Hollow-shell-structured nanospheres: A recoverable heterogeneous catalyst for rhodium-catalyzed tandem reduction/lactonization of ethyl 2-acylarylcarboxylates to chiral phthalides

Article abstract of DOI:10.1002/asia.201301543

Chiral organorhodium-functionalized hollow-shell-structured nanospheres were prepared by immobilization of a chiral N-sulfonylated diamine-based organorhodium complex within an ethylene-bridged organosilicate shell. Structural analysis and characterization reveal its well-defined single-site rhodium active center, and transmission electron microscopy images reveal a uniform dispersion of hollow-shell-structured nanospheres. As a heterogenous catalyst, it exhibits excellent catalytic activity and enantioselectivity in synthesis of chiral phthalides by a tandem reduction/lactonization of ethyl 2-acylarylcarboxylates in aqueous medium. The high catalytic performance is attributed to the synergistic effect of the high hydrophobicity and the confined chiral organorhodium catalytic nature. The organorhodium-functionalized nanospheres could be conveniently recovered and reused at least 10 times without loss of catalytic activity. This feature makes it an attractive catalyst in environmentally friendly organic reactions. The results of this study offer a new approach to immobilize chiral organometal functionalities within the hollow-shell-structured nanospheres to prepare materials with high activity in heterogeneous asymmetric catalysis. Shell strategy: Hollow-shell-structured chiral organorhodium-functionalized nanospheres present excellent catalytic efficiency in the aqueous asymmetric synthesis of chiral phthalides by a tandem reduction/lactonization reaction of 2-acylarylcarboxylates (see figure). The superior catalytic performance and the enhanced enantioselectivity are attributed to the salient hydrophobic function of nanospheres and confined chiral catalytic nature of the organorhodium functionality.

Full text of DOI:10.1002/asia.201301543

FULL PAPER
DOI: 10.1002/asia.201301543
Hollow-Shell-Structured Nanospheres: A Recoverable Heterogeneous
Catalyst for Rhodium-Catalyzed Tandem Reduction/Lactonization of Ethyl
2-Acylarylcarboxylates to Chiral Phthalides
Rui Liu, Ronghua Jin, Juzeng An, Qiankun Zhao, Tanyu Cheng, and Guohua Liu*[a]
Abstract: Chiral organorhodium-func-
tionalized hollow-shell-structured
selectivity in synthesis of chiral phtha-
lides by a tandem reduction/lactoniza-
tion of ethyl 2-acylarylcarboxylates in
aqueous medium. The high catalytic
performance is attributed to the syner-
gistic effect of the high hydrophobicity
and the confined chiral organorhodium
catalytic nature. The organorhodium-
functionalized nanospheres could be
conveniently recovered and reused at
least 10 times without loss of catalytic
activity. This feature makes it an attrac-
tive catalyst in environmentally friend-
ly organic reactions. The results of this
study offer a new approach to immobi-
lize chiral organometal functionalities
nanospheres were prepared by immobi-
lization of a chiral N-sulfonylated dia-
mine-based organorhodium complex
within an ethylene-bridged organosili-
cate shell. Structural analysis and char-
acterization reveal its well-defined
single-site rhodium active center, and
within
the
hollow-shell-structured
transmission
images reveal a uniform dispersion of
hollow-shell-structured nanospheres.
As a heterogenous catalyst, it exhibits
excellent catalytic activity and enantio-
electron
microscopy
nanospheres to prepare materials with
high activity in heterogeneous asym-
metric catalysis.
Keywords: asymmetric catalysis
·
heterogeneous catalysis · immobili-
zation · silicates · supported cata-
lysts
Introduction
perior.[4] In addition to the general benefits of a large specif-
ic surface area/pore volume and tunable pore dimensions/
well-defined pore arrangement, PMO-supported functional-
ized materials have high hydrophobicity due to the intrinsic
organosilicate walls. These characteristics can facilitate
asymmetric reactions in two-phase catalytic systems and can
promote organic transformations in aqueous media.[5] More
importantly, various self-assembly strategies offer extensive
opportunities to anchor chiral organometallic complexes
within organosilicate networks to construct supported heter-
ogeneous catalysts. In addition, many structural morpholo-
gies introduce various possibilities to enhance enantioselec-
tive performance through the potential confinement effect
of mesoporous materials.[6] Thus, by screening morphologies
and by utilizing the significant hydrophobicity of PMOs, it is
possible to develop a suitable PMO-supported heterogene-
ous catalyst with high catalytic efficiency in asymmetric re-
actions in aqueous medium.
Phthalide (1(3H)-isobenzofuranone) is a versatile building
block used extensively in the synthesis of valuable pharma-
cological molecules.[1] Recently, a number of successful
ruthenium- or rhodium-catalyzed syntheses of chiral phtha-
lides by a reduction/lactonization of ethyl 2-acylarylcarboxy-
lates have been explored.[2] However, the intrinsic disadvan-
tages of using organoruthenium and organorhodium, such as
the need to reuse an expensive catalyst and the leaching of
metals into products, still hinder their applications. Thus, de-
velopment of a practical approach to overcome these disad-
vantages represents a promising strategy in asymmetric cat-
alysis.
Recently, studies of mesoporous silica supports have led
to efficient, reusable organometallic catalysts in asymmetric
reactions.[3] In particular, periodic mesoporous organosilica
(PMO) as a support has been shown to be significantly su-
In an effort to develop highly efficient heterogeneous cat-
alysts,[7] we recently discovered that two chiral organonickel-
functionalized PMOs can markedly increase the catalytic
performance and that one ethylene-bridged PMO possesses
higher enantioselective performance.[7f–h] On the basis of
these findings and the potential confinement effect of
hollow-shell-structured materials,[8] we report PMO-type
[a] Dr. R. Liu, Dr. R. Jin, Dr. J. An, Dr. Q. Zhao, Dr. T. Cheng,
Prof. G. Liu
Key Laboratory of Resource Chemistry of Ministry of Education
Shanghai Key Laboratory of Rare Earth Functional Materials
Shanghai Normal University
No.100 Guilin Rd., Shanghai 200234 (P. R. China)
Fax : (+86)2164322722
[Cp*RhTsDPEN]-functionalized
hollow-shell-structured
E-mail: ghliu@shnu.edu.cn
nanospheres ([Cp*RhTsDPEN]:[9] Cp*=pentamethylcyclo-
pentadiene and TsDPEN=4-methylphenylsulfonyl-1,2-di-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301543.
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Guohua Liu et al.
phenylethylenediamine) that display high catalytic activity
and enantioselectivity in the tandem reduction/lactonization
of ethyl 2-acylarylcarboxylates to chiral phthalides in aque-
ous medium. As expected, the high hydrophobicity could
significantly enhance catalytic performance and the hollow-
shell structure could enhance enantioselectivity. The catalyst
could be readily reused at least 10 times without reducing
its catalytic performance. These characteristics make it an
attractive catalyst in environmentally friendly organic reac-
tions.
Figure 1. 13C CP MAS NMR spectra of TsDPEN-functionalized PMO (2;
gray line) and catalyst 3 (black line).
Results and Discussion
Synthesis and Structural Characterization of the
Heterogeneous Catalyst
Ethylene-bridged PMO-type organorhodium-functionalized
to the TsDPEN moiety could be observed clearly in the
spectra of both materials. The signal at d=94 ppm in the
spectrum of catalyst 3 is ascribed to the carbon atoms in the
Cp ring, and that at d=10 ppm is attributed to the carbon
atom of the CH3 groups attached to the Cp ring (which is
absent in the spectrum of 2). These results suggest the for-
mation of the [Cp*RhTsDPEN] complex. The chemical
shifts of catalyst 3 are strongly similar to those of its homo-
geneous counterpart [Cp*RhTsDPEN],[11] which demon-
strates that catalyst 3 has the same well-defined single-site
active species as [Cp*RhTsDPEN]. Signals around d=17
and 58 ppm are attributed to the nonhydrolyzed ethoxy
groups (CH3CH2O) that often appeared in the 13C CP/MAS
spectra.
hollow-shell-structured
nanospheres,
abbreviated
as
[Cp*RhTsDPEN]-PMO (3), were synthesized as outlined in
Scheme 1. First, chiral silane, 4-(trimethoxysilyl)ethyl)phe-
The organosilicate network and composition of catalyst 3
could be confirmed by the solid-state 29Si MAS NMR spec-
tra. As shown in Figure 2, TsDPEN-functionalized PMO (2)
and catalyst 3 produced one group of exclusive T signals
that were derived from organosilica, which suggests that all
Si species were covalently attached to carbon atoms. Typical
isomer shift values for catalyst 3 are similar to those report-
ed in the literature[12] (d=À48.5, À58.5, and À67.5 ppm for
Scheme 1. Preparation of catalyst 3.
T1, T2, and T3 (R(HO)2SiOSi, R(HO)Si
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
nylsulfonyl-1,2-diphenylethylene-diamine (1) was prepared
according to our previous report.[7c] Co-condensation[10] of
1 and 1,2-bis(triethoxysilyl)ethane by using Pluronic F127 as
the surfactant and mesitylene as the micelle-swelling agent
afforded hollow-shell-structured TsDPEN-functionalized
PMO (2) as a white powder. Finally, heterogeneous catalyst
3 was obtained by direct complexation of [Cp*RhCl2]2 and 2
followed by further Soxhlet extraction to clear the nano-
channels (see Figures S1–S3 in the Supporting Information).
Incorporation of single-site [Cp*RhTsDPEN] active cen-
ters within an ethylene-bridged organosilicate shell could be
confirmed by using solid-state 13C CP/MAS NMR spectros-
copy. As shown in Figure 1, TsDPEN-functionalized PMO
(2) and catalyst 3 produced characteristic strong carbon sig-
nals for the SiCH2CH2Si groups around d=5 ppm, which
corresponds to ethylene-bridged organosilica. Carbon atoms
of NCH groups between d=66 and 71 ppm corresponding
2
3
À
À
respond to T (R SiACHTNUTRGENNUG(OSi)2(OH)) and T (R SiAHCUTGNTREN(NUGN OSi)3) (R=
TsDPEN, [Cp*RhTsDPEN], or ethylene-bridged groups)
and are strongly similar to reported signals.[12b,c] These obser-
vations demonstrated that TsDPEN-functionalized PMO (2)
Figure 2. 29Si CP MAS NMR spectrum of TsDPEN-functionalized PMO
(2; black line) and catalyst 3 (gray line).
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Guohua Liu et al.
À
and catalyst 3 possessed organosilicate networks with R Si-
À
A
R
cate shells. Furthermore, the absence of signals for Q series
from d=À90 to À120 ppm indicated that the carbon–silicon
bond was not cleaved during the hydrolysis-condensation
process.
To investigate the electronic state of the rhodium center,
a X-ray photoelectron spectroscopy (XPS) study of catalyst
3 and its homogeneous [Cp*RhTsDPEN] were performed.
As shown in Figure 3, catalyst 3 had Rh3d5/2 binding energy
Figure 5. Nitrogen adsorption–desorption isotherms of TsDPEN-func-
&
^
tionalized PMO (2; ) and catalyst 3 ( ).
small-angle XRD patterns and nitrogen adsorption–desorp-
tion isotherms revealed that the hollow-shell-structured
nanospheres of catalyst 3 had diffraction peaks and a typical
type IV isotherm, similar to pure hollow-shell-structured
nanospheres.[10] This suggests that the structure observed in
the pure hollow-shell-structured nanospheres could be pre-
served after functionalization. The TEM image (Figure 6)
Figure 3. XPS spectra of catalyst
[Cp*RhTsDPEN] (gray line).
3 (black line) and homogeneous
similar to its homogeneous counterpart (309.4 versus
309.3 eV), but both energies were obviously different from
that of the parent [Cp*RhCl2]2 (308.8 eV; see Figure S4 in
the Supporting Information). These results indicate the for-
mation of the single-site Cp*RhTsDPEN active center
within the organosilicate shell of catalyst 3.
The porosity and hollow-shell-structured nanospheres
were further investigated by using X-ray diffraction (XRD),
nitrogen adsorption–desorption techniques, and transmission
electron microscopy (TEM). As shown in Figures 4 and 5,
Figure 6. a) TEM image of catalyst 3 and b) TEM image with a chemical
mapping of catalyst 3 that shows the distribution of Si (white) and Rh
(red).
Figure 4. Small-angle powder XRD patterns of TsDPEN-functionalized
PMO (2; black line) and catalyst 3 (gray line).
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Table 1. Asymmetric synthesis of phthalides in the enantioselective
tandem reduction/lactonization of ethyl 2-acylarylcarboxylates.[a]
further confirmed that catalyst 3 was composed of uniformly
dispersed hollow-shell-structured nanospheres with an aver-
age size of about 20 nm (Figure 6a; also see Figure S3 in the
Supporting Information). The TEM image with a chemical
mapping technique indicated that Rh centers were uniform-
ly distributed within the hollow-shell-structured nanospheres
(Figure 6b).
Entry
Ar
R
t
Conv. [%][b]
ee [%][b]
Based on the above characterizations and analyses,
1[c]
2[d]
3[e]
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Ph
Ph
Ph
4-FPh
4-ClPh
4-BrPh
4-CH3Ph
4-OCH3Ph
3,4-(OCH3)2Ph
2-ClPh
3-OCH3Ph
4-CF3Ph
2-naphthyl
1-naphthyl
3-CF3Ph
2-thienyl
4-CF3Ph
4-BrPh
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cl
Cl
8 (10)
16
8
8
8
>99 (>99)
92
99 (96)
98
96
96
97
97
99
97
98
93
98
96
99
96
95
97
96
hollow-shell-structured nanospheres with
a well-defined
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
single-site [Cp*RhTsDPEN] active center could be readily
prepared.
8
10
10
10
10
10
10
12
12
10
10
12
12
Catalytic Properties of the Heterogeneous Catalyst
Catalyst Screening and Catalytic Performance
Chiral N-sulfonylated diamine-based homogeneous catalysts
have been used extensively in asymmetric transfer hydroge-
nation in aqueous medium.[9,13] In the present study, chiral
TsDPEN-functionalized PMO (2) was screened by complex-
ation with the extensively studied organometallic complexes
[Cp*MCl2]2 or [AreneMCl2]2 (M=Ru, Rh, Ir) in aqueous
medium, respectively (see Figure S5 in the Supporting Infor-
mation). It was found that only [Cp*RhCl2]2 showed en-
hanced enantioselectivity relative to its homogenous coun-
terpart. More importantly, tandem reduction/lactonization
of ethyl 2-(2-phenylacetyl)benzoate catalyzed by catalyst 3
could be completed with the same conversion and ee value
(Table 1, entry 1). Of particular note was the tandem reac-
tion catalyzed by catalyst 3, whichcould be run at high sub-
strate-to-catalyst molar ratio without obviously affecting the
ee value, as exemplified by the reduction/lactonization of
ethyl 2-(2-phenylacetyl)benzoate at a substrate-to-catalyst
molar ratio of 500 (Table 1, entry 2).
Given its excellent catalytic efficiency, heterogeneous cat-
alyst 3 was used in the tandem reduction/lactonization of
ethyl 2-acylarylcarboxylates to chiral phthalides in a further
investigation.[2] Excellent conversions, no side products, and
high enantioselectivities were obtained with the tested sub-
strates under similar conditions. The structures and electron-
ic properties of substituents at the Ar moiety did not signifi-
cantly affect the enantioselectivity, that is, various electron-
withdrawing and -donating substituents at the Ar moiety
were equally efficient (Table 1, entries 4–18).
93
[a] Reaction conditions: catalyst 3 (6.87 mg, 2.0 mmol of Rh based on the
ICP analysis), HCOONa (1.0 mmol), cetyltrimethylammonium bromide
(8.0 mmol), 2-acylarylcarboxylate (0.20 mmol), and water (2.0 mL), T=
408C. [b] Determined by using chiral HPLC analysis (see Figure S6 in
the Supporting Information). [c] Data in parentheses were obtained by
using homogeneous [Cp*RhTsDPEN] as a catalyst. [d] Data were ob-
tained at a substrate-to-catalyst mole ratio of 500. [e] Data were obtained
by using 2 plus homogeneous [Cp*RhTsDPEN] as a catalyst
same reaction system and the reaction was continued for
a further 5.0 h for comparison through the same process.
Tandem reduction/lactonization of ethyl 2-(2-phenylacetyl)-
benzoate gave chiral (S)-3-benzylphthalide at 83% conver-
sion and 92% ee before filtration, whereas the same reac-
tion after filtration afforded only tiny yields of product. This
observation demonstrated that residual homogeneous com-
plex remaining after noncovalent physical adsorption on cat-
alyst 3 could be eliminated because catalyst 3 had passed
through a stricter Soxhlet extraction during preparation.
This fact eliminates the role of the homogeneous catalyst
due to noncovalent adsorption and reveals that heterogene-
ous catalysis is indeed derived from the heterogeneous cata-
lyst itself.
Investigation of the Nature of Heterogeneous Catalysis
Investigation of Factors Affecting Catalytic Performance
In a general heterogeneous catalysis system, residual homo-
geneous complex remaining after noncovalent physical ad-
sorption often disturbs heterogeneous catalysis. To eliminate
this possibility and to gain better insight into the nature of
the heterogeneous catalysis, we performed a comparable ex-
periment by using a controlling filtration method. In this
case, a mixture of pure hollow-shell-structured nanospheres
plus homogeneous [Cp*RhTsDPEN] was used as a catalyst
in the same reaction system. After the reaction mixture was
stirred for 5.0 h and was filtered, the products in the solution
were analyzed. The collected solids were resuspended in the
It is worth mentioning that the tandem reduction/lactoniza-
tion of ethyl 2-(2-phenylacetyl)benzoate catalyzed by cata-
lyst 3 had a reaction ratio higher those attained with inorga-
nosilica-based mesoporous heterogeneous catalysts in gener-
al,[7a–d] even higher than that with its homogenous counter-
part (the reaction catalyzed by 3 could be completed within
8 h, whereas 10 h was needed with the homogeneous cataly-
sis system: Table 1, entry 1 vs. value in parentheses). This
characteristic, which is rare in general heterogeneous cataly-
sis systems, suggests that the high hydrophobicity of ethyl-
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Guohua Liu et al.
ene-bridged hollow-shell-structured nanospheres significant-
ly promoted the catalytic performance. This phenomenon is
strongly similar to the observed hydrophobicity of organosil-
ica-bridged PMO in PMO-supported heterogeneous cata-
lysts in our previous studies.[7f–h] Evidence to support this
view came from an investigation done on a Pickering emul-
sion (Figure 7). Because the stabilized Pickering emulsion
action catalyzed by catalyst 3 resulted in higher initial activi-
ty than that achieved with [Cp*RhTsDPEN] (the initial
TOFs within 1.0 h were 59.8 and 48.7 molmolÀ1 hÀ1, respec-
tively). This investigation confirmed that the enhanced reac-
tion rate in this asymmetric organic transformation is attrib-
uted to the high hydrophobicity of ethylene-bridged hollow-
shell-structured nanospheres that is in agreement with the
result of the Pickering emulsion experiment.
In addition to the increased reaction ratio discussed
above, a slightly enhanced ee value was also observed in this
case. Taking the synthesis of (S)-3-benzylphthalide as an ex-
ample, tandem reduction/lactonization of ethyl 2-(2-phenyla-
cetyl)benzoate catalyzed by catalyst 3 afforded (S)-3-ben-
zylphthalide with an ee value of more than 99%. This value
was slightly higher than that of its homogeneous counterpart
(Table 1, entry 1 vs. value in parentheses) and markedly
higher that of its analogue [AreneRuTsDPEN] (93% ee;
Arene=cymenebenzene) reported in the literature.[2b] This
result indicates that the synergistic effect of the high hydro-
phobicity of the hollow-shell-structured nanospheres dis-
cussed above and the confined chiral organorhodium cata-
lytic nature are responsible for enhanced enantioselectivity.
Indirect evidence to support the hypothesis came from an
XPS investigation (Figure 3); catalyst 3 had the nearly same
Rh3d5/2 binding energy as that of [Cp*RhTsDPEN] (309.57
vs. 309.55 eV), as discussed above. This similarity indicated
that the active center of catalyst 3 had the same microenvir-
onment as that of the active center of [Cp*RhTsDPEN].
Thus, the enhanced enantioselectivity was due to the syner-
gistic effect of the high hydrophobicity of hollow-shell-struc-
tured nanospheres and the uniformly dispersed single-site
active Rh species proved by the TEM image with a chemical
mapping (Figure 6b). To further demonstrate this conclu-
sion, a parallel experiment with TsDPEN-functionalized-
PMO (2) plus its homogeneous counterpart as a catalyst was
performed. The result showed that tandem reduction/lacto-
nization of ethyl 2-(2-phenylacetyl)benzoate within 8.0 h af-
forded (S)-3-benzylphthalide with greater than 99% conver-
sion and 96% ee (Table 1, entry 3). This ee value is very sim-
ilar to its homogenous counterpart (Table 1, entry 1, value
in parentheses), but slightly lower than that of catalyst 3
(Table 1, entry 1). Notably, it is the same value as that ob-
tained with the homogeneous counterpart due to the nature
of homogeneous catalysis. The slightly higher ee value ob-
tained with catalyst 3 further demonstrated that the en-
hanced enantioselectivity originates from the hollow-shell-
structured nanospheres with the uniformly dispersed single-
site active Rh species.
Figure 7. Optical microscope fluorescent image of the Pickering emulsion
with ethyl 2-(2-phenylacetyl)benzoate marked by using BODIPY dye.
marked by a fluorescent BODIPY dye (BODIPY: 4,4-di-
fluoro-8-(2-{2-[(1,3-dioxoisoindolin-2-yl)oxy]acetamido}-
phenyl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene)
could be observed under an optical microscope,[14] a large
amount of colloids were formed and all ethyl 2-(2-phenyla-
cetyl)benzoate was microencapsulated in the 2-acylarylcar-
boxylate-in-water system. This observation demonstrated
that formation of an oil-in-water emulsion in the reaction
system played an important role in promoting the catalytic
performance that originated from the high hydrophobicity
of ethylene-bridged shells. To further confirm this conclu-
sion, the kinetic profile of the tandem reduction/lactoniza-
tion of ethyl 2-(2-phenylacetyl)benzoate catalyzed by cata-
lyst 3 and homogeneous [Cp*RhTsDPEN] were also investi-
gated as shown in Figure 8. The results showed that the re-
Catalyst Recycling and Reuse
An important aim of the design of any heterogeneous cata-
lyst is its easy separation by simple filtration and retention
of catalytic activity and enantioselectivity of the recovered
catalyst after multiple cycles. As shown in Table 2, catalyst 3
was recovered easily and reused in the reaction with 2-(2-
phenylacetyl)benzoate. Remarkably, after 10 consecutive re-
actions, recycled catalyst 3 still afforded the desired products
Figure 8. Comparison of the asymmetric transfer hydrogenation of ethyl
*
2-(2-phenylacetyl)benzoate catalyzed by catalyst 3 ( ) and homogeneous
&
[Cp*RhTsDPEN] ( ). Conditions: T=408C, catalyst (10.0 mmol),
HCOONa (5 equiv), and
(5.0 mL).
a substrate/catalyst ratio of 200 in water
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Guohua Liu et al.
Table 2. Reusability of catalyst 3 with ethyl 2-(2-phenylacetyl)benzoate
that, the mixture was aged at 1008C for 24 h. The resulting solid was fil-
tered and rinsed with excess deionized water before being dried over-
night on a filter. The surfactant template was removed by Soxhlet extrac-
tion with ethanol for 24 h. The solid was filtered and rinsed with ethanol
again, then dried at 608C in vacuum overnight to afford TsDPEN-func-
tionalized PMO material (2) in the form of a white powder (yield:
2.56 g). 13C CP MAS NMR (161.9 MHz): d=126.6 (CH of Ph), 66.6–70.9
(N-CH-Ph), 57.3 (O-CH2CH3), 27.8 (CH2Ph), 16.8 (O-CH2CH3), 4.5 ppm
(CH2Si); 29Si MAS NMR (79.4 MHz): T1 (d=À49.3 ppm), T2 (d=
as a substrate.[a]
Cycle
Conversion [%][b]
ee [%][b]
1
2
3
4
5
6
7
8
99.9
99.7
99.6
99.3
99.4
99.3
99.3
99.1
99.0
95.4
99.8
99.7
99.4
99.5
98.0
99.0
98.2
98.2
97.9
98.1
À59.5 ppm), T3 (d=À65.7 ppm); IR (KBr): n=3419.4 (s), 2963.8 (m),
~
2934.3 (m), 2868.8 (m), 1648.2 (m), 1464.3 (w), 1346.8 (w), 1082.1 (s),
942.8 (s), 802.8 (m), 560.6 cmÀ1 (w); SBET: 113.8 m2 gÀ1, dpore: 15.19 nm,
Vpore: 0.28 cm3 gÀ1
.
9
10
Preparation of Heterogeneous Catalyst 3
[a] Reaction conditions: catalyst 3 (68.70 mg, 20.0 mmol of Rh based on
ICP analysis), HCOONa (10.0 mmol), cetyltrimethylammonium bromide
(80.0 mmol), 2-acylarylcarboxylate (2.0 mmol), and water (10.0 mL), T=
408C, t=8.0 h. [b] Determined by using chiral HPLC analysis.
In a typical synthesis, [Cp*RhCl2]2 (0.20 g, 0.32 mmol) was added to
a stirred suspension of 2 (1.0 g) in dry CH2Cl2 (20 mL) at RT. The result-
ing mixture was stirred at RT for 12 h. The mixture was then filtered
through filter paper and rinsed with excess dry CH2Cl2. After Soxhlet ex-
traction for 24 h with dry CH2Cl2 to remove the starting materials, the
solid was dried at 608C under vacuum overnight to afford catalyst 3 as
a light yellow powder (yield: 1.02 g). ICP analysis shows that the Rh
loading amount is 29.97 mg (0.291 mmol) per gram of catalyst.
13C CP MAS NMR (161.9 MHz): d=126.4 (CH of Ph), 93.5 (C of Cp
ring), 66.7–71.0 (N-CH-Ph), 58.2 (O-CH2CH3), 29.1 (CH2Ph), 16.6 (O-
CH2CH3), 10.0 (CpCH3), 4.5 ppm (CH2Si); 29Si MAS NMR (79.4 MHz):
T1 (d=À49.7 ppm), T2 (d=À59.8 ppm), T3 (d=À64.7 ppm); IR (KBr):
with 95.4% conversion and 98.1% ee. The high recyclability
is due to the uniformly dispersed single-site active Rh spe-
cies within the hollow-shell-structured nanospheres through
a covalent-bonding immobilization approach, which could
efficiently reduce leaching of Rh as shown by using induc-
tively coupled plasma (ICP) analysis. The amount of Rh re-
maining after the tenth cycle was 28.83 mg per gram of cata-
lyst and the amount of Rh lost was only 3.8%, which dem-
onstrates that its high recyclability was attributed to the
minimized leaching of Rh.
~
n=3440.2 (s), 2961.8 (w), 2802.3 (w), 1632.1 (m), 1456.8 (w), 1406.3 (w),
1265.6 (m), 1153.8 (s), 1087.4 (s), 1013.9 (s), 910.3 (m), 792.8 (m),
696.8 cmÀ1 (m); SBET: 103.8 m2 gÀ1, dpore: 15.09 nm, Vpore: 0.27 cm3 gÀ1
.
General Procedure for the Asymmetric Synthesis of Chiral Phthalides
In a typical procedure, heterogeneous catalyst 3 (2.0 mmol of Rh based
on ICP analysis), HCOONa (1.0 mmol), cetyltrimethylammonium bro-
mide (4.0 mol%), 2-acylarylcarboxylate (0.20 mmol), and water (2.0 mL;
with 0.10 mL of CH2Cl2 as cosolvent if the substrates were in solid or vis-
cous liquid state) were added to a 5 mL flask and purged with nitrogen.
The mixture was allowed to react at 408C for 8–12 h. During that time,
the reaction was monitored constantly by using TLC. After completion
of the reaction, the heterogeneous catalyst was separated by using a cen-
trifuge (10000 rpm) for the recycling experiment. The aqueous solution
was extracted with Et2O (3ꢁ3.0 mL). The combined Et2O fractions were
washed with twice brine and dried over Na2SO4. After evaporation of
Et2O, the residue was purified by using silica gel flash column chromatog-
raphy to afford the desired product. The ee value could be determined by
using HPLC analysis with a Photo-Diode Array detector using a Daicel
chiralcel column (0.46ꢁ25 cm).
Conclusion
We used a hollow-shell-structured nanosphere as a support
and developed an organorhodium-functionalized heteroge-
neous catalyst. The catalyst displays excellent catalytic effi-
ciency and high recyclability in the asymmetric synthesis of
chiral phthalides by the reduction/lactonization of 2-acylar-
ylcarboxylates in aqueous medium. As expected, the superi-
or catalytic performance is attributed to the synergistic
effect of the high hydrophobicity and the confined chiral or-
ganorhodium catalytic nature. The organorhodium-function-
alized nanospheres could be conveniently recovered and
reused at least 10 times without loss of catalytic activity.
This study offers a new approach to immobilize various
chiral complexes onto a hollow-shell-structured functional-
ized nanosphere, which represents our future efforts.
Acknowledgements
We are grateful to the China National Natural Science Foundation
(20673072), the Shanghai Sciences and Technologies Development Fund
(12nm0500500 and 13ZR1458700), CSIRT (IRT1269), and Shanghai Mu-
nicipal Education Commission (12ZZ135, 14YZ074, SK201329) for finan-
cial support.
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Received: November 18, 2013
Revised: January 15, 2014
Published online: March 12, 2014
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim