Superparamagnetic nanoparticle-Supported (S)-diphenyl- prolinol trimethylsilyl ether as a recyclable catalyst for asymmetric Michael addition in water

Article abstract of DOI:10.1002/adsc.201000508

A new superparamagnetic nanoparticle-supported (S)-diphenylprolinol trimethylsilyl ether (Jorgensen-Hayashi catalyst) was synthesized and applied for the asymmetric Michael addition of aldehydes to nitroalkenes in water, which gives products in moderate t

Full text of DOI:10.1002/adsc.201000508

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DOI: 10.1002/adsc.201000508
Superparamagnetic Nanoparticle-Supported (S)-Diphenyl-
prolinol Trimethylsilyl Ether as a Recyclable Catalyst for
Asymmetric Michael Addition in Water
Bang Gen Wang,a Bao Chun Ma,a,* Qiong Wang,a and Wei Wanga,*
a
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou, Gansu 730000, Peopleꢀs Rupublic of China
Fax : (+86)-931-891-5557; e-mail: wang_wei@lzu.edu.cn or mabaochun@lzu.edu.cn
Received: June 30, 2010; Revised: September 5, 2010; Published online: November 12, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000508.
Abstract: A new superparamagnetic nanoparticle-
supported (S)-diphenylprolinol trimethylsilyl ether
(Jørgensen–Hayashi catalyst) was synthesized and
applied for the asymmetric Michael addition of al-
dehydes to nitroalkenes in water, which gives prod-
ucts in moderate to good yields (up to 96%), good
enantioselectivity (up to 90% ee) and diastereose-
lectivities (up to 99:1). The immobilized catalyst
could be easily separated from the reaction by an
external magnet and recycled for four times without
significant loss of catalytic efficiency.
efficiently and economically. A solution to this prob-
lem could be catalyst immoblization[7] onto some solid
supports, such as mesoporous silica or polymers.
After the reaction is completed, the working catalyst
could be re-collected by filtration or centrifugation
and further used for the next run. For example,
Mager and Zeitler[8] have very recently developed a
highly efficient approach for the immobilization of
Jørgensen–Hayashi catalyst onto the MeOPEG poly-
mer. More strikingly, the recyclable MeOPEG-sup-
ported catalyst shows comparable reactivity and selec-
tivity as the homogeneous Jørgensen–Hayashi coun-
terpart.
Keywords: asymmetric Michael addition; immobili-
zation; magnetic nanoparticles; organocatalysis
On the other hand, magnetic nanoparticles (MNPs)
of Fe3O4 have emerged recently as a new support[10]
[9]
for heterogeneous catalyst. The MNPs of Fe3O4 are
inexpensive, non-toxic, chemically stable and can be
simply prepared. Most importantly, the MNP-support-
During the past decade, organocatalysis has become ed catalyst could be re-collected by using an external
one of the most rapidly growing research areas in syn- magnet conveniently without filtration or centrifuga-
thetic organic chemistry.[1] Many asymmetric organo- tion. Due to the above-mentioned advantages, immo-
catalysts have been developed and applied in a wide bilization of organocatalysts,[11] enzymes,[12] and transi-
range of organic processes and methodologies, provid- tion-metal catalysts[13] onto the MNP support has
ing efficient and environmentally friendly access to been exploited. To the best of our knowledge, how-
chiral compounds including many drugs and bioactive
ACHTUNGTRENeNGNU ver, there is only one example reported so far for an
natural products. In this aspect, one of the most versa- MNP-supported asymmetric organocatalyst, which
tile organocatalysts is the Jørgensen–Hayashi cata- was developed by Luo et al. in 2008.[11c] Immobiliza-
lysts, i.e., a,a-diarylprolinol ethers[2,3], which activate tion of chiral cyclohexanediamine onto the MNPs of
aldehydes via an enamine mechanism[4] and a,b-unsa- Fe3O4 coated with SiO2 was successfully achieved and
turated carbonyl substrates via an iminium ion mech- the immobilized catalyst was further applied in the
anism.[5] Moreover, the Jørgensen–Hayashi catalysts asymmetric aldol reaction of cyclohexanone in water.
have been utilized in tandem and multi-component The reaction gives the aldol product with up to 98%
reactions, through which many complicated com- ee and up to 98% yield. Moreover, the MNP-support-
pounds have been conveniently constructed.[6]
In most cases of organocatalyzed reactions, how- times without significant loss in activity and stereose-
ever, a large amount of the organocatalyst (typically, lectivities.
ed asymmetric organocatalyst could be reused for 11
ACHTUNGTRENNUNG
10–30 mol%) is needed, which presents a challenge
As described above, the Jørgensen–Hayashi cata-
for organic chemists to utilize organocatalysts more lysts, i.e., a,a-diarylprolinol ethers are of great impor-
Adv. Synth. Catal. 2010, 352, 2923 – 2928
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Scheme 1. Preparation of the MNP-supported Jørgensen–Hayashi catalyst 7.
tance and of general use in organocatalysis. Exploring XRD images indicated that the MNPs of Fe3O4 were
new approaches for the immobilization of these cata- intact after immobilization (Figure 3). According to
lysts may, therefore, offer further possibilities for the elemental analysis of nitrogen, the loading of cat-
their applications in asymmetric organocatalysis.[14] alyst 7 was 0.66 mmolgÀ1.
Herein, we report on the MNP-supported (S)-diphe-
nylprolinol trimethylsilyl ether (catalyst 7) and its ap- asymmetric Michael addition of propanal to trans-b-
plication in asymmetric Michael addition[2c,15] of alde- nitrostyrene was selected as
model reaction
hydes to nitroalkenes in water. (Table 1). Firstly, the effect of solvent on this reaction
To evaluate the catalytic activity of catalyst 7, the
a
The MNPs of Fe3O4 coated with SiO2 (denoted as was examined and the obtained results are summar-
Fe3O4@SiO2) were prepared according to the litera- ized in Table 1. In CH3CN or toluene (Table 1, en-
ture.[13a,16] As described in Scheme 1, the MNP-sup- tries 1 and 2), only a trace of product was observed.
ported Jørgensen-Hayashi catalyst 7 was accordingly On the contrary, moderate yields could be achieved
synthesized and immobilized onto the MNPs of when CH2Cl2, CHCl3 or ethanol was applied as the
Fe3O4@SiO2. To a solution of compound 6 in anhy- solvent (Table 1, entries 3–5). More strikingly, we
drous toluene, the MNPs of Fe3O4@SiO2 were added found that the reaction proceeded smoothly in water
and refluxed for 48 h. When the reaction was com- and gave the desired product in 85% yield (Table 1,
pleted, the MNP-supported catalyst 7 was washed entry 6). In all the cases tested, stereoselectivities for
with ethyl acetate and CH2Cl2 three times in turn, the obtained products were, however, at the same
and dried under vacuum at 558C for 24 h. The ob- level (82% to 91% ee; 84:16 to 90:10 dr). The effect
tained catalyst was then characterized by transmission of co-solvent on the reaction was also examined:
electron microscopy (TEM), magnetization measure- CH2Cl2:H2O (v/v=1:1) gave the desired product in
ment, and powder X-ray diffraction (XRD). As 49% yield, 91% ee, and 90:10 dr (Table 1, entry 7);
shown in Figure 1, the nanometer dimensions of while EtOH:H2O (v/v=1:1) gave 85% yield, 87% ee,
MNPs before and after immobilization were con- and 86:14 dr (Table 1, entry 8).[17] Therefore, water
firmed by TEM analysis. Dynamic light scattering was chosen as the solvent for the asymmetric Michael
(DLS) analysis indicates that the diameters of MNPs addition of aldehydes to nitroalkenes using catalyst 7
before and after immobilization are 145Æ20 nm and at ambient temperature.
190Æ10 nm, respectively. Magnetization curves mea-
With the optimized conditions at hand, we further
sured at ambient temperature showed that the MNP- examined the substrate scope for the asymmetric Mi-
supported catalyst 7 is superparamagnetic (Figure 2). chael addition catalyzed by 7 and the obtained results
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Superparamagnetic Nanoparticle-Supported (S)-Diphenylprolinol Trimethylsilyl Ether
Figure 1. TEM images for MNPs of Fe3O4@SiO2 (a) and MNP-supported catalyst 7 (b).
Table 1. Screening of solvents for the asymmetric Michael
addition reaction in the presence of catalyst 7.[a]
Entry Solvent
Yield [%][b] ee [%][c] dr[d]
1
2
3
4
5
6
7
8
CH3CN
C6H5CH3
CH2Cl2
CHCl3
EtOH
trace
trace
49
n.d.[e]
n.d.
83
82
84
84 (90)
91
87
n.d.
n.d.
84:16
85:15
86:14
84:16
90:10
86:14
49
44
H2O
85 (80)[f]
Figure 2. Magnetization curves for MNPs of Fe3O4@SiO2
(open symbols) and MNP-supported fresh catalyst 7 (solid
symbols).
CH2Cl2:H2O (1:1) 49[g]
EtOH:H2O (1:1)
85[g]
[a]
Reaction conditions: trans-b-nitrostyrene (0.2 mmol),
propanal (2.0 mmol) and catalyst 7 (0.04 mmol) in sol-
vent (0.5 mL) at ambient temperature for 72 h.
Isolated yields after silica gel column chromatography.
Determined by chiral HPLC (Daicel chiral OD-H
column). The absolute configuration of reaction product
is determined according to ref.[2c]
Determined by H NMR.
Not determined.
1.0 equivalent of benzoic acid was added.
Reaction time of 48 h.
[b]
[c]
[d]
[e]
[f]
1
[g]
are listed in Table 2. When the aromatic ring of the
arylnitroalkene was substituted by either electron-
withdrawing groups or electron-donating groups
(Table 2, entries 2–7), the yield of reaction product
was a little lower (53% to 79%) than that without
substitution (85%, Table 2, entry 1), but the enantio-
selectivity and diastereoselectivity remained at the
Figure 3. XRD patterns of for MNPs of Fe3O4@SiO2
(bottom) and MNP-supported catalyst 7 (top).
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Table 2. Asymmetric Michael addition of aldehydes to nitroalkenes using heterogeneous catalyst 7.[a]
Entry
Ar
R
Yield [%][b]
ee [%][c]
dr[d]
1
2
3
4
5
6
7
8
Ph
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH2CH2CH3
85 (80)[e]
64
79
67
84
90
80
75
77
83
81
82
81
84
76
85
86
84:16
82:18
85:15
76:24
75:25
86:14
86:14
92:8
> 99:1
77:23
80:20
79:21
> 99:1
o-NO2C6H4
p-ClC6H4
o-ClC6H4
o-BrC6H4
p-CH3OC6H4
m-CH3C6H4
Ph
Ph
1-naphthyl
2-furyl
70
53 (32)[e]
64
57
54
67
96
74
70
9
A
10
11
12[f]
13[f]
CH3
CH3
CH3
p-CH3OC6H4
Ph
A
[a]
Reaction conditions: nitroalkene (0.2 mmol), aldehyde (2.0 mmol) and catalyst 7 (0.04 mmol) in H2O (0.5 mL) at ambient
temperature for 72 h.
Isolated yields after silica gel column chromatography.
[b]
[c]
Determined by chiral HPLC analysis (Daicel chiral columns, see Supporting Information for details). The absolute con-
figuration is determined according to ref.[2c]
[d]
[e]
[f]
1
Determined by H NMR.
The yields were given in parentheses when addition of 1.0 equiv of benzoic acid was applied in the reaction.
Reactions carried out in EtOH:H2O (0.5 mL, v/v=1:1) at ambient temperature for 48 h.
Table 3. Recycle use of catalyst 7.[a]
same level (75% to 90% ee; 75:25 to 86:14 dr). Pro-
longation of the chains of the aldehydes caused a de-
crease of the reaction yields (54% and 57%, respec-
tively, see Table 2, entries 8 and 9), but the dr value
could be increased to 99:1 (Table 2, entry 9). The ni-
troalkene with
a naphthyl substituent (Table 2,
entry 10) gave a slightly lower yield (67%) but good
enantioselectivity (84% ee). A nitroalkene with a het-
erocyclic substituent, such as the 2-furyl group,
achieved a high yield of 96% but gave only 76% ee
(Table 2, entry 11).
Next we investigated the recyclability of catalyst 7.
The re-collection of superparamagnetic catalyst 7 is
very facile with an external magnet. When the reac-
tion is completed, a small magnet was put nearby the
reaction flask to concentrate catalyst 7 so that the
aqueous solution can easily be decanted. In the simi-
lar way, the used catalyst 7 was repeatedly washed by
ethyl acetate and further dried under vacuum at 558C
for 24 h for the next run. During the recycle experi-
ments (Table 3), we found that addition of benzoic
acid to the reaction system is crucial for the recycle
Cycle
Yield [%][b]
ee [%][c]
dr[d]
1
2
3
4
80
83
54
42
90
88
85
84
84:16
86:14
83:17
80:20
[a]
Reaction conditions: trans-b-nitrostyrene (0.2 mmol),
propanal (2.0 mmol), benzoic acid (0.2 mmol) and cata-
lyst 7 (0.04 mmol) in H2O (0.5 mL) at ambient tempera-
ture for 72 h.
Isolated yields after silica gel column chromatography.
Determined by chiral HPLC (Daicel chiral OD-H
column). The absolute configuration of reaction product
is determined according to ref.[2c]
[b]
[c]
[d]
1
Determined by H NMR.
use of catalyst 7 in the asymmetric Michael addition 54% in the third cycle and further to 42% in the
reaction.[18] When 1.0 equivalent of benzoic acid was fourth cycle.[19] On the contrary, no significant loss of
added, the recycle reaction proceeded smoothly. The stereoselectivities occurred in each cycle (Table 3,
results of recycle experiments with the addition of cycles 1–4).
benzoic acid are summarized in Table 3. The reaction
In summary, we have immobilized, for the first
yields remained at the same level in the first two time, the Jørgensen–Hayashi catalyst of (S)-diphenyl-
cycles (80% and 83%, respectively), but decreased to prolinol trimethylsilyl ether onto the MNPs of
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Superparamagnetic Nanoparticle-Supported (S)-Diphenylprolinol Trimethylsilyl Ether
K. A. Jørgensen, J. Am. Chem. Soc. 2005, 127, 18296–
Fe3O4@SiO2 and applied the immobilized catalyst in
the asymmetric Michael addition of aldehydes to ni-
troalkenes. Moderate to good yields (up to 96%),
good enantioselectivity (up to 90% ee) and diastereo-
selectivities (up to 99:1) were obtained. The NMP-
supported catalyst could be reused in the asymmetric
Michael addition reaction for four times without sig-
nificant loss of stereoselectivities.
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General Procedure for the Asymmetric Michael
Addition Reaction using Catalyst 7
To a solution of aldehyde (2.0 mmol) in water (0.5 mL) was
added nitroalkene (0.2 mmol) and heterogeneous catalyst 7
(60 mg, 0.04 mmol). After the resulting mixture had been
stirred at ambient temperature for 72 h, catalyst 7 was con-
centrated by using an external magnet and the aqueous solu-
tion was decanted. Ethyl acetate was then added to wash
catalyst 7 for several times and the aqueous solution de-
ACHTUNGTRENNUNGcanted was extracted with ethyl acetate three times. The or-
ganic layers were then combined, dried over anhydrous
Na2SO4 and further purified by column chromatography on
silica to obtain the desired products, with petroleum ether/
EtOAc=4:1 to 10:1 as the eluent.
Recycle Use of Catalyst 7
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After being washed with ethyl acetate for several times, the
used catalyst 7 was dried under vacuum at 558C for 24 h.
The recycled catalyst 7 (60 mg, 0.04 mmol) was then added
to the solution of propanal (116 mg, 2.0 mmol), trans-b-ni-
trostyrene (30 mg, 0.2 mmol), and benzoic acid (24 mg,
0.2 mmol) in water (0.5 mL). After being stirred for 72 h at
ambient temperature, the reaction mixture and the recycled
catalyst 7 were treated as described above.
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 20972064), the 111 project,
and the Program for New Century Excellent Talents in Uni-
versity (NCET-06-0904).
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[19] According to the elemental analysis on nitrogen, the
loading of the Jørgensen–Hayashi catalytic moiety onto
catalyst 7 after the 4th cycle is 0.59 mmolgÀ1 which is
slightly lower than that for the fresh catalyst
(0.66 mmolg)À1. Therefore, catalyst leaching should not
be the main reason for the reactivity loss of the cata-
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