Functionalized periodic mesoporous organosilica: A highly enantioselective catalyst for the michael addition of 1,3-dicarbonyl compounds to nitroalkenes

Article abstract of DOI:10.1002/chem.201202407

A functionalized periodic mesoporous organosilica with incorporated chiral bis(cyclohexyldiamine)-based Ni^II complexes within the silica framework was developed by the co-condensation of (1R,2R)-cyclohexyldiamine- derived silane and ethylene-br

Full text of DOI:10.1002/chem.201202407

DOI: 10.1002/chem.201202407
Functionalized Periodic Mesoporous Organosilica: A Highly Enantioselective
Catalyst for the Michael Addition of 1,3-Dicarbonyl Compounds to
Nitroalkenes
Ketang Liu,[a] Ronghua Jin,[a] Tanyu Cheng,[a] Xiangming Xu,[a] Fei Gao,[a]
Guohua Liu,*[b] and Hexing Li*[b]
Abstract:
mesoporous organosilica with incorpo-
rated chiral bis(cyclohexyldiamine)-
A
functionalized periodic
FTIR and solid-state NMR spectrosco-
py demonstrated the incorporation of
well-defined single-site bis(cyclohexyl-
diamine)-based NiII active centers
within periodic mesoporous organosili-
ca. As a chiral heterogeneous catalyst,
this functionalized periodic mesopo-
rous organosilica showed high catalytic
activity and excellent enantioselectivity
in the asymmetric Michael addition of
1,3-dicarbonyl compounds to nitroal-
kenes, comparable to those with homo-
geneous catalysts. In particular, this
heterogeneous catalyst could be recov-
ered easily and reused repeatedly up to
nine times without obviously affecting
its enantioselectivity, thus showing
good potential for industrial applica-
tions.
AHCTUNGTRENNUNG
based NiII complexes within the silica
framework was developed by the co-
condensation of (1R,2R)-cyclohexyldia-
mine-derived silane and ethylene-
bridge silane, followed by the complex-
ation of NiBr2 in the presence of
(1R,2R)-N,N’-dibenzylcyclohexyldi-
Keywords: asymmetric catalysis
heterogeneous catalysis · immobili-
zation mesoporous materials
Michael addition
·
ACHTUNGTRENNUNGamine. Structural characterization by
XRD, nitrogen sorption, and TEM dis-
closed its orderly mesostructure, and
·
·
Introduction
actions. In particular, as a typical organosilane-based meso-
porous material, periodic mesoporous organosilicas (PMOs)
with a bridging chiral moiety within the silica framework
have shown some salient features in asymmetric catalysis.[5]
Besides the general advantages of inorganosilane-based
mesoporous materials, such as relatively large surface area,
tunable pore volume, and well-defined pore arrangement,
periodic mesoporous organosilicas have obvious hydropho-
bicity, owing to their intrinsic organic functionality on their
inner surface, which can promote catalytic activity. More im-
portantly, functionalized periodic mesoporous organosilicas
with chiral ligands embedded within the silica framework
possess special rigidity, which is beneficial for the construc-
tion of suitable chiral microenvironments to enhance enan-
tioselectivity. Moreover, they also exhibit superior thermal
and mechanical stability relative to polymer-supported cata-
lysts, which makes them easy to separate and convenient to
recycle. Although lots of periodic mesoporous organosilicas
with achiral-functionalized alkyl and aryl groups embedded
within silica frameworks have been reported,[6] only a few
examples have successfully incorporated chiral functionality
within the silica framework.[5,7] Moreover, most of these still
frameworks suffer from lower catalytic activities and enan-
tioselectivities than their corresponding homogeneous cata-
lysts. Thus, exploiting suitable chiral-functionalized periodic
mesoporous organosilicas for highly efficient asymmetric
catalysis remains a scientific and technological challenge.
Great success has been achieved in the immobilization of
homogeneous catalysts within silica-based mesoporous ma-
terials for asymmetric catalysis.[1] Various self-assemblies of
functionalized inorganosilanes and organosilanes through
postsynthesis and co-condensation methods have dominated
the preparation of heterogeneous catalysts. Some of these
inorganosilane-based mesoporous materials, such as MCM-
41,[2] SBA–15,[3] and SBA-16,[4] have been successfully used
to immobilize homogeneous chiral catalysts and have exhib-
ited excellent enantioselectivities in various asymmetric re-
[a] K. Liu, R. Jin, T. Cheng, X. Xu, F. Gao
Department of Chemistry
College of Life and Environmental Science
Shanghai Normal University
No.100 Guilin Rd., Shanghai 200234 (P.R. China)
[b] Prof. G. Liu, Prof. H. Li
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)2164321819
E-mail: ghliu@shnu.edu.cn
Hexing-Li@shnu.edu.cn
À
As one type of important carbon carbon bond-formation
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202407.
reactions, chiral bis(cyclohexyldiamine)-based NiII com-
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FULL PAPER
plexes exhibit high catalytic ac-
tivity and excellent enantiose-
lectivity in the asymmetric Mi-
chael addition of 1,3-dicarbonyl
compounds to nitroalkenes.[8] In
particular, their chiral products
are used extensively as useful
chiral building blocks to gener-
ate chiral amino acids and five-
membered nitrogen heterocy-
cles.[9] More importantly, the in-
vestigation of its catalytic mech-
anism has disclosed that single-
site
bis(cyclohexyldiamine)-
based NiII active species play an
exclusive role in this asymmet-
ric addition reaction.[8a] Thus,
the immobilization of chiral
Scheme 1. Synthesis of heterogeneous catalyst 5.
bis(cyclohexyldiamine)-based
NiII complexes within periodic
mesoporous organosilicas could not only eliminate unex-
pected disturbances to solve the complicated compatibility
of functionalities in a heterogeneous catalysis system but it
could also further elucidate the catalytic mechanism from
the viewpoint of solid-state chemistry.
ized periodic mesoporous organosilica DACH-functional-
ized PMO 3 as a white powder.[10b] Finally, the heterogene-
ous catalyst (5) was successfully obtained by the direct com-
plexation of NiBr2 with compound 3 in the presence of
(1R,2R)-N,N’-dibenzylcyclohexyldiamine (4). To eliminate
any unreacted starting materials, the heterogeneous catalyst
was purified by Soxlet extraction under reflux conditions in
toluene, thus affording pure DACH-Ni-PMO (5) as a pale
powder. Inductively coupled plasma (ICP) optical-emission
spectrometry analysis showed that the loading of Ni in cata-
lyst 5 was 19.68 mg (0.34 mmol), which was approximately
consistent with the thermogravimetric (TG) analysis (see
the Supporting Information, Figure S2). In addition, the
molar ratio of Ni atoms to N atoms (almost 25%), as calcu-
lated from 1.91% N, suggested the generation of well-de-
fined single-site active centers within the periodic mesopo-
rous organosilica.
We are interested in mesoporous-silica-supported hetero-
geneous catalysts;[10] in particular, their use in the develop-
ment of silica-based heterogeneous catalysts. Recently, we
have reported a series of chiral, heterogeneous, inorganosili-
ca-based ruthenium, rhodium, and iridium catalysts and
some of these have showed highly catalytic activity and ex-
cellent enantioselectivity in the asymmetric transfer hydro-
genation of ketones.[11] Herein, we report the successful in-
corporation of chiral bis(cyclohexyldiamine)-based NiII com-
plexes within ethylene-bridged periodic mesoporous organo-
silica to afford an organosilane-based chiral heterogeneous
NiII catalyst that has shown high catalytic activity and excel-
lent enantioselectivity in the asymmetric Michael addition
of 1,3-dicarbonyl compounds to nitroalkenes. Moreover, an
exploration of the heterogeneous mono(cyclohexyldiamine)-
based NiII catalyst and an investigation of the recyclability
of the heterogeneous bis(cyclohexyldiamine)-based NiII cat-
alyst were also performed.
As shown in Figure 1, FTIR spectroscopy of the (1R,2R)-
DACH-functionalized PMO (3) and of heterogeneous cata-
lyst 5 showed the characteristic bands of PMO-type materi-
Results and Discussion
Synthesis and structural characterization of the heterogene-
ous catalyst: The chiral heterogeneous bis(cyclohexyldia-
mine)-based ethylene-bridged NiII catalyst, abbreviated as
DACH-Ni-PMO (5, DACH=(1R,2R)-1,2-diaminocyclohex-
ane), was prepared as outlined in Scheme 1. First, (1R,2R)-
N,N’-dibenzylcyclohexyldiamine-derived silane, with tri-
methlysilyl groups at the 4-positon of the aromatic ring, was
synthesized according to a literature procedure.[3a] Then, the
co-condensation of (1R,2R)-DACH-derived silane 1 and eth-
ylene-bridged silane 2 afforded (1R,2R)-DACH-functional-
Figure 1. FTIR spectra of compounds 3 and 5.
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15547

G. Liu, H. Li et al.
als at around 3421, 1189, 1609, and 440 cmÀ1 for n
ACTHNUGTENR(NUG O H),
À
[12]
À
À
À
nACHTUNGTRENNUNG(Si C), nACHTUNGTRENNUNG(Si O) and wACHTNUGTNER(NUGN C H), respectively. The relative-
ly weak bands between 3100–2800 cmÀ1 were assigned to
À
asymmetric and symmetric stretching vibrations of the C H
bonds, whilst the bands between 1610–1400 cmÀ1 were at-
tributed to stretching vibrations of the C=C bonds both in
the ethylene groups and in the aromatic rings. In compari-
son with compound 3, the bands at 1609, 1453, and
1403 cmÀ1 in compound 5 were consistently more intense,
thus implying that (1R,2R)-N,N’-dibenzylcyclohexyldiamine
(4) participated in the coordination of Ni centers to form
bis(cyclohexyldiamine)-based NiII complexes within the pe-
riodic mesoporous organosilica.
The incorporation of bis(cyclohexyldiamine)-based NiII
complexes within periodic mesoporous organosilica was fur-
ther confirmed by solid-state NMR spectroscopy. As shown
in Figure 2, the 13C cross-polarization magic angle spinning
Figure 3. Solid-state 13C CP MAS NMR spectrum of compound 5 after
treatment with acetylacetone.
amine)-based NiII–acetylacetone intermediate showed the
same typical peaks as the bis(cyclohexyldiamine)-based NiII
complexes and only the intensity of the peaks varied. The
obviously less-intense peaks (denoted by asterisks, Figure 3)
should be ascribed the protons of the Ph groups outside the
silica framework, thus indicating that parts of the bis(cyclo-
hexyldiamine)-based NiII complexes were displaced by ace-
tylacetone molecules. This behavior suggested that the po-
tential ligand displacement between (1R,2R)-N,N’-dibenzyl-
cyclohexyldiamine and the substrates would play an impor-
tant role in the catalytic performance (see below). In addi-
tion, the undisplaced peaks of the protons on the aromatic
rings remained at their original values, thus demonstrating
that these protons in the aromatic rings were inside the
silica framework. In particular, two new peaks at d=193
and 102 ppm corresponded to the C atoms of the C=O
group and the enolized position, respectively, which was evi-
dence for the formation of the mono(cyclohexyldiamine)-
based NiII–acetylacetone intermediate. All of these observa-
tions demonstrated the successful incorporation of bis(cyclo-
hexyldiamine)-based NiII complexes within periodic meso-
porous organosilica.
Figure 2. Solid-state 13C CP MAS NMR spectrum of compound 5.
(CP MAS) NMR spectrum of compound 5 clearly showed
the characteristic peaks of bis(cyclohexyldiamine)-based NiII
complexes. Notably, the protons of the aromatic rings inside
and outside the silica framework gave obviously different
chemical-shift values. In sharp contrast to those of (1R,2R)-
DACH-functionalized PMO 3 (see the Supporting Informa-
tion, Figure S1), the chemical shifts of the Ph groups outside
the silica framework (denoted by asterisks, Figure 2) were at
their normal values, whilst those inside the silica framework
were obviously shifted down-field, owing to the deshielding
effect of the ethylene groups within the silica framework.
Similarly, a difference between the protons on the methyl-
ene moiety of the benzyl groups inside and outside the silica
framework was also observed. To further confirm these
structural arrangements, the 13C CP MAS NMR spectrum of
a mono(cyclohexyldiamine)-based NiII–acetylacetone inter-
mediate,[8a] which was obtained by the treatment of hetero-
geneous catalyst 5 with acetylacetone, was also investigated.
As shown in Figure 3, we found that the mono(cyclohexyldi-
The 29Si CP MAS NMR spectra (Figure 4) clearly showed
that DACH-functionalized PMO (3) and heterogeneous cat-
Figure 4. Solid-state 29Si MAS NMR spectra of compounds 3 and 5.
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Functionalized Periodic Mesoporous Organosilica
FULL PAPER
alyst 5 possessed the organosilicate framework.[5,10a,b] In the
case of the heterogeneous catalyst (5), the characteristic sig-
nals of the T series at d=À74.5 and À82.9 ppm, which cor-
2
3
À
À
responded to T (R SiACTHNUTRGENN(UG OAr)2(OH)) and T (R SiAHCUTNGTREN(NUGN OAr)3),
suggested that all of the Si species were covalently attached
to carbon atoms,[10a–b] whereas the most intense T2 signal re-
À
vealed that the R Si
ly inside the silica framework. Furthermore, the absence of
G
signals for the Q series from d=À90 to À120 ppm indicated
À
that no cleavage of the carbon silicon bond occurred during
the hydrolysis/condensation process.
The (1R,2R)-DACH-functionalized PMO material (3) and
the heterogeneous catalyst (5) showed the same intense d100
diffraction peak as those of typical PMO materials in their
small-angle XRD patterns (Figure 5), thus suggesting that
Figure 7. Nitrogen adsorption–desorption isotherms of compounds 3 and
5.
mesopores (Figure 7). As shown by the structural parame-
ters (Figure 5), the complexation of NiBr2 within the period-
ic mesoporous organosilicas caused a decrease in mesopore
size, surface area, and pore volume, presumably owing to
the coverage of the pore surface with bis(cyclohexyldia-
mine)-based NiII complexes, thereby leading to an increase
in the wall-thickness.[13] Based on these above studies, a
chiral heterogeneous PMO-based NiII catalyst with a highly
ordered mesostructure could be readily obtained in this
case.
Catalytic properties of the heterogeneous catalysts: The
asymmetric Michael addition of malonate to nitroalkenes
has been investigated extensively as one type of important
Figure 5. Powder XRD patterns of compounds 3 and 5.
[8,14]
À
carbon carbon bond-formation reactions.
With hetero-
the hexagonal pore structure (p6mm) could be preserved
after the co-condensation and complexation.[10b] TEM analy-
sis further confirmed that both the (1R,2R)-DACH-function-
alized PMO material (3) and the heterogeneous catalyst (5)
formed highly ordered mesostructures with hexagonal ar-
rangements (Figure 6). In addition, nitrogen-adsorption/-de-
geneous catalyst 5 in hand, we first examined its catalytic ac-
tivity and enantioselectivity in the asymmetric Michael addi-
tion of diethylmalonate to nitrostyrene. As shown in
Table 1, entry 1, the catalytic addition of diethylmalonate to
nitrostyrene gave chiral products with more than 99% con-
version and 94% ee, which were comparable to the values
with the parent homogeneous catalyst (Table 1, entry 1, in
parentheses[8a]). In this case, the high enantioselectivity
might be due to the fact that the bis(cyclohexyldiamine)-
based NiII active centers that were incorporated within the
periodic mesoporous organosilicas remained in the same
chiral microenvironment as the parent homogeneous cata-
lyst. To confirm this conclusion, X-ray photoelectron spec-
troscopy (XPS) of the heterogeneous catalyst (5) and the
homogeneous bis(cyclohexyldiamine)-based NiII complex,
abbreviated as DACHNiDACH, was performed. As shown
in Figure 8, the XPS spectra of the heterogeneous catalyst
(5) showed almost the same Ni 2p3/2 electron-binding energy
as the parent DACHNiDACH complex (855.53 versus
855.51 eV),[15] thus confirming that the high enantioselectivi-
ty originated from the unchanged chiral-coordination micro-
environment.
Figure 6. TEM images of compounds 3 and 5, viewed along the [100] and
[001] directions.
sorption isotherms of both the (1R,2R)-DACH-functional-
ized PMO material (3) and the heterogeneous catalyst (5)
exhibited typical type-IV isotherms with a H1 hysteresis
loop and a visible step at P/P0 =0.30–0.70, which corre-
sponded to the capillary condensation of nitrogen in the
Based on this excellent result, heterogeneous catalyst 5
was further investigated systemically in this asymmetric ad-
dition reaction. We found that most of the tested substrates
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15549

G. Liu, H. Li et al.
Table 1. Asymmetric Michael addition of malonates to nitroalkenes.[a]
(R) became enlarged, their asymmetric reactions needed
longer reaction times to reach completion. Indeed, in the
case where the R group was a tert-butyl group (Table 1,
entry 7), the reaction was not complete within three days. A
reasonable explanation for this result was that the steric
bulk of the mesopores was responsible for this trend, in
which the large, sterically substituted malonates found it dif-
ficulty to enter the nanopores, thereby leading to prolonged
reaction times. Evidence to support this view came from
two comparable experiments, in which physical mixtures of
the (1R,2R)-DACH-functionalized PMO (3) and an equimo-
lar amount of the malonate (diethylmalonate or di-tert-bu-
tylmalonate) were suspended in the solution under the same
conditions. After stirring for 4 h and filtering, the molar
ratio of the two reactants in solution was 1:11.5, as detected
by GC analysis, thus confirming that di-tert-butylmalonate
found it more difficult to penetrated the mesopores, owing
to the steric bulk of the mesopores. Furthermore, the elec-
tronic properties of the substituents on the nitroalkenes did
not affect its catalytic activity and enantioselectivity. The re-
actions of diethyl malonate with electron-rich and electron-
poor substituents on the nitroalkenes were equally efficient
in the asymmetric Michael addition of diethylmalonate to
nitroalkenes (Table 1, entries 8–11).
To gain better insight into the nature of this heterogene-
ous catalysis and to eliminate the disturbance that originates
from mono(cyclohexyldiamine)-based NiII complexes within
PMO materials, two control experiments were also carried
out in which the asymmetric addition of diethylmalonate to
nitrostyrene was performed with (1R,2R)-DACH-functional-
ized PMO (3) and NiBr2 in the presence or absence of
(1R,2R)-N,N’-dibenzylcyclohexyldiamine as the heterogene-
ous catalysts. We found that the former reaction afforded
the corresponding products with 89% conversion and 94%
ee (Table 1, entry 2), whilst the latter reaction gave the cor-
responding products with 45% conversion and 89% ee
(Table 1, entry 3). The former result suggested that the cata-
lyst that was synthesized by an in situ postmodification
method also showed moderate catalytic performance. Its
lower catalytic activity than that of catalyst 5 should be due
to the fact that a small amount of NiBr2 had not coordinated
during the catalytic process, in which the loss of Ni was de-
tected by ICP analysis in solution. The latter reaction indi-
cated that the presence of mono(cyclohexyldiamine)-based
NiII complexes was a key factor in determining chiral per-
formance. Obviously, its lower catalytic efficiency than that
of the heterogeneous catalyst (5) should be ascribed to the
nature of the mono(cyclohexyldiamine)-based NiII complex-
es,[8a] in which the absence of additional (1R,2R)-N,N’-diben-
zylcyclohexyldiamine group as a Brçnsted base could not ac-
tivate diethylmalonate to form deprotonated metal-bound
acetylacetone, thereby resulting in decreased catalytic per-
formance. In other words, bis(cyclohexyldiamine)-based NiII
active centers within periodic mesoporous organosilicas
played a dual role as the homogeneous catalyst:[8a] 1) the
displaced ligands could activate diethylmalonate to enhance
the catalytic activity, as shown by the 13C CP MAS NMR
Entry
R
Ar
Run
t
Conv.
[%][b]
ee
TOF
[h]
[%][b]
1
2
3
4
5
6
7
8
Et
Et
Et
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1
1
1
1
1
1
1
1
1
1
1
1
2
3
4
5
6
7
8
9
8
>99(99)
89
45
94(94)[c]
94[d]
89[e]
1[f]
95
94
95
95
95
95
6.3
3.7
0.5
0.3
6.3
4.2
3.1
0.3
4.1
4.1
4.2
4.2
4.2
4.2
4.2
4.2
4.2
4.2
4.2
4.1
12
48
48
8
Et
29
Me
Bn
iPr
tBu
Et
>99
>99
>99
76
99
98
>99
>99
>99
>99
>99
>99
>99
>99
>99
98
12
16
120
12
12
12
12
12
12
12
12
12
12
12
12
9
4-MePh
4-OMePh
4-BrPh
2-ClPh
Ph
10
11
12
13
14
15
16
17
18
19
20
Et
Et
93
95
Et,
Me
Me
Me
Me
Me
Me
Me
Me
95[g]
95[g]
95[g]
94[g]
94[g]
93[g]
94[g]
94[g]
Ph
Ph
Ph
Ph
Ph
Ph
Ph
[a] Reaction conditions: catalyst (58.8 mg, 0.02 mmol of Ni, based on ICP
analysis), nitroalkene (1.0 mmol), malonate (1.0 mmol), toluene (4.0 mL),
408C, 12–120 h; turnover frequency (TOF=mole of substrate converted
per mole of Ni complex per hour). [b] Determined by chiral HPLC anal-
ysis (Chiralcel AD-H column; see the Supporting Information, Figures S3
and S4); conversion was calculated by using an external standard.
[c] Data are taken from reference [8a]. [d] Data were obtained by using a
physical mixture of (1R,2R)-DACH-functionalized PMO (3) and NiBr2 in
the presence of (1R,2R)-N,N’-dibenzylcyclohexyldiamine as a catalyst.
[e] Data were obtained by using a physical mixture of (1R,2R)-DACH-
functionalized PMO (3) and NiBr2 in the absence of (1R,2R)-N,N’-diben-
zylcyclohexyldiamine. [f] Data were obtained by using 20% mol of
(1R,2R)-DACH-functionalized PMO (3) as a catalyst. [g] Data were ob-
tained by using the reused DACH-Ni-PMO (5) as a catalyst.
Figure 8. XPS spectra of DACHNiDACH and compound 5.
afforded their corresponding products with high conversions
and excellent enantioselectivities under similar conditions.
More interestingly, the steric properties of substituted malo-
nates significantly affected their catalytic activities. As
shown in Table 1, entries 1 and 4–7, as the alkyl ester group
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Functionalized Periodic Mesoporous Organosilica
FULL PAPER
spectrum of the mono(cyclohexyldiamine)-based NiII-
neous inorganosilica-supported NiII catalyst was prepared by
A
the similar co-condensation of (1R,2R)-DACH-derived
hexyldiamine)-based NiII complexes could control the chiral
microenvironment to improve the enantioselectivity.
silane (1) with tetraethylorthosilicate, in which O Si O
À À
groups acted as the inner wall rather than the phenyl moie-
An important feature of the design of heterogeneous cata-
lyst 5 is its easy and reliable separation by simple filtration
and that the reused catalyst could still retain its catalytic ac-
tivity and enantioselectivity over multiple cycles. As shown
in Table 1, entries 12–19, heterogeneous catalyst 5 clearly
showed good reusability for the addition of dimethylmalo-
nate to nitrostyrene, in which the ee values, as well as the
conversions, did not drop notably after its continuous nine
runs. To elucidate that the recycling efficiency was derived
from the heterogeneous catalyst itself, rather than form non-
covalent adsorption, a hot-filtration experiment was carried
out. In this case, the heterogeneous catalyst (5) was filtered
from the reaction mixture after 8 h and the reaction was
continued for a further 12 h. We found that there was no ap-
preciable change in either the conversion or ee values of the
chiral products. This result ruled out the role of noncovalent
physical adsorption, thereby confirming that the high recy-
clability was derived from the heterogeneous catalyst itself.
Importantly, this heterogeneous catalyst (5) could also be
applied to the asymmetric Michael addition of b-ketoesters
to nitroalkenes. As shown in Table 2, their corresponding
products were formed with high conversions and excellent
enantioselectivities under similar conditions. Taking dime-
ACHTUNGTRENtNUGN ies. With this catalyst, a reaction time of 18 h was needed to
complete this reaction, thus confirming that the enhanced
reaction rate in the case of heterogeneous catalyst 5 should
be ascribed the high hydrophobicity of the nanopores.
Conclusion
In conclusion, we have successfully immobilized chiral bis-
AHCTUNGTRENNUNG
(cyclohexyldiamine)-based NiII complexes within periodic
mesoporous organosilica to afford a chiral, heterogeneous,
PMO-supported NiII catalyst (5). This catalyst exhibited
high catalytic activity and excellent enantioselectivity in the
asymmetric Michael addition of 1,3-dicarbonyl compounds
to nitroalkenes, comparable to those with homogeneous cat-
alysts. In particular, this research also disclosed that chiral
bis(cyclohexyldiamine)-based NiII complexes within the pe-
riodic mesoporous organosilica played a crucial role in de-
termining both the catalytic and enantioselective perform-
ance. More importantly, heterogeneous catalyst 5 could be
recovered and reused up to nine times without obviously af-
fecting its enantioselectivity, thus showing good potential for
industrial applications.
Table 2. Asymmetric Michael addition of b-ketoesters to
a
nitroalkene.[a]
Experimental Section
Preparation of (1R,2R)-DACH-functionalized PMO (3): In a
typical synthesis, the structure-directing agent (pluronic P123,
2.0 g) was fully dissolved in a mixture of 0.2m hydrochloric
acid (80 mL) and KCl (6.0 g). Then, bis(triethyoxysilyl)ethy-
lene (3.46 g, 9.00 mmol) was added as the silica precursor at
Entry
R’
R’’
Ar
t
Conv.
[%][b]
d.r.
(a/b)[b]
ee [%]
ACHTUNGTNER(NUNG a/b)
[h]
G
1
2
3
4
5
Me
Me
Me
Me
iPr
Me
Et
iPr
tBu
Et
Ph
Ph
Ph
Ph
Ph
6
6
8
8
8
>99
>99
>99
>99
96
1.2:1 (1:1)
1.3:1
1.5:1
1.7:1
1:1
95 (93) (94 (94))
94 (93)
408C. After
DACH-[(CH2PhSi
a
pre-hydrolysis period of 40 min, (1R,2R)-
(OMe)3]2 (1, 0.53 g, 1.00 mmol) was added,
AHCTUNGTRENNUNG
95 (95)
95 (92)
97 (97)
in which the initial molar ratio of Si/P123/KCl/HCl/water in
the mother solution was 1.0:0.017:4.0:0.80:218 (Si refers to the
total silica source). The reaction mixture was stirred at 408C
for 24 h and aged at 1008C for 24 h. The resulting solid was fil-
tered and rinsed with excess EtOH before being dried over-
night on a filter funnel. The surfactant template was removed
by heating at reflux in acidic EtOH (400 mL per gram) for
24 h. The solid was filtered, rinsed again with EtOH, and dried
at 608C under reduced pressure overnight to afford (1R,2R)-
DACH-functionalized PMO (3, 1.26 g) as a white powder. 13C
CP MAS NMR (100.6 MHz): d=145.4 (CH=CH), 133.5 (CH
[a] Reaction conditions: catalyst (58.8 mg, 0.02 mmol of Ni, based on ICP analysis), ni-
troalkene (1.0 mmol), b-ketoester (1.0 mmol), toluene (4.0 mL), 408C, 6–8 h. [b] De-
termined by chiral HPLC analysis (Chiralcel AD-H column; see the Supporting Infor-
mation, Figure S5); conversion was calculated by using an external standard. [c] Data
refer to those in Ref. [8a].
thylacetoacetate as an example, the catalytic reaction afford-
of the Ph rings inside the framework), 73.9, 69.3 (C atoms of the cyclo-
ed chiral products with more than 99% conversion and
95% ee, which was comparable to that with a homogeneous
catalyst (Table 2, entry 1, in parentheses[8a]). Furthermore,
this catalytic reaction could be completed within the same
reaction time as that with the homogeneous catalyst, thus
demonstrating that the ethylene moiety worked as the hy-
drophobic inner wall to rapidly draw reactants into the
nanopore, thereby accelerating the reaction rate relative to
other heterogeneous catalysts, which needed prolonged re-
action times owing to the general features of heterogeneous
catalysis.[11b] To confirm this conclusion, a similar heteroge-
À
À
hexyl group that are connected to the N atom), 57.3 (O CH3 and O
À
CH2CH3), 51.2–49.0 (N CH2Ph), 35.2–20.5 (C atoms in the cyclohexyl
À
group that are not connected to the N atom), 16.6 ppm (O CH2CH3);
29Si MAS NMR (79.5 MHz): d=À74.3 (T2), À81.0 ppm (T3); IR (KBr):
n˜ =3420.1 (s), 2984.5 (w), 2942.9 (w), 2869.4 (w), 1609.9 (m), 1452.9 (w),
1403.0 (w), 1189.4 (s), 1049.3 (s), 925.8 (m), 802.3 (m), 440.2 cmÀ1 (m);
S
BET =514.2 m2gÀ1; dpore =3.67 nm; Vpore =0.46 cm3gÀ1; elemental analysis
found (%) found: C 32.60, H 4.52, N 0.81.
Preparation of heterogeneous catalyst 5: To a stirring suspension of
(1R,2R)-DACH-functionalized PMO (3, 1.0 g) in dry MeCN (50 mL) was
added NiBr2 (51.84 mg, 0.24 mmol) and N,N’-dibenzyl diamine (70.56 mg,
0.24 mmol) at room temperature. The resulting mixture was heated at
Chem. Eur. J. 2012, 18, 15546 – 15553
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15551

G. Liu, H. Li et al.
reflux for 12 h. After cooling to room temperature, the mixture was fil-
tered through filter paper and rinsed with excess CH2Cl2. After Soxlet ex-
traction (toluene) for 24 h to remove any homogeneous and unreacted
starting materials, the solid was dried overnight at 608C under vacuum to
afford heterogeneous catalyst 5 (1.04 g) as a pale powder. ICP analysis
showed that the Ni loading was 19.68 mg (0.34 mmol) per gram of cata-
lyst. 13C CP MAS NMR (161.9 MHz): d=146.9 (CH=CH), 135.3 (CH of
the Ph rings inside the framework), 130.4 (CH of the Ph rings outside the
framework), 74.9, 70.9 (C of the cyclohexyl group that are connected to
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À
À
À
the N atom), 59.7 (O CH3 and O CH2CH3), 53.1, 50.6, 48.5 (N CH2Ph),
33.7, 30.4, 25.4 (C atoms in the cyclohexyl group that are not connected
to the N atom), 18.8 ppm (O CH2CH3); 29Si MAS NMR (300 MHz): d=
À
À74.5 (T2), À82.9 ppm (T3); IR (KBr): n˜ =3413.2 (s), 3083.0 (w), 2976.2
(w), 2942.8 (w), 2869.3 (w), 1609.7 (m), 1452.9 (w), 1403.0 (w), 1189.4 (s),
1049.3 (s), 925.8 (m), 802.3 (m), 695.5 (w), 662.2 (w), 440.2 cmÀ1 (m);
S
BET =498.8 m2gÀ1; dpore =3.64 nm; Vpore =0.44 cm3gÀ1; elemental analysis
found (%): C 38.50, H 5.38, N 1.91.
Treatment of heterogeneous catalyst 5 with acetylacetone: To heteroge-
neous catalyst 5 (300 mg) was added acetylacetone (2.0 mL) and the mix-
ture was stirred for 12 h at room temperature. The mixture was then fil-
tered through filter paper and rinsed with excess CH2Cl2. After Soxlet ex-
traction (toluene) for 24 h to remove any homogeneous and unreacted
start materials, the solid was dried overnight at 608C under vacuum to
afford the mono(cyclohexyldiamine)-based NiII–acetylacetone intermedi-
ate as a pale powder. 13C CP MAS NMR (161.9 MHz): d=192.7 (C of
the C=O group), 146.8 (CH=CH), 134.9 (CH of the Ph rings inside the
framework), 130.1 (CH of the Ph rings outside the framework), 101.8
(CH atom at the enolized position), 73.1, 70.3 (C atoms of the cyclohexyl
À
À
group that are connected to the N atom), 59.2 (O CH3 and O CH2CH3),
À
56.4, 52.4, 52.0, 50.0 (N CH2Ph substituted by acetylacetone and
À
N CH2Ph unsubstituted by acetylacetone), 27.3 (CH3 of acetylacetone),
32.1, 29.7, 24.9 (C atoms in the cyclohexyl group that are not connected
À
to the N atom), 18.5 ppm (O CH2CH3).
General procedure for asymmetric Michael addition of 1,3-dicarbonyl
compounds to nitroalkenes: In a typical procedure, heterogeneous cata-
lyst 5 (58.8 mg, 0.02 mmol of Ni, based on ICP analysis), nitrostyrene
(0.15 g, 1.0 mmol), dimethylmalonate (1.0 mmol), and toluene (4.0 mL)
were added into a 10 mL round-bottomed flask and the mixture was
heated at 408C for 8–28 h. During that time, the reaction was monitored
constantly by TLC. After the completion of the reaction, the heterogene-
ous catalyst was filtered through filter paper for the recycling experi-
ments. The aqueous solution was extracted with Et2O (3ꢁ3.0 mL) and
the combined Et2O extracts were washed twice with brine and dehydrat-
ed with Na2SO4. After the evaporation of Et2O, the residue was purified
by column chromatography on silica gel to afford the desired product.
Conversion was calculated by using an external standard and the ee value
was determined by chiral HPLC analysis with a UV/Vis detector and a
Daicel AD-H chiralcel column (F=0.46 cmꢁ25 cm). The absolute con-
figurations of the compounds were assigned by comparison of their opti-
cal rotations to literature values.[8a]
Acknowledgements
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We are grateful to the National Natural Science Foundation of China
(20673072), the Shanghai Sciences and Technologies Development Fund
(10J1400103, 10JC1412300), and the Shanghai Municipal Education Com-
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Received: July 6, 2012
Published online: October 2, 2012
Chem. Eur. J. 2012, 18, 15546 – 15553
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15553