A superhydrophobic mesostructured silica as a chiral organometallic immobilization platform for heterogeneous asymmetric catalysis

Article abstract of DOI:10.1039/c8cy00648b

Immobilization of molecular catalysts in a superhydrophobic material can efficiently overcome the shortage of low catalytic efficiency in heterogeneous catalysis. In this study, by taking advantage of a superhydrophobic mesostructured silica as a support,

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A super–hydrophobic mesostructured silica as a chiral organometallics
immobilization platform for heterogeneous asymmetric catalysis
Bing Han, Lei Zhao, Yongkang Song, Zhongrui Zhao, Dongfeng Yang, Rui Liu,* Guohua Liu*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
Immobilization of molecule catalysts in a super–hydrophobic material can efficiently overcome the
shortage of low catalytic efficiency in heterogeneous catalysis. In this work, by taking advantage of
a super–hydrophobic mesostructured silica as a support, we incorporate conveniently chiral diamine
10 within its silicate network, constructing two hydrophobic rhodium/diamine– and
ruthenium/diamine–functionalized heterogeneous catalysts. Analyses via solid–state carbon spectra
disclose the well–defined single-site active species in their silicate framework, and that with water
contact angle measurements reflects their highly hydrophobicity nature. Characterizations via
scanning and transmission electron microscopy reveal their monodispersed feature. As presented in
15 the study, the hydrophobic rhodium/diamine–functionalized catalyst promotes greatly the
enantioselective tandem reduction/lactonization of ethyl 2–acylarylcarboxylates to give various
chiral phthalides, whereas the hydrophobic ruthenium/diamine–functionalized catalyst boosts an
efficient asymmetric transfer hydrogenation–dynamic kinetic resolution process for construction of
1,2–distereocentered diethyl α–benzoyl–β–hydroxyphosphonates. As we envisaged, the as–made
20 catalysts with high hydrophobicity and uniformly distributed single–site catalytically active nature
make combinational contributions in their catalytic performances, affording chiral products in high
yields with up to 99% enantioselectivity. Moreover, catalyst can be also recovered easily and
recycled repeatedly, making it an attracting feature in an efficiently organic transformation.
catalytic efficiency. Moreover, in some special cases, for example,
in the case of reaction with water–soluble products, the super–
strong water repellent ability guarantees a timely desorption of
55 products, boosting greatly catalytic performance.6 For the
mesostructured silica–support benefit, large surface area and pore
volume, tunable pore dimension and well-defined pore
arrangement, and high thermal and mechanical stabilities not only
allow an efficient immobilization of chiral organometallic
60 complexes for a maintainable catalytic performance but also
realize a reliable recycling via simple nanofiltration. Despite some
scattered reports made in construction of super–hydrophobic
mesoporous silicas for catalysis,3 exploration of their applications
in an enantioselective reaction has not been explored yet. Thus,
1. Introduction
25 Development of mesostructured silicas as supports to immobilize
chiral organometallic complexes for heterogeneous asymmetric
catalysis has made great achievement recently.1 Especially, some
prominent properties of mesostructured silica, such as confinement
effect and synergistic effect, have produced many superior
30 heterogeneous catalysts to their homogeneous ones, which
complement nicely the drawbacks of heterogeneous catalysis.2
Therefore, utilization of functional mesoporous silica as a support
fabricating highly efficient heterogeneous catalysts represents an
attracting research direction in heterogeneous asymmetric catalysis.
Super–hydrophobic mesostructured silicas3 possess the feature
of super–hydrophobic materials4 and advantage of inorganosilicate
nanoparticles.5 Combining both benefits in the construction of the
super–hydrophobic mesostructured silica–supported molecule
catalysts has great superiority in heterogeneous asymmetric
40 catalysis.6 For the hydrophobic benefit, heterogeneous catalysts
enable a highly efficient mass transfer in support, which can
greatly overcome the disadvantage of slow reaction rate in a
general heterogeneous catalysis. Furthermore, super–hydrophobic
feature also ensures a high dispersion in organic reaction system,
45 which can result in a homo–like catalytic environment and enhance
35
65 fabrication of
a super–hydrophobic mesostructured silica–
supported chiral molecule catalyst with enhanced catalytic activity
and selectivity in an enantioselective reaction is a significant
challenge in heterogeneous asymmetric catalysis.
As efforts aimed at exploration of heterogeneous catalysts for
70 enantioselective reactions,7 in this contribution, we utilize a super–
hydrophobic mesostructured silica as a support, and combine
chiral diamine wihin its silicate network to establish a chiral
diamine–modified mesostructured silica platform. It not only
offers
a practical approach to prepare two hydrophobic
75 heterogeneous catalysts, but also performs two type of efficiently
enantioselective reactions. As demonstrated in this study, the chiral
rhodium/diamine–functionalized heterogeneous catalyst promotes
geatly the enantioselective tandem reduction/lactonization of ethyl
2–acylarylcarboxylates to construct chiral phthalides, and the
80 chiral ruthenium/diamine–functionalized heterogeneous catalyst
Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai
Key Laboratory of Rare Earth Functional Materials, Shanghai Normal
University, Shanghai, 200234, China.
50 E–mail: rliu@shnu.edu.cn; ghliu@shnu.edu.cn, Fax: 86–21–64322511;
Tel: 86–21–64322280.
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enables a asymmetric transfer hydrogenation–dynamic kinetic
resolution process for synthesis of various 1,2–distereocentered
diethyl α–benzoyl–β–hydroxyphosphonates.
in CH2Cl2 to remove homogeneous and unreacted starting
materials, the solid was dried at ambient temperature under
vacuum overnight to afford Ph@Cp*RhArDPEN@MSNs (5)
(0.52 g) light–yellow powder. ICP analysis showed that the Rh–
60 loading was 9.516 mg (0.09239 mmol of Rh) per gram of catalyst.
13C CP/MAS NMR (161.9 MHz): 154.9−125.5 (C of −SiPh2, Ph
and Ar groups), 96.1 (C of Cp ring), 69.2−58.3 (C of –NCHPh, and
of –NCH2– in CTAB molecule), 40.1−25.2 (C of −CH2Ar, and of
–CH2– in CTAB molecule), 20.6−12.5 (C of CH3− in CTAB
2. Experimental
5
2.1. Characterization
Ru and Rh loading amounts in this catalysts were analyzed using
an inductively coupled plasma optical emission spectrometer
(ICP–OES, Varian VISTA−MPX). Fourier transform infrared 65 molecule), 10.7 (C of –CH3 in Cp(CH3)5), 2.2 (C of –CH2Si) ppm.
(FT−IR) spectra were collected on a Nicolet Magna 550
10 spectrometer using a KBr method. Scanning electron microscopy
(SEM) image was obtained using JEOL JSM−6380LV
29Si MAS NMR (79.4 MHz): D1 (δ = −40.2 ppm), D2 (δ = −47.9
ppm), T2 (δ = −61.4 ppm), T3 (δ = −71.1 ppm), Q2 (δ = −94.9 ppm),
Q3 (δ = −105.1), Q4 (δ = −113.4) ppm.
a
microscope operating at 20 kV. Transmission electron microscopy
(TEM) image was performed on a JEOL JEM2010 electron
microscope at an acceleration voltage of 220 kV. Nitrogen
15 adsorption isotherms were measured at 77 K with a Quantachrome
Nova 4000 analyzer. The samples were measured after being
outgassed at 423 K overnight. Pore size distributions were
calculated by using the BJH model. The specific surface areas
(SBET) of samples were determined from the linear parts of BET
20 plots (p/p0 = 0.05−1.00). Solid−state NMR experiments were
explored on a Bruker AVANCE spectrometer at a magnetic field
strength of 9.4 T with 1H frequency of 400.1 MHz, 13C frequency
of 100.5 MHz and 29Si frequency of 79.4 MHz with 4 mm rotor at
two spinning frequency of 5.5 kHz and 8.0 kHz, TPPM decoupling
25 is applied in the during acquisition period. 1H cross
polarization in all solid−state NMR experiments were employed
using a contact time of 2 ms and the pulse lengths of 4μs.
2.3. Catalyst 8 preparation
70 Prepared according to the above general procedure 5 using
(mesityleneRuCl2)2 (7) instead of (Cp*RhCl2)2 (4), the solid was
dried under reduced pressure overnight to afford
Ph@MesityleneRuArDPEN@MSNs (8) light−yellow powder.
ICP analysis showed that the Ru–loading was 4.002 mg (0.03923
75 mmol of Ru) per gram of catalyst. 13C CP/MAS NMR (161.9
MHz): 154.8−125.6 (C of −SiPh2, Ph and Ar groups), 106.1, 102.3
(C of arometic carbons in Mesitylene groups), 64.7−60.1 (C of –
NCHPh, and of –NCH2– in CTAB molecule), 39.3−23.5 (C of
−CH2Ar, and of –CH2– in CTAB molecule), 20.8 (C of –CH3 in
80 Mesitylene(CH3)3), 14.6 (Cof CH3− in CTAB molecule), 1.5 (C of
–CH2Si) ppm. 29Si MAS NMR (79.4 MHz): D1 (δ = −38.5 ppm),
D1 (δ = −43.9 ppm), T2 (δ = −60.1 ppm), T3 (δ = −67.8 ppm), Q2
(δ = −93.6 ppm), Q3 (δ = −101.3 ppm), Q4 (δ = −110.5) ppm.
2.2. Catalyst 5 preparation
2.4. General procedure for the asymmetric synthesis of chiral
85 phthalides
In a typical synthesis, (The first step for the synthesis of 3) 0.40 g
30 (0.27 mmol) of cetyltrimethylammonium bromide (CTAB) was
added to an aqueous solution (180 mL) of NaOH (1.40 mL, 2 M)
at 70 °C. After dissolution of CTAB, 1.136 mL (0.50 mmol) of
mesitylene (TMB) was added to the system. The mixture was
sonicated for one hour to form stable white emulsion.
35 tetraethoxysilane (TOES) (2. 00 mL, 9.0 mmol) and ethyl acetate
(1.60 mL) was then added, and the mixture was stirred for 10
A typical procedure was as follows. Catalyst 5 (21.65 mg, 2.0 μmol
of Rh based on ICP analysis), HCO2Na (1.0 mmol), 2–
acylarylcarboxylates (0.20 mmol) and 3.0 mL of the mixed
solvents (MeOH/H2O, v/v = 2/1) were added in 5 mL flask purged
90 with nitrogen in turn. The mixture was allowed to react at 40 °C
for 5–10 h. During that time, the reaction was monitored constantly
by TLC. After completion of the reaction, the heterogeneous
catalyst was separated via centrifuge (10000 r/minute) for the
recycle experiment. The aqueous solution was extracted by Et2O
95 (3 × 3.0 mL). The combined Et2O was washed with brine twice and
dehydrated with Na2SO4. After the evaporation of Et2O, the residue
was purified by silica gel flash column chromatography to afford
the desired products.
minute. After that, 0.50
g
(1.0 mmol) of (S,S)–4–
(trimethoxysilyl)ethyl)phenylsulfonyl–1,2–
diphenylethylenediamine (1) was added dropwise to the system,
40 the mixture was stirred for another 10 minute. Finally, 2.10 mL
(10.0 mmol) of diphenyldichlorosilane (2) was added to the system.
After being stirred for another two hours at 70 °C, the mixture was
transferred to the autoclaves and kept aging at 100 °C for 24 h.
After cooling to room temperature, the solids were collected by
45 centrifugation and washed repeatedly with excess distilled water.
The surfactant template was removed by refluxing in a solution
(160.0 mg of ammounium nitrate in 250 mL of ethanol) at 60 °C
for 12 h. The solids was filtered and washed with excess water and
ethanol, and dried at ambient temperature under vacuum overnight
50 to afford Ph@ArDPEN@MSNs (3) as a white powder (1.86 g).
(The second step for the synthesis of 5) The collected solids (0.50
g) was suspended in 20.0 mL of dry CH2Cl2, 61.80 mg (0.10 mmol)
of (Cp*RhCl2)2 (4) was added and the resulting mixture was stirred
at 25 °C for 12 h. The mixture was filtered through filter paper and
55 then rinsed with excess CH2Cl2. After Soxhlet extraction for 24 h
2.5. General procedure for the DKR–ATH of α–benzoyl–β–
100 ketophosphonates
A typical procedure was as follows. Catalyst 8 (12.75 mg, 0.50
μmol of Ru based on ICP analysis), α–benzoyl–β–
ketophosphonates (0.10 mmol), and 0.10 mL (20.0 mmol) of
HCOOH–Et3N (5:2), and 3.0 mL of CH2Cl2 were added
105 sequentially to a 10.0 mL round−bottom flask. The mixture was
then stirred at 35 °C for 24–30 h. During this period, the reaction
was monitored constantly by TLC. After completion of the
reaction, the catalyst was separated by centrifugation (10,000 rpm)
for the recycling experiment. The aqueous solution was extracted
110 with ethyl ether (3 × 3.0 mL). The combined ethyl ether extracts
2
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were washed with brine twice and then dehydrated with Na2SO4.
After evaporation of ethyl ether, the residue was purified by silica
gel flash column chromatography to afford the desired products.
of the well–defined single–site Cp*RhArDPEN species because
these chemical shifts are similar to those of its homogeneous
Cp*RhTsDPEN.9 These findings confirmed that immobilization of
chiral Cp*RhArDPEN–functionality within its silicate network
45 could keep the original chemical environment of the corresponding
homogeneous Cp*RhTsDPEN.
3. Results and discussion
5
3.1. Synthesis and structural characterization of the
heterogeneous catalysts
Fig. 1 Solid−state 13C CP/MAS NMR spectra of 3 and catalyst 5.
Scheme 1. Preparation of heterogeneous catalyst 5.
Assembly of chiral catalytically active centers within the network
10 of super–hydrophobic mesostructured silica to produce the
rhodium/diamine–functionalized
heterogeneous
catalyst,
abbreviated as Ph@Cp*RhArDPEN@MSNs
(5)
(Cp*RhArDPEN:8 Cp* = pentamethyl cyclopentadiene and
ArDPEN = (S,S)–4–((trimethoxysilyl)ethyl)phenylsulfonyl–1,2–
15 diphenylethylene–diamine) could be performed via a simple two–
step procedure as outlined in Scheme 1. In first step, three–
component condensation of tetraethoxysilane (TOES), (S,S)–
TsDPEN–siloxane (1) and Ph2SiCl2 (2) through the use of
cetyltrimethylammonium bromide (CTAB) as
a structure–
20 directing template and mesitylene (TMB) as micelle swelling agent 50 Fig. 2 Solid−state 29Si MAS NMR spectra of 3 and catalyst 5.
afforded the chiral ArDPEN–modified Ph@ArDPEN@MSNs (3)
Figure 2 showed the solid–state 29Si MAS NMR spectra 3 and
catalyst 5, which further demonstrated the compositions of their
silicate networks. It was found that both 3 and catalyst 5 had three
groups of typical signals (Q–, T– and D–series), where Q signals
55 were attributed to inorganosilica, T signals were corresponding to
–SiCH2– groups of organosilica, and D signals ascribed to
hydrophobic –SiCPh2 groups of organosilica, respectively. As
compared these values of 5 with those typical ones in the
literature,10 the strong Q3−Q4 signals at −105.1 and −113.4 ppm
60 demonstrated the (HO)Si(OSi)3 and Si(OSi)4 species as its
inorganosilicate wall, whereas the T3 signal at −71.1 ppm indicated
the R–Si(OSi)3 species (R = Cp*RhTsDPEN–linked alkyl–
functionality) as its one part of organosilica. Characteristic D
signals for D1 and D2 at −40.2 and −47.9 ppm confirmed that
65 super–hydrophobic –SiCPh2 groups had been incorporated
successfully into its silicate network.3 These findings elucidated
that the silicate compositions of catalyst 5 was the inorganosilicate
networks of (OH)Si(OSi)3 and Si(OSi)4 with the organosilicate R–
Si(OSi)3 and –SiCPh2 groups as its main part of silica walls.3, 10
as a white powder. In second step, direct complexation of 3 with
(Cp*RhCl2)2 (4), followed by a purified Soxhlet extraction,
provided the pure catalyst 5 as a light–yellow powder (see Fig. S1
25 of ESI).
Well–defined single–site chiral rhodium/diamine active center
incorporated in its hydrophobic silicate network of 5 could be
proven by its solid–state 13C cross–polarization (CP)/magic angle
spinning (MAS) NMR spectroscopy. As shown in Figure 1, the
30 solid–state 13C CP/ MAS NMR spectra of 3 and catalyst 5
produced the strong carbon signals of phenyl groups in the –
SiCPh2 moiety around 136 ppm, which were correspond to the
hydrophobic organosilica. In the part of chiral rhodium/diamine
complex, besides the general carbon signals around 63 ppm and
35 around 130 ppm for the carbon atoms of the –NCHPh groups and
of the –C6H5 groups in ArDPEN moiety, the characteristic peak at
96.1 ppm in the spectrum of 5 ascribed to the carbon atoms of the
Cp* rings while that at 10.7 ppm was attributed to the carbon atom
of the CH3 groups attached to the Cp* ring. These characteristic
40 peaks were absent in the spectrum of 3, suggesting the formation
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In order to demonstrate the morphology, pore structure and
rhodium distribution of catalyst 5, its nitrogen
10 adsorption−desorption isotherms, scanning electron microscopy
(SEM), and transmission electron microscopy (TEM) images were
further investigated. As shown in Figure 3, the nitrogen
adsorption–desorption isotherms exhibited that both 3 and catalyst
5 were mesoporous due to the presence of typical IV characters
15 with an H1 hysteresis loop, which are the similar to the
corresponding pure material.3 As compared their structural
parameters, the catalyst 5 relative to its parent material 3 had a
decrease in the mesopore size (3.66 nm versus 3.69 nm), surface
area (443.2 cm2/g versus 462.1 cm2/g), and pore volume (0.49
20 cm3/g versus 0.51 cm3/g), suggesting the complexation of 3 with
(Cp*RhCl2)2 made the nanopore of catalyst 5 narrow. As shown in
Figure 4, the SEM image revealed that catalyst 5 was composed of
the uniformly dispersed nanoparticules with an average size of
about 80 nm (Figure 4a), where its water contact angle was 130 °C
25 (Figure 4b). The TEM image (Figure 4c) further confirmed its
mesostructure, where the TEM image with a chemical mapping
technique disclosed that the rhodium centers were uniformly
distributed within its silicate network (Figure 4d).
Fig. 3 Nitrogen adsorption−desorption isotherms of of 3 and catalyst 5.
All these structural analyses and characterizations expatiate that
30 a hydrophobic heterogeneous catalyst could be constructed
steadily through the incorporation of chiral rhodium/diamine
active center with the silicate network, where the well–defined
single–site chiral rhodium/diamine−functionality verified by the
13C CP/MAS NMR spectrum, the hydrophobic nature proved by
35 the water contact angle, uniformly dispersed nanoparticules
confirmed by the SEM image, and highly dispersive active centers
indicated at a TEM mapping, would govern its efficiently catalytic
performance discussed below.
3.2. Catalytic performance of the heterogeneous catalysts
40 Chiral
N–sulfonylated
diamine–based
(TsDPEN–based)
organometallic complexes, as a kind of efficient asymmetric
transfer hydrogenation (ATH) catalysts, had been applied
11
extensively in various enantioselective reactions.8,
With the
heterogeneous catalyst 5 in hand, we chose enantioselective
45 tandem reduction/lactonization of ethyl 2–acylarylcarboxylates as
a model reaction to test its catalytic performance.12 According to
the reported method,12a the reduction/lactonization of ethyl 2–(2–
phenylacetyl)benzoate was carried out through the use of 1.0 mol%
of 5 as a catalyst and the HCOONa as a hydrogen source. It was
50 found that this enantioselective reaction could afford the chiral
products of (S)–3–benzylisobenzofuran–1(3H)–one in 99% yield
and 99% ee, which was better than that of its homogeneous
counterpart, Cp*RhTsDPEN12a (Entry 1 vs Entry 2, Table 1).
Notably, such a high level of enantioselectivity was attributed to
55 the well–defined single–site chiral rhodium/diamine species in 5
confirmed by its 13C CP/ MAS NMR. This judgement could be
further proved by a comparison of their XPS investigations. It was
found that both catalyst 5 and its homogeneous Cp*RhTsDPEN
had the similar Rh 3d5/2 electron binding energy (309.2 eV versus
60 309.3 eV) (see Fig. S2 of ESI), further confirming that the well–
defined single–site Cp*RhArDPEN–functionality within its
silicate network was responsible for the highly enantioselective
performance.
5 Fig. 4 (a) SEM image of 5, (b) TEM image of 5, (c) the water contact angle
of 5, and (d) SEM image with a chemical mapping of 5 showing the
distribution of Si (white) and Ru (red).
4
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Catalysis Science & Technology
On the basis of the above efficient catalytic performance in the
enantioselective tandem reduction/lactonization of ethyl 2–(2–
procedure (see experiment part of ESI and Figure S6). Only
difference from the catalyst 5 is that 5' is an inorganosilicate
phenylacetyl)benzoate, a series of aryl–substituted substrates were 35 analogue and its inorganosilicate network has not the hydrophobic
further investigated under the same reaction conditions for the
general applicability of catalyst 5. As shown in Table 1, it was
found that all the tested substrates could be converted into the
corresponding chiral phthalides in high yields with excellent
–SiCPh2 groups. Taking use of this analogue 5' as a parallel
catalyst, we compared its catalytic performance in the
enantioselective tandem reduction/lactonization of ethyl 2–(2–
phenylacetyl)benzoate. The result showed that the reaction
5
enantioselectivities under the same reaction conditions. It was 40 catalyzed by its analogue 5' within 10 h only afforded the
noteworthy that the structural and electronic properties of
corresponding chiral products in 45% yield with 83% ee (Entry 3).
This finding suggested that the absence of hydrophobic –SiCPh2
groups in its silicate network of 5' led to a poor catalytic
performance, indicating the hydrophobic benefit of the designed
10 substituents at the aromatic ring did not affect significantly their
enantioselectivities, where the reactions with various electron–
withdrawing and electron–donating substituents at the aromatic
ring were equally efficient (Entries 4–14). In addition, the other 45 catalyst 5. A direct evidence supports this judgment coming from
substrates, such as ethyl 2–(2–(naphthalen–1–yl)acetyl)benzoate
15 and ethyl 2–(2–(naphthalen–2–yl)acetyl)benzoate could also be
converted into the corresponding chiral phthalides with excellent
enantioselectivities (Entries 15–16).
a kinetic investigation in the enantioselective reaction of ethyl 2–
(2–phenylacetyl)benzoate. As shown in Figure 5, it was found that
the reaction catalysed by 5 resulted in initial activity higher than
that of its homogeneous Cp*RhTsDPEN, and markedly better than
50 that attained with 5' (the initial TOFs within 1 h were 73, 48 and
13 molmol–1h–1, respectively), futher confirming the benefit the
hydrophobic advantage of 5.
Table 1. The enantioselective tandem reduction/lactonization of ethyl 2–
acylarylcarboxylates.a
100
80
20
Entry
Ar (6)
Time (h)
%Yield
%eeb
60
1
Ph (6a)
5
99
97
45
97
94
96
91
93
95
96
93
92
93
91
93
93
99
96c
83d
99
98
94
99
99
99
98
98
99
99
98
98
99
40
2
Ph (6a)
10
10
5
Catalyst 5
Cp*RhArDPEN
Its analogue 5'
20
3
Ph (6a)
4
4–FPh (6b)
0
5
2,4–F2C6H3 (6c)
4–ClPh (6d)
2–ClPh (6e)
4–BrPh (6f)
4–CF3Ph (6g)
3–CF3Ph (6h)
4–MePh (6i)
4–OMePh (6j)
3–OMePh (6k)
3,4–(MeO)2C6H3 (6l)
1–naphthyl (6m)
2–naphthyl (6n)
5
0
2
4
6
8
10
Reaction time (h)
6
5
7
5
Fig. 5 Comparison of the enantioselective reaction of ethyl 2–(2–
55 phenylacetyl)benzoate catalyzed by catalyst 5, its homogeneous
Cp*RhTsDPEN and its analogue 5' (Reactions were carried out using 1.0
mol% of the catalyst at 40 °C).
8
5
9
5
10
11
12
13
14
15
16
5
5
5
5
5
5
5
a Reaction conditions: catalyst 5 (21.65 mg, 2.0 μmol of Rh based on the
ICP analysis), HCO2Na (1.0 mmol), 2–acylarylcarboxylates (0.20 mmol),
and 3.0 mL MeOH/H2O (2:1), reaction temperature (40 °C). b Determined
by chiral HPLC analysis (see Fig. S7 and S11 of ESI). c Data were obtained
Scheme 2. Preparation of heterogeneous catalyst 8.
d
25 using homogeneous Cp*RhTsDPEN as a catalyst. Data were obtained
using its analogue 5' as a catalyst.
60
It was worth mentioning that the super–hydrophobic
mesostructured silica as a chiral organometallics immobilization
platform could be used to construct the other hydrophobic
heterogeneous catalysts for enantioselective reaction. As shown in
Scheme 2, the hydrophobic chiral ruthenium/diamine–
To gain insight into the hydrophobic nature of catalyst 5 and to
investigate the factors affecting catalytic performance,
comparable SiO2–based Cp*RhArDPEN–functionalized
a
30 inorganosilicate analogue 5' as a parallel catalyst was also
synthesized. In this case, 5' was prepared by a two–component
condensation of tetraethoxysilane and 1 following the similar
65 functionalized mesostructured nanoparticles, abbreviated as
Ph@MesityleneRuArDPEN@MSNs (8)
(MesityleneRuArDPEN:11 mesitylene = 1,3,5–trimethylbenzene,
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where
ArDPEN
=
(S,S)–4–
and its ability to retain its catalytic activity and enantioselectivity
((trimethoxysilyl)ethyl)phenylsulfonyl–1,2–diphenylethylene–
after multiple recycles. As we expected, catalyst 5 and 8 could be
diamine), was also prepared from Ph@ArDPEN@MSNs (3) via a 40 recovered through high–speed centrifugation and recycled
similar procedure. In this case, direct complexation of 3 with
(mesityleneRhCl2)2 (7), followed by a Soxhlet extraction, afforded
catalyst 8 as a light–yellow powder (see Fig. S1, S3–S6 of ESI).
repeatedly. As shown in Figure 6, in eighth consecutive reactions,
the recycled catalyst 5 still gave 90% yield and 98% ee in the
enantioselective tandem reduction/lactonization of ethyl 2–(2–
phenylacetyl)benzoate (see Table S1 and Fig. S9 of ESI). Also, the
45 recycled catalyst 8 could be used repeatedly for seven times in the
5
Table 2. The DKR–ATH of α–benzoyl–β–ketophosphonates.a
DKR–ATH
of
diethyl
(2–(benzyloxy)–3–oxo–3–
phenylpropanoyl)phosphonite (see Table S2 and Fig. S10 of ESI).
Entry
Ar (9)
%Yield
%eeb
dr
1
Ph (9a)
99
98
94
96
95
93
92
96
95
94
91
93
99
99
93
96
96
95
99
98
98
95
99
99
99:1
99:1c
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
2
Ph (9a)
3
4–FPh (9b)
4–ClPh (9c)
4–ClPh (9d)
4–CNPh (9e)
4–CO2Me (9f)
4–MePh (9g)
3–MePh (9h)
4–OMePh (9i)
2–naphthyl (9j)
2–thienyl (9k)
4
5
6
7
Fig.
6 Reusability of catalyst 5 in the entioselective tandem
50 reduction/lactonization of ethyl 2–(2–phenylacetyl)benzoate as a substrate.
8
9
Conclusions
10
11
12
In conclusions, by utilizing a super–hydrophobic mesostructured
silica, we develop two rhodium/diamine– and ruthenium/diamine–
functionalized heterogeneous catalysts through the combination
55 chiral organometallic complexes within its silicate networks. As
demonstrated in the present study, both heterogeneous catalysts
exhibit excellent catalytic and enantioselective performance in two
types of asymmetric reactions, where chiral rhodium/diamine–
functionalized catalyst promotes greatly the enantioselective
60 tandem reduction/lactonization of ethyl 2–acylarylcarboxylates to
various chiral phthalides and chiral ruthenium/diamine–
functionalized catalyst boosts an efficient asymmetric transfer
hydrogenation–dynamic kinetic resolution process for
construction of 1,2–distereocentered diethyl α–benzoyl–β–
a Reaction conditions: catalyst 8 (12.75 mg, 0.50 μmol of Ru based on ICP
10 analysis),α–benzoyl–β–ketophosphonates (0.10 mmol), 0.10 mL of
HCOOH–Et3N (5:2), and 3.0 mL of CH2Cl2, reaction temperature (35 °C),
b
reaction time (24–30 h). Determined by chiral HPLC analysis (see Fig.
c
S8 and S11 of ESI).
Data were obtained using homogeneous
MesityleneRuTsDPEN as a catalyst.
15
Having obtained catalyst 8, we then explored a challenging
dynamic kinetic resolution by asymmetric transfer hydrogenation
(a DKR–ATH method13) for construction of 1,2–distereocentered
diethyl α–benzoyl–β–hydroxyphosphonates14 based on those
well–established DKR–ATH process in preparations of various
65 hydroxyphosphonates.
As
designed,
the
combined
20 1,2–distereocentered
α–substituted
β–hydroxy
multifunctionalities of high hydrophobicity, and well–defined
single–site active catalytic nature and uniformly distributed
nanoparticles contribute cooperatively the highly catalytic
performance. Furthermore, the heterogeneous catalysts could be
70 recovered conveniently and reused repeatedly. The study described
here highlights hydrophobic heterogeneous catalysts realize highly
efficient asymmetric reactions.
ketones/esters/amides.15 In this case, we investigated the 8–
catalysed DKR–ATH process using the dynamic reduction of
diethyl (2–(benzyloxy)–3–oxo–3–phenylpropanoyl)phosphonate
as a model substrate. The reaction was carried out with 0.5 mmol%
25 of 8 as a catalyst and the azeotropic mixture of HCO2H–NEt3 (5:2)
as a hydrogen source. The result showed that chiral products of
diethyl
(R,R)–(2–(benzyloxy)–3–hydroxy–3–
phenylpropanoyl)phosphonite in 99% yield with high levels of
stereoselectivity (99:1 dr and 99% ee) could be obtained, which
30 was comparable to that attained with its homogeneous counterpart,
MesityleneRuTsDPEN14 (Entry 1 vs Entry 2, Table 2). Similarly,
a series of aryl–substituted substrates could be converted into the
corresponding chiral products in high yields with high levels of
stereoselectivity under the same reaction conditions, suggesting its
35 general applicability.
Acknowledgements
We are grateful to China National Natural Science Foundation
75 (21672149), and Ministry of Education of China (PCSIRT–IRT–
16R49) for financial support.
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Graphical Abstract：
A super–hydrophobic mesostructured silica as a chiral organometallics
immobilization platform for heterogeneous asymmetric catalysis
Bing Han, Lei Zhao, Yongkang Song, Zhongrui Zhao, Dongfeng Yang, Rui Liu* and Guohua Liu*
Super-hydrophobic mesostructured silica-supported molecule catalysts are developed and their
applications in enantioselective organic transformation are investigated.