A new type of self-supported, polymeric Ru-carbene complex for homogeneous catalysis and heterogeneous recovery: Synthesis and catalytic activities for ring-closing metathesis

Article abstract of DOI:10.1039/b809025d

A novel 2^nd generation Grubbs-type catalyst tethering an isopropoxystyrene has been synthesized and automatically polymerized in solution to form a self-supported polymeric Ru-carbene complex, which catalyzed ring-closing metathesis homogeneously, but was recovered heterogeneously. The Royal Society of Chemistry.

Full text of DOI:10.1039/b809025d

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A new type of self-supported, polymeric Ru-carbene complex for
homogeneous catalysis and heterogeneous recovery: synthesis and catalytic
activities for ring-closing metathesis†
Shu-Wei Chen,a Ju Hyun Kim,a Hyunik Shinb and Sang-gi Lee*a
Received 28th May 2008, Accepted 17th June 2008
First published as an Advance Article on the web 30th June 2008
DOI: 10.1039/b809025d
A novel 2nd generation Grubbs-type catalyst tethering an
isopropoxystyrene has been synthesized and automatically
polymerized in solution to form a self-supported polymeric
Ru-carbene complex, which catalyzed ring-closing metathesis
homogeneously, but was recovered heterogeneously.
Another promising and recent strategy involves heterogeniza-
tion of the homogeneous catalysts or precursors by homocombi-
nation of multi-topic organic ligands to generate “self-supporting”
metal–organic polymeric networks.10 Compared to other methods,
the self-supporting strategy can produce heterogeneous catalysts
without using any supports with a high density of catalyti-
cally active units. However, these heterogeneous self-supported
catalysts often have decreased catalytic activity compared to
their homogeneous counterparts. Taking advantage of this self-
supporting strategy, it could conceptually be possible to generate
new types of self-supported catalysts or catalyst precursors to
conduct catalysis homogeneously with heterogeneous recovery. We
anticipated that the structural features and “release and return”
metathesis mechanism could provide an excellent opportunity for
the generation of a new type of self-supported Ru-complexes,
allowing homogeneous catalysis and heterogeneous recovery.
Quite recently, we demonstrated for the first time the validity of
this strategy for ring-closing metathesis with Ru-carbene catalyst
3 (type A in Fig. 2).11 The catalytically active dimeric Ru-
methylidene and dimeric isopropoxybenzylidene were dissociated
from the polymer matrix, and after catalysis, could be re-associated
to form the polymeric catalyst precursor 3, which was recovered
heterogeneously. In principle, homo-coupled, self-supported poly-
meric Ru-carbene complexes such as 4 (type B in Fig. 2) would
also be possible, in which the catalytically active Ru-methylidene
Olefin metathesis has become one of the simplest and most
effective synthetic methods for carbon–carbon double bond
construction and is widely employed in a variety of fields
of chemistry including natural products, pharmaceuticals, and
polymer chemistry.1 Much of the recent successful progress in
olefin metathesis stems largely from the availability of several,
well-defined ruthenium catalysts such as Grubbs-type ruthenium
alkylidenes 12 and Hoveyda/Grubbs-type ruthenium catalysts 2,3
which are easy to handle and are tolerant towards different kinds of
functional groups (Fig. 1). Despite the general superiority offered
by these Ru catalysts, they share some disadvantages associated
with difficulties in the recovery and reuse of the expensive metal
catalysts as well as product contamination caused by metal
leaching. Hence, from an economic and environmental point of
view, development of immobilization technology for recycling of
the catalyst is of great importance.4 A common approach for
immobilization of homogeneous ruthenium catalysts is grafting of
the catalyst, particularly Hoveyda/Grubbs-type2 to supports such
as polymers,5 dendrimers,6 or nanoparticles.7 The use of tagged
catalysts has also been employed as alternative immobilization
methods for fluorous8 or ionic liquid phase catalysis,9 in which the
tagged catalyst can be easily separated from untagged products by
liquid–liquid partition.
Fig. 1 Representative Ru-complexes for olefin metathesis.
aDepartment of Chemistry and Nano Science (BK21), Ewha Womans
University, Seoul, 120-750, Korea. E-mail: sanggi@ewha.ac.kr; Fax: +82-
2-3277-3419; Tel: +82-2-3277-4505
bChemical Development Division, LG Life Sciences, Ltd./R&D, Daejeon,
305-380, Korea
† Electronic supplementary information (ESI) available: Details of syn-
Fig. 2 Schematic presentation of self-supported catalysts for homoge-
thetic procedures for 4 and 5. See DOI: 10.1039/b809025d
neous catalysis and heterogeneous recovery.
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=
species, having a chelating isopropoxystyrene group, would be
generated. Herein we report the synthesis of this new-type of
2nd generation Ru-carbene complex 5 tethering an isopropoxy
styrene group, and its conversion to a self-supported polymeric
Ru-carbene complex 4, and their catalytic activity and reusability
in ring-closing metathesis.
and a new Ru CH– resonance peak at 16.1 ppm corresponding
to the isopropoxybenzylidene Ru-complex appeared after 5 min
(Fig. 3b). After 30 min, the resonance peak at 19.2 ppm was
completely converted to the peak at 16.1 ppm (Fig. 3c). Moreover,
disappearance of the vinyl proton resonance signals at 5.69 ppm
and 5.24 ppm in 5 clearly indicated formation of complex 4 (see
ESI for full scale spectrum†).
As a precursor for the homo-coupled, self-supported polymeric
Ru-carbene complex 4, the Grubbs’ 2nd generation-type Ru-
carbene complex 5, tethering an isopropoxystyrene with a butyl
linker, was designed and synthesized as shown in Scheme 1.
The 5-hydroxy-2-isopropoxystyrene 65d was alkylated with 1-
chloro-4-iodobutane, and the resulting chloride reacted with
1-mesitylimidazole12 to form imidazolium chloride 7. For the
synthesis of complex 5, the NHC-carbene was generated in situ
by reaction of imidazolium chloride 7 with t-BuOK in toluene
◦
at 0 C, then reacted with Grubbs’ 1st generation catalyst at
room temperature for 3 h. After short silica-gel chromatographic
purification, the desired Ru-carbene complex 5, bearing an iso-
propoxystyrene moiety linked with a butyl tether, was isolated in
60% yield.‡ In the 1H NMR spectrum, the characteristic resonance
=
signals of Ru CH-Ph at 19.2 ppm and the free vinyl protons of
=
styrene, CH2, at 5.69 ppm (dd, J = 17.8, 1.3 Hz) and 5.24 ppm
(dd, J = 11.1 and 1.3 Hz) were observed.2c However, it was
found that 5 was not stable in CDCl3, and converted to the
homo-coupled self-supported polymeric Ru-carbene complex 4.
As shown in Fig. 3, during the NMR measurement, the intensity
1
=
Fig. 3 H NMR (250 MHz, CDCl3) spectra of the Ru CH– region of
(a) 5; (b) after 5 min; (c) after 30 min.
Based on these observations, we anticipated that the poly-
meric Ru-complex 4 could be prepared during ring-closing
metathesis (RCM) with monomeric Ru-complex 5 (Scheme 2).
Thus, the RCM of the benchmark substrate, N,N -bisallyl p-
toluenesulfonamide, was carried out in methylene chloride at 40 ◦C
in the presence of 5.0 mol% of 5, and the reaction was complete in
4 h with >98% yield (entry 1 in Table 1). After RCM, complex 5
was converted to polymeric complex 4, which could be recovered
via precipitation using ethyl acetate, as previously observed in
hetero-coupled, self-supportedpolymeric Ru-carbenecomplex 3.11
Inductive coupled plasma atomic emission spectroscopic (ICP-
AES) analysis of isolated 4 showed 18 wt% incorporation of Ru
into the polymer matrix. The recovered polymeric Ru-carbene
complex showed slightly decreased catalytic activity compared to
monomeric 5 (entry 2). The decreased catalytic activity of 4 could
be ascribed to the slow dissociation of the catalytically active Ru-
species from the polymer matrix. The recovered 4 can be reused
2 more times without loss of catalytic activity (entries 3 and 4).
However, ICP-AES analysis indicated significant amounts of Ru
=
of the Ru CH– peak at 19.2 ppm (Fig. 3a) rapidly decreased,
Scheme
1
Synthesis of Ru-carbene complex 5 tethering an iso-
propoxystyrene group.
Table 1 Catalyst recycling in ring-closing metathesis of N,N -bisallyl p-
toluenesulfonamide using Ru-carbene complexes 4 and 5a
Entry
Catalyst
Time/hb
Yield (%)c
Ru leachingd
1
5
4
4
4
4
4
4
6
6
6
8
>98
>98
>98
>98
>98
>98
2e
3e
4e
5e
6e
25 ppm
20 ppm
—
—
—
12
a Reactions were carried out using 5.0 mol% of Ru at 40 ◦C in methylene
chloride (0.5 M). b Time for completion. c Determined by 1H NMR
analysis. d Determined by ICP-AES analysis of the ethyl acetate layer.
e Recovered catalyst from previous entry was used.
Scheme 2 Synthesis of homo-coupled polymeric Ru-carbene complex 4.
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leached out into the ethyl acetate layer in each cycle, thus requiring
longer times to complete the reaction from 5th (entry 5) and 6th
runs (entry 6). Although reusability of the polymeric Ru-carbene
complex was not sufficiently high, this approach might provide an
alternative way to generate self-supported recoverable Ru-catalysts
for metathesis.
In summary, we have synthesized a new type of Grubbs’ 2nd
generation Ru-carbene complex 5 tethering an isopropoxystyrene
moiety, which was rapidly converted to the homo-coupled,
self-supported polymeric Ru-carbene complex 4. Polymeric Ru-
carbene complex 4 catalyzed ring-closing metathesis homoge-
neously and was recovered heterogeneously. Further studies on
the impact of a linker on both structure of assemblies and their
catalytic performance are under way.
Ru-complexes 4 was conducted in the same way as described for the first
run. This reaction was repeated, each time using the catalyst recovered
from a previous cycle. The results are listed in Table 1.
1 Selected reviews on olefin metathesis (a) R. R. Schrock and A. H.
Hoveyda, Angew. Chem., Int. Ed., 2003, 42, 4592; (b) T. M. Trnka and
R. H. Grubbs, Acc. Chem. Res., 2001, 34, 18.
2 (a) P. Schwab, M. B. France, J. W. Ziller and R. H. Grubbs, Angew.
Chem., Int. Ed. Engl., 1995, 34, 2039; (b) P. Schwab, R. H. Grubbs and
J. W. Ziller, J. Am. Chem. Soc., 1996, 118, 100; (c) J. Huang, E. D.
Stevens, S. P. Nolan and J. L. Petersen, J. Am. Chem. Soc., 1999, 121,
2674; (d) M. Scholl, T. M. Trnka, J. P. Morgan and R. H. Grubbs,
Tetrahedron Lett., 1999, 40, 2247.
3 (a) J. S. Kingsbury, J. P. A. Harrity, P. J. Bonitatebus and A. H. Hoveyda,
J. Am. Chem. Soc., 1999, 121, 791; (b) S. B. Garber, J. S. Kingsbury,
B. L. Gray and A. H. Hoveyda, J. Am. Chem. Soc., 2000, 122, 8168.
4 Review on supported Ru-catalysts, see: H. Clavier, K. Grela, A.
Kirschning, M. Mauduit and S. P. Nolan, Angew. Chem., Int. Ed.,
2007, 46, 6786.
5 Selected papers (a) S. Randl, N. Buschmann, S. J. Connon and S.
Blechert, Synlett, 2001, 10, 1547; (b) J. Dowden and J. Savovic, Chem.
Commun., 2001, 146; (c) S. J. Connon and S. Blechert, Bioorg. Med.
Chem. Lett., 2002, 12, 1873; (d) Q. Yao, Angew. Chem., Int. Ed., 2000,
39, 3896; (e) Q. Yao and A. R. Motta, Tetrahedron Lett., 2004, 45, 2447;
(f) S. J. Connon, A. M. Dunne and S. Blechert, Angew. Chem., Int. Ed.,
2002, 41, 3835; (g) S. Varray, R. Lazaro, J. Martinez and F. Lamaty,
Organometallics, 2003, 22, 2426.
This work was supported by the Korean Research Foundation
(KRF-2006–312-C00587).
Notes and references
‡ Synthesis of 5 and its polymeric Ru-carbene complex 4 via RCM. A
solution of imidazolium salt 7 (0.12 g, 0.26 mmol) in toluene (10 mL)
t
=
was treated with BuOK (29.5 mg, 0.26 mmol) and (PCy3)2Cl2Ru CHPh
6 S. B. Garber, J. S. Kingsbury, B. L. Gray and A. H. Hoveyda, J. Am.
Chem. Soc., 2000, 122, 8168.
7 B. S. Lee, S. K. Namgoong and S.-g. Lee, Tetrahedron Lett., 2005, 46,
4501.
(0.18 g, 0.22 mmol) at 0 ◦C. The reaction mixture was stirred at room
temperature for 3 h under N2. The suspension was filtered and the solvent
was evaporated by vacuum. The resulting residue was purified by silica
gel chromatography using EtOAc–hexane (1 : 3) as the eluent to afford
5 (0.075 g, 0.13 mmol, 60%). 1H NMR (CDCl3, 250 MHz) d 19.24 (s,
1H), 7.41 (m, 2H), 7.16–6.99 (m, 6H), 6.81 (m, 4H), 6.47 (br s, 1H), 5.74
(dd, 1H, J = 1.3, 17.8 Hz), 5.27 (dd, 1H, J = 1.3, 11.1 Hz), 4.82 (t, 2H,
J = 7.2 Hz), 4.38 (sept, 1H, J = 6.1 Hz), 4.07 (t, 2H, J = 5.7 Hz), 2.41–
2.34 (m, 6H), 2.04 (m, 2H), 1.93 (m, 8H), 1.63 (m, 24H), 1.32 (d, 6H,
J = 6.1 Hz), 1.25–1.21 (m, 6H). A 10 mL oven-dried round-bottom flask
equipped with a reflux condenser was charged with polymeric Ru-catalyst
5 (13.5 mg, 0.014 mmol). The flask was evacuated and filled with N2.
N,N-Diallyl-p-toluenesulfonamide (120.0 mg, 0.279 mmol) and anhydrous
CH2Cl2 (5.0 mL) were added, the flask was then heated to gentle reflux for
6 h, and the solvent was evaporated. Ethyl acetate (5 mL) was added to the
residue, and the precipitated polymeric Ru-complex 4 was recovered by
filtration and washed with ethyl acetate (5 mL × 2). After evaporation of
the ethyl acetate, 1H NMR analysis of the crude residue revealed complete
conversion, and the product was isolated by flash chromatography (n-
hexane–EtOAc = 3 : 1) to afford pure product as a white crystalline solid
(61 mg, 99%). A second run of the metathesis using the recovered polymeric
8 (a) Q. Yao and Y. Zhang, J. Am. Chem. Soc., 2004, 126, 74; (b) M.
Matsugi and D. P. Curran, J. Org. Chem., 2005, 70, 1636.
9 (a) Q. Yao and Y. Zhang, Angew. Chem., Int. Ed., 2003, 42, 3395;
(b) Q. Yao and M. Sheets, J. Organomet. Chem., 2005, 690, 3577;
(c) N. Audic, H. Clavier, M. Mauduit and J.-C. Guillemin, J. Am.
Chem. Soc., 2003, 125, 9248; (d) H. Clavier, N. Audic, M. Mauduit
and J.-C. Guillemin, Chem. Commun., 2004, 2282; (e) H. Clavier,
N. Audic, M. Mauduit and J.-C. Guillemin, J. Organomet. Chem.,
2005, 690, 3585; (f) A. Michrowska, Ł. Gułajski, Z. Kaczmarska, K.
Mennecke, A. Kirschning and K. Grela, Green Chem., 2006, 8, 685;
(g) D. Rix, H. Clavier, Y. Coutard, Ł. Gułajski, K. Grela and M.
Mauduit, J. Organomet. Chem., 2006, 691, 5397.
10 Highlight (a) L.-X. Dai, Angew. Chem., Int. Ed., 2004, 43, 5726; (b) K.
Ding, Pure Appl. Chem., 2006, 78, 293.
11 S.-W. Chen, J. H. Kim, C. E. Song and S.-g. Lee, Org. Lett., 2007, 9,
3845.
12 M. C. Perry, X. Cui, M. T. Powell, D.-R. Hou, J. H. Reibenspies and
K. Burgess, J. Am. Chem. Soc., 2003, 125, 113.
2678 | Org. Biomol. Chem., 2008, 6, 2676–2678
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The Royal Society of Chemistry 2008
©