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Polyethylene glycol-bound Ru catalyst for asymmetric transfer hydrogenation of aromatic ketones in water.

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Full Paper
Received: 11 July 2010
Revised: 16 September 2010
Accepted: 19 September 2010
Published online in Wiley Online Library: 27 January 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1747
Polyethylene glycol-bound Ru catalyst
for asymmetric transfer hydrogenation
of aromatic ketones in water
Zhongqiang Zhou∗ and Qiong Ma
A new polyethylene glycol-supported chiral monosulfonamide was synthesized from (R,R)-1,2-diaminocyclohexane and shown
to act as a ligand for ruthenium(II)-catalyzed asymmetric transfer hydrogenation of aromatic ketones in neat water using
sodium formate as the hydrogen source. Good enantioselectivities were obtained and the catalyst could be easily separated
c 2011 John Wiley & Sons, Ltd.
from the reaction mixture and reused several times. Copyright Keywords: asymmetric transfer hydrogenation; polyethylene glycol; chiral monosulfonamide; water; ketones
Introduction
Enantioselective reduction of prochiral ketones to yield optically
pure secondary alcohols is of interest because of the significance
of these alcohols as intermediates for the manufacture of
advanced materials and pharmaceuticals. Asymmetric transfer
hydrogenation (ATH) of ketones is a highly efficient and practical
method for this transformation.[1 – 4] Water has been considered
to be an ideal reaction media since it is abundant, inexpensive,
non-flammable, and environmentally friendly.[5 – 9] ATH reaction
in water is an important means of organic synthesis that
meets green chemistry conditions.[3] Recently, ligands derived
from (R,R)-1,2-diaminocyclohexane have been developed and
applied to this reaction, providing good conversion rates and
enantioselectivities with sodium formate as the hydrogen source
in aqueous solution, 2-propanol or a formic acid-triethylamine
azeotrope as hydrogen donor as well as solvents (Scheme 1).[10 – 26]
Gao retorted the preparation of water-soluble poly(acrylic acid
salt)-supported chiral ruthenium complex catalyst using (R,R)-1,
2-diaminocyclohexane as a chiral source.[27] The catalyst was used
in ATH of acetophenone in 2-propanol with good to excellent
yield and ee. The catalyst could be reused twice with some loss
of activity and enantioselectivity. It is worthwhile developing new
water-soluble catalysts that can simplify the purification step and
be reused without significant loss of activity and enantioselectivity.
Polyethylene glycol has been previously used to support ligands
for many homogeneous reactions.[28] Herein, a new polyethylene
glycol-supported chiral monosulfonamide was synthesized from
(R,R)-1,2-diaminocyclohexane and shown to act as ligand for
ruthenium(II)-catalyzed asymmetric transfer hydrogenation of
aromatic ketones in neat water using sodium formate as the
hydrogen source. Good enantioselectivities were obtained and
the catalyst could be easily separated from the reaction mixture
and reused for the following reaction.
Results and Discussion
Appl. Organometal. Chem. 2011, 25, 233–237
N-(p-toluenesulfonyl)-1,2-diaminocyclohexane] catalyst can be
performed in an open atmosphere using water as a solvent with very good results.[19] However, the Ru-TsCYDN catalyst cannot be easily separated from products due to the
catalyst being soluble in common solvents, which renders
catalyst separation by extraction impossible. To facilitate catalyst/product separation, we have successfully synthesized a
new polyethylene glycol-supported chiral monosulfonamide. The
polyethylene glycol-bound ruthenium catalyst prepared in-situ
from water-soluble polyethylene glycol-supported chiral monosulfonamide and [RuCl2 (p-cymene)]2 could be easily separated
from the reaction mixture due to its insolubility in hexane and
reused for the following reaction.
The synthetic route for polyethylene glycol-supported chiral
monosulfonamide (6) is summarized in Scheme 2. Polyethylene glycol monomethyl ether (MeOPEG) of molecular weight
1900 reacted with succinic anhydride in the presence of 4dimethylaminopyridine in methylene chloride to afford MeOPEG
monosuccinate (4). Reaction of MeOPEG monosuccinate with (R,R)N-Boc-N -(4-aminophenylsulfonyl)-1,2-diaminocyclohexane in the
presence of 4-dimethylaminopyridine and dicyclohexylcarbodiimide in methylene chloride produced polyethylene glycolsupported chiral monosulfonamide (5). Then, removal of the Boc
∗
Correspondence to: Zhongqiang Zhou, Key Laboratory of Catalysis and
Materials Science of the State Ethnic Affairs Commission and Ministry of
Education, College of Chemistry and Materials Science, South-Central University
for Nationalities, Wuhan 430074, China. E-mail: zhou-zq@hotmail.com
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs
Commission and Ministry of Education, College of Chemistry and Materials
Science, South-Central University for Nationalities, Wuhan 430074, China
c 2011 John Wiley & Sons, Ltd.
Copyright 233
It has been recently reported that the asymmetric transfer
hydrogenation of aromatic ketones using Ru-TsCYDN [(R,R)-
Scheme 1. Asymmetric transfer hydrogenation of ketones.
Z. Zhou and Q. Ma
O
a
H3N
H2N
NH3
HN
S
NO2
b
O
tartrate
1
O
BocHN
HN
S
O
c
NO2
BocHN
HN
O
2
S
NH2
O
3
O
HO
OMe
n
O
d
O
HO
Mw=1900
O
OMe
n
O
OMe
n
O
4
O
O
e
BocHN
HN
S
O
N
H
O
O
5
O
O
f
H3N
HN
S
O
O
N
H
CF3CO2
O
OMe
n
O
6
Scheme 2. Synthesis of polyethylene glycol supported chiral monosulfonamide. (a) 4-O2 N-PhSO2 Cl, triethylamine, CH2 Cl2 ; (b) (Boc)2 O, triethylamine,
CH2 Cl2 , r.t.; (c) Pd/C, HCOONH4 , MeOH, r.t.; (d) succinic anhydride, 4-dimethylaminopyridine, CH2 Cl2 ; (e) (R,R)-N-Boc-N -(4-aminophenylsulfonyl)-1,2diaminocyclohexane (3), dicyclohexylcarbodiimide, 4-dimethylaminopyridine, CH2 Cl2 ; (f) CF3 COOH, CH2 Cl2 .
234
group of 5 by treatment with trifluoroacetic acid provided a
new polyethylene glycol-supported chiral monosulfonamide (6)
in good yield. The loading of polyethylene glycol-supported chiral
monosulfonamide was established by elemental analysis.
According to Xiao’s procedure,[19] polyethylene glycol-bound
water-soluble ruthenium catalyst was prepared in-situ by reacting
[RuCl2 (p-cymene)]2 with polyethylene glycol-supported chiral
monosulfonamide in neat water at 40 ◦ C for 1 h under argon
atmosphere. The ATH was initiated by introducing 5 mmol of
sodium formate and 1 mmol of acetophenone. After completion
of the reaction, the mixture was cooled to room temperature.
The organic compounds were extracted with hexane using a
syringe. The conversions were determined by GC-MS analysis.
Enantiomeric excesses were determined by GC analysis with a
chiral Chirasil-Dex CP 7502 column. We have also examined
the asymmetric transfer hydrogenation reaction of aromatic
ketones. As shown in Table 1, for various ketones, including
2-substituted, electron-rich and electron-deficient variants, the
catalyst afforded quantitative conversion in 6 h reaction time. For
the asymmetric transfer hydrogenation reaction of acetophenone
(entry 1), the conversion (100%) was excellent and the ee
(86.5%) was good. For the asymmetric transfer hydrogenation
wileyonlinelibrary.com/journal/aoc
reaction of propiophenone (entry 2), the conversion (100%)
was excellent but the ee (72.1%) was not comparable to
that of acetophenone. Thus, as the steric bulkiness increased,
the enantioselectivities decreased. For the asymmetric transfer
hydrogenation reaction of meta-methoxyacetophenone and paramethoxyacetophenone (entries 10 and 11), the ee values (84.5
and 78.5%, respectively) were different from each other but
the conversions were the same (100%). The halo group on
the 2-position slightly improved the enantioselectvity. For the
asymmetric transfer hydrogenation reaction of ortho-substituted
acetophenones (entry 8), the conversion was excellent but the ee
values were poor. Thus, the ortho-substitution by Me decreased
the enantioselectivity. This observation is quite similar to ATH
of ketones with Rh-TsCYDN by HCOONa in water.[19] When RhTsCYDN was used as a catalyst, a low enantioselectivity was
obtained for ortho-substituted aromatic ketones compared with
meta- and para-substituted aromatic ketones.[19] Although the
reactions took longer to complete than Ru-TsCYDN-catalyzed
asymmetric transfer hydrogenation, the results found in this study
are comparable with those obtained using Ru-TsCYDN catalyst.[19]
From the viewpoint of green chemistry it is highly desirable that
the catalyst can be recovered and reused. The recycling ability of
c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 233–237
Polyethylene glycol-bound Ru catalyst
Table 1. Asymmetric transfer hydrogenation of aromatic ketones in watera
OH
O
R2
[RuCl2(p-cymene)]2 / 6
R1
Entry
1
2
3
4
5
6
7
8
9
10
11
12
13
R2
R1
HCOONa, H2O
R1
R2
Time (h)
Conversion (%)b
ee (%)c
Configurationd
H
H
H
para-CH3
para-Cl
meta-Br
para-Br
ortho-CH3
para-CH3
meta-CH3 O
para-CH3 O
para-Cl
H
H
CH3
Cl
Cl
H
H
H
H
H
H
H
Cl
Br
6
6
6
6
6
6
6
6
6
6
6
6
6
100
100
100
100
100
100
100
100
100
100
100
100
100
86.5
72.1
88.6
90.2
86.4
86.4
87.1
53.5
77.9
84.5
78.5
87.4
91.7
R
R
S
S
R
R
R
R
R
R
R
S
S
Reactions were carried out using 1 mmol of ketone, 5 equiv. of HCOONa, and an S/C ratio of 100 in 2 ml of water under argon atmosphere at 40 ◦ C.
Determined by GC-MS analysis on a HP-5 ms column.
c Determined by GC equipped with a chiral column (CP-Chirasil-Dex CB CP7502).
d Absolute configuration was assigned by comparison of the sign of the specific rotation with that reported.[29 – 31]
a
b
Table 2. Catalyst recycle in asymmetric transfer hydrogenation of
acetophenone in watera
Run
1
2
3
4
Time (h)
Conversion (%)b
ee (%)c
Configurationd
6
6
6
24
100
100
100
73.8
86.5
86.8
86.5
86.8
R
R
R
R
a
Reactions were carried out using 1 mmol of acetophenone, 5 equiv of
HCOONa, and a S/C ratio of 100 in 2 ml of water under argon atmosphere
at 40 ◦ C in the first run. Since the second run, 1.1 mmol HCOOH was
added to regenerate sodium formate in every recycling run.
b Determined by GC-MS analysis on an HP-5 ms column.
c Determined by GC equipped with a chiral column (CP-Chirasil-Dex CB
CP7502).
d
Absolute configuration was assigned by comparison of the sign of
the specific rotation with that reported.[29 – 31]
the catalyst has also been evaluated. Polyethylene glycol-bound
ruthenium catalyst could be easily recovered by extraction with
hexane and recycled through four cycles with acetophenone as a
substrate; the results are shown in Table 2. There was no decrease
in the reaction rates in the first three runs; however, the fourth
run gave a 73.8% conversion in 24 h. Catalyst could be used for
at least four runs with completely maintained enantioselectivities.
Catalyst recycle led to loss of catalytic activity, presumably due to
the decomposition of active Ru–6 complex.
catalyzed asymmetric transfer hydrogenation of aromatic ketones
in neat water using sodium formate as the hydrogen source. Good
enantioselectivities have been obtained and the catalyst could
be easily separated from the reaction mixture and reused several
times.
Experimental
General Methods
Melting points were determined on a melting point apparatus
and were uncorrected. Reactions were monitored by thin-layer
chromatography using precoated silica plates (Silica gel for thinlayer chromatography, GF254 , Qingdao Haiyang Chemical Co. Ltd).
Specific rotations were measured on a WZZ-3 digital polarimeter.
IR spectra were recorded on a Nicolet Nexus 470 FTIR spectrometer.
NMR spectra were recorded on a Bruker Avance III 400 with TMS
as internal standard. Elemental analysis was performed using
Vario EL Series III. The conversions were measured by GC-MS on
an Agilent 5973N (HP-5 ms capillary column). The enantiomeric
excesses were determined by GC (Agilent 6890N) equipped with
a chiral column (CP-Chirasil-Dex CB CP7502). The chemicals used
in this work were purchased from the Alfa Aesar Chemical and
Sinopharm Chemcial Reagent Co. Ltd.
(R,R)-N-(4-Nitrophenylsulfonyl)-1,2-Diaminocyclohexane (1)
Dichloromethane (10 ml) was added to a stirred solution of the
salt of (R,R)-1,2-diaminocyclohexane (1.32 g, 5 mmol) in
5.3 ml of 2 M NaOH solution. The mixture was cooled to 0 ◦ C and
a solution of 4-nitrophenylsulfonyl chloride (554 mg, 2.5 mmol) in
10 ml of dichloromethane was added dropwise over 20 min. After
the addition was completed, the mixture was allowed to warm
to room temperature and stirred overnight. The resulting solution
was washed with water (3 × 50 ml) and dried over anhydrous
L-tartrate
Conclusions
Appl. Organometal. Chem. 2011, 25, 233–237
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
235
In summary, a new polyethylene glycol-supported watersoluble chiral monosulfonamide is synthesized from (R,R)-1,2diaminocyclohexane and shown to act as ligand for ruthenium(II)-
Z. Zhou and Q. Ma
MgSO4 . After concentration, the residue was dried to afford 1 as
a pale yellow powder (632 mg, 84.5%). M.p. 180 ◦ C (lit.[18] , m.p.
177.5–178 ◦ C), [α]D = +26.7 (c 0.49, C2 H5 OH). IR (KBr); ν: 3429,
3359, 3297, 1528, 1349, 1162, 1092 cm−1 .
(R,R)-N-Boc-N -(4-nitrophenylsulfonyl)-1,2diaminocyclohexane (2)
(R,R)-N-Boc-N -(4-nitrophenylsulfonyl)-1,2-diaminocyclohexane
(2) was prepared from compound 1 according to the literature procedure.[18] Yield 75.5%, m.p. 159–162 ◦ C (lit.[18] , m.p.
159–162 ◦ C), [α]D = +42.8 (c 0.42, C2 H5 OH).
(R,R)-N-Boc-N -(4-aminophenylsulfonyl)-1,2diaminocyclohexane (3)
(R,R)-N-Boc-N -(4-aminophenylsulfonyl)-1,2-diaminocyclohexane
(3) was prepared from compound 2 according to the literature
procedure.[18] Yield 95.7%, m.p. 187 ◦ C (lit.[18] , m.p. 176–177 ◦ C),
[α]D = +69.6 (c 0.39, C2 H5 OH).
MeOPEG Monosuccinate (4)
Succinic anhydride (0.4 g, 4 mmol) and 4-dimethylaminopyridine
(48.4 mg, 0.4 mmol) were added to a stirred solution of polyethylene glycol monomethyl ether (Mw = 1900; 7.6 g, 4 mmol) in
methylene chloride (40 ml). The reaction mixture was stirred at
reflux for 48 h and allowed to cool to room temperature. The
solution was added to diethyl ether (100 ml) drop by drop. The
solid was filtered and dried in vacuo to afford the white solid
4 (7.7 g, 96%); IR(KBr), ν(cm−1 ): 3427, 2886, 1735, 1467, 1348,
1281, 1240, 1113, 1064, 958. 1 H NMR (CDCl3 , 400 MHz), δ: 2.62 (s,
4H, COCH2 CH2 CO), 3.36 (s, 3H, CH3 O), 3.45 (s, 2H, COOCH2 CH2 ),
3.63 (s, 166H, OCH2 CH2 O), 4.24 (s, 2H, COOCH2 ). 13 C NMR (CDCl3 ,
100 MHz), δ: 59.0 (CH3 ), 63.4 (COOCH2 ), 69.1 (CH2 ), 71.9 (CH2 ), 72.4
(CH2 ), 73.1 (CH2 OCH3 ), 173.2 (C O). Anal. calcd for 4: C, 53.94; H,
8.89. Found: C, 54.47; H, 9.05.
Polyethylene Glycol-supported Chiral Monosulfonamide (6)
236
The solution of (R,R)-N-Boc-N -(4-aminophenylsulfonyl)-1,2-diaminocyclohexane (590 mg, 1.6 mmol) in methylene chloride
(10 ml) was added to a stirred solution of 4 (1.6 g, 0.8 mmol)
in methylene chloride (30 ml) at room temperature. Then, 4dimethylaminopyridine (261 mg, 2.14 mmol) and dicyclohexylcarbodiimide (337 mg, 1.6 mmol) were added. The solid was filtered
off after the mixture stirred at room temperature for 48 h. The
filtrate was added to diethyl ether (100 ml) drop by drop. The
precipitate was filtered off and washed with diethyl ether several
times, and then dried in vacuo to afford the pale yellow solid 5
(1.7 g, 90.4%). IR (KBr), ν (cm−1 ): 3368, 2886, 1688, 1639, 1597, 1465,
1346, 1112, 957 and 843. Trifluoroacetic acid (8 ml) was added to a
stirred solution of compound 5 (1.69 g, 0.72 mmol) in methylene
chloride (8 ml). The mixture was stirred at room temperature for
6.5 h. After trifluoroacetic acid was distilled out, toluene (10 ml)
was added to the mixture and the volatile was removed under reduced pressure to give 6 as pale yellow waxy solid (1.70 g, 99.8%).
IR(KBr), ν(cm−1 ): 3440, 3098, 2916, 1736, 1692, 1647, 1535, 1459,
1352, 1120, 951, 842, 804. 1 H NMR (CDCl3 , 400 MHz), δ: 1.26–2.15
[m, 8H, -CH(CH2 )4 CH-], 2.66–2.76 (m, 4H, COCH2 CH2 CO), 3.12 (s,
1H, NCH), 3.29 (s, 3H, CH3 O), 3.57 (s, 2H, COOCH2 CH2 ), 3.66 (s, 166H,
OCH2 CH2 O), 3.83 (s, 1H, NCH), 4.25 (s, 2H, COOCH2 ), 7.73–7.79 (m,
2H, ArH), 7.81–7.94 (m, 2H, ArH). 13 C NMR (CDCl3 , 100 MHz), δ: 24.8
wileyonlinelibrary.com/journal/aoc
(CH2 CH2 CHN), 25.6 (CH2 CH2 CHN), 29.7 (CH2 CHN), 31.3 (CH2 CHN),
33.1 (COCH2 CH2 COO), 33.9 (COCH2 CH2 COO), 49.1 (CHN), 51.8
(CHN), 59.0 (CH3 ), 63.7 (COOCH2 ), 69.7 (CH2 ), 70.4 (CH2 ), 72.0
(CH2 OCH3 ), 121.4 (Ar–C), 128.7 (Ar–C), 133.9 (Ar–C), 140.9 (Ar–C),
173.0 (C O). Anal. calcd for 6: C, 52.72; H, 8.28; N, 1.77. Found: C,
45.6; H, 9.25; N, 1.67. Nitrogen loading: 0.399 mmol g−1 .
General Procedure for Asymmetric Transfer Hydrogenation
Under an argon atmosphere, [RuCl2 (p-cymene)]2 (3.1 mg,
0.005 mmol) and polyethylene glycol-supported chiral monosulfonamide 6 (36 mg, 0.015 mmol) were dissolved in 2 ml of
degassed water. After the solution was stirred at 40 ◦ C for 1 h,
HCO2 Na·2H2 O (520 mg, 5 mmol) and ketone (1.0 mmol) were
added to the solution. The solution was allowed to react at 40 ◦ C
for a certain period of time. After the solution was cooled to room
temperature, the organic compounds were extracted with hexane
(3 × 5 ml). The conversion was determined by GC-MS analysis (HP5 ms capillary column). The enantioselectivity was determined by
GC analysis (CP-Chirasil-Dex CB CP7502 column). In the case of recycle, following each reduction the aqueous phase was extracted
with hexane (3 × 5 ml) using a syringe, and the new reduction
was started by introducing another portion of acetophenone
(1.0 mmol) along with 1.1 equiv of HCOOH.
Chiral GC Analyses of Chiral Aromatic Alcohols
(R)-1-phenylethanol
The GC conditions were: inlet pressure, 87 kPa; flow rate,
1 ml min−1 ; injector temperature, 250 ◦ C; detector temperature,
250 ◦ C; column temperature, 120 ◦ C; tR , 8.67 min for (R), 9.73 min
for (S).
(R)-1-phenylpropanol
The GC conditions were: inlet pressure, 58 kPa; flow rate,
0.6 ml min−1 ; injector temperature, 250 ◦ C; detector temperature,
250 ◦ C; column temperature, 130 ◦ C; tR 13.08 min for (R), 13.79 min
for (S).
(S)-2-chloro-1-phenylethanol
The GC conditions were: inlet pressure, 96 kPa; flow rate,
1 ml min−1 ; injector temperature, 250 ◦ C; detector temperature,
250 ◦ C; column temperature, 150 ◦ C; tR 7.45 min for (S), 8.08 min
for (R).
(S)-2-Chloro-1-(p-methylphenyl)ethanol
The GC conditions were: inlet pressure, 96 kPa; flow rate,
1 ml min−1 ; injector temperature, 250 ◦ C; detector temperature,
250 ◦ C; column temperature, 150 ◦ C; tR , 9.78 min for (S), 10.83 min
for (R).
(R)-1-(p-chlorophenyl)ethanol
The GC conditions were: inlet pressure, 62 kPa; flow rate,
0.6 ml min−1 ; injector temperature, 250 ◦ C; detector temperature,
250 ◦ C; column temperature, 140 ◦ C; tR 15.30 min for (R), 17.34 min
for (S).
c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 233–237
Polyethylene glycol-bound Ru catalyst
(R)-1-(m-bromophenyl)ethanol
The GC conditions were: inlet pressure, 96 kPa; flow rate,
1 ml min−1 ; injector temperature, 250 ◦ C; detector temperature,
250 ◦ C; column temperature, 150 ◦ C; tR 9.67 min for (R), 10.45 min
for (S).
250 ◦ C; column temperature, 150 ◦ C; tR 11.34 min for (S), 12.02 min
for (R).
Acknowledgments
Financial support of this work by the Natural Science Foundation
of Hubei Province (2007ABA291) is gratefully acknowledged.
(R)-1-(p-bromophenyl)ethanol
The GC conditions were: inlet pressure, 93 kPa; flow rate,
1 ml min−1 ; injector temperature, 250 ◦ C; detector temperature,
250 ◦ C; column temperature, 140 ◦ C; tR 16.41 min for (R), 18.86 min
for (S).
(R)-1-(o-methylphenyl)ethanol
The GC conditions were: inlet pressure, 60 kPa; flow rate,
0.6 ml min−1 ; injector temperature, 250 ◦ C; detector temperature,
250 ◦ C; column temperature, 130 ◦ C; tR 14.79 min for (R), 17.87 min
for (S).
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water, asymmetric, bound, transfer, ketone, polyethylene, glycol, hydrogenation, aromatic, catalyst
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