A Multifunctional Metal-Organic Framework for Oxidative C-O Coupling Involving Direct C-H Activation and Synthesis of Quinolines

Article abstract of DOI:10.1021/acs.inorgchem.8b00683

A robust paddle-wheel Cu(II)-based metal-organic framework (MOF) 1, having dual functionalities, namely, Lewis acid and basic sites, has been explored as a heterogeneous catalyst. This MOF, because of its large void volume (10298 ?³, 67.6%), la

Full text of DOI:10.1021/acs.inorgchem.8b00683

Article
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
A Multifunctional Metal−Organic Framework for Oxidative C−O
Coupling Involving Direct C−H Activation and Synthesis of
Quinolines
Vivekanand Sharma, Dinesh De, and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, Uttar Pradesh, India
S
* Supporting Information
ABSTRACT: A robust paddle-wheel Cu(II)-based metal−organ-
ic framework (MOF) 1, having dual functionalities, namely, Lewis
acid and basic sites, has been explored as a heterogeneous catalyst.
This MOF, because of its large void volume (10298 Å3, 67.6%),
large surface area (1480 m2/g), and high thermal stability,
encouraged us to see its applicability in two catalytic reactions,
namely, oxidative C−O coupling (cross-dehydrogentaive coupling
̈
reaction) involving direct C−H activation and Friedlander
reaction under solvent free and ambient conditions. This study
demonstrates the green aspect of MOFs in coupling reactions
because of the simplified recovery, shorter reaction time,
minimum waste, and smooth activation of the C−H bond,
which is very challenging in synthetic chemistry.
starting materials that require additional steps and produce
severe environmental hazards.9−11 Also, in the transition metal-
catalyzed C−heteroatom coupling reaction, C−H bond
activation is a highly challenging process because of the
inertness. Thus, the direct activation of the C−H bond is a
possible way to solve this problem.12,13 To overcome these
synthetic difficulties, we aim to develop such a catalyst that
could offer a sustainable and green perspective in terms of such
cross-coupling reactions with improved synthetic protocols. To
illustrate the aforementioned arguments, we have chosen the
cross-dehydrogenative coupling reaction for C−H activation.
Such coupling reactions offer short and efficient synthetic
schemes, which are key aspects for the next generation of C−C
or C−heteroatom bond formations.14 In addition, considering
the robustness and Lewis acidity of MOF 1, quinoline
INTRODUCTION
■
Today, it is highly desirable to obtain a catalyst that can exhibit
multifunctional activity toward various catalytic transforma-
tions. Among these, metal−organic frameworks (MOFs) have
captured considerable attention within the scientific commun-
ity across a range of different laboratories. The past several
years have witnessed vigorous efforts to develop and apply
MOFs in gas storage, CO2 sequestration, gas separation,
sensors, optics, biomedical devices, and catalysis.1−3 However,
the utilization of MOFs in catalysis has not been explored that
much compared to their possible applications in gas storage
and capture. The nanoporosity of a MOF coupled with a
replete number of transition metal sites in MOF can be
applicable as catalytic sites unless the free coordination
positions are not utilized in the construction of MOFs. It
has been reported that many MOFs were employed for
transition metal-catalyzed reactions, which are routinely used
in the synthesis of a number of valuable compounds.4−7 Thus,
the MOF has been examined as a heterogeneous catalyst.
Some MOFs are more prone to heat and chemicals, and the
lack of availability of a free coordination position retards their
application in catalysis. In the green chemistry context, it is
imperative to develop more environmentally friendly and
versatile catalytic systems; we are hopeful we can examine
these aspects of MOFs, which are vitally important because of
simplified recovery and reusability of a catalyst.
̈
synthesis has been performed via Friedlander reaction to
probe its further applicability in catalysis. Quinoline and its
derivatives are well-known compounds that display a broad
range of biological significance.15−18
EXPERIMENTAL SECTION
■
Materials and General Methods. All the required chemicals for
the synthesis of MOF and catalytic reactions were purchased from
Sigma-Aldrich or S. D. Fine Chemicals. The various spectroscopic
data were collected for characterization of the compounds as
described in our previous report.19 Field emission scanning electron
microscopy (FE-SEM) images were collected on a CARL ZEISS EVO
Transition metal-catalyzed C−C and C−heteroatom cross-
coupling reactions have attracted significant attention for the
synthesis of a variety of valuable chemicals and important
intermediates.8 Despite the great achievement in classical C−C
bond formation methods, generally they use prefunctionalized
Received: March 14, 2018
© XXXX American Chemical Society
A
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Inorganic Chemistry
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(BET) surface area of this MOF was found to be 1480 m2
g−1, with a pore volume of 0.88 cm3 g−1. From a topological
point of view, it has a 3,3,4,4-connected net with mfj topology
and can be considered as a close packing of channels, shown in
Figure 1c. The framework was endowed with large spherical
cages (Figure 1b) with a diameter of ∼11 Å (atom−atom
distances excluding van der Waals radii). The Lewis acidity of
these cages increased because of the vacant coordination
positions (Figure 1d) on copper, and hence, the compound
was explored as a nanomolecular vessel for C−O bond and C−
C bond coupling reactions, viz., cross-dehydrogentaive
50 instrument, at an acceleration voltage of 15.00 kV with 200000×
magnification.
Synthesis of {[Cu6(L)3(H2O)6]·(14DMF)(9H2O)}n (1). The syn-
thesis of 1 was achieved following our previous report.19
General Procedure for Cross-Dehydrogenative Coupling
Reactions. A solution of 2-hydroxybenzaldehyde (1.0 mmol) in 1,4-
dioxane (4 mL, 50 mmol) was placed in a 10 mL round-bottom flask
preloaded with catalyst 1′ (1 wt %). Then, tert-butyl hydroperoxide
(70 wt % in water, 0.436 mL, 3.0 mmol) was added to the resulting
mixture. The mixture was stirred at 100 °C for 1 h. To monitor the
reaction, thin layer chromatography (TLC) (using a 2:1 n-hexane/
ethyl acetate mixture as the eluent) was performed. Then, the reaction
mixture was filtered and washed successively with CH2Cl2 to recover
the catalyst. The filtrate was passed through anhydrous Na2SO4, and
the solvent was evaporated. The crude product was purified by silica
gel column chromatography (eluent being 10% ethyl acetate in n-
hexane) to give the product. The recovered catalyst was washed
thoroughly and reused after reactivation at 130 °C under vacuum for
6 h.
̈
coupling (CDC) and Friedlander reactions, respectively.
Cross-Dehydrogenative Coupling Reactions. Direct
C−C and C−heteroatom bond formation through C−H bond
activation has emerged as a versatile tool for introducing
complexities into molecules from simple precursors in
synthetic organic chemistry.20,21 In particular, cross-dehydro-
genative coupling reactions under oxidative conditions are
atom economical and prevent prefunctionalization of the
coupling partner and hence offer a superior pathway for
coupling reactions.22−26
̈
General Procedure for Friedlander Reaction. Acetylacetone
(5 mmol), 1′ (1 wt %), and 2-aminoarylketone (1 mmol) were added
to the 25 mL round-bottom flask, and the mixture was heated at 80
°C under solvent free conditions until the total consumption of the
reactants was detected by TLC, which took ∼8 h. After cooling, the
reaction mixture was filtered and washed thoroughly with CH2Cl2 to
remove any possible adhered products on the catalyst surface. The
filtrate was passed through Na2SO4, and the solvent was evaporated.
The crude product was purified by silica gel column chromatography
using an n-hexane/ethyl acetate mixture (19:1 ratio) as the eluent to
give the desired product. The recovered catalyst was washed
thoroughly and reused after reactivation at 130 °C under vacuum
for 6 h.
We have explored the cross-dehydrogenative coupling
reactions by direct C−H activation of 1,4-dioxane with 2-
carbonyl-substituted phenols (Table 1) under solvent free
Table 1. CDC Reactions under Solvent Free Conditions at
a
100 °C
RESULTS AND DISCUSSION
■
b
In our previous work,19 we had synthesized the thermally
robust MOF {[Cu6(L)3(H2O)6]·(14DMF)(9H2O)}n (1) from
a bent amino-functionalized tetracarboxylate ligand, H4L
(Figure 1a), and Cu(NO3)2 under solvothermal conditions.
entry
catalyst
1′
1′
1′
1′
R1
R2
yield (%)
1
2
3
4
5
6
7
8
H
H
Cl
NO2
H
OMe
H
H
−
H
H
H
96
92
84
65
10
73
71
1′
unsubstituted phenol
Cu(OAc)2
Cu(NO3)2
no catalyst
H
H
H
no reaction
a
Reaction conditions: hydroxybenzaldehyde (1.0 mmol), 1′ (1 wt %),
1,4-dioxane (4 mL, 50 mmol), tert-butyl hydroperoxide (70 wt % in
water, 0.436 mL, 3.0 mmol). Yield of the isolated product.
b
conditions using tert-butyl hydroperoxide (TBHP) as an
oxidant. In a typical experiment, a mixture of substituted 2-
hydroxybenzaldehyde (1.0 mmol) and 1,4-dioxane (4 mL, 50
mmol) was added to a flask containing catalyst 1′ (1 wt %).
Then, TBHP (70 wt % in water, 0.436 mL, 3.0 mmol) was
added. The reaction mixture was stirred at 100 °C for 1 h, and
its progress was observed by TLC.
A look at the results indicated that the unsubstituted 2-
hydroxybenzaldehyde (Table 1, entry 1) and electron-donating
group on 2-hydroxybenzaldehyde (Table 1, entry 2) afforded
comparable yields, while the presence of electron-withdrawing
groups at the para position to the phenolic -OH group (Table
1, entries 3 and 4) led to a decrease in the yield of the desired
product. Phenol was almost unreactive with only 10%
conversion after 4 h (Table 1, entry 5). In the absence of
catalyst 1′, the reaction showed no progress (Table 1, entry 8),
which indicated the catalytic nature of 1′. The identity of the
product in each case was confirmed by electrospray ionization
Figure 1. (a) Ligand H4L. (b) Nanospherical cages in 1. (c) mfj
topology in 1 (closely packed by nanospherical cages). (d) Replete
number of open metal sites (polyhedra).
The framework consisted of paddle-wheel Cu2(CO2)4
secondary building units (SBUs). Water molecules occupying
the axial positions of the SBU could be easily removed by
heating, leading to the formation of highly porous (67.6% void
volume) 1′ in which the metal ions became coordinatively
unsaturated with enough coordination space available for
heterogeneous catalysis. The Brunauer−Emmett−Teller
1
mass spectrometry (ESI-MS), H nuclear magnetic resonance
B
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Inorganic Chemistry
Article
(NMR), and 13C NMR (Figures S1−S12) after silica gel
column chromatography.
Friedlander Reactions. The Friedlander reaction offers an
efficient and straightforward synthetic protocol for the
synthesis of various nitrogen heterocycles and quinoline
derivatives that are of paramount importance in various
biological functions.15−18 In addition to their biological
activity, they are also useful in the synthesis of nanostructured
material with enhanced optoelectronic properties.29−31 Be-
cause of their importance, several methods have been reported
using a diverse range of catalysts such as Al2O3,32 H2SO4/
SiO2,33 NaHSO4/SiO2,34 MCM-41,35 etc., but none of these
methods are free from harsh reaction conditions or longer
reaction times. However, only a few studies have reported
using MOF as a catalyst.36−40 Therefore, it is straightforward
and logical to develop a catalyst for better catalytic efficiency.
̈
̈
Catalyst 1′ exhibited profound catalytic activity compared to
that of other paddle-wheel Cu MOFs such as Cu3(BTC)2,
Cu(BDC), and Cu(BPDC) and catalytic activity superior to
that of Cu2(BPDC)2(BPY) as reported by the Phan group.27
Also, 1′ offers advantages over homogeneous copper salts such
as Cu(OAc)2 and Cu(NO3)2 (Table 1, entries 6 and 7) in
terms of both catalytic activity and catalyst recyclability. The
recyclability of a catalyst makes it viable for commercial
applications; hence, catalyst 1′ was investigated for its
recoverability and recyclability in the coupling reaction. After
the coupling reaction was complete, the catalyst was separated
by simple filtration, washed with methanol followed by DCM
to remove the physisorbed product, and then activated by
being heated at 130 °C for 6 h under vacuum to reactivate the
catalyst. The recovered catalyst was then reused in the
successive reactions under the same conditions that were
used for the first run. In fact, an 85% yield was still achieved in
the fifth run (Figure S13); nonetheless, crystallinity was
maintained during the course of the reaction, which was
evident after assessment via powder X-ray diffraction (PXRD)
(Figure S14).
̈
Here, the synthesis of quinoline derivatives via the Friedlander
heteroannulation reactions between aminoaryl ketones and
acetylacetone were attempted using 1′ (1 wt %) as the catalyst
(Table 2). Initially, the reaction of acetylacetone with 2-
̈
Table 2. Friedlander Reactions under Solvent Free
a
Conditions at 80 °C
The heterogeneity of catalyst 1′ was justified by a hot
filtration test, following the standard protocol.28 The reaction
mixture was filtered after 15 min (∼38% yield) to separate the
catalyst. The resulting reaction mixture did not afford the
further progress, establishing the heterogeneity (Figure S15).
A simple and straightforward probable mechanism for the
CDC reaction is given in Scheme 1. The open Cu(II) site first
b
entry
catalyst
R1
Ph
CH3
Ph
Ph
Ph
R2
yield (%)
1
2
3
4
5
1′
1′
1′
1′
1′
H
H
NO2
Cl
H
95
89
76
90
11
Scheme 1. Plausible Mechanism for the CDC Reaction
a
Reaction conditions: acetylacetone (5.0 mmol), 2-aminoarylketone
(1 mmol) reacted at 80 °C in the presence of 1′ (1 wt %). No
b
solvents were taken. Yield of the isolated product.
aminobenzophenone was chosen as a model reaction that
afforded the corresponding product in an excellent yield
̈
(95%). The profound catalytic activity of 1′ for Friedlander
synthesis could be attributed to the replete number and easy
accessibility of the empty Cu(II) binding sites and also
accessible amine groups promoting facile substrates and active
site interactions. Thereafter, a variety of 2-aminoaryl ketones
that afforded the desired products in satisfactory isolated yields
were used. With 4-nitro and 4-chloro derivatives of 2-
aminobenzophenone, the yields were 76 and 90%, respectively
(Table 2, entries 3 and 4, respectively). Furthermore, in the
absence of 1′, the reaction gave an 11% yield (Table 2, entry
5).
In general, 1′ (1 wt %) was added to a mixture of 2-
aminoarylketone (1 mmol) and acetylacetone (5 mmol), and
the mixture was heated at 80 °C under solvent free conditions
for 8 h. After the mixture had cooled, the catalyst was removed
by filtration and the reaction mixture was washed thoroughly
with CH2Cl2 to remove the possible adhered product from the
catalyst surface. The desired product was purified by silica gel
column chromatography using an n-hexane/ethyl acetate
mixture (19:1 ratio) as the eluent. In each case, the catalytic
forms a chelate complex with hydroxyaldehyde (step A). At the
same time, TBHP generates hydroxyl and tert-butoxyl radicals.
The produced tert-butoxyl radical abstracts hydrogen from
carbon adjacent to the oxygen atom of dioxane and forms a
radical dioxane species. The hydroxyl radical takes hydrogen
from the chelate complex and leaves a water molecule (step B).
The formation of the desired product takes place during the
reaction of the dioxane radical with the proposed chelate
complex.
1
product was characterized by ESI-MS, H NMR, and 13C
NMR (Figures S16−S27). The recovered catalyst was dried at
130 °C for 6 h under vacuum for its reuse. No significant
decrease in catalytic activity or crystallinity was observed after
up to four cycles (Figures S28 and S29), and the
C
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Inorganic Chemistry
Article
heterogeneous nature of the catalyst was again established by a
CONCLUSIONS
In summary, a robust Cu MOF possessing mfj topology is
systematically investigated as a molecular vessel for oxidative
■
hot filtration test (Figure S30).
A probable mechanism of Friedlander reaction has also been
proposed (Scheme 2) on the basis of the work reported by the
̈
̈
̈
CDC and Friedlander reactions. In CDC and Friedlander
reactions, the catalyst could be easily recovered and recycled
without a significant loss of catalytic activity after up to five and
four times, respectively. This study provides an ideal platform
for further improving catalyst design, and the catalysts can
serve as single versatile tools for various catalytic needs.
̈
Scheme 2. Plausible Mechanism for the Friedlander
Reaction
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00683.
■
S
Several types of spectroscopic data, PXRD patterns, and
additional figures (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: pkb@iitk.ac.in.
■
ORCID
Parimal K. Bharadwaj: 0000-0003-3347-8791
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the financial support
received from the MNRE (New Delhi, India) (to P.K.B. and
D.D.). P.K.B. thanks DST for a JCB Fellowship. V.S. thanks
the University Grants Commission (UGC) (New Delhi,
India).
Cejka group.36 Concerted effects of a pair of two paddle-wheel
units here lead to the proximity of reactants to each other and
enhance the reaction rate. The proposed mechanism involves
first stabilization of both reactants on neighboring open metal
sites (A). This aids the formation of intermolecular aldol
intermediate B, followed by loss of a water molecule to form C.
Subsequently, intramolecular proton transfer generates D, from
which the loss of a water molecule takes place, resulting in the
desired product quinoline.
The crystallinity and morphology of the recovered catalyst
were further analyzed by FE-SEM. FE-SEM images in the case
of the CDC reaction before and after the fifth run of catalytic
cycle were compared (Figure 2). The crystalline nature and
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̈
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DOI: 10.1021/acs.inorgchem.8b00683
Inorg. Chem. XXXX, XXX, XXX−XXX