Simply modified chiral diphosphine: Catalyst recycling via non-covalent absorption on carbon nanotubes

Article abstract of DOI:10.1002/adsc.200700617

A new type of recyclable chiral catalyst system was developed by absorption of pyrene-modified Pyrphos rhodium catalyst onto carbon nanotubes via π-π stacking interaction. This modified catalyst was successfully applied in the asymmetric hydrogenation of

Full text of DOI:10.1002/adsc.200700617

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DOI: 10.1002/adsc.200700617
Simply Modified Chiral Diphosphine: Catalyst Recycling via
Non-covalent Absorption on Carbon Nanotubes
Liang Xing,a Jian-Hua Xie,a Yong-Sheng Chen,b Li-Xin Wang,a and Qi-Lin Zhoua,*
a
State Key Laboratory and Institute of Elemento-organic Chemistry, Nankai University, Tianjin 300071, Peopleꢀs Republic
of China
Fax : +86-22-2350-6177; e-mail: qlzhou@nankai.edu.cn
Center of Nanoscale Science and Technology and Institute of Polymer Chemistry, College of Chemistry, Nankai
b
University, Tianjin 300071, Peopleꢀs Republic of China
Received: December 29, 2007; Published online: March 18, 2008
Dedicated to Professor Andreas Pfaltz on the occasion of his 60th birthday.
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A new type of recyclable chiral catalyst
system was developed by absorption of pyrene-
modified Pyrphos rhodium catalyst onto carbon
nanotubes via p-p stacking interaction. This modi-
fied catalyst was successfully applied in the asym-
metric hydrogenation of a-dehydroamino esters for
nine cycles without obvious deterioration of activity
and enantioselectivity.
convenient recycling of the catalyst from the reaction
mixture) have become a focus of intense research in
recent years. For example, Chan and co-workers re-
ported the synthesis of a soluble polyester-supported
BINAP ligand based on the concept of “one-phase
catalysis and two-phase separation”. This supported
chiralilgand showed a higher activity than the corre-
sponding “free” ligand in the Ru(II)-catalyzed asym-
metric hydrogenation of a-(6-methoxyl-2-naphthyl)-
Keywords: absorption; asymmetric catalysis; carbon
A
nanotube; catalyst recycling; pyrene
Recent advances such as thermomorphic catalyst
systems,[4] dendritic catalysts,[5] magnetically recovera-
ble catalysts,[6] and self-assembled supported cata-
lysts[7] have provided interesting alternatives for re-
covering and reusing chiralcatayl sts. However, the so-
Asymmetric catalysis is a powerful, economically fea- phisticated designs generally result in increased cost
sible tool for the synthesis of optically active mole- of the chiral catalyst and/or catalyst system. In this
cules that serve as precursors for pharmaceuticals, communication, we describe a very simple and effi-
agrochemicals, and flavorings, as well as functional cient approach to recycle and reuse a chiral Rh(I)/di-
materials. Many efficient, homogeneous chiral cata- phosphine catalyst for asymmetric hydrogenation via
lysts that exhibit high activities and enantioselectivi- non-covalent absorption on carbon nanotubes
ties have been developed in the past few decades.[1] (CNTs).
However, the considerable difficulty involved in sepa-
rating these expensive chiralcataylsts from products
Since their discovery in 1991, CNTs have attracted
considerable attention because of their unique me-
for reuse has restricted their applications in industrial chanical, electronic, and chemical stability proper-
processes.
ties.[8] These properties have made CNTs very promis-
The immobilization of chiral catalysts on various in- ing nanomaterials for many applications in chemis-
soluble solid supports such as inorganic materials, or- try.[9] However, only a few applications of CNTs in
ganic polymers, or membranes can solve the problems catalytic asymmetric reactions have been reported.
arising from homogeneous catalysis.[2] However, these García and co-workers[10] utilized single-well nano-
“heterogenized” catalysts usually suffer from de- tubes (SWNTs) to support chiralvanadylsaeln com-
creased catalytic activities and enantioselectivities due plexes via a covalent link for the heterogeneous asym-
to restriction by the solid matrix, which limits mobili- metric cyanosilylation of aldehydes, affording chiral
ty and accessibility of the active sites. Thus, chiral cat- a-hydroxy cyanides in 66% enantiomeric excess.
alysts that combine the advantages of homogeneous
To develop a new method for recycling chiral cata-
catalysts (high catalytic activity and enantioselectivi- lysts, we designed a diphosphine ligand connected to
ty) and heterogeneous catalysts (easy separation and a polyaromatic ring, which can be adsorbed onto
Adv. Synth. Catal. 2008, 350, 1013 – 1016
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Liang Xing et al.
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Scheme 1. Synthesis of pyrene-modified Pyrphos ligand 2.
CNTs via p-p stacking.[11] Pyrene-modified Pyrphos viding the corresponding products 4b–d with good
ligand (R,R)-2 was synthesized, and its complex with enantioselectivities (92–96% ee) (entries 3–5).
Rh(I) was found to be strongly adsorbed on CNTs in
The absorption of the chiralcataylst onto CNTs
several solvents and weakly adsorbed in other sol- was studied. The Rh(I) catalyst 5 was generated in
vents. This novel platform allows the “homogeneous” situ by the reaction of 1.0 mg (0.0025 mmol) Rh-
performance of the asymmetric reaction and the recy-
cling of the chiral catalyst via general solid/liquid sep- (0.0025 mmol) ligand (R,R)-2, followed by treatment
aration simply by changing the solvent. with 15 mg CNTs in 4 mL solvent. After stirring for
(COD)2BF4 (COD=1,5-cyclooctadiene) and 1.8 mg
The synthesis of the pyrene-modified Pyrphos 15 min, the concentration of catalyst 5 in the upper
ligand (R,R)-2 is very straightforward (Scheme 1). solution was analyzed by UV/vis spectroscopy.
The reaction of (3R,4R)3,4-bis(diphenylphosphino)- Among the solvents tested, CH2Cl2 gave the lowest
pyrrolidine (Pyrphos, R,R-1)[12] with commercially absorption of catalyst 5 on CNTs (50%), and the
available 1-pyrenebutyric acid in the presence of 1,3- color of the upper solution was yellow (Figure 1 a). In
dicyclohexylcarbodiimine (DCC) and 4-(dimethyl- contrast, catalyst 5 was mostly absorbed on CNTs in
ACHTREUNG
not affect its enantioselectivity (Table 1). The uses of CNTs were used in the hydrogenation of 3a
pyrene-modified Pyrphos (R,R)-2 and N-acetyl-Pyr- (0.25 mmol) with catalyst 5 (0.0025 mmol) in 4 mL
phos (Ac-Pyrphos) as ligands gave hydrogenation CH2Cl2, the reaction was complete in 2 h at 10 atm of
product 4a in similar enantioselectivities (entries 1 H2 and 208C, providing the hydrogenation product in
and 2). The a-dehydroamino esters 3b–d with elec- 93% ee. After the reaction, catalyst 5 was recovered
tron-donating or -withdrawing substituents can also by changing the solvent to EtOAc. The CH2Cl2 was
be hydrogenated efficiently with ligand (R,R)-2, pro- evaporated, 4 mL EtOAc were added, and the mix-
ture was stirred for 15 min. Catalyst 5 was separated
by filtration. Increasing the amount of CNTs was fa-
vorable for recovering the catalyst but decreased the
Table 1. Asymmetric hydrogenation of a-dehydroamino
esters 3.[a]
reaction rate and enantioselectivity (Table 2). Consid-
ering the recovery of catalyst, the reaction rate, and
the enantioselectivity, we chose 15 mg CNTs and
Entry
1
2
3
4
5
Ligand
Ar
Product
ee [%][b]
Ac-Pyrphos
C6H5
C6H5
4-MeC6H4
4-MeOC6H4
4-ClC6H4
4a
4a
4b
4c
4d
93
94
95
96
92
A
C
ACHTREUNG
ACHTREUNG
[a]
[b]
All reactions were carried out at a substrate concentra-
tion of 0.063M; (substrate/catalyst/ligand=100:1:1)
under 10 atm of H2 at 208C for 2 h. The conversion of
substrate for all reaction was >99%.
Figure 1. The absorption of chiralcatayl st onto CNTs in dif-
ferent solvents. (a) CH2Cl2: no CNTs (left); added CNTs
(right). (b) EtOAc: no CNTs (left); added CNTs (right).
Determined by ChiralGC using
column.
a Supceol C.L.V.
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Catalyst Recycling via Non-covalent Absorption on Carbon Nanotubes
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Table 2. Optimizing the conditions for hydrogenation and
AHCTREUNG
recovery of catalyst.[a]
Entry CNTs
[mg]
Cat. 5 in solution Time Conv.
ee
[%]
[%][b]
[h]
[%]
1
2
3
4
5
6
7
0
100
65
50
36
33
2
2
2
3
4
6
12
100
100
100
100
100
99
94
94
93
91
90
86
40
10
15
20
25
30
100
Experimental Section
General Procedure for Hydrogenation Reaction and
CNTs-Assistant Catalyst Recycling
27
<1.0
4
A mixture of [Rh(COD)2]BF4 (1.0 mg, 0.0025 mmol) and
A
[a]
General conditions: The catalyst 5 generated in situ from
Rh(COD)2BF4 (1.0 mg, 0.0025 mmol) and (R,R)-2
(1.8 mg, 0.0025 mmol) was treated with CNTs in 4 mL
CH2Cl2 and stirred for 15 mins. The substrate 3a (55 mg,
0.25 mmol) was hydrogenated under 10 atm of H2 at
208C for a proper time.
ligand (R,R)-2 (1.8 mg, 0.0025 mmol) was stirred at room
temperature for 15 min in CH2Cl2 (4 mL) under nitrogen.
The resulting solution of catalyst 5 was directly transferred
to a 20-mL glass vessel charged with substrate 3a (55 mg,
0.25 mmol), CNTs (15 mg)[14] and a magnetic stirring bar.
The vessel was put into a stainless steel autoclave, and the
autoclave was closed and pressurized with H2 to 10 atm. The
reaction mixture was stirred (1100 rpm) under the H2 pres-
sure at 208C for a proper time. After the reaction, the hy-
drogen pressure was carefully released and 20 mL COD
were added immediately. The CH2Cl2 was removed under
reduced pressure, 4 mL EtOAc were added and the mixture
was stirred for 15 min. The catalyst 5 absorbed onto the
CNTs was recovered via filtration under nitrogen atmos-
phere and dried under reduced pressure, which was directly
used for next cycle. The filtered EtOAc layer was used to
determine the conversion of substrate and the enantiomeric
excess of the hydrogenation product by GC on a Supelco
C.L.V. column.
ACHTREUNG
[b]
Determined by UV/vis spectrum.
Table 3. Recycling catalyst experiments.[a]
Run
Time [h]
Conv. [%]
ee [%]
1
2
3
4
5
6
7
8
9
10
2
2
3
4
5
6
8
10
12
12
100
100
99
100
95
98
97
97
95
93
93
93
93
93
93
93
92
92
92
15
Acknowledgements
[a]
Reaction conditions: Rh
A
ACHTREUNG
This work was supported by the National Natural Science
Foundation of China, the Major Basic Research Develop-
ment Program (Grant No. 2006CB806106), the “111” project
(B06005) of the Ministry of Education of China. We thank
Prof. Qing-Hua Fan for helpful discussions.
0.0025 mmol/0.0025 mmol/0.25 mmol/15 mg
CH2Cl2 under 10 atm H2 at 208C for the appropriate
time.
0.0025 mmolcataylst 5 for the catalyst recycling ex-
periment.
References
By absorption on CNTs, catalyst 5 was successfully
recycled and reused in the asymmetric hydrogenation
of 3a. As shown in Table 3, catalyst 5 was reused for
nine cycles without obvious deterioration in activity
or enantioselectivity (entries 1–9). To our knowledge,
this represents the best reported result for the recy-
cling and reuse of the highly air-sensitive Pyrphos-de-
rived catalyst. When recovered catalyst 5 was reused
for 10th run, the activity of the catalyst droped dra-
matically, although the high enantioselectivity (92%)
remained (entry 10).
In conclusion, a new type of recyclable chiral cata-
lyst system was developed that utilizes absorption of a
pyrene-modified Pyrphos rhodium catalyst onto
CNTs by p-p stacking interactions. With this catalyst
system, the asymmetric hydrogenation of a-dehydro-
[1] a) R. Noyori, Angew. Chem. 2002, 114, 2108–2123;
Angew. Chem. Int. Ed. 2002, 41, 2008–2022; b) W. S.
Knowles, Adv. Synth. Catal. 2003, 345, 3–13; c) E. N.
Jacobsen, A. Pfaltz, H. Yamamoto (Eds.), Comprehen-
sive Asymmetric Catalysis, Vols. I–III, Springer-Verlag,
Berlin, 2000.
[2] a) Q. H. Fan, Y. M. Li, A. S. C. Chan, Chem. Rev. 2002,
102, 3385–3466; b) C. E. Song, S. Lee, Chem. Rev.
2002, 102, 3495–3524.
[3] Q. H. Fan, C. Y. Ren, C. H. Yeung, W. H. Hu, A. S. C.
Chan, J. Am. Chem. Soc. 1999, 121, 7407–7408.
[4] For selected examples of thermomorphic chiral catalyst
systems, see: a) G. Pozzi, F. Cinato, F. Montanari, S.
Quici, Chem. Commun. 1998, 877–878; b) M. Cavazzi-
ni, A. Manfredi, F. Montarani, S. Quici, G. Pozzi,
Chem. Commun. 2000, 2171–2172; c) Y. Y. Huang,
Adv. Synth. Catal. 2008, 350, 1013 – 1016
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1015

Liang Xing et al.
COMMUNICATIONS
Y. M. He, H. F. Zhou, L. Wu, B. L. Li, Q. H. Fan, J.
Org. Chem. 2006, 71, 2874–2877; d) L. S. Zu, J. Wang,
H. Li, W. Wang, Org. Lett. 2006, 8, 3077–3079.
matarchis, A. Bianco, M. Prato, Chem. Rev. 2006, 106,
1105–1136.
[9] a) P. M. Ajayan, Chem. Rev. 1999, 99, 1787–1799;
b) P. M. Ajayan, O. Z. Zhou, Top. Appl. Phys. 2001, 80,
391–425; c) R. H. Baughman, A. A. Zakhidov, W. A.
de Heer, Science 2002, 297, 787–792; d) N. Robertson,
C. A. McGowan, Chem. Soc. Rev. 2003, 32, 96–103.
[10] C. Baleizao, B. Gigante, H. García, A. Corma, Tetrahe-
dron 2004, 60, 10461–10468.
[5] For selected examples of dendritic chiral catalysts with
a positive dendrimer effect, see: a) R. Breinbauer,
E. N. Jacobsen, Angew. Chem. 2000, 112, 3750–3753;
Angew. Chem. Int. Ed. 2000, 39, 3604–3607; b) Q. H.
Fan, Y. M. Chen, X. M. Chen, D. Z. Jiang, F. Xi,
A. S. C. Chan, Chem. Commun. 2000, 789–790; c) Y.
Ribourdouille, G. D. Engel, M. Richard-Plouet, L. H.
Gade, Chem. Commun. 2003, 1228–1229; d) Y. Wu, Y.
Zhang, M. Yu, G. Zhao, S. Wang, Org. Lett. 2006, 8,
4417–4420; e) Z. J. Wang, G. J. Deng, Y. Li, Y. M. He,
W. J. Tang, Q. H. Fan, Org. Lett. 2007, 9, 1243–1246.
[6] For recent examples of magnetically recoverable chiral
catalysts, see: a) A. G. Hu, G. T. Yee, W. B. Lin, J. Am.
Chem. Soc. 2005, 127, 12486–12487; b) D. W. Lee, J. W.
Lee, H. G. Lee, S. M. Jin, T. H. Hyeon, B. M. Kim,
Adv. Synth. Catal. 2006, 348, 41–46.
[7] For selected examples of self-assembly supported chiral
catalysts, see: a) J. S. Seo, D. Whang, H. Lee, S. I. Jun,
J. Oh, Y. J. Jeon, K. Kim, Nature 2000, 404, 982–986;
b) S. Takizawa, H. Somei, D. Jayaprakash, H. Sasai,
Angew. Chem. 2003, 115, 5889–5892; Angew. Chem.
Int. Ed. 2003, 42, 5711–5714; c) Y. Liang, Q. Jing, X.
Li, L. Shi, K. Ding, J. Am. Chem. Soc. 2005, 127, 7694–
7695; d) H. Y. Peng, C. K. Lam, T. C. W. Mak, Z. W.
Cai, W. T. Ma, Y. X. Li, H. N. C. Wong, J. Am. Chem.
Soc. 2005, 127, 9603–9611; e) 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.
[11] For other examples involving CNTs-pyrene p-p stack-
ing, see: a) R. J. Chen, Y. Zhang, D. Wang, H. Dai, J.
Am. Chem. Soc. 2001, 123, 3838–3839; b) C. Ehli,
G. M. A. Rahman, N. Jux, D. Balbinot, D. M. Guldi, F.
Paolucci, M. Marcaccio, D. Paolucci, M. Melle-Franco,
F. Zerbetto, S. Campidelli, M. Prato, J. Am. Chem. Soc.
2006, 128, 11222–11231; c) H. Paloniemi, M. Lukkari-
nen, T. Aaritalo, S. Areva, J. Leiro, M. Heinonen, K.
Haapakka, J. Lukkari, Langmuir 2006, 22, 74–83.
[12] U. Nagel, E. Kinzel, J. Andrade, G. Prescher, Chem.
Ber. 1986, 119, 3326–3343.
1
[13] Mp 152–1548C; [a]20: +117 (c 1.0, CH2Cl2); H NMR
(400 MHz, CDCl3): Dd=8.33 (d, J=9.2 Hz, 1H), 8.18–
7.97 (m, 7H), 7.83 (d, J=7.6, 1H), 7.40–7.28 (m, 8H),
7.23–7.10 (m, 12H), 4.04–3.94 (m, 1H), 3.86–3.70 (m,
2H), 3.37 (t, J=7.2, 2H), 3.27–3.21 (m, 1H), 2.92–2.82
(m, 2H), 2.25–2.19 (m, 2H), 2.18–2.10 (m, 2H);
13C NMR (75 MHz, CDCl3): d=170.2, 135.5, 135.3,
135.1, 134.9, 134.5, 134.4, 132.5, 130.4, 130.0, 128.8,
128.3, 128.1, 127.9, 127.6, 126.5, 126.3, 125.6, 124.7,
124.0, 123.7, 122.6, 47.6, 47.0, 38.0, 36.3, 32.8, 31.8, 25.5;
31P NMR (121 MHz, CDCl3): d=À11.57, À11.63; HR-
+
MS (ESI): m/z=710.2743, calcd. for C48H42NOP2
[8] a) S. Iijima, Nature 1991, 354, 56–58; b) M. Dressel-
haus, G. Dresselhaus, P. Avouris, Carbon Nanotubes:
Synthesis Properties and Applications, Springer-Verlag,
Berlin, 2001; c) S. Reich, C. Thomsen, J. Maultzsch,
Carbon Nanotubes: Basic Concepts and Physical Prop-
erties, Wiley-VCH, Weinheim, 2004; d) D. Tasis, N. Tag-
([M+H]+): 710.2736.
[14] Purchased from Shenzhen Nanotech Port Co. Ltd. Mul-
tiwall carbon nanotubes prepared by the chemical
vapor deposition method, 3–5 walls, <10 nm diameter,
>95% purity, <5% amorphous, <0.2% ash, 5–15 mm
length.
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Adv. Synth. Catal. 2008, 350, 1013 – 1016