Mesoporous cross-linked polymer copolymerized with chiral BINAP ligand coordinated to a ruthenium species as an efficient heterogeneous catalyst for asymmetric hydrogenation

Article abstract of DOI:10.1039/c2cc35192g

We report here a successful preparation of a heterogeneous chiral catalyst from copolymerization of mesoporous cross-linked polymer with chiral BINAP ligands, followed by coordination of the BINAP with a ruthenium species, which exhibits high activity, excellent enantioselectivity, and extraordinary recyclability in asymmetric hydrogenation.

Full text of DOI:10.1039/c2cc35192g

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Mesoporous cross-linked polymer copolymerized with chiral BINAP
ligand coordinated to a ruthenium species as an efficient heterogeneous
catalyst for asymmetric hydrogenationw
Qi Sun,a Xiangju Meng,*a Xiao Liu,b Xiaoming Zhang,b Yan Yang,b Qihua Yang*b and
Feng-Shou Xiao*a
Received 19th July 2012, Accepted 4th September 2012
DOI: 10.1039/c2cc35192g
We report here a successful preparation of a heterogeneous chiral
catalyst from copolymerization of mesoporous cross-linked polymer
with chiral BINAP ligands, followed by coordination of the BINAP
with a ruthenium species, which exhibits high activity, excellent
enantioselectivity, and extraordinary recyclability in asymmetric
hydrogenation.
More recently, a series of unique and stable nanoporous
PCPs with superhydrophobicity, hierarchical mesoporosity, high
surface area, large pore volume, and extraordinary adsorption
capacity for organic compounds has been reported.6 Herein, we
report an alternative route for the preparation of chiral meso-
porous PCPs by successful copolymerization of divinylbenzene
(DVB) with chiral BINAP [2,20-bis(diphenylphosphino)-1,10-
binaphthyl] ligands (PCP-BINAP), one of the most important
and efficient ligands in asymmetric reactions.7 The synthesis of
PCP-BINAP was begun from 5,50-diacryloylamino BINAP
dioxide, which was synthesized from oxidation of BINAP
with H2O2, nitration with nitric acid, reduction with hydrazine
monohydrate, and acylation with acryloyl chloride. After
copolymerization with DVB and reduction with HSiCl3,
PCP-BINAP was finally obtained, as illustrated in Scheme 1.
Interestingly, PCP-BINAP is porous and insoluble, having a
large surface area and good swelling properties (Fig. S1, ESIw),
which is quite different from conventional polymer-supported
Porous materials have undergone revolutionary growth in the
past decade because of their vital importance in many applica-
tions such as catalysis, gas separation, and gas storage.1 As
a typical example, metal–organic-frameworks (MOFs) with
unprecedentedly high porosity have been designed and evalu-
ated for a host of applications including gas separation and
storage, catalysis and biomedical applications.2 Inspired by the
advances in MOF design, framework materials with more
robust linkages such as covalent-organic frameworks (COFs)
and porous cross-linked polymers (PCPs) have recently
emerged.3–5 Compared with their MOF counterparts, PCPs
show potential advantages over MOFs as heterogeneous cata-
lysts because of their enhanced stability, which greatly facilitates
their recovery and reuse. As a result, PCPs can be used as an
ideal platform for incorporating molecular catalytic modules
into highly stable and recyclable heterogeneous catalyst systems
by taking advantage of their permanent porosity and the ability
to tune their compositions and properties at the molecular
level.4,5 It is worth noting that most of the research is focused
on microporous chiral PCPs, but syntheses of mesoporous
chiral PCPs are still scarce.5 Normally, relatively small micro-
pores influence mass transfer, which limits asymmetric catalytic
performance. In addition, relatively bulky reactants make
contact with active sites in such small micropores difficult.1
To solve this problem, the synthesis of mesoporous or macro-
porous chiral PCPs is strongly desirable.
a Department of Chemistry, Zhejiang University, Hangzhou 310028,
China. E-mail: mengxj@zju.edu.cn, fsxiao@zju.edu.cn;
Fax: +86 431-8516-8624; Tel: +86 431-8516-8590
b State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Dalian 116023, P.R. China
w Electronic supplementary information (ESI) available: Synthesis
details, characterizations, and activities. See DOI: 10.1039/c2cc35192g
Scheme 1 Synthesis of the porous and insoluble polymer of (R)-BINAP
as the backbone (PCP-BINAP).
c
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BINAP catalysts with soluble features.8 After loading the
ruthenium species, the heterogeneous chiral catalysts exhibited
superior catalytic performance in asymmetric hydrogenation.
Fig. 1A shows N2 isotherms of PCP-BINAPO and PCP-BINAP
samples, exhibiting typical type-IV isotherms with a hysteresis loop
at 0.60–0.95, which indicates the presence of mesoporosity in the
samples. Correspondingly, pore sizes are mainly distributed around
10–30 nm (Fig. S2 and S3, ESIw). Their BET surface areas and pore
volumes are very similar, at 524–585 m2 gꢀ1 and 0.63–0.65 cm3 gꢀ1
(Tables S1 and S2, ESIw). Fig. 1B shows a scanning electron
microscope (SEM) image of the PCP-BINAP sample, giving
direct evidence of abundant mesoporosity (10–30 nm, Fig. S4,
ESIw), in good agreement with the results of N2 sorption isotherms.
Fig. 1C shows the 13C magic angle spinning (MAS) NMR
spectrum of PCP-BINAPO, giving a broad peak at 135 ppm
assigned to the carbon connected to heteroatoms (P or N) and
a very weak peak at 175 ppm assigned to carbonyl groups,
which suggest the successful copolymerization of BINAPO
with DVB.9 Fig. 1D shows the 31P MAS NMR spectra
of PCP-BINAP and PCP-BINAPO samples. The strong sig-
nals confirm the presence of the element P in the samples,
which is consistent with its mapping (Fig. S5, ESIw). In
addition, PCP-BINAPO exhibits a broad peak centered at
about 29.7 ppm assigned to phosphorus of phosphine oxide,10
while PCP-BINAP gives the signal at ꢀ16.5 ppm assigned
to the unprotected phosphine. These results indicate the successful
transformation from BINAPO to BINAP groups in the sample.
Fig. 1E shows IR spectra of PCP-BINAPO and PDVB samples.
Compared with PDVB, PCP-BINAPO shows additional new
bands at ca. 1111, 1201, and 1680 cmꢀ1, which are attributed
to PQO, C–N, and CQO, respectively.11 These results further
demonstrate the successful functionalization of BINAPO groups
in the sample.
To determine the chirality of the PCP-BINAP sample, the
enantiomers of BINAP were used as starting materials. As a
result, both PCP-(S)-BINAP and PCP-(R)-BINAP were
obtained. Fig. 1F shows (circular dichroism) CD spectra of
PCP-(S)-BINAP and PCP-(R)-BINAP samples. As expected,
both show mirror signals at ca. 240, 275 and 335 nm. In
addition, UV-visible spectroscopy shows that the PCP-BINAP
sample has adsorption peaks at 240, 275 and 335 nm (Fig. S6,
ESIw), suggesting that the mirror CD peaks associated PCP-
(S)-BINAP and PCP-(R)-BINAP are not artificial (Fig. S7 and
S8, ESIw).12 As observed from the N2 sorption isotherm, SEM
image, NMR, IR, and CD results (Fig. 1), it is concluded that
mesoporous chiral PCP-BINAP was successfully synthesised.
To evaluate the efficiency of the mesoporous PCP-BINAP
with chiral BINAP ligands, the asymmetric hydrogenation of
methyl methacrylate as a model was chosen. Notably, the
ruthenium species [RuCl2(benzene)]2 was effectively coordi-
nated with PCP-BINAP in methanol at room temperature for
2 h, as evidenced by an obvious shift in UV-visible spectra
between PCP-BINAP and Ru/PCP-BINAP (Fig. S9, ESIw).
Interestingly, a number of different b-keto esters could be
completely converted into the corresponding chiral alcohols
on Ru/PCP-BINAP with very high enantioselectivity in the
range of 94–99% under a very high S/C (substrate/catalyst)
ratio of 2000 (Table 1, entries 1–8). For example, in the hydro-
genation of methyl acetoacetate, the catalyst shows a full conver-
sion with 94.6% ee (entry 1). Moreover, even if the S/C ratio was
increased to 5000, methyl acetoacetate could still be completely
transformed into methyl-3-hydroxybutyrate with 90.1% ee
(entry 10). Heterogeneous asymmetric catalysts with such high
enantioselectivities at very high S/C ratios (e.g. 5000) have not
been reported yet.10,14 The high enantioselectivity could be attrib-
uted to the following unique features of the catalyst: (1) chiral
BINAP ligands are incorporated into the polymer backbones
rather than grafted onto the surface of the supports, which offers
a more uniform distribution for catalytically active sites;13 (2) the
ruthenium species could be effectively coordinated with chiral
BINAP ligands in the PCP-BINAP sample, which is quite similar
to the case in homogeneous catalysts. In contrast, the hydroxyl
groups in silica-based heterogeneous catalysts strongly influence
the coordination of ruthenium species with BINAP ligands.10
Moreover, the effect of various parameters (e.g. Ru/BINAP molar
ratio, the amount of solvent, H2 pressure and S/C ratio) on ee
values have been systemically investigated (Fig. S10–S13, ESIw).
Furthermore, it is observed that Ru/PCP-BINAP has extra-
ordinary recyclability in the hydrogenation of methyl acetoacetate
(Table S3, Fig. S14, ESIw). Very importantly, the Ru/PCP-BINAP
catalyst is not only limited to the use of asymmetric hydrogenation
of b-keto esters. Other reactions, such as asymmetric isomeriza-
tion, transfer hydrogenation of ketones, and hydrogenation of
olefins, could be also applied.7,14
Fig. 1 (A) N2 isotherms, (B) SEM image, (C) 13C MAS NMR, (D)
31P MAS NMR, (E) IR spectra, and (F) CD spectra of (a) PCP-
BINAPO, (b) PCP-(R)-BINAP (abbreviation: PCP-BINAP), (c)
PDVB, (d) PCP-(S)-BINAP. *Side bands. (Line b in A has been offset
by 200 cm3 gꢀ1 along the vertical axis for clarity).
c
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Table 1 Asymmetric hydrogenation of b-keto esters catalyzed by Ru/
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PCP-BINAP catalysta
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Conv.c Sel.c
S/C b (%)
(%)
eec
(%)
Entry
R
1
2
3
4
5
6
7
8
R1 = R2 = Me
R1 = Me, R2 = Et
2000 >99.5
2000 >99.5
>99.5 94.6
>99.5 95.1
>99.5 97.7
>99.5 94.3
>99.5 95.3
>99.5 95.4
>99.5 99.0
>99.5 97.0
>99.5 91.3
>99.5 90.1
>99.5 99.0
>99.5 94.3
>99.5 95.3
R1 = 40-OMe–Ph, R2 = Et 2000 >99.5
R1 = CH2Cl, R2 = Me
R1 = Pr, R2 = Me
2000 >99.5
2000 >99.5
2000 >99.5
2000 >99.5
2000 >99.5
3000 >99.5
5000 >99.5
2000 >99.5
2000 >99.5
2000 >99.5
t
R1 = Me, R2 = PhCH2
R1 = Me, R2 = Bu
t
i
R1 = Me, R2 = Pr
9
R1 = R2 = Me
R1 = R2 = Me
R1 = R2 = Me
R1 = R2 = Me
R1 = R2 = Me
10d
11e
12f
13g
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a
The reaction was carried out under a hydrogen pressure of 2 MPa
in 2.0 mL methanol at 50 1C for 20 h (the use of 0.005 mmol of Ru
with BINAP/Ru at 1.01). Molar ratio of substrate to catalyst.
b
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c
d
Determined by GC on a Supelco g-DEX 225 capillary column. The
use of 2.5 mL of methanol. The use of homogeneous Ru/BINAP
e
f
catalyst. Reuse. Recycled 6 times.
g
More importantly, this methodology is not limited to BINAP.
Other chiral ligands such as 1,2-diaminocyclohexane and 1,2-
diphenylethylenediamine groups (Fig. S15 and S16, ESIw)
could also been copolymerized into the mesoporous PCPs as
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coordinated with a ruthenium species gives complete conversion
with an ee value at 94% in asymmetric transfer hydrogenation
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This work is supported by NSFC (U1162201 and 21003107),
State Basic Research Project of China (2009CB623507) and
Fundamental Research Funds for the Central Universities
(2010QNA3035).
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Chem. Commun., 2012, 48, 10505–10507 10507