An imidazolium-modified chiral rhodium/diamine-functionalized periodic mesoporous organosilica for asymmetric transfer hydrogenation of α-haloketones and benzils in aqueous medium

Article abstract of DOI:10.1039/c6gc01589a

The use of a hydrophobic, imidazolium-functionalized periodic mesoporous organosilica for immobilization of chiral organometallic complexes as a heterogeneous catalyst is highly desirable as this catalyst can greatly promote an aqueous organic transformation due to its hydrophobic function and phase-transfer feature in an aqueous medium. Herein, by utilizing a three-component co-condensation strategy, we conveniently incorporate 1,2-bis(triethoxysilyl)ethane, (R,R)-4-((trimethoxysilyl)ethyl)phenylsulfonyl-1,2-diphenylethylene-diamine and 1,3-bis(3-(triethoxysilyl)propyl)-1H-imidazol-3-ium iodide within its silicate network, which is coordinated with (Cp?RhCl₂)₂, leading to an imidazolium-modified chiral rhodium/diamine-functionalized periodic mesoporous organosilica. A solid-state carbon spectrum discloses its well-defined chiral rhodium/diamine active species, and its X-ray diffraction; nitrogen adsorption-desorption measurement and transmission electron microscopy images reveal its ordered dimensional-hexagonal mesostructure. As a bifunctional heterogeneous catalyst, this periodic mesoporous organosilica significantly boosts asymmetric transfer hydrogenation of α-haloketones and benzils in water, where the hydrophobic periodic mesoporous organosilica, phase-transfer-featured imidazolium-functionality, and the confined chiral organorhodium catalytic nature are responsible for its catalytic performance. Furthermore, the catalyst can be recovered and recycled seven times without the loss of its catalytic activity, making it an attractive heterogeneous catalyst for asymmetric transfer hydrogenation in an environmentally friendly manner.

Full text of DOI:10.1039/c6gc01589a

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ARTICLE TYPE
An ImidazoliumꢀModified Chiral Rhodium/diamineꢀFunctionalized
Periodic Mesoporous Organosilica for Asymmetric Transfer
Hydrogenation of α–Haloketones and Benzils in Aqueous Medium
Feng Zhou, Xiaoying Hu, Ming Gao, Tanyu Cheng*, 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
Use of
a hydrophobic, imidazoliumꢀfunctionalized periodic mesoporous organosilica for
immobilizations of chiral organometallic complexes as a heterogeneous catalyst is a highly
desirable as this catalyst can promote greatly an aqueous organic transformation due to its
hydrophobic function and phaseꢀtransfer feature in an aqueous medium. Herein, by utilizing a
threeꢀcomponent
coꢀcondensation
strategy,
we
incorporate
conveniently
1,2ꢀ
bis(triethoxysilyl)ethane,
(R,R)ꢀ4ꢀ(trimethoxysilyl)ethyl)phenylsulfonylꢀ1,2ꢀdiphenylethyleneꢀ
diamine and 1,3ꢀbis(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀimidazolꢀ3ꢀium iodide within its silicate network,
which is coordinated with (Cp*RhCl2)2 to lead to an imidazoliumꢀmodified chiral
rhodium/diamineꢀfunctionalized periodic mesoporous organosilica. Solidꢀstate carbon spectrum
discloses its wellꢀdefined chiral rhodium/diamine active species, and its Xꢀray diffraction, nitrogen
adsorption–desorption measurement and transmission electron microscopy image reveal its ordered
dimensionalꢀhexagonal mesostructure. As a bifunctional heterogeneous catalyst, this periodic
mesoporous organosilica boosts significiently asymmetric transfer hydrogenation of α–haloketones
and benzils in water, where the hydrophobic periodic mesoporous organosilica, phaseꢀtransferꢀ
featured imidazoliumꢀfunctionality, and confined chiral organorhodium catalytic nature are
responsible for its catalytic performance. Furthermore, catalyst can be recovered and recycled for
seven times without loss of its catalytic activity, making it an attracting heterogeneous catalyst for
asymmetric transfer hydrogenation in an environmentally friendly manner.
organosilicas (PMOs) as supports for immobilizations of chiral
organometallic complexes have led to many efficient, reusable
heterogeneous catalysts.[3] Prominent feature of these
heterogeneous catalysts possesses PMOꢀsupported organosilicate
networks, which ensures a high reaction rate in an aqueous
reaction system relative to an SiO2ꢀsupported inorganosilicate
networks.[4] This superiority is due to the hydrophobic nature of
PMOꢀsupport, which can concentrate greatly organic substrate
into pore channels and reach a highly catalytic performance.[5] In
particular, some imidazoliumꢀfunctionalized PMOs can offer
additional contributions for catalytic reactions. These
imidazoliumꢀfunctionalities within PMOs can function as a
phaseꢀtransferꢀfeatured catalyst to promote catalytic performance
in an aqueous medium.[6] Although lots of imidazoliumꢀ
functionalized PMOs have been reported,[7] immobilization of
chiral organometallic complex within an imidazoliumꢀ
functionalized PMO for asymmetric catalysis is still not explored
yet. Accordingly, taking advantage of imidazoliumꢀfunctionality,
together with benefit of hydrophobic organosilicate network, it is
reasonable to expect that an imidazoliumꢀmodified chiral
rhodium/diamineꢀfunctionalized PMO has an efficiently
enantioselective reaction in an aqueous medium.
1. Introduction
Asymmetric transfer hydrogenation as
a
powerful
methodology has been studied extensively in enantioselective
reduction of ketones and imines, where chiral Nꢀsulfonylated
diamineꢀbased organometallic complexes, such as η5ꢀCp*Mꢀ
series (Cp* = pentamethyl cyclopentadiene) and η6ꢀareneꢀMꢀ
series (η6ꢀarene = aromatic ring; M = Ru, Rh and Ir), are wellꢀ
known as efficiently homogeneous catalysts.[1] Recently, some
of them have been expanded successfully to various
enantioselective reactions in an environmentally friendly
medium.[2] In spite of great efforts have been made on
asymmetric transfer hydrogenation, contributions from
industrial fields are much lower than expected. Main
limitations are due to the expensive transitionꢀmetal recycling
and product contamination arising from metal leaching.
Therefore, exploration of an immobilization strategy to
overcome these limitations and development of a practical
approach to realize an green catalysis are still an unmet
challenge in asymmetric catalysis.
Recent developments through use of periodic mesoporous
We are interesting various heterogeneous catalysts for
green asymmetric catalysis.[8] Recently, we found that
hydrophobic PMOꢀsupported organosilicate network could
enhance reaction rate in asymmetric reactions.[8a–e]
Meanwhile, we also found that some imidazoliumꢀ
Key Laboratory of Resource Chemistry of Ministry of Education,
Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai
Normal University, Shanghai, 200234, China. Eꢀmail: ghliu@shnu.edu.cn,
tycheng@shnu.edu.cn, Fax: 86ꢀ21ꢀ64322511;Tel: 86ꢀ21ꢀ64322280.
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functionality
performance.[8f–g] Consideration both, in this contribution, we
incorporate the imidazoliumꢀfunctionality within
in
materials
could
boost
catalytic
ambient temperature. The resulting mixture was stirred for 12 h.
The mixture was filtered through filter paper and then rinsed with
excess water and CH2Cl2. After Soxhlet extraction for 24 h in
CH2Cl2 to remove homogeneous and unreacted starting materials,
the solid was dried at ambient temperature under vacuum
overnight to afford catalyst 5 (1.02 g) as a light−yellow powder.
ICP analysis showed that the Rh loading was 9.64 mg (0.093
mmol) per gram of catalyst. IR (KBr) cm−1: 3430.7 (s), 3064.5
(w), 2977.7 (w), 2930.4(w), 2898.4(w), 1630.5 (m), 1566.5 (w),
1496.2 (w), 1464.2 (w), 1416.8 (w), 1329.9 (w), 1052.2 (s),
1194.2 (s), 1098.5 (s), 925.6 (m), 789.9 (m), 695.3 (m), 576.3 (m),
441.9 (m). 13C CP/MAS NMR (161.9 MHz): 147.1, 136.3, 128.1
(CH of imidazolium, and C of Ph or Ar), 94.7 (C of Cp ring),
a
hydrophobic PMO network, developing an imidazoliumꢀ
modified chiral rhodium/diamineꢀfunctionalized PMO. As we
envisaged, this PMO material possesses bifunctionality of
hydrophobic function and imidazoliumꢀphaseꢀtransfered
catalytic function in an aqueous reaction condition. As a
heterogeneous catalyst, it displays excellent catalytic activity
and high enantioselectivity in asymmetric transfer
hydrogenation of α–haloketones and benzils in water, where
its bifunctionality together with chiral rhodium/diamine
catalytic nature promote synergistically its catalytic
performance.
70.8−75.2
(−NCHPh−),
59.1
(−OCH2CH3),
51.9
(−NCH2CH2CH2Si−), 20.1−31.7 (−NCH2CH2CH2Si−, and
−CH2Ar), 17.9 (−OCH2CH3), 9.6 (CpCH3), 12.3−0.5 (−CH2Si)
ppm. 29Si MASNMR (79.4 MHz): T1 (δ = −48.2 ppm), T2 (δ =
−57.4 ppm), T3 (δ = −65.5 ppm.
2. Experimental
2.1. Characterization
Rh loading amounts in the catalyst were analyzed using an
inductively coupled plasma optical emission spectrometer (ICP,
Varian VISTAꢀMPX). Transmission electron microscopy (TEM)
images were performed on a JEOL JEM2010 electron microscope
at an acceleration voltage of 220 kV. Nitrogen 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 plots (p/p0
= 0.05ꢀ1.00). Solid state NMR experiments were explored on a
Bruker AVANCE spectrometer at a magnetic field strength of 9.4
2.3. General procedure for the asymmetric transfer
hydrogenation.
A typical procedure was as follows. Catalyst 5 (21.50 mg, 2.0
ꢁmol of Rh, based on ICP analysis), HCO2Na (0.68 mg, 1.0
mmol), α–haloketones or benzils (0.20 mmol), and 2.0 mL of
water were added sequentially to a 10.0 mL round−bottom flask.
The mixture was then stirred at 40 °C for 1−12 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 with ethyl ether (3 × 3.0 mL). The
combined ethyl ether extracts 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. The ee values
were determined by a HPLC analysis using a UVꢀVis detector and
a Daicel chiralcel column (Φ 0.46 × 25 cm).
1
T with H 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 is applied
1
in the during acquisition period. H cross polarization in all solid
state NMR experiments was employed using a contact time of 2
ms and the pulse lengths of 4ꢁs.
2.2. Preparation of catalyst 5
3. Results and discussion
In a typical synthesis, 2.0 g of structureꢀdirecting agent, pluronic
P123
(CH2ꢀCH2O)20(CH2(CH3)CH2O)70(CH2CH2O)20),
was
3.1. Synthesis and structural characterization of the
heterogeneous catalyst
completely dissolved in a mixture of 80 mL of hydrochloric acid
(0.2 N) and 6.0 g of KCl. The mixture was stirred at room
temperature for 1.0 h. Subsequently, 3.0 g (8.47 mmol) of the
silica precursor 1,2ꢀbis(triethoxysilyl)ethane was added at 40 °C.
After a preꢀhydrolysis period of 40 minute, 0.25 g (0.50 mmol) of
(R,R)ꢀ4ꢀ(trimethoxysilyl)ethyl)phenylsulfonylꢀ1,2ꢀ
diphenylethyleneꢀdiamine (1) and 0.25 g (0.27 mmol) of 1,3ꢀ
bis(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀimidazolꢀ3ꢀium iodide (2) was
added. The reaction mixture was stirred at 40 °C for 24 h and
then aged at 100 °C for 24 h. The resulting solid was filtered,
rinsed with excess ethanol, and then dried overnight on a filter.
The surfactant template was removed by refluxing in acidic
ethanol (400 mL per gram) for 24 h. The solid was filtered, rinsed
with ethanol again, and then dried at 60 °C under reduced
pressure overnight to afford Imidazolium@ArDPEN@PMO (3)
(1.78 g) in the form of a yellow powder. The part of collected
solids (1.0 g) was suspended in 20 mL of dry CH2Cl2, and 145.0
mg (0.25 mmol) of [CpRh*Cl2]2 was added to the solution at
Scheme 1. Preparation of catalyst 5.
2
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Imidazoliumꢀmodified chiral Cp*RhArDPENꢀfunctionalized
periodic mesoporous organosilica, abbreviated as
Imidazolium@Cp*RhArDPEN@PMO (5) (Cp*RhArDPEN:[9]
ppm, corresponding to the imidazoliumꢀmodified, ethyleneꢀ
bridged organosilica. In the part of chiral rhodium/diamine
complex, signals between 70 and 75 ppm were attributed to the
carbon atoms of NCH groups in the TsDPEN moiety, whilst
peaks at 94.7 and 9.6 ppm in the spectrum of 5 ascribed the
carbon atoms of the Cp ring and to the carbon atoms of the CH3
groups attached to Cp ring (in contrast, no carbon signals in the
latter case in the spectrum of 3 demonstrates the formation of the
Cp*RhArDPEN complexes in 5). Their chemical shifts in catalyst
Cp*
=
pentamethylcyclopentadiene and ArDPEN
=
Nꢀ4ꢀ
was
ethylenephenylsulfonylꢀ1,2ꢀdiphenylethylenediamine),
synthesized as outlined in Scheme 1. Threeꢀcomponent coꢀ
condensation of 1,2ꢀbis(triethoxysilyl)ethane, (R,R)ꢀ4ꢀ
(trimethoxysilyl)ethyl)phenylsulfonylꢀ1,2ꢀdiphenylethyleneꢀ
diamine (1) and 1,3ꢀbis(3ꢀ(triethoxysilyl)propyl)ꢀ1Hꢀimidazolꢀ3ꢀ
ium iodide (2),[10] led to chiral diamineꢀfunctionalized PMO
(Imidazolium@ArDPEN@PMO (3)) in the form of a white
powder. Direct complexation of 3 with (Cp*RhCl2)2 afforded then
the crude heterogeneous catalyst. This crude one was treated with
a strict Soxhlet extraction to give its pure catalyst 5 as a lightꢀ
yellow powder (see SI in experimental part and Figure S1).
5
were similar to those of its homogeneous counterpart
(Cp*RhTsDPEN),[11] disclosing that both had the similar wellꢀ
defined chiral rhodium/diamine species.
250
200
150
b
100
3
a
i
3
Catalyst 5
50
Catalyst 5
a
k
j
0
f
h
c
e
d
1.0
1.5
2.0
2.5
3.0
3.5
4.0
g
2 Theta/degree
Figure 3. Smallꢀangle powder XRD patterns of 3 and catalyst 5.
200
150
100
50
0
ppm
500
450
400
350
Figure 1. Solidꢀstate 13C CP/MAS NMR spectra of 3 and catalyst 5.
4
6
8
10
2 Pore Diameter (nm)
300
250
200
150
100
T2
T3
3
Catalyst 5
Catalyst 5
3
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
T1
Figure 4. Nitrogen adsorptionꢀdesorption isotherms of 3 and catalyst 5.
ppm
ꢀ100
ꢀ20
ꢀ40
ꢀ60
ꢀ80
Figure 2 showed the silicate compositions of catalyst 5 in its
solidꢀstate 29Si MAS NMR spectroscopy. It was found that both 3
and 5 only presented one group of characteristic T signals for
organosilica, suggesting that all Si species were bonded
covalently to carbon atoms.[12] In constrast these values with
those isomer values reported in the literature (−48.5, −58.5, and
−67.5 ppm for T1, T2, and T3 of R(HO)2SiOSi, R(HO)Si(OSi)2,
and RSi(OSi)3, respectively),[13] the observed strong T2–T3
signals at −57.4, and −65.5 ppm in the spectrum of 5
demonstrated that R–Si(OSi)2(OH) and R–Si(OSi)3 species (R =
propylꢀlinked imidazolium or ethylꢀlinked chiral functionality or
Figure 2. Solidꢀstate 29Si CP/MAS NMR spectra of 3 and catalyst 5.
Assembly of threeꢀcomponent siloxanes, especially wellꢀ
defined chiral rhodium/diamine species, within its silicate
network of 5 could be proven by solidꢀstate 13C crossꢀpolarization
(CP)/magic angle spinning (MAS) NMR spectroscopy. As shown
in Figure 1, both 3 and 5 produced carbon signals of SiCH2CH2Si
groups at ~5 ppm and of imidazolium moiety at ~147 and ~136
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ethyleneꢀbridged group) were its main silicate compositions in
catalyst 5. It is noteworthy that there were not Q signals from −90
to −120 ppm, indicating that the carbon–silicon bond was not
cleaved during the co–condensation process.
HCO2Na worked as a hydrogen source and 1.0% mol 5 acted as a
catalyst according to the reported method.[14a] It was found that
the asymmetric transfer hydrogenation of 2ꢀbromoꢀ
phenylethanone catalyzed by 5 gave (S)ꢀ2ꢀbromophenylethanol
with 95% yield and 99% ee value. Such a result was comparable
to that attained with its homogenous counterpart of
Cp*RhTsDPEN in the presence of sodium dodecyl sulfate as
phase transfer catalyst,[14a] better than that in the absence of
sodium dodecyl sulfate as phase transfer catalyst (Table 1, entry
1 vs entry 1 in brackets). This finding suggested the superiority of
catalyst 5. Furthermore, the asymmetric reaction could also be
run at much higher substrateꢀtoꢀcatalyst mole ratio without
affecting its ee value, as exemplified by the asymmetric transfer
hydrogenation of 2ꢀbromoꢀphenylethanone at substrateꢀtoꢀ
catalyst mole ratio of 200 (Table 1, entry 2).
Meanwhile, Xꢀray diffraction (XRD), nitrogen adsorption–
desorption measurement, and transmission electron microscopy
(TEM), further demonstrated the ordered mesostructure and wellꢀ
defined pore arrangement of catalyst 5. As shown in Figure 3, the
smallꢀangle XRD patterns revealed that both 3 and 5 produced an
intense d100 diffraction peak along with two similar weak
diffraction peaks (d110
, d200), indicating their dimensionalꢀ
hexagonal pore structures (p6mm).[8aꢀb] Nitrogen adsorption–
desorption isotherms (Figure 4) suggested their mesoporous
structures due to the presence of typical type IV isotherms with
an H1 hysteresis loop and visible step at P/P0 = 0.50–0.80. Their
structural parameters were listed in a Table that was an inset in
Figure 3. TEM images further confirmed above judgement
(Figure 5), where the highly ordered mesostructure with
dimensionalꢀhexagonal arrangements of catalyst 5 would be
observed clearly.
Table 1. Asymmetric transfer hydrogenation of aryl–substituted αꢀ
haloketones.a
Catalyst 5
OH
O
X
X
HCOONa
H2O, 40 oC
Ar
X = Br, Cl
Ar
6aꢀ6l
Entry
Ar (6)
X
Time (h)
%Yieldb
%ee b
99(92) c
1
Ph (6a)
Br
1.5 (1.5)
95(76)
Ph (6a)
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
5
88
93
92
96
92
93
90
88
91
93
83
91
92
90
96
93
98d
98e
96f
94
95
94
95
93
97
96
93
97
98
98
94
97
2
3
Ph (6a)
2
4
Ph (6a)
6
5
4ꢀFPh (6b)
3ꢀFPh (6c)
4ꢀClPh (6d)
4ꢀBrPh (6e)
3ꢀBrPh (6f)
4ꢀCF3Ph (6g)
3,4ꢀCl2Ph (6h)
4ꢀNO2Ph (6i)
4ꢀCH3Ph (6j)
4ꢀOCH3Ph (6k)
2ꢀnaphthyl (6l)
4ꢀFPh (6m)
4ꢀClPh (6n)
1.5
1.5
1.5
1.5
1.5
1.5
2
6
7
8
Figure 5. TEM images of catalyst 5 viewed along [100] (a) and [001] (b)
directions.
9
10
11
12
13
14
15
16
17
3.2. Catalytic performance of the heterogeneous catalyst
3.2.1. Catalytic properties
2
Chiral Nꢀsulfonylated diamineꢀbased organometallic complexes
as a type of highly efficient catalysts are wellꢀdocumented in
2
9]
various asymmetric reactions.[2,
Some applications in the
3
asymmetric transfer hydrogenation of α–haloketones have also
been explored.[14] In the present study, we first examined the
catalytic and enantioselective performance of catalyst 5 in the
asymmetric transfer hydrogenation of 2ꢀbromoꢀphenylethanone
in water. Because this asymmetric reaction in water is a typical
twoꢀphase reaction system, it often needs surfactant as phase
transfer catalyst, such as sodium dodecyl sulfate or
cetyltrimethylammonium bromide, promote its catalytic
performance in a homogeneous catalysis.[14a] In this case, due to
the designed heterogeneous catalyst 5 containing the hydrophobic
PMO silicate network and phaseꢀtransferꢀfeatured imidazoliumꢀ
functionality, this enantioselective reaction catalyzed by 5 was
carried out in the absence of surfactant system, where the
3
1.5
1.5
a Reaction conditions: catalyst 5 (21.50 mg, 2.0 ꢁmol of Rh based on the
ICP analysis), HCO2Na (1.0 mmol), α–haloketones (0.20 mmol) and 2.0
b
1
mL water, reaction temperature (40 °C). Yield was determined by H
NMR analysis and the ee values were determined by chiral HPLC
c
analysis (see SI in Figures S2). Data in bracket were obtained using
d
homogeneous Cp*RhTsDPEN as a catalyst. Data were obtained at
e
substrateꢀtoꢀcatalyst mole ratio of 200. Data were obtained using the
parallel PMOꢀsupported analogue as a catalyst. f Data were obtained using
SBAꢀ15ꢀsupported analogue as a catalyst.
4
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On the basis of above excellent catalytic and enantioselective
performance, catalyst 5 was further investigated with various
aryl–substituted 2–bromoketones as substrates. As shown in
Table 1, various 2–bromoketones could be smoothly converted to
chiral products with high yields and enantioselectivities under the
same reaction conditions. Also, it was found that the electronic
properties of substituents at the Ar moiety did not affect
enantioselectivities, that is, various electronꢀwithdrawing and ꢀ
donating substituents at the Ar moiety led to the same catalytic
efficiency (Table 1, entries 5–14). In addition, αꢀchloroketones
catalyzed by 5 could be smoothly converted to chiral products
with high yields and enantioselectivities in aqueous medium
(entries 16ꢀ17).
supported analogue. Further evidence supports this judgment
coming from kinetic profile investigation, where the
a
asymmetric transfer hydrogenation of 2ꢀbromoꢀphenylethanone
catalyzed by catalyst 5, PMOꢀsupported analogue and SBAꢀ
supported analogue were performed as shown in Figure 6. The
results showed that the asymmetric reaction catalyzed by catalyst
5 resulted in an initial activity higher than that achieved with
PMOꢀsupported and SBAꢀ15ꢀsupported analogues (the initial
TOFs within 1.0 h were 83.7, 69.1 and 47.2 molmolꢀ1hꢀ1,
respectively). Based on above these investigations, a conclusion
could be drawn that the enhanced reaction rate of catalyst 5
relative to PMOꢀsupported and SBAꢀ15ꢀsupported analogues
should be attributed to a synergistic result of the hydrophobic
PMO silicate network and imidazoliumꢀfunctionality in catalyst 5.
3.2.2. Investigation of the factors affecting catalytic
performance
100
80
An important aim for incorporation of imidazoliumꢀfunctionality
within PMO network as a heterogeneous catalyst expects to
enhance reaction rate in an aqueous medium because the
hydrophobic benefit PMO network and phaseꢀtransferꢀfeatured
imidazoliumꢀfunctionality is beneficial to an aqueous catalysis.
As expected, the asymmetric transfer hydrogenation of 2ꢀbromoꢀ
phenylethanone in water catalyzed by catalyst 5 could be
completed within 1.5 h, which is slightly shorter than that
attained with its homogeneous counterpart (2.0 hours). This
phenomenon is rare since a general heterogeneous catalyst
relative to its homogeneous counterpart often needs an obviously
enlarged reaction time due to the common diffuse process of
substrates during the catalysis process. This finding indicated that
the high reaction rate obtained with catalyst 5 was attributed to
the hydrophobic benefit and of phaseꢀtransferꢀfeatured
imidazoliumꢀfunctionality in catalyst 5. In order to confirm their
role, two analogues of catalyst 5, a parallel PMOꢀsupported
analogue (without imidazoliumꢀfunctionality in PMO network)
and a SBAꢀ15ꢀsupported analogue (without hydrophobic silicate
network in the similar dimensionalꢀhexagonal pore arrangements),
were employed to compare their catalytic performances in the
asymmetric transfer hydrogenation of 2ꢀbromoꢀphenylethanone.
The results showed that the former case catalyzed by its parallel
PMOꢀsupported analogue could be reached to its catalytic
completion in 2 hours, demonstrating a slightly slower reaction
rate than that with catalyst 5 (Table 1, entry 3 versus entry 1).
This finding proved the benefit of imidazoliumꢀfunctionality in
catalyst 5 because its parallel PMOꢀsupported analogue has not
imidazoliumꢀfunctionality in the similar ethyleneꢀbridged PMO
organosilicate network. The latter case catalyzed by its SBAꢀ15ꢀ
supported analogue needed an obviously long reaction time
relative to that catalyzed by 5 (Table 1, entry 4 versus entry 1).
This observation revealed the hydrophobic advantage of PMOꢀ
60
40
Catalyst 5
PMOꢀsupported analog
SBAꢀ15ꢀsupported analog
20
0
0
40
80
120
160
200
Reaction time (min)
Figure 6. Asymmetric transfer hydrogenation of 2ꢀbromoꢀ
phenylethanone catalyzed by catalyst 5, PMOꢀsupported analogue and
SBAꢀsupported analogue. Reactions were carried out at 40 °C, using 10.0
ꢁmol of the catalyst, 5 equiv. HCOONa, and at an S/C ratio of 100 in 5.0
mL of water.
3.2.3. Catalyst’s stability and recyclability
Another important consideration for design of the heterogeneous
catalyst 5 is easy separation, where the recycled catalyst 5 can
keep its catalytic activity and enantioselectivity after multiple
cycles. As we envisaged, the heterogeneous catalyst 5 could be
recovered easily through centrifugation and reused in the
asymmetric transfer hydrogenation of 2ꢀbromoꢀphenylethanone.
As shown in Table 2, in eight consecutive reactions, the recycled
catalyst 5 still afforded (S)ꢀ2ꢀbromophenylethanol with 92%
conversion and 93% ee (see SI in Figure S3). An inductively
coupled plasma (ICP) optical emission spectrometer analysis
found that the low leaching of Rh was responsible for its high
recyclability, where the amounts of Rh in catalyst 5 after the
eighth recycle was 9.31 milligrams per gram of catalyst and only
3.4 % of Rh loss.
supported organosilicate network in catalyst
5 since this
organosilicate network had a high hydrophobicity relative to the
SiO2ꢀsupported inorganosilicate network in the SBAꢀ15ꢀ
Table 2. Reusability of catalyst 5 for transfer hydrogenation of 2ꢀbromoꢀphenylethanone.a,b
Run time
1
2
3
4
5
6
7
8
9
99
98
99
97
98
94
95
92
85
Conversion [%]
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DOI: 10.1039/C6GC01589A
99
98
97
97
95
94
93
93
91
ee [%]
a Reaction conditions: catalyst 5 (215.0 mg, 20.0 ꢁmol of Rh based on the ICP analysis), HCO2Na (10.0 mmol), 2ꢀbromoꢀphenylethanone (2.0 mmol) and
10.0 mL water, reaction temperature (40 oC), reaction time (1.5 h). [b] The ee values were determined by chiral HPLC analysis (see SI in Figures S3).
In conclusions, by utilizing hydrophobic benefit and phaseꢀ
3.2.4. Expansion of asymmetric transfer hydrogenation
transferꢀfeatured imidazoliumꢀfunctionality, we develop an
imidazoliumꢀmodified chiral rhodium/diamineꢀfunctionalized
As a highly efficient PMOꢀsupported heterogeneous catalyst,
periodic mesoporous organosilica. As
a
bifunctional
catalyst
5 was further explored in asymmetric transfer
heterogeneous catalyst, it displays high catalytic activity and
enantioselectivity in the asymmetric transfer hydrogenation of α–
haloketones and benzils, where the hydrophobic organosilicate
network, phaseꢀtransferꢀfeatured imidazoliumꢀfunctionality, and
wellꢀdefined chiral organorhodium catalytic nature are
responsible for highly catalytic performance. Moreover, the
convenient recovery and high recyclability make this
heterogeneous catalyst an attractive feature in practical organic
transformations in an environmentally friendly manner.
hydrogenation that is difficult to be transformed. In this case, the
asymmetric transfer hydrogenation of benzils in water was
investigated according to the reported methods,[2a, 15] where the
asymmetric reaction with the HCO2Na as a hydrogen source and
1.0% mol 5 as a catalyst was performed. As shown in Table 3, it
was found that catalyst
5 was still highly efficient in
transformation of benzils to various chiral diols. Taking the
asymmetric transfer hydrogenation of benzil as an example, the
enantioselective reaction catalyzed by 5 gave (1S,2S)ꢀ1,2ꢀ
diphenylethaneꢀ1,2ꢀdiol with 99% ee, where was comparable to
that attained with its homogenous counterpart of
Cp*RhTsDPEN.[2a] Similarly, the synergistic advantages of
hydrophobicity and phaseꢀtransferꢀfeatured imidazoliumꢀ
functionality in catlayt 5 could also be observed, where a kinetic
profile in the asymmetric transfer hydrogenation of benzil
catalyzed by catalyst 5, PMOꢀsupported analogue, SBAꢀ
supported analogue confirmed this trends (see SI in Figure S4).
Furthermore, catalyst 5 could be recovered easily and reused in
the asymmetric transfer hydrogenation of benzils, where, the
recycled catalyst 5 in eight consecutive reactions still afforded
(S)ꢀ2ꢀbromophenylethanol with 89% yield and 99% ee (see SI in
Table S1 and Figure S6).
Acknowledgements
We are grateful to China National Natural Science Foundation
(21402120), Shanghai Sciences and Technologies Development
Fund (13ZR1458700), the Shanghai Municipal Education
Commission (14YZ074) for financial support.
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Ar (7)
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Ph (7a)
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4ꢀFPh (7b)
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DOI: 10.1039/C6GC01589A
Graphical Abstract：
An ImidazoliumꢀModified Chiral Rhodium/diamineꢀFunctionalized Periodic Mesoporous
Organosilica for Asymmetric Transfer Hydrogenation of α–Haloketones and Benzils in
Aqueous Medium
Feng Zhou, Xiaoying Hu, Ming Gao, Tanyu Cheng*, Guohua Liu*
An imidazoliumꢀmodified chiral rhodium/diamineꢀfunctionalized periodic mesoporous
organosilica is developed, which promotes greatly asymmetric transfer hydrogenation of α–
haloketones and benzils in aqueous medium.