Ag NPs decorated on a COF in the presence of DBU as an efficient catalytic system for the synthesis of tetramic acids via CO2 fixation into propargylic amines at atmospheric pressure

Article abstract of DOI:10.1039/c9dt00017h

CO₂ fixation reactions by inserting it in reactive organic compounds are very challenging for the utilization of this abundant and harmful gas present in air and thus to mitigate this greenhouse gas responsible for global warming. This can be achieved by appropriate design of functionalized porous nanocatalysts having high surface areas and porosity and good CO₂ uptake capacity. Herein, we first report the decoration of silver nanoparticles (NPs) over the surface of a covalent organic framework (COF) material TpPa-1 synthesized through the polycondensation of 2,4,6-triformylphloroglucinol (TFP) and p-phenylenediamine. The resulting material Ag@TpPa-1 was thoroughly characterized by N₂ adsorption/desorption, powder X-ray diffraction (PXRD), FE-SEM, TEM, UV-Vis, FT IR and thermogravimetric techniques. This Ag NP decorated porous COF in the presence of DBU exhibited excellent catalytic activity for the synthesis of tetramic acids from a variety of propargylic amine derivatives at 60 °C under atmospheric pressure of carbon dioxide via formation of oxazolidinones, where CO₂ acts as a C1 reagent. The Ag@TpPa-1 catalyst exhibited excellent recycling efficiency for the synthesis of tetramic acid with no leaching of Ag from the catalyst surface.

Full text of DOI:10.1039/c9dt00017h

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Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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a Zr-metal–organic
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Ag NPs decorated COF in the presence of DBU as an efficient
catalytic system for the synthesis of tetramic acids via CO2 fixation
into propargylic amines at atmospheric pressure
Received 00th January 20xx,
Accepted 00th January 20xx
Swarbhanu Ghosha, Rostam Ali Mollab, Utpal Kayalc, Asim Bhaumikc* and Sk. Manirul Islama*
DOI: 10.1039/x0xx00000x
CO2 fixation reactions by inserting it in reactive organic compounds are very challenging for the utilization of this abundant
and harmful gaspresent in air and thus to mitigate this greenhouse gas responsible for global warming. This can be achieved
by proper designing of functionalized porous nanocatalysts having high surface area, porosity and good CO2 uptake capacity.
Herein, we first report the decoration of silver nanoparticles (NPs) over the surface of a covalent organic framework (COF)
material TpPa-1 synthesized through the polycondensation of 2,4,6-triformylphloroglucinol (TFP) and p-phenylenediamine.
Resulting material Ag@TpPa-1 has been thoroughly characterized by N2 adsorption/desorption, powder X-ray diffraction
(PXRD), FE-SEM and TEM, UV-Vis, FT IR and thermogravimetric tools. This Ag NPs decorated porous COF in the presence of
DBU exhibited excellent catalytic activity for the synthesis of tetramic acids from variety of propargylic amine derivatives at
60oC under atmospheric pressure of carbon dioxide via formation of oxazolidinones, where CO2 act as a C1 reagent. The
Ag@TpPa-1 catalyst exhibited excellent recycling efficiency for the synthesis of tetramic acid together with no leaching of
Ag from the catalyst surface.
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addition of intramolecular nature,11 and enantioselective cyclization
by using SmI2 mediated coupling12 are employed for the synthesis of
tetramic acids. In this regard, fixation of the carbon dioxide
Introduction
Tetramic acid is an important building block in several natural
products, which displayed significant activity in various biological
systems.1 Decalin containing tetramic acids with cytotoxic and
antimicrobial activity were separated from the fungus Coniochaeta
cephalothecoides.2 The isolation of spiroscytalin (Scheme 1) was
performed from solid-phase cultures of the filamentous fungus
Scytalidium cuboideum, which is bioactive in nature.3 For example,
discodermide hinders the population of Candida fungi4 and
reutericyclin hinders the growth of Gram-positive bacteria.5
Moreover, spirotetramat has been utilized as a valuable chemical for
the agricultural systems.6 As a result, a large number of attempts has
been made for the preparation of tetramic acid derivatives
synthetically, viz. through intramolecular rearrangement via base-
catalyzed condensation (Dieckmann condensation)7 or amidation
reactions.8 However, a wide range of derivatives of starting material
are necessary and often the reaction need high temperature in
alkaline media. Accordingly, new and eco-friendly routes for the
synthesis of tetramic acids are very demanding. Recently, cyclization
of oxidative nature with hypervalent iodine agents,9 cyclization of
reductive nature with samarium(II) iodide,10 aza-anti-Michael
(greenhouse gas, responsible for global warming) by inserting it in
reactive organic compounds13 has huge potentialto utilize this
abundant and harmful CO2present in air. Hence, Ag-catalysed
activation of alkyne followed by intramolecular cyclizationcan offer a
very useful strategy in the CO2 fixation reactions.14 On the other
hand, the design and synthesis of functionalized porous
nanocatalysts having high surface area, porosity and good CO2
uptake capacity is very promising field of research.A vast array of
porous organic materials are reported via polymerization of building
blocks bearing multiple reactive groups viz. amine, azine, enamine,
boronate esters, β-ketoenamine, hydrazine and aldehydes.15 A large
number of covalent triazine frameworks (CTFs),16 porous organic
polymers (POPs),17 and conjugated microporous polymers (CMPs)18
has been developed over the years to explore these
polycondensation reactions. However, due to the periodicity of the
porous network, low framework density, excellent thermal stability,
nanoscale porosity and high specific surface area covalent organic
frameworks (COFs)19 synthesized through modular association of a
wide range of symmetrical and rigid building blocks via covalent
bonds can offer a huge scopein supporting a reactive metallic NPs at
its surface.
a. Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B.,
India. E-mail: manir65@rediffmail.com
Thus, we have synthesized the COF material TpPa-1having 2D-
hexagonal nanostructure via one-pot polycondensation of the 2,4,6-
triformylphloroglucinol (TFP) and p-phenylenediamine. This
chemically stable TpPa-1 has been selected as a support material for
the proof-of-concept of the utilization of COF as a heterogeneous
catalyst for CO2 fixation reaction. Ag NPsare decorated at the surface
b. Department of Chemistry, The University of Burdwan, Burdwan, West Bengal,
India.
c. School of Materials Science, Indian Association for the Cultivation of Science,
Kolkata - 700032, India. E-mail: msab@iacs.res.in
†
Electronic Supplementary Information (ESI) available: Experimental,
characterization of the catalyst and NMR of the products.See
DOI: 10.1039/x0xx00000x
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of TpPa-1 to obtain Ag@TpPa-1 (Scheme 1) catalyst. This Ag- collected after centrifugation using ethanol as the washing solvent
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DOI: 10.1039/C9DT00017H
nanocatalyst is extremely efficient for CO2insertion into the reactive (Scheme 2).
propargylic amines for the selective synthesis of tetramic acid
derivatives. Further, the Ag@TpPa-1 catalyst is recyclable and
retained its catalytic efficiency for several reaction cycles suggesting
its future potential inCO2 fixation reactions.
Scheme 1. Bioactive compounds containing tetramic acid moieties.
Experimental
Synthesis of Ag@TpPa-1
Initially, TFP was synthesized by following
a reported
literature.20 In brief, the required amounts of trifluoroacetic
acid and phloroglucinol were poured into 250 mL round
bottomed flask containing hexamethylenetetramine in DMF.
The resulting reaction mixture was placed in oil bath at 100°C
for 2 h under continuous stirring. The whole reaction was
carried out in nitrogen atmosphere. Hydrochloric acid was
added into the resulting solution very carefully under
continuous magnetic stirring at fixed temperature for next 1 h.
Then the reaction mixture was filtered through the cold celite
bed. The extraction of filtrate part was performed with DCM for
five times, and anhydrous Na2SO4 was utilized to absorb water
from the mixture. After evaporation of the solvent with the help
of rotavapor, the red color product was observed. Monomers
are characterized by using 1H and 13C NMR and FTIR
spectroscopic tools. For the synthesis of TpPa-1, the mixture of
anhydrous dimethylformamide (DMF, 15 mL) andTFP (1 mmol)
was taken in a two neck round bottomed flask. After that p-
phenylenediamine in dry DMF was poured into the resulting
reaction mixture using a micropipette. The above mixture was
refluxed in inert atmosphere at 140 oC for about 24 h to obtain
the red solid product. Then this product was filtered and
washed with DMF and ethanol for multiple times to eliminate
the unreacted compounds. Then soxhlet extraction was carried
out by using ethanol as solvent to remove the organic and
inorganic impurities and solvents from the COF surface. Then
the material was dried in 60 oC for 4 h and it is labelled as TpPa-1
(Scheme 1).
Scheme 2. Synthetic route for the production of Ag@TpPa-1 nanomaterial.
Results and discussion
3.1. Characterizations
The schematic outline for the synthesis of silver nanoparticles
decorated COF (Ag@TpPa-1)21 is shown in Scheme 1. The Ag@TpPa-
1 material is insoluble in all common solvents. The PXRD, UHR-TEM,
FE-SEM, FT IR, UV-Vis spectroscopic and thermogravimetric analysis
have been performed in solid state. Ag loading in Ag@TpPa-1 has
been estimated by using atomic absorption spectroscopy and the
loading of Ag in Ag@TpPa-1 was 2.12 wt%. Further, the carbon,
hydrogen and nitrogen contents in Ag@TpPa-1 have been measured
through CHN analysis and it showed: C= 67.25, H= 5.5 and N= 7.76 %,
which totally agrees with the framework of Ag@TpPa-1.
The PXRD pattern of Ag@TpPa-1 exhibited a first peak of high
intensity at ∼4.8° (2θ), which could be attributed to the most intense
reflection from (100) planes (Figure 1).22 Further, a very weak
intensity peak appeared at 2θ value ( ∼ 27.6°) and this could be
attributed to the π−π stacking coming from (001) Miller plane.The
powder X-ray diffraction patterns of AgNPs of Ag@TpPa-1 could be
assigned to a FCC (face-centered cubic) (Figure 1). Therefore,
Ag@TpPa-1 exhibited three additional diffraction peaks at 38.1°,
44.4° and 64.4° corresponding to the (111), (202), and (220) sets of
Miller planes, and it may be indexed as the peak for FCC structure
(face-centered cubic) of metallic silver.23 The powder XRD data thus
clearly demonstrated the loading of metallic silver nanoparticles
over the surface ofAg@TpPa-1, which are crystalline in nature.
Impregnation of the silver nanoparticles were performed by
pouring silver nitrate (100 mg) into water (10 ml) in the presence of
tris(hydroxymethyl)aminomethane (TRIS, 0.5 mmol). It was allowed
to stir for five min after which 0.25 ml solution of NaBH4 (0.08%) was
poured dropwise. After that, the resulting mixture was subjected to
stir for 10 min. The resulting nanocolloid was allowed to keep at 4 °C.
TpPa-1 (600 mg) was dispersed in 10 ml AgNPs solution stabilized in
TRIS buffer. It was allowed to keep at room temperature for 1 h
under continuous stirring. Orange coloured product Ag@TpPa-1 was
In order to investigate the permanent porosity in the Ag-loaded
COF material N2 adsorption measurements were performed at 77 K.
Figure 2 exhibits a sharp N2 uptake at low P/P024 followed by steady
increase in N2 uptake at high P/P0.25 Nitrogen adsorption data further
revealed that the Ag@TpPa-1 had highly porous structure with BET
surface area 205 m2g-1 (Figure 2). The distribution of pore size of
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Ag@TpPa-1 was derived by employingthe nonlocal density large micropores of ca.1.5 nm. The TEM images of Ag@TpPa-1 at
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DOI: 10.1039/C9DT00017H
functional theory (NLDFT) from adsorption branch of the isotherm. different magnifications are shown in Figure 4. As seen from Figure
As seen from this plot that Ag@TpPa-1displayed the presence of 4(a-e) the micropores of dimensions ca.1.5 nm are spread
extra large micropores of dimension ca. 1.5 nm.
throughout the entire specimen. The boundaries of the TEM images
(Figure 4c) exhibit the arrangement of layered-sheet of Ag@TpPa-1.
As noticed from the TEM images the well-dispersed Ag nanoparticles
(black dots) are clearly seen (Figures 4c and 4d). The TEM images of
the reused Ag@TpPa-1 catalyst (Figures S1) suggest that the Ag
nanoparticles were still extremely dispersed. Additionally, the black
dots observed in the TEM images of Ag@TpPa-1 (Figure 4d) could
also be attributed to the presence of Ag(0) nanoparticles.
Figure 1. Powder X-ray diffraction pattern of TpPa-1 and Ag@TpPa-1.
Figure 4. The UHR-TEM images of Ag@TpPa-1 at several scales (a,b) 1 μm, (c)
100 nm, (d) 20 nm, (e) 5 nm and (f) SAED pattern of Ag@TpPa-1.
Figure2. The N2 adsorption/desorption isotherm of Ag@TpPa-1. The plot of the
distribution of pore sizesusing NLDFT method is demonstrated in the inset of the figure
Further, the spatial distribution of elements in a framework is
obtained from the elemental mapping analysis from the UHR-TEM
image analysis of Ag@TpPa-1 (Figure 5). Figure 5c–f indicates the
FE-SEM images of Ag@TpPa-1 (Figure 3) demonstrated
nanofiber-like particle morphology with diameter of ca. 25-34 nm
and length of ca. 150-250 nm, and its random distributions
throughout the entire specimens to generate a network of a bunch
of wires.
Figure 5. EDS mapping images of Ag@TpPa-1 (a) electron image, (b) EDS
layered image of Ag@TpPa-1, (c), (d), (e) and (f) C, O, N and Ag element EDS
mapping.
Figure 3. The FE-SEM images of TpPa-1 at two magnifications of different
range: (a) 50000 and (b) 25000. The FE-SEM images of Ag@TpPa-1 at two
magnifications of different range: (c) 100000 and (d) 50000.
Additionally, the UHR-TEM images of Ag@TpPa-1 (Figure 4)
suggested morphology of particle of nanofibre type having extra
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band at 2903 cm-1 for –C-H bonds and 1585 cm-1 for thVeie–wCA=rtCiclbeoOnndlinse.
An intense peak at 1283 cm-1 could be attribDuOtaI:b1l0e.1t0o3t9h/eC9CD-NT0b0o0n17dHs
present in the framework of Ag@TpPa-1. The FTIR spectra of the
Ag@TpPa-1 and reused Ag@TpPa-1 are almost similar in nature.
Figure 6. The EDAX pattern of Ag@TpPa-1.
Electron mapping images of carbon, oxygen, nitrogen and silver,
respectively.Amounts of various elements obtained from the
elemental mapping analysis agreed with the EDAX pattern as shown
in Figure 6.
Figure 8. FT IR spectrum of TpPa-1.
The diffuse UV-Vis reflectance spectrum of the Ag@TpPa-1 is
shown in Figure 7 using barium sulphate (BaSO4) coatings as a ready-
to-use white diffusive reflecting material as reference. As noticed
from this spectrum that the adsorption band of covalent organic
frameworks in the UV-visible region indicated σ→σ* and π→π*
transitions.
Figure 9. FT IR spectrum of Ag@TpPa-1.
Catalytic Activity
Synthesis of tetramic acids catalysed by Ag@TpPa-1
Tetramic acids are generally been produced through intramolecular
cyclization of via amidation (Scheme 3, eq. 1) or conventional
Dieckmann condensation (Scheme 2, eq. 2) reactions. These
strategies are very well-known for the synthesis of tetramic acids. On
the other hand, various types of derivatives of starting materials play
an important role and the reaction requires elevated temperature in
Figure 7. The DRS-UV-visible absorption spectrum of Ag@TpPa-1.
alkaline media. Here we have employed Ag@TpPa-1 as
a
The DRS-UV-visible absorption spectrum suggested the presence
of surface bound silver nanoparticles in Ag@TpPa-1. FTIR of
Ag@TpPa-1 is shown in Figure 8 and it showed the presence of
similar stretching and bending vibrations as that of the parent TpPa-
1. The intense peaks at ∼1254 cm−1 (−C−N) and ∼1586 cm-1 (−C=C)
undoubtedly suggested the presence of the β-ketoenamine linked
COF structure in Ag@TpPa-1. In the FTIR spectrum of TpPa-1 (Figure
8) showed the appearance of a broad band at 3417 cm-1 owing to the
stretching of N-H bonds and absence of imine stretching (-C=N) in the
solid matrix.21 The IR band at 3200-3600 cm−1 could be attributed due
to the stretching vibration of H-bonded OH-groups ν(O−H)26. This band
may overlap with the broad –N-H band appeared at 3417 cm-1. Thus,
the presence of some residual –OH groups in TpPa-1 framework can
not be ruled out. The intense band at 1586 cm-1 could clearly be
attributed to -C=C stretching vibrations for enol-keto
tautomerization. The significant peak at 1254 cm-1 could be assigned
owing to existance of newly produced -C-N group. The FTIR spectrum
of the Ag@TpPa-1 is shown in Figure 9. The FTIR spectrum of
Ag@TpPa-1 exhibited a broad band at 3409 cm-1 owing to stretching
of N–H bonds. The Ag@TpPa-1 exhibited the significantly strong
heterogeneous catalyst for the insertion of CO2 into various types of
derivatives of propargylic amines to produce tetramic acids. This
intramolecular rearrangement was shown to be applicable to cyclic
compounds of five membered rings to facilitate production of
tetramic acids from the primary propargylic amine derivatives. As
noticed from previously reported literatures carbon dioxide
isinserted into different propargylic amines, which yield theresulting
oxazolidinone with a silver catalyst in the absence of any alkali
(Scheme 4, eq. 3). Yamada et al. have shown that in the presence of
DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene) resulting oxazolidinone
derivatives could undergo an intramolecular rearrangement for the
generation of desired tetramic acidderivatives with homogeneous
silver catalyst(Scheme 3, eq. 4).14b
The reaction was performed in the presence of a strong organic
alkaline system and silver catalyst would be shown to produce the
corresponding oxazolidinone derivatives that could conduct a
rearrangement of intramolecular nature for the synthesis of desired
derivatives of tetramic acid under ambient reaction conditions from
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common and simple starting materials (Scheme 5). Thus our acid synthesis; instead, the corresponding oxazolidinone was formed
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DOI: 10.1039/C9DT00017H
in excellent yield (Table 1, entries 1, 3 and 4). When weak base
experimental results suggested an eco-friendly method for the
Scheme 6. Production of tetramic acid from CO2 and primary derivatized
propargylic amines by Ag@TpPa-1
triethylamine was employed, the desired yield was not observed, but
oxazolidinone was produced in 91% yield (Table 1, entries 2). When
DBU (strong organic base) was employed, the conversion of
propargylic amine was high and resulting oxazolidinone can be
converted easily into the tetramic acid (Table 1, entry 5).
Scheme 3. Conventional synthesis of tetramic acids.
Table 1. Optimization of bases for the production of tetramic acida
Yielda
(%)
Scheme 4. Incorporation of CO2 into various alkynyl amines.
Entry
Base
Solvent
(a)
(b)
1
NaOH
Et3N
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
0
0
85
2
3
4
5
6
7
91
tBuOK
Na2CO3
DBU
0
94
0
96
98
98
96
trace
trace
trace
Scheme 5. Production of tetramic acid from CO2 and primary derivatized
propargylic amines.
DBU
generation of tetramic acids, which is facilitated by the Ag-catalyzed
fixation of CO2 onto alkyne, followed by an intramolecular
rearrangement.
DBU(1
equiv)
We have started the carbon dioxide fixation of various propargylic
amine derivatives in the presence of DBU as a strong organic base in
the presence of Ag@TpPa-1 catalyst. Initially we have taken 2.6
mmol of 1-phenylethynyl-cyclohexylamine, 5.2 mmol of NaOH, 40
mg of Ag@TpPa-1and 5 ml of MeCN at 0.1 MPa CO2 pressure. The
result of this reaction was quite attractive and we have obtained 85
% of the oxazolidinone product of primary propargylic amine
(Scheme 6). To obtain high yield of the product we have optimized
all other reaction conditions one by one. The conversion of starting
material is dependent on the nature of base. Inorganic bases such as
NaOH, t-BuOK and Na2CO3 were not much effective for the tetramic
8
DBU
DBU
DBU
THF
87
98
92
trace
trace
trace
9
DMSO
DMF
10
Reaction conditions: 1-Phenylethynyl-cyclohexylamine (2.6 mmol),
temperature (60 0C), base (5.2 mmol), CO2 (0.1 MPa), 18 h. aIsolated
yield.
A wide range of solvents was employed for this reaction using
Ag@TpPa-1 as catalyst (Table 1). In THF solvent the observed yield of
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tetramic acid was 87% (Table 1, entry 8). Additionally, at 0.1 MPa
carbon dioxide pressure (atmospheric pressure) the reaction was
carried out in commonly used solvent like dimethyl sulfoxide, instead
of acetonitrile, without any significant decrease in the yield of
tetramic acid (Table 1, entry 9). The reaction smoothly proceeds in
polar aprotic solvents like MeCN, DMSO or DMF to obtain desired
yields of tetramic acid. Finally, the best suited solvent was
acetonitrile in the selection of solvent due to it being simply removed
under the application of reduced pressure.
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(a)
(b)
1
2
3
4
5
6
1
1
1
1
2
2
42
42
18
18
18
5
30
40
50
60
60
60
39
66
64
96
98
91
52
31
Effect of catalyst on the reaction is shown in Figure10. When the
reaction was conducted with 40 mg of Ag@TpPa-1, the
desiredtetramic acid was afforded in 98% yield. The reactions were
conducted with 30, 20 and 10 mg of Ag@TpPa-1, and the desired
product was obtained in moderate yield after 18 h. 30 mg of
Ag@TpPa-1 could efficiently accelerate the reaction to form the
desired tetramic acid in 84 % yield in 18 h. The reaction without
Ag@TpPa-1 produced the desired product in trace yield despite
elevated reaction time. This result suggested that Ag@TpPa-1 is truly
act as a heterogeneous catalyst for the synthesis of desired tetramic
acids from respective propargylic amine derivatives.
26
trace
trace
trace
Reaction conditions: 1-Phenylethynyl-cyclohexylamine (2.6 mmol),
Ag@TpPa-1 (40 mg) MeCN (5 ml), CO2 (0.1 MPa), 18 h.
These outcomes indicated that the rate-limiting step is the
intramolecular rearrangement of oxazolidinone triggered by the
removal of a proton of the amide. Almost completion of the reaction
was observed in the presence of DBU (2 equiv.) in 18 h (Table 2, entry
5).
To conduct the removal of a proton of the amide, the DBU
loading was enhanced (Table 2). Reactions at 30, 40, and 50 °C
provided the desired tetramic acid; oxazolidinone also remained
(Table 2, entries 1−3). Subsequently, the reaction should be carried
out at 60 °C to achieve the corresponding tetramic acid in high yield
(Table 2, entry 4). These outcomes suggested that the rate-
determining step for this cycloaddition reaction is the rearrangement
of oxazolidinone triggered by deprotonation of the amide.26 When
Figure 10. Effect of amount of catalyst on the synthesis of tetramic acid.
Moreover, optimization of several key parameters such as time 1.0 equivalent of DBU was utilized in the reaction, the desired
and reaction temperature has been studied in depth (Table 2). When product was observed in 96% yield (Table 2, entry 4). When 2.0
temperature of reaction was tried to reduce from 60 °C to 30 °C, the equivalents of DBU were utilized, this yielded the desired product in
intermediate oxazolidinone was produced in very high yield along satisfactory amount. In presence of Strong organic base DBU (2
with the desired tetramic acid, despite increasing the reaction time equiv.), reaction time was decreased to 5 h thereby leading to
and a larger quantity of DBU base (Table 2, entries 1 and 2). This desired tetramic acid in 91% yield (Table 2, entry 6). Completion of
result suggested that necessary temperature for fixing CO2on the reaction was observed in the presence of 2 equivalent of DBU
propargylic aminesis 60 °C to obtain the corresponding tetramic acids and 40 mg of Ag@TpPa-1 in 18 h at the temperature of 60 °C (Table
in excellent yields (Table 2, entry 4).
2, entry 5).
Table 2. Optimization of reaction conditions for the production of
The propargylic amine derivative was transformed into the
tetramic acida
desired product in 98% yield in presence of DBU under 0.1 MPa
pressure of CO2 in MeCN (Table 3, entry 1). The propargylic amine
containing terminal alkyne yielded the desired tetramic acid in 62%
yield (Table 3, entry 4). The influences of various substituents on the
aromatic ring in group R was investigated. When aromatic ring
contains a –NO2 substituent as an electron-attracting substutuent at
the p-position, the reaction took place rapidly to generate the
desired product in 96% yield (Table 3, entry 3). An electron releasing
methyl substituent or methoxy substituent when used as a para
substituent on the aromatic group CO2 fixation of both substrates
resulted desired products in satisfactory yields (Table 3, entries 5 and
2).
Yielda
Entry
base
time
(h)
temperature
(oC)
(%)
(equiv)
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Table 3. Synthesis of tetramic acid over Ag@TpPa-1 under optimized Table 4. Synthesis of tetramic acid over Ag@TpPa-1 under optimized
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conditionsa
conditionsa
Entry
Product
Time
(h)
Yieldb
(%)
Entry
Product
Time
(h)
Yieldb
(%)
V (h)
W (h)
1
18
98
1
28
5
95
2
3
26
10
5
5
76
92
2
3
18
78
12
96
4
10
4
74
4
5
18
42
62
92
5
6
10
10
4
6
94
77
aReaction conditions: Starting material (2.4 mmol), Ag@TpPa-1 (40
mg, 0.00786 mmol based on silver metal), DBU (1.0 equiv), CO2
(0.1MPa), MeCN (5 ml). bAll yields correspond to isolated products.
aReaction conditions: Starting material (2.6 mmol), Ag@TpPa-1 (40
mg, 0.00786 mmol based on silver metal), DBU (2.0 equiv), CO2
(0.1Mpa), MeCN (5 ml). bAll yields correspond to isolated products.
In the presence of Ag@TpPa-1 without strong organic base DBU,
the starting material was transformed into the intermediate
oxazolidinone by catalytic incorporation of CO2. Degassing is
desirable to perform this method by freeze-deaeration to get rid of
dissolved CO2. 1.0 equivalent of DBU was poured into the reaction
mixture at 25 °C. When we conducted this method the starting
material afforded the corresponding tetramic acid in 95% yield.
Propargylic amines having an electron releasing methyl group in the
para-, meta-, or ortho-positions underwent reaction to generate the
desired tetramic acids in satisfactory yield (Table 4).
When no group is substituted on the propargylic position in
propargylic amines, the desired tetramic acid was not observed;
instead the reactions lead to mixtures of complex. Under optimized
the reaction condition, the substrate mentioned in Table 4 was
tested with a 1 equivalent of DBU and different reaction time at 25
0C temperature.
Proposed mechanism for the synthesis of the tetramic acids
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Journal Name
A proposed reaction mechanism for the synthesis of the tetramic Recyclability of Ag@TpPa-1 material
View Article Online
At the end of the reaction the catalyst AgD@OTI:p1P0a.1-0139w/aCs9DcTo0ll0e0c1t7eHd
from the mixture by applying centrifugation technique. After that,
washing of Ag@TpPa-1 with water and acetone was conducted to get
rid of the contamination and allowed to dry in air oven at 70 0C for 8
h followed by continuation of the next run under ambient reaction
conditions. As noticed from Figure 12 that Ag@TpPa-1 can be easily
reused for five times without considerable loss in yields, which
suggestedthe catalysis over Ag@TpPa-1 is entirely heterogeneous.
Our experimental outcomes revealed outstanding recycling
efficiency of Ag@TpPa-1 for the production of tetramic acids.In our
recycling experiments, we have simultaneously studied the leaching
of silver (if any) in fromAg@TpPa-1 catalyst. Measured Ag in the
reaction mixtures varied within 2-5 ppm, suggesting negligible
leaching of Ag from catalyst surface.
acids is shown in Figure 11 on the basis of previously reported
literature results.27 The mechanism for the reaction was assumed to
be that the intermediate (carbamate) derived from propargylic
amine with CO2 reacted with the alkyne activated by the Ag catalyst
to achieve the oxazolidinone via an intramolecular ring-closing
reaction. Though DBU base would assist with the carbamate
production, in equilibrium between free propargylic amine and the
corresponding ammonium carbamate under carbon dioxide
atmosphere, the amine could work as analkali to assist with the
carbamate production without DBU.28 The reaction was carried out
in the presence of an alkali and Ag@TpPa-1, which resulted an
oxazolidinone intermediate that could undergo an intramolecular
rearrangement to generate the desired tetramic acids under ambient
conditions from general and simple starting materials (Scheme 4).
The present result suggested a novel route for the synthesis of
tetramic acid derivatives using supported Ag-nanocatalyst for the
fixation of CO2 involving the intramolecular rearrangement. This
intramolecular rearrangement usually leads to cyclic compounds of
five membered systems, which facilitates the production of tetramic
acids from the primary propargylic amines.
Figure 12.Recyclability of Ag@TpPa-1 catalyst for the chemical fixation
of CO2 over 1-phenylethynyl-cyclohexylamine under optimized reaction
conditions
Conclusions
In conclusion, we have decorated Ag NPs at the surface of acovalent
organic framework material to obtain
a robust one-pot
heterogeneousnanocatlyst Ag@TpPa-1. It exhibited outstanding
catalytic efficiency and recyclability for the generation of tetramic
acids from propargylic amine derivatives using CO2 as reagent at
atmospheric pressure. The environmentally benign and highly
efficient catyalytic pathway for the fixation of CO2 over Ag@TpPa-1
reported herein for the synthesis of tetramic acids has huge future
potential in green chemical synthesis.
Figure 11.Proposed mechanistic pathway for the synthesis of tetramic acids
over Ag@TpPa-1.
Heterogeneity test of Ag@TpPa-1 catalyst
To check the heterogeneous nature of Ag@TpPa-1 catalyst, we have
performed hot filtration test in this CO2 fixation reaction. From this
leaching limit study by AAS instrument we can conclude that after
the first reaction cycle, leaching of silver was observed in negligible
amount in ppm scale. The AAS analysis recorded that after several
reaction runs, a very negligible quantity of silver was leached out in
our experiment compared to the other reported systems. For further
confirmation we have done ICP-AES analysis of the filtrate and the
recovered catalyst. We did not get any considerable metal trace in
the filtrate and in Ag@TpPa-1 catalyst got same metal loading as that
of fresh catalyst (≈2.12 %). All these experimental data clearly
confirmed the heterogeneous nature of the Ag@TpPa-1 together
with one-pot design in the catalyst synthesis.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
SMI thankfully acknowledges Department of Science and Technology
DST-SERB
EMR/2016/004956), New Delhi, Govt. of India, Board of Research in
Nuclear Sciences (BRNS), Project
reference No: 37(2)/14/03/2018-BRNS/37003, Govt. of India and
Council of Scientific and Industrial and
Research, CSIR (project reference no 02(0284)2016/EMR-II dated
06/12/2016) New Delhi, Govt. of India
(project
referenceno.
for providing financial support. RAM and UK acknowledges DST for
8 | J. Name., 2012, 00, 1-3
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ARTICLE
G. Zhu, Chem. Commun., 2013, 49, 9773; (h) R. Gomes, P.
their respective NPDF grants. We acknowledge Department of
Science and Technology (DST) and University Grant Commission
(UGC) New Delhi, India for providing support to the Department of
Chemistry, the University of Kalyani under PURSE, FIST and SAP
program. AB thanks DST-SERB for providing extramural project grant.
Bhanja and A. Bhaumik, Chem. Commun., 2015,V5ie1w,A1rt0ic0le5O0n;li(nie)
DOI: 10.1039/C9DT00017H
H. Zhong, Y. Q. Su, X. W. Chen, X. J. Li and R. H. Wang,
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H. Baek and S. Yoon, Green Chem., 2016, 18, 6456; (d) R. Xu,
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