Surfactant-accelerated asymmetric transfer hydrogenation with recyclable water-soluble catalyst in aqueous media

Article abstract of DOI:10.1039/c2ra22432a

Water-soluble ligands (R,R)-2 were successfully prepared, in which the bis-meta-sulphonated ligand was definitely detected as the major product. The corresponding transition-metal complexes containing the ligands displayed excellent catalytic performance in asymmetric transfer hydrogenation (ATH) of aromatic ketones. Especially, the aromatic ketones with a bromine group in the α position could be smoothly reduced to the expected alcohol, keeping the bromine group intact with excellent enantioselectivities (up to 96% ee). The catalyst could be reused at least 21 times without erosion of the enantioselectivity in high conversion. Moreover, it was found that cationic surfactant and proper pH values were necessary for the maintenance of high reactivity.

Full text of DOI:10.1039/c2ra22432a

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Surfactant-accelerated asymmetric transfer
hydrogenation with recyclable water-soluble catalyst in
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1
825
aqueous media3
ad
c
a
a
a
ad
a
Jiahong Li, Xuefeng Li, Yaping Ma, Jiashou Wu, Fei Wang, Jing Xiang, Jin Zhu,
a
b
Qiwei Wang* and Jingen Deng*
Water-soluble ligands (R,R)-2 were successfully prepared, in which the bis-meta-sulphonated ligand was
definitely detected as the major product. The corresponding transition-metal complexes containing the
ligands displayed excellent catalytic performance in asymmetric transfer hydrogenation (ATH) of aromatic
ketones. Especially, the aromatic ketones with a bromine group in the a position could be smoothly
reduced to the expected alcohol, keeping the bromine group intact with excellent enantioselectivities (up
to 96% ee). The catalyst could be reused at least 21 times without erosion of the enantioselectivity in high
conversion. Moreover, it was found that cationic surfactant and proper pH values were necessary for the
maintenance of high reactivity.
Received 8th October 2012,
Accepted 23rd November 2012
DOI: 10.1039/c2ra22432a
www.rsc.org/advances
4
d
one (Fig. 1, 1b) is cationic bearing two positive charges.
Although they exhibit excellent catalytic performance in case
of enantioselective reduction of ketones and imines, both of
these two ligands fail to be effectively reused, despite
recyclability being a critical criteria for an efficient aqueous
catalyst. With this issue in mind, we envisioned that the
introduction of an anionic group to the ligand should be an
abstractive strategy if the anionic ligand could retain water-
solubility and improve the recyclability at the same time.
Bujoli and co-workers developed a water-soluble version of
N,N-dimethyl-1,2-diphenylethane-1,2-diamine by introducing
phosphonic acid moieties on the para-position of the aromatic
Introduction
The interest of aqueous phase asymmetric catalysis becomes
increasingly significant because of many potential advantages.
Water is safe, benign, non-toxic, cheap, environmentally
friendly and easily separated, and is thus an ideal choice
instead of organic solvents. As a result, great effort has been
focused on this field and a great number of elegant and
effective aqueous phase catalytic reactions have been docu-
1
mented, especially for the asymmetric transfer hydrogenation
2
–4
(ATH) of ketones and imines. Although a disadvantage often
associated with catalysis in water is the decrease in catalytic
activity and/or stereoselectivity on going from organic solvents
to water, the use of water soluble ligands/catalysts can
overcome the drawbacks, leading to high reactivity and even
better selectivity superior to those in organic media in some
5
rings. Moreover, the Williams group also reported a water-
soluble ligand containing an additional sulfonic acid group
located on the benzene ring of a tosylated group.
these ligands have to be prepared via multistep processes and
further evolution is demanded for satisfactory enantioselec-
tivity and reusability.
3a,b
All of
1
c,2e–g,3,4
cases.
In our continuing effort on aqueous-phase
ATH, two well-defined ligands have been explored succes-
4
4
c
sively, which one of them (Fig. 1, 1a) is neutral and the other
6
Sulfonation is a classic reaction for aromatic compounds
a
and thus two sulphate groups could be conveniently
embedded into the benzene rings of DPEN (DPEN = 1,2-
diphenylethylenediamine) after direct bissulfonation. Herein,
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu,
6
10041, P. R. China. E-mail: wqw@cioc.ac.cn
b
Key Laboratory of Drug-Targeting of Education Ministry, West China School of
Pharmacy, Sichuan University, Chengdu, 610041, P. R. China.
E-mail: jgdeng@cioc.ac.cn
c
College of Chemistry and Environment Protection Engineering, Southwest University
for Nationalities, Chengdu, 610041, P. R. China
d
Graduate University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
3
Electronic Supplementary Information (ESI) available: Sulfonation of DPEN
with SO oleum, influence of pH values on ATH of acetophenone, recycling
results of ATH applying surfactant, influence of metal precursor for ATH of
3
1
13
a-bromoacetophenones and H NMR, C NMR, H–H COSY, C–H COSY, HRMS
and IR spectra of reported ligands (2, 2a, 3, 3a-c). See DOI: 10.1039/c2ra22432a
Fig. 1 Structures of the water-soluble ligands 1a and 1b.
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we would like to report the anionic ligand 2 (Fig. 2) derived
from DPEN skeleton bearing two hydrophilic anionic sulphate
groups, which displayed excellent catalytic performance in
terms of reactivity and enantioselectivity in the presence of
cationic surfactant CTAB (CTAB = cetyltrimethylammonium
bromide). In particular, the catalyst could be reused 21 times
without obvious erosion of the enantioselectivity in high
conversion. Especially, the aromatic ketones with a bromine
group in the a position could be smoothly reduced to the
expected alcohol, keeping the bromide group intact with
excellent enantioselectivities (up to 96% ee).
with
[RhCl
different
metal precursors
, and [IrCl Cp*] (Cp* = 1,2,3,4,5-pentamethylcy-
2
2 2
[RuCl (p-cymene)] ,
Cp*]
2
2
2
clopentadiene) in degassed water at 40 uC for 1 h. To our
delight, (R,R)-2 displayed good solubility in water and the
reaction occurred without any surfactant using HCO Na as the
2
hydrogen source (entries 1–3). Almost identical catalytic
reactivity was observed for three different metal precursor
regardless of slightly better enantioselectivity obtained in the
presence of [RuCl (p-cymene)] . In order to enhance the
2 2
reactivity of the catalytic system, the following research
involved a surfactant as the additive which can form micelles
4,8
to increase the solubility of nonpolar substrates in water.
Again three different metal precursors were investigated for
comparison purposes. [RuCl (p-cymene)] exhibited better
2
2
Results and discussion
performance, leading to (R)-1-phenylethanol (5a) with 96%
enantiomeric excess (ee) and 93% conversion after 1 h when
Synthesis and purification of water-soluble ligands 2
4b
CTAB was added (entries 4-6). Inspired by this improvement,
The chiral water-soluble ligands (R,R)-2 were easily obtained
different surfactants were investigated (entries 7–14). As
expected, the reactivity depended closely on the property of
the surfactants, which decrease in the order cationic surfac-
tants (CTAB, CPB) . zwitterionic surfactant (DDAPS) .
anionic surfactants (SDBS, SDS). Among them, the cationic
surfactants exerted significant positive effect on the conver-
sion and enantioselectivity compared with the control experi-
ment (entries 6 and 7 vs. entry 1). It should be caused by
elevated concentration of hydrogen source around the micelles
due to the attraction between formate ions and positive
charges around the micellar surface. In contrast, reactions
involving neutral and anionic surfactants gave results inferior
to the reaction conducted without surfactants (entries 8–10).
Especially for SDBS, an almost sluggish reaction was observed
probably because of the repulsion between the negatively
charged formates and the anionic micelles formed from the
anionic surfactants. Interestingly, zwitterionic surfactant
DDAPS and phase-transfer catalysts TBAB, BBC led to
acceptable reactivity (entries 11–13). Because vesicles could
be formed by mixing simple single-tailed cationic and anionic
4a,7
via a two-step procedure shown in Scheme 1.
The
sulfonated product (R,R)-3 was provided as a mixture of
different substituents on the aromatic rings in 87–90% yield
after treatment of commercially available (R,R)-DPEN with
oleum. Fortunately, we succeeded in separating the sulfonated
mixture, which contained three sulfonated products, after
careful purification by preparative HPLC (High Performance
Liquid Chromatography). Similar with our reported nitration
4
d
of DPEN, the bis-meta-sulfonated product (R,R)-3a, rather
7
than the bis-ortho-substituted product was surprisingly
identified as the major product in a ratio of 75% (Fig. 3). At
the same time, (R,R)-3b possessing a meta-sulfonated benzene
ring and a para-sulfonated benzene ring was furnished (23%)
along with a small amount of bis-para-sulfonated (R,R)-3c
1
13
(
2%). All of them had been confirmed by H NMR, C NMR,
H–H COSY, and C–H COSY spectra. The successive tosylation
proceeded smoothly and the pure mono-N-tosylated derivative
(R,R)-2a was obtained in 75% yield after purification by silica
gel column chromatography. Meanwhile, we also subjected the
crude sulfonated product (R,R)-2 to p-toluenesulfonyl chloride
and obtained a mixture of mono-N-tosylated derivatives (R,R)-2
after simple purification by silica gel column chromatography
9
surfactants, the mixture of SDS and CTAB was therefore
4
e
tested in the transfer hydrogenation of 4a. However, even the
optimal combination of the mixture previously reported by our
group displayed an unsatisfactory reactivity (entry 14,
(for more details about water-soluble ligands (R,R)-3a-c and
4
e,9
mono-N-tosylated derivatives, please see the ESI ).
3
SDS : CTAB = 2 : 1).
CTAB took a leading place among all
the surfactants investigated concerning reactivity and enan-
tioselectivity, as a result, it was employed in the further
optimization. An issue always associated with water-soluble
ligand was that it was difficult to purify, which had hampered
its widespread application. It was indeed the case for our
catalytic system, thus we specifically compared the catalytic
effect of pure ligand (R,R)-2a with the impure ligand (R,R)-2
under the same conditions. In particular, an almost identical
result was represented for conversion and enantioselectivity
Asymmetric aqueous phase transfer hydrogenation
With the pure ligand and the mixture of ligands in hand, we
sequentially examined their catalytic performance in the case
of aqueous-phase ATH of acetophenone (4a) (Table 1). The
initial transformation was performed with (R,R)-2 as the ligand
and the precatalyst was prepared in situ by treatment of (R,R)-2
(entry 15 vs. 6). Therefore, time-consuming extra purification
was avoided and it provided a facile access to this water-
soluble ligand. The influence of the concentration of CTAB
was further studied. Initially, reactivity increased significantly
as the concentrations obtained using 0.02 M of CTAB (Fig. 4).
The conversions declined as the CTAB concentration further
increased. Interestingly, the enantioselectivity increased from
90% ee to 96% ee as the concentrations of the CTAB increased
Fig. 2 Structure of the water-soluble ligand (R,R)-2.
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Scheme 1 Synthesis of anionic ligands (R,R)-2a and (R,R)-2.
from 0 to 0.005 M, and there was no decrease of enantios-
electivity when CTAB was further increased to 0.2 M. The
declined reactivity was probably due to a dilution effect of
substrate in the micelle and increase in the viscosity of the
solution, which may interfere with the impingement of
different ratios keeping the total concentration of formate
constant. As shown in Fig. 5, the reduction of 4a with Ru-(R,R)-
1
0b
2 complex was also pH dependent.
The pH value of the
solution was found to be slightly increased when the reaction
was complete, it may be due to the decomposition of formate
8
j,8l
10a
reactant molecules.
2
by Ru-(R,R)-2 with proton abstract from H O. When the pH
It was reported that the reactivity and enantioselectivity of
aqueous-phase ATH of aromatic ketones was dependent on the
value was under 4, the reduction proceeded very slowly. The
conversion and enantioselectivities were increased when pH
1
0
pH value of the aqueous phase. The pH dependence of
reactivity and enantioselectivity has also been found in
value increase from 3.9 to 8.1 (see, Table 2S, ESI,
3
entries 3–7).
When the starting pH was 8.1 (Table 2S, ESI, entry 7), optimal
3
8
g
aqueous micellar media. This prompted us to investigate
the correlation of pH value with the reactivity and enantios-
electivity in the presence of surfactant. The pH value of the
solution was controlled by adjusting HCOONa and HCOOH at
conversion and enantioselectivity were obtained. When pH
value further increased to 9.8 by adding sodium carbonate
(Table 2S, ESI,
3
entry 8), a decrease in conversion was
observed. Under acidic conditions, a protonation of the amide
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Fig. 4 Influence of concentration of CTAB on the conversion and enantioselec-
Fig. 3 HPLC spectrum of the crude sulfonated products 3.
tivity.
nitrogen of the catalyst resulted in a ring with formate
The water-soluble catalyst (R,R)-2 was effectivly recycled
when employing cationic surfactants CTAB and CPB (Fig. 7).
After the reaction was complete, 1.1 equivalents of formic acid
was added to the reaction system to regenerate sodium
formate, 92% conversion and 96% ee were obtained on the
tenth run when CTAB was used. To our surprise, high
conversion (89%) and enantioselectivity (94% ee) remained
even on the 21st run when CPB was applied. These results
show that there may be a strong interaction between the
anionic catalyst 2 and the cationic micelles formed from CTAB
and CPB respectively to enhance the stability and solubility of
1
0c
coordination to Ru(II) under more basic conditions.
All
these lead to decrease the reactivity. The pH dependence is
similar to that of the ATH catalyzed by Ru–TsDPEN complex in
1
0
neat water and the reaction is obviously influenced by the pH
value of the surrounding water phase. Moreover, the anionic
catalyst 2, probably because of charge pairing, interacts with
the surface of the cationic micelles without being buried in the
hydrophobic core, acting as counter anion for the ammonium
cations of the CTAB. This implied that the catalytic reduction
most likely proceeds in the phase border layer between the
surface of the micelles and the water phase (stern layer) via a
8j
2 in water. To the best of our knowledge, it has been seldom
1
1
10c
concerted or step-wise mechanism (Fig. 6).
reported that a water-soluble catalyst was reused so many
1
2
times in aqueous media.
Under the optimal conditions, we subsequently examined
the substrate scope and a wide range of ketones 4b–m,
especially a-bromoacetophenone 4h–m were reduced
smoothly to the desired products 5b–m with good to excellent
enantioselectivities (Fig. 8). A variety of aromatic ketones (4b–
g), which failed to achieve good reactivities employing SDS as
surfactant in our previous report, were smoothly reduced
with high reactivities and enantioselectivities using water-
soluble catalyst (R,R)-2 with CTAB as surfactant. Optically pure
a
Table 1 Asymmetric transfer hydrogenation of acetophenone (4a)
b
c
c
Entry
Metal
Surfactant
Conv.%
Ee%
4
a
1
2
3
4
5
6
7
8
9
Ru
Rh
Ir
Rh
Ir
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
—
—
—
24
26
25
63
43
93
78
15
20
10
74
84
82
29
95
90
88
88
95
92
96
96
92
96
94
96
95
95
95
96
CTAB
CTAB
CTAB
CPB
Triton X-100
SDS
SDBS
DDAPS
BBC
TBAB
2-bromo-1-arylethanols are the key intermediates of b-adre-
1
1
1
1
1
0
1
2
3
4
SDS:CTAB = (2 : 1)
CTAB
d
1
5
a
Unless otherwise noted, all the reactions were conducted in 1 mL
of degassed water under argon atmosphere at 40 uC for 1h. [(R,R)-
2
] : [surfactant] : [4a] : [HCOONa?2H
2
O] = 1 : 4 : 100 : 500; [(R,R)-2] =
b
0
.005 M. CTAB = cetyltrimethylammonium bromide; CPB =
cetylpyridine bromide; Triton X-100 = poly(ethylene glycol)mono[4-
1,1,3,3-tetramethylbutyl)phenyl]ether; SDS = sodium dodecyl sulfate;
(
SDBS = sodium dodecylbenzene sulfonate; DDAPS = dodecyldimethyl
ammonio propanesulfonate; BBC = benzyltributylammonium
chloride; TBAB = tetrabutylammonium bromide. The conversion
c
and ee values were determined by GC analysis (CP-cyclodex B-236
d
M). Pure (R,R)-2a was employed as ligand.
Fig. 5 Influence of solution pH on conversion and enantioselectivity of ATH.
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4
a,e
tion approach for a-bromoacetophenones was established.
The Rh-complex 6b was found to be the best choice in the
presence of CTAB concerning the chemoselectivity, conversion
and enantioselectivity (Table 5S, ESI
3
). a-Bromoacetophenone
4h was hydrogenated to the corresponding alcohol within 2 h
in 95% isolated yield and 95% ee and no formate displace-
ment nor debromination occurred, while the formate product
was obtained by using Noyori–Ikariya’s catalyst in 18% yield
4
e
only with CTAB as the surfactant. However, the electronic
property exerted influence on the chemoselectivity to certain
extent. For the substrates with electron-donating groups on
the para-site of the benzene ring, 4i and 4j, good yields (81%
for 4i and 85% for 4j) and excellent enantioselectivities (94%
ee for 4i and 96% ee for 4j) were obtained within 1 h.
Dehalogenated products (9% for 4i and 8% for 4j) were
Fig. 6 Proposed mechanism for reduction of 4a in CTAB formed micelle.
1
detected via the H NMR spectra analysis, but the amounts are
nergic receptor agonists, which are usually obtained by
lower than those observed in the reduction with Rh–TsDPEN
reduction of the corresponding a-bromomethylaromatic
4
e
complex as catalyst. On the contrary, 4k containing an
electron-withdrawing nitro group gave a clean transformation
in excellent enantioselectivity (95% ee). The reduction pro-
ducts of 4l and 4m were the key medicinal intermediates of
1
3
ketones.
However, as reported previously, the ATH of
a-bromoacetophenone could not be performed in the azeo-
1
4,15
trope system due to the formate displacement reaction.
It
was well-known that surfactants could construct micelles or
vesicles and embed ketones substrates in these hydrophobic
1
6
17
anti-asthma drugs, terbutaline and salbutamol . Although
prolonged time was required for full conversion of 4l and 4m,
the corresponding reduction products were generated both
with 93% ee in satisfactory yields.
4
,8,9
microreactors.
As a sequence, the microreactors should
separate the hydrophobic a-bromoacetophenone from hydro-
philic hydrogen source, thus the undesired formate displace-
ment should be efficiently suppressed. Based on this well-
defined protocol, an exceptionally efficient asymmetric reduc-
Conclusion
In summary, the highly efficient water-soluble ligands (R,R)-2
were easily accessed via classical sulfonation and successive
tosylation. It was interesting that bis-meta-substituted com-
pound (R,R)-2a, rather than bis-ortho-substituted product was
confirmed as the main product. The mixture of ligands (R,R)-2
almost displayed equal catalytic performance compared with
pure one, therefore the time-consuming extra purification was
avoided during the course of preparing ligands (R,R)-2. A range
of aromatic ketones, especially a-bromomethylaromatic
ketones, were successfully converted to the corresponding
alcohols employing this ligand with the cationic surfactant
CTAB. Considerable enhancement of activity was observed in
the presence of the cationic surfactant (CTAB), it may caused
by accelerated transformation of formate ions from aqueous
solution to micellar phases. It was notable that the catalytic
system derived from the water-soluble ligands (R,R)-2 could be
reused with a high conversion and enantioselectivity remain-
ing even on the 21st run in the ATH of acetophenone with CPB
as the surfactant. To the best of our knowledge, it has been
seldom reported that a water-soluble catalyst could be reused
so many times in aqueous media.
Experimental Section
General procedure for synthesis of water-soluble ligands
Fig. 7 Reuse of catalysts in ATH of acetophenone in aqueous media. (a) CTAB
Synthesis of mixture of ligands (R,R)-3:. Method A. A 50 mL
was used as the surfactant. (b) CPB was applied as the surfactant.
3
round-bottomed flask charged with 20 mL of oleum (50% SO )
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Fig. 8 Asymmetric transfer hydrogenation of the aromatic ketones in aqueous media. Unless otherwise noted, the reaction was carried out in 1 mL of degassed water
under argon atmosphere at 40 uC for a certain time using 0.5 mmol substrate, 5 equivalents of HCO Na as hydrogen donor, 0.02 mmol CTAB, S : C = 100 : 1. The
2
alcohols were obtained with (R)-configuration. All the yields were isolated yields and the ee values were determined by HPLC or GC analysis. (a) 6a was employed as
the catalyst. (b) 6b was used as the catalyst. (c) The data in parenthesis was the yield of dehalogenated products.
was cooled down to 0 uC using an ice bath. Later (R,R)-DPEN (2
g, 9.43 mmol) was added batchwise. After stirring for 10 min,
the resulting reaction mixture was warmed up to 30 uC. The
solution was stirred for an additional 22 h and the viscous
mixture was poured onto 250 g of crushed ice with vigorous
stirring. After the pH value of the solution was adjusted to 9
with solid NaOH, the aqueous phase was washed with DCM
crude product, this process was repeated twice to obtain a 3.6 g
(yield = 90%) mixture of (R,R)-3 as a white or yellow solid.
Method B. A 50 mL round-bottomed flask charged with 20
mL of oleum (50% SO ) was cooled down to 0 uC using an ice
3
bath. Later (R,R)-DPEN (2 g, 9.43 mmol) was added batchwise.
After stirring for 10 min, the resulting reaction mixture was
warmed up to 30 uC. The solution was stirred for an additional
22 h and the viscous mixture was poured onto 250 g of crushed
ice with vigorous stirring. After the pH value of the solution
(
50 mL 6 3). Removal of the water under reduced pressure,
the white or yellow solid was obtained. The crude product was
redissolved in 50 mL of CH OH and the insoluble compound
was removed by filtration. After concentrated to afford the
3
3
was adjusted to 7 with excess solid BaCO , the insoluble solid
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2
D
0
was removed by filtration. The filtrate was concentrated to ca.
(R,R)-3b: White solid, m.p. .280 uC (decomposed); [a]
=
1
5
0 mL and 1.34 g (9.4 mmol) Na SO was added. After
2 3 2
+87.6 (c 0.250, H O); H NMR (CD OD + D O, 300 MHz) d 7.88
2
4
filtration of the precipitated BaSO , water was removed under
reduced pressure and a 3.5 g (yield = 87%) mixture of (R,R)-3
3 3 4’ 3’
(s, 1H, H ), 7.75–7.70 (m, 3H, H , H ), 7.35–7.30 (m, 3H, H ,
H ), 7.16 (d, J = 7.7 Hz, 2H, H ), 4.58 (s, 2H, H , H ); C NMR
6 7 1 1’
4
1
3
was obtained as a white solid .
(D O, 125 MHz) d 143.2 (C ), 142.5 (C ), 141.0 (C ), 138.5 (C ),
2
4
5’
2’
2
1
7 6 5
30.9 (C ), 129.6 (C ), 128.1 (C3’), 126.0 (C ), 125.6 (C5’), 124.1
1
+
Mixture of (R,R)-3. White or yellow solid, m.p. .280 uC; H
3 1’ 1
(C ), 59.7 (C ), 59.6 (C ); ESI-HRMS [M+H] calcd. for
NMR (CD
3
OD + D
2
O, 300 MHz) d 7.78 (s, 2H), 7.69–7.63 (m,
14 15 2 2 6 2
C H N Na O S : 417.0161, found: 417.0159.
1
2
H), 7.32–7.26 (m, 2H), 7.18 (d, J = 7.9 Hz, 2H), 4.18 and 4.16
(
R,R)-3c: White solid, m.p. .280 uC (decomposed); H NMR
1
3
(s, 2H); C NMR (CD OD + D O, 75 MHz) d 145.4, 144.2, 143.2,
3 2
(D O, 600 MHz) d 7.53 (d, J = 8.4 Hz, 4H, H ), 7.17 (d, J = 8.2 Hz,
2
4
1
3
143.1, 131.1, 129.6, 128.6, 126.8, 125.8, 125.2, 62.6, 62.5.
4
H, H ), 4.37 (s, 2H, H ); C NMR (CD OD + D O, 75 MHz) d
3 1 3 2
Synthesis of mono-N-tosylated derivative (R,R)-2. Compound
5 2 3 4 1
146.1 (C ), 141.7 (C ), 128.6 (C ), 127.5 (C ), 60.7 (C ); ESI-
+
(
R,R)-3 (1.288 g, 3.1 mmol) and Na
2
CO
3
(1.3 g, 12.4 mmol) were
HRMS [M+H] calcd. for C14
17.0159.
General procedure for asymmetric transfer hydrogenation of
ketone. (R,R)-2 (3.3 mg, 0.0055 mmol), RuCl (p-cymene)] (1.5
15 2 2 6 2
H N Na O S : 417.0161, found:
dissolved in 15 mL of H
2
O, then a THF solution (15 mL) of
4
TsCl (709 mg, 3.7 mmol) was added to the aqueous solution
dropwise with vigorously stirring at 0 uC. Once the addition
was completed, the resulting mixture was warmed up to 40 uC
and stirred for 24 h. This was followed by washing the aqueous
2
2
mg, 0.0025 mmol) and surfactant (0.02 mmol) was added to a
reaction tube and degassed three times. 1 mL of degassed
water was added and the mixture was stirred at 40 uC for 1 h
layer with 30 mL of Et O and removal of the water under
2
reduced pressure. The residue was redissolved in 30 mL of
under argon. Ketone 4a–g (0.5 mmol) and HCOONa?2H O (260
2
CH OH and removal of the insoluble compound by filtration.
3
mg, 2.5 mmol) were added to the solution. The mixture was
After concentration under reduced pressure, the resulting
degassed three times and was stirred under argon at 40 uC
solid was purified by silica gel chromatography (H
2
O : THF =
21
with 800 r min stirring . When the aromatic ketone was
1
(
: 10) and 1.4 g (yield = 75%) white or yellow solid product
R,R)-2 was obtained.
completely consumed (determined by TLC), the reaction
mixture was extracted with CH Cl (5 mL) three times. The
2
2
conversion and enantioselectivity were determined by GC
analysis (CP-cyclodex-236 M). Isolated yield was obtained by
flash chromatography.
Mixture of (R,R)-2. White or yellow solid, m.p. .280 uC
1
(
7
decomposed); H NMR (D O, 300 MHz) d 7.57–7.47 (m, 2H),
2
.29–7.11 (m, 6H), 6.91 (brs, 3H), 6.76–6.73 (m, 1H), 4.54–4.46
1
3
Recycle experiment for asymmetric transfer hydrogenation
of acetophenone. The mixture of (R,R)-2 (3.3 mg, 0.0055
mmol), RuCl (p-cymene)] (1.5 mg, 0.0025 mmol) and surfac-
(
(
1
1
1
5
m, 1H), 4.34–4.28 (m, 1H), 2.13 and 2.11 (s, 3H); C NMR
O, 75 MHz) d 145.4, 143.5, 143.3, 140.6, 138.5, 135.9, 135.8,
31.6, 131.4, 130.5, 129.9, 129.7, 127.4, 125.8, 125.2, 124.9,
D
2
2
2
2
1
tant (0.02 mmol) was added to a reaction tube and degassed
three times. 1 mL of degassed water was added and the
24.7, 64.6, 60.4, 21.4; IR (cm ): 3451.3, 1634.0, 1190.6,
+
040.1; ESI-HRMS [M+H] calcd. for
71.0250, found: 571.0254.
21 21 2 2 8 3
C H N Na O S :
mixture was stirred at 40 uC for
1 h under argon.
Acetophenone 4a (60 mL, 0.5 mmol) and HCOONa?2H O (260
2
2
3
mg, 2.5 mmol) were added to the solution. The mixture was
(
R,R)-2a. White solid, M.p. .280 uC (decomposed); [a]
=
D
1
degassed three times and was stirred under argon at 40 uC
+
7
7
1
3
1
1
1
2 2
104.6 (c 1.02, H O); H NMR (D O, 600 MHz) d 7.50 (s, 1H),
2
1
with 800 r min stirring. After the reaction was completed
determined by TLC), HCO H (22.5 mL, 0.55 mmol) was added
.45 (d, J = 7.8 Hz, 1H), 7.23–7.20 (m, 3H), 7.15–7.12 (m, 2H),
.05 (d, J = 7.8 Hz, 1H), 6.89–6.86 (m, 3H), 6.69 (d, J = 7.7 Hz,
H), 4.49 (d, J = 9.4 Hz, 1H), 4.29 (d, J = 9.4 Hz, 1H), 2.07 (s,
2
O, 150 MHz) d 144.8, 142.9, 142.7, 137.0,
34.9, 130.9, 130.7, 129.7, 129.4, 128.9, 126.6, 125.5, 124.6,
24.1, 123.8, 62.8, 59.2, 20.6; IR (cm ): 3446.2, 3130.9, 1631.9,
162.1, 1118.8, 1037.1, 995.1, 698.7, 618.5, 555.9; ESI-HRMS
M+H] calcd. for C H N Na O S : 571.0255, found:
71.0246; [M+Na] calcd. for C H N Na O S : 593.0069,
(
2
to regenerate sodium formate, and the product was extracted
1
3
with n-hexane : Et O (v : v = 1 : 1) (3 mL) three times.
2
H); C NMR (D
Acetophenone (60 mL, 0.5 mmol) was added to the aqueous
solution for a new reaction cycle, and then the mixture was
degassed three times and stirred at 40 uC under an argon
2
1
2
1
+
atmosphere with 800 r min
stirring. The conversion and
[
5
2
1
21
2
2 8 3
+
enantioselectivity were determined by GC analysis.
21 20 2 3 8 3
General procedure for asymmetric transfer hydrogenation of
a-bromomethylaromatic ketone. The mixture of (R,R)-2 (3.3
mg, 0.0055 mmol), [RhCl Cp*] (1.6 mg, 0.0025 mmol) and
2 2
found: 593.0075.
Purification of water-soluble diamines (R,R)-3 (a–c). Pure
R,R)-3(a–c): The mixture of (R,R)-3 was separated by prepara-
(
tive HPLC on Varian Dynamax 250 6 10 mm column, 0.02 mol
CTAB (7.3 mg 0.02 mmol) was added to a reaction tube and
degassed three times. 1 mL of degassed water was added and
the mixture was stirred at 40 uC for 1 h under argon.
a-Bromomethylaromatic ketone 4h–m (0.5 mmol) and
2
1
L
KCl, UV 220 nm.
R,R)-3a: White solid, m.p. .280 uC (decomposed); [a]
2
D
0
(
=
1
+64.4 (c 0.252, H
2
O); H NMR (CD
3
OD + D
2
O, 300 MHz) d 7.87
(
s, 2H, H ), 7.70 (d, J = 7.8 Hz, 2H, H
3
5
), 7.30 (t, J = 7.7 Hz, 2H,
HCOONa?2H
2
O (260 mg, 2.5 mmol) were added to the
1
3
H
6
), 7.19 (d, J = 7.8 Hz, 2H, H
OD + D
30.1 (C ), 127.1 (C
Calcd. For C14
7
), 4.48 (s, 2H, H
1
); C NMR
solution. The mixture was degassed three times and was
stirred under argon at 40 uC with 800 r min stirring. When
the reaction was completed (determined by TLC), the reaction
2 2
mixture was extracted with CH Cl (5 mL) three times. The
2
1
(
1
CD
3
2
O, 75 MHz) d 146.5 (C
), 125.4 (C ), 60.6 (C
Na : 417.0161, found: 417.0142.
4
), 139.2 (C
2
), 131.4 (C ),
7
+
6
5
3
1
); ESI-HRMS [M+H]
H
15
N
2
2
O
6
S
2
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combined organic phases were dried over anhydrous Na SO4
and evaporated under reduced pressure. The crude product
Cyclodextrin-2,3,6-M-19, 0.25 mm 6 25 m, column tempera-
ture = 170 uC (isothermal), inject temperature = 240 uC,
2
was dissolved in 2 mL of CDCl and the yield was determined
by H NMR analysis using 1,3,5-trimethylbenzene as an
detector temperature = 260 uC, inlet pressure = 12 psi; t = 10.7
3
1
1
min, t = 11.1 min.
2
internal standard. Isolated yield was obtained by flash
chromatography.
4
e
23
(R)-2-Bromo-1-phenylethanol (5h).
[a]D 250.7 (c 1.0,
1
CHCl
3
); 95% yield, 95% ee; H NMR (CDCl , 300M Hz) d
3
4
e
23
(
R)-1-Phenylethanol (5a). [a]D = + 53.4 (c 1.07, CHCl ); 96%
7.31–7.20 (m, 5H), 4.93 (dd, J = 3.3, 8.8 Hz, 1H), 3.56 (dd, J =
3.4, 10.4 Hz, 1H), 3.46 (dd, J = 8.9, 10.4 Hz, 1H), 2.63 (br, 1H);
GC analysis: CP-Chirasil-DEX CB, 0.25 mm 6 25 m, column
temperature = 100 uC (isothermal), inject temperature = 240
uC, detector temperature = 260 uC, inlet pressure = 12.1 psi;
tmajor = 7.9 min, tminor = 8.3 min.
3
1
ee; H NMR (CDCl , 300 MHz) d 7.42–7.30 (m, 5H), 4.94 (q, J =
3
6.3 Hz, 1H), 1.52 (d, J = 6.3 Hz, 3H); GC analysis: CP-cyclodex
B-236 M, 0.25 mm 6 25 m, column T = 115 uC (isothermal),
inject T = 220 uC, detector T = 240 uC, inlet pressure = 10.5 psi;
t = 5.0 min, t = 5.3 min.
1 2
4
e
23
(
R)-1-Indanol (5b). [a]D 234.9 (c 0.95, CHCl ); 99% yield,
4e
23
3
(R)-1-[4-(Benzyloxy)phenyl]-2-bromoethanol (5i). [a]D 229.8
1
9
7
2
7% ee; H NMR (CDCl
.19 (m, 3H), 5.17 (t, J = 6.0 Hz, 1H), 2.94–2.99 (m, 1H), 2.75–
.78 (m, 1H), 2.40–2.42 (m, 1H), 1.84–1.91 (m, 1H); GC
analysis: CP-Chirasil-DEX CB, 0.25 mm 6 25 m, column T =
30 uC, inject T = 220 uC, detector T = 240 uC, inlet pressure =
1.5 psi; t = 9.9 min, t = 10.2 min.
3
, 300 MHz) d 7.29–7.36 (m, 1H), 7.14–
1
(
7
4
c 1.0, CHCl
3
); 81% yield, 94% ee; H NMR (CDCl
.44–7.29 (m, 7H), 6.98 (dd, J = 2.1, 6.7 Hz, 2H), 5.07 (s, 2H),
.88 (dd, J = 3.9, 9.0 Hz, 1H), 3.60 (dd, J = 3.6, 10.4 Hz, 1H), 3.53
3
, 300 MHz) d
(
dd, J = 9.2, 10.1 Hz, 1H), 1.72 (brs, 1H); HPLC analysis:
Chiralcel OD, i-PrOH/n-hexane = 10/90 (v/v), 1.0 mL min , 23
uC, UV 230 nm; tminor = 13.8 min, tmajor = 15.7 min.
1
1
21
1
2
4
e
23
(
R)-1-Phenyl-propan-1-ol (5c).
[a]D = +38.5 (c 1.1, CHCl
9% yield, 91% ee; H NMR (CDCl , 300 MHz) d 7.48–7.39 (m,
H), 4.73 (t, J = 6.5 Hz, 1H), 1.96–1.88 (m, 3H), 1.05 (t, J = 7.5
3
);
4e
23
(R)-2-Bromo-1-(4-methoxyphenyl)ethanol (5j).
[a]D 235.8 (c
1
9
5
3
1
1
7
8
.0, CHCl ); 85% yield, 96% ee; H NMR (CDCl , 300 MHz) d
3 3
.23 (d, J = 8.6 Hz, 2H), 6.83 (d, J = 8.6 Hz, 2H), 4.80 (dd, J = 3.8,
.8 Hz, 1H), 3.73 (s, 3H), 3.52 (dd, J = 3.6, 10.4 Hz, 1H), 3.44
dd, J = 8.2, 10.4 Hz, 1H), 2.53 (br, 1H); GC analysis: CP-
cyclodex B-236 M, 0.25 mm 6 25 m, column temperature =
60 uC (isothermal), inject temperature = 220 uC, detector
Hz, 3H); GC analysis: CP-Chirasil-DEX CB, 0.25 mm 6 25 m,
column temperature = 140 uC (isothermal), inject temperature
240 uC, detector temperature = 260 uC, inlet pressure = 11 psi;
(
=
t = 4.8 min, t = 5.0 min.
1
2
1
temperature = 240 uC, inlet pressure = 10.5 psi; tminor = 13.9
min, tmajor = 14.3 min.
4
e
23
(
R)-2-(2’-Naphthyl)ethanol (5d). [a]D = +46.5 (c 1.0, CHCl
3
);
) d 7.86–7.80 (m,
H), 7.54–7.47 (m, 3H); 04 (q J = 6.5 Hz, 1H), 2.26 (s, 1H), 1.59
1
9
4
(
9% yield, 94% ee; H NMR (300 MHz, CDCl
3
4
e
23
(
R)-2-Bromo-1-(4-nitrophenyl)ethanol (5k). [a]D 233.5 (c 1.0,
d, J = 6.5 Hz, 3H); GC analysis, CP-cyclodex B-236 M, 0.25 mm
25 m, column temperature = 170 uC (isothermal), inject
temperature = 240 uC, detector temperature = 260 uC, inlet
1
CHCl
3
3
); 93% yield, 95% ee; H NMR (CDCl , 300 MHz) d 8.24
6
(d, J = 8.7 Hz, 2H), 7.60 (d, J = 8.7 Hz, 2H), 5.08–5.05 (m, 1H),
3.71–3.66 (m, 1H), 3.57–3.51 (m, 1H), 2.93 (br, 1H); GC
pressure = 12 psi, t = 5.0 min, t = 5.2 min.
1
2
analysis: CP-cyclodex B-236 M, 0.25 mm 6 25 m, column
2
1
4
e
23
temperature = 145 uC, hold 14 min, 10 uC min to 200 uC,
hold 20 min, inject temperature = 240 uC, detector temperature
(
R)-1,2,3,4-Tetrahydro-1-naphthol (5e).
CHCl ); 99% yield, 97% ee; H NMR (CDCl , 300 MHz) d 7.45–
.42 (m, 1H), 7.26–7.19 (m, 2H), 7.13–7.11 (m, 1H), 4.78 (t, J =
.0 Hz, 1H), 2.87–2.75 (m, 2H), 2.02–1.79 (m, 5H); GC analysis:
[a]D 234.2 (c 1.1,
1
3
3
=
3
260 uC, inlet pressure = 11.5 psi; tminor = 32.2 min, tmajor
2.7 min.
=
7
5
CP-Cyclodextrin-2,3,6-M-19, 0.25 mm
temperature = 130 uC (isothermal), inject temperature = 220
uC, detector temperature = 260 uC, inlet pressure = 11 psi;
6 25 m, column
4
e
(
R)-1-(3,5-Bis-benzyloxy-phenyl)-2-bromo-ethanol(5l).
White
2
3
1
solid, m.p: [a]D 220.5 (c 1.1, CHCl
NMR (CDCl
.60 (s, 1H), 5.00 (s, 4H), 4,88–4.84 (m, 1H), 3.65–3.55 (m, 1H),
3.52–3.18 (m, 1H), 2,80 (br, 1H); HPLC analysis: Chiralcel OD,
3
); 93% yield 93% ee; H
3
, 300 MHz) 7.46–7.38 (m, 10H), 6.83–6.61 (m, 2H),
t
minor = 13.3 min, tmajor = 13.6 min.
6
4
a
23
(
R)-1-(thiophen-2-yl)ethanol (5f). [a]D = +21.7 (c 1.1, CHCl3);
2
1
1
i-PrOH/n-hexane = 10/90 (v/v), 1.0 mL min , 23 uC, UV 230
nm; tmajor = 23.5 min, tminor = 30.4 min.
9
1
1
3
9% yield, 96% ee; H NMR (CDCl , 300 MHz) d 7.26–7.22 (m,
H), 6.97–6.94 (m, 2H), 5.12 (q, J = 6.4 Hz, 1H), 2.27 (br., 1H),
.59 (d, J = 6.4 Hz, 3H); GC analysis: CP-Cyclodextrin-2,3,6-
(
R)-Methyl 5-(2-bromo-1-hydroxyethyl)-2-hydroxybenzoate
5m). White solid; mp: 73–74 uC; [a]D 239.7 (c 1.0, CHCl );
3
1% yield, 93% ee; H NMR (CDCl , 300 MHz) d 10.7 (s, 1H),
.82 (d, J = 2.3 Hz, 1H), 7.43 (dd, J = 2.3, 8.6 Hz, 1H), 6.95 (d, J =
.6 Hz, 1H), 4.83 (dd, J = 3.6, 8.6 Hz, 1H), 3.93 (s, 3H), 3.56 (dd,
J = 3.7, 10.5 Hz, 1H), 3.48 (dd, J = 8.6, 10.4 Hz, 1H), 2.94 (brs,
H); GC analysis: CP-Chirasil-DEX CB, 0.25 mm 6 25 m,
M-19, 0.25 mm 6 25 m, column temperature = 130 uC
isothermal), inject temperature = 220 uC, detector tempera-
4
e
23
(
(
1
9
7
8
ture = 260 uC, inlet pressure = 11 psi; t
1
= 3.6 min, t
2
= 3.7 min.
3
4
a
23
(
R)-1-(p-Nitroyphenyl)ethanol (5g).
[a]D = + 33.7 (c 1.0,
1
3 3
CHCl ); 99% yield, 85% ee; H NMR (CDCl , 300 MHz) d 8.10
1
(d, J = 8.7 Hz, 2H), 7.47 (d, J = 8.7 Hz, 2H), 4.95 (q, J = 6.5 Hz,
1H), 2.83 (brs, 1H), 1.45 (d, J = 6.5 Hz, 3H); GC analysis: CP-
column temperature = 180 uC, inject temperature = 240 uC,
1
832 | RSC Adv., 2013, 3, 1825–1834
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detector temperature = 260 uC, inlet pressure = 10 psi; tmajor
=
compound which was purified on silica-gel column
chromatography and preparative HPLC respectively.
16.9 min, tminor = 17.3 min.
8
(a) T. Dwars, E. Paetzold and G. Oehme, Angew. Chem., Int.
Ed., 2005, 44, 7174; (b) T. Dwars and G. Oehme, Adv. Synth.
Catal., 2002, 344, 239; (c) G. Oehme, I. Grassert,
E. Paetzold, R. Meisel, K. Drexler and H. Fuhrmann,
Coord. Chem. Rev., 1999, 185–186, 585; (d) For selected
examples involved surfactant, see: K. Manabe, M. S. Xiang
and S. Kobayashi, J. Am. Chem. Soc., 2001, 123, 10101; (e)
K. Manabe, S. Iimura, X. M. Sun and S. Kobayashi, J. Am.
Chem. Soc., 2002, 124, 11971; (f) K. Ahlforda, J. Lind,
L. M ¨a ler and H. Adolfsson, Green Chem., 2008, 10, 832; (g)
I. Grassert, J. Kov ´a cs, H. Fuhrmann and G. Oehme, Adv.
Synth. Catal., 2002, 344, 312; (h) M. Ludwig, R. Kadyrov,
H. Fiedler, K. Haage and R. Selke, Chem.–Eur. J., 2001, 7,
3298; (i) H. Y. Rhyoo, H. J. Park, W. H. Suh and Y.
K. Chung, Tetrahedron Lett., 2002, 43, 269; (j) F. Trentin, A.
M. Chapman, A. Scarso, P. Sgarbossa, R. A. Michelin,
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Acknowledgements
This work was financially supported by the National Nature
Science Foundation of China (Grant No. 20972154, 21172152),
the National Basic Research Program of China (973 Program,
No. 2010CB833300), Sichuan University and the Fundamental
Research Funds for the Central Universities, Southwest
University for Nationalities (No. 11NZYBS05).
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In this work, we have also confirmed that the ligand used
in both literatures 4a and 4b is really the bis-meta-
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