Homochiral metal-organic frameworks with enantiopure proline units for the catalytic synthesis of -lactams

Article abstract of DOI:10.1021/ic501849g

Two enantiopure organic ligands integrating flexible proline units and rigid isophthalate units have been rationally designed and employed for the construction of four homochiral porous metal-organic frameworks (MOFs), respectively. One pair of these MOFs

Full text of DOI:10.1021/ic501849g

Article
pubs.acs.org/IC
Homochiral Metal−Organic Frameworks with Enantiopure Proline
Units for the Catalytic Synthesis of β‑Lactams
Zhong-Xuan Xu,†,‡ Yan-Xi Tan,† Hong-Ru Fu,† Juan Liu,† and Jian Zhang*,†
†State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, People’s Republic of China
‡Department of Chemistry, Zunyi Normal College, Zunyi, 563002, People’s Republic of China
S
* Supporting Information
ABSTRACT: Two enantiopure organic ligands integrating flexible proline
units and rigid isophthalate units have been rationally designed and
employed for the construction of four homochiral porous metal−organic
frameworks (MOFs), respectively. One pair of these MOFs is used as
heterogeneous catalysts to construct β-lactam derivatives by oxidative
coupling reactions.
ligand tends to connect paddle-wheel type units (e.g.,
Cu2(COO)4) into large cages, which is a very successful
building strategy on MOFs.11 Such a combination of
enantiopure proline and isophthalate unit may provide a new
and feasible approach to design and construct HMOFs with
large porosity and specific functions.
In this contribution, we report the successful synthesis of a
pair of enantiopure 5-(2-carboxypyrrolidine-1-carbonyl) iso-
phthalic acid (denoted as (S)-H3PIA and (R)-H3PIA) (Scheme
1) and two enantiomeric pairs of HMOFs, namely, [Cu3((S)-
PIA)2(1,4-dioxane)(H2O)2]·2(1,4-dioxane)·H2O (L-1),
[Cu3((R)-PIA)2(1,4-dioxane)(H2O)2]·2(1,4-dioxane)·H2O
(D-1), [Cu4((S)-PIA)2.5(H2O)3]·x(guest) (L-2) and [Cu4((R)-
PIA)2.5(H2O)3]·x(guest) (D-2).12,13 All HMOFs exhibit three-
dimensional (3D) porous structures containing paddle wheel
[Cu2(COO)4] units linked by the enantiopure ligands, and
large cages are presented in these homochiral structures. The
structural details of L-1 and L-2 are described below.
INTRODUCTION
■
Current interest in homochiral metal−organic frameworks
(HMOFs) is rapidly expanding, because of their potential
applications in enantioselective processes.1−4 The most
effective method to synthesize HMOFs is to select an
enantiopure ligand as the primary linker to impart homochir-
ality to the frameworks.1−3 Rational design of enantiopure
ligands is very important for the construction of HMOFs with
special functions. For example, the coordination fashion of
enantiopure ligands should not only determine the homochiral
environment, but also control the framework stability and the
generation of active sites.5 So far, determining how to design
the new enantiopure ligands and then construct functional
HMOFs is still a huge challenge for chemists.
Natural amino acids may be the inexpensive and ideal
enantiopure linkers for the formation of HMOFs.3 However,
the pore sizes of these resulting HMOFs are always limited by
the flexible nature of amino acids. Adding rigid auxiliary ligands
(e.g., 4,4′-bipyridine) to support the porous structures of such
HMOFs is an effective approach,3,6 and another typical way is
to modify the functional groups (−NH2 or −COOH) of amino
acids with suitable aromatic parts.7 Although some efforts focus
on the synthesis of HMOFs based on the derivatives of amino
acids,8 large porous structures integrating catalytic properties
remain rarely explored.
EXPERIMENTAL SECTION
■
General Procedures. All of the reagents and solvents used in
reactions were purchased from Energy-Chemical, Sigma−Aldrich,
Acros, TCI, or Alfa Aesar and used without purification, unless
1
otherwise indicated. H NMR (400 MHz) and 13C NMR (100 MHz)
spectra were recorded in CDCl3 (δ 7.26) or DMSO (δ 2.49) solutions
using a Bruker Model Avance 400 spectrometer. Elemental analyses
and mass spectra were performed by the analysis center of our
institute. Chemical shifts are reported as δ values in parts per million
(ppm), relative to tetramethylsilane (TMS) for all recorded NMR
Inspired by the outstanding MOF structure, HKUST-1,9 we
try to modify the 1,3,5-benzenetricarboxylate ligand in
HKUST-1 into an enantiopure linker via adding one proline
group (see Scheme 1). It is well-known that proline and its
derivatives are very promising catalysts for asymmetric organic
synthesis.10 In addition, the remaining isophthalate unit of the
Received: September 12, 2014
© XXXX American Chemical Society
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Scheme 1. Synthetic Routes to the Ligands (S)-H3PIA and (R)-H3PIA
spectra. FT-IR spectra were measured as KBr pellets on a Nicolet
Magna 750 FT-IR spectrometer in the range of 350−4000 cm−1. All
powder X-ray diffraction (PXRD) analyses were recorded on a Rigaku
Dmax 2500 diffractometer with Cu Kα radiation (λ = 1.54056 Å).
Thermal stability studies were carried out on a Netzsch Model STA-
449C thermoanalyzer with a heating rate of 10 °C/min under an
nitrogen atmosphere. Gas adsorption measurement was performed in
the Micromeritics ASAP 2020 system.
Synthesis of Trimethyl-1,3,5-benzenetricarboxylate (2). Benzene-
1,3,5-tricarboxylic acid (1) (21.0 g, 100 mmol) and concentrated
sulfuric acid (5 mL) were dissolved in dry methanol (400 mL), and
then the solution was refluxed for 24 h at 120 °C. After most of solvent
was removed by rotary evaporation, the resulting residue was slowly
added into saturated sodium bicarbonate (800 mL). The mixture was
stirred at room temperature for 1 h, then filtered under reduced
pressure to give the desired product, trimethyl-1,3,5-benzenetricarbox-
ylate (compound 2), as a white powder (22.7 g, 90%): 1H NMR (400
MHz, CDCl3), δ (ppm): 8.80 (3H, s), 3.96 (9H, s); 13C NMR (100
MHz, CDCl3), δ (ppm): 165.32, 134.51, 131.12, 52.62; LRSM (ESI):
Mass calcd for C12H13O6 [M+H]+, 253.2; found 253.3.
Synthesis of 3,5-Bis(methoxycarbonyl)benzoic Acid (3). Com-
pound 2 (10.1 g, 40 mmol) was dissolved in methanol (500 mL), then
aqueous sodium hydroxide (35 mL, 35 mmol) was slowly added over a
period of 24 h. After the mixture was stirred vigorously for 36 h, the
solvent was moved by rotary evaporation. Sodium bicarbonate (10.6 g,
100 mmol) and water (200 mL) were added in the resulting residue,
and the suspension was stirred for 2 h at 50 °C. The suspension was
filtered under reduced pressure to get unreacted starting material (1.2
g, 4.8 mmol). After acidizing to pH 1.0 with concentrated HCl, the
precipitated solid was separated by filtration to give pure 3,5-
bis(methoxycarbonyl)benzoic acid (compound 3) as a white powder
(6.7 g, 70%): 1H NMR (400 MHz, DMSO), δ (ppm): 13.68 (1H, brs),
8.59−8.51 (3H, sss), 3.91 (6H, s); 13C NMR (100 MHz, DMSO), δ
(ppm): 165.93, 165.05, 134.02, 133.45, 132.56, 131.13, 53.17; LRSM
(ESI): Mass calcd for C12H11O6 [M+H]+, 239.2; found 239.4.
Synthesis of Dimethyl-5-(methoxycarbonyl)prrolidine-1-
carbonyl)isophthalate (6). To a round-bottomed flask containing
compound 3 (7.14 g, 30 mmol) and freshly distilled SOCl2 (60 mL),
four drops of dimethylformamide (DMF) was added under a nitrogen
atmosphere. The reaction mixture was heated at 90 °C for 2 h, then
the excess SOCl2 was removed under in vacuo, giving dimethyl-5-
(chlorocarbonyl)isophthalate (4) as a white solid. To a solution of
methyl ester of L-proline or D-proline hydrochloride (5) (5.45 g, 33
mmol) in the dry CH2Cl2 (100 mL) and triethylamine (6.67 g, 66
mmol) under a nitrogen atmosphere and ice-water bath, compound 4
in dry CH2Cl2 (40 mL) was added dropwise over a period of 2 h. The
reaction mixture was washed with 1.0 M HCl (2 × 30 mL) and
saturated NaCl (2 × 30 mL), then dried over anhydrous sodium
sulfate. After filtration and removal of the solvent in vacuo, the residue
was purified by flash column chromatography (EtOAc:petroleum ether
= 1:3) to give dimethyl-5-(methoxycarbonyl)prrolidine-1-carbonyl)-
isophthalate (compound 6) as an amber-colored oil (8.90 g, 85%): 1H
NMR (400 MHz, CDCl3), δ (ppm): 8.8−8.25 (3H, m), 4.71−4.28
(1H, m), 3.96 (6H, s), 3.80−3.62 (3H, ss), 3.66−3.54 (2H, m), 2.37−
2.33 (1H, m), 2.12−1.31 (4H, m); 13C NMR (100 MHz, CDCl3), δ
(ppm): 172.38, 167.66, 165.49, 165.41, 136.31, 134.93, 132.46, 132.13,
131.80, 131.15, 130.94, 61.38, 59.32, 52.56, 52.39, 49.90, 46.85, 31.47,
29.34, 25.29, 22.65; LRSM (ESI): Mass calcd for C12H11O6 [M+H]+,
349.1; found 349.4.
Synthesis of 5-(2-carboxypyrrolidine-1-carbonyl)isophthalic Acid
(7). Compound 6 (3.49 g, 10 mmol), methanol (10 mL), water (40
mL), and solid sodium hydroxide (1.8 g, 45 mmol) were added to a
100-mL round-bottomed flask containing a stirring bar. The reaction
mixture was stirred and heated at 50 °C for 10 h, and then the result
solution was slowly acidified to pH 1−2 with concentrated aqueous
HCl in an ice bath. The precipitated solid was separated by filtration to
1
give pure compound 7 (2.6 g, 8.5 mmol, 85%) as a white solid: H
NMR (400 MHz, DMSO), δ (ppm): 13.33 (3H, brs), 8.64−8.13 (3H,
m), 4.46−4.30 (1H, m), 3.60−3.49 (2H, m), 2.51−2.28 (1H, m),
1.93−1.84 (3H, m); 13C NMR (100 MHz, DMSO), δ (ppm): 173.96,
173.54, 167.06, 166.51, 166.42, 138.48, 137.66, 132.19, 132.04, 131.91,
131.68, 131.59, 131.10, 61.25, 59.50, 50.07, 47.01, 31.51, 29.42, 25.47,
22.79; LRSM (ESI): Mass calcd for C12H11O6 [M+H]+, 308.1; found
308.2.
Synthesis of [Cu3((S)-PIA)2(1.4-dioxane)(H2O)2]·(1.4-doxane)2·H2O
(L-1). A mixture of (S)-H3PIA (31 mg, 0.1 mmol) and Cu(NO3)2·
2.5H2O (47 mg, 0.2 mmol) was dissolved in a solvent mixture of 1.4-
dioxane and H2O (4 mL/1 mL) with two drops of pyridine in a screw-
capped vial. The reaction mixture was heated at 100 °C for 3 days and
then cooled to room temperature. Green-blue polygonal crystals (40
mg, 70%, based on (S)-H3PIA) were obtained after filtration.
Elemental analysis calcd (%) for L-1: C 43.05, H 4.48, N 2.51;
found C 43.44, H 4.62, N 2.35. IR (solid KBr pellet, cm−1): 3412.4m,
2360.7w, 1621.0s, 1442m, 1398s, 1363.6s, 1307.3w, 717.7m, 487.2w.
Synthesis of [Cu3((R)-PIA)2(1.4-dioxane)(H2O)2]·(1.4-doxane)2·
H2O (D-1). The same procedure as that for L-1 was used, except
that (R)-H3PIA was used instead of (S)-PIA). Green-blue polygonal
crystals (42 mg, 73%, based on (R)-H3PIA) were obtained after
filtration. Elemental analysis calcd (%) for L-1: C 43.05, H 4.48, N
2.51; found C 43.61, H 4.67, N 2.45.
B
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Table 1. Crystal Data and Structure Refinement for L-1, D-1, L-2, and D-2
compound reference
chemical formula
L-1
D-1
L-2
D-2
C40H51Cu3N2O23
1118.45
monoclinic
21.7630(6)
11.0749(4)
19.5074(5)
90.00
C40H51Cu3N2O23
1118.45
monoclinic
21.7800(3)
10.8551(1)
19.4405(3)
90.00
C35H35Cu4N2.5O22.5
1104.8
C35H35Cu4N2.5O22.5
1104.8
formula mass
crystal system
tetragonal
12.4736(1)
12.4736(1)
70.2884(8)
90.00
tetragonal
12.5085(1)
12.5085(1)
70.2284(15)
90.00
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
92.760(3)
90.00
92.0880(10)
90.00
90.00
90.00
90.00
90.00
unit-cell volume (Å3)
temperature (K)
space group
4696.3(2)
293(2)
C2
4593.15(10)
293(2)
10936.22(18)
293(2)
P41212
10988.1(3)
293(2)
P43212
C2
Z
4
4
8
8
density, calcd. (g/cm3)
1.572
1.617
1.332
1.326
radiation type
Mo Kα
1.432
Cu Kα
Cu Kα
2.372
Cu Kα
2.361
absorption coefficient (μ/mm−1
F(000)
)
2.384
2276
2304
4376
4376
θ range data collection
No. of reflections measured
No. of independent reflections
Rint
2.74−25
9329
2.27−76.57
22470
3.60−74.61
22338
2.52−74.62
22739
6214
8781
10787
10680
0.0198
0.0292
0.0422
0.0398
final R1 values (I > 2σ(I))
final wR(F2) values (I > 2σ(I))
final R1 values (all data)
final wR(F2) values (all data)
goodness of fit on F2
Flack parameter
0.0363
0.0359
0.0848
0.0874
0.0960
0.1136
0.2432
0.2544
0.0414
0.0376
0.0894
0.0924
0.1000
0.1186
0.2519
0.2620
1.005
1.023
1.007
1.013
−0.018(13)
−0.029(19)
0.00(1)
0.00(1)
Synthesis of [Cu4((S)-PIA)2.5(H2O)3](guest) (L-2). A mixture of (S)-
H3PIA (31 mg, 0.1 mmol) and Cu(NO3)2·2.5H2O (47 mg, 0.2 mmol)
was dissolved in a solvent mixture of N,N-diethylformamide and H2O
(1 mL/3 mL) in a screw-capped vial. The reaction mixture was heated
at 100 °C for 3 days and then cooled to room temperature. Green-blue
octahedral crystals (32 mg, 80%, based on (S)-H3PIA) were obtained
after filtration. Elemental analysis calcd (%) for L-2: C 41.19, H 2.45,
N 3.43; found C 42.56, H 2.78, N 3.92. IR (solid KBr pellet, cm−1):
3125.3m, 2968.9w, 2362.2w, 2337.3w, 1629.7s, 1586.6m, 1452.7m,
1370.3s, 1301.5w, 713.9m, 480.1w.
ligands (see Figure 1a). It is notable that there are two types of
[Cu2(COO)4] units. One [Cu2(COO)4] unit has three
Synthesis of [Cu4((R)-PIA)2.5(H2O)3](guest) (D-2). The same
procedure as that used for L-2 was employed, except that (R)-
H3PIA was used instead of (S)-H3PIA. Green-blue octahedral crystals
(30 mg, 78%, based on (R)-H3PIA) were obtained after filtration.
Elemental analysis calcd (%) for D-2: C 41.19, H 2.45, N 3.43; found
C 42.66, H 2.88, N 3.62.
X-ray Crystallographic Analysis. The diffraction data for the
compounds were collected on an SuperNova diffractometer. The
structures were solved by direct methods and refined on F2 full-matrix
least-squares using the SHELXTL-97 program package. All non-
hydrogen atoms were refined anisotropically. Crystal data for the
compounds are summarized in Table 1.
Figure 1. Schematic illustrations of L-1: (a) coordination environ-
ment; (b) cage substructure; (c) 3D framework, showing the packing
of cages; and (d) topological net.
RESULTS AND DISCUSSION
■
Both L-1 and L-2 were synthesized solvothermally at the same
reaction temperature, but different solvents were used to
produce two distinct structures. The prominent structural
feature of L-1 is the packing of irregular cages building from
[Cu2(COO)4] units and (S)-PIA ligands. Single-crystal X-ray
diffraction (XRD) study confirmed that compound L-1
crystallized in the chiral space group C2 with a Flack parameter
of −0.018(13). In the structure of L-1, each (S)-PIA ligand is a
μ6-linker and connects three paddle wheel [Cu2(COO)4] units,
while each [Cu2(COO)4] unit is surrounded by four (S)-PIA
carboxylate groups from three isophthalate parts and one
carboxylate group from the proline part. Meanwhile, two apical
sites of this [Cu2(COO)4] unit are located by two water
molecules. Another [Cu2(COO)4] unit has C2 symmetry and
links to pairs of isophthalate parts, proline parts and 1,4-dioxane
molecules. Interestingly, the (S)-PIA ligands alternately connect
the [Cu2(COO)4] units into irregular cage-type substructures
C
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(see Figure 1b), and the cages are packing into a 3D porous
framework (see Figure 1c). From the viewpoint of structural
topology, the (S)-PIA ligands and the [Cu2(COO)4] units can
be regarded as the 3- and 4-connected nodes, respectively.
Thus, the entire framework of L-1 can be topologically
(BET) surface areas are 352.6 and 250.7 m2/g for L-1, and
249.9 and 165.3 m2/g for L-2, respectively.
Considering the presence of [Cu2(CO2)4] units with Lewis
acid sites and chiral environment in these structures, further
catalytic properties of these compounds were investigated.
Here, we are interested in the catalytic synthesis of β-lactam
through an oxidative carbon−carbon bond formation of
phenolic amide derivatives. The β-lactam is not only observed
in biologically active natural products, but also incorporated
into numerous pharmaceutical ingredients, such as penicillins
and carbapenems.13 It is very important to create some new
type of compounds containing a β-lactam unit for bacterial
resistance. Although some homogeneous catalytic methods
have been successfully applied to form β-lactam building
blocks,14 heterogeneous catalysis to fabricate this aim is still
rarely explored up to date.
represented as a (3,4)-connected net with point (Schlafli)
̈
symbol of (4.82)2(4.85)2(83)2(86) (see Figure 1d).
The outstanding structural feature of L-2 compound is the
presence of octahedral cages in the framework, which is unlike
the irregular cages in L-1. Similar paddle wheel [Cu2(COO)4]
units are also the main inorganic building blocks in the
structure of L-2 (Figure 2a), and the (S)-PIA ligands link these
Since the carbon−carbon bond formation during the
synthesis of β-lactam is largely dependent on proper oxidant
and solvent, several oxidants (e.g., tert-butyl hydroperoxide
(TBHP), H2O2, and iodobenzene diacetate (IBD)) and
solvents (Table 2) are used to test the catalytic ability of two
a
Table 2. Optimization of the Catalyst
b
c
entry
catalyst
L-1
oxidant
solvent
temperature
yield (%)
d
1
IBD
IBD
IBD
IBD
IBD
IBD
IBD
IBD
IBD
IBD
IBD
ethanol
CH3OH
acetone
dioxane
CH2Cl2
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
RT
56
Figure 2. Schematic illustrations of L-2: (a) coordination environ-
ment; (b) cage substructure; (c) 3D framework, showing the packing
of cages; and (d) topological net.
2
L-1
L-1
L-1
L-1
L-1
L-1
RT
trace
3
RT
0
0
0
4
RT
5
RT
d
6
0 °C
50 °C
RT
58
[Cu2(COO)4] units into a 3D porous framework with
octahedral cages (see Figures 2b and 2c). Each octahedral
cage with an inner diameter of 7 Å consists of 12 (S)-PIA ligand
fragments and 6 [Cu2(COO)4] units (see Figure 2b). It is
worthy of noting that one independent (S)-PIA ligand in the
asymmetric unit of L-2 is a μ7-linker, with its acyl O atom
bonding to one [Cu2(COO)4] unit, so it acts as a 4-connected
node in the topological representation and the related
[Cu2(COO)4] unit becomes a 5-connected node. Another
independent (S)-PIA ligand and the secondary [Cu2(COO)4]
unit are the 3- and 4-connected nodes, respectively. Thus, the
entire framework of L-2 is topologically represented as a
7
complexity
0
8
d
9
D-1
RT
55
10
11
L-2
RT
trace
trace
HKUST-1
RT
a
Reactions were carried out at room temperature with amide 8a (0.5
mmol), catalyst (0.025 mmol), and oxidant (0.6 mmol) in 10 mL of
solvent for 3 h, except for entry 6, which was carried out at 0 °C for 10
h. RT = room temperature. On the basis of TLC detection. Yields
b
c
d
represent isolated yields of 9a.
tetranodal (3,4,5)-connected net with point (Schlafli) symbol
pairs of homochiral compounds. Our initial studies commenced
with the reaction of compound 8a. The results indicated that
the reaction did not take place at all by using H2O2 or TBHP as
the oxidizing agent in various solvents (see Table S3 in the
Supporting Information). Fortunately, the IBD is effective in
regard to promoting the reaction in ethanol at room
temperature (Table 2, entry 1), giving product 9a in moderate
yield, but it still failed in other solvents (acetone, methanol,
dioxane, and dichloromethane (DCM); see Table 2, entries 2−
5). The efforts to enhance yields were also proved fruitless,
when the reactions were conducted at lower or higher
temperature (see Table 2, entries 6 and 7).
̈
of (42.63.8)2(42.65.83)2(62.8)2(64.82)3 (Figure 2d).
Both L-1 and L-2 are porous frameworks and exhibit solvent-
accessible volumes of ∼43% and ∼38%, as calculated by
PLATON, respectively.12 They are insoluble and stable in water
and common organic solvents, such as methanol, ethanol,
CH2Cl2, etc. (see Figures S4−S7 in the Supporting
Information). The thermogravimetic analysis (TGA) indicated
that both compounds also have high thermal stability (∼240
°C) (see Figures S10 and S11 in the Supporting Information).
The permanent porosity of L-1 (or L-2) was further
demonstrated by the N2 gas sorption at 77 K. The desolvated
samples of L-1 and L-2 show type-I sorption isotherms,
respectively (see Figures S8 and S9 in the Supporting
Information). The Langmuir and Brunauer−Emmett−Teller
To demonstrate the heterogeneous nature of this catalytic
system, compound L-1 was stirred in ethanol for 3 h and
removed by filtration, and then the reaction substrate (8a) and
D
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oxidizing agent IBD were subsequently added into the filtrate
and stirred for another 3 h at room temperature. As a result,
only a trace of product 9a was detected via thin-layer
chromatography (TLC) (see Table 2, entry 8).
With alkoxy or alkyl, the phenol amides only afforded the
corresponding products in moderate yields under the standard
reaction conditions (see Table 3, compounds 8b−8e). When
the R2 group is phenyl or 4-methyl phenyl, the reaction
performance is very efficient and all the yields are over 75%
(see Table 3, 8f−8l). However, phenol amides possessing a
strong electron-withdrawing nitryl group on the phenyl ring
afforded the products in trace amounts (see Table 3,
compounds 7m−7n). The screening of the R2 groups revealed
that various substituent groups, such as benzyl, p-methyl benzyl,
p-nitryl benzyl, 2-methylene naphthalene, and so forth, were all
suitable for the C−C bond coupling reaction.
Furthermore, crystals of compounds 8g, 8i, 8j, and 8l were
successfully obtained after careful recrystallization, and their β-
lactam structures were characterized by single-crystal X-ray
diffraction (see Figure S3 and Table S2 in the Supporting
Information). Because the large and small reagents showed
similar reactivity, all these catalytic processes should take place
on the surface of the catalyst L-1.15
After being recovered using filtration and washed with
ethanol, compound L-1 could be subsequently used in the
successive runs (see Figure S12 in the Supporting Information).
The catalytic activity of L-1 experienced only a slight
degradation after four cycles; meanwhile, it always retained its
crystallinity, as determined by PXRD (see Figure S13 in the
Supporting Information). Encouraged by these results, both L-
2 and HKUST-1 containing [Cu2(CO2)4] units were further
screened under the same reaction condition, respectively.
However, both of them cannot effectively catalyze the reaction
(see Table 2, entries 10 and 11). These results might be
correlated with the possible generation of Lewis acid sites from
the [Cu2(CO2)4] units in the structures. For L-1 (or D-1), the
weak bond between the [Cu2(CO2)4] unit and 1,4-dioxane
makes it easy to generate exposed Cu(II) sites for catalysis.
On the basis of the above results, the generality of this
reaction was tested by a wide range of substituted phenol
amides under the optimized condition. For catalyst L-1, the
reaction has broad tolerance toward a variety of functional
groups (see Table 3, compounds 8b−8l). We found that the R2
group of phenol amides had significant effects on the reaction.
CONCLUSION
■
In summary, two enantiopure organic ligands ((S)-H3PIA and
(R)-H3PIA), integrating flexible proline units and rigid
isophthalate units, have been rationally designed and employed
to the construction of four homochiral porous MOFs (L-1 and
D-1, and L-2 and D-2), respectively. Among them, L-1 and D-1
were used as heterogeneous catalysts for C−C oxidative
coupling reaction. To the best of our knowledge, it is the
first C−C oxidative forming procedure in the presence of
porous MOFs. Further studies on the mechanistic aspects and
applications of these compounds in organic synthesis are
currently underway in our laboratory.
a
Table 3. Substrate Scope
ASSOCIATED CONTENT
* Supporting Information
■
S
1H NMR, 13C NMR spectra of all key intermediates, analytical
data, and X-ray crystallographic files (CCDC-1011428−
CCDC-1011431 (L-1 and D-1, L-2 and D-2) and CCDC-
1011453−CCDC-1011456 (compounds 8g, 8i, 8j, and 8l)).
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* E-mail: zhj@fjirsm.ac.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the 973 Program (Nos.
2012CB821705 and 2011CB932504) and NSFC (Nos.
21221001 and 21233009).
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