A new copper-based metal-organic framework as a promising heterogeneous catalyst for chemo- and regio-selective enamination of β-ketoesters

Article abstract of DOI:10.1039/c3cc45310c

Assembly of 5-nitro-1,2,3-benzenetricarboxylic acid (H₃nbta) with Cu^II in the presence of 1,3-bis(1,2,4-triazol-1-yl)propane (1,3-btp) leads to a new metal-organic framework, [Cu(Hnbta)(1,3-btp)] ·2H₂O (A₁), which is shown to be an efficient and recyclable heterogeneous catalyst for enamination of β-ketoesters with excellent product yields and selectivity.

Full text of DOI:10.1039/c3cc45310c

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A new copper-based metal–organic framework as a
promising heterogeneous catalyst for chemo- and
regio-selective enamination of b-ketoesters†
Cite this: DOI: 10.1039/c3cc45310c
Received 14th July 2013,
Ying Zhao,ab Dong-Sheng Deng,b Lu-Fang Ma,*b Bao-Ming Jib and Li-Ya Wang*bc
Accepted 31st August 2013
DOI: 10.1039/c3cc45310c
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Assembly of 5-nitro-1,2,3-benzenetricarboxylic acid (H3nbta) with synthetic method. Consequently, various catalysts including InBr3,
CuII in the presence of 1,3-bis(1,2,4-triazol-1-yl)propane (1,3-btp) Sc(OTf)3, Zn(OAc)2ꢀ6H2O/MgSO4, Si(OEt)4, NaAuCl4, ZrOCl2ꢀ8H2O,
leads to a new metal–organic framework, [Cu(Hnbta)(1,3-btp)]ꢀ Ca(CF3COO)2, and so on9 have been employed to promote this
2H2O (A1), which is shown to be an efficient and recyclable reaction. However, the reaction occurs often under homogeneous
heterogeneous catalyst for enamination of b-ketoesters with excellent conditions, which would lead to several problems, such as difficulty
product yields and selectivity.
in separation and recovery and disposal of spent catalysts. There-
fore, a process featuring an efficient and easy-to-handle catalytic
There is much interest in the design of hybrid materials for system in combination with a reusable and eco-friendly catalyst
selective and multi-step catalytic processes. Currently, the study would be highly desirable. Until now, no MOFs have been
on MOFs as catalysts is one of the hot topics.1 Compared with reported as effective heterogeneous catalysts for condensation
conventional homogeneous catalysts, MOFs as heterogeneous of b-dicarbonyl compounds with amines. Here, we report a new
catalysts have many advantages, including separation and recovery, 3D complex, [Cu(Hnbta)(1,3-btp)]ꢀ2H2O (A1), which displays
disposal of spent catalysts, and so on.2 Generally, to design and remarkable capability for heterogeneous selective enamination
synthesize catalytically active MOFs, coordinatively unsaturated of b-ketoesters with excellent product yields and selectivity.
metal centers are often used because they may exhibit Lewis
Reaction of 5-nitro-1,2,3-benzenetricarboxylic acid (H3nbta,
acidity and associated catalytic functionality, well-organized and 0.1 mmol) and 1,3-bis(1,2,4-triazol-1-yl)propane (1,3-btp, 0.1 mmol)
well-maintained active sites and thus are expected to be effective with Cu(II) acetate at 130 1C for 3 days under hydrothermal condi-
heterogeneous catalysts.3 On the other hand, Cu(II) ions exhibit tions generates blue crystalline product [Cu(Hnbta)(1,3-btp)]ꢀ2H2O
dynamic Jahn–Teller distortion4 and can provide open Lewis (A1).‡ Single-crystal X-ray analysis reveals that compound A1 crystal-
acid sites which are effective for catalysis of various organic lizes in the orthorhombic form with a chiral space group P212121.§
reactions such as azide–alkyne cycloaddition, oxidative coupling The Flack parameter of 0.007(12) demonstrates the homochirality of
reaction, ring opening of epoxides, Henry reaction, Diels–Alder the single crystal. The randomly selected single crystals from the
reaction, etc.5 As is well known, b-enaminoesters are key skeletons of same batch show that spontaneous resolution occurs here and the
some biologically natural products such as therapeutic agents6 and bulk sample is racemic.
synthons of different important antibacterial, anti-inflammatory,
The asymmetric unit of A1 contains one crystallographically
and antitumour agents.7 Therefore, many efficient strategies have independent CuII ion, one Hnbta, one 1,3-btp and two free
been established to obtain these compounds.8 Among these, the water molecules, as shown in Fig. S1, ESI.† The Cu1 center is
condensation of b-dicarbonyl compounds with amines was the best coordinated by two nitrogen atoms of two 1,3-btp [Cu1–N2 =
2.001(2) and Cu1–N7B = 2.003(2) Å] and four oxygen atoms of
two Hnbta [Cu1–O1 = 1.966(18), Cu1–O2 = 2.667(2), Cu1–O5A =
2.716(2) and Cu1–O6A = 1.953(18) Å], showing a distorted [4+2]
a College of Chemistry and Molecular Engineering, Zhengzhou University,
Zhengzhou 450001, P. R. China
b College of Chemistry and Chemical Engineering, Luoyang Normal University,
octahedral geometry. The Cu1–O2 and Cu1–O5A bond lengths are
much longer, suggesting negligible interaction. The 2-carboxylic
group of Hnbta remains undeprotonated and is free of coordina-
tion. Each Hnbta takes a m2-bridging mode to coordinate two Cu(II)
ions via its two carboxylic groups in bis-monodentate modes. Thus
the Cu(II) ions are connected by Hnbta to produce a 1D chain along
the b axis. These polymeric chains are arranged at parallel levels
and are further cross-linked to one another by bridging 1,3-btp into
Luoyang 471022, P. R. China. E-mail: mazhuxp@126.com
c College of Chemistry and Pharmaceutical Engineering, Nanyang Normal
University, Nanyang 473061, P. R. China. E-mail: wlya@lynu.edu.cn
† Electronic supplementary information (ESI) available: An X-ray crystallographic
file in the CIF format, NMR data, and additional structural figures, some of the
results of preparation of different b-enaminoesters in the presence of various
catalysts, PXRD and TGA curves. CCDC 926307. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c3cc45310c
c
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Table 1 Synthesis of different b-enaminoesters using A1 as catalyst under
solvent-free conditions
Entry Amine
1
Product
Time (h) Yielda (%)
0.5
1
97
93
Fig. 1 (a) Perspective view of the 3D network showing its two types of
rectangular channels. (b) Schematic view of the 4-connected framework with a
diamond topology, blue bonds represent the 1D chains, while red spheres
represent the Cu atoms.
2
3
4
2
2
92
92
a 3D network with a 4-connected dia topology (see Fig. 1a and b),
featuring two types of rectangular channels (see Fig. S2, ESI†).
The nitro groups of Hnbta protrude inside the voids of one type
of channel and another type of channel encapsulates free water
molecules as guest molecules. Two crystallographically unique
free water molecules are observed in the unit of A1, which are
stabilized by maintaining hydrogen bonds to the carboxylic
oxygen atoms of Hnbta in the hydrophilic channel. The effective
free volume of A1 was calculated by PLATON analysis to be
11.9% of the crystal volume (251.6 out of the 2105.3 Å3 unit cell
volume).
5
2
91
6
7
8
1
1
1
94
95
94
Thermogravimetric analysis (TGA, see Fig. S3, ESI†) of A1
indicates that the first weight loss of 7.1% (calculated: 6.8%)
corresponds to the loss of two free water molecules. From ca.
200 1C, the expulsion of organic components occurs. The pure
compound A1 was confirmed by powder X-ray diffraction
(PXRD) measurement in which diffraction peaks of experimental
data are in agreement with the simulated data from single-
crystal X-ray data (see Fig. S4, ESI†). After a sample of A1 was
treated under vacuum at 120 1C for 4 h, a PXRD analysis of the
9
1
96
0
resultant crystalline solid A1 showed a sharp diffraction pattern
similar to that of the as-synthesized sample (see Fig. S4, ESI†).
This indicates that the framework is maintained after removal of
the free water molecules. Furthermore, the desorption process
was also monitored by TGA. The TGA curve of A1 shows no
weight loss up to 200 1C.
10
2
2
92
95
0
11
Recent studies have shown that coordinatively unsaturated
copper sites can exhibit interesting catalytic activities.3 To
evaluate the catalytic activity of A1, the reaction of ethyl
a
Isolated yield. Selectivity% for each reaction: 100%.
acetoacetate with benzyl amine was performed under solvent b-ketoesters. And reaction with the ester part was not observed
free conditions at room temperature for 0.5 h, which generated in all cases to give the corresponding amide.
the corresponding product in excellent yield (Table 1, entry 1).
In order to confirm the role of catalyst, a blank reaction was
In order to further explore the versatility of A1 as catalyst and performed under similar reaction conditions with benzyl amine
the effect of substrates on the condensation reaction, different and ethyl acetoacetate. At room temperature, no b-enaminoester
types of amines with different molecular dimensions were used was observed even after stirring for a long reaction time (24 h).
as substrates under solvent-free conditions. The results are However, at 80 1C, the reaction can proceed to afford a mixture with
summarized in Table 1. All condensation reactions between 76% conversion with low selectivity, which contains imine (62.6%),
ethyl acetoacetate and various amines including aromatic amide (8.2%) and only 29.2% selectivity of b-enaminoester (see
(entries 1–10) and aliphatic (entry 11) were completed within Scheme S1, ESI†). The result clearly shows the role of A1 as catalyst
0.5–2 h at room temperature to afford b-enaminoesters in in the activation of the carbonyl group and the orientation of the
excellent yields (up to 97%). Further, it is noted that all of reaction toward the desired enaminoester.
these reactions display excellent chemoselectivity. The nucleo-
Secondly, to prove the heterogeneous mechanism of the
philic reactant amine only reacts with the ketone part of the condensation catalyzed by A1, the catalyst was filtered off after
c
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catalytic studies: In a typical experiment 1 mmol ethyl acetoacetate,
1 mmol amine and 0.05 mmol A1 were stirred at room temperature for
appropriate time. At the end of the reaction, 10 mL of distilled water was
added to the residue. After the catalyst was filtered off, the filtrate was
extracted with ethyl acetate (3 ꢂ 10 mL). The organic layer was dried over
MgSO4. The solvent was removed under reduced pressure; pure
b-enaminoester was obtained by column chromatography over the silica
gel using hexane/ethyl acetate as an eluent. All isolated pure products
were fully characterized by 1H and 13C NMR or otherwise compared with
the known compounds. The recovered catalyst was washed with ethyl
acetate, dried, and reused without further purification or regeneration.
Moreover, the recovered catalysts were characterized by powder X-ray
diffraction and showed identical results to those of the fresh samples.
15 min reaction time and stirring of the filtrate continued under the
same conditions. The result confirms the assumption of a hetero-
geneous mechanism since neither additional ethyl acetoacetate is
consumed nor the product is formed after filtration. A plausible
mechanism of enamination of b-ketoester reactions is that
unsaturated Cu(II) acts as a Lewis acid active site.5 Finally, for
a more comprehensive study of the catalytic activity of A1 in the
condensation of b-ketoesters with primary amines, a recycling
test with three consecutive runs was performed (see the experi-
mental section). As mentioned before, a product yield of 97% is
achieved in the first run after 0.5 h. In the second and the third
run the product yields, determined after the same reaction time
(0.5 h), decrease to 95% and 93%, respectively. The PXRD
patterns of the recovered A1 suggest that its structure is well
maintained after several cycles of reactions (see Fig. S4, ESI†).
In summary, we have reported a pertinent method for
preparation of b-enaminoesters by condensation of b-ketoesters with
primary amines under solvent-free conditions at room temperature
with A1 as a novel heterogeneous catalyst. Compared with the
reported homogeneous catalysts for the condensation reaction, A1
has some fascinating features because of its low environmental
impact, recovery and reusability, and high chemo-selectivity. To
our knowledge, this is the first report on selective enamination of
b-ketoesters performed using Cu(II) with unsaturated coordination
spheres as active sites.
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This work was supported by the National Natural Science
Foundation of China (21073082, 21271098 and 21272109), and
sponsored by the Program for New Century Excellent Talents in
University (NCET-11-0947), Innovation Scientists and Technicians
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Notes and references
‡ Synthesis of A1: [Cu(Hnbta)(1,3-btp)]ꢀ2H2O. A mixture of H3nbta
(0.1 mmol, 25.5 mg), 1,3-btp (0.1 mmol, 17.8 mg), Cu(OAc)2ꢀH2O
(0.1 mmol, 19.8 mg) and H2O 12 mL was placed in a Teflon-lined
stainless steel vessel, heated to 130 1C for 3 days, and then cooled to
room temperature for over 24 h. Blue block crystals of A1 were obtained.
Yield: 18.5 mg, 51% (based on Cu). Elemental analysis (%): calcd for
C16H17CuN7O10 C 36.20, H 3.23, N 18.47; found C 36.29, H 3.15, N 18.40.
IR (cmꢁ1): 3118 m, 1723 s, 1574 s, 1537 s, 1432 m, 1338 s, 1132 m,
995 m, 734 s, 665 m.
§ Crystal data for A1: C16H17CuN7O10, M = 530.91, orthorhombic, space
group P212121, a = 5.7467(5) Å, b = 17.9631(16) Å, c = 20.3944(19) Å, V =
2105.3(3) Å3, Z = 4, S = 1.057, Flack parameter = 0.007(12), Dc
=
1.675 g cmꢁ3, F(000) = 1084, m = 1.109 mmꢁ1, 3809 reflections were
used in the refinement. Rint = 0.0277, R1 = 0.0277, and wR2 = 0.0696 for
[I > 2s (I)], and R1 = 0.0326, wR2 = 0.0719 for all data, Drmax = 0.241 e Åꢁ3
,
Drmin = ꢁ0.246 e Åꢁ3. Single crystal X-ray diffraction analysis of A1 was
carried out on a Bruker APEX II CCD diffractometer equipped with
graphite monochromated Mo-Ka radiation (l = 0.71073 Å) using the
f/o scan technique at room temperature. A total of 3809 reflections were
used in the refinement for A1. The structure was solved by direct methods
with SHELXS-97. The hydrogen atoms were included in the final refine-
ment by use of geometrical restraints and assigned with isotropic
displacement factors. The hydrogen atoms of water molecules were
located by different maps and the bond length of O–H was restrained
to 0.85 Å and the bond angle was 1071 and then refined by a riding mode
with Ueq = 1.5Ueq(O). A full-matrix least-squares refinement on F2 was
carried out for the non-H atoms using SHELXL-97. A typical procedure for
c
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