Enantioselective addition of malonates and β-keto esters to nitroalkenes over an organonickel-functionalized periodic mesoporous organosilica

Article abstract of DOI:10.1002/adsc.201200222

A periodic mesoporous organosilica (PMO) with chiral cyclohexyldiamine- based nickel(II) complexes incorporated within the silica framework was prepared through a co-condensation of (1R,2R)-cyclohexyldiamine-derived silane and Ph-bridged silane followed b

Full text of DOI:10.1002/adsc.201200222

FULL PAPERS
DOI: 10.1002/adsc.201200222
Enantioselective Addition of Malonates and b-Keto Esters to Ni-
troalkenes over an Organonickel-Functionalized Periodic Meso-
porous Organosilica
Ronghua Jin,a Ketang Liu,a Daquan Xia,a Qingqian Qian,a Guohua Liu,a,*
and Hexing Lia,*
a
Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai Key Laboratory of Rare Earth Functional
Materials, Shanghai Normal University, Shanghai 200234, Peopleꢀs Republic of China
Fax : (+86)-21-6432-1819; phone: (+86)-21-6432-1819; e-mail: ghliu@shnu.edu.cn or HeXing-Li@shnu.edu.cn
Received: May 25, 2012; Revised: August 9, 2012; Published online: November 13, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200222.
Abstract:
(PMO)
A
with
periodic mesoporous organosilica within the PMO material. In particular, as a hetero-
chiral cyclohexyldiamine-based geneous chiral catalyst, this periodic mesoporous or-
nickel(II) complexes incorporated within the silica ganosilica showed high catalytic activity and excel-
framework was prepared through a co-condensation lent enantioselectivity in asymmetric Michael addi-
of (1R,2R)-cyclohexyldiamine-derived silane and Ph- tion of 1,3-dicarbonyl compounds to nitroalkenes
bridged silane followed by complexation of nickel(II) (more than 92% conversions and up to 99% ee
bromide in the presence of (1R,2R)-N,N’-dibenzylcy- values). More importantly, this heterogeneous cata-
clohexyldiamine. Structural analyses by X-ray lyst could be recovered easily and reused repeatedly
powder diffraction, nitrogen sorption and transmis- nine times without obviously affecting its ee value,
sion electron microscopy disclosed its orderly meso- showing good potential for industrial applications.
structure while characterization by solid-state NMR
and X-ray photoelectron spectroscopy demonstrated Keywords: asymmetric catalysis; heterogeneous cat-
the well-defined single-site chiral bis(cyclohexyldi- alysis; immobilization; mesoporous materials; Mi-
ACHTUNGTRENNUNGamine)-based nickel(II) active centers incorporated chael addition
Introduction
also exhibit superior thermal and mechanical stability
relative to polymer-supported catalysts, showing a po-
Periodic mesoporous organosilicas (PMOs) with tential application in industry. Up to now, although
a chiral moiety bridged within the silica framework lots of periodic mesoporous organosilicas containing
for asymmetric catalysis have attracted a great deal of alkyl and aryl groups have been reported[2] and some
interest due to their relatively large surface area and periodic mesoporous organometalsilicas with incorpo-
pore volume, orderly pore arrangement and salient rated chiral functionality within silica framework have
surface chemistry.[1] Large surface area and pore been explored,[1,3] however, to the best of our knowl-
volume can enhance effectively the loadings and the edge, most of them still suffer from lower catalytic ac-
dispersibility of chiral metal complexes while tuneable tivities and enantioselectivities than the respective ho-
pore dimensions and well-defined pore arrangement mogeneous catalysts. Main problems are attributed to
are beneficial to control the chiral microenvironment mismatched hydrophobicity/hydrophilicity of the het-
of catalytic active centers, exhibiting a superiority in erogeneous catalyst, complicated compatibility of
stereocontrol performance. More importantly, relative functionalities and an undesired chiral microenviron-
to inorganosilica-supported catalysts, periodic meso- ment of active centers, which will inevitably decrease
porous organosilicas with organic functionality em- catalytic activity and enantioselectivity. Thus, exploit-
bedded within the silica framework possess an obvi- ing a suitable periodic mesoporous organosilica to
ous hydrophobicity that can promote significantly the adjust its hydrophobicity, to optimize its compatibility
reaction rate of organic reactions, presenting a high and to control its chiral microenvironment plays an
efficiency in catalytic performance. In addition, they
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important role in the design of PMO-supported chiral carbonyl compounds to nitroalkenes. Meanwhile,
catalysts. comparison of mono(cyclohexyldiamine)-based
a
Chiral bis(cyclohexyldiamine)-based Ni(II) com- Ni(II) heterogeneous catalysts and an investigation of
plexes are one type of highly effective catalysts for reusability of bis(cyclohexyldiamine)-based Ni(II)
asymmetric Michael addition of 1,3-dicarbonyl com- heterogeneous catalyst are also discussed in detail.
pounds to nitroalkenes,[4] in which chiral products as
useful chiral building blocks are used extensively to
generate chiral amino acids and five-membered nitro- Results and Discussion
gen heterocycles.[5] More importantly, the investiga-
tion of the catalytic mechanism discloses that single- Synthesis and Characterization of the Catalyst
site bis(cyclohexyldiamine)-based Ni(II) active species
play an exclusive role in this asymmetric addition re- The chiral bis(cyclohexyldiamine)-based phenyl-
action, which is beneficial to eliminate unexpected bridged heterogeneous Ni(II) catalyst, abbreviated as
disturbance in a heterogeneous catalysis system. DACH-Ni-PMO (5) [DACH=(1R,2R)-1,2-diaminocy-
Based on the high catalytic efficiency and exclusive clohexane], was prepared as outlined in Scheme 1.
function of bis(cyclohexyldiamine)-based Ni(II) active Firstly, the (1R,2R)-DACH-functionalized PMO (3)
species, making use of the hydrophobicity of PMO was synthesized by the co-condensation of (1R,2R)-
materials, it is reasonable to expect that immobiliza- DACH-derived silane (1)[3c] and Ph-bridged silane (2)
tion of chiral bis(cyclohexyldiamine)-based Ni(II) using
pluronic
P123
{(CH2-CH2O)20ACHTUNGTRENNUNG[CH2
complexes within periodic mesoporous organosilicas (CH3)CH2O] CHUTNGTRENNNUG
N
70A(CH2CH2O)20} as a structure-directing
will exhibit high catalytic efficiency and desired enan- agent according to our previous reports (see Experi-
tioselectivity in the asymmetric Michael addition of mental Section).[6c] The raw catalyst was then ob-
1,3-dicarbonyl compounds to nitroalkenes.
tained by direct complexation of 3 with 0.93 equiva-
We are interested in mesoporous silica-supported lents of NiBr2 in the presence of 0.93 equivalents of
heterogeneous catalysts,[6] especially, a series of meso- (1R,2R)-N,N’-dibenzylcyclohexyldiamine (4). Finally,
porous inorganosilica-supported chiral Ru, Rh and Ir Soxhlet extraction of the raw catalyst under reflux
catalysts have shown high catalytic activity and enan- conditions in toluene afforded pure catalyst DACH-
tioselectivity in asymmetric transfer hydrogenation of Ni-PMO (5) in the form of a pale powder. The induc-
ketones.[7] As an extension of our previous studies, we tively coupled plasma (ICP) optical emission spec-
herein immobilized successfully chiral bis(cyclohexyl- trometer analysis showed that the loading of Ni in the
diamine)-based Ni(II) complexes within the phenyl- catalyst 5 was 20.51 mg (0.35 mmol) per gram of cata-
bridged periodic mesoporous organosilica and pre- lyst while the elemental analysis disclosed that the
pared a PMO-supported chiral Ni(II) catalyst, which mole amount of N was 1.414 mmol per gram of cata-
showed high catalytic activity and excellent enantiose- lyst calculated from the mass% of N (1.98%). The 1:4
lectivity in the asymmetric Michael addition of 1,3-di- mole ratio of Ni to N atoms suggested generation of
Scheme 1. Preparation of 3 and 5.
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Enantioselective Addition of Malonates and b-Keto Esters to Nitroalkenes
Figure 2. The 13C CP MAS NMR spectrum of 5 treated with
acetylacetone.
Figure 1. The FT-IR spectra of 3 and 5.
the well-defined single-site active centers within the
periodic mesoporous organosilica.
Figure 1 shows the FT-IR spectra of the (1R,2R)-
DACH-functionalized PMO (3) and the heterogene-
ous catalyst 5. Generally, both 3 and 5 exhibited the
characteristic bands of the PMO-type materials
À
around 3423, 1648, 1080 and 522 cmÀ1 for n
ACHTUNGTNRE(UNNG O H),
[8]
À
À
À
dACHTUNGTRENNUNG(O H), nACHTUNGTRENNUNG(Si O) and wACHTUNGTREN(NUNG C H), respectively. The
relatively weak bands between 3100–2800 cmÀ1 were
assigned to asymmetric and symmetric stretching vi-
À
brations of C H bonds while the bands between
1610–1450 cmÀ1 were attributed to breathing vibra-
tions of C=C bonds in the aromatic ring.[9] The peaÀk1s
À
indicative of n
(Si C) should appear at 1100 cm ,
however, they were difficult to be distinguished be-
cause of the overlapping by the absorbance from n-
[10]
À
G
Scheme 2. The catalyst 5 treated with acetylacetone.
sorbance at 1602, 1561 and 1451 cmÀ1 in the 5 (as-
signed to the vibration of C=C bonds in aromatic
ring) increased consistently, implying that (1R,2R)- ca was the 13C CP MAS NMR spectrum of the mon-
N,N’-dibenzylcyclohexyldiamine (4) participated in o(cyclohexyldiamine)-based Ni(II)-acetylacetone in-
the coordination of Ni centers. All these observations termediate[4a] obtained by treatment of the heteroge-
demonstrated the successful incorporation of bis(cy- neous catalyst 5 with acetylacetone. As shown in
clohexyldiamine)-based Ni(II) complexes within the Figure 2, it was found that the mono(cyclohexyldia-
periodic mesoporous organosilica.
mine)-based
Ni(II)-acetylacetone
intermediate
The incorporation of bis(cyclohexyldiamine)-based (Scheme 2) showed the typical peaks around 18–
Ni(II) complexes within the periodic mesoporous or- 25 ppm corresponding to the C atoms in the cyclohex-
ganosilica could be further confirmed by the solid- yl group and the peaks at 60 ppm corresponding to
state NMR spectra. Although there were the relative- the C atoms in the cyclohexyl group connected to ni-
ly broad peaks in 13C cross polarization (CP) MAS trogen atom. Meanwhile, the peak around 31 ppm de-
NMR spectrum of 5 due to the paramagnetic charac- rived from the C atoms in the CH2 group connected
ter of bis(cyclohexyldiamine)-based Ni(II) complexes, to the aromatic rings and the peak around 134 ppm
it was still clear that each characteristic peak could be corresponding to the C atoms in the aromatic rings
observed, in which the typical chemical shift values were also observed. More importantly, there were two
were strongly similar to those of (1R,2R)-DACH- new peaks at 193 and 101 ppm, which were assigned
functionalized PMO (3) (see Figure S1 in the Sup- to the C atoms of C=O and the C atoms of the enoli-
porting Information). A further evidence to support zation. These structural assignments confirmed the
the presence of bis(cyclohexyldiamine)-based Ni(II) formation of mono(cyclohexyldiamine)-based Ni(II)-
complexes within the periodic mesoporous organosili- acetylacetone intermediate[4a,b] due to a ligand dis-
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placement between (1R,2R)-N,N’-dibenzylcyclohexyl- Ni(II) complexes within the periodic mesoporous or-
diamine and acetylacetone, proving indirectly the ganosilica.
presence of bis(cyclohexyldiamine)-based Ni(II) com-
In order to investigate the electronic state of the
plexes within this periodic mesoporous organosilica. nickel center within the periodic mesoporous organo-
In addition, the 29Si CP MAS NMR spectrum of the silica, the heterogenous catalyst 5 and the correspond-
heterogeneous catalyst 5 (see Figure S1 in the Sup- ing homogeneous bis(cyclohexyldiamine)-based Ni(II)
porting Information) demonstrated that the heteroge- complex (DACHNiDACH) were examined by X-ray
neous catalyst 5 possessed an organosilicate frame- photoelectron spectroscopy (XPS). As shown in
work[1a,3b,6c] with R-Si
ACTHNUTRGEN(UNG OAr)2(OH) (R=chiral organo- Figure 4, the XPS spectra of the heterogeneous cata-
metallic functionality) as the main part of silica lyst 5 gave the Ni 2p3/2 binding energy around
framework. The characteristic signals of the T-series 855.54 eV, indicating that the nickel center presented
at À62.2, À71.0 and À79.9 ppm corresponding to T1 in the divalent state.[11] Meanwhile, it was easily found
À
À
[R Si
[R Si
(OAr)(OH)2], T2 [R Si
(OAr)2(OH)] and T3 that the heterogeneous catalyst 5 had the nearly same
À
A
lently attached to carbon atoms[3b,6c] while the most in- NiDACH complex, suggesting that the nickel center
tense T signal revealed that R Si
2
À
A
were predominant in the silica framework. Further- its original chiral coordination microenvironment that
more, the absence of signals for the Q-series from would dominate the chiral performance discussed
À90 to À120 ppm indicated that no cleavage of the later.
carbon-silicon bond occurred during the hydrolysis-
condensation process.
The mesostructural morphology and the pore ar-
rangements of (1R,2R)-DACH-functionalized PMO
The TG/DTA curves of the heterogeneous catalyst (3) and the heterogeneous catalyst 5 were further
5 are presented in Figure 3. An endothermic peak characterized using transmission electron microscopy
around 361 K with a weight loss of 8% could be at- (TEM), X-ray diffraction (XRD), and nitrogen ad-
tributed to the release of physically adsorbed water. sorption-desorption techniques. The small-angle XRD
In addition, an exothermic peak around 545 K with patterns (Figure 5) revealed that the heterogeneous
a weight loss of 4% could be assigned to the oxidation
of chiral diamine ligands embedded in the pore wall
while another exothermic peak at 782 K with the
weight loss of 41% resulted from the oxidation of the
phenyl fragments embedded in the pore wall.[1a] All
the peaks observed here were strongly similar to
those of the parent (1R,2R)-DACH-functionalized
PMO materials (3) (see Figure S2 in the Supporting
Information). It was worth mentioning that a new
exothermic peak around 626 K with a weight loss of
9% could be assigned to the oxidation of bis(cyclo-
hexyldiamine)-based Ni(II) complexes in the pore
channel when compared to the TG/DTA curve of 3.
These observations further demonstrated the success-
ful incorporation of bis(cyclohexyldiACTHNUTRGNEaNUG mine)-based Figure 4. The XPS spectra of DACHNiDACH and 5.
Figure 3. The TG/DTA curves of 5.
Figure 5. The powder XRD patterns of 3 and 5.
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Enantioselective Addition of Malonates and b-Keto Esters to Nitroalkenes
Figure 6. The TEM images of 3 and 5 viewed along the [100] and [001] directions.
with ordered mesostructure could be readily obtained
in this case.
Catalytic Properties of the Heterogeneous Catalyst
The asymmetric Michael addition of 1,3-dicarbonyl
compounds to nitroalkenes is one kind of important
carbon-carbon bond formation reactions.[4,13] With the
heteogeneous catalyst 5 in hand, we examined its cat-
alytic activity and enantioselectivity in the asymmetric
Michael addition of malonates to nitroalkenes at first.
As shown in Table 1, when the mole amount of the
heterogeneous catalyst 5 was decreased down to
2.0 mol%, the catalytic reaction of diethyl malonate
addition to nitrostyrene gave a chiral product with
more than 99% conversion and 94% ee value
(entry 1). Such an enantioselectivity was comparable
to that of the homogeneous catalyst (entry 1 versus
Figure 7. The nitrogen adsorption-desorption isotherms of 3
and 5.
catalyst 5 presented the same d100 diffraction peak as the value in brackets[4a]). It is worth mentioning that
(1R,2R)-DACH-functionalized PMO materials (3), the catalytic reaction could be completed wihtin 8 h,
suggesting that the dimensional-hexagonal pore struc- suggesting that the phenyl moiety working as a hydro-
ture (p6mm) observed in PMO-type materials could phobic inner wall could accelerate reaction rate. In
be preserved after the complexation.[3b] The TEM order to prove this assumption the condensation of
morphologies further confirmed that 5 had a highly (1R,2R)-DACH-derived silane (1) and tetraethylor-
À À
ordered mesostructure with the dimensional-hexago- thosilicate was compared, in which O Si O groups
nal arrangement as shown in Figure 6. Nitrogen ad- worked as the inner wall rather than the phenyl
sorption-desorption isotherms of both
3
and
5
moiety . The experiment showed that 18 h of reaction
(Figure 7) exhibited typical IV type isotherms with H1 time were needed to complete this catalytic reaction,
hysteresis loop and a visible step at P/P0 =0.40–0.90, confirming that the enhanced reation rate in the case
corresponding to capillary condensation of nitrogen in of the heterogeneous catalyst 5 should be ascribed the
mesopores. As shown by the structural parameters good hydrophobicity of the nanopores. Of particular
listed in the table inserted in Figure 5, the complexa- note is that the reaction could also be run at a high
tion of NiBr2 within the periodic mesoporous organo- substrate/catalyst ratio without affecting obviously the
silicas caused a decrease in mesopore size, surface ee value, as exemplified by the asymmetric Michael
area, and pore volume, obviously due to coverage of addition with 0.5 mol% of catalyst (entry 2). The high
pore surface with bis(cyclohexyldiamine)-based Ni(II) enantioselectivity in these cases should be due to the
complexes, leading to an increase of the wall thick- fact that bis(cyclohexyldiamine)-based Ni(II) active
ness.[12] Based on the above studies, we can safely con- centers incorporated within the periodic mesoporous
clude that the PMO-supported chiral Ni(II) catalyst organosilicas retained the same chiral microenviron-
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Table 1. Asymmetric Michael addition of malonates to nitro-
explanation is that the size of mesopores delayed the
entrance of the substrate, leading to a prolonged reac-
tion time. Evidence to support the view was provided
by three comparable experiments, in which each mix-
ture of (1R,2R)-DACH-functionalized PMO (3) mate-
rial and the equivalent mole reactant (diethyl malo-
nate or di-isopropyl malonate or di-tert-butyl malo-
nate) was suspended in the solution under the same
conditions, respectively. It was found that, after stir-
ring for 4 h and filtering, the mole ratio of the three
reactants in the solution was 1:1.5:9.6 as detected by
the GC analyses, demonstrating that the large steric
substituted malonates were more difficult to pene-
trate into the mesopores due to the size effect of the
mesopores. However, the electronic properties of sub-
stituents at the nitroalkenes did not affect catalytic ac-
tivity and enantioselectivity, in which the reactions of
diethyl malonate with electron-rich and electron-poor
substituted nitroalkenes were equally efficient (en-
tries 11–14).
alkenes.[a]
Entry R, Ar
Time [h] Conv. [%][b] ee [%][b]
1
2
3
4
5
6
7
8
Et, Ph
Et, Ph
Et, Ph
Et, Ph
Et, Ph
Et, Ph
Me, Ph
Bn, Ph
i-Pr, Ph
t-Bu, Ph
Et, 4-MeC6H4
Et, 4-MeOC6H4 12
Et, 4-BrC6H4
Et, 2-ClC6H4
8 (5)
24
36
36
36
36
8
12
16
120
12
>99 (99)
83
76
89
38
94 (94)
92[c]
88[d]
91[e]
1[f]
61
89[g]
94
>99
>99
>99
63
99
98
94
95
95
95
95
93
95
9
10
11
12
13
14
12
12
>99
>99
As shown in Table 2, this heterogeneous catalyst 5
could also be applied to the asymmetric Michael addi-
tion of b-keto esters to nitroalkenes, in which high
conversions and the excellent enantioselectivities
were obtained. Taking dimethyl acetoacetate as an ex-
ample, the catalytic reaction afforded chiral products
with more than 99% conversion and more than 99%
ee values (entry 1), in which both enantioselectivities
were higher than those of the homogeneous catalyst
(both 99% ees in entry 1 versus 94% ees in the litera-
ture[4a]). This behaviour suggested that there was
a positive confinement effect with bis(cyclohexyldi-
[a]
Reaction conditions: catalysts (57.0 mg, 0.02 mmol of Ni,
based on the ICP analysis), nitroalkenes (1.0 mmol), mal-
onate (1.20 mmol) and 4.0 mL toluene, reaction tempera-
ture (408C), reaction time (8–120 h).
Determined by chiral HPLC analysis using Chiralcel
AD-H columns (see Figure S4 in the Supporting Infor-
mation) and the conversions were calculated by the inter-
nal standard method.
Data were obtained with 0.5 mol% of catalyst.
Data were obtained using a physical mixture of (1R,2R)-
DACH-functionalized PMO (3) plus NiBr2 in the pres-
ence of (1R,2R)-N,N’-dibenzylcyclohexyldiamine as a cat-
alyst.
Data were obtained using a physical mixture of (1R,2R)-
DACH-functionalized PMO (3) plus the homogeneous
bis(cyclohexyldiamine)-based Ni(II) complexes as a cata-
lyst.
[b]
[c]
[d]
AHCTUNGTREGaNNUN mine)-based Ni(II) active centers within PMO-type
[e]
mesoporous materials, in which a similar positive con-
finement effect of an enhanced enantioselectivity had
also been observed in the inorganosilicas-type meso-
porous materials.[14] More importantly, the catalytic
reaction could be completed within the same reaction
time as that of the homogeneous catalyst, further con-
firming that the phenyl moiety worked as a hydropho-
bic inner wall that could draw rapidly reactants into
the nanopore, thereby greatly accelerating the reac-
[f]
Data were obtained using 20 mol% of (1R,2R)-DACH-
functionalized PMO (3) as a catalyst.
Data were obtained using mono(cyclohexyldiamine)-
based Ni(II) complex as a catalyst.
[g]
ment as in the homogeneous catalyst as was proven tion rate. Similarly, the steric properties of two sub-
by the XPS studies mentioned above. stituents at the b-keto ester did affect significantly the
On the basis of the above excellent results, the het- catalytic activity due to the size effect of the nano-
erogeneous catalyst 5 was further investigated system- pores discussed above (entries 1–6). For example,
ically in the asymmetric addition of malonates to ni- ethyl isobutyrylacetate was more difficult to convert
troalkenes (Table 1). In general, high conversions and even in a prolonged reaction time due to the relative-
no side products, and excellent enantioselectivities ly large steric substituents (entry 6).
were obtained under similar conditions for all tested
To gain better insight into the nature of the hetero-
substrates. Also it was found that the steric properties geneous catalysis and to eliminate the non-covalent
of substituted malonates did significantly affect the adsorption during the catalytic process, two control
catalytic activity. From entries 1 and 7–10, it is seen experiments for the asymmetric addition of diethyl
that the reaction time must be prolonged as the alkyl malonate to nitrostyrene were also carried out using
ester group (R) is enlarged. In particular, the reaction (1R,2R)-DACH-functionalized PMO (3) plus NiBr2 in
could not be completed when R was tert-butyl the presence of (1R,2R)-N,N’-dibenzylcyclohexyldi-
(entry 10), even on prolongation to 3 days. A possible
ACHTUNGTRENaNUNG mine and (1R,2R)-DACH-functionalized PMO (3)
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Enantioselective Addition of Malonates and b-Keto Esters to Nitroalkenes
Table 2. Asymmetric Michael addition of b-ketoester to nitroalkenes.[a]
Entry
R’
R’’
Ar
Time(h)
Conv. [%]
dr (a:b)
ee [%] (a:b)
1
2
3
4
5
6
Me
Me
Me
Me
Me
i-Pr
Me
Me
Et
t-Bu
t-Bu
Et
Ph
4-BrC6H4
Ph
Ph
4-BrC6H4
Ph
6 (6)
4
5
10
10
12
98 (94)
>99
>99
>99
>99
92
1.3:1 (1:1)
2.7:1
1.2:1
1.8:1
2:1
>99 (99) [94 (94)][4a]
>99 (>99)
93 (94)
94 (96)
93 (96)
1:1
91 (92)
[a]
Reaction conditions: catalysts (57.0 mg, 0.02 mmol of Ni, based on the ICP analysis), nitroalkenes (1.0 mmol), b-keto
ester (1.20 mmol) and 4.0 mL toluene, reaction temperature (408C), reaction time (4–12 h).
Determined by chiral HPLC analysis using Chiralcel AD-H columns (see Figure S4 in the Supporting Information) and
the conversion was calculated by the internal standard method
[b]
plus the homogeneous bis(cyclohexyldiamine)-based that the catalytic reaction could afford 38% of prod-
Ni(II) complexes as heterogeneous catalysts. It was uct although without any enantioselective background
found that the former afforded the corresponding (entry 5 in Table 1), which was strongly similar to that
products with 76% conversion and 88% ee value of (1R,2R)-N,N’-dibenzylcyclohexyldiamine itself as
(entry 3 in table 1) while the latter gave the corre- a Brønsted base in a homogeneous catalysis system.[4a]
sponding products with 89% conversion and 91% ee This behaviour demonstrated that (1R,2R)-DACH-
value (entry 4 in Table 1). The former result suggested functionalized PMO (3) itself acts as a Brønsted base
that the catalyst synthesized by an in situ post-modifi- that could deprotonate diethyl malonate and accord-
cation method could result in a medium catalytic per- ingly anticipate the addition of nitrostyrene. In the
formance. Lower catalytic activity and enantioselec- case of (1R,2R)-DACH-functionalized PMO (3) plus
tivity than those of the catalyst 5 should be due to the NiBr2 in the absence of (1R,2R)-N,N’-dibenzylcyclo-
fact that the small part of NiBr2 had not been coordi- hexyldiamine as a heterogeneous catalyst, we treated
nated during the catalytic process, in which the loss of this heterogeneous catalyst via a Soxhlet extraction
Ni was detected by ICP analysis in solution. The process to keep a pure mono(cyclohexyldiamine)-
latter indicated that the chiral microenvironment had based Ni(II) complex within the PMO material. The
not been changed as derived from non-covalent ad- result showed that the chiral product with 61% con-
sorption. Lower conversion than that of the catalyst 5 version and 89% ee value (entry 6 in Table 1) was ob-
suggested that part of the homogeneous catalyst did served, however, the reaction needed 36 h. Obviously
not participate in the catalytic reaction due to jam- the lower catalytic efficiency than that of the hetero-
ming from physical absorption.[7a,b] However, when geneous catalyst 5 should be ascribed to the nature of
both heterogeneous catalysts of the above control ex- mono(cyclohexyldiamine)-based Ni(II) complexes in
periments were allowed to undergo a process of Soxh- the absence of one equivalent (1R,2R)-N,N’-dibenzyl-
let extraction, it was found that the catalytic reactions cyclohexyldiamine as a Brønsted base. All these ob-
using the treated heterogeneous catalysts only gave servations confirmed that bis(cyclohexyldiamine)-
traces of products. This fact ruled out the role of the based Ni(II) active centers within periodic mesopo-
non-covalent physical adsorption, explaining that the rous organosilicas played a crucial role in both cata-
nature of this heterogeneous catalysis was derived lytic and enantioselective performances.
from the heterogeneous catalyst 5 itself.
An important feature of the design of the heteroge-
To further consolidate the role of bis(cyclohexyldi- neous catalyst 5 was its easy and reliable separation
ACHTUNGTRENNUNG
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Ronghua Jin et al.
FULL PAPERS
Table 3. Reusability of the catalyst 5 for dimethylmalonate addition to nitrostyrene.[a]
Run time
1
2
3
4
5
6
7
8
9
10
Conv. [%]
>99.9
94.4
>99.9
94.0
>99.9
93.7
>99.9
93.5
>99.9
93.3
99.8
93.2
>99.9
93.1
>99.9
92.7
99.1
92.6
89.9
92.4
ee [%][b]
[a]
Reaction conditions: catalysts (57.0 mg, 0.02 mmol of Ni, based on the ICP analysis), nitrostyrene (0.15 g, 1.0 mmol), di-
methyl malonate (0.16 g, 1.20 mmol) and 4.0 mL toluene, reaction temperature (408C), reaction time (8 h).
Determined by chiral HPLC analysis using Chiralcel AD-H columns (see Figure S5 in Supporting Information) and the
conversions were calculated by the internal standard method.
[b]
case, the heterogeneous catalyst 5 was filtered from the homogeneous catalyst. In particular, this research
the reaction mixture after 4 h and the reaction was did also disclose that the nature of this heterogeneous
continued for further 12 h. It was found that there catalysis was derived from the heterogeneous catalyst
was no appreciable change either in conversion or ee itself, demonstrating that chiral bis(cyclohexyldia-
value of the chiral products. This fact eliminated the mine)-based Ni(II) complexes within the periodic
role of homogeneous mono(cyclohexyldiamine)-based mesoporous organosilica played a crucial role in both
Ni(II) complexes or the homogeneous bis(cyclohexyl- catalytic and enantioselective performance. More im-
diamine)-based Ni(II) complexes, confirming that the portantly, the heterogeneous catalyst 5 could be re-
recycle efficiency was derived from heterogeneous covered and reused nine times without affecting obvi-
catalyst itself. In order to analysis the reasons for the ously its ee value, showing good potential in industrial
decreased reactivity at the tenth recycle, an ICP anal- application.
ysis was undertaken. The result showed that the Ni
amount of the catalyst 5 in the solid state at the tenth
recycle was 16.73 mg Ni per gram catalyst, meaning Experimental Section
that the leached amount at the tenth reuse was 18.4%
relative to the starting amount. This behaviour indi- General Remarks
cated that the leaching of Ni metal might be responsi-
ble for the decreased reactivity at the tenth recycle.
were carried out under an argon atmosphere using standard
All experiments, which were sensitive to moisture or air,
In order to further consolidate this judgment, the Ni
amount at the ninth recycle was also investigated. It
was found that the leaching amount at ninth recycle
was 8.27 mg Ni per gram catalyst, suggesting that the
leached amounts at the ninth recycle was 9.1%. This
value was nearly consistent with the 9.6% loss of
(1R,2R)-N,N’-dibenzylcyclohexyldiamine detected by
elemental analysis (1.79% of N at the ninth recycle
means 9.5% of the lost amounts relative to 1.98% of
N in the starting amount). These observations con-
firmed that the decreased reactivity originated from
the leaching of Ni metal.
Schlenk techniques. 1,4-Bis(triethyoxysilyl)benzene, surfac-
tant P123 {(CH2-CH2O)20A[CH2A(CH3)CH2O]70A(CH2CH2O)20},
(1R,2R)-1,2-cyclohexyldiamine and NiBr2 were purchased
G
E
CHTUNGTRENNUNG
from Sigma–Aldrich Company Ltd. and used as received.
Compound 1 {(1R,2R)-DACH-[(CH2PhSiACHTUNGRTNEUNG(OMe)3]2} was syn-
thesized according to the reported literature.[3c] The products
were analyzed by using HPLC with a UV-Vis detector using
a Daicel AD-H or OD-H chiralcel columns(F 0.46ꢂ25 cm).
Characterization
The Ni loading amount in the catalyst was analyzed using an
inductively coupled plasma optical emission spectrometer
(ICP, Varian VISTA-MPX). Fourier transform infrared (FT-
IR) spectra were collected on a Nicolet Magna 550 spec-
trometer using the KBr method. X-ray powder diffraction
(XRD) was carried out on a Rigaku D/Max-RB diffractom-
eter with CuKa radiation. Transmission electron microscopy
(TEM) images were performed on a JEOL JEM2010 elec-
tron microscope at an acceleration voltage of 220 kV. X-ray
photoelectron spectroscopy (XPS) measurements were per-
formed on a Perkin–Elmer PHI 5000C ESCA system. All
the binding energies were calibrated by using the contami-
nant carbon (C1s =284.6 eV) as a reference. Thermal gravi-
metric analysis (TGA) was performed with a Perkin–Elmer
Conclusions
In conclusion, we have immobilized successfully the
chiral bis(cyclohexyldiamine)-based Ni(II) complexes
within the periodic mesoporous organosilica and ob-
tained a PMO-supported chiral Ni(II) catalyst (5).
This heterogeneous catalyst exhibited highly catalytic
activity and excellent enantioselectivity in the asym-
metric Michael addition of 1,3-dicarbonyl compounds
to nitroalkenes, which were comparable to those of Pyris Diamond TG analyzer under an air atmosphere with
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Adv. Synth. Catal. 2012, 354, 3265 – 3274

Enantioselective Addition of Malonates and b-Keto Esters to Nitroalkenes
a heating ramp of 5 KminÀ1. Solid-state 13C (100.5 MHz)
General Procedure for Asymmetric Michael Addition
of 1,3-Dicarbonyl Compounds to Nitroalkenes
and 29Si (79.4 MHz) CP-MAS NMR were obtained on
a Bruker DRX-400 spectrometer.
A typical procedure was as follows: the heterogeneous cata-
lyst 5 (57.0 mg, 0.02 mmol based on Ni from ICP), nitrostyr-
ene (0.15 g, 1.0 mmol), dimethyl malonate (1.20 mmol) and
4.0 mL toluene were added in a 10-mL round-bottom flask
in turn. The mixture was allowed to react at 408C for 8–
28 h. During that time, the reaction was monitored constant-
ly by TLC. After completion of the reaction, the heteroge-
neous catalyst was filtered through filter paper for the recy-
cle experiment. The aqueous solution was extracted by Et2O
(3ꢂ3.0 mL). The combined Et2O layer was washed with
brine twice and dehydrated with Na2SO4. After the evapora-
tion of Et2O, the residue was purified by silica gel flash
column chromatography to afford the desired product. The
conversion was calculated by the internal standard method
and the ee value could be determined by chiral HPLC analy-
sis with a UV-Vis detector using a Daicel OD-H chiralcel
column (F 0.46ꢂ25 cm).
Preparation of (1R,2R)-DACH-Functionalized
Organosilicas (3)
In a typical synthesis, 2.0 g of structure-directing agent (plur-
onic P123) were fully dissolved in a mixture of 80 mL hydro-
chloric acid (0.2 N) and 6.0 g KCl. Then, 3.62 g (9.00 mmol)
of 1,4-bis(triethyoxysilyl)benzene were added as the silica
precursor at 408C. After a pre-hydrolysis period of 40 min,
0.53 g (1.00 mmol) of (1R,2R)-DACH-[(CH2PhSiACHTNURTGNEUNG(OMe)3]2
(1) were added, in which the initial molar ratio is
Si:P123:KCl:HCl:H2O in the mother solution is
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 filtered and rinsed
with excess ethanol before being dried overnight on a filter.
The surfactant template was removed by refluxing in acidic
ethanol (400 mL per gram) for 24 h. The solid was filtered
and rinsed with ethanol again, and dried at 608C under re-
duced pressure overnight to afford (1R,2R)-DACH-func-
tionalized organosilicas (3) in the form of a white powder;
yield: 1.38 g. IR (KBr): n=3423.5 (s), 3062.1 (w), 2933.7
(w), 2851.1 (w), 1648.7 (m), 1602.3(w), 1561.5 (w), 1451.3
(w), 1383.3 (w), 1155.6 (s), 1079.2 (s), 949.7 (m), 861.3
(w),810.3 (w), 698.9 (w), 620.4 (w), 536.5 (w), 520.1 cmÀ1
Acknowledgements
We are grateful to the China National Natural Science Foun-
dation (20673072), Shanghai Sciences and Technologies De-
velopment
Fund
(10DJ1400103,
10JC1412300
and
12nm0500500) and Shanghai Municipal Education Commis-
sion (No. 12ZZ135, S30406) for financial support.
(m); elemental analysis (%): C 50.62, H 4.41, N 1.05; SBET
:
396.7 m2 gÀ1; dpore: 5.95 nm; Vpore: 0.71 cm3 gÀ1 29Si MAS
;
NMR (79.5 MHz): T1 (d=À64.3 ppm), T2 (d=À72.3 ppm),
T3 (d=À79.7 ppm); 13C CP MAS NMR (100.6 MHz): d=
132.7, 69.8, 57.3, 51.2, 33.5, 23.1, 16.1 ppm.
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