A Polymer-Coated Rhodium/Diamine-Functionalized Silica for Controllable Reaction Switching in Enantioselective Tandem Reduction-Lactonization of Ethyl 2-Acylarylcarboxylates

Article abstract of DOI:10.1021/acscatal.5b02797

The design of a smart heterogeneous catalyst for controllable reaction switching is highly desirable in asymmetric catalysis. In this work, by taking advantage of the thermoresponsive behavior of a water-soluble polymer coating and the confined feature of

Full text of DOI:10.1021/acscatal.5b02797

Research Article
pubs.acs.org/acscatalysis
A Polymer-Coated Rhodium/Diamine-Functionalized Silica for
Controllable Reaction Switching in Enantioselective Tandem
Reduction−Lactonization of Ethyl 2‑Acylarylcarboxylates
Lingyu Kong, Junwei Zhao, Tanyu Cheng, Jingrong Lin, and Guohua Liu*
Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai Key Laboratory of Rare Earth Functional Materials,
Shanghai Normal University, Shanghai 200234, China
S
* Supporting Information
ABSTRACT: The design of a smart heterogeneous catalyst
for controllable reaction switching is highly desirable in
asymmetric catalysis. In this work, by taking advantage of the
thermoresponsive behavior of a water-soluble polymer coating
and the confined feature of silica nanoparticles, we have
constructed a silica material with chiral rhodium/diamine
functionality on SiO2 nanospheres coated with a water-soluble
thermoresponsive polymer. The solid-state 13C NMR spec-
trum of the product demonstrated well-defined single-site
chiral rhodium active centers within the thermoresponsive
polymer, and scanning electron microscopy and transmission
electron microscopy revealed its uniformly dispersed morphology. As a smart heterogeneous catalyst, it enables catalyst-based
temperature-controlled reaction switching in the enantioselective tandem reduction−lactonization of ethyl 2-acylarylcarboxylates
in water. At 40 °C, the catalyst promotes highly enantioselective tandem reduction−lactonization by adopting the extended form
of the thermoresponsive polymer coating, whereas at 15 °C the reaction is terminated and the heterogeneous catalyst can be
recycled because of its closed form. This feature endows this catalyst with high efficiency and recyclability for the synthesis of
chiral phthalides in an environmentally friendly medium.
KEYWORDS: asymmetric catalysis, heterogeneous catalyst, immobilization, silica, polymer
ature-controlled reaction switching are key scientific and
technological challenges in heterogeneous asymmetric catalysis.
Recently, chiral organometallic complexes immobilized on
silica-based supports have been extensively applied to various
asymmetric reactions.4 A significant benefit of this method is
that a homogeneous chiral organometallic complex is efficiently
confined on its support, avoiding loss of its catalytically active
species. Furthermore, unlike polymer-based catalysts,5 relatively
rigid silica-based supports can overcome the negative influence
on enantioselectivity that arises from the flexible nature and the
swelling effect of soluble polymers, resulting in comparable
enantioselective performance.6 Thus, by the combination of the
reversible temperature responsiveness of a thermoresponsive
polymer and the confining effect of silica nanoparticles, it is
reasonable to expect that a thermoresponsive-polymer-coated
chiral organometal-functionalized silica may serve as a smart
catalyst to perform controllable reaction switching based on
temperature (Scheme 1).
INTRODUCTION
■
Thermoresponsive polymers show a reversible response to
temperature, whereupon their properties can be drastically
altered with changes in temperature.1 This feature is beneficial
for the design of a polymer-based smart heterogeneous catalyst
for controllable reaction switching based on temperature, which
has not been explored to date. Generally, two types of polymer-
based heterogeneous catalysts have been applied to catalytic
reactions,2 namely, polymer-supported catalysts (homogeneous
complexes supported on polymers) and polymer-encapsulated
catalysts (homogeneous complexes encapsulated within poly-
mers). However, both cases have some intrinsic shortcomings.
In the case of polymer-supported catalysts, the active species
are randomly bound to a polymer, making it impossible for this
polymer to completely entrap these active species within its
interior, and therefore, it is difficult to rigorously regulate its
catalytic behavior.2a,3 As a result, it is not feasible to achieve
reaction switching by exploiting the extended and closed forms
of a thermoresponsive polymer. In the case of polymer-
encapsulated catalysts, loss of active species from the extended
form of a thermoresponsive polymer is unavoidable during the
catalytic process.2b,c Therefore, the development of a
thermoresponsive-polymer-based catalyst to overcome these
limitations and the realization of real catalyst-based temper-
Our recent efforts have indicated that silica-based catalysts
can maintain highly enantioselective performance in asym-
Received: December 10, 2015
Revised: February 22, 2016
© XXXX American Chemical Society
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DMSO solvent three times and with 10.0 mL of methanol three
times. The collected solid was suspended in 20 mL of deionized
H2O, and 0.12 g (0.20 mmol) of [Cp*RhCl2]2 was added. The
resulting mixture was stirred at 40 °C for 12 h. The resulting
solids were filtered, rinsed with excess distilled H2O, and
washed with excess CH2Cl2. After Soxhlet extraction in CH2Cl2
solvent for 24 h to remove the remaining [Cp*RhCl2]2, the
solid was dried at 60 °C under reduced pressure overnight to
afford catalyst 3 (2.58 g) as a light-yellow powder. Inductively
coupled plasma (ICP) analysis showed that the Rh loading was
9.15 mg (0.089 mmol) per gram of catalyst. IR (KBr) cm−1:
3421.5 (s), 3028.1 (w), 2933.6 (w), 2862.7 (w), 1628.4 (m),
1494.6 (w), 1455.3 (w), 1101.1 (s), 951.6 (w), 802.1 (m),
691.8 (w), 668.3 (w), 565.9 (w), 471.5 (m). 13C CP/MAS
NMR (161.9 MHz): 179.1 (CO), 146.9, 138.1, 128.0 (C of
Ar and Ph and −CN), 95.3 (−CH of Cp ring), 75.1, 70.1
(−NCHCHN−), 66.9−56.4 (−OCH2CH3), 49.6−22.8
(−CH2CH2−, −CH2CONH2, and −CH2Ar), 16.9
(CH3CH2−, and −OCH2CH3), 9.2 (−CH3Cp, −CH2Si) ppm.
General Procedure for the Enantioselective Tandem
Reduction−Lactonization of Ethyl 2-Acylarylcarboxy-
lates. A typical procedure was as follows: The heterogeneous
catalyst 3 (22.50 mg, 2.0 μmol of Rh based on ICP analysis),
HCO2Na (1.0 mmol), 2-acylarylcarboxylate (0.20 mmol), and
2.0 mL of water were added to a 10 mL flask, which in turn was
purged with nitrogen. The mixture was allowed to react at 40
°C for 8−12 h. During that time, the reaction was monitored
constantly by thin-layer chromatography. After completion of
the reaction, the heterogeneous catalyst was separated from the
mixture via centrifugation (10000 rpm) for the recycle
experiment. The aqueous solution was extracted with Et2O (3
× 3.0 mL). The combined Et2O was washed with brine twice
and dried with Na2SO4. After evaporation of the Et2O, the
residue was purified by silica gel flash column chromatography
to afford the desired product. The yields were determined by
1H NMR analysis, and the ee values were determined by HPLC
analysis with a photodiode array detector using a Daicel
Chiralcel column (Φ = 0.46 cm × L = 25 cm).
Scheme 1. Schematic Illustration of the Reversible Response
to Temperature of a Polymer-Coated Organometal-
Functionalized Silica for Controllable Catalysis
metric reactions.7 In this work, by combining the benefits of a
thermoresponsive polymer and silica nanoparticles, we have
constructed a thermoresponsive-polymer-coated rhodium/
diamine-functionalized silica material as a smart heterogeneous
catalyst. This catalyst enables controllable reaction switching in
the enantioselective tandem reduction−lactonization of ethyl 2-
acylarylcarboxylates in water. At 40 °C, the polymer coating is
in an extended form, allowing the asymmetric reaction to
proceed with high catalytic efficiency because of the
homogeneous-like catalytic environment. At 15 °C, however,
the polymer coating is converted to the closed form,
terminating the reaction, and the catalyst can be efficiently
recycled. Moreover, the catalyst displays an enhanced reaction
rate and comparable enantioselective performance relative to its
homogeneous counterpart, which can be attributed to the
water-soluble polymer coating and the confined chiral
rhodium/diamine catalyst. Furthermore, the heterogeneous
catalyst can be recycled at least eight times without loss of its
enantioselective performance.
EXPERIMENTAL SECTION
■
Preparation of Vinyl@ArDPEN@SiO2 (2). In a typical
synthesis, the obtained SiO2 nanospheres were treated before
use as follows. The nanospheres were immersed in a 1.0% HCl
solution for 8 h at room temperature to remove contaminants,
followed by washing and immersion in deionized water for
approximately 1 h to clean and hydrolyze the surface. The
nanospheres were then filtered and dried at 110 °C in a vacuum
oven overnight to remove excess surface water, affording clean
hydroxylated SiO2 nanospheres. Subsequently, 2.0 g of the
hydroxylated SiO2 nanospheres was added to a solution of 0.20
g (0.40 mmol) of (S,S)-4-((trimethoxysilyl)ethyl)-
phenylsulfonyl-1,2-diphenylethylenediamine (1) in 20 mL of
xylene, and the mixture was stirred at room temperature for 1 h,
refluxed at 137 °C for 12 h, and cooled to room temperature.
Then 0.40 g (2.10 mmol) of triethoxy(vinyl)silane was added,
and the mixture was stirred at room temperature for another 1
h and then at 70 °C for 12 h. The resulting solid was filtered,
rinsed with excess xylene, and then dried at 110 °C in a vacuum
oven for 2 days to to afford 2 (2.49 g) in the form of a white
powder.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization of Hetero-
geneous Catalyst 3. Water-soluble thermoresponsive-poly-
mer-coated Cp*RhArDPEN-functionalized silica, abbreviated
as P(EAamAn)@Cp*RhArDPEN@SiO2 (3), [P(EAamAn)8 =
poly(ethene-co-acrylamide-co-acrylonitrile); Cp*RhArDPEN:9
Cp* = pentamethylcyclopentadiene, ArDPEN = (S,S)-4-
((trimethoxysilyl)ethyl)phenylsulfonyl-1,2-diphenylethylenedi-
amine (1)], was prepared as outlined in Scheme 2. First, SiO2
nanospheres were obtained through condensation of tetrae-
thoxysilane according to the reported method.10 Continuous
postgraftings of triethoxy(vinyl)silane and 1 then afforded
Vinyl@ArDPEN@SiO2 (2).11 Finally, the free radical polymer-
ization of 2 with acrylamide and acrylonitrile12 followed by
complexation with [Cp*RhCl2]2 at 40 °C led to catalyst 3 in
the form of a light-yellow powder (see the the experimental
section and Figure S1 in the Supporting Information). For
comparison, a parallel polymer-supported analogue without the
SiO2 support, P(EAmAn)@Cp*RhArDPEN (3′), was also
prepared by free radical polymerization of N-((S,S)-2-amino-
1,2-diphenylethyl)-4-vinylbenzenesulfonamide with acrylamide
and acrylonitrile following a similar procedure.
Preparation of Catalyst 3. In a typical synthesis, 2.0 g of
2, 1.02 g (14.4 mmol) of acrylamide, and 0.19 g (3.6 mmol) of
acrylonitrile were weighed into a 100 mL nitrogen flask and
dissolved in 20 mL of distilled dimethyl sulfoxide (DMSO).
Then 65.5 mg (2% mol) of 2,2-azobis(isobutyronitrile) (AIBN)
was added at room temperature. After a degassing process
involving three freeze−pump−thaw cycles, the flask was placed
into an oil bath for polymerization at 60 °C for 6.0 h. The
resulting solid was filtered and then rinsed with 10.0 mL of
Incorporation of single-site rhodium active species in the
polymer-coated SiO2 nanospheres was proved by solid-state
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Figure 2, SEM images revealed that catalyst 3 was composed of
uniformly dispersed nanospheres with an average diameter of
Scheme 2. Preparation of Heterogeneous Catalyst 3
13C cross-polarization/magic-angle spinning (CP/MAS) NMR
spectroscopy. As shown in Figure 1, catalyst 3 produced strong
Figure 2. (a) SEM image, (b) TEM image, and (c) enlarged TEM
image of catalyst 3 and (d, e) photographs showing dispersions of
catalyst 3 in water at (d) 15 °C and (e) 40 °C.
Figure 1. Solid-state 13C CP/MAS NMR spectrum of catalyst 3.
347 nm (Figure 2a), while TEM images confirmed that these
nanospheres had a core−shell structure in which the core was
coated by a polymer shell with a thickness of about 15 nm
(Figure 2b,c). It is noteworthy that catalyst 3 could be better
dispersed in water at 40 °C than at 15 °C (cf. Figure 2d,e),
suggesting that it adopted its extended form at the higher
temperature and its closed form at the lower temperature.
Catalytic Property of the Heterogeneous Catalyst.
Chiral N-sulfonylated diamine-based organometallic complexes
are extensively used for various asymmetric reactions, and some
of them have been applied to the enantioselective reduction−
lactonization of ethyl 2-acylarylcarboxylates.7a,15 In the present
study, we first examined the catalytic activity and enantiose-
lectivity of 3 toward the enantioselective tandem reduction−
lactonization of ethyl 2-(2-phenylacetyl)benzoate as a model
reaction, where the reaction with HCO2H as a hydrogen source
and 1.0 mol % 3 as a catalyst was investigated according to a
reported method.7a It was found that catalyst 3 produced the
target product (S)-3-benzylisobenzofuran-1(3H)-one in quan-
titative yield with 98% ee after 10 h. This ee value is comparable
to that achieved with the homogeneous catalyst (Table 1, entry
1 vs entry 1 in parentheses). This behavior indicated that the
Cp*RhArDPEN functionality appended onto the SiO2 nano-
spheres of catalyst 3 retained the original chiral environment of
carbon signals of the −CHCONH2 moiety at δ = 179.1 ppm
and the −CHCN moiety at δ = 128.0 ppm, corresponding to
the characteristic carbon atoms of the coated polymer.13 In
addition, strong signals for the alkyl carbon atoms of the coated
polymer could be observed, as marked in the spectrum. In
particular, all of the carbon atoms of the chiral rhodium/
diamine complex showed their characteristic signals. Specifi-
cally, the signals between δ = 70 and 75 ppm could be
attributed to the NCHPh carbon atoms in the ArDPEN moiety,
and the peaks at δ = 95.3 and 9.2 ppm could be ascribed to the
carbon atoms of the Cp ring and the attached CH3 groups,
respectively. These chemical shifts of catalyst 3 are similar to
those of the homogeneous counterpart, Cp*RhTsDPEN,14
demonstrating that both have the same well-defined single-site
active species. Similarly, the parallel polymer-supported catalyst
3′ also showed the same chemical shifts in its 13C CP/MAS
NMR spectrum (see Figure S2), suggesting that it could be
regarded as a standard analogue for comparison of catalytic
performances.
The nanostructural morphology of catalyst 3 was further
investigated by means of scanning electron microscopy (SEM)
and transmission electron microscopy (TEM). As shown in
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the extended form of its water-soluble polymer coating. As a
result, the Cp*RhArDPEN functionality appended onto the
SiO2 nanospheres was fully exposed to the reaction system,
leading to a homogeneous-like catalytic environment for the
active centers and hence the same catalytic behavior in its
catalytic performance. The consistency of the results also
implied that the extended form of the water-soluble polymer
coating in catalyst 3 did not disturb the configuration of the
Cp*RhArDPEN active centers during the catalytic process at
40 °C, resulting in similar enantioselectivities. Thus, heteroge-
neous catalyst 3 retained structural and electronic properties
similar to those of its homogeneous counterpart, enabling the
same catalytic and enantioselective performance in the
asymmetric reaction.
Table 1. Enantioselective Tandem Reduction−Lactonization
a
of Ethyl 2-Acylarylcarboxylates
b
b
entry
Ar
time (h)
yield (%)
ee (%)
c
1
2
Ph (5a)
Ph (5a)
Ph (5a)
Ph (5a)
Ph (5a)
10 (24)
48
99 (97)
8
98 (96)
d
85
e
3
48
85
95
f
4
48
63
93
g
5
8
98
94
To gain better insight into the nature of the catalyst-based
temperature-controlled reaction switching, the tandem reduc-
tion−lactonization of ethyl 2-(2-phenylacetyl)benzoate cata-
lyzed by 3 at 15 °C was performed. It was found that only an
8% yield of (S)-3-benzylisobenzofuran-1(3H)-one could be
obtained, even when the reaction time was prolonged to 48 h
(Table 1, entry 2). This yield was obviously much lower than
that of 99% obtained at 40 °C (entry 1) and even lower than
that of 85% obtained with the homogeneous counterpart
Cp*RhArDPEN at 15 °C (entry 3). These findings implied that
the water-soluble thermoresponsive polymer coating of catalyst
3 at 15 °C was in its closed form because the reaction at 15 °C
only afforded a small amount of product. The very low yield at
15 °C relative to that attained at 40 °C (8% vs 99% yield)
should be due to the fact that the closed polymer coating
greatly restricted the attack of ethyl 2-(2-phenylacetyl)benzoate
to the Cp*RhArDPEN active species. As a result, it was difficult
for the reaction to proceed at 15 °C, and thus, a very poor yield
was obtained. Further evidence supporting this view was
provided by the parallel experiment, where the reaction
catalyzed by the polymeric analogue 3′ at 15 °C produced
(S)-3-benzylisobenzofuran-1(3H)-one in 63% yield (entry 4 vs
entry 2). The obviously higher yield obtained with catalyst 3′ at
15 °C not only confirmed the closed nature of 3 at this
temperature but also indicated that the general polymeric
analogue 3′ was not amenable to catalyst-based temperature-
controlled reaction switching. On the other hand, the extended
form of smart catalyst 3 could also be proven easily by a similar
method, where the asymmetric reactions catalyzed by 3 and 3′
at 40 °C both reached catalytic completion within 10 h (entries
1 and 5) because they had the similar extended form of the
thermoresponsive polymer at 40 °C.
More importantly, direct evidence to detect the extended and
closed forms of the polymer coating in catalyst 3 could also be
offered by an investigation of the hydrodynamic diameter (Dh)
distributions at 15 and 35 °C using dynamic laser scattering
(DLS). As shown in Figure 3, catalyst 3 exhibited steady
temperature responsiveness over five consecutive runs, where
the average Dh increased from 347 to 427 nm as the
temperature was increased from 15 to 35 °C (also see Figure
S4). In sharp contrast with original average size of catalyst 3 at
15 °C (average diameter 347 nm with a coated layer thickness
of 15 nm as determined by SEM and TEM), the increase in the
average diameter (50 nm) at 40 °C [average diameter at 40 °C
(427 nm) − average diameter at 15 °C (347 nm) − twice the
polymer coating thickness (2 × 15 nm)] should be ascribed to
the extended length of the polymer coating. These observations
confirmed the adoption of the extended and closed forms of the
polymer coating in catalyst 3, as differential scanning
6
4-FPh (5b)
12
99
99
99
98
98
99
96
99
99
98
99
7
4-ClPh (5c)
12
99
8
2-ClPh (5d)
14
98
9
4-CF3Ph (5e)
4-MePh (5f)
12
99
10
11
12
13
14
15
14
97
4-MeOPh (5g)
3-MeOPh (5h)
3,4-(MeO)2Ph (5i)
1-naphthyl (5j)
2-naphthyl (5k)
14
98
14
97
14
93
14
97
14
98
a
Reaction conditions: catalyst 3 (22.50 mg, 2.0 μmol of Rh based on
ICP analysis), HCO2Na (1.0 mmol), ethyl 2-acylarylcarboxylate (0.20
mmol), and 2.0 mL water at the reaction temperature (15 °C in entries
2 and 3 and 40 °C in the others). Yields were determined by H
NMR analysis (see Figure S5) and ee values by chiral HPLC analysis
(see Figure S6). Data in parentheses were obtained using
homogeneous Cp*RhTsDPEN as the catalyst. Data were obtained
b
1
c
d
e
at 15 °C using 3 as the catalyst. Data were obtained at 15 °C using
homogeneous Cp*RhTsDPEN as the catalyst. Data were obtained
f
g
using 3′ as the catalyst at 15 °C. Data were obtained using 3′ as the
catalyst at 40 °C.
the homogeneous analogue and led to a similar enantioselective
performance, further confirming retention of the same well-
defined single-site Cp*RhArDPEN active species, as indicated
by 13C CP/MAS NMR spectroscopy. Upon comparison of the
enantioselective performances of catalyst 3 and its polymeric
analogue 3′, it was found that the asymmetric reaction
catalyzed by 3 had a higher ee than that attained with 3′,
although the latter had a shorter reaction time (98% ee vs 94%
ee, 10 h vs 8 h; entries 1 and 5). This difference indicated that
the designed catalyst 3 could overcome the disadvantage of its
polymeric analogue in terms of enantioselective performance
because the flexible nature of polymer-supported analogue 3′
affects the chiral environment of the active species and thereby
results in a lower ee value.
On the basis of the above excellent results, heterogeneous
catalyst 3 was further investigated with a series of substrates in
the enantioselective tandem reduction−lactonization. As shown
in Table 1, all of the tested substrates were smoothly
transformed to the corresponding chiral phthalides in high
yields with excellent enantioselectivities (>96% ee). Also, it was
found that the structures and electronic properties of
substituents on the Ar moiety did not affect the enantiose-
lectivity, as various electron-withdrawing and -donating
substituents on the Ar moiety were equally efficient (entries
6−15). This behavior mirrors that attained with its
homogeneous counterpart,15a suggesting that during the
catalytic process at 40 °C the heterogeneous catalyst 3 adopted
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recovered conveniently by simple centrifugation. As shown in
Figure 5, in eight consecutive recycles, catalyst 3 still afforded
Figure 3. Average hydrodynamic diameter distributions of catalyst 3 at
15 and 35 °C in water.
calorimetry analysis showed that the melting point of catalyst 3
was 168 °C (see Figure S3).12b
Figure 5. Reusability of catalyst 3 for the tandem reduction−
lactonization of ethyl 2-(2-phenylacetyl)benzoate.
It is noteworthy that the asymmetric reaction catalyzed by 3
proceeded rapidly in the absence of Bu4NBr, reaching
completion within 10 h, whereas the reaction required 24 h
with its homogeneous counterpart Cp*RhArDPEN (Table 1,
entry 1 versus entry 1 in parentheses). This behavior is rare in
this tandem asymmetric reaction because Bu4NBr is necessary
to promote the catalytic performance in a homogeneous
catalysis system. This observation demonstrated that the
extended polymer coating in catalyst 3 at 40 °C could play a
similar role as Bu4NBr in the homogeneous catalytic system,
greatly promoting the catalytic performance. In order to
elucidate the role of the polymer coating during the catalytic
process, parallel reactions at 40 °C were compared through
kinetic profiles of the enantioselective tandem reduction−
lactonization of ethyl 2-(2-phenylacetyl)benzoate catalyzed by
3 in the absence of Bu4NBr and catalyzed by Cp*RhArDPEN
in the presence or absence of Bu4NBr. As shown in Figure 4,
the desired products in 91% yield with 97% ee in the
enantioselective reduction−lactonization of ethyl 2-(2-(4-
bromophenyl)acetyl)benzoate (see Table S1 and Figure S7).
Importantly, the closed form of the water-soluble polymer
coating in catalyst 3 efficiently decreased the leaching of Rh, as
only 3.6% of the Rh had been lost after the eighth recycle as
determined by ICP optical emission analysis.
In conclusion, by taking advantage of the thermoresponsive
behavior of a water-soluble polymer coating and the confining
effect of SiO2 nanoparticles, we have constructed a smart
heterogeneous catalyst that enables catalyst-based temperature-
controlled reaction switching in the enantioselective tandem
reduction−lactonization of ethyl 2-acylarylcarboxylates in
water. The extended form of the polymer coating of catalyst
3 at 40 °C facilitates highly efficient tandem asymmetric
catalysis, whereas the closed form at 15 °C terminates the
reaction. The heterogeneous catalyst could be easily recovered
and reused repeatedly at least eight times without loss of
enantioselectivity. The highly dispersed extended polymer
coating and the confined chiral rhodium/diamine catalytic
centers combined to further boost the catalytic performance.
The work described here offers a new concept for catalyst-
based temperature-controlled reaction switching that may
potentially be applied to other asymmetric reactions.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b02797.
■
S
Figure 4. Comparison of the kinetic profiles for the enantioselective
tandem reduction−lactonization of ethyl 2-(2-phenylacetyl)benzoate
catalyzed by 3 in the absence of Bu4NBr and catalyzed by
Cp*RhArDPEN in the presence or absence of Bu4NBr. Reactions
were carried out at substrate-to-catalyst mole ratio of 200.
Experimental procedures and analytical data for chiral
products (PDF)
AUTHOR INFORMATION
Corresponding Author
*Tel: + 86 21 64322280. E-mail: ghliu@shnu.edu.cn.
■
the initial turnover frequency (TOF) values were 64, 50, and 40
mol mol−1 h−1), respectively (TOF = number of moles of
substrate converted per mole of catalyst per hour). The results
confirmed that the enhanced reaction rate attained with 3 at 40
°C could be attributed to its highly dispersed nature (Figure
2e) because of its extended water-soluble polymer coating.
Another important consideration concerns the ability to
recycle the heterogeneous catalyst through this catalyst-based
temperature-controlled reaction switching. As observed, the
asymmetric reaction could be terminated easily and catalyst 3
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21402120), the Shanghai Sciences and Technologies
Development Fund (13ZR1458700), and the Shanghai
Municipal Education Commission (14YZ074, 13CG48,
Young Teacher Training Project) for financial support.
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DOI: 10.1021/acscatal.5b02797
ACS Catal. 2016, 6, 2244−2249