Pd NPs-Loaded Homochiral Covalent Organic Framework for Heterogeneous Asymmetric Catalysis

Article abstract of DOI:10.1021/acs.chemmater.7b02131

We herein report the fabrication of a palladium nanoparticles (Pd NPs) loaded homochiral covalent organic framework using S-(+)-2-methylpiperazine and cyanuric chloride (Pd@CCOF-MPC) (2) via a very facile chemical approach. The chiral COF support of 1 (CC

Full text of DOI:10.1021/acs.chemmater.7b02131

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Article
Pd NPs-Loaded Homochiral Covalent Organic
Framework for Heterogeneous Asymmetric Catalysis
Hui-Chao Ma, Jing-Lan Kan, Gong-Jun Chen, Cheng-Xia Chen, and Yu-Bin Dong
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b02131 • Publication Date (Web): 07 Jul 2017
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Pd NPs-Loaded Homochiral Covalent Organic Framework for
Heterogeneous Asymmetric Catalysis
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a
a
a
b
a
Hui-Chao Ma, Jing-Lan Kan, Gong-Jun Chen,* Cheng-Xia Chen, and Yu-Bin Dong*
a
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functional-
ized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Min-
istry of Education, Shandong Normal University, Jinan 250014, P. R. China. E-mail: yubindong@sdnu.edu.cn,
gongjchen@126.com
b
MOE Laboratory of Bioinorganic and Synthetic Chemistry Lehn Institute of Functional Materials, School of Chem-
istry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, P. R. China.
ABSTRACT: We herein report the fabrication of a palladium nano particles (Pd NPs) loaded homochiral covalent organic
framework using S-(+)-2-methylpiperazine and cyanuric chloride (Pd@CCOF-MPC) (2) via a very facile chemical ap-
proach. The chiral COF support of 1 (CCOF-MPC) is synthesized by the combination of cyanuric chloride and S-(+)-2-
methylpiperazine at 90⁰C for 36 h, and the Pd NPs-loaded Pd@CCOF-MPC (2) is prepared via sequential solution im-
pregnation and NaBH reduction at room temperature. 2, as a highly active reusable asymmetric heterogeneous compo-
4
site catalyst, is able to effectively promote the Henry reaction and reductive Heck reaction in high yield with excellent
stereoselectivity.
able to upload and stabilize nanosized species such as
metal NPs. In this way, the CCOF templating and metal
INTRODUCTION
Over the past decades, palladium nanoparticles (Pd NPs)
have been intensively pursued in a wide range of im-
portant catalytic processes owing to their excellent cata-
NPs catalytic functionality would be logically integrated
into one composite system, consequently, making the
metal NPs involved NPs@CCOFs to be highly active and
reusable multifunctional catalytic material for heteroge-
neous asymmetric catalysis.
1
-8
lytic performance. However, they are known to easily
aggregate and form Pd black because of their high surface
9-10
energy.
In this contribution, we report, the first of its kind,
such a composite asymmetric catalytic system Pd@CCOF-
MPC (2) which is generated from a new chiral nitrogen-
rich CCOF (CCOF-MPC (1)) and Pd NPs. The obtained
Pd@CCOF-MPC (2) is a highly active heterogeneous
asymmetric catalyst for the Henry reaction and reductive
Heck reaction with excellent yield and stereoselectivity.
One of the most practical ways for addressing such a
problem is to immobilize Pd NPs in the porous supports
such as metal-organic frameworks (MOFs), which has
11-20
been well demonstrated by many previous studies.
Very recently, covalent organic frameworks (COFs), as a
novel class of polymeric porous crystalline organic mate-
21-25
rials, have currently gained considerable attention.
Although both MOFs and COFs are constructed from the
molecular building blocks via intermolecular covalent
binding interactions, and they all feature reticular crystal-
line extended solids, COFs, however, might be more sta-
EXPERIMENTAL SECTION
Synthesis of CCOF-MPC (1). An anhydrous 1,4-dioxane
(
20 mL) solution of cyanuric chloride (4 mmol) was
dropwise added -(+)-2-methylpiperazine (6 mmol) and
anhydrous K CO (12 mmol) in anhydrous 1,4-dioxane (50
S
ble to water, organic solvents and even strong acidic or
2
3
26-39
alkaline media.
Such features could endow COFs more
mL) at room temperature. After that, the mixture was
heated at 90°C for 36 h. The resulting product was col-
lected by filtration and washed with dichloromethane,
ethanol, and water to completely remove the unreacted
advantage to be the ideal supports to upload active cata-
lytic species for catalysis, especially those catalytic reac-
tions are carried out under relatively rigorous conditions.
-
1
It is similar to chiral MOFs (CMOFs), chiral COFs
starting precursors. IR (KBr pellet cm ): 2927(s), 2821(s),
1737(m), 1652(s), 1492(m), 1380 (s). Elemental Analysis (%)
calcd for C H N : C, 54.53; N, 39.75; H, 5.72. Found (%): C,
(
CCOFs) can also be synthesized by directly linking the
chiral molecular building blocks. In principle, the optical-
ly pure CCOFs can be the asymmetric template to guide
the reaction substrates in the specific spatial orientations.
8
10
5
55.04; N, 38.95; H, 6.01.
Synthesis of Pd@CCOF-MPC (2). 1 (10 mg, 0.002 mmol)
40-41
On the other hand, the porous crystalline structure
was added to a CH OH (2 mL) solution of palladium ni-
3
together with heteroatom donors on CCOFs should be
trate (6.7 mg, 0.12 mmol). The mixture was stirred for 1 h
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at room temperature. The resulting solid was isolated by
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centrifugation and washed with CH CN. The obtained
3
green-yellow crystalline solid was mixed with NaBH (25
4
mg, 0.65 mmol) in water (2 mL) and the mixture was
stirred at room temperature for additional 5 h to afford 2
as dark brown crystalline solids. The obtained crystalline
solids were washed with CH CN and EtOH and dried in
3
-
1
air. IR (KBr pellet cm ): 2923(s), 2819(s), 2152(m),
009(m), 1724(w), 1652(m), 1477(w), 1384(w). ICP meas-
2
urement indicated that the encapsulated amount of Pd
NPs in 2 is 34.1 wt %.
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General procedure for the Henry reaction catalyzed
by 2. Ethanol solution (2 mL) of Ar-CHO (0.80 mmol),
nitromethane (59 µL, 1.1 mmol), K CO (311 mg, 2.4 mmol)
2
3
and 2 (0.75 mol % Pd) was stirred at room temperature
for 8 h to afford the corresponding nitroaldol adducts.
Yield was determined by the GC measurement on HP-5
column, and enantiomeric excess was determined by
HPLC with a Chiralcel OD-H column (85 : 15 = n-hexane :
isopropanol, 1.0 mL/min, 254 nm), respectively (Support-
ing Information).
General procedure for the reductive Heck reaction
catalyzed by 2. Ethanol-H O (2 mL, 1:1) solution of 2-
cyclohexen-1-one (49 µL, 0.50 mmol), Ar-X (2.5 mmol),
2
Scheme 1. Schematic representation for the synthesis of
CCOF-MPC (1) and Pd@CCOF-MPC (2). Pd NPs are shown
as yellow balls. The photographs of bulk samples and SEM
images of 1 and 2 are inserted.
K CO (0.345 g, 2.5 mmol) and 2 (3.2 mol % Pd) was heat-
2
3
ed at 80°C for 10 h to afford corresponding products. Yield
was determined by the GC measurement on DB-WAX
column, and enantiomeric excess was determined by
HPLC with a Chiralcel OD-H column (98 : 2 = n-hexane :
isopropanol, 1.0 mL/min, 254 nm), respectively (Support-
ing Information).
RESULTS AND DISCUSSION
Synthesis and Characterization of 1 and 2.
As shown in Scheme 1, homochiral CCOF-MPC (
was synthesized in high yield (95 %) by heating the
mixture of -(+)-2-methylpiperazine and cyanuric
chloride in anhydrous 1,4-dioxane at 90⁰C in the
presence of K CO for 36 h. was obtained by filtra-
1)
S
1
2
3
tion and successively washed with dichloromethane,
ethanol, and water to afford white crystalline pow-
ders that were insoluble in common organic solvents
and water. Scanning electron microscopy (SEM) was
used to visualize the as-prepared 1, and a particle
morphology was observed (Scheme 1 and Supporting
Information). In addition, thermogravimetric analysis
Figure 1. a) IR spectra of cyanuric chloride (blue), S-(+)-
methylpiperazine (red) and 1 (black). b) C NMR of 1. c)
simulated and measured PXRD patterns of 1 and 2.
13
(TGA) indicated that almost no weight loss of 1 was
observed up to 350⁰C (Figure S2, Supporting Infor-
mation).
-1
(2821 and 2927 cm for methylpiperazinyl C-H bonds)
-1
and C-N stretching vibrations (1482 and 1380 cm for
CCOF-MPC (
1
) was characterized by Fourier trans-
13
form infrared (FT-IR) and C cross-polarization mag-
ic angle spinning (CP-MAS) spectroscopies. The IR
triazinyl C-N bond) indicated that both the
methylpiperazinyl and triazinyl units existed in
1
Figure 1a). Solid-state C CP-MAS NMR was also
13
spectrum of
stretching vibration at 3214 cm
methylpiperazine and C-Cl stretching vibration of
1 shows that the characteristic N-H
(
-1
of
S-(+)-2-
used to establish the connectivity of the COF species.
The existence of the methyl, triazinyl and piperazinyl
groups in 1 was well confirmed via the characteristic
-1
cyanuric chloride at 850 cm disappeared after the
reaction. Meanwhile the C-H stretching vibrations
2
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ductively coupled plasma (ICP) measurement, is up to
resonances at 16, 46, 55, 68 and 162 ppm, respectively
(Figure 1b).
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34.1 wt %. The oxidation state of the encapsulated Pd spe-
cies after reduction was determined by X-ray photoelec-
tron spectroscopy (XPS) (Figure 3b). The observation of
Pd d5/2 and d3/2 peaks at 335 and 341 eV demonstrated the
In addition, CCOF-MPC (1) exhibits good crystallinity
as revealed by the powder X-ray diffraction (PXRD) pat-
tern (Figure 1c). The structural modeling was then con-
ducted with the software of Materials Studio (ver. 5.0).
4
4
42
existence of Pd(0) in 2. Figure 3a shows that
2 exhibits
positive Cotton effect at 220 and 236 nm, and nega-
tive dichroic signal at 210 and 228 nm in its CD spec-
trum, which is basically identical to those of 1.
The most probable structure of 1 was simulated, analo-
gous to that of 1 as a 2D staggered layered-sheets using
the chiral space group of P63 with the optimized parame-
ters of a = b = 15.406 Å and c = 8.400 Å (residuals wRp =
0
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1
.05 % and Rp = 3.19 %, Supporting Information). As indi-
cated in Figure 1c, the calculated PXRD pattern of this
hexagonal unit cell matches well with the experimental
profile, but it is distinctly different from its starting mate-
rials of cyanuric chloride, S-(+)-2-methylpiperazine (Fig-
ure S2, Supporting Information) and PAF-6 COF generat-
43
ed from cyanuric chloride and piperazine. As shown in
Figure 1c, 1 displays intense peaks at 6.6, 12.0, 15.1, 16.5 and
20.7⁰ that correspond to the (1,0,0), (1,0,1), (2,-1,1), (2,0,1)
and (0,0,2) planes, respectively. This 2D sheet contains
hexagonal structural unit, in which the opposite Ntria-
zinyl···Ntriazinyl distance is ca. 1.5 nm (Figure 2).
Figure 3. a) The CD spectra of 1 (red line) and 2 (blue line).
b) XPS spectrum of 2. c) HRTEM images of 2 and simulated
the structure of 2.
In addition, we used high resolution transmission elec-
tron microscopy (HRTEM) to investigate the Pd NPs dis-
persion and size in 2. As indicated in Figure 3c, the Pd
NPs in 2 were highly dispersed with particle size of 2-5
nm. The atomic lattice fringes with an interplanar spacing
of 0.24 nm corresponding to the 1/3 (422) fringes of face-
Figure 2. Left: single hexagonal unit. Right: crystal packing
patterns viewed down the crystallographic c and b axes.
The AB-type layer staggered model was determined by
the force field-based molecular mechanics calculations.
The results indicated that eclipsed model showed a dis-
tinct energetic preference to the AB type staggered-
structure. The staggered AB-layer stacking model gave
the interlayer distance of 4.2 Å at low-energy minimum,
which is consistent with the (002) diffraction peak at
20.7⁰, indicating that adjacent layers in 1 are in van der
Waals contact (Figure 2). In 1, the S configuration of S-
(+)-2-methylpiperazine (C7, Figure 2) was retained, which
is well supported by the solid circular dichroism (CD)
measurement. The CD spectrum of 1 unambiguously
demonstrated its enantiomeric nature of the optical activ-
ity. As indicated in Figure 3a, 1 exhibits positive Cotton
effect at 220 and 240 nm, and negative dichroic signal at
207 and 227 nm.
4
5
centered cubic (fcc) Pd (Figure 3b).
Owing to the Pd
NPs (2-5 nm) sizes are larger than that of pore size (1.5
nm) in 2, so the involved Pd NPs should be located be-
tween the 2D COF layers and stabilized by the nitrogen
atoms in the layers, and inevitably led to an increase in
the interlayer spacing of 2, which was well demonstrated
by the PXRD pattern of 2. As shown in Figure 1c, the
(
0,0,2) reflection shifted from 20.7⁰ (1) to 16.7⁰ (2), corre-
sponding to an interlayer distance increase to ~5.7 Å in 2.
The existence of the characteristic bands for CCOF
-1
(C-H stretching vibrations at 2819 and 2923 cm for
methylpiperazinyl C-H bond and C-N stretching vi-
-1
brations at 1477 and 1384 cm for triazinyl C-N bond)
in IR spectrum (Figure S4, Supporting Information)
of 2 further confirmed that the COF support is stable
Pd NPs embedded Pd@CCOF-MPC (2) was simply pre-
pared by the reduction (NaBH , H O, 5h, r.t.) of
Pd(II)@CCOF-MPC that was obtained by impregnating of
1 in a solution of Pd(NO ) in MeOH at room temperature
for 1 h. The colour of 1 changed from white to dark of 2
after the impregnation and reduction processes (Scheme
1). The total uploaded Pd amount, as determined by in-
4
2
after Pd(0) loading.
3
2
The porosity difference before and after Pd loading
was further supported by the gas adsorption–
desorption measurement. As shown in Figure 4, N2
adsorption by
1 and 2 at 77 K revealed absorption
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3
amounts of 56.70 and 213.38 cm /g by CCOF-MPC (
and Pd@CCOF-MPC ( ), respectively. The surface
areas calculated on the basis of the BET model are 101
1
)
cycles. After each run, the catalyst was collected by centrifu-
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gation, washed with ethanol and dichloromethane, dried at
90°C in vacuum for the next catalytic run under the same
conditions. The unactivated catalyst resulted in very low
yield (Supporting Information). Reaction conditions: 2 (0.75
wt % Pd), benzaldehyde (82 µL, 0.8 mmol), nitromethane (59
µL, 1.1 mmol), K CO (331 mg, 2.4 mmol), ethanol (2 mL) and
2
2
2
m /g for 1 and 686 m /g for 2, and the total pore vol-
3
umes are 0.10 and 0.33 cm /g, respectively. As men-
tioned above, the Pd NPs loading in 1 caused an appar-
ent interlayer distance increase, which might be the rea-
son to result in a surface area and N adsorption increase
in 2. The pore size distribution curves, calculated
2
3
room temperature. The solid catalyst was filtrated from the
reaction solution after 4 h, whereas the filtrate was trans-
ferred to a new vial and reaction was carried out under the
same conditions for an additional 8 h.
2
from Barrett-Joyner-Halenda analysis, showed that
and possessed the narrow pore diameter distribu-
tions with pore widths of 0.41 ( ) and 1.26 nm ( ), re-
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To optimize the reaction conditions, nitroaldol re-
spectively (Figure 4). Notably, although the involved
Pd NPs significantly changed the framework porosity,
the optical activity of the framework was well main-
tained.
action of benzaldehyde with nitromethane (Table 1,
entry 1) was chosen as the model reaction (Figure 5).
When the reaction was carried out in the presence of
2
(0.75 mol % Pd) at room temperature in methanol,
dichloromethane or methylbenzene in the absence of
base, the desired product was obtained in quite low
yields with very poor enantioselectivities. The similar
result was obtained once the reaction was performed
in ethanol with Na CO . The reaction was also failed
2
3
to afford the expected product when strong base of
NaOH was used instead of Na CO in ethanol. We
2
3
were pleased to find that
the desired nitroaldol adduct of
2
(0.75 mol % Pd) afforded
)-2-nitro-1-
(R
Figure 4. Left: N adsorption isotherms 1 and 2 at 77 K.
phenylethan-1-ol in high yield (97 %) with excellent
enantiomeric excess (95 % ee) in the presence of
K CO (K CO /benzaldehyde = 3 : 1) in ethanol at
2
Right: the pore widths of 1 and 2 are centered at 0.41 nm and
1.26 nm, respectively.
2
3
2
3
room temperature for 8 h (Figure 5, Figures S5-S7,
Supporting Information).
Asymmetric Henry reaction catalyzed by 2. By tak-
ing advantage of the chiral feature
46-48
and the embedded
The turnover number (TON, defined as the molar
ratio of converted substrate to catalyst loading) and
turnover frequency (TOF, defined as the molar ratio
of converted substrate to catalyst loading per unit of
highly active Pd NPs, we employed 2 for the asymmetric
Henry reaction. It is well-known that the optically active
β-nitro alcohols derived from the Henry reaction is an
important class of intermediates that might be trans-
formed into valuable species such as bioactive natural
products and pharmaceutical agents. The enantioselective
Henry reactions are usually triggered by homogeneous
chiral metal complexes and sometimes chiral organocata-
lysts (Table S1, Supporting Information). The asymmet-
ric catalysis for Henry reaction based on the COF-
supported Pd NPs composite heterogeneous catalyst
herein could be a valuable complement to this field.
-1
time) for the model reaction are 48.5 and 6.1 h , re-
spectively. In order to gain insight into the heteroge-
neous nature of
out. As indicated in Figure 5, no further reaction took
place without after ignition of the reaction at 4 h,
indicating exhibits a typical heterogeneous catalyst
2, the hot leaching test was carried
2
2
nature herein.
Table 1. Henry reactions of nitromethane with
[a]
different aromatic aldehydes catalyzed by 2
[
b]
[b]
entry
ArꢀCHO
yield (%)
ee (%)
config
1
2
3
4
5
6
7
PhꢀCHO
97
95
R
R
R
R
R
R
R
4ꢀ(Me)ꢀPhꢀCHO
3ꢀ(Me)ꢀPhꢀCHO
4ꢀ(MeO)ꢀPhꢀCHO
3ꢀ(MeO)ꢀPhꢀCHO
88
81
87
82
82
81
80
79
4ꢀ(NO
2
)ꢀPhꢀCHO
)ꢀPhꢀCHO
99
97
3ꢀ(NO
2
94
88
Figure 5. Left: reaction time examination (black line) and
leaching test (red line) for Henry reaction. Right: catalytic
4
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yields (80 - 99 %, entries 2 - 13) with excellent enanti-
omeric excesses ( -configuration, 79 - 97 % ee, en-
tries 2 - 13) are observed for those phenylaldehydes
bearing either electron-donating or electron-
withdrawing substituents (entries 2 - 13). For the larg-
er sized aromatic aldehydes such as 4-
8
4ꢀ(F)ꢀPhꢀCHO
4ꢀ(Br)ꢀPhꢀCHO
2ꢀ(Br)ꢀPhꢀCHO
4ꢀ(Cl)ꢀPhꢀCHO
4ꢀ(CN)ꢀPhꢀCHO
4ꢀ(OH)ꢀPhꢀCHO
biphenylꢀCHO
anthraceneꢀCHO
96
92
90
94
99
96
5
95
93
90
95
97
90
ND
ND
R
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
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R
9
1
R
0
R
11
R
biphenylcarboxaldehyde
(entry
14)
and
9-
12
13
14
15
R
anthraldehyde (entry 15), no expected nitroaldol ad-
ducts were generated under the optimized reaction
conditions. This demonstrated that the nitroaldol
reaction herein might be an internal surface catalytic
process.
R
[
c]
ND
ND
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
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3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ND
[
a]
Compared to various reported chiral catalysts for
the asymmetric Henry reaction of benzaldehyde with
nitromethane (Table S1, Supporting Information), 2
herein exhibits good catalytic activity and enantiose-
lectivity, which demonstrated by the milder reaction
conditions (r.t. and short reaction time) and excellent
yield with high ee value (Table S1). Therefore, it can
be a valuable complement for the asymmetric cata-
lysts to the heterogeneous homochiral catalysts for
the asymmetric Henry reaction.
Reaction conditions: 2 (0.75 mol % Pd), aromatic benzaldehydes (0.8
mmol), nitromethane (59 ꢁL, 1.1 mmol), K CO (331 mg, 2.4 mmol),
Yield and ee values are deterꢀ
mined by GC and chiral HPLC, respectively (Figures S11ꢀS12, Supportꢀ
2
3
[b]
ethanol (2 mL), room temperature.
[
c]
ing Information). Not detected.
As heterogeneous catalyst, the recyclability of
2
was also examined (Figure 5). After each run, the sol-
id catalyst was readily recovered by centrifugation,
washed with ethanol and dichloromethane, and then
dried at 90°C in vacuum for the next catalytic run
under the same reaction conditions. The yield was
still up to 91 % (91 % ee) even after five catalytic cy-
cles (Figures S8-S9, Supporting Information). Alt-
Asymmetric Reductive Heck reaction catalyzed
by 2. Next, we examined the asymmetric catalytic
activity of 2 for the reductive Heck reaction, which is
an important type of coupling reaction between aryl
halides and alkenes to form the addition products
under the similar Heck reaction conditions. Although
some progresses have been made in the homogene-
hough the size of the Pd NPs in
-
2 slightly increased (5
10 nm) after five catalytic runs, no valence change
for Pd species was detected based on XPS analysis,
and the atomic lattice fringe (0.24 nm) of Pd NPs was
still clearly observed (Figure S10, Supporting Infor-
mation). The diffraction (ca. 40⁰) of Pd NPs was de-
tected from PXRD pattern due to the decrease of
crystallinity of the CCOF support after multiple cata-
lytic cycles caused by the Pd NPs aggregation (Figure
S10, Supporting Information). The Pd loss is only ca.
ous palladium catalyzed intermolecular Heck reac-
tion,
49-52
and no heterogeneous asymmetric Pd-
catalytic system for this type of reactions has been
reported so far.
8
.5 % which was determined by ICP-AES. Thus, 1
could be considered as an ideal platform to load Pd
NPs for this asymmetric Henry reaction.
On the other hand, only tiny amount of target ni-
troaldol adduct was obtained when the reaction was
carried out in the presence of 1 (10 mol %), indicating
that the sandwiched Pd NPs in the CCOF-MPC ma-
trix are responsible for the catalytic activity. Notably,
yield of the asymmetric Henry reaction of PhCHO
with CH NO was also quite low (6 %) when the reac-
tion was carried out in the presence of only Pd NPs
under the same reaction conditions, which was re-
sulted from the Pd NPs agglomeration without COF
support. For further demonstrating the function of
chiral COF support, the Henry reaction of PhCHO
with CH NO was performed in the presence of
3
2
Figure 6. Left: reaction time examination (black line) and
leaching test (red line) for the synthesis of (R)-3-
phenylcyclohexan-1-one catalyzed by 2. Right: catalytic cy-
cles. After each run, the catalyst was collected by centrifuga-
tion, washed with ethanol and dichloromethane, dried at
3
2
Pd@PAF-6 which is an achiral COF supported Pd NPs
43
catalyst
was obtained in high yield (93%) but without stereoselec-
tivity (Figures S11-S13, Supporting Information).
.
The result showed the nitroaldol adduct
9
0°C in vacuum for the next catalytic run under the same
conditions. The unactivated catalyst resulted in very low
yield (Supporting Information). Reaction conditions: 2 (3.2
mol % Pd), 2-cyclohexen-1-one (49 µL, 0.5 mmol), bromo-
With optimized conditions in hand, the scope of
the reaction was explored (Table 1). In general, high
5
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Chemistry of Materials
Page 6 of 9
benzene (258 µL, 2.5 mmol), K CO (345 mg, 2.5 mmol), eth-
[a]
2
3
Reaction conditions:
2 (3.2 mol % Pd), 2ꢀcyclohexenꢀ1ꢀone (49
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
anol (1 mL) and H O (1 mL) and 80°C. The solid catalyst was
2
ꢁL, 0.5 mmol), aromatic halides (2.5 mmol), K CO (345 mg, 2.5
2
3
[
b]
filtrated from the reaction solution after 6 h, whereas the
filtrate was transferred to a new vial and reaction was carried
out under the same conditions for an additional 4 h.
2
mmol), ethanol (1 mL) and H O (1 mL) and 80°C. Yield and ee
values are determined by GC and chiral HPLC, respectively (Figꢀ
[c]
ures S19ꢀS20, Supporting Information). Not detected.
It is similar to previous case, Pd NPs aggregation
was also observed based on TEM image after the cata-
lytic cycles. The Pd atomic lattice fringe, however,
was still clearly visible and the zero valence of Pd
In this context,
2 catalysed asymmetric reductive
Heck reaction of 2-cyclohexen-1-one with bromoben-
zene (Table 2, entry 1) was first studied for the opti-
mal reaction conditions (Figure 6). To our delight,
species in
2 was maintained based on the XPS meas-
when the reaction was performed in EtOH/H O
2
urement after cyclic utilization for five times (Figure
S21, Supporting Information). ICP-AES analysis
showed that the loaded Pd amount decreased to 20.8
wt % which might be resulted from the existence of
strong base in reaction media at relatively high tem-
perature. Again, the diffraction (ca. 40⁰) of Pd NPs
species was detected from PXRD pattern due to their
aggregation, which led to the decrease of crystallinity
of the CCOF support after catalytic runs (Figure S21,
Supporting Information).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
mixed solvent system with K CO at 80⁰C, the desired
2
3
arylative product of (R)-3-phenylcyclohexan-1-one
was obtained in excellent yield (99 %) and enantiose-
lectivity (94 % ee) after 10 h (Figures S16-S18, Sup-
porting Information). The corresponding TON and
-1
TOF values for the reaction are 30.9 and 3.09 h , re-
spectively. The reaction in ethanol or methanol under
the same reaction conditions, however, afforded the
arylative product in very low yield. It is reasonable to
suppose that water in this reaction can effectively
improve the solubility of potassium carbonate, more-
A control experiment indicated that only
significantly promote this reaction (yield of 3-
henylcyclohexan-1-one, < 8 %). On the other hand,
1 cannot
over, provided the hydride source. Indeed, when D O
2
p
was used instead of H O to performed the reaction,
2
the desired product of 3-phenylcyclohexan-1-one was
obtained in only 7 % in the presence of only Pd NPs
under the same reaction conditions, which might be
resulted from the serious reunion of Pd NPs without
CCOF support under reaction conditions. This
strongly suggests that the uniform distributed encap-
sulated Pd NPs are the catalytically active species and
the CCOF support played a key role in stabilizing Pd
NPs during this reductive Heck reaction process. The
the deuterated arylative product was obtained, which
was unambiguously confirmed by the MS analysis
(Figure S16, Supporting Information).
In addition, the hot leaching test confirmed
2 in
this reaction is the heterogeneous catalyst (Figure 6)
and it can be reused for five catalytic cycles without
significant loss its activity even over five catalytic cy-
cles (93 % yield with 90 % ee at the fifth run) (Figure
6, Figures S19-S20, Supporting Information).
chiral templating effect of CCOF-MPC (1) was also
demonstrated. When the reaction of 2-cyclohexen-1-
one with bromobenzene was catalysed by an achiral
catalyst Pd@PAF-6, 3-phenylcyclohexan-1-one was
generated in 90% yield, however, without any stere-
Table 2. Reductive Heck reactions of 2-
cyclohexen-1-one with different aromatic halides
catalyzed by 2
[a]
oselectivity (Figure S22, Supporting Information).
[
b]
[b]
entry
ArꢀX
yield (%)
ee (%)
config
R
Again, we studied the scope of the reaction under
the optimal reaction conditions, and the obtained
results are summarized in Table 2. Compared to io-
dobenzene (entry 3), bromobenzene and chloroben-
zene provided higher yields (entries 1 and 2), but all
with the excellent enantioselectivity (over 95 % ee).
In comparison with the reactions of the substrates
bearing electron-donating groups (entries 6 - 9),
higher yields were obtained for those equipped with
electron-withdrawing substituents on the benzene
ring at para- or meta-position (entries 4 and 5). The
lowest ee value for those smaller-sized substrates
containing single benzene ring is 88 % (entry 8).
Meanwhile, the larger-sized biphenyl and polycyclic
aromatic halides were unfavourable for the reaction,
and the desired products were obtained in quite low
yields without any enantioselectivity (entries 10 - 12),
further demonstrating that the reaction could be an
inner pore catalytic process.
1
2
3
4
5
6
7
8
9
bromobenzene
99
94
chlorobenzene
96
95
R
Iodobenzene
72
96
R
4ꢀ(CN)ꢀbromobenzene
3ꢀ(CN)ꢀbromobenzene
4ꢀ(Me)ꢀbromobenzene
3ꢀ(Me)ꢀbromobenzene
4ꢀ(MeO)ꢀbromobenzene
3ꢀ(MeO)ꢀbromobenzene
4ꢀbromobiphenyl
99
91
R
99
97
R
80
89
R
90
95
R
82
88
R
89
90
R
[c]
10
11
12
15
ND
ND
ND
ND
9ꢀbromophenanthrene
9ꢀbromoanthracene
21
ND
ND
20
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Chemistry of Materials
Palladium Nanoparticles. Angew. Chem., Int. Ed. 2012, 51, 8500–
8504.
(5) Chung, M. K.; Orlova, G.; Goddard, J. D.; Schlaf, M.; Harris,
R.; Beveridge, T. J.; White, G.; Hallett, F. R. Regioselective Silyla-
tion of Sugars through Palladium Nanoparticle-Catalyzed Silane
Alcoholysis. J. Am. Chem. Soc. 2002, 124, 10508–10518.
(6) Mondal, B.; Acharyya, K.; Howlader, P.; Mukherjee, P. S.
Molecular Cage Impregnated Palladium Nanoparticles: Efficient,
Additive-Free Heterogeneous Catalysts for Cyanation of Aryl
Halides. J. Am. Chem. Soc. 2016, 138, 1709–1716.
(7) Scheuermann, G. M.; Rumi, L.; Steurer, P.; Bannwarth, W.;
Mülhaupt, R. Palladium Nanoparticles on Graphite Oxide and Its
Functionalized Graphene Derivatives as Highly Active Catalysts
for the Suzuki−Miyaura Coupling Reaction. J. Am. Chem. Soc.
CONCLUSION
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
We developed a facile method for the fabrication of
asymmetric CCOF-supported Pd NPs heterogeneous
catalyst, which was readily derived from a homochiral
COF support and Pd NPs. The nitrogen-rich CCOF
framework might be an ideal platform for uploading
and stabilizing the Pd NPs for those organic trans-
formations under mild conditions, for example,
asymmetric Henry reaction herein. It was demon-
strated that the obtained Pd@CCOF-MPC (2) is a
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
highly active asymmetric heterogenous catalyst
which can effectively promote the Henry and reduc-
tive Heck reactions with excellent yield and stereose-
lectivity under mild reaction conditions. We expect
this approach to be viable for the construction of
many more new and practical chiral composite heter-
ogeneous catalytic systems, and studies toward the
preparation of new asymmetric catalysts of this type
containing other CCOF supports and/or metal NPs
are underway.
2009, 131, 8262–8270.
(8) Wang, Z. J.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I.
Photocatalytic Suzuki Coupling Reaction Using Conjugated Mi-
croporous Polymer with Immobilized Palladium Nanoparticles
under Visible Light. Chem. Mater. 2015, 27, 1921−1924.
(9) Iwasawa, T.; Tokunaga, M.; Obora, Y.; Tsuji, Y. Homoge-
neous Palladium Catalyst Suppressing Pd Black Formation in Air
Oxidation of Alcohols. J. Am. Chem. Soc. 2004, 126, 6554-6555.
(10) Tian, N.; Zhou, Z.-Y.; Yu, N.-F.; Wang, L.-Y.; Sun, S.-G. Di-
rect Electrodeposition of Tetrahexahedral Pd Nanocrystals with
High-Index Facets and High Catalytic Activity for Ethanol Elec-
trooxidation. J. Am. Chem. Soc. 2010, 132, 7580–7581.
(11) Zhao, M.; Yuan, K.; Wang, Y.; Li, G.; Guo, J.; Gu, L.; Hu, W.;
Zhao, H.; Tang, Z. Metal-Organic Frameworks as Selectivity
Regulators for Hydrogenation Reactions. Nature, 2016, 539, 76-
80.
ASSOCIATED CONTENT
Supporting Information. Additional details such as, mate-
rials and instruments, additional characterization data of 1
and 2, leaching test, characterization of model and expanded
asymmetric Henry reaction catalyzed by 2, characterization
of the model and expanded asymmetric reductive Heck reac-
tion catalyzed by 2, structural analysis of 1, and control ex-
periments for catalysis. This material is available free of
charge via the Internet at http://pubs.acs.org.
(12) Dhakshinamoorthy, A.; Garcia, H. Catalysis by metal na-
noparticles embedded on metal–organic frameworks. Chem. Soc.
Rev. 2012, 41, 5262-5284.
(13) Liu, Y.; Zhang, W.; Li, S.; Cui, C.; Wu, J.; Chen, H.; Huo, F.
Designable Yolk–Shell Nanoparticle@MOF Petalous Hetero-
structures. Chem. Mater. 2014, 26, 1119–1125.
AUTHOR INFORMATION
Corresponding Author
(14) Zhu, Q. L.; Li, J.; Xu, Q. Immobilizing Metal Nanoparti-
cles to Metal–Organic Frameworks with Size and Location Con-
trol for Optimizing Catalytic Performance. J. Am. Chem. Soc.
*
*
yubindong@sdnu.edu.cn
gongjchen@126.com
2013, 135, 10210-10213.
Notes
(15) Huang, Y.-B.; Shen, M.; Wang, X.; Shi, P.-C.; Cao, R. Hier-
The authors declare no competing financial interest.
archically micro- and mesoporous metal-organic framework-
supported alloy nanocrystals as bifunctional catalysts: Toward
cooperative catalysis. J. Catal. 2015, 330, 452-457.
(16) Huang, G.; Yang, Q. H.; Xu, Q.; Yu, S. H.; Jiang, H. L. Pol-
ydimethylsiloxane Coating for a Palladium/MOF Composite:
Highly Improved Catalytic Performance by Surface Hydrophobi-
zation. Angew. Chem., Int. Ed. 2016, 55, 7379-7383.
ACKNOWLEDGMENT
We are grateful for financial support from NSFC (Grant Nos.
2
2
1671122, 21475078, 21301109, 21501111), 973 Program (Grant No.
013CB933800), Taishan scholar’s construction project.
(17) Yuan, B. Z.; Pan, Y. Y.; Li, Y. W.; Yin, B. L.; Jiang, H. F. A
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