Filtration-free recyclable catalytic asymmetric dihydroxylation using a ligand immobilized on magnetic mesocellular mesoporous silica

Article abstract of DOI:10.1002/adsc.200505354

A new magnetic mesocellular mesoporous silica support featuring a 3D open-pore structure has been developed for highly efficient, filtration-free recycling of chiral ligands for catalytic asymmetric dihydroxylation. Reactions using the ligand immobilized on this magnetic silica system exhibited almost the same reactivity and enantioselectivity as those obtained in the homogeneous reaction. Magnetically recovered ligand could be recycled eight times with good to excellent conservation of reaction rates and enantioselectivities.

Full text of DOI:10.1002/adsc.200505354

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DOI: 10.1002/adsc.200505354
Filtration-Free Recyclable Catalytic Asymmetric Dihydroxylation
Using a Ligand Immobilized on Magnetic Mesocellular
Mesoporous Silica
Daewon Lee,a, c Jinwoo Lee,b, c Honggeun Lee,a Sunmi Jin,b Taeghwan Hyeon,b,*
B. Moon Kima,*
a
School of Chemistry, College of Natural Sciences, Seoul National University, Seoul 151-747, South Korea
Fax: (þ82)-2-872-7505, e-mail: kimbm@snu.ac.kr
b
National Creative Research Initiative Center for Oxide Nanocrystalline Materials and School of Chemical and Biological
Engineering, Seoul National University, Seoul 151-747, South Korea
Fax: (þ82)-2-888-1604, e-mail: thyeon@plaza.snu.ac.kr
c
These two individuals contributed equally to this work
Received: September 9, 2005; Accepted: November 21, 2005
Abstract: A new magnetic mesocellular mesoporous
silica support featuring a 3D open-pore structure has
been developed for highly efficient, filtration-free re-
cycling of chiral ligands for catalytic asymmetric di-
hydroxylation. Reactions using the ligand immobi-
lized on this magnetic silica system exhibited almost
the same reactivity and enantioselectivity as those
obtained in the homogeneous reaction. Magnetically
recovered ligand could be recycled eight times with
good to excellent conservation of reaction rates
and enantioselectivities.
recycling through magnetic separation for asymmetric
synthesis has rarely been documented.[5,7]
The Sharpless catalytic asymmetric dihydroxylation
(AD) is one of the most efficient asymmetric transfor-
mations generating enantiomerically enriched vicinal
diols.[8] To achieve repetitive use of the catalytic osmi-
um-ligand system, several approaches have been docu-
mented in regard to the immobilization of the alkaloid
ligand,[9,10] or osmium.[11] In recycling of the ligand-cata-
lyst system through immobilization, filtration of the het-
erogenized catalyst-ligand complex is often tricky due to
the high toxicity and volatility of the osmium compo-
nent.
Herein we report on the preparation of a new type of
magnetically separable, hierarchically-ordered meso-
cellular mesoporous silica (M-HMMS) and its applica-
tion towards a filtration-free, recyclable AD reaction.
Hierarchically ordered mesocellular mesoporous silica
(HMMS), which is mesocellular silica foam[12] with or-
Keywords: asymmetric dihydroxylation; filtration-
free; magnetic separation; mesoporous silca; recy-
cling
Much effort has been focused on the recycling of cata- dered mesoporous walls, was synthesized using a single
lysts through immobilization for the purpose of develop- template as reported recently from one of these labora-
ing practical catalytic systems.[1] Although catalysts tories.[13] Then, magnetic nanoparticles were grafted on
grafted on heterogeneous supports allow for recycling the pore surface of HMMS through the thermal decom-
and simplify purification steps, they often suffer from re- position of an iron propionate complex (Figure 1).[14]
duced reactivity and lower stereoselectivity. Further-
Several peaks typical for g-Fe2O3 were observed from
more, in many instances recovery of heterogenized cat- the XRD pattern (Figure 1). The large peak widths are
alysts through filtration is cumbersome, especially in the attributed to the formation of nanosized particles.
case of air-sensitive catalysts. In this regard, magnetic Mean particle size calculated by Debye–Scherrer equa-
separation of catalysts could be an attractive alternative tion using the (311) peak is 5.4 nm. It is also found that
to the filtration.[2] Recently, the development of uni- about 5 nm-sized nanoparticles are homogeneously dis-
formly sized magnetic nanoparticles has been intensive- tributed all over the pores of HMMS as shown by subse-
ly pursued[3] and they have been exploited in the immo- quent transmission electron microscopy studies. The
bilization of proteins, enzymes, bioactive agents,[4] and magnetization curve of M-HMMS exhibited typical su-
Pd catalysts.[5] Very recently, magnetic nanoparticle-in- perparamagnetic behavior, showing no observed hyste-
corporated mesoporous silica and carbon materials resis at room temperature. The saturation magnetiza-
have been reported.[6] However, efficient chiral ligand tion value at room temperature is 15.12 emu/g, which
Adv. Synth. Catal. 2006, 348, 41 – 46
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Daewon Lee et al.
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Reaction of M-HMMS functionalized with a chloro-
benzyl linker and the previously reported Cinchona al-
kaloid DHQ-PHAL-DHQ-S-(CH2)5CH2OH monomer
ligand L6[10a] (Figure 2b) in refluxing toluene for 24 h
followed by extensive washing provided the silica load-
ed with the ligand (M-HMMS-L6, Scheme 1). Elemen-
tal analysis of M-HMMS-L6 indicated a ligand loading
of 0.27 mmol/g, which was higher than that obtained
with the mesoporous SBA-15 (0.17 mmol/g),[10a] in spite
of the lower surface area (Figure 2c, below 253 m2/g) of
the M-HMMS than that of SBA-15 (>600 m2/g). The
loading of the organic linker and the ligand renders
the surface of the M-HMMS-L6 hydrophobic, a desira-
ble feature for the AD reaction, which requires the li-
gand to be well dispersed in organic (t-BuOH) layer.
To investigate the reactivity of the M-HMMS-L6/
OsO4 complex, the dihydroxylation of styrene was first
investigated under the standard conditions employing
K3Fe(CN)6 as a co-oxidant,[16] changing the molar ratio
of ligand and OsO4 from 1.0 to 0.2 mol %. The results
were quite consistent regardless of the relative amounts
of osmium and the ligand (97.3, 96.4 and 95.9% ees with
1.0, 0.4 and 0.2 mol % of OsO4, respectively). Even with
0.2 mol % each of M-HMMS-L6 and OsO4, the reaction
proceeded almost at the same rate (less than 2 h) as the
homogeneous reaction. Encouraged by this result, ADs
of various olefins were investigated with M-HMMS-L6
Figure 1. Preparation of iron-nanoparticle encapsulated mes-
ostructured material and physical data: wide-angle XRD pat-
tern of M-HMMS and magnetization curve of M-HMMS.
is sufficiently high for magnetic separation (Figure 1) and the results were compared with those obtained
and much higher than those of previously reported mag- from the reactions using homogeneous ligand and ligand
netic nanoparticles loaded onto MCM-41[15a] and Cab-o- immobilized onto SBA-15 (Table 1).
Sil.[15b] We envisioned that the fact that M-HMMS has a
Reactions using M-HMMS-L6 exhibited almost the
high magnetization value with a high ligand loading ca- same rates and enantioselectivities as those observed
pability would warrant its general applicability for effi- in the reactions using homogeneous ligand. Most reac-
cient catalyst/ligand recovery.
tions were complete within 8 h. The high reactivity and
Scheme 1. Immobilization of chiral ligand L6 on M-HMMS.
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Filtration-Free Recyclable Catalytic Asymmetric Dihydroxylation
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Figure 2. (a) Pictorial representation of M-HMMS, (b) structure of the modified ligand L6 and the TEM image of M-HMMS-
*
~
L6, (c) N2 adsorption-desorption isotherm of magnetic M-MMS ( ) and M-MMS-L6 ( ) including calculated pore diameter by
the BJH method.
enantioselectivity of M-HMMS-L6 were attributed to ceeded with 95, 81 and 19% yields for three consecutive
the well-connected three-dimensional (3D) pore struc- runs, respectively, thus a precipitous yield drop was ob-
tures and small particle size of the mesostructured silica served at the third run, indicating a significant loss of os-
support (~500 nm), which render the active sites well mium. The quick drop in reactivity was similarly ob-
dispersed on the surface and readily accessible. Finally, served with other types of ligand immobilization from
to explore the repetition capability of the magnetically this and other laboratories.[10] For this reason, a new
separable ligand, we performed reactions consecutively batch of OsO4 was employed in each recycling experi-
using the recycled ligand. The filtration-free M-HMMS- ment to obtain a reasonable reaction rate.
L6 was almost quantitatively recovered through simple
As shown in Table 1, the recovered chiral ligand could
magnetic separation (Figure 3) and then reused for the be recycled eight times with good to excellent conserva-
next reaction employing a new batch of K3Fe(CN)6. Re- tion of reaction rates and enantioselectivities. In the case
actions of styrene using the recycled M-HMMS-L6 pro- of trans-stilbene, the enantioselectivity remained within
Adv. Synth. Catal. 2006, 348, 41 – 46
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Daewon Lee et al.
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Table 1. Results of AD using M-HMMS-L6 in comparison with those using homogeneous and SBA-15 immobilized ligand,
and recycling results.
Entry
Olefin
% ees with
homogeneous
reagent[a]
% ees
% ees with consecutive use of M-HMMS-L6[c]
with SBA-15[b]
1st
2nd
3rd
4th
5th
6th
7th
8th
1
>99.5
>99.5
>99.5
99.2
98.8
98.8
98.8
98.6
>99.5
98.5
2
3
97
97
96
98
97.3
96.9
95.2
93.1
94.7
93.2
94.4
93.5
94.0
95.2
93.8
93.1
93.5
96.2
94.1
93.8
4
93
90
92.7
87.9
81.5
81.5
80.1
81.2
80.7
80.9
5
97
94
95.5
93.0
91.8
92.0
90.6
91.5
92.2
91.9
[a]
Ref.[8a]
[b]
[c]
Ref.[10a]
All reactions were carried out with molar ratio of olefin/OsO4/M-HMMS L6¼1/0.01/0.015 under standard K3Fe(CN) 6 con-
ditions. Recycling experiments were performed using a new batch of OsO4 and oxidant. The ee values were determined
through HPLC analysis.
Figure 3. (a) AD reaction mixture containing M-HMMS-L6, and (b) M-HMMS-L6 collected using an external magnet after
the reaction.
a 1% margin of the initial run (>99.5 to 98.5%) with favorable reaction kinetics over other mesoporous ma-
>95% yields even at the eighth recycle (entry 1). These terials. The immobilized ligand for asymmetric dihy-
results are comparable to the best results obtained with droxylation exhibited almost the same activity and
other immobilized ligands on silica materials.[1,9,10] The enantioselectivity for ADs of several olefins as those ob-
same alkaloid ligand L6 anchored on SBA-15 has shown tained in the homogeneous reaction. The magnetically
a considerable decrease of ees (>99.5 to 94%) even at recovered ligand could be recycled eight times with
the 2nd recycle in the reaction of trans-stilbene.[10a] The good to excellent conservation of reaction rates and
highly conserved reactivity and enantioselectivity ob- enantioselectivities in most cases. This filtration-free
served with M-HMMS-L6 may be due to the efficient, heterogeneous ligand system may pave the way to the
filtration-free magnetic separation of the ligand system. development of other magnetically separable heteroge-
In summary, we have developed a new type of magnet- neous ligand/catalyst systems free from filtration prob-
ically separable ligand system using magnetic mesocel- lems, thus providing a practical means to highly recycla-
lular mesoporous silica (M-HMMS) as a ligand support, ble and efficient catalyst systems.
which has easily accessible 3D open pores allowing for
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Filtration-Free Recyclable Catalytic Asymmetric Dihydroxylation
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and the organic layer was dried with anhydrous MgSO4. After
removal of the solvent, the crude product was purified through
chromatography on silica gel with EtOAc/hexanes affording
the desired cis-diol. The enantiomeric excesses of the diols
were determined through HPLC analysis using chiral columns.
The magnetically separated ligand (M-HMMS-L6) was reused
for successive reactions.
Experimental Section
Magnetically Separable Hierarchically Mesocellular
Mesoporous silica (M-HMMS)
Fe(NO3)3 ·9 H2O (1.34 g, 3.3 mmol) dissolved in methanol was
impregnated onto 0.5 g HMMS[13] silica and dried in a drying
oven at 858C until the material was free from methanol. The
Fe(NO3)3-impregnated silica was allowed to react with pro-
pionic acid at 858C for 3 h to form the iron propionate com-
plex. The composite was heated to 3008C under air (18C/
min) for decomposition of the iron propionate complex and al-
lowed to remain at that temperature for 30 min. The obtained
silica was denoted as M-HMMS (magnetic HMMS). BET sur-
face area: 253 m2/g. Mean pore diameter: 27 and 12 nm.
Acknowledgements
T. H. would like to express thanks for the financial support from
the Korean Ministry of Science and Technology through the Na-
tional Creative ResearchInitiativeProgram. B. M. K. thanks the
Ministry of Science and Technology of Korea through the Re-
search Center for Nanocatalysis, one of the National Science
Programs for Key Nanotechnology.
M-HMMS Functionalized withChlorobenzyl Linker
(M-HMMS-F)
References and Notes
M-HMMS (500 mg) was suspended in dry toluene and 2-(4-
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brown solid was dried under vacuum. Grafted amount
(1.06 mmol/g) was calculated from the mass% of carbon. Ele-
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using M-HMMS-L6
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Immobilized ligand M-HMMS-L6 (30 mg, 0.008 mmol),
K3Fe(CN)6 (494 mg, 1.5 mmol), K2CO3 (207 mg, 1.5 mmol)
and MeSO2NH2 (47 mg, 0.5 mmol) (not necessary for terminal
olefins) were dissolved in t-BuOH-H2O (v/v¼1:1, 3 mL). This
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and OsO4 (4 wt % aqueous solution, 0.005 mmol) were added
to the solution. After completion of the reaction, the reaction
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aqueous sodium sulfite solution. The layers were separated
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