Controlled reversible anchoring of η6-arene/TsDPEN- ruthenium(II) complex onto magnetic nanoparticles: A new strategy for catalyst separation and recycling

Article abstract of DOI:10.1002/adsc.201100317

A new catalyst separation and recycling protocol combining magnetic nanoparticles and host-guest assembly was developed. The catalyst, (η⁶-arene)[N-(para-toluenesulfonyl)-1,2-diphenylethylenediamine] ruthenium trifluoromethanesulfonate [Ru(OTf)(TsDPEN)(η⁶-arene)] bearing a dialkylammonium salt tag, was easily separated from the reaction mixtures by magnet-assisted decantation, on basis of the formation of a pseudorotaxane complex by using dibenzo[24]crown-8-modified Fe₃O ₄ nanoparticles. The ruthenium catalyst has been successfully reused at least 5 times with the retention of enantioselectivity but at the expense of relatively low catalytic activities in the asymmetric hydrogenation of 2-methylquinoline. Copyright

Full text of DOI:10.1002/adsc.201100317

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DOI: 10.1002/adsc.201100317
Controlled Reversible Anchoring of h6-Arene/TsDPEN-
Ruthenium(II) Complex onto Magnetic Nanoparticles:
A New Strategy for Catalyst Separation and Recycling
Lei Wu,a,b Yan-Mei He,a and Qing-Hua Fana,*
a
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function,
Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, Peopleꢀs Republic of China
Fax : (+86)-10-6255-4449; phone: (+86)-10-6255-4472; e-mail: fanqh@iccas.ac.cn
The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150080, Peopleꢀs
b
Republic of China
Received: April 25, 2011; Revised: June 8, 2011; Published online: October 20, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100317.
Abstract: A new catalyst separation and recycling
protocol combining magnetic nanoparticles and
host-guest assembly was developed. The catalyst,
(h6-arene)[N-(para-toluenesulfonyl)-1,2-diphenyl-
quently observed. Secondly, in case of catalyst degra-
dation or deactivation (especially for moisture- or air-
sensitive catalysts) during the reaction and/or the cat-
alyst recycling, all of the vehicles together with the
catalysts have to be discarded. As a result, non-cova-
lent anchoring has been developed as an interesting
alternative approach.[2–4] Unlike the covalent method,
the homogeneous catalysts are immobilized via non-
covalent interactions, such as ion pairing, hydrogen
bonding and so on. After simple removal of the deac-
tivated catalyst, the supports can be recovered and
reused. In addition, such an approach can enable the
reactions to be carried out in a homogeneous manner
if the catalysts is unloaded from the support during
the reaction.[3b,d,4e] For example, Zhou and co-workers
recently reported the recycling of a chiral rhodium
catalyst via non-covalent absorption on carbon nano-
tubes.[4e] The asymmetric hydrogenation was per-
formed homogeneously, and the catalyst was recov-
ered via solid/liquid separation simply by changing
the solvent.
A
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
the reaction mixtures by magnet-assisted decanta-
tion, on basis of the formation of a pseudorotaxane
complex by using dibenzo[24]crown-8-modified
Fe3O4 nanoparticles. The ruthenium catalyst has
been successfully reused at least 5 times with the re-
tention of enantioselectivity but at the expense of
relatively low catalytic activities in the asymmetric
hydrogenation of 2-methylquinoline.
Keywords: asymmetric catalysis; catalyst recycling;
homogeneous catalysis; nanoparticles; self-assembly
In recent years, magnetic nanoparticles (MNPs)
The recycling of homogeneous catalysts is one of the have emerged as a robust, readily available, high-sur-
vital issues in chemical and pharmaceutical processes, face-area heterogeneous support in catalytic transfor-
especially when the expensive noble metal complexes mations.[5] These MNPs can be simply magnetically
are employed.[1] Immobilization of homogeneous cat- separated from a liquid reaction mixture, which
alysts on solid supports, including cross-linked poly- makes the catalyst recovery and recycling much more
mers and inorganic materials, is usually the preferred easier. Although significant progress has been made,
method of choice since the immobilized catalysts can examples for MNP-bound chiral catalysts are still
be facilely recovered via a simple filtration process scarce.[6] In addition, the semi-heterogeneous MNP
after the reaction. Despite the great success achieved, support can also cause a decrease in the activity and/
the covalent immobilization method usually suffers or enantioselectivity.[7]
from some major shortcomings. Firstly, due to the het-
As part of our continuing interest in the synthesis
erogeneous nature of the supported catalyst in the re- of metal nanoparticle catalysts[8] and the development
action media, a substantial decrease in catalytic activi- of easily recyclable chiral organometallic catalysts,[1b,9]
ty and selectivity of the immobilized catalysts is fre- herein, we report a conceptually new catalyst recy-
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cling protocol by combining MNPs and self-assembly.
The synthesis of chiral ruthenium catalyst bearing a
As shown in Figure 1, the catalytic reaction is per- dialkylammonium salt tag is summarized in Scheme 1.
formed in a homogeneous manner with a catalyst Ligand 1 was firstly synthesized from 2-(cyclohexa-
bearing a proper affinity tag. Upon completion of the 1,4-dien-1-yl)ethyl 4-methylbenzenesulfonate and tert-
Scheme 1. Synthetic route to the h6-arene/TsDPEN-Ru(II)
catalyst bearing a dialkylammonium salt tag.
butyl benzyl(4-hydroxybenzyl) carbamate in three
steps modified from the published method.[12] The re-
sulting ligand was then refluxed with RuCl3 in anhy-
drous methanol for 10 h to afford ruthenium dimer 2.
Further reaction of 2 with (R,R)-TsDPEN gave the
desired Ru catalyst 3, which was confirmed by
1H NMR, FT-IR and HR-MS (for details, see the Sup-
porting Information).
As shown in Scheme 2, the synthesis of DB24C8-
Figure 1. Illustration of homogeneous catalysis and catalyst functionalized MNPs 5 began with the preparation of
recycling by combining MNPs and self-assembly.
MNPs by co-precipitation of iron(II) and ironACHTUNGTRENNUNG(III) in
a basic solution at 608C as reported by Massart.[13]
Treatment of the performed nanoparticles with
reaction, MNPs equipped with complementary tags DB24C8 terminated catechol ligand (4) afforded 5
were added to facilitate molecular assembly with the DB24C8-MNPs (5). Ethough the average diameters
catalyst. The assembled heterogeneous catalyst is of MNPs from TEM images showed no significant
then separated from the product by simple rinse and change after functionalization (around 20 nm in both
decantation with the aid of an external magnet. After cases), DB24C8-MNPs were found to be more uni-
being released from the MNP support, the disassem- form as compared with the unmodified ones [Figure 2
bled catalyst can be applied in the next catalytic (a) and (b)]. The IR spectrum of the functionalized
cycle. To exemplify our protocol (Figure 1), a pseu- MNPs showed strong absorptions at 1694 and
dorotaxane complex of dialkylammonium salt and di- 1590 cmÀ1 due to ester and benzene asymmetric
benzo-24-crown-8 (DB24C8) was chosen as the bind- stretching, which indicated covalent bonding of
ing motif,[10] which could be modulated by way of the DB24C8 to the surface of MNPs. The loading of
pH value and was previously used to prepare supra- DB24C8 on MNPs was determined by TGA as
molecular chiral catalyst by us,[11] and h6-arene/ 0.26 mmolgÀ1. Magnetization curves measured at
TsDPEN-Ru(II) as the catalyst, which has proven to room temperature showed that the DB24C8-MNPs is
be an excellent catalyst for the asymmetric hydroge- super-paramagnetic [Figure 2 (c)]. The MNP-support-
nation of quinolines.[9f]
ed catalysts are well dispersed in CH2Cl2 and THF,
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Controlled Reversible Anchoring of h6-Arene/TsDPEN-Ruthenium(II) Complex onto Magnetic Nanoparticles
Scheme 2. Preparation of MNPs equipped with DB24C8.
Figure 2. TEM images of (a) co-precipitation prepared Fe3O4 MNPs; (b) DB24C8-functionalized MNPs 5 (scale bar=
100 nm); (c) magnetic curves of MNPs and DB24C8-functionalized MNPs; (d) MNP-supported catalyst 3 dispersion in
CH2Cl2; and (e) catalyst separation in CH2Cl2/CH3CN with an external magnet.
but badly dispersed in MeOH, Et2O, and CH3CN (see (0.0047 mmol) catalyst 3 and 104 mg (0.034 mmol of
the Supporting Information). The DB24C8-MNPs DB24C8) DB24C8-MNPs was stirred in 5 mL CH2Cl2.
with catalysts could be easily separated by a small After 12 h, the MNP-supported catalyst was isolated
magnet placed nearby [Figure 2 (e)].
and 98% of the catalyst was found to be anchored
With these complementary catalyst and MNPs in onto the surface of MNPs. Further disassembly of the
hand, we first examined the feasibility and efficiency MNP-supported-catalyst was performed in a mixture
of the assembly (complexation of ruthenium catalyst of Et3N/CH2Cl2/CH3CN for 12 h. After removal of the
3 with DB24C8-MNPs 5) by inductively coupled unloaded DB24C8-MNPs, 95.6% of the catalyst was
plasma (ICP) determinations. A mixture of 4.5 mg released into organic solvents, indicating that less
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Lei Wu et al.
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than 5% Ru catalyst was still absorbed on the surface tries 3–7). Gratifyingly, the same enantioselectivity
of the recycled DB24C8-MNPs.
was observed for each run.
Having demonstrated their decent complexation ef-
In summary, a conceptually new catalyst separation
ficiency, the Ru catalyst bearing a dialkylammonium and recycling protocol was developed through con-
salt tag was then employed in the asymmetric hydro- trolled reversible anchoring of the h6-arene/TsDPEN-
genation of 2-methylquinoline.[9f] Considering the re- Ru(II) complex onto magnetic nanoparticles after the
quirement of pseudorotaxane assembly in the catalyst catalytic reaction. With this protocol, the asymmetric
recycling process, aprotic CH2Cl2 was chosen as the hydrogenation of 2-methylquinoline was performed
solvent. As shown in Table 1, in the presence of homogeneously, and the chiral catalyst bearing a di-
1 mol% catalyst 3, very good enantioselectivity was
ACHTUNGTRENaNGNU lkylammonium salt tag was readily recovered at the
observed, albeit the conversion is moderate (entry 1). end by magnet-assisted decantation through the for-
After we increased the catalyst loading to 2 mol%, mation of a pseudorotaxane complex on the surface
of the DB24C8-modified Fe3O4 nanoparticles. This
strategy for catalyst recycling has potential wide-
spread applications in other organic catalytic reac-
tions.
Table 1. Asymmetric hydrogenation of 2-methylquinoline
and catalyst recycling.[a]
Experimental Section
Entry
Catalyst
Conversion [%][c]
ee [%][d]
Synthesis of Chiral Ruthenium Catalyst Bearing a
Secondary Ammonium Salt Tag
1[b]
2
3
4
5
3
3
63%
88 (R)
89 (R)
89 (R)
89 (R)
88 (R)
89 (R)
88 (R)
>99%
>99%
>99%
>99%
>99%
95%
3 (run 1)
3 (run 2)
3 (run 3)
3 (run 4)
3 (run 5)
Refluxing of compound 1 (1.54 g, 3.3 mmol) and 282 mg
RuCl3 in 20 mL anhydrous methanol for 10 h afforded com-
pound 2 as an orange powder, which was precipitated from
the solution upon completion of the reaction; yield: 90%.
1H NMR (300 MHz, DMSO-d6): d=2.92 (t, J=5.97 Hz,
2H), 4.03 (s, 2H), 4.06 (s, 2H), 4.34 (t, J=6.00 Hz, 2H),
5.82 (t, J=5.40 Hz, 1H), 5.91 (d, J=5.82 Hz, 2H), 6.03 (t,
J=5.70 Hz, 2H), 6.99 (d, J=8.55 Hz, 2H), 7.40–7.55 (m,
7H), 9.48 (br s, 2H); 13C NMR (75 MHz, DMSO-d6): d=
32.2, 49.4, 49.6, 66.1, 84.2, 86.5, 88.1, 103.0, 114.6, 124.6,
128.5, 128.6, 129.9, 131.6, 132.6, 158.4.
6
7
[a]
Reaction conditions: 0.7 mmol 2-methylquinoline in
CH2Cl2 (5 mL), 2 mol% 3, 2 mol% AgOTf, 50 atm H2,
158C for 16 h.
1 mol% 3 was used.
Determined by H NMR.
[b]
[c]
[d]
1
Determined by HPLC with a Chiralcel-OJ-H column.
Compound 2 (200 mg) and 115 mg (R,R)-TsDPEN were
dissolved in 5 mL DMF, and stirred for 30 min at room tem-
perature. After removal of the solvent, the functionalized
chiral RuCl(R, R-TsDPEN)ACHTNUGRTNEUNG
(h6-arene) catalyst 3 was precipi-
complete conversion and the same enantioselectivity
were obtained (entry 2).
tated with anhydrous ether as an orange powder; yield:
1
90%. H NMR (300 MHz, DMSO-d6): d=2.21 (s, 3H), 3.06
Finally, separation and recycling of the MNP-sup-
ported Ru catalyst was studied. Considering the inevi-
table leaching of the catalyst due to the defective as-
sembly/disassembly process as described above and to
obtain full conversion for the reused catalyst, the re-
action was performed in the presence of 2 mol% cata-
lyst 3 at room temperature for 16 h. After the comple-
tion of the hydrogenation, DB24C8-MNPs were
added to the reaction mixtures and stirred for 12 h to
(t, J=6.78 Hz, 2H), 4.10 (s, 2H), 4.12 (s, 2H), 4.19–4.58 (m,
4H), 5.58–6.02 (m, 5H), 6.46–7.48 (m, 23H), 8.95 (br s, 2H);
MS (ESI): m/z=930.18 ([MÀCl]+), 819.18 ([MÀHPF6]),
784.17 ([MÀClÀHPF6]+); HR-MS (ESI): m/z=819.1924,
calcd. for C43H45ClO3N3SRu ([MÀHPF6]): 819.1835.
General Procedure for Hydrogenation Reaction and
Catalyst Recycling
assure complete assembly. Then the hydrogenation Ruthenium catalyst 3 (15.1 mg, 2 mol%), AgOTf (3.6 mg,
2 mol%), 2-methylquinoline (100 mg, 0.7 mmol) and 5 mL
CH2Cl2 were charged into an autoclave, purged with hydro-
gen for three times and then pressurized to 50 atm. After
stirring for 16 h at room temperature and careful release of
hydrogen, 208 mg DB24C8-functionalized magnetic nano-
particles were added. The mixture was stirred for a further
12 h to assure the complete assembly. Hexane (15 mL) was
then added. The assembled magnetic nanoparticles were col-
lected with an external magnet, and were rinsed twice with
DCM/CH3CN (1:2, v:v). The combined layer was applied in
product was easily collected by rinsing and decanting
of the liquid with the aid of an external magnet.[14]
The obtained MNP-supported catalyst was then disas-
sembled in a mixture of Et3N/CH2Cl2/CH3CN for 12 h
to release the catalyst 3, and the unloaded support
was readily separated by using a small magnet again.
The resulting solution was acidified by addition of
HPF6 and concentrated to give the crude catalyst for
the next reaction. With this recycling mode, five con-
secutive catalytic runs were performed (Table 1, en- the determination of yield and enantiomeric excess.
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Controlled Reversible Anchoring of h6-Arene/TsDPEN-Ruthenium(II) Complex onto Magnetic Nanoparticles
The obtained MNP-supported catalysts were disassembled
in 40 mL Et3N/DCM/CH3CN solution for 12 h. The unload-
ed MNPs were collected with an external magnet again. The
released catalyst 3 was acidified by adding 119 mg 60%
HPF6 and evaporation under vacuum to give the crude Ru
catalyst for the next catalytic run.
1016; f) Y. Li, Y.-M. He, Z.-W. Li, F. Zhang, Q.-H. Fan,
Org. Biomol. Chem. 2009, 7, 1890–1895.
[5] For selected reviews, see: a) S. Shylesh, V. Schꢃne-
mann, W. R. Thiel, Angew. Chem. 2010, 122, 3504–
3537; Angew. Chem. Int. Ed. 2010, 49, 3428–3459; b) A.
Schꢄtz, O. Reiser, W. J. Stark, Chem. Eur. J. 2010, 16,
8950–8967; c) Y. H. Zhu, L. P. Stubbs, F. Ho, R. Z. Liu,
C. P. Ship, J. A. Maguire, N. S. Hosmane, ChemCat-
AHCTUNGTREGCNNUN hem 2010, 2, 365–374.
Acknowledgements
[6] For selected examples on magnetically recoverable
chiral catalysts, see: a) A. Hu, G. T. Yee, W. Lin, J. Am.
Chem. Soc. 2005, 127, 12486–12487; b) S. Luo, X.
Zheng, J.-P. Cheng, Chem. Commun. 2008, 5719–5721;
c) O. Gleeson, R. Tekoriute, Y. K. Gun’ko, S. J.
Connon, Chem. Eur. J. 2009, 15, 5669–5673; d) T. Zeng,
L. Yang, R. Hudson, G. Song, A. R. Moores, C.-J. Li,
Org. Lett. 2011, 13, 442–445.
This work is supported by the National Natural Science
Foundation of China (Grant No 20973178), the National
Basic Research Program of China (973 Program, No
2011CB808600), the Ministry of Health (Grant No
2009ZX09501-017) and the Chinese Academy of Sciences.
[7] For a magnetically recoverable catalysts based on a
catch-release system, see: S. Wittmann, A. Schꢄtz,
R. N. Grass, W. J. Stark, O. Reiser, Angew. Chem. 2010,
122, 1911–1914; Angew. Chem. Int. Ed. 2010, 49, 1867–
1870.
[8] a) L. Wu, B.-L. Li, Y.-Y. Huang, H.-F. Zhou, Y.-M. He,
Q.-H. Fan, Org. Lett. 2006, 8, 3605–3608; b) L. Wu, Z.-
W. Li, F. Zhang, Y.-M. He, Q.-H. Fan, Adv. Synth.
Catal. 2008, 350, 846–862.
[9] a) Q.-H. Fan, Y.-M. Chen, X.-M. Chen, D.-Z. Jiang, F.
Xi, A. S. C. Chan, Chem. Commun. 2000, 789–790;
b) G.-J. Deng, Q.-H. Fan, X.-M. Chen, D.-S. Liu,
A. S. C. Chan, Chem. Commun. 2002, 1570–1571; c) H.-
F. Zhou, Q.-H. Fan, W.-J. Tang, L.-J. Xu, Y.-M. He, G.-
J. Deng, L.-W. Zhao, L.-Q. Gu, A. S. C. Chan, Adv.
Synth. Catal. 2006, 348, 2172–2182; d) Z.-J. Wang, G.-J.
Deng, Y. Li, Y.-M. He, W.-J. Tang, Q.-H. Fan, Org.
Lett. 2007, 9, 1243–1246; e) Y.-Y. Huang, Y.-M. He, H.-
F. Zhou, L. Wu, B.-L. Li, Q.-H. Fan, J. Org. Chem.
2006, 71, 2874–2877; f) H.-F. Zhou, Z.-W. Li, Z.-J.
Wang, T.-L. Wang, L.-J. Xu, Y.-M. He, Q.-H. Fan, J.
Pan, L.-Q. Gu, A. S. C. Chan, Angew. Chem. 2008, 120,
8592–8595; Angew. Chem. Int. Ed. 2008, 47, 8464–8467.
[10] For magnetic nanoparticle-supported crown ethers, see:
M. Kawamura, K. Sato, Chem. Commun. 2007, 3404–
3405.
References
[1] For selected reviews, see: a) D. J. Cole-Hamilton, Sci-
ence 2003, 299, 1702–1706; b) Q.-H. Fan, Y. M. Li,
A. S. C. Chan, Chem. Rev. 2002, 102, 3385–3466; c) K.
Ding, Y. Uozumi, Handbook of Asymmetric Heteroge-
neous Catalysis, Wiley-VCH, Weinheim, 2008.
[2] For recent reviews on non-covalent catalyst immobili-
zation, see: a) R. van de Coevering, R. J. M. K. Geb-
bink, G. van Koten, Prog. Polym. Sci. 2005, 30, 474–
490; b) J. Horn, F. Michalek. C. C. Tzschucke, W. Bann-
warth, Top. Curr. Chem. 2004, 242, 43–75; c) J. M.
Fraile, J. I. Garcꢂa, J. A. Mayoral, Chem. Rev. 2009, 109,
360–417.
[3] For selected examples on achiral non-covalent catalyst
immobilization, see: a) R. F. Chen, R. P. J. Bronger,
P. C. J. Kamer, P. W. N. M. van Leeuwen, J. N. H. Reek,
J. Am. Chem. Soc. 2004, 126, 14557–14566; b) J. H.
Yoon, Y. J. Park, J. H. Lee, J. Yoo, C.-H. Jun, Org. Lett.
2005, 7, 2889–2892; c) R. van de Coevering, A. P.
Alfers, J. D. Meeldijk, E. Martꢂnez-Viviente, P. S. Pre-
gosin, R. J. M. K. Gebbink, G. van Koten, J. Am. Chem.
Soc. 2006, 128, 12700–12713; d) B. W. T. Gruijters,
M. A. C. Broeren, F. L. van Delft, R. P. Sijbesma,
P. H. H. Hermkens, F. P. J. T. Rutjes, Org. Lett. 2006, 8,
3163–3166.
[4] For selected reviews and examples on chiral non-cova-
lent catalyst immobilization, see: a) S. Kobayashi, R.
Akiyama, Chem. Commun. 2003, 449–460; b) Z. Wang,
G. Chen, K. L. Ding, Chem. Rev. 2009, 109, 322–359;
c) L. Shi, X. Wang, C. A. Sandoval, M. Li, Q. Qi, Z. Li,
K. Ding, Angew. Chem. 2006, 118, 4214–4218; Angew.
Chem. Int. Ed. 2006, 45, 4108–4112; d) S. Luo, J. Li, L.
Zhang, H. Xu, J.-P. Cheng, Chem. Eur. J. 2008, 14,
1273–1281; e) L. Xing, J.-H. Xie, Y.-S. Chen, L.-X.
Wang, Q.-L. Zhou, Adv. Synth. Catal. 2008, 350, 1013–
[11] Y. Li, Y. Feng, Y.-M. He, F. Chen, J. Pan, Q.-H. Fan,
Tetrahedron Lett. 2008, 49, 2878–2881.
[12] a) T. Watabe, A. Hiratsuka, T. Aizawa, T. Sawahata,
Tetrahedron Lett. 1982, 23, 1185–1188; b) T. J. Geld-
bach, P. J. Dyson, J. Am. Chem. Soc. 2004, 126, 8114–
8115.
[13] R. Massart, IEEE Trans. Magn. 1981, 17, 1247–1248.
[14] For highly viscous reaction mixtures, we used an equal
amount of hexane to facilitate the magnet-assisted sep-
aration.
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