Ru coordinated with BINAP in knitting aryl network polymers for heterogeneous asymmetric hydrogenation of methyl acetoacetate

Article abstract of DOI:10.1039/c5ra23597a

A facile method for the preparation of heterogeneous asymmetric hydrogenation catalysts was presented. BINAP was knitted with aryl compounds using formaldehyde dimethyl acetal (FDA) as a cross-linker by Friedel-Crafts reaction without any pre-modification. The prepared catalysts showed different catalytic activities, and excellent recyclablilty results could be achieved in asymmetric hydrogenation.

Full text of DOI:10.1039/c5ra23597a

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Ru coordinated with BINAP in knitting aryl network
polymers for heterogeneous asymmetric
hydrogenation of methyl acetoacetate†
a
Cite this: RSC Adv., 2016, 6, 28447
Tao Wang,ac Yuan Lyu, Xingkun Chen,ac Cunyao Li,ac Miao Jiang,a Xiangen Songa
*
Received 9th November 2015
Accepted 11th March 2016
ab
*
and Yunjie Ding
DOI: 10.1039/c5ra23597a
www.rsc.org/advances
A facile method for the preparation of heterogeneous asymmetric asymmetric hydrogenation of methyl acetoacetate.6 According
hydrogenation catalysts was presented. BINAP was knitted with aryl to previous works, complicated modications of BINAP were
compounds using formaldehyde dimethyl acetal (FDA) as a cross- generally essential to attach this chiral ligand to a polymer,
linker by Friedel–Crafts reaction without any pre-modification. The which made the traditional strategies of preparing chiral
prepared catalysts showed different catalytic activities, and excellent organic polymers costly and time consuming. Inspired by Tan's
recyclablilty results could be achieved in asymmetric hydrogenation.
work, who used a simple Friedel–Cras reaction of a low-cost
cross-linker (FDA) with aromatic compounds to produce
microporous polymers, a facile strategy was presented here to
address this problem.7 BINAP was directly cross-linked with
aromatic compounds, such as benzene, biphenyl and 1,3,5-tri-
phenylbenzene, followed by loading a ruthenium species, and
methyl acetoacetate was selected as a classic substrate to test
asymmetric hydrogenation activities of these varied polymer-
supported catalysts. To the best of our knowledge, this is the
rst time to incorporate the un-modied BINAP in the polymer
backbone, and apply it for asymmetric hydrogenation.
During the preparation of materials, FDA was used to link
(S)-BINAP and aromatic compounds. Typically, (S)-BINAP (0.4
mmol, 0.249 g), benzene (0.781 g, 10 mmol), and FDA (2.283 g,
30 mmol) were dissolved in 20 mL dichloromethane and reac-
ted to form BINAP–benzene cross-linked aromatic polymers
(KAP-1) under the catalysis of FeCl3 (4.486 g, 30 mmol)
(Scheme 1). Using biphenyl and 1,3,5-triphenylbenzene to
Asymmetric hydrogenation catalysis is an important method for
the synthesis of pharmaceuticals and other ne chemicals.
Although homogeneous catalysts have good reactivity and high
TON (turnover number), the losses of noble metals and chiral
ligands have limited their further application in industry.1 Thus
considerable effort has been devoted to look for strategies to
develop heterogeneous asymmetric catalysts, such as covalently
and non-covalently immobilized catalysts.2 As one of the most
successful chiral ligands, BINAP [2,20-bis(diphenylphosphino)-
1,10-binaphthyl] is frequently used to explore new ways of
developing heterogeneous asymmetric catalysis.3 Wherein,
chiral BINAP that was immobilized in organic polymers, has
exhibited extraordinary stability and activity.4 Chan et al.
synthesized several soluble polymer-supported catalysts by the
polycondensation of the amine group substituted BINAP, ter-
ephthaloyl chloride and several diols, such as (2S,4S)-pentane-
diol, poly-(ethylene glycol) or MEO-PEG-OH (Mw ¼ 5000) for
asymmetric hydrogenation, which showed
a comparable
activity to corresponding homogeneous catalyst.5 Xiao et al.
presented a chiral cross-linked mesoporous polymer by copo-
lymerization of the diacryloylamino substituted BINAP with
divinylbenzene. Aer loading a ruthenium species, the catalyst
exhibited a high activity and satisfactory recyclability in
aDalian National Laboratory for Clean Energy, Dalian, 116023, P. R. China. E-mail:
dyj@dicp.ac.cn; luyuan@dicp.ac.cn; Fax: +86-411-84379143; Tel: +86-411-84379143
bState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian, 116023, P. R. China
cUniversity of Chinese Academy of Sciences, Beijing 100039, P. R. China
† Electronic supplementary information (ESI) available: Experimental procedures,
characterization data and activities. See DOI: 10.1039/c5ra23597a
Scheme 1 The synthesis of network structures.
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replace benzene comonomer, BINAP–biphenyl based knitted another signal at d ¼ 23.5 ppm due to phosphorous oxide. The
aromatic polymers (KAP-2) and BINAP–1,3,5-triphenylbenzene other two materials, KAP-2 and KAP-3, also give two resonance
based knitted aromatic polymers (KAP-3) were obtained peaks at the same location approximately. Both signals are
according to the above strategies, respectively.
strong and also conrm the presence of element P in the
The pore properties of three materials are analysed by N2 samples. Since the Friedel–Cras reaction was catalysed by
physisorption analysis (Fig. 2A). The polymer materials are Lewis acid FeCl3, which could accelerate the oxidation of BINAP
insoluble and porous, having a large surface area at 1100–1200 even though only trace oxygen remained in the reaction system.8
m2 gꢀ1 (Table S1, ESI†). However, the pore volume of KAP-1 is as As the gure shows, the molar ratio of the unprotected phos-
high as 1.941 cm3 gꢀ1, which is much higher than the pore phine and phosphorous oxide in the materials decreased in the
volume sum of KAP-2 and KAP-3. A sharp N2 uptake at low order KAP-3 > KAP-1 > KAP-2. To test the catalytic properties of
relative pressure (P/P0 < 0.001) of KAP-2 and KAP-3 was observed materials obtained by this method, we did not manage to
due to the lling of micropores, which was conrmed by the reduce the phosphorous oxide in the polymers articially. As
pore size distribution curves calculated by DFT method a representative catalyst, the 31P MAS NMR spectrum of Ru/KAP-
(Fig. S1†). KAP-2 and KAP-3 were basically all made of micro- 1 shows a relatively broad peak at 49.6 ppm, which can be
pores. This was in sharp contrast to the pore size distribution of assigned to the unprotected phosphine coordinated with Ru
KAP-1, the isotherms of which showed a hysteresis loop at 0.6– species (Fig. S6†).9 It should be noted that the presence of
0.95 (P/P0). The scanning electron microscope (SEM) images of element Fe in the catalyst can produce a magnetic eld inter-
those materials (Fig. S2†) are in good agreement with the results ference in the NMR instrument, which may result in the peak
of N2 sorption isotherms, showing porous knitting networks. broadening. This may be the reason of the peaks at 49.6 ppm
The transmission electron microscope (TEM) images of polymer and 25.9 ppm have partially overlapping.
supported catalysts show that the materials still have abundant
Excellent thermal stability is a crucial requirement for the
pores aer loading with ruthenium species (Fig. S3–S5†). The polymer materials used as catalyst supports. The curves of
EDS on TEM conrms the presence of Ru metal in catalysts, and thermogravimetric analysis (TG) under air (Fig. 1D) show that
its content is generally consistent with the amount of added these materials have superior thermal stability without any
[RuCl2(benzene)]2, which approve that the ruthenium species decomposition at the temperature of 400 ꢁC. Compared with
are highly dispersed in the catalysts. Notably, the particles of Ru aryl-based knitting networks that Tan et al. reported, which
cannot be observed in the high resolution TEM images of the decomposition temperature were before 300 ꢁC, the results
three catalysts.
demonstrate that the BINAP-based aryl networks are even more
The 13C magic angle spinning (MAS) NMR spectrums of stable than aryl-based knitted networks.7 Interestingly, the
three polymers (Fig. 1B) show a broad resonance peaks near 139 curves slightly rise at 350 ꢁC, and the phenomenon might be
and 131 ppm assigned to the substituted aromatic carbon and attributed to the oxidation of unprotected phosphine in the
non-substituted aromatic carbon respectively. And the reso- materials.
nance peak at 20 ppm can be attributed to the carbon in the
The MW dispersity of materials also has a great impact on
methylene linker formed from the Friedel–Cras reaction.7 The their catalytic application. We attempt to determine the char-
31P MAS NMR spectrum of KAP-1 (Fig. 1C) gives a resonance acter by using several means, like gel permeation chromatog-
signal at d ¼ ꢀ14.3 ppm due to the unprotected phosphine and raphy (GPC) and matrix-assisted laser desorption/ionization
time of ight mass spectrometry (MALDI-TOF-MS). However,
the solubility of our polymer materials are very poor, so no
satisfactory results are obtained.
To determine the asymmetric hydrogenation activities of
these series polymer-supported catalysts, we chose methyl ace-
toacetate as a classic substrate. Aer stirred for 24 hours, the
ruthenium precursor, [RuCl2(benzene)]2, was successfully
coordinated with polymers in mixed solution of N,N-dime-
thylformamide and dichloromethane (v : v ¼ 1 : 2) at room
temperature (Fig. S6†). By optimizing the reaction conditions,
like temperature and pressure, Ru/KAP-1 catalyst exhibited
excellent activity (yield > 99.5%, ee ¼ 96.7%) at 80 ꢁC under low
H2 pressure of 0.5 MPa (Table 1, entry 1). Ru/KAP-2 and Ru/KAP-
3 catalysts obtained low product yield (45.1% and 45.7%
respectively) under the same reaction conditions (entries 2 and
3). When the reaction pressure was increased to 2 MPa, all the
catalysts exhibited excellent activity (yield > 99.5%, ee > 94%)
(entries 4–6). It was also notably that the Ru/KAP-1 catalyst
demonstrated a satisfactory activity at very high substrate/
catalyst molar ratios (S/C ¼ 6000) for 10 hours. Different pore
structures may be one of the reasons that these catalysts showed
Fig. 1 (A) N2 isotherms, (B) 13C MAS NMR, (C) 31P MAS NMR, (D) TG
analysis of (a) KAP-1, (b) KAP-2, (c) KAP-3. Asterisks denote spinning
sidebands.
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Table 1 Asymmetric hydrogenation of b-keto esters
This maybe because the catalyst still contains trace amounts of
Lewis acid FeCl3, which may accelerate the side reaction,
although the material have been thoroughly washed during its
preparation process. Meanwhile, the reuse property of Ru/KAP-1
catalyst was preliminary explored, and it could be reused for
four times with the high yield (higher than 99.5%) and a slight
decrease on ee value (from 96.0% to 94.7%) (Fig. 2).
Entry Catalyst
R
S/Cb
Yieldc (%) eec (%)
1d
Ru/KAP-1 R1 ¼ R2 ¼ Me
Ru/KAP-2 R1 ¼ R2 ¼ Me
Ru/KAP-3 R1 ¼ R2 ¼ Me
Ru/KAP-1 R1 ¼ R2 ¼ Me
Ru/KAP-2 R1 ¼ R2 ¼ Me
Ru/KAP-3 R1 ¼ R2 ¼ Me
Ru/KAP-1 R1 ¼ R2 ¼ Me
Ru/KAP-1 R1 ¼ R2 ¼ Me
Ru/KAP-1 R1 ¼ iPr, R2 ¼ Me
Ru/KAP-1 R1 ¼ Me, R2 ¼ et
Ru/KAP-1 R1 ¼ Me, R2 ¼ iPr
1000 >99.5
96.4
95.7
94.8
97.0
94.1
96.8
97.0
97.9
96.4
98.7
94.0
94.8
Conclusions
2d
1000
1000
45.1
45.7
3d
In summary, a series of novel porous polymers (KAP-1, KAP-2,
KAP-3) with no-modied BINAP were successfully synthesized
by Friedel–Cras reaction for the rst time. These polymer-
supported catalysts demonstrated excellent activities and
enantioselectivities on the asymmetric hydrogenation of b-keto
esters. Further efforts are underway with a focus on reduction of
partly oxidized BINAP in knitting networks to improve the
stability of catalysts.
4a
1000 >99.5
1000 >99.5
1000 >99.5
4000 >99.5
6000 >99.5
5a
6a
7a
8e
9a
1000
91.9
10f
11g
12h
1000 >99.5
1000
96.5
90.9
Ru/KAP-1 R1 ¼ Me, R2 ¼ tBu 1000
a
Reaction conditions: 2 mL methanol, initial H2 pressure 2 MPa,
b
0.00086 mmol Ru with BINAP/Ru at 10, 80 ꢁC, 10 h. Molar ratio of
Acknowledgements
c
substrate to catalyst. Determined by GC on a Cyclosil-B capillary
d
e
f
g
column. 0.5 MPa H2. 4 mL methanol. 2 mL ethanol. 2 mL
This work was supported by the National Natural Science
Foundation of China (21303190).
isopropanol. h 2 mL t-butanol.
Notes and references
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as the solvent rather than methanol. Otherwise trans-
esterication reaction could easily take place (entries 9–12).
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