Organorhodium-functionalized periodic mesoporous organosilica: High hydrophobicity promotes asymmetric transfer hydrogenation in aqueous medium

Article abstract of DOI:10.1002/asia.201300738

Three organosilica-bridged periodic mesoporous organosilicas were prepared by the immobilization of a chiral N-sulfonylated diamine-based organorhodium complex within their silicate network. Structural analysis and characterization confirmed their well-defined single-site active rhodium centers, whilst electron microscopy revealed their highly ordered hexagonal mesostructures. Among these three different organosilica-bridged periodic mesoporous organosilicas, the ethylene-bridged periodic mesoporous organosilica catalyst exhibited excellent heterogeneous catalytic activity and high enantioselectivity in the aqueous asymmetric transfer hydrogenation of aromatic ketones. This superior catalytic performance was attributed to its salient hydrophobicity, whilst its comparable enantioselectivity relative to the homogeneous catalyst was derived from the confined nature of the chiral organorhodium catalytic sites. Furthermore, this ethylene-bridged periodic mesoporous organosilica could be conveniently recovered and reused at least 12 times without the loss of its catalytic activity. This feature makes this catalyst attractive for practical organic synthesis in an environmentally friendly manner. This study offers a general way of optimizing the bridged organosilica moiety in periodic mesoporous organosilicas, thereby enhancing its catalytic activity in heterogeneous catalysis. Copyright

Full text of DOI:10.1002/asia.201300738

FULL PAPER
DOI: 10.1002/asia.201300738
Organorhodium-Functionalized Periodic Mesoporous Organosilica: High
Hydrophobicity Promotes Asymmetric Transfer Hydrogenation in Aqueous
Medium
Rui Liu, Ronghua Jin, Lingyu Kong, Jinyu Wang, Chen Chen, Tanyu Cheng,* and
[
a]
Guohua Liu*
Abstract: Three organosilica-bridged
periodic mesoporous organosilicas
were prepared by the immobilization
of a chiral N-sulfonylated diamine-
based organorhodium complex within
their silicate network. Structural analy-
sis and characterization confirmed
their well-defined single-site active rho-
dium centers, whilst electron microsco-
py revealed their highly ordered hexag-
onal mesostructures. Among these
three different organosilica-bridged pe-
riodic mesoporous organosilicas, the
ethylene-bridged periodic mesoporous
organosilica catalyst exhibited excellent
heterogeneous catalytic activity and
high enantioselectivity in the aqueous
asymmetric transfer hydrogenation of
aromatic ketones. This superior catalyt-
ic performance was attributed to its sa-
lient hydrophobicity, whilst its compa-
rable enantioselectivity relative to the
homogeneous catalyst was derived
from the confined nature of the chiral
organorhodium catalytic sites. Further-
more, this ethylene-bridged periodic
mesoporous organosilica could be con-
veniently recovered and reused at least
12 times without the loss of its catalytic
activity. This feature makes this cata-
lyst attractive for practical organic syn-
thesis in an environmentally friendly
manner. This study offers a general
way of optimizing the bridged organo-
silica moiety in periodic mesoporous
organosilicas, thereby enhancing its cat-
alytic activity in heterogeneous cataly-
sis.
Keywords: asymmetric catalysis
·
heterogeneous catalysis · immobili-
zation · mesoporous materials
·
silicas
[
4]
Introduction
aqueous solution. More importantly, various organosilicate
walls offer potential additional interactions with chiral
active centers that are incorporated in the PMO silicate net-
works, such as pÀp interactions and hydrogen bonds, which
Periodic mesoporous organosilicas (PMOs) that contain
chiral organic moieties embedded within their silicate net-
works have attracted a great deal of interest and have
are beneficial for adjusting the chiral microenvironment
around the active center and can enhance its enantioselec-
tive performance. Until now, although a large number of
achiral-organosilica-bridged periodic mesoporous organosili-
cas have been reported and some periodic mesoporous or-
ganosilicas that incorporated chiral functionalities within
their silica networks for asymmetric catalysis have also been
explored, to the best of our knowledge, most of them still
suffer from lower catalytic activities and enantioselectivities
than their homogeneous counterparts. Thus, the utilization
of the significant features of periodic mesoporous organosili-
ca and the systemic adjustment of the organic bridge to ach-
ieve high catalytic activity and enantioselectivity remain
unmet challenges in heterogeneous asymmetric catalysis.
We are interested in heterogeneous catalysts, in particular
the development of various silica-based mesoporous hetero-
geneous catalysts. Recently, we have developed a series of
inorganosilica-based chiral mesoporous heterogeneous cata-
lysts, some of which have shown excellent catalytic activities
[1]
shown some salient features in asymmetric catalysis. Like
[2]
inorganosilicate mesoporous materials, they also possess
large specific surface areas/pore volumes, tunable pore di-
mensions/well-defined pore arrangements, and high thermal/
mechanical stabilities. These properties open up a variety of
possibilities for the immobilization of various homogeneous
complexes onto periodic mesoporous organosilicas as sup-
ports. Unlike inorganosilicate mesoporous materials, peri-
odic mesoporous organosilicas have a highly hydrophobic
inner surface, owing to their intrinsic organosilicate walls,
which may facilitate asymmetric reactions in a two-phase
catalytic system and can promote organic transformations in
[5]
[6]
[3]
[
a] Dr. R. Liu, Dr. R. Jin, Dr. L. Kong, Dr. J. Wang, Dr. C. Chen,
Dr. T. Cheng, Prof. G. Liu
Key Laboratory of Resource Chemistry of
The Ministry of Education
Shanghai Key Laboratory of Rare Earth Functional Materials
Shanghai Normal University
Shanghai 200234 (P. R. China)
[7]
and enantioselectivities in asymmetric reactions, in particu-
lar SBA-16-based chiral-organorhodium-functionalized mes-
oporous heterogeneous catalysts. However, these inorga-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201300738.
[7h]
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Tanyu Cheng, Guohua Liu et al.
nosilica-based
mesoporous
chiral heterogeneous catalysts
often showed low reaction
rates, owing to the nature of
the inorganosilicate materials.
Interestingly, in organosilica-
based chiral-organonickel-func-
tionalized periodic mesoporous
[8]
organosilica, the superior hy-
drophobicity of the periodic
mesoporous organosilica af-
fords much-higher catalytic per-
formance in the asymmetric Mi-
chael addition of 1,3-dicarbonyl
compounds to nitroalkenes.
Herein, we utilize the salient
hydrophobicity of periodic mes-
oporous organosilica to facili-
tate an asymmetric transfer-hy-
Scheme 1. Synthesis of heterogeneous catalysts 3–5.
drogenation reaction in a typical
two-phase catalytic system and
we develop three different organosilica-bridged chiral-
Cp*RhTsDPEN-functionalized (Cp*=pentamethyl cyclo-
pentadiene, TsDPEN=4-methylphenylsulfonyl-1,2-dipheny-
functionalized PMOs (2a–2c) as white powders. Finally, het-
erogeneous catalysts 3–5 were successfully obtained by the
direct complexation of [(Cp*RhCl ) ] with compounds 2a–
2c, followed by further Soxhlet extraction to clear their
nanochannels (see the Supporting Information, Figures S1–
S4).
2
2
[9]
lethylenediamine) periodic mesoporous organosilicas. As
expected, the superior hydrophobicity of these periodic mes-
oporous organosilicas affords greatly enhanced catalytic per-
formance in the asymmetric transfer hydrogenation of aro-
matic ketones in aqueous medium. Moreover, by systemical-
ly optimizing the organosilica bridge, we found that ethyl-
ene-bridged chiral-Cp*RhTsDPEN-functionalized periodic
mesoporous organosilica was the best chiral promoter,
thereby affording excellent catalytic activity and comparable
enantioselectivity to its homogeneous counterpart. Further-
more, this heterogeneous catalyst could be conveniently re-
covered and reused at least 12 times without loss of its cata-
lytic activity, thus showing its potential industrial applica-
tion. Moreover, a systemic comparison of three bridged pe-
riodic mesoporous organosilicas in terms of their enantiose-
lective performance is also discussed in detail.
The
incorporation
of
well-defined
single-site
Cp*RhTsDPEN active centers within the organosilicate net-
1
3
works was confirmed by solid-state C cross-polarization
(CP) magic angle spinning (MAS) NMR spectroscopy (see
the Supporting Information, Figure S2). Taking TsDPEN-
functionalized PMO 2a and catalyst 3 as representative ex-
amples in Figure 1, we found that both compounds showed
strong characteristic signals of the SiCH2 groups at d
ꢀ7 ppm, which were ascribed to ethylene-bridged silica that
was embedded within the silicate networks. A comparison
of the spectrum of TsDPEN-functionalized PMO 2a and
catalyst 3 indicated that the peak at dꢀ95 ppm was derived
from carbon atoms of the Cp rings, whilst the peak at d
ꢀ
9 ppm corresponded to the carbon atoms of CH groups
3
Results and Discussion
Synthesis and Structural Characterization of the
Heterogeneous Catalysts
Three different organosilica-bridged N-sulfonylated dia-
mine-based organorhodium-functionalized periodic mesopo-
rous organosilicas, abbreviated as Cp*RhTsDPEN-PMO (3–
5
), were synthesized as shown in Scheme 1. Firstly, the key
chiral TsDPEN-derived silane (S,S)-4-(trimethoxysilyl)-
ACHTUNGTRENNUNGe thyl)phenylsulfonyl-1,2-diphenylethylenediamine was ob-
[7f]
tained according to our previously reported procedure.
Then, the co-condensation of TsDPEN-derived silane and
various bridged silanes (1a–1c; a: 1,2-bis(triethoxysilyl)-
ethane>, b: 1,2-bis(triethoxysilyl)ethylene, c: 1,4-bis(trie-
thoxysilyl)benzene) afforded their corresponding TsDPEN-
13
Figure 1. C CP MAS NMR spectra of TsDPEN-functionalized PMO 2a
and catalyst 3.
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Tanyu Cheng, Guohua Liu et al.
that were connected to the Cp rings, thus suggesting that
complexation of the CpMe moiety and a chiral ligand had
5
occurred. Moreover, catalyst 3 clearly displayed characteris-
tic peaks at dꢀ78–82 ppm, which corresponded to the
carbon atoms of NCH groups that were connected to phenyl
groups. In addition, the characteristic peaks at dꢀ127 ppm
corresponded to the carbon atoms of the phenyl rings. Inter-
estingly, in sharp contrast to the shifts of the signals for the
[10]
CpMe and NCH moieties, the spectrum of catalyst 3 was
5
largely similar to that of its homogeneous counterpart, thus
confirming that catalyst 3 had the same well-defined single-
site active center. In addition, the peaks at dꢀ16 and
58 ppm should be attributed to nonhydrolyzed ethoxy
groups (CH CH O) that often appear in the CP MAS spec-
3
2
[
11]
tra.
Figure 3. XPS spectra of Cp*RhTsDPEN and catalysts 3–5.
In addition, the organosilicate networks and compositions
of heterogeneous catalysts 3–5 were confirmed by solid-state
29
Si MAS NMR spectroscopy (see the Supporting Informa-
tion, Figure S3). As shown in Figure 2, TsDPEN-functional-
lysts 3–5 presented similar Rh 3d5/2 binding energies to that
of Cp*RhTsDPEN, which were clearly different from that
of the parent [(Cp*RhCl ) ] (see the Supporting Informa-
2
2
tion, Figure S5), thus demonstrating the formation of single-
site Cp*RhTsDPEN active centers within their organosili-
cate networks. Notably, catalyst 3 had a comparable Rh 3d5/2
binding energy to its homogeneous counterpart (309.57 eV
versus 309.55 eV), whilst catalysts 4 and 5 showed slight de-
viations from their homogeneous counterpart (309.10 and
3
09.37 eV versus 309.55 eV); these differences might be
a key factor in determining their enantioselective perfor-
mance (see below).
Furthermore, the ordered mesostructures and well-de-
fined pore arrangements of these materials were further
AHCTUNGTRENNUNGi nvestigated by using X-ray diffraction (XRD), TEM, and
2
9
nitrogen-adsorption/desorption techniques. As shown in
Figure 4, the small-angle XRD patterns revealed that cata-
lysts 3 and 4 presented one similar intense d100 diffraction
Figure 2. Si CP MAS NMR spectra of TsDPEN-functionalized PMO 2a
and catalyst 3.
peak, along with two similar weak diffraction peaks (d110
,
ized PMO (2a) and catalyst 3 each showed one group of ex-
clusive T signals that were derived from organosilica, thus
suggesting that all of the Si species were covalently attached
d200), thus suggesting that the dimensional hexagonal pore
structure (p6mm) that was observed in the pure PMO mate-
rials was preserved after co-condensation. The TEM mor-
phologies of catalysts 3 and 4 further confirmed their well-
ordered mesostructures with dimensional hexagonal ar-
[8]
[4]
to carbon atoms. From a comparison with typical isomeric
[
12]
shifts reported in the literature
(d=À48.5/À58.5/
1
2
3
À67.5 ppm
for
T /T /T ,
[R(HO) SiOSi]/[R(HO)Si-
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OSi) ]/[RSiACHTUNGTRENNUNG
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OSi) ]), the T signals at d=À58 and À64 ppm
2
3
2
3
corresponded to T [RSi ACHTUNGETRNN(UNG OSi) (OH)] and T [RSi ACTHUNGTRENNUNG( OSi) ]
2 3
(
R=Cp*RhTsDPEN or ethylene-bridged groups), respec-
[13]
tively, similar to literature values.
All of these observa-
tions demonstrated that TsDPEN-functionalized PMO 2a
and catalyst 3 possessed organosilicate networks with [RSi-
ACHTUNGTRENNUNG( OSi) (OH)] and [R-Si ACHTUNETGRNNNU(G OSi) ] species as their main organosi-
2 3
licate walls. Furthermore, the absence of signals for the Q-
series from d=À90 to À120 ppm indicated that no cleavage
of the carbonÀsilicon bond occurred during the hydrolysis/
condensation process.
To investigate the electronic state of the rhodium center,
X-ray photoelectron spectroscopy (XPS) of catalysts 3–5
and its homogeneous counterpart (Cp*RhTsDPEN) were
investigated. As shown in Figure 3, the XPS spectra of cata-
Figure 4. Small-angle powder-XRD patterns of catalysts 3 and 4.
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Tanyu Cheng, Guohua Liu et al.
Catalytic Properties of the Heterogeneous Catalysts
Catalysts Screen and Catalytic Performance
As a type of highly active and enantioselective catalysts,
chiral N-sulfonylated diamine-based organometallic com-
pounds have been extensively studied in the asymmetric hy-
[9]
drogenation of ketones. Herein, we examined their catalyt-
ic activities and enantioselectivities in the aqueous asymmet-
ric transfer hydrogenation of aromatic ketones according to
Figure 5. TEM images of catalysts 3 and 4, viewed along the [100] and
001] directions.
[15]
a literature procedure, with which various chiral alcohols
[
and their standard ee values have been established in our
[7]
previous reports. Owing to their different types of bridging
organosilicate walls, the as-prepared Cp*RhTsDPEN-PMO
catalysts (3–5) were first screened in the asymmetric reac-
tion of acetophenone; the reactions were performed by
using 1.0 mol% of the catalyst in the absence of Bu NBr
4
(
Table 1, entry 1). Among these three heterogeneous cata-
lysts, we found that the asymmetric reactions catalyzed by
catalysts 4 and 5 afforded (S)-1-phenyl-1-ethanol with lower
ee values than that of their homogenous counterpart. How-
ever, to our delight, catalyst 3 gave the corresponding prod-
ucts with 95% ee, comparable to that of its homogenous
[15a]
counterpart,
thus suggesting that ethylene-bridged heter-
ogeneous catalyst 3 was the best promoter of this asymmet-
ric reaction. Notably, the asymmetric reaction catalyzed by
catalyst 3 could be performed at a much higher substrate/
catalyst (S/C) ratio without affecting the ee value, as shown
by the asymmetric transfer hydrogenation of acetophenone
at S/C=500:1 (Table 1, entry 1 in parentheses).
Figure 6. Nitrogen-adsorption/desorption isotherms of catalysts 3 and 4.
rangements (Figure 5). In addition, the nitrogen-ad-
sorption/desorption isotherms of catalysts 3 and 4
[
a]
Table 1. Asymmetric transfer hydrogenation of aromatic ketones.
(
Figure 6) exhibited typical type-IV isotherms with
a H hysteresis loop and a visible step at P/P =
1
0
0.50–0.90, which corresponded to the capillary con-
densation of nitrogen in the mesopores; structural
parameters are listed in Figure 4, inset. Similarly, an
ordered mesostructure and well-defined pore ar-
rangement were also observed for heterogeneous
catalyst 5 (see the Supporting Information, Fig-
ure S6–S8). In particular, additional peaks were
also observed at 2q=10–508 in the wide-angle
powder XRD pattern for catalyst 5, which could be
assigned to a periodic phenylene-bridged structure
and were very similar to literature values (see the
Entry Substrate
Catalyst 3
Conversion ee [%]
Catalyst 4
Catalyst 5
Conversion ee
[
b]
Conversion ee
[b]
[b]
[b]
[b]
[b]
[
%]
[%]
[%]
[%]
[%]
[
c]
[c]
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
>99 (92)
85
95 (94)
94
>99
87
91
90
90
90
91
85
92
91
91
>99
87
89
88
88
85
84
82
88
89
85
[d]
[e]
>99
>99
>99
94
96
93
93
95
96
95
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
4-FPh
4-ClPh
4-BrPh >99
3-BrPh >99
4-MePh >99
[14]
Supporting Information, Figure S6).
4-
>99
Based on the above characterization and analysis,
chiral organorhodium-functionalized periodic meso-
OMePh
3-
OMePh
1
0
>99
95
>99
90
>99
88
porous
ACHTUNGTRENNUNGo rganosilicas that contained well-defined
11
2
4-CNPh >99
4-NO Ph >99
4-CF Ph >99
90
85
97
>99
>99
>99
80
79
84
98
95
>99
78
72
83
single-site Cp*RhTsDPEN active centers in or-
dered dimensional hexagonal pore structures could 13
1
2
3
be readily constructed with various types of bridged [a] Reaction conditions: Catalyst (2.00 mmol Rh based on ICP analysis), HCO Na
2
silane through a co-condensation strategy.
(0.68 g, 10.0 mmol), ketone (0.20 mmol), water (2.0 mL), 408C, 0.5–4.0 h. [b] Deter-
mined by chiral GC analysis (see the Supporting Information, Table S1 and Figure S9).
[
c] Homogeneous Cp*RhTsDPEN was used as the catalyst without the Bu
tive in aqueous medium. [c] S/C=500:1. [d] TsDPEN-functionalized PMO 2a–2c and
(Cp*RhCl ] were used as the catalysts. [e] TsDPEN-functionalized PMO 2a–2c and
homogeneous Cp*RhTsDPEN were used as the catalysts.
4
NBr addi-
[
2 2
)
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Tanyu Cheng, Guohua Liu et al.
Based on these results, catalyst 3 was further investigated
in the aqueous asymmetric transfer hydrogenation of vari-
ous substituted acetophenones. In general, excellent conver-
sions (with no side products) and high enantioselectivities
were obtained under similar conditions. Moreover, the elec-
tronic properties of the substituents did not affect their
enantioselectivities, that is, the asymmetric reactions with
electron-rich and electron-poor substituents on acetophe-
none were equally efficient (Table 1, entries 4–13). For com-
parison, the asymmetric reactions catalyzed by catalysts 4
and 5 were also performed (Table 1, columns 4 and 5).
in the absence of Bu NBr, catalyst 3 gave (S)-1-phenyl-1-
4
ethanol within 0.5 h with more than 99% conversion, higher
than that with its homogeneous counterpart (88% conver-
[7a]
sion).
This improvement was more noticeable for the
aqueous asymmetric reactions of 4-methoxylacetophenone.
[15a]
As reported previously,
this reaction needed 20 h to com-
plete the asymmetric organic transformation, owing to its
homogeneous catalytic nature. Herein, this asymmetric reac-
tion of 4-methoxylacetophenone catalyzed by catalyst 3 was
complete within 4.0 h. Notably, catalyst 3 showed a much-
faster reaction rate than the homogeneous catalyst. To fur-
ther confirm this result, the kinetic profiles for the asymmet-
ric reaction of 4-methoxylacetophenone catalyzed by cata-
lyst 3 and its homogeneous counterpart (Cp*RhTsDPEN)
were also investigated (Figure 7). These results showed that
Investigation of the Nature of the Heterogeneous Catalysis
Generally, in a heterogeneous catalysis system, residual ho-
mogeneous catalyst often disturbs the catalytic nature of the
heterogeneous catalysis through non-covalent physical ad-
sorption during the preparation of the heterogeneous cata-
lyst. In this case, to eliminate this possibility and to under-
stand the nature of the heterogeneous catalysis, two types of
parallel experiments, that is, reactions with TsDPEN-func-
tionalized PMOs 2a–2c plus [(Cp*RhCl ) ] and TsDPEN-
2
2
functionalized
PMOs
2a–2c
plus
homogeneous
Cp*RhTsDPEN as catalysts, were also investigated (Table 1,
entries 2 and 3). Taking the reactions of compound 2a plus
[
(Cp*RhCl ) ] and compound 2a plus Cp*RhTsDPEN as
2 2
examples, we found that the asymmetric reaction of aceto-
phenone afforded the corresponding alcohol with 85% and
Figure 7. Comparison of the asymmetric transfer hydrogenation reactions
of 4-methoxylacetophenone (squares) catalyzed by compound 3 and its
homogeneous counterpart (Cp*RhTsDPEN). The reactions were per-
formed at 408C with the catalyst (4.0 mmol, S/C=500:1) and HCOONa
>
99% conversion, respectively, and 94% ee in both cases.
The former reaction suggested that the catalyst that was syn-
thesized through an in situ complexation method could lead
to high enantioselectivity. The lower catalytic activity than
that of catalyst 3 should be ascribed to the fact that part of
(
5 equiv) in water (2 mL).
[
(Cp*RhCl ) ] was not coordinated during the catalysis pro-
catalyst 3 presented a higher initial activity than homogene-
2
2
cess, in which the loss of Rh in solution was detected by ICP
analysis. The latter case indicated that the homogeneous cat-
alyst could result in good catalytic performance through
non-covalent adsorption. However, when this catalyst under-
went Soxhlet extraction, the reused catalyst only gave tiny
products, clearly different from heterogeneous catalyst 3.
These results ruled out the role of noncovalent physical ad-
sorption, thus demonstrating that the nature of the hetero-
geneous catalysis was indeed derived from the heterogene-
ous catalyst itself.
ous Cp*RhTsDPEN: The initial TOFs after 1.0 h were 394.5
and 208.0 mol h , respectively. Clearly, catalyst 3 showed
À1 À1
a reaction rate that was almost twice that of its homogene-
ous counterpart, thus demonstrating that the enhanced reac-
tion rate in this asymmetric organic transformations was at-
tributed to its superior hydrophobicity; this result was con-
firmed by water/benzene gas-adsorption experiments and
Pickering emulsion experiments (see the Supporting Infor-
[16]
mation, Figures S10–S12).
Moreover, notably catalysts 3–5 showed clear differences
in their enantioselective performance. As shown in Table 1,
the asymmetric reaction catalyzed by catalyst 3 afforded
higher enantioselectivities for all of the tested ketones than
catalysts 4 and 5. According to the elemental and ICP analy-
ses, the total amount of chiral-organorhodium-functionalities
that were used in these three asymmetric reactions was
same; only the bridged organosilicate walls varied. Thus,
a reasonable prediction is that the bridged organosilicate
walls play an important role in determining their enantiose-
lective performance. A possible explanation for this result is
that the different organosilica-bridged functionalities offer
different additional interactions with chiral active centers
that are incorporated into their silicate networks, which
Investigation of the Factors that Affect Catalytic
Performance
More attractively, the asymmetric reactions catalyzed by
heterogeneous catalysts 3–5 presented clearly faster reaction
rates than those of typical inorganosilica-based mesoporous
[7a–d]
heterogeneous catalysts.
In this case, owing to the salient
hydrophobic features of heterogeneous catalysts 3–5, the
asymmetric reactions were performed in the absence of
Bu NBr, unlike the homogeneous systems, which often
4
needed Bu NBr as a phase-transfer catalyst to enhance their
4
catalytic performance. Taking acetophenone as an example,
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Tanyu Cheng, Guohua Liu et al.
result in a change in the chiral microenvironment within the
active center, thus affording differences in their enantiose-
lective performance. Evidence to support this view came
from X-ray photoelectron spectroscopy (XPS) investigations
fact that this co-condensation strategy, which proceeded
through a covalent-bonding immobilization approach, could
efficiently decrease the leaching of Rh. Evidence to support
this view came from ICP analysis, which showed that the
amount of Rh after the 13th cycle was 26.97 mg per gram of
catalyst and only 3.7% of the Rh content was lost, thus
demonstrating that its high recyclability was attributed to
the low leaching of Rh. Similarly, heterogeneous catalysts 4
and 5 were easily recovered and repeatedly reused when
acetophenone was chosen as a substrate. Recycled catalysts
4 and 5 retained their catalytic activities and enantioselectiv-
ities after eight cycles (see the Supporting Information,
Table S2).
(
Figure 3). Catalyst 3 had almost same Rh 3d5/2 binding
energy as its homogeneous counterpart (309.57 eV versus
09.55 eV). As a result, catalyst 3 presented comparable
3
enantioselectivity to its homogeneous counterpart. Howev-
er, in the cases of heterogeneous catalysts 4 and 5, the devi-
ations in their Rh 3d5/2 electron-binding energies from those
of their homogeneous counterparts (309.10 and 309.37 eV
versus 309.55 eV) were responsible for their low enantiose-
lectivities, owing to changes in the chiral microenvironment
within the active center. Among these three heterogeneous
catalysts, the main compositional differences were that cata-
lysts 4 and 5 had unsaturated ethenylene- or phenylene-
bridged organosilicas as their silica walls, whilst catalyst 3
had saturated ethylene-bridged organosilica as its silica
walls. Clearly, in catalysts 4 and 5, there were potential pÀp
Conclusions
In
conclusions,
three
organosilica-bridged
chiral-
Cp*RhTsDPEN-functionalized periodic mesoporous orga-
nosilicas have been prepared and their use in the aqueous
asymmetric transfer hydrogenation of aromatic ketones has
been investigated. As expected, these three heterogeneous
catalysts showed excellent catalytic performance, owing to
the hydrophobic nature of the periodic mesoporous organo-
silica, whilst the additional pÀp interactions between the un-
[5c]
interactions
between ethenylene- or phenylene-bridged
moieties and the Ph group of 1,2-diphenylethylenediamine,
which enforced a certain configurational transformation of
the active center, thereby resulting in a decrease in facial se-
lectivity. To confirm the presence of such pÀp interactions,
a comparable inorganosilica-based SBA-15-type mesoporous
heterogeneous catalyst was prepared by using a similar strat-
egy, in which inorganosilicate OÀSiÀO groups acted as its
saturated ethenylene-bridged or phenylene-bridged organo-
silica and the phenyl moiety on the chiral ligand played an
important role in their enantioselective performance. Fur-
thermore, ethylene-bridged heterogeneous catalyst 3 could
be conveniently recovered and reused at least 12 times with-
out loss of its catalytic activity. This study offers a general
way of optimizing the bridging organosilane moiety in peri-
odic mesoporous organosilicas to tune their catalytic activity
in heterogeneous reactions.
silicate walls rather than ethenylene- or phenylene-bridged
organosilicate moieties. These results showed that the ee
[7d]
value returned to normal value (95% ee) when the com-
parable inorganosilica-based SBA-15-type mesoporous het-
erogeneous catalyst was employed in the asymmetric trans-
fer hydrogenation of acetophenone, thus demonstrating that
the pÀp interactions in catalysts 4 and 5 were responsible
for their lower enantioselectivities.
Experimental Section
Catalyst Recycling and Reuse
Preparation of (S,S)-TsDPEN-Functionalized PMO 2a
An important feature of any heterogeneous catalyst is its
easy separation by simple filtration and the recovered cata-
lyst should retain its catalytic activity and enantioselectivity
after multiple cycles. As shown in Table 2, heterogeneous
catalyst 3 was easily recovered and repeatedly reused when
acetophenone was chosen as the substrate. Remarkably,
after 12 consecutive reactions, the recycled catalyst still af-
forded (S)-1-phenyl-1-ethanol with 99% conversion and
In a typical synthesis, the structure-directing agent pluronic P123 (2.0 g)
was completely dissolved in a mixture of 0.2 m HCl (80 mL) and KCl
(
6.0 g) and the mixture was stirred at RT for 1.0 h. Then, 1,2-bis(triethox-
ysilyl)ethane (3.36 mL, 9.10 mmol) was added as the silica precursor at
408C. After pre-hydrolysis period of 40 min, (S,S)-DPEN-SO Ph-
(CH Si(OMe) (0.48 g, 0.96 mmol) was added. The reaction mixture
a
A
H
U
G
R
N
U
G
2
A
H
U
G
R
N
N
2
)
2
A
T
N
T
E
N
N
3
was stirred at 408C for 24 h and aged at 1008C for 24 h. The resulting
solid was filtered and rinsed with excess EtOH before being dried over-
night on a filter. The surfactant template was removed by heating at
À1
92% ee. Notably, this high recyclability should be due to the
reflux in acidic EtOH (400 mLg ) for 24 h. The solid was filtered, rinsed
again with EtOH, and dried at 608C overnight under reduced
Table 2. Reusability of catalyst 3 in the asymmetric transfer hydrogenation of aceto-
phenone.
pressure to afford TsDPEN-functionalized PMO material 2a
(1.42 g) as a white powder. Si MAS NMR (79.4 MHz): T
[
a,b]
2
9
2
3
13
(
(
(
(
1
(
d=À58.6 ppm),
161.9 MHz): d=127.4 (CH of Ph), 74.8, 70.1 (NCHPh), 57.4
OCH CH ), 28.8 (CH Ph), 16.6 (OCH CH ), 14.6–0.5 ppm
CH Si); IR (KBr): n˜ =3434.4 (s), 2986.0 (w), 2896.0 (w),
657.6 (m), 1415.6 (w), 1263.3 (m), 1155.2 (s), 1068.7 (s), 913.1
T
(d=À63.9 ppm); C CP MAS NMR
Run
1
2
3
4
5
6
7
8
9
10
11
12
Conversion
%]
ee [%]
99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9
2
3
2
2
3
[
2
95.0 94.6 94.4 94.0 93.7 93.3 93.4 93.3 93.0 92.7 92.3 92.2
À1
2
À1
m), 766.1 (m), 697.7 (s), 448.1 cm (m); SBET =618.7 m g
pore =5.59 nm; Vpore =0.72 cm g
;
[
a] Reaction conditions: catalyst 3 (73.5 mg, 20.0 mmol Rh based on ICP analysis),
HCO Na (0.68 g, 100.0 mmol), acetophenone (2.0 mmol), water (20.0 mL), 408C, 0.5–
.0 h. [b] Determined by chiral GC analysis (see the Supporting Information, Fig-
3
À1
d
.
2
4
ure S13).
Chem. Asian J. 2013, 8, 3108 – 3115
3113
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Tanyu Cheng, Guohua Liu et al.
Preparation of (S,S)-TsDPEN-Functionalized PMO 2b
(10J1400103, 10JC1412300, and 12nm0500500), the CSIRT (IRT1269),
and the Shanghai Municipal Education Commission (12ZZ135 and
SK201329) for financial support.
Compound 2b was prepared according to the general procedure for com-
pound 2a by using 1,2-bis(triethoxysilyl)ethylene instead of 1,2-bis(trie-
thoxysilyl)ethane. The solid was dried overnight under reduced pressure
2
9
to afford compound 2b as a white powder; Si MAS NMR (79.4 MHz):
1
2
3
13
T
(d=À63.5 ppm),
T
(d=À72.5 ppm),
T
(d=À81.5 ppm); C CP
MAS NMR (161.9 MHz): d=145.8 (CH of Ph), 132.9 (CH=CH), 72.2,
[1] a) A. Thomas, Angew. Chem. 2010, 122, 8506; Angew. Chem. Int.
Ed. 2010, 49, 8328; b) N. Mizoshita, T. Taniab, S. Inagaki, Chem.
Soc. Rev. 2011, 40, 789; c) F. Hoffmann, M, Frçba, Chem. Soc. Rev.
2011, 40, 608; d) F. Hoffmann, M. Cornelius, J. Morell, M. Frçba,
Angew. Chem. 2006, 118, 3290; Angew. Chem. Int. Ed. 2006, 45,
3216; e) M. Kruk, Acc. Chem. Res. 2012, 45, 1678; f) J. A. Melero,
R. van Grieken, G. Morales, Chem. Rev. 2006, 106, 3790–3812;
g) Q. Yang, J. Liu, L. Zhang, C. Li, J. Mater. Chem. 2009, 19, 1945;
h) P. Van Der Voort, D. Esquivel, E. De Canck, F. Goethals, I. Van
Driessche, F. J. Romero-Salguero, Chem. Soc. Rev. 2013, 42, 3913.
6
9.9 (NCHPh), 58.7 (OCH
2 3 2
CH ), 30.7 (CH Ph), 20.8–10.5 ppm
(
OCH CH and CH Si); IR (KBr): n˜ =3416.8 (s), 3084.2 (w), 2976.8 (w),
2
3
2
2
1
4
931.9 (w), 1657.6 (m), 1496.3 (w), 1433.8 (w), 1388.9 (w), 1155.2 (s),
À1
074.5 (s), 922.2 (m), 814.2 (m), 679.4 (m), 518.0 cm (m); SBET
=
2
À1
3
À1
12.5 m g ; dpore =5.65 nm; Vpore =0.58 cm g
.
Preparation of Heterogeneous Catalyst 3
In a typical synthesis, to a stirring suspension of TsDPEN-functionalized
PMO material 2a (1.0 g) in dry CH Cl (20 mL) was added
(Cp*RhCl ] (0.10 g, 0.16 mmol) at RT. The resulting mixture was
stirred at RT for 12 h. Then, the mixture was filtered through filter paper
and rinsed with excess CH Cl and dry toluene. After Soxhlet extraction
in toluene to remove homogeneous and unreacted starting materials for
4 h, the solid was dried at 608C overnight under vacuum to afford cata-
lyst 3 (1.04 g) as a light-yellow powder. ICP analysis showed that the Rh
2
2
[
2] a) F. Kleitz in Handbook of Asymmetric Heterogeneous Catalysis,
Wiley-VCH, Weinheim, 2008, pp. 178; b) D. E. De Vos, M. Dams,
B. F. Sels, P. A. Jacobs, Chem. Rev. 2002, 102, 3615; c) H. Zou, S. S.
Wu, J. Shen, Chem. Rev. 2008, 108, 3893; d) S. Minakata, M. Komat-
su, Chem. Rev. 2009, 109, 711; e) A. Corma, Chem. Rev. 1997, 97,
[
2 2
)
2
2
2
2
373; f) J. M. Thomas, R. Raja, Acc. Chem. Res. 2008, 41, 708; g) C.
2
9
Li, H. D. Zhang, D. M. Jiang, Q. H. Yang, Chem. Commun. 2007,
547.
loading was 28.01 mg (0.272 mmol) per gram of catalyst. Si MAS NMR
2
3
13
(
(
(
(
2
9
5
79.4 MHz): T (d=À58.4 ppm), T (d=À63.7 ppm); C CP MAS NMR
[
[
3] a) M. E. Davis, Nature 2002, 417, 813; b) S. Inagaki, S. Guan, T.
Ohsuna, O. Terasaki, Nature 2002, 416, 304.
4] a) J. L. Huang, F. X. Zhu, W. H. He, F. Zhang, W. Wang, H. X. Li, J.
Am. Chem. Soc. 2010, 132, 1492; b) X. S. Yang, F. X. Zhu, J. L.
Huang, F. Zhang, H. X. Li, Chem. Mater. 2009, 21, 4925.
161.9 MHz): d=127.3 (CH of Ph), 94.4 (C of Cp ring), 81.3, 77.7
NCHPh), 57.4 (OCH
CpCH ), 14.6–0.5 ppm (CH
896.3 (w), 1657.1 (m), 1409.2 (w), 1271.6 (m), 1160.9 (s), 1087.7 (s),
13.3 (m), 766.8 (m), 702.2 (s), 445.3 cm (m); SBET =524.4 m g ; dpore
.24 nm; Vpore =0.66 cm g
2
CH
3
), 27.7 (CH
Si); IR (KBr): n˜ =3429.2 (s), 2978.8 (w),
2 2 3
Ph), 16.1 (OCH CH ), 8.8
3
2
À1
2
À1
=
3
À1
[5] a) Y. F. Lu, H. Y. Fan, N. Doke, D. A. Loy, R. A. Assink, D. A.
Lavan, C. J. Brinker, J. Am. Chem. Soc. 2000, 122, 5258; b) B. D.
Hatton, K. Landskron, W. Whitnall, D. D. Perovic, G. A. Ozin, Adv.
Funct. Mater. 2005, 15, 823; c) M. P. Kapoor, Q. H. Yang, S. Inagaki,
J. Am. Chem. Soc. 2002, 124, 15176; d) P. Borah, X. Ma, K. T.
Nguyen, Y. Zhao, Angew. Chem. 2012, 124, 7876; Angew. Chem. Int.
Ed. 2012, 51, 7756; e) M. Waki, N. Mizoshita, T. Tani, S. Inagaki,
Angew. Chem. 2011, 123, 11871; Angew. Chem. Int. Ed. 2011, 50,
.
Preparation of Heterogeneous Catalyst 4
Catalyst 4 was prepared according to the general procedure for catalyst
3
4
3
T
. The solid was dried overnight under reduced pressure to afford catalyst
as a brown powder. ICP analysis showed that the Rh loading was
2
9
9.04 mg (0.379 mmol) per gram of catalyst. Si MAS NMR (79.4 MHz):
1
2
3
13
(d=À65.2 ppm),
T
(d=À74.6 ppm),
T
(d=À82.6 ppm); C CP
1
1667; f) M. Guan, W. Wang, E. J. Henderson, ꢂ. Dag, C. Kꢃbel,
MAS NMR (161.9 MHz): d=146.4 (CH=CH),128.1 (CH of Ph), 94.7 (C
V. S. K. Chakravadhanula, J. Rinck, I. L. Moudrakovski, J. Thomson,
J. McDowell, A. K. Powell, H. Zhang, G. A. Ozin, J. Am. Chem.
Soc. 2012, 134, 8439; g) M. Mandal, Mi. Kruk, Chem. Mater. 2012,
of Cp ring), 78.6, 75.5 (NCHPh), 58.2 (OCH
2
CH
3 2
), 28.6 (CH Ph), 20.8–
1
3
1
S
0.5 (OCH CH and CH Si), 9.2 ppm (CpCH ); IR (KBr): n˜ =3392.5 (s),
089.5 (w), 2978.8 (w), 2942.3 (w), 1647.7 (m), 1455.4 (w), 1427.3 (w),
2
3
2
3
2
4, 123.
À1
381.4 (w), 1160.9 (s), 1069.8 (s), 922.6 (m), 794.1 (m), 435.8 cm (m);
[
6] a) A. Kuschel, S. Polarz, J. Am. Chem. Soc. 2010, 132, 6558; b) X.
Wu, T. Blackburn, J. D. Webb, A. E. Garcia-Bennett, C. M. Crud-
den, Angew. Chem. 2011, 123, 8245; Angew. Chem. Int. Ed. 2011, 50,
8095; c) X. Liu, P. Y. Wang, L. Zhang, J. Yang, C. Li, Q. H. Yang,
Chem. Eur. J. 2010, 16, 12727; d) R. A. Garcia, R. van Grieken, J. Ig-
lesias, V. Morales, N. Villajos, J. Catal. 2010, 274, 221; e) P. Y. Wang,
X. Liu, J. Yang, Y. Yang, L. Zhang, Q. H. Yang, C. Li, J. Mater.
Chem. 2009, 19, 8009; f) T. Seki, K. McEleney, C. M. Crudden,
Chem. Commun. 2012, 48, 6369.
2
À1
3 À1
BET =374.3 m g ; dpore =5.58 nm; Vpore =0.45 cm g
.
General Procedure for the Asymmetric Transfer Hydrogenation of
Aromatic Ketones
A typical procedure is as follows: Heterogeneous catalyst 3 (2.00 mmol
Rh based on ICP analysis), HCO
2
Na (0.68 g, 10.0 mmol), ketone
(
0.2 mmol), and water (2.0 mL) were successively added into a 10 mL
round-bottomed flask. The mixture was heated at 408C for 0.5–4.0 h,
during which time the reaction was monitored by TLC. After completion
of the reaction, the heterogeneous catalyst was separated by centrifuga-
[
7] a) H. S. Zhang, R. H. Jin, H. Yao, S. Tang, J. L. Zhuang, G. H. Liu,
H. X. Li, Chem. Commun. 2012, 48, 7874; b) W. Xiao, R. H. Jin,
T. Y. Cheng, D. Q. Xia, H. Yao, F. Gao, B. X. Deng, G. H. Liu,
Chem. Commun. 2012, 48, 11898; c) Y. Q. Sun, G. H. Liu, H. Y. Gu,
T. Z. Huang, Y. L. Zhang, H. X. Li, Chem. Commun. 2011, 47, 2583;
d) J. Long, G. H. Liu, T. Y. Cheng, H. Yao, Q. Q. Qian, J. L. Zhuang,
F. Gao, H. X. Li, J. Catal. 2013, 298, 41; e) S. Tang, R. H. Jin, H. S.
Zhang, H. Yao, J. L. Zhuang, G. H. Liu, H. X. Li, Chem. Commun.
À1
tion (10000 rmin ) for the recycling experiment. The aqueous solution
was extracted with Et
twice with brine and dehydrated with Na
Et O, the residue was purified by column chromatography on silica gel to
2
O (3ꢁ3.0 mL). The combined Et
2
O was washed
2
SO . After the evaporation of
4
2
afford the desired product. The conversion and ee value were determined
by chiral GC on a chiral Supelco b-Dex 120 column (30 mꢁ0.25 mm i.d.,
0
.25 mm film) or by HPLC analysis with a UV/Vis detector on a Daicel
2
012, 48, 6286; f) G. H. Liu, J. Y. Wang, T. Z. Huang, X. H. Liang,
OD-H chiralcel column (F=0.46ꢁ25 cm).
Y. L. Zhang, H. X. Li, J. Mater. Chem. 2010, 20, 1970; g) D. Q. Xia,
T. Y. Cheng, W. Xiao, K. K. Liu, Z. L. Wang, G. H. Liu, H. L. Li, W.
Wang, ChemCatChem 2013, 5, 1784–1789; h) Y. L. Xu, T. Y. Cheng,
J. Long, K. T. Liu, Q. Q. Qian, F. Gao, G. H. Liu, H. X. Li, Adv.
Synth. Catal. 2012, 354, 3250.
Acknowledgements
[8] a) R. H. Jin, K. T. Liu, D. Q. Xia, Q. Q. Qian, G. H. Liu, H. X. Li,
Adv. Synth. Catal. 2012, 354, 3265; b) K. T. Liu, R. H. Jin, T. Y.
Cheng, X. M. Xu, F. Gao, G. H. Liu, H. X. Li, Chem. Eur. J. 2012,
18, 15546.
We are grateful to the National Natural Science Foundation of China
20673072), the Shanghai Sciences and Technologies Development Fund
(
Chem. Asian J. 2013, 8, 3108 – 3115
3114
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Tanyu Cheng, Guohua Liu et al.
[
9] a) T. Ikariya, A. J. Blacker, Acc. Chem. Res. 2007, 40, 1300;
b) J. E. D. Martins, G. J. Clarkson, M. Wills, Org. Lett. 2009, 11, 847;
c) R. Malacea, R. Poli, E. Manoury, Coord. Chem. Rev. 2010, 254,
[15] a) X. F. Wu, X. H. Li, A. Zanotti-Gerosa, A. Pettman, J. Liu, A. J.
Mills, J. L. Xiao, Chem. Eur. J. 2008, 14, 2209; b) X. F. Wu, J. Liu,
D. D. Tommaso, J. A. Iggo, C. R. Catlow, J. Bacsa, J. L. Xiao, Chem.
Eur. J. 2008, 14, 7699; c) P. N. Liu, J. G. Deng, Y. Q. Tu, S. H. Wang,
Chem. Commun. 2004, 2070; d) N. A. Cortez, G. Aguirre, M. Parra-
Hake, R. Somanathan, Tetrahedron Lett. 2009, 50, 2228.
7
29.
[
[
10] J. Mao, D. C. Baker, Org. Lett. 1999, 1, 841.
11] P. F. W. Simon, R. Ulrich, H. W. Spiess, U. Wiesner, Chem. Mater.
2
001, 13, 3464.
12] O. Krçcher, O. A. Kçppel, M. Frçba, A. Baiker, J. Catal. 1998, 178,
84.
13] a) B. J. Melde, B. T. Holland, C. F. Blanford, A. Stein, Chem. Mater.
999, 11, 3302; b) N. Linares, A. E. Sepffllveda, J. R. Berenguer, E.
Lalinde, J. Garcia-Martinez, Microporous Mesoporous Mater. 2012,
58, 300.
14] a) Q. Yang, M. P. Kapoor, S. Inagaki, J. Am. Chem. Soc. 2002, 124,
694.
[16] a) Y. Kuwahara, J. Aoyama, K. Miyakubo, T. Eguchi, T. Kamegawa,
K. Mori, H. Yamashita, J. Catal. 2012, 285, 223; b) L. A. Fielding,
S. P. Armes, J. Mater. Chem. 2012, 22, 11235.
[
[
2
1
Received: June 1, 2013
Revised: July 4, 2013
Published online: September 2, 2013
1
[
9
Chem. Asian J. 2013, 8, 3108 – 3115
3115
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim