Efficient heterogeneous asymmetric transfer hydrogenation catalyzed by recyclable silica-supported ruthenium complexes

Article abstract of DOI:10.1002/ejoc.200500074

Easily accessible N-(p-tolylsulfonyl)-1,2-diphenylethylenediamine-(TsDPEN-) derived organic-inorganic hybrid ligands were prepared and applied to the heterogeneous asymmetric transfer hydrogenation in traditional HCO ₂H/NEt₃ and in w

Full text of DOI:10.1002/ejoc.200500074

FULL PAPER
Efficient Heterogeneous Asymmetric Transfer Hydrogenation Catalyzed by
Recyclable Silica-Supported Ruthenium Complexes
Pei-Nian Liu,[a] Pei-Ming Gu,[a] Jin-Gen Deng,[b] Yong-Qiang Tu,*[a] and Ya-Ping Ma[b]
Keywords: Asymmetric catalysis / Heterogeneous catalysis / Hydrogenation / Immobilization
Easily accessible N-(p-tolylsulfonyl)-1,2-diphenylethylenedi-
amine-(TsDPEN-)derived organic–inorganic hybrid ligands
were prepared and applied to the heterogeneous asymmetric
transfer hydrogenation in traditional HCO2H/NEt3 and in
water with HCO2Na as the hydrogen source and sodium do-
decyl sulfate (SDS) as the phase-transfer catalyst. The reac-
tions demonstrated high catalytic activities and excellent
enantioselectivities for most aromatic ketones and the imine
tested, with results similar to those in homogeneous catalysis.
In particular, this heterogeneous catalyst can be readily recov-
ered and used in multiple consecutive catalytic runs with the
high enantioselectivity maintained both in HCO2H/NEt3 and
in water.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
most important enantioselective catalytic transformations
because of its high enantioselectivity, high product yield
and operational simplicity. Among the great progress
achieved in this area,[7] the most significant to date is per-
haps the discovery of the catalyst consisting of a rutheni-
um(ii) complex with (R,R)- or (S,S)-N-(p-tolylsulfonyl)-1,2-
diphenylethylenediamine (TsDPEN, Figure 1) reported by
Noyori et al.[8] Although some reports have appeared con-
cerning heterogeneous asymmetric transfer hydrogenation
with polymer- or dendrimer-supported catalysts,[3,9] exam-
ples of catalysts supported on inorganic supports are still
rare.[4,10] Moreover, the catalytic activities or enantio-
selectivities, especially the recycling of these supported cata-
lysts, are not ideal. In a recent preliminary communica-
tion[11] we reported studies of the highly recyclable silica-
supported Ru-TsDPEN for asymmetric transfer hydrogena-
tion of ketones in HCO2H/NEt3 (formic acid-triethylamine
azeotrope), which demonstrates high reactivity and excel-
lent enantioselectivity. In this paper we wish to present the
details of the preparation and further expanded use of the
supported Ru-TsDPEN catalyst for asymmetric transfer hy-
drogenation in HCO2H/NEt3, as well as in water with
HCO2Na as the hydrogen source and sodium dodecyl sul-
fate (SDS) as the phase-transfer catalyst (PTC).
Introduction
During the last decades thousands of chiral homogen-
eous catalysts have been developed for a great variety of
enantioselective transformations, and many of them are
known to be highly effective.[1] However, the application of
such catalysts in industry is greatly limited, partly due to
the problems of separation and recycling of the often ex-
pensive or toxic catalysts. To overcome such drawbacks, co-
valent immobilization of homogeneous chiral catalysts on
insoluble supports has received very fast growing interest in
recent years as it greatly simplifies the separation of the
catalysts from the reaction mixtures and allows the efficient
recovery and reuse of the expensive catalysts.[2–4] Moreover,
if the heterogeneous asymmetric catalytic reactions using
the supported catalysts can be performed in water,[5] they
meet the requirement of green chemistry very well and are
therefore very attractive for industry because they combine
the advantages of both aqueous and heterogeneous switch-
ing in one system and provide risk-free and environmentally
friendly processes, as well as easy work-up and efficient cat-
alyst recycling.[6]
Asymmetric transfer hydrogenation, which affords im-
portant chiral secondary alcohols or amines, is one of the
[a] State Key Laboratory of Applied Organic Chemistry and De-
partment of Chemistry, Lanzhou University,
Lanzhou, 730000, P. R. China
Fax: +86-931-8912582
E-mail: tuyq@lzu.edu.cn
[b] Key Laboratory of Asymmetric Synthesis & Chirotechnology
of Sichuan Province and Union Laboratory of Asymmetric
Synthesis, Chengdu Institute of Organic Chemistry, Chinese
Academy of Science,
Chengdu, 610041, P. R. China
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Figure 1. The structure of ligand TsDPEN (1).
Eur. J. Org. Chem. 2005, 3221–3227
DOI: 10.1002/ejoc.200500074
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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P.-N. Liu, P.-M. Gu, J.-G. Deng, Y.-Q. Tu, Y.-P. Ma
FULL PAPER
Scheme 1. Synthesis of the supported ligands 5–8.
fugation and can be reused up to five times with the same
high enantioselectivity and without recharging the Ru me-
tal, although the reactivity decreases slightly in subsequent
reactions. We presume that the excellent performance of the
amorphous silica gel-supported Ru-5 might be due to the
comparably large pore size and the amorphous structure of
silica gel, which allows better accessibility of substrates and
reactants.
Results and Discussion
Preparation of Supported Ligands 5–8
As shown in Scheme 1,[11] the reaction of (R,R)-DPEN
(1,2-diphenylethylenediamine) (2) with 3 in CH2Cl2, with
triethylamine as base, smoothly gave the TsDPEN-derived
ligand 4, which was then immobilized onto amorphous sil-
ica gel and mesopores of MCM-41 and SBA-15 by re-
fluxing in toluene for 24 h to afford the supported ligands
Table 1. Supported ruthenium(ii) complexes (Ru-n)-catalyzed
5–7, respectively. We also modified the remaining free sil- asymmetric transfer hydrogenation of acetophenone in HCO2H/
NEt3.[a]
anol sites on ligand 5 to alkylsilanes by treating 5 with a
large excess of hexamethyldisiloxane (HMDSO) at reflux
temperature for 18 h to afford the ligand 8. Subsequent ele-
mental analysis of ligands 5–8 based on the wt.-% of N
demonstrated that the loading ratios of the chiral ligands
were 0.15, 0.14, 0.10 and 0.12 mmolg–1, respectively. Al-
though the amounts of organic moieties contained in the
organic–inorganic hybrid materials are rather low, the FT-
IR spectra of the ligands still show some small new peaks
at 2960, 2860, 1464, 702 cm–1 in comparison with the inor-
ganic supports before grafting, which indicate the forma-
tion of the organic–inorganic hybrid ligands.
Entry
Ligand
Run
Time
[h]
Conv.
[%][b]
ee
[%][c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
5
5
5
5
5
1
4
6
6
6
7
7
7
7
8
8
1
2
3
4
5
1
1
1
2
3
1
2
3
4
1
2
6
8
9
99
Ͼ99
97
99
94
Ͼ99
Ͼ99
99
99
44
Ͼ99
99
92
45
98
97
97
97
97
97
97
97
96
96
96
97
97
97
96
96
–
22
44
5.5
5.5
8
24
88
8
11
24
72
9
The Asymmetric Transfer Hydrogenation of Acetophenone
Catalyzed by Ru-n in HCO2H/NEt3
With the supported ligands 5–8 in hand, the ruthenium
complexes Ru-5–Ru-8 were prepared in situ by mixing the
corresponding ligands and [RuCl2(p-cymene)]2 in CH2Cl2
at 28 °C for 1 h with triethylamine as the base. Sub-
sequently, Ru-5–Ru-8 (1 mol-%) were examined in the tradi-
60
5
tional HCO2H/NEt3 solvent mixture for the asymmetric [a] The reactions were carried out at 40 °C with 0.4 mmol of aceto-
phenone in 0.2 mL of HCO2H/NEt3 with an Ru/ligand/acetophe-
transfer hydrogenation of the model substrate acetophen-
none ratio of 1:1.7:100. [b] Based on GC analysis. [c] Determined
one at 40 °C; the results are listed in Table 1.[11] We were
by GC with a CP-Chirasil-DEX CB column (25 m×0.32 mm); the
(R)-configuration was determined from the optical rotation value.
very pleased to find that the cheapest amorphous silica gel
gave the best results, 99% yield and 97% ee after just 6 h,
which is as good as the results obtained with the non-immo-
To investigate the reasons for the decrease of the catalytic
bilized homogeneous catalysts Ru-1 and Ru-4 (entries 1, 6 activity after five recycling runs we measured the leaching
and 7). Moreover, Ru-5 can be readily recovered by centri- of Ru metal by inductively coupled plasma atomic emission
3222
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Asymmetric Transfer Hydrogenation Catalyzed by Recyclable Silica-Supported Ru Complexes
FULL PAPER
spectrometry (ICP-AES). The results demonstrated that the drogenation of 4-methylacetophenone (9) occurs with an
amount of Ru metal contained in Ru-5 before reaction and excellent 95% ee and quantitative yield after 14 h, and in
after five runs was 0.54% and 0.35%, which means that 30– the third run 94% ee and 98% yield could still be observed
40% of the Ru has leached from the catalyst after five uses. (entries 1–3). For 4-methoxyacetophenone (10), the asym-
The amount of Ru metal leached in the five reaction metric reduction gives an excellent enantiomeric excess
batches (including the workup procedures) was measured to (96% ee) with a shorter reaction time than for the homo-
be 0.011 mg, 0.015 mg, 0.017 mg, 0.024 mg and 0.034 mg, geneous reaction,[8b] and three uses of Ru-5 (entries 4–6).
respectively, which matches with the trend of reactivity de- In the asymmetric reaction of propiophenone (11), a high
crease of the catalyst in the recycling reactions. Moreover, ee value (94% ee) and fairly low reactivity were obtained,
a colour change of the catalyst from yellow to red could be and the catalyst could be used one more time with 94% ee
observed in the recycling routine and the FT-IR spectrum and 94% yield (entries 7 and 8). We also investigated the
of Ru-5 after five uses shows new strong peaks at 1660 and heterogeneous transfer hydrogenation of ethyl benzoylfor-
1609 cm–1. In addition, the addition of further [RuCl2(p- mate (12), which shows that the strong electron-with-
cymene)]2 to the catalytic system does not regain the cata-
lytic activity. Such phenomena indicate that decomposition
or deactivation of the active species in the catalyst occurs
irreversibly in the recycling routine, which causes a gradual
decrease of the activity of the catalyst.
Table 2. Supported ruthenium(ii) complex (Ru-5)-catalyzed asym-
metric transfer hydrogenation of ketones and imine in HCO2H/
NEt3.[a]
For the further examination of the reaction rates a com-
parative experiment between Ru-5 and the non-immobilized
catalysts Ru-1 and Ru-4 was conducted. The results (Fig-
ure 2) reveal that the heterogeneous reaction containing im-
mobilized catalyst Ru-5 possesses a very similar reaction
rate at 40 °C to the homogeneous reactions containing the
non-immobilized ones. In fact, this supported catalyst with
a similar catalytic activity and enantioselectivity as the
homogeneous catalysts is distinguished because the immo-
bilization of chiral catalysts often results in lower reactivi-
ties and enantioselectivities.
Figure 2. Reaction rates of the asymmetric transfer hydrogenation
of acetophenone catalyzed by Ru-1, Ru-4 and Ru-5.
[a] The reactions were carried out at 40 °C with 0.4 mmol substrate
in 0.2 mL of HCO2H/NEt3 with a Ru/5/substrate ratio of 1:1.7:100.
[b] Based on GC analysis. [c] Determined by GC with a CP-Chira-
sil-DEX CB column (25 m×0.32 mm) unless noted; the configura-
tions of the products were determined as R from the optical rota-
tion values, unless noted. [d] The configuration was determined as
S from the optical rotation value. [e] Determined by HPLC on a
The Asymmetric Transfer Hydrogenation of Ketones and
Imine Catalyzed by Ru-5 in HCO2H/NEt3
As shown in Table 2, a variety of aromatic ketones 9–12
and imine 13 could be reduced to the corresponding chiral
alcohols or sultam in the presence of the amorphous silica
gel-supported complex Ru-5. The asymmetric transfer hy- Chiralcel OD column. [f] Isolated yield.
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P.-N. Liu, P.-M. Gu, J.-G. Deng, Y.-Q. Tu, Y.-P. Ma
FULL PAPER
drawing ester group linked directly to the carbonyl carbon sorptive nature,[14] we tried to apply our silica-supported
atom of the ketone reduces the enantioselectivity greatly to catalysts in water (Table 3). The ligands 5–8 were respec-
70–74% ee, although the reaction activity was remarkably tively stirred with [RuCl2(p-cymene)]2 in CH2Cl2 at 28 °C
high (Ͼ99% yield after 1 h) and the same high reactivity for 1 h with triethylamine as the base to afford the com-
could even be observed in the fifth run (entries 9–14). We plexes Ru-5–Ru-8 after the removal of CH2Cl2 and triethyl-
also prepared the imine 13 from saccharin and applied it to amine by evaporation. The catalysts were then used in the
the transfer hydrogenation catalyzed by Ru-5 to give the transfer hydrogenation of acetophenone in water with
useful chiral sultam auxiliary.[12] The results showed the re- HCO2Na (5 equiv.) as the hydrogen source. We were very
action could achieve good enantioselectivity (92% ee) and pleased to find that the organic-solvent-free catalytic reac-
very high reactivity. The recycling use of the catalyst for this tion in water (Table 4) catalyzed by Ru-5 proceeded
imine was very good (seven runs), although the ee values smoothly with Ͼ99% yield and 91% ee after 9 h. Moreover,
decreased a little in the sixth and seventh runs (entries 15– the catalyst could be recovered readily and used in two
21).
more runs with a slightly enhanced enantioselectivity (92%
ee) and high yields (entries 1–3). We then added the surfac-
tant SDS (4% mol) to the reaction mixture and achieved
better enantioselectivity (94% ee). The catalyst could be
used in three runs with the same 94% ee, although the cata-
lytic activity in the third run was rather low (entries 4–6).
When the catalyst was prepared in water, the transfer hydro-
The Asymmetric Transfer Hydrogenation of Acetophenone
Catalyzed by Ru-n in Water
Recently, asymmetric transfer hydrogenation in water has
been attracting increasing interest as it meets the require-
ments of green chemistry.[13] As the silica would be effective
as an organic reaction medium in water based on its ad-
Table 4. Supported ruthenium(ii) complex (Ru-5)-catalyzed asym-
metric transfer hydrogenation of ketones in water.[a]
Table 3. Supported ruthenium(ii) complex (Ru-n)-catalyzed asym-
metric transfer hydrogenation of acetophenone in water.[a]
Entry
Ligand
Temp.
[°C]
Run
Time
[h]
Conv.
[%][b]
ee
[%][c]
1[d,e]
2[d,e]
3[d,e]
4
5
6
5
5
5
5
5
5
5
5
5
6
7
7
7
7
8
8
5
5
5
5
5
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
28
28
28
20
20
1
2
3
1
2
3
1
2
3
1
1
2
3
4
1
2
1
2
3
1
2
9
Ͼ99
Ͼ99
98
Ͼ99
Ͼ99
70
Ͼ99
97
36
Ͼ99
Ͼ99
Ͼ99
99
43
Ͼ99
77
91
92
92
94
95
94
94
94
94
87
92
93
94
94
95
95
95
95
95
95
95
21
79
9
21
72
9
36
72
22
8
11
22
47
13
53
11
50
77
35
53
7[f]
8[f]
9[f]
10
11
12
13
14
15
16
17
18
19
20
21
Ͼ99
89
8
Ͼ99
55
[a] The reactions were carried out with 0.4 mmol of acetophenone
in 0.4 mL of water with 2.0 mmol of HCO2Na·2H2O, 0.016 mmol
of SDS and a Ru/ligand/acetophenone ratio of 1:1.7:100. [b] Based
on GC analysis. [c] Determined by GC with a CP-Chirasil-DEX
CB column (25 m×0.32 mm); the (R)-configuration was deter-
mined from the optical rotation value. [d] Without SDS. [e] The
catalyst was prepared by stirring ligand and [RuCl2(p-cymene)]2 in
H2O at 80 °C for 1 h. [f] The catalyst was prepared by stirring li-
gand and [RuCl2(p-cymene)]2 in H2O at 40 °C for 1 h.
[a] The reactions were carried out at 40 °C with 0.4 mmol of aceto-
phenone in 0.4 mL of water with 2.0 mmol of HCO2Na·2H2O,
0.016 mmol of SDS and a Ru/5/ketone ratio of 1:1.7:100. [b] Based
on GC analysis. [c] Determined by GC with a CP-Chirasil-DEX
CB column (25 m×0.32 mm); the configurations of all the products
were determined as R from the optical rotation values.
3224
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Asymmetric Transfer Hydrogenation Catalyzed by Recyclable Silica-Supported Ru Complexes
FULL PAPER
genation gave worse results with regards to the catalytic re- metric transfer hydrogenation in HCO2H/NEt3 or in water
cycling (entries 7–9). Similar to the reactions in HCO2H/ catalyzed by Ru-5 demonstrates high catalytic activities and
NEt3, the application of the mesoporous-material-sup- excellent enantioselectivities for most aromatic ketones and
ported Ru-6 and Ru-7 for the heterogeneous asymmetric the imine tested, which were comparable with the results
transfer hydrogenation in water with SDS gave worse obtained in homogeneous catalysis. Furthermore, this
enantioselectivities than Ru-5 (entries 10–14), although Ru- heterogeneous catalyst can be readily recovered and used in
7 supported on mesopores of SBA-15 exhibited a better re- multiple consecutive catalytic runs with the high enantio-
cycling performance (4 runs). Ru-8, whose silanol sites have selectivity maintained both in HCO2H/NEt3 and in water,
been capped, gave high enantioselectivity (95% ee) in this which is beneficial for possible industrial applications. Fur-
aqueous reaction, but with only 77% yield in the second ther exploration of this green and recyclable heterogeneous
run (entries 15 and 16). When the temperature was lowered asymmetric transfer hydrogenation in water are in progress
to 28 °C the enantioselectivity of Ru-5 increased to 95% in our laboratory.[15]
ee, but the catalytic activity and recycling use were affected
negatively (entries 17–19). A further decrease of the tem-
Experimental Section
perature to 20 °C had no further effect on the enantio-
selectivity but the catalytic activity and recovery perform-
ance of the catalyst became worse (entries 20 and 21).
General: (1R,2R)-TsDPEN (1) and HCO2H/NEt3 were prepared by
standard methods. Acetophenone was distilled from KMnO4 and
CH2Cl2 was distilled from CaH2. Triethylamine and H2O were dis-
tilled before use. [RuCl2(p-cymene)]2 and imine 13 were prepared
according to literature procedures.[16] Amorphous silica gel (60–
100 mesh, purchased from Qingdao Haiyang Chemical Co., Ltd.),
mesoporous silicas of MCM-41 and SBA-15 (provided by Prof.
D.-Y. Zhao of the Chemistry Department of Fudan University)
were heated at 150 °C for 3 h and cooled under argon before use.
Toluene was freshly distilled from a deep-blue solution of sodium-
The Asymmetric Transfer Hydrogenation of Ketones
Catalyzed by Ru-5 in Water
The ketones 9, 11 and 14–16 were applied to the organic-
solvent-free heterogeneous asymmetric transfer hydrogena-
tion in water in the presence of Ru-5 and SDS. The aqueous
reduction of 2-fluoroacetophenone (14) occurred with benzophenone under argon. Other reagents and chemicals were
used as received. 1H and 13C NMR spectra were recorded on a
Varian Mercury-plus 300 BB with TMS as internal standard.
lower enantioselectivity (83–84% ee) than the reaction in
HCO2H/NEt3,[11] but its reactivity and recycling perform-
HRMS data were measured with ESI techniques (Bruker Apex II).
ance (six runs with the same enantioselectivity) were both
Elemental analysis was performed with an elementar vario EL and
the best amongst the substrates tested in this aqueous reac-
FT-IR spectra were recorded with a Nicolet NEXUS 670. The pore
tion. In the investigation of 2-bromoacetophenone (15) we
were pleased to find that the reaction in water possesses
sizes and surface areas were determined with a Micromeritics
ASAP 2010 Accelerated Surface Area and Porosimetry System.
Optical rotation values were measured with a Perkin–Elmer 341
polarimeter.
high reactivity and excellent enantioselectivity (93% ee) that
are just as good as the results obtained in HCO2H/NEt3.[11]
With this substrate Ru-5 could be used in four runs with
Synthesis of (1R,2R)-4: (1R,2R)-DPEN (2; 0.64 g, 3.0 mmol) and
the same ee values and quantitative yields. For 3-methoxy-
acetophenone (16), 93% ee and Ͼ99% yield were obtained
after 7 h and the catalyst could be used for four runs in
water. As for the transfer hydrogenation of 4-methylace-
triethylamine (0.6 mL) were dissolved in CH2Cl2 (30 mL), and 2-
(4-chlorosulfonylphenyl)ethyltrimethoxysilane (3; 0.75 g, 2.3 mmol)
in CH2Cl2 (30 mL) was then added dropwise at 0 °C. The reaction
mixture was then allowed to warm to room temperature slowly and
tophenone (9), good enantioselectivity (92% ee) and fairly stirred for 3 h. After removal of the solvent in vacuo, the residue
was passed quickly through a short column (silica gel; eluent: Et3N/
CH3OH/CH2Cl2 = 1:10:100) and concentrated in vacuo to afford
(R,R)-4 as a beige glass. Yield: 86%. [α]2D0 = –36 (c = 0.8, CH2Cl2).
1H NMR (300 MHz, CDCl3): δ = 0.89–0.95 (m, 2 H), 1.50 (s, 2 H,
NH2), 2.65–2.70 (m, 2 H), 3.59 (s, 9 H), 4.12 (d, J = 5.4 Hz, 1 H),
4.37 (d, J = 5.4 Hz, 1 H), 6.99–7.34 (m, 14 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 11.0, 28.5, 50.6, 60.4, 63.2, 126.5, 126.9,
low reactivity (Ͼ99% yield after 20 h) were observed. For
propiophenone (11), the asymmetric reaction gave relatively
poor results (87% ee and 99% yield after 27 h), while a
much worse result (80% ee and 19% yield after 20 h) was
observed in the transfer hydrogenation in water catalyzed
by the water-soluble Ru-TsDPEN-derived catalyst.[13e]
127.3, 127.4, 127.8, 128.2, 128.3, 137.3, 139.1, 141.3, 148.8. IR: ν
˜
= 3352, 3296, 3171, 3060, 3028, 2941, 2841, 1902, 1691, 1600, 1495,
1454, 1410, 1343, 1322, 1265, 1191, 1157, 1088, 1013 cm–1. ESI-
HRMS calcd. for C25H32N2O5SSi + H+ 501.1874; found 501.1881.
Conclusions
In summary, we have synthesized easily accessible
TsDPEN-derived organic–inorganic hybrid ligands by at-
taching the homogeneous chiral ligand covalently onto sil-
ica gel and mesopores of MCM-41 and SBA-15. The ruthe-
nium complex of ligand 5 supported on cheap amorphous
silica gel has proved to be the most effective catalyst for
asymmetric transfer hydrogenation not only in traditional
HCO2H/NEt3, but also in water with HCO2Na as the hy-
General Procedure for the Synthesis of Supported Ligands 5–7: The
predried silica (3 g) and ligand (R,R)-4 (0.25 g) were added to dry
toluene (40 mL) and the mixture was refluxed for 24 h under argon.
After filtration, the white solid was washed with toluene (60 mL)
and a mixture of CH2Cl2 and CH3OH (1:1, 100 mL). The solid was
then suspended in a mixture of CH2Cl2 and CH3OH (1:1, 50 mL)
and stirred overnight. After filtration and thorough washing
(CH2Cl2/CH3OH = 1:1, 50 mL and CH3OH, 50 mL), the solid was
drogen source and SDS as PTC. The heterogeneous asym- dried at 60 °C in vacuo for 24 h to give the supported ligand.
Eur. J. Org. Chem. 2005, 3221–3227
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P.-N. Liu, P.-M. Gu, J.-G. Deng, Y.-Q. Tu, Y.-P. Ma
FULL PAPER
Ligand 5: IR: ν = 3440, 2960, 2856, 1642, 1509, 1464, 1097, 801,
˜
Acknowledgments
702, 559, 469 cm–1. Elemental analysis: C 4.71, H 0.59, N 0.41, S
0.48. Average pore size: 90.2 Å. SBET: 324 m2 g–1.
We are grateful for financial support from the Natural Science
Foundation of China (NSFC no. 30271488, 20021001, 203900501)
and the Cheung-Kong Scholars Programme. We thank Prof. Ying-
Chun Chen for valuable suggestions.
Ligand 6: IR: ν = 3442, 2962, 2860, 1627, 1461, 1081, 812, 698,
˜
455 cm–1. Elemental analysis: C 6.52, H 1.04, N 0.38, S 0.47.
Average pore size: 18.7 Å. SBET: 1150 m2 g–1.
Ligand 7: IR: ν = 3438, 2958, 2856, 1635, 1462, 1084, 811, 701,
˜
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462 cm–1. Elemental analysis: C 5.74, H 0.54, N 0.28, S 0.31.
Average pore size: 61.8 Å. SBET: 497 m2 g–1.
Synthesis of the Modified Ligand 8: Supported ligand 5 (0.3 g) was
added to hexamethyldisiloxane (HMDSO, 10 mL) and the mixture
was refluxed for 18 h under argon. After filtration and thorough
washing with CH3OH (30 mL), the solid was dried at 60 °C in
vacuo for 24 h to give ligand 8. IR: ν = 3439, 2970, 2845, 1637,
˜
1460, 1095, 965, 802, 700, 467 cm–1. Elemental analysis: C 5.55, H
1.10, N 0.33, S 0.41. Average pore size: 87.4 Å. SBET: 313 m2 g–1.
General Procedure for the Transfer Hydrogenation in HCO2H/NEt3
Using the Supported Catalysts: Supported ligand (0.0068 mmol),
[RuCl2(p-cymene)]2 (0.002 mmol) and triethylamine (0.016 mmol)
were stirred in dry CH2Cl2 (0.4 mL) for 1 h under argon at 28 °C.
Then, HCO2H/NEt3 (0.2 mL) and the ketone (or imine, 0.4 mmol)
were added and the mixture was stirred at 40 °C and monitored by
TLC. After the completion of the reaction, dry CH2Cl2 (1 mL) was
added and the mixture was stirred for 1–2 min, then the reactor
was centrifuged (3000 min–1) for 2–3 min and the solution was re-
moved with a syringe. The catalyst was then washed with CH2Cl2
(1 mL) twice and the CH2Cl2 was removed; a new reaction could
be conducted by adding HCO2H/NEt3 (0.2 mL) and ketone (or
imine, 0.4 mmol) in turn to the recovered catalyst. The solution
containing the product was passed rapidly through a short column
(silica gel, eluent: Et2O) and the conversion and the ee value were
then determined by chiral GC on a CP-Chirasil-DEX CB column
(25 m×0.32 mm) or by HPLC with a Chiralcel OD column on Ag-
ilent 1100 Series.
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General Procedure for the Transfer Hydrogenation of Ketones in
Water Using the Supported Catalysts: Supported ligand
(0.0068 mmol), [RuCl2(p-cymene)]2 (0.002 mmol) and triethylamine
(0.016 mmol) were stirred in dry CH2Cl2 (0.4 mL) for 1 h under
argon at 28 °C and then CH2Cl2 and triethylamine were evaporated
under reduced pressure. HCO2Na·2H2O (2.0 mmol), SDS
(0.016 mmol), H2O (0.4 mL) and ketone (0.4 mmol) were added
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by TLC. After completion of the reaction, H2O (1 mL) was added
and the mixture was stirred for 1 min, then the reactor was centri-
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a syringe. The catalyst was washed with CH3OH (1 mL, twice) and
H2O (1 mL, twice) and the solutions were removed with a syringe.
A new reaction could be conducted by adding HCO2Na·2H2O
(2.0 mmol), SDS (0.016 mmol), H2O (0.4 mL) and ketone
(0.4 mmol) in turn to the recovered catalyst. The solvent CH3OH
was removed in vacuo, the residue was dissolved in Et2O and the
aqueous solution was extracted with Et2O. The combined Et2O
fractions were washed twice with brine and dehydrated with
Na2SO4. After the evaporation of Et2O, the conversion and the ee
value could be determined directly by chiral GC on a CP-Chirasil-
DEX CB column (25 m×0.32 mm).
Supporting Information Available (see also footnote on the first
page of this article): [α]D, 1H and 13C NMR spectra, analytical
conditions and retention times of GC or HPLC of the obtained
chiral aromatic alcohols.
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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Asymmetric Transfer Hydrogenation Catalyzed by Recyclable Silica-Supported Ru Complexes
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Received: January 30, 2005
Eur. J. Org. Chem. 2005, 3221–3227
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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