Conjugated microporous polymers with chiral BINAP ligand built-in as efficient catalysts for asymmetric hydrogenation

Article abstract of DOI:10.1039/c5cy00038f

A series of chiral conjugated microporous polymers (CMPs) based on the chiral (R)-BINAP ligand (BINAP-CMPs) were synthesized with tunable BET surface areas. These solid catalysts show high activities and enantioselectivities for the asymmetric hydrogenation of β-keto esters after coordination with ruthenium species. Moreover, CMPs can realize spatial isolation. Through preventing the formation of dimers and trimers, BINAP-CMPs show much higher activity than BINAP for the Ir-catalyzed asymmetric hydrogenation of quinaldine.

Full text of DOI:10.1039/c5cy00038f

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Conjugated microporous polymers with chiral
BINAP ligand built-in as efficient catalysts for
asymmetric hydrogenation†
Cite this: DOI: 10.1039/c5cy00038f
Received 9th January 2015,
Accepted 27th February 2015
Xu Wang, Sheng-mei Lu, Jun Li, Yan Liu* and Can Li*
DOI: 10.1039/c5cy00038f
www.rsc.org/catalysis
A series of chiral conjugated microporous polymers (CMPs) based
on the chiral (R)-BINAP ligand (BINAP-CMPs) were synthesized
with tunable BET surface areas. These solid catalysts show high
exposed, unlike the common mesoporous organosilicas
(PMOs), which always have some of the functional groups
1
2
buried in the pore wall. On the other hand, compared with
crystalline COFs and MOFs, amorphous CMPs can be synthe-
sized more easily by linking the appropriate organic building
units to achieve functionalization. Since the pioneering work
activities
and
enantioselectivities
for
the
asymmetric
hydrogenation of β-keto esters after coordination with ruthenium
species. Moreover, CMPs can realize spatial isolation. Through
preventing the formation of dimers and trimers, BINAP-CMPs
show much higher activity than BINAP for the Ir-catalyzed asym-
metric hydrogenation of quinaldine.
1
0
on CMPs in 2007 by Cooper, various chemical reactions,
building blocks and synthetic methods have been developed
for synthesizing CMPs with different structures and specific
properties. Consequently, CMPs have shown many potential
applications in the fields of gas storage, molecular separa-
Asymmetric catalysis has been developed as a powerful
strategy to obtain organic molecules in their optically active
forms. Compared to the significant developments in
13
tion, light harvesting, supercapacitive energy storage, etc.
Moreover, CMPs functionalized with catalytic units can be
regarded as heterogeneous catalysts. For example, Jiang and
co-workers reported the synthesis of CMP-type iron porphyrin
homogeneous
asymmetric
catalysis,
heterogeneous
asymmetric catalysis has received less attention and has
developed slowly, in spite of its potential practicality. Actually,
in most of the heterogeneous asymmetric systems, the
catalytic units are normally distributed in the pore structures
(
FeP–CMP) networks via a Suzuki–Miyaura coupling reac-
14
tion. The FeP–CMP showed high activity and selectivity as a
heterogeneous catalyst for the oxidation of sulfides to sulfox-
ide under ambient conditions. Cooper's group developed
bpy-ligated metal–organic CMPs, which can be regarded as
1
,2
of the supports,
which may cause pore blocking, thus
3
leading to a decrease in the selectivities and activities.
Recently, the emergence of porous organic materials,
including porous organic polymers (POPs),
frameworks (MOFs), and covalent organic frameworks
COFs), offers opportunities for developing new strategies in
15
efficient heterogeneous catalysts for reductive amination.
4
–6
metal–organic
The same group also integrated Rose Bengal dye into the
skeleton of CMPs. The resulting polymers showed high activ-
ity for heterogeneous photocatalytic aza-Henry reactions for a
7
8
(
heterogeneous asymmetric catalysis.
16
wide range of substrates. Deng's group synthesized a class
Conjugated microporous polymers (CMPs) are new kinds
of POPs, which have captured great interest. Apart from the
advantages of common POPs, such as large surface areas and
high stability, the porous structure and surface area of CMPs
can be finely controlled at molecular level through the rigid
of metal-functionalized CMPs with salen–Co/Al complexes
and 1,3,5-triethynylbenzene, which showed excellent activity
for the conversion of propylene oxide to propylene carbonate
17
under mild experimental conditions. For other catalytic
CMPs, see ref. 18. Despite these important advances, how-
ever, to the best of our knowledge, applying chiral CMPs as
heterogeneous catalysts has not been reported. We
envisioned that chiral organometallic catalysts could be intro-
duced into the structure of CMPs. Not only do these chiral
CMPs act as heterogeneous catalysts with fine-tunable struc-
tures, but also the connatural topological structure can pre-
vent the formation of dimers and trimers, which are regarded
as deactivated species in some cases (Scheme 1).
9
–11
node–strut topology of the monomer structure.
Moreover,
the functional groups embedded on CMPs can be fully
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian 116023, China. E-mail: yanliu503@dicp.ac.cn,
canli@dicp.ac.cn
†
Electronic supplementary information (ESI) available: General remarks,
experimental procedure, physical and spectroscopic data for new compounds,
and NMR spectra. See DOI: 10.1039/c5cy00038f
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Scheme 2 Synthetic route for the BINAP-CMPs.
materials. Different optimal conditions were found for the
different substituted alkynes coupling with IJR)-4,4′-dibromo
BINAPO. After repeated rinsing with chloroform, water, meth-
anol and acetone, the CMPs were rigorously washed by
Soxhlet extraction with methanol to remove any unreacted
monomer or catalyst residues, and then dried at 50 °C for 24
hours under vacuum. These CMPs are insoluble in water and
common organic solvents due to their cross-linked networks.
TGA results showed that the decomposition of these frame-
work starts at 300 °C (Fig. S5, ESI†).
Scheme 1 The design of chiral CMPs.
Herein, we report the synthesis of four robust chiral CMPs
based on the chiral (R)-BINAP ligand (BINAP-CMPs). These
solids could be utilized as recyclable heterogeneous chiral
catalysts, after being post-modified with Ru ions, for the
asymmetric hydrogenation of β-ketoesters with up to 99% ee.
Moreover, we also demonstrate that CMPs can realize spatial
isolation, thus preventing the formation of dimers and tri-
mers. As a result, the BINAP-CMPs show higher activity than
BINAP for the Ir-catalyzed asymmetric hydrogenation of
quinaldine.
The surface properties of the BINAPO-CMPs were investi-
gated by nitrogen adsorption/desorption experiments at 77 K
(Fig. 1A). The Brunauer–Emmett–Teller surface areas ranged
2
−1
2
−1
from 391 m
g
to 509 m
g
(Table 1). By comparing
BINAPO-CMP-S (monomer 2) and BINAPO-CMP-M (monomer
3), we found that the CMP with a longer strut length had a
2
−1
2
−1
2
,2′-BisIJdiphenylphosphino)-1,1′-binaphthalene
(BINAP)
higher BET surface area (391 m g vs. 407 m g ). Similar
results were found for BINAPO-CMP-3D-1 (monomer 4) and
with axial chirality was chosen as the catalytic unit for the
synthesis of the CMPs because its backbone can be modified
by functional groups to connect each other by linkers. More-
over, its linear molecular structure will reduce the possibility
of any potential space-efficient packing, which will benefit
the construction of polymer porosity. Inspired by Cooper's
pioneering work where the micropore size and surface area
of CMPs can be tuned by varying the strut length of the
2
−1
2
−1
BINAPO-CMP-3D-2 (monomer 5) (474 m g vs. 509 m g ).
Correspondingly, the pore sizes were mainly distributed
around 1–2 nm for BINAPO-CMP-S and BINAPO-CMP-M. For
BINAPO-CMP-3D-1 and BINAPO-CMP-3D-2, a distribution at
about 0.6 nm was found as well as a distribution around 1–2
nm (Fig. 1B).
All polymers in the series were characterized at the molec-
9
–11
1
13
linker,
linkers (alkynes) with different lengths were used
ular level by H– C CP/MAS NMR spectroscopy. As shown in
(2, 3, 4, and 5). Thus, we could synthesize a series of chiral
CMPs with tunable BET surface areas and pore size distribu-
tions, and investigate the influence of the material properties
(such as the BET surface area and pore size distribution) on
the activity and selectivity.
The preparation of the BINAPO-CMPs was carried out by a
Sonogashira–Hagihara reaction of IJR)-4,4′-dibromo BINAPO
(1) with alkynes of different lengths (Scheme 2). Such poly-
condensation reactions led to four inherent porous polymers
with built-in chiral BINAP platforms. We examined the cross-
coupling polycondensation reactions under different condi-
tions, including base, solvents, temperature, etc., with the
aim of achieving maximum surface areas for the resulting
Fig. 1 (A) Nitrogen adsorption isotherms measured at 77 K for the
BINAPO-CMPs. (B) Pore size distribution (PSD) for the BINAPO-CMPs,
calculated by non-local density functional theory (NLDFT).
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Table 1 Physical properties of the BINAPO-CMPs
a
2
−1
b
2
−1
c
3
−1
b
3
−1
Polymer
S
BET (m g
)
S
micro (m g
)
V
total (cm g
)
Vmicro (cm g )
BINAPO-CMP-S
391
407
474
509
146
261
298
334
0.56
0.36
1.00
0.88
0.07
0.13
0.15
0.17
BINAPO-CMP-M
BINAPO-CMP-3D-1
BINAPO-CMP-3D-2
a
b
2
Surface area calculated from the N adsorption isotherm using the Brunauer–Emmett–Teller method. Micropore surface area and micropore
c
0
volume calculated using the t-plot method based on the Halsey thickness equation. Total pore volume at P/P = 0.99.
Fig. 2A, the region ranging from about 110 ppm to 150 ppm
corresponds to the aromatic groups in the networks. The
peaks emerging at about 90 ppm represent the alkynyl groups
in the polymers. The results prove the successful synthesis of
the chiral CMP networks. The FT-IR spectra, in which the
multi-substituted aromatic alkyne shows sharp peaks at
around 3300 cm which are weakened in the networks, also
confirm the high copolymerization of dibromo BINAPO with
multi-substituted aromatic alkynes in the synthesized poly-
mers (Fig. 2C).
most of the BINAPO groups are exposed in the pore as we
proposed.
With the BINAP-CMPs in hand, to evaluate their catalytic
activity as robust heterogeneous catalysts, we accordingly
chose the asymmetric hydrogenation reaction of methyl
1
9
acetoacetate as a model reaction. Under the same reaction
conditions, all the BINAP-CMPs combined with ijRuIJbenzene)-
Cl ] were tested as chiral catalysts. The results are shown in
−
1
2
2
Table 2. We found that all the CMPs showed similar conver-
sions ranging from 24% to 30% in 2 hours, but varying ee
values. The Ru/BINAP-CMPs-3D-2 catalyst achieved a much
better ee value than the others. In view of the structural data
of BINAP-CMPs-3D-2 (Table S2, ESI†), the large BET surface
area and micropore volume may play important roles in the
better activity and enantioselectivity. It is understandable that
the CMPs with larger BET surface areas and micropore vol-
umes may benefit from the diffusion of the substrate and
product, which makes it easier to access the active center and
then improve the reaction performance.
Additionally, a number of different substituted β-keto
esters could be completely converted into the corresponding
chiral alcohols on Ru/BINAP-CMP-3D-2 with 0.1 mol% cata-
lyst loading (Table 3). The structures of the β-keto esters sig-
nificantly affect the catalytic performance of Ru/BINAP-CMP-
Next, we needed to reduce the BINAPO group embedded
in the CMPs into BINAP for the asymmetric catalysis applica-
tion. The BINAPO-CMPs were deoxygenated using the
1
2
reported procedure and the whole process was monitored
3
1
31
by P MAS NMR spectroscopy (Fig. 2B). In the P MAS NMR
spectrum, the BINAPO-CMP shows a broad peak centered at
about 25.1 ppm assigned to phosphorus in phosphine oxide.
After the reduction, a clean signal at −15.8 ppm, which can
be assigned to the reduced phosphine, shows the complete
reduction of BINAPO to BINAP. This result also suggests that
2
3D-2. Keto esters bearing larger substituents in the R posi-
tion result in a higher enantioselectivity than those bearing
smaller ones. For the asymmetric hydrogenation of tert-butyl
3
-oxobutanoate (Table 3, entry 4), the ee value could reach as
high as 99%. We also found that with appropriate steric hin-
drance in the R position, the ee value could reach as high as
2% (Table 3, entry 3). The above results suggest that
1
9
Ru/BINAP-CMP-3D-2 is an efficient catalyst for asymmetric
hydrogenation. Upon completion of the reaction, the solid
Ru/BINAP-CMP-3D-2 catalyst could be readily recovered by
centrifugation and a regular filter. The colorless filtrate from
the asymmetric hydrogenation of methyl acetoacetate did not
afford any additional product, suggesting the heterogeneous
nature of the reaction system. After washing with THF and
heating under vacuum, the solid catalyst was reused for the
next cycle. We did not observe any significant deterioration
in the activity for the recovered catalyst even after three
cycles. However, the enantioselectivity decreased from 94% to
90%, (Table S1, ESI†) which may be due to the unknown
changing of the microenvironment of the activity center
when processing.
1
13
Fig. 2 (A) Solid-state H– C CP/MAS NMR spectra for the BINAPO-
3
1
CMPs, (B) solid-state P CP/MAS NMR spectra for BINAPO-CMP-3D-2
and BINAP-CMP-3D-2, (C) FT-IR spectra of IJR)-4,4-dibromo-BINAPO 1,
1
,3,5,7-tetrakisIJ4-ethynylphenyl)adamantine 5 and the BINAPO-CMPs.
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Table 2 Reactivity comparison experiment of the synthesized BINAP-CMPsa
b
b
Entry
Catalyst
Conversion (%)
ee (%)
1
2
3
4
Ru/BINAP-CMP-S
26
27
24
30
67
71
72
91
Ru/BINAP-CMP-M
Ru/BINAP-CMP-3D-1
Ru/BINAP-CMP-3D-2
a
b
Reaction conditions: 5 mmol of methyl acetoacetate in 2 mL of MeOH, 0.0025 mmol ijRuIJbenzene)Cl
2 2
] , 5 MPa H
2
, 52 °C for 2 h. The
conversion and ee values were determined by GC on a Supelco γ-DEX 225 capillary column.
a
Table 3 Asymmetric hydrogenation of β-keto esters catalyzed by the Ru/BINAP-CMP-3D-2 catalyst
b
b
Entry
R
Conversion (%)
ee (%)
1
2
3
4
5
6
7
R
1
R
1
R
1
R
1
R
1
R
1
R
1
R
1
R
1
= Me, R
= Me, R
2
2
= Me
= Et
99
99
99
99
99
99
99
99
99
94
93
92
99
95
94
90
90
99
= ClCH
= Me, R
= Me, R
= Me, R
2
, R
= Bu
2
= Me
t
2
2
2
= CH
2
-Ph
i
= Pr
= Et, R
= Pr, R
= Me, R = Me
2
2
= Me
2
= Me
c
i
8
d
9
a
b
Reaction conditions: 5 mmol of β-keto esters in 2 mL of MeOH, 0.0025 mmol ijRuIJbenzene)Cl
2
]
2
, 5 MPa H
2
, 52 °C for 24 h. The conversion
c
d
and ee values were determined by GC on a Supelco γ-DEX 225 capillary column. Reaction time = 48 h. The use of homogeneous Ru/BINAP
catalyst.
In Ir-catalyzed asymmetric hydrogenation, irreversible for- Conclusions
mation of inactive dimers and trimers through hydride-
bridged bonds is regarded as the deactivation factor, which
In summary, we have synthesized a series of BINAP-CMPs
with tunable BET surface areas, which show high activities
and enantioselectivities for the asymmetric hydrogenation of
β-keto esters after coordination with ruthenium species.
Moreover, we found that the catalytic performance of the
CMPs was related to the structure properties of the CMPs.
The BINAP-CMPs were also used for the iridium-catalysed
asymmetric reactions of quinaldine and they show much
higher activity than BINAP because they prevent the forma-
tion of dimers and trimers.
2
0
results in high catalyst loadings. For example, in the asym-
metric hydrogenation of quinaldine, Ir-BINAPs are always
2
1
used as catalysts with high loadings. Encouraged by the
natural structure of BINAP-CMPs, which should inhibit the
formation of dimers and trimers, we applied the BINAP-CMPs
to the Ir-catalyzed asymmetric hydrogenation of quinaldine. In
the preliminary investigations for this reaction, the BINAP-
CMPs combined with Ir showed much higher activity than the
homogeneous BINAP/Ir catalytic system (Table 4).
a
Table 4 Asymmetric hydrogenation of quinaldine by the Ir/BINAP-CMP-3D-2 catalyst
b
c
Entry
Ligand
Conversion (%)
ee (%)
1
2
BINAP
BINAP-CMP-3D-2
23
99
71
70
a
Reaction conditions: 0.25 mmol of quinaldine in 3 mL of CH
Determined by H NMR analysis of the crude product. Determined by HPLC analysis with a Chiralpak OJ-H column.
2
Cl
2
, 0.00125 mmol ijIrIJCOD)Cl]
2
, 0.0125 mmol I
2
2
, 4 MPa H , 25 °C for 2 h.
b
1
c
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