-Carbene@ IM(PF6)-f-MWCNT 4b: X = PF6
Scheme 1. Synthesis of ionic CNT-supported Ru–carbene complexes.
IM-f-MWCNTs 3a having bromide anion. Anion exchange with
NaPF6 provided IM-f-MWCNTs 3b with PF6 anion. The Ru–carbene
moiety was introduced successfully through the metathesis of 3a
and 3b with Grubb’s second-generation catalyst, RuCl2(@CHPh)(P-
Cy3)(NHC), to give the corresponding IM-f-MWCNT-supported
Ru–carbene complexes 4a and 4b. Scanning electron microscopy
(SEM) (Fig. 1A) and transmission electron microscopy (TEM)
(Fig. 1B) analyses of 4a indicated that the tubular structure of CNTs
was retained during the synthetic procedures. Moreover, detection
of both Cl and Br, and the Ru elements in energy-dispersive X-ray
spectroscopy (EDX) clearly supports the incorporation of the Ru-
complex (Fig. 1C). X-ray photoelectron spectroscopy (XPS) analysis
(Ru3p(3/2) at 463.4 eV, Ru3p(1/2) at 485.8 eV, Cl2p at 197.9 eV,
Br3d(5/2) at 68 eV) further confirmed that the Ru(IV) species of
the Ru–carbene complex was attached without alteration of the
oxidation state (Fig. 1D and E, see also Supplementary data).11
Inductive-coupled plasma mass spectroscopy (ICP-MS) analysis
indicated that 0.0856 mmol/g of Ru was incorporated onto the
IM-f-MWCNTs. As shown in Figure 1F, the IM-f-MWCNT-supported
Ru-catalyst 4a was well-dispersed in both methylene chloride (a in
Fig. 1F) and an ionic liquid, 1-buthyl-2-methylimidazolium hexa-
fluoro phosphate ([bmim][PF6]) (b in Fig. 1F), which ascribe to the
ionic imidazolium bromide linker.
We first investigated the catalytic activity of IM-f-MWCNT-
supported Ru catalysts 4a and 4b having Br and PF6 counter anions,
respectively, for ring-closing metathesis (RCM) of diene 5a in
methylene chloride. It has been found that the anion of imidazoli-
um salt moiety did not significantly influence to the catalytic
Figure 1. (A) SEM, (B) TEM, and (C) EDX images of 4a. XPS spectrum of (D) 4a and (E) 4b (inset: Ru3p, Br3d(5/2), and F1s regions). (F) Photo for the solution of 4a (1.0 mg/
1.0 mL) in (a) CH2Cl2 and (b) [bmim][PF6] (bmim:1-butyl-3-methylimidazolium).

686
S. Lee et al. / Tetrahedron Letters 54 (2013) 684–687
activity (compare the entries 1 and 2, Table 1), and the diene 5a
was completely converted into 6a within 1 h. We next carried
out the RCM of various dienes 5 with 4a. As shown in Table 1,
the RCMs of terminal dienes 5a–5f were completed within 1 h in
the presence of 2.5 mol % of catalyst 4a to afford the corresponding
cyclic olefins 6a–6f in almost quantitatively (entries 2–7, Table 1).
The RCMs of dienes 5g–5i providing the corresponding trisubsti-
tuted olefins 6g–6i were also completed in slightly prolonged reac-
tion time (entries 8–10, Table 1). Moreover, the sterically more
demanding diene 5j, known to be a difficult substrate for RCM,
could also cyclized smoothly to afford the tetrasubstituted cyclic
olefin 6j in 71% yield (entry 11, Table 1). The high catalytic activity
of the CNT-supported Ru-catalyst 4a was partly ascribed to the
high compatibility of the imidazolium salt-functionalized CNTs
with reaction medium.
Next, the recyclability of the IM-f-MWCNT-supported Ru cata-
lyst 4a was investigated for RCM of N,N-bisallylsulfonamide 5a in
CH2Cl2 (Table 2). After each reaction, the solvent was evaporated
and the reaction product was extracted with diethyl ether. The
recovered catalyst can be reused 10 times without any significant
Table 1
RCMs of various dienes 5 using IM-f-MWCNT-supported Ru–catalyst 4aa
R2
R1
n
n
m
4
Cat. (2.5 mol%)
X
CH2Cl2, 40 o
C
R1
R2
m
X
6
5
X = NTs, C(CO2Et)2; n, m = 1, 2; R1, R2 = H, Me
Entry
5
6
Timeb (h)
Conversionc (%)
1
2
1
1
>99 (97)d
99e
5a
6a
6b
N
N
Ts
Ts
N
3
5b
1
>99
Ts
N
Ts
4
5
6
5c
5d
5e
6c
6d
6e
1
1
1
>99
>99
>99
N
Ts
N
Ts
EtO2C
EtO2C
CO2Et
CO2Et
EtO2C
CO2Et
CO2Et
CO2Et
EtO2C
7
5f
6f
1
>99
EtO2C
CO2Et
EtO2C
N
8
9
5g
5h
6g
6h
1.5
1.5
>99
>99
N
Ts
Ts
N
N
Ts
Ts
10
11
5i
5j
6i
6j
2
>99
71
EtO2C CO2Et
CO2Et
EtO2C
36
N
N
Ts
Ts
a
b
c
Reactions were carried out with diene 5 (0.1 mmol) at 40 °C in CH2Cl2 (2.0 mL) in the presence of 2.5 mol % (based on Ru content) of catalyst 4a.
Time for completion.
Determined by 1H NMR analysis.
d
e
Parenthesis is isolated yield by silica column chromatography.
Reaction was carried out with 4b.

S. Lee et al. / Tetrahedron Letters 54 (2013) 684–687
687
Table 2
Recycling of the CNT-supported Ru catalyst 4a after RCM of 5a in CH2Cl2 and [bmim][PF6]a
Sol/run
1
2
3
4
5
6
7
8
9
10
CH2Cl2
>99
>99c
1
>99
57
1
>99
—
1
>99
—
1
>99
>99
>99
>99
>99
96
Conv.b (%)
Time (h)
1
1
1
1
1.5
96
2
[bmim][PF6]/CH2Cl2
Conv.b (%)
Time (h)
>99
>99c
1
>99
95
1
>99
86
1
>99
40
1
>99
>99
>99
>99
94
1
1
1
1
1
1
a
b
c
Reactions were carried out with diene 5a (0.1 mmol) with 2.5 mol % 4a (based on Ru contents) at 40 °C in CH2Cl2 (2.0 mL) or CH2Cl2/[bmim][PF6] (1:1 v/v, 2.0 mL).
Determined by 1H NMR analysis.
Reaction was carried out in the presence of N-butylimidazolium bromide-functionalized MWCNTs and 2.5 mol % Grubbs–Hoveyda catalyst instead of the supported
catalyst 4a.
626; American Chemical Society: Washinton, DC, 1996.; (b)Green Chemistry:
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loss of catalytic activity. The ICP-MS analysis of the extracts from
the first three runs indicated that approximately 100 ppm of Ru
species leached out from each run. When the catalyst was recov-
ered by filtration, leaching of Ru increased as the catalytically
active Ru-species were not completely returned to the resting
pre-catalyst form. Taking the advantage of high-dispersity of the
IM-f-MWCNT-supported Ru catalyst 4a in an ionic liquid,
[bmim][PF6] (b in Fig. 1F), we examined recyclability of 4a in an io-
nic liquid. The RCM of 5a, conducted in a mixture of [bmim][PF6]/
CH2Cl2 (1/1, v/v) in the presence of 2.5 mol % of 4a, was completed
within 1 h. After evaporation of CH2Cl2, the product was extracted
with diethyl ether, and the catalyst immobilized in [bmim][PF6]
was reused for the next run, allowing 10 times of reuse without
a significant loss of catalytic activity. As control experiments for
catalyst recycling, the RCMs of 4a with homogeneous Grubbs–
Hoveyda Ru-catalyst, in the presence of N-butylimidazolium bro-
mide-functionalized MWCNTs, were carried out. It has been found
that the conversion was dramatically decreased upon recycling of
the catalyst, suggesting significant amounts of Ru-catalysts were
leached out during the extraction of product with ethyl ether.
These results further confirmed the fact that the covalent immobi-
lization of Ru-catalysts onto the support material could be one of
the effective ways to recover and reuse of the Ru-catalyst.
In summary, the Hoveyda-type Ru–carbene complex was
immobilized successfully onto the imidazolium salt-functionalized
ionic MWCNTs. The ionic CNT-supported Ru–carbene complexes
showed high dispersity in methylene chloride and ionic liquid en-
abling the RCMs of various dienes in both methylene chloride and
ionic liquid solvents with excellent catalytic activity and recycla-
bility. Studies on further applications of the ionic CNTs as support
materials for other organometallic catalysts are currently
underway.
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Acknowledgements
This work was supported by the National Research Foundation
(NRF-20120005673 and SRC Program 2012-0000648).
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
12.002.
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References and notes
1. (a) For selected recent reviews and monographs on green chemistry, see: Green
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