Phosphonate-containing polystyrene copolymer-supported Ru catalyst for asymmetric transfer hydrogenation in water

Article abstract of DOI:10.1039/c3ra22057e

A series of phosphonate-containing copolystyrenes with chiral ligand (1R,2R)-(+)-N¹-toluenesulfonyl-1,2-diphenylethylene-1,2-diamine were prepared by radical copolymerization. The supported Ru catalysts with excellent catalytic performances (94-98% yields, 93.9-97.8% ee and 100% chemoselectivity) in aqueous asymmetric transfer hydrogenation could be easily recycled by centrifugal separation.

Full text of DOI:10.1039/c3ra22057e

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Phosphonate-containing polystyrene copolymer-
supported Ru catalyst for asymmetric transfer
Cite this: RSC Advances, 2013, 3, 6747
hydrogenation in water3
Received 5th September 2012,
Accepted 11th March 2013
Xiao Xu, Rui Wang, Jingwei Wan, Xuebing Ma* and Jingdong Peng*
DOI: 10.1039/c3ra22057e
www.rsc.org/advances
A series of phosphonate-containing copolystyrenes with chiral
ligand (1R,2R)-(+)-N1-toluenesulfonyl-1,2-diphenylethylene-1,2-
diamine were prepared by radical copolymerization. The sup-
ported Ru catalysts with excellent catalytic performances (94–
98% yields, 93.9–97.8% ee and 100% chemoselectivity) in
aqueous asymmetric transfer hydrogenation could be easily
recycled by centrifugal separation.
protic and highly polar media such as water, it is necessary that
the polarity of the polystyrene support is modified by the
introduction of highly polar functional groups.8
In this communication, by introducing a phosphonate group
(PO3H2) and the chiral ligand (1R,2R)-(+)-N1-toluenesulfonyl-1,2-
diphenylethylene-1,2-diamine into the backbone of polystyrene, a
novel type of phosphonate-containing polystyrene copolymer, 2a–
d, bearing chiral ligands with different arm chain lengths and 2a1–
a4 with different compositions (y/x) of functional groups (PO3H2
and NH2), has been prepared for the first time (Scheme 1). Due to
the introduction of phosphonate (PO3H2) and amino (NH2)
groups, the polarity of polystyrene-based supports 2a–d and 2a1–
a4 was strengthened to afford good solubility in alcohol and good
dispersion properties in water. The supported ruthenium catalysts
3a–d and 3a1–a4 possessed mesoporous structures and excellent
catalytic performances (94–98% yield, 93.9–97.8% ee and 100%
In the past decades thousands of chiral homogeneous catalysts
have been developed and many of them are known to be highly
effective in numerous asymmetric transformations. However, the
application of homogeneous catalysts in industry is limited, partly
due to the problems of separation, recycling and limited stability
of the catalysts.1 One efficient way to overcome the problem of
isolation of homogeneous catalysts is the heterogenization of the
active catalytic molecules. To date, heterogenization is commonly
achieved by non-covalent entrapment or covalent grafting of active
molecules on the surface or inside the pores of a solid support,2
such as organic polymers,3 silica,4 and magnetic nanoparticles,5 as
it greatly simplifies the separation of the catalyst from the reaction
mixture and allows the efficient recovery and reuse of expensive
homogeneous catalysts. Furthermore, from the perspective of
green chemistry, reactions in which water is used as a solvent have
attracted
a great deal of attention because water is an
environmentally friendly and safe medium, which avoids the
problem of pollution that is inherent with organic solvents.6
Although polystyrene (PS), which has good solubility in THF,
dichloromethane, chloroform, benzene and ethyl acetate, is an
attractive and popular organic polymer support in the field of
asymmetric catalysis,7 it suffers from strong shrinkage and poor
swelling in highly polar solvents such as water and alcohol,
resulting in poor accessibility of the reactants. In order to enhance
the swelling properties of the polystyrene support and the
accessibility of the substrate into the catalytic site, especially in
College of Chemistry and Chemical Engineering, Southwest University, Chongqing,
400715, P. R. China. E-mail: zcj123@swu.edu.cn; Fax: (+86)23-68253237;
Tel: (+86)23-68253237
Scheme 1 The synthetic route to polystyrene-bound Ru catalysts 3a–d and 3a1–
a4.
Electronic supplementary information (ESI) available: NMR, IR, GPC, SEM, AFM,
3
GC, nitrogen adsorption–desorption isotherm plots. See DOI: 10.1039/c3ra22057e
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(Fig. 1). As expected, the surface areas, average pore diameters and
pore volumes of 2a1–a4 increased from 3.3 to 8.3 m2 g21, 0.7 to 1.7
nm and 4.6 to 8.3 6 1023 cc g21, respectively, as the loading
amount of 1-phosphonate styrene in the copolystyrene backbone
was increased.
Taking into account the intimate relationship between the
physical surface properties of the support and its catalytic
performance, polystyrene copolymers 2a1–a4 were well charac-
terised by SEM, TEM and AFM to understand their surface
morphologies and particle sizes in different existential states. As a
representative example, after being well-dispersed in water (1 mg
sample in 5 mL of H2O), the AFM image of copolymer 2a3
efficiently reflected its surface morphology in water, which seemed
to simulate the ‘‘true’’ state of 2a3 in the aqueous catalytic system.
Among the statistically analyzed 265 particles for 2a3, 90% of
particle sizes were in the range of 10 to 130 nm (Fig. 1). The SEM
images showed that the insoluble aggregates with particle sizes of
1–5 mm dispersed in THF and could be considered to mirror the
surface morphology of 2a3 in the solid state.10
Fig. 1 The characterization of polystyrene copolymers 2a1–a4 by 31P NMR, AFM and
Upon the immobilization of [RuCl2(p-cymene)]2 into polystyr-
ene copolymers 2a1–a4, the chemical composition, porous
structure and surface morphology were characterized by DTG,
SEM, AFM, XPS and nitrogen adsorption–desorption isotherms
and some of them are shown in Fig. 2. Some nanopores (0.8–8
nm) disappeared, especially at 6.4 nm for 2a1–a4, which
demonstrated that the [RuCl2(p-cymene)]2 particles were trapped
inside those nanopores. It is particularly worth noting that four
nitrogen adsorption–desorption isotherm.
chemoselectivity) in the aqueous asymmetric transfer hydrogena-
tion of aromatic ketones. Furthermore, these polystyrene-bound
Ru catalysts could be easily recovered by simple centrifugal
separation in quantitative yields and reused five times without a
sharp loss of their catalytic properties.
Phosphonate-containing polystyrene copolymers 2a–d and 2a1–
a4 were prepared, respectively by simple radical copolymerization
of 1-phosphonate styrene with styrenes 1a–d, bearing chiral amino
groups with different arm chain lengths, and styrene 1a under
different molar ratios in THF at 80 uC for 24 h and in the presence
of BPO as a radical initiator. The chemical compositions (y/x) in
polystyrene copolymers 2a–d (y/x = 3.66–4.08) and 2a1–a4 (y/x =
0.21–5.72), regulated by altering the amount used of 1-phospho-
nate styrene, could be determined by quantitative 31P NMR. The
structural distribution of phosphonate groups (PO3H2) in the
polystyrene backbone could be also elucidated by 31P NMR (Fig. 1).
Compared to 31P NMR of 1-phosphonate polystyrene at 12.5 (q),
5.1 (s), 1.0 (s) and 0.0 ppm (s), the new and wide 31P NMR peak of
2a1–a4 emerged in the range of d 20–35 ppm beside the same
sharp four peaks as 1-phosphonate polystyrene, which resulted
from the 31P absorption of the chain segment copolymerized by
monomer 2a with 1-phosphonate styrene. GPC analysis showed
that the weight-average molecular weights (Mw) of copolymers 2a–
d and 2a1–a4 were between 759.5 and 974.1 kDa and the PDI
values were between 1.06 and 1.28, which may be not accurate
since monodispersed polysaccharide standards were used for
calibration while the copolymers 2a–d and 2a1–a4 are brush
polymers. The isotherm plots of 2a1–a4 were linear to the P/P0 axis
at a relatively low P/P0 range (0–0.8) and convex to the P/P0 axis at a
high P/P0 range (0.8–1.0), which were beyond the classic
definitions.9 From the calculated data, the pore size distributions
(PSDs) of as-synthesized 2a1–a4 suggest the existence of 0.8–8 nm
irregular micropores with 0.7–1.3 nm average pore diameters
Fig. 2 The comparative XPS, AFM and nitrogen adsorption–desorption isotherms of
support 2a3 and its supported catalyst 3a3.
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Table 1 The catalytic performances of 3a–d and 3a1–a4 in the asymmetric transfer hydrogenation of acetophenonea
Entry
Cat.
R
S/C
Solvent
Conv. (%)b
Yield (%)c
(% ee)d
1
2
3
4
5
6
7
8
3a
3b
3c
3d
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
660
660
660
660
660
660
660
660
660
660
660
660
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
100
100
100
100
100
100
100
100
100
100
93.8
95.2
94.8
99.2
90.1
44.2
100
100
100
100
100
100
100
100
99.2
100
100
100
100
100
98
98
98
97
98
98
98
98
98
99
91
92
90
97
87
40
97
97
98
96
97
97
98
97
95
98
96
94
96
96
97.2 (R)
96.8 (R)
95.1 (R)
95.0 (R)
96.7 (R)
96.8 (R)
97.2 (R)
97.8 (R)
97.2 (R)
97.6 (R)
97.2 (R)
96.3 (R)
96.9 (R)
97.2 (R)
97.4 (R)
97.3 (R)
95.1 (R)
96.2 (R)
95.8 (R)
87.1 (R)
82.8 (R)
95.2 (R)
96.1 (R)
95.8 (R)
97.9 (R)
97.6 (R)
93.9 (R)
96.9 (R)
292.3 (S)
97.3 (R)
3a1
3a2
3a3(3a)
3a4
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
3a3
9
MeOH–H2Oe
MeOH
CHCl3
DMF
THF
H2O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
660
2.0 6 103
3.3 6 103
6.6 6 103
660
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
p-Br
m-Br
o-Br
p-NO2
m-NO2
p-Cl
m-Cl
o-Cl
p-CH3
m-CH3
o-CH3
p-OCH3
o-OCH3
m-OCH3
660
660
660
660
660
660
660
660
660
660
660
660
660
a
Reaction conditions: cat. (1.5 wt% Ru, 9 mg, 1.3 6 1023 mmol), acetophenone (0.1 mL, 0.86 mmol), HCO2H–Et3N (0.5 mL, v/v = 1 : 3), 50
b
c
d
e
uC, 6 h. Monitored by GC. Isolated yield. Determined by chiral GC, chiral Cyclodex-B (30 m 6 0.25 nm 6 0.25 mm, Supelco). v/v = 1 : 1.
typical nanopores inside 2a1–a4, at about 0.6–0.7, 1.0–1.3, 1.8–2.4
and 3.2–4.0 nm, were reconstructed (Fig. 2 (3)). Those nanopores
provided the diffusional channels for substrates to smoothly
access the catalytic sites in the catalytic process. In order to cast
light on the immobilized [RuCl2(p-cymene)]2 particles, support 2a3
and its supported Ru catalyst 3a3 (1.5 wt% Ru) were scanned by
XPS as a pair of comparative examples. As expected, some N1s
electron binding energies of 2a3 increased from 399.8 to 401.1 eV
with 1.3 eV increment after the immobilization of
[RuCl2(p-cymene)]2 particles (Fig. 2 (1)). However, there were only
0.1–0.4 eV changes for S2p, O1s and P2s binding energies.
Therefore, it was concluded that the [RuCl2(p-cymene)]2 particles
successfully complexed with some of the amino groups. Due to the
coordination reaction between [RuCl2(p-cymene)]2 and the amino
groups, the AFM images showed the change of surface
morphology in length, width and height owing to the interaction
force among supported Ru catalyst particles. The mean length of
supported Ru catalyst 3a3 decreased from 390.0 nm to 179.7 nm.
However, the mean height and width slightly increased from 0.6
and 112.5 nm to 1.9 and 138.6 nm, respectively (Fig. 2 (2)).
Aqueous transfer hydrogenation of aromatic ketones was used
as a model reaction to investigate the catalytic performances of
supported Ru catalysts 3a–d and 3a1–a4.11 Due, in part, to good
solubility in MeOH and good dispersion properties in H2O,
phosphonate-containing copolymer-supported Ru catalysts 3a–d
Fig. 3 The conversion and enantioselectivity of acetophenone in the overall catalytic
process.
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Table 2 Literature survey for asymmetric hydrogenation of acetophenone in watera
Support
Reaction conditions
Efficiency
Repeatability (fifth cycle)
Sulfonated polystyrene13
Poly(ethylene glycol)14
Silica15
HCOONa, 40 uC, 3 h
S/C = 100, 100% conv., 97% ee
S/C = 100, 99% conv., 94% ee
S/C = 100, 99% conv., 96% ee
S/C = 660, 99% conv., 97.8% ee
98% conv., 97–98% ee
98% conv., 94% ee
70% conv., 96% ee
90% conv., 96.6% ee
HCOONa, 40 uC, 3 h
HCOONa, 40 uC, 2 h, 4 mol % TBAB,
HCO2H–Et3N (v/v = 1/3), 50 uC, 6 h
Phosphonate-containing polystyrene
a
Over supported [RuCl2(p-cymene)]2 catalyst.
and 3a1–a4 exhibited excellent catalytic conversions, enantioselec-
tivities and chemoselectivities in protic polar solvents. As the
hanging arm chain lengths increased from 3a to 3d, the
enantioselectivities decreased from 97.1 to 95.0% ee owing to
the successively improved flexibilty (Table 1, entries 1–4).
Furthermore, due to the enhanced phosphonate-containing
content (y/x) from 3a1 to 3a4, good dispersion in water was found
and the enantioselectivities increased from 96.7 to 97.8% ee
(Table 1, entries 5–8). Unfortunately, the conversion of acetophe-
none decreased to 93–95% in aprotic polar solvents (Table 1,
entries 11–13). Especially, there was good conversion (90.1%), yield
(87%) and excellent enantioselectivity (97.4% ee) even at the low S/
C = 3300 (0.05 mol% Ru) (Table 1, entries 14–16). The optimized
protocol was expanded to a wide variety of aromatic ketones. All of
them possessed excellent conversions (y100%), yields (94–98%)
and enantioselectivities (93.9–97.9% ee, except for the nitro-
substituted aromatic ketones) for either electron-rich or -poor and
o-, m- or p-position aromatic ketones (Table 1, entries 17–30).
In particular, the used amount of Et3N played an important
role in the conversion of acetophenone. At volume ratios of Et3N–
HCO2H , 2, the asymmetric transfer hydrogenation of acetophe-
none barely took place like in other pH-controlled reactions in
water.12 From the TG curves of fresh catalyst 3a3 and one dealing
with Et3N, the weight loss between 150 and 800 uC decreased from
90.8% to 64.2%, which demonstrated that the phosphonate
moieties on the polystyrene backbone in catalyst 3a3 underwent
deprotonation with Et3N to form phosphonates (PO3H2N+HEt3)
and resulted in difficult thermal decomposition. On the other
hand, the formation of phosphonate acting as a surfactant was
identified by the good dispersion upon adding Et3N in the
catalytic reaction. The small dosage of catalyst 3a3 had little
influence on the used amount of Et3N (Et3N/HCO2H, v/v = 3), and
Et3N was enough to react with formic acid as a hydrogen source.
In the overall catalytic process, the catalytic intermediates were
detected using HPLC. It was found that the specifically selective
hydrogenation of the CLO bond was observed without side
reactions such as further hydrogenolysis of C–O and the aromatic
ring. The conversions of acetophenone and the enantioselectivity
of R-1-phenylethanol in the catalytic process are shown in Fig. 3.
A comparison of the present system with other reported
relevant catalytic systems using [RuCl2(p-cymene)]2 as a precursor
in terms of the catalyst’s preparation, easiness and efficiency
including the reaction time, temperature, yield, selectivity and
repeatability is summarized in Table 2. The phosphonate-contain-
ing polystyrene copolymer-supported Ru catalyst, in which
hydrophilic phosphonate rendered itself dispersed in water,
provided an excellent example with high S/C = 660 efficiency for
the asymmetric transfer hydrogenation of aromatic ketones in
water.
Catalyst 3a3 could be easily and quantitatively recycled by
centrifugal separation and reused in five consecutive runs without
a sharp loss of catalytic performance (90% conv., 86% yield and
96.6% ee in the 5th cycle).
Acknowledgements
The financial support from NSFC (grant 21071116) and CSTC
(2010BB4126) is gratefully acknowledged.
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