Chiral ruthenium porphyrin encapsulated in ordered mesoporous molecular sieves (MCM-41 and MCM-48) as catalysts for asymmetric alkene epoxidation and cyclopropanation

Article abstract of DOI:10.1039/b209276j

Encapsulation of chiral ruthenium porphyrin [Ru^II(D₄-Por*)CO] in modified mesoporous silica supports such as MCM-41 and MCM-48 achieves active catalysts for asymmetric epoxidation of alkenes by 2,6-dichloropyridine N-oxide and intram

Full text of DOI:10.1039/b209276j

Chiral ruthenium porphyrin encapsulated in ordered mesoporous
molecular sieves (MCM-41 and MCM-48) as catalysts for asymmetric
alkene epoxidation and cyclopropanation†
Jun-Long Zhang, Yun-Ling Liu and Chi-Ming Che*a
a
ab
a
Department of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular
Technology for Drug Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong,
P.R. China. E-mail: cmche@hku.hk; Fax: (852)2857-1586
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, 119 Jiefang
b
Road, Changchun 130023, P.R.China
Received (in Cambridge, UK) 24th September 2002, Accepted 15th October 2002
First published as an Advance Article on the web 5th November 2002
II
Encapsulation of chiral ruthenium porphyrin [Ru (D
4
-
preparation and reactivities of chiral metalloporphyrin catalysts
encapsulated in ordered mesoporous molecular sieves 1a–b
(Scheme 1). To compare the effect of different silica supports,
Por*)CO] in modified mesoporous silica supports such as
MCM-41 and MCM-48 achieves active catalysts for asym-
metric epoxidation of alkenes by 2,6-dichloropyridine N-
oxide and intramolecular cyclopropanation, which is the
first example of chiral metalloporphyrin supported on
ordered molecular sieves.
II
*
we also anchored [Ru (D
As previously reported, treatment of [Ru (D
with surface modified (by 3-aminopropyltriethoxysilane) me-
soporous silica in CH Cl led to immobilization of I on the
4
-Por )(CO)] on silica surface 1c.
II
*
4
-Por )(CO)] (I)
2
2
7
support through a ligand exchange reaction (Scheme 1). The
catalysts (1a–d) were characterized by FT-IR, powder-XRD
(for 1a and 1b) and solid diffuse-reflectance UV–VIS spectra.
And the loading amount of ruthenium porphyrin was deter-
mined by a published method. IR spectra of the catalysts show
stretching vibrations at 1940, 1942, 1938 and 1939 cm
Although significant progress has been made in transition
metal-catalyzed enantioselective organic reactions, the search
for efficient, robust and recyclable chiral metal catalysts
1
7
remains a difficult task in asymmetric catalysis. Recent studies
2
1
showed that the
D
4
-symmetric ruthenium complex of
[
5,10,15,20-tetrakis-(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-
,2:5,8-dimeth ano-anthracen-9-yl] porphyrin H (D -Por ) is an
2 4
(nCNO), respectively. Solid diffuse-reflectance UV–VIS spectra
show the absorption bands at ca. 418 and 528 nm. These
*
1
II
*
excellent catalyst for its reactivity, versatility, and enantiose-
lectivity in asymmetric alkene epoxidation and cyclopropana-
spectroscopic data are close to that of [Ru (D
indicating that the molecular structure of [Ru (D
4
-Por )(CO)],
II
*
4
-Por )(CO)]
2
tion. However, the tedious preparation and high cost of the
remains intact during the process of encapsulation. The XRD
patterns of 1a–b exhibit the characteristic peaks of MCM-41
and MCM-48, without the peaks due to free ruthenium
porphyrin, which demonstrates that the ordered structure of
*
H
2
(D
4
-Por ) ligand obstruct its application in organic syntheses.
An approach to circumvent this problem is to immobilize
II
*
[
Ru (D
4
-Por )(CO)] on solid support, facilitating the product-
3
II
*
catalyst separation and recycling of the chiral catalyst.
Although some progress in heterogeneous organic oxidations
catalyzed by soluble and insoluble polymer supported ruthe-
nium porphyrins have been made,4 a similar strategy is not
suitable to develop polymer supported chiral metalloporphyrins
MCM-41 and MCM-48 was retained and [Ru (D
4
-Por )(CO)]
II
molecules should be dispersed in the channels. For 1c, [Ru (D
4
-
*
Por )(CO)] was anchored on the surface of silica gel. Using sol–
,5
II
*
gel method, we encapsulated [Ru (D
4
-Por )(CO)] into the
matrix of silica gel (1d).
because of the difficulty involved in attaching chiral porphyrin
The asymmetric epoxidation of styrene with 2,6-Cl
catalyzed by 1a–1d were carried out in benzene for 24 h at room
2
pyNO
*
ligand particularly the D
4
-symmetric H
2
(D
4
-Por ) onto the
polymer chain.
temperature. The results are summarized in Table 1. From
An alternative method is to attach ruthenium porphyrin to
mesoporous silica material by coordinative grafting; this would
circumvent the problem encountered in the structural modifica-
entries 1–3, the reactivity and enantioselectivity of styrene
II
epoxide are the best when the loading amount of [Ru (D
4
-
*
Por )(CO)] of 1a reaches 1.6 wt%; a further increase in loading
3
tion of the porphyrin ligand. We previously encapsulated chiral
amount results in decrease of product yield (entry 3). Therefore,
IV
*
II
*
[
Ru (D
4
-Por )Cl
2
] in a sol–gel matrix which exhibited lower
4
the loading amount of [Ru (D -Por )(CO)] in 1b, 1c and 1d are
reactivity by physical host–guest interaction; the obtained
heterogenous catalyst exhibited lower reactivity in asymmetric
epoxidation of alkenes compared to its homogenous counter-
1.7, 1.6 and 1.7 wt%, respectively, for the studies. In this work,
the reaction performed in benzene gave a better ee than that in
CH
2
Cl
2
(Table S1†). Entries 4–6 of Table 1 show the results of
pyNO with catalysts 1b–d and
6
part. This was ascribed to a blocking of the reactant to the
styrene epoxidation by 2,6-Cl
2
active sites of amorphous silica. However, in the case of MCM-
free ruthenium porphyrin as catalysts. Notably, 1b, with
modified MCM-48 as solid support, gave the highest ee and
yield, while 1a exhibited similar reactivity and enantioselectiv-
4
1, an ordered mesoporous silica support, attachment of
ruthenium porphyrins [Ru (Por)CO] (Por = p-Cl-TPP and
II
TDCPP) or chiral chromium Schiff base could achieve effective
7
catalysts for alkene epoxidation. We envision the structure of
solid support could play an important role in heterogeneous
organic oxidations catalyzed by chiral ruthenium porphyrins.
In contrast to the MCM-41 silica phase with a one-
dimensional straight channel system, MCM-48 contains a three-
dimensional network, which provides a large surface area for
high accessibility of reactant(s) to active sites within the porous
8
network structure. These features make MCM-48 a good solid
support for heterogenous catalysis. Herein, we report the
†
Electronic supplementary information (ESI) available: experimental
section. See http://www.rsc.org/suppdata/cc/b2/b209276j/
Scheme 1
2
906
CHEM. COMMUN., 2002, 2906–2907
This journal is © The Royal Society of Chemistry 2002

View Article Online
II
II
ity to the homogenous counterpart. But 1c, with [Ru (D
4
-
and 1b could not catalyze the reactions but free [Ru (D
4
-
*
*
Por )(CO)] anchored on the silica gel surface, could not
efficiently catalyze the reaction and most of ruthenium complex
leached from the surface of silica gel after the reaction.
Complex 1d exhibited stability in this reaction and little free
ruthenium porphyrin released from the solid catalyst was
observed after 24 h. However, its reactivity was low (conver-
sion 10%). From entries 7 and 8, we found that, in the presence
Por )(CO)] gave high epoxide yield ( > 99%) with complete b
selectivity (supporting information†). The results suggest that
ruthenium porphyrin is encapsulated in the channels of
mesoporous molecular sieves and cannot leach off from the
solid support during the reaction. The steroid molecule is
unlikely to freely diffuse into the inner channels of MCM-41 or
MCM-48 despite their pore sizes being ca. 30–50 Å.
of HCl, 1a and 1b could give high turnover numbers (up to 1.0
4
3
10 , Table S2† and entries 7–9 in Table 1). For 1b, when total
4
turnovers reached to 2.6 3 10 after two recycles, reactivity
remained almost unchanged and ee decreased to 66%. Com-
IV
*
4 2
pared to the previously reported [Ru (D -Por )Cl ]/sol–gel
catalyst, the reactivity and stability of 1b exhibited in
3
epoxidation of styrene are better (entry 8). Extension of the
epoxidation of styrene to other alkene substrates also gave high
Scheme 2
reactivity and comparable selectivity (entries 10–16) than those
II
*
obtained using free [Ru (D
4
-Por )(CO)] catalyst. Similar to
We note that there has been no report on using silica
supported metal catalysts for intramolecular cyclopropanations.
To demonstrate the versatility of these ruthenium porphyrin
modified catalysts in organic transformation reactions, 1b was
employed as catalyst for intramolecular cyclopropanation of
trans-cinnamyl diazoacetate. The reaction was found to proceed
with high enantioselctivity (Scheme 3) (85%) and product
II
*
free [Ru (D
4
-Por )(CO)] catalyst, 1b gave better enantiose-
lectivity and reactivity towards cis-alkenes than trans-counter-
parts (Table S3†). This shows that, upon immobilization on the
II
*
inside surface of channels of MCM-48, [Ru (D
4
-Por )(CO)]
performed like the homogenous condition and the unique
environment constituted by [Ru (D -Por )(CO)] and the chan-
4
II
*
3
nels of MCM-48 leads to maintain the chiral introduction,
enhance its reactivity and stabilize the catalyst in asymmetric
epoxidation of alkenes.
turnover number reaches 1.5 3 10 (for homogenous counter-
2
2
part: 1.5 3 10 ). Furthermore, 1b could be reused 4 times
(support information†) and after two cycles, ee decreased to
76%. We envision that structural modification of ordered
molecular sieves such as enlarging pore size and enhancing
thermal stability will provide a new class of heterogeneous
ruthenium catalysts for organic reactions.
We have examined the epoxidation of cholesterol acetate
Scheme 2) using the ruthenium modified silica materials; 1a
(
2
Table 1 Asymmetric epoxidation of alkenes by 2,6-Cl pyNO catalysed by
a
1a–1d
Conver- Yield(%,
sion% TON)
b
b
Ee%c
Scheme 3
Entry Catalyst Substrate
1
2
3
4
5
6
7
8
9
0
1
1a
1a
1a
1b
1c
1d
1a
1b
1b
1b
1b
X = H
X = H
X = H
X = H
X = H
X = H
X = H
X = H
X = H
X = 4-Cl
X = 4-CF
79
76
56
84
42
10
72
82
76
76
62
86(3390) 72 (R)
86(3260) 46 (R)
88(2460) 66 (R)
86(3610) 75 (R)
79(1650) 43 (R)
This work was supported by the Generic Drug Research
Program (HKU), the Hong Kong Research Grants Council
d
e
(
1
HKU 7298/99P) and Area of Excellence Scheme (AoE/P-
0/01), University Grants Committee of Hong Kong SAR,
68(340)
66 (R)
China.
f
f
f
80(11520) 70 (R)
82(13450) 74 (R)
83(12620) 66 (R)
93(3530) 68 (R)
98(3030) 70 (R)
Notes and references
1
1
1
T. Katsuki, in, Catalytic Asymmetric Synthesis, ed. I. Ojima, Wiley-VCH,
New York, 2000, ch. 6B, p. 287.
3
2
A. Berkessel and M. Frauenkron, J. Chem. Soc., Perkin Trans. 1, 1997,
12
1b
94
94(4420) 70 (n.d)
2
265; T. S. Lai, R. Zhang, K. K. Cheung, H. L. Kwong and C. M. Che,
Chem. Commun., 1998, 1583; R. Zhang, Y. W. Yu, H. Z. Sun, W. S. Liu
and C. M. Che, Chem.–Eur. J., 2002, 8, 2495; C. M. Che, J. S. Huang, F.
W. Lee, Y. Li, T. S. Lai, H. L. Kwong, P. F. Teng, W. S. Lee, W. C. Lo,
S. M. Peng and Z. Y. Zhou, J. Am. Chem. Soc., 2001, 123, 4119.
Y. R. de Miguel, E. Brule and R. G. Margue, J. Chem. Soc., Perkin Trans.
1
1
1
3
4
5
1b
1b
1b
92
80
99(4600) 75 (n.d.)
92(3680) 77 (R)
3
1
, 2001, 3085; Chiral Catalyst Immobilization and recycling, ed. D. E. de
90
85(3820) 74 (1S, 2R)
98(4900) 76 (1R, 2S)
Vos, I. F. J. Vankelecom and P. A. Jacobs, Wiley-VCH, Weinheim,
2000.
1
6g 1b
> 99
a
4 O. Nestler and K. Severin, Org. Lett., 2001, 3, 3907; X. Q. Yu, J. S.
Huang, W. Y. Yu and C. M. Che, J. Am. Chem. Soc., 2000, 122, 5337; J.
L. Zhang, H. B. Zhou, J. S. Huang and C. M. Che, Chem.–Eur. J., 2002,
8, 1554.
All reactions were performed in benzene for 24 h by using 0.2 mmol
pyNO. The loading
-Por )(CO)] for catalysts 1a–1d are 1.6, 1.7, 1.6 and 1.7
catalyst, 1.0 mmol of substrate, and 1.1 mmol 2,6-Cl
amount of [Ru (D
wt %, respectively, unless otherwise noted Conversions were determined
by GC using 1,4-dichlorobenzene as standard; yields were based on the
amount of substrates consumed. c Ee% of epoxides were determined by GC
equipped with chiral capillary column (J&W Scientific cyclodex-B) or H-
NMR using Eu(hfp)
2
II
*
4
b
5
J. L. Zhang and C. M. Che, Org. Lett., 2002, 4, 1911.
6 R. Zhang, W. Y. Yu, K. Y. Wong and C. M. Che, J. Org. Chem., 2001,
66, 8145.
1
7
C. J. Liu, S. G. Li, W. Q. Pang and C. M. Che, Chem. Commun., 1997,
65; C. J. Liu, W. Y. Yu, S. G. Li and C. M. Che, J. Org. Chem., 1998, 63,
7364; X. G. Zhou, X. Q. Yu, J. S. Huang, S. G. Li, L. S. Li and C. M. Che,
Chem. Commun., 1999, 1789.
3
as chiral shift reagent. Absolute configuration was
d
determined by comparing with authentic chiral samples. The loading
amount of [Ru (D -Por )(CO)] is 0.8 wt %. The loading amount of
4
II
*
e
II
*
f
[
Ru (D
mmol 2,6-Cl
a and 1b, room temperature for 24 h. cis Isomer.
4
-Por )(CO)] is 2.4 wt %. Reaction condition: 2.0 mmol alkene, 2.2
8
R. Khön and M. Fröba, Catalysis Today, 2001, 68, 227; A. Sayari, Chem.
Mater., 1996, 8, 1840; J. Y. Ying, C. P. Mehnert and M. S. Wong, Angew.
Chem., Int. Ed., 1999, 38, 56.
2
pyNO, a catalytic amount of HCl (0.1% equiv.) and 0.1 mmol
g
1
CHEM. COMMUN., 2002, 2906–2907
2907