Copper-catalyzed efficient access to 2,4,6-triphenyl pyridinesviaoxidative decarboxylative coupling of aryl acetic acids with oxime acetates

Article abstract of DOI:10.1039/d1nj01987b

An efficient and concise approach for the synthesis of 2,4,6-triphenyl pyridines has been developed through copper-catalysed oxidative decarboxylative coupling of C(sp³) aryl acetic acids with oxime acetates in DMF at 150 °C under an oxygen atm

Full text of DOI:10.1039/d1nj01987b

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Copper-catalyzed efficient access to 2,4,6-
triphenyl pyridines via oxidative decarboxylative
coupling of aryl acetic acids with oxime acetates†
Cite this: New J. Chem., 2021,
45, 15205
Received 28th April 2021,
Accepted 16th July 2021
a
Bodala Varaprasad,ab Karasala Bharat Kumar,a Vidavalur Siddaiah,
*
Pulipaka Shyamalab and Lekkala Chinnaria
DOI: 10.1039/d1nj01987b
rsc.li/njc
An efficient and concise approach for the synthesis of 2,4,6- TCT,5c TiO2–SO3H,5d Fe3O4@TiO2@O2PO2(CH2)2NHSO3H5e etc.5f–h,6
triphenyl pyridines has been developed through copper-catalysed However, most of the methods have some drawbacks, such as the
oxidative decarboxylative coupling of C(sp3) aryl acetic acids with usage of expensive transition metal catalysts, oxidants, unfriendly
oxime acetates in DMF at 150 8C under an oxygen atmosphere. additives, poor yields, and harsh conditions. Recently, oxime
Various functional groups were well tolerated and provided the acetates were proved to be versatile building blocks for the con-
corresponding 2,4,6-triphenyl pyridines in good to excellent yields. struction of various heterocyclic compounds, in particular
triphenylpyridines.7 Thus, plenty of methods have been reported
for the synthesis of tri-substituted pyridines from oxime acetates by
the coupling of various precursors such as aldehydes, toluenes,
benzyl amines, and p-toluene-sulfonylhydrazones using various
Introduction
transition metals as catalysts (Scheme 1a).8–12
Nitrogen-heterocyclic compounds are one of the most important
classes of compounds and they are found extensively in natural
products, pharmaceutical drugs and several functional materials.
Among the various heterocycles, pyridines have been recognized
as privileged scaffolds because of their widespread biological and
synthetic applications.1a–c Pyridines exhibit significant biological
properties such as antioxidant, anticancer, anticoagulant, anti-
inflammatory, anti-HIV, anti-malarial, anticonvulsant, vasodilator
and antiepileptic activities.1d–g In addition, some of the
compounds that have pyridine core units are being used as
therapeutic drugs (Fig. 1).2
On the other hand, transition metal catalysed decarboxylative
coupling of carboxylic acids,13 in particular, C(sp3) aryl acetic
acids, with nucleophiles has recently emerged as a powerful tool
for the synthesis of various heterocyclic compounds because of
the readily available starting materials, nontoxic byproduct
(CO2), and easy handling. Moreover, aryl acetic acids are more
stable than aldehydes.8 Despite the prevalence of the
synthetic utility of the phenyl acetic acids in the synthesis of
various heterocyclic compounds, methods that directly form
On the other hand, some of the substituted pyridines are
used as tuneable dye lasers, organic light-emitting diodes, and
fluorescent chromophores and as ligands in coordination
chemistry and aqua chemistry.3 Due to the interesting applications
of pyridine derivatives, a large number of methods have been
developed for their synthesis.4 Conventionally, these compounds
are synthesized by a three-component cyclocondensation of
acetophenones, benzaldehydes, and ammonium acetate in
the presence of various catalysts such as AcOH,5a ZrOCl2,5b
a Department of Organic Chemistry & FDW, Andhra University, Visakhapatnam,
530003, India. E-mail: sidduchem@gmail.com
b Department of Physical Nuclear and Chemical Oceanography, Andhra University,
Visakhapatnam, 530003, India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj01987b
Fig. 1 Some of the bioactive compounds containing pyridine core units.
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Table 1 Optimization of the conditionsa
Entry Catalyst (mol%) Oxidant Base
Solvent T (1C) Yieldb (%)
1
2
3
4
5
6
7
8
CuI (10)
O2
O2
O2
O2
O2
O2
O2
O2
NaHSO3 DMF
NaHSO3 DMF
NaHSO3 DMF
NaHSO3 DMF
NaHSO3 DMF
Cs2CO3 DMF
Na2CO3 DMF
150
150
150
150
150
150
150
150
150
150
150
150
150
150
150
40
58
65
25
32
30
35
32
CuBr (10)
CuCl (10)
CuCl2 (10)
Cu(OAc)2 (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (20)
CuCl (30)
CuCl (20)
—
K2CO3
Et3N
DMF
DMF
9
O2
40
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
DTBP
TBHP
PIDA
Air
N2
Ar
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
NaHSO3 DMF
NaHSO3 DMF
NaHSO3 DMF
NaHSO3 DMF
NaHSO3 DMF
NaHSO3 DMF
NaHSO3 CH3CN 150
NaHSO3 DCE 150
NaHSO3 DMSO 150
NaHSO3 Dioxane 150
NaHSO3 Toluene 150
50c
NRd
NRd
15e
Tracee
Tracee
NR
NR
35
NR
NR
40
52
90
86
NR
NR
NaHSO3 DMF
NaHSO3 DMF
NaHSO3 DMF
NaHSO3 DMF
120
160
150
150
150
150
Scheme 1 Synthesis of 2,4,6-triphenyl pyridines.
—
DMF
2,4,6-triphenyl pyridines have not been explored very well in the
literature. However, Phan14 and co-workers have recently synthesized
2,4,6-triphenyl pyridines from phenyl oxime acetates by using
strontium-doped lanthanum cobaltite as a recyclable pervoskite
heterogeneous catalyst, and DTBP as an oxidant (Scheme 1b).
However, this protocol has some drawbacks, such as the usage
of less abundant, toxic and highly expensive catalysts and
oxidants. Therefore, the development of an efficient and eco-
friendly protocol for the synthesis of tri-substituted pyridine
derivatives from oxime acetates and phenyl acetic acids is
highly desirable. In continuation of our research work on the
development of new synthetic methodologies for heterocyclic
compounds, herein, we describe a simple and efficient method
for 2,4,6-tri-substituted pyridines from oxime acetates and
C(sp3) aryl acetic acids in the presence of copper chloride as a
catalyst, and NaHSO3 as a base in DMF at 150 1C for 2 h under
an oxygen atmosphere (Scheme 1c).
NaHSO3 DMF
a
Reaction conditions: 1a (0.2 mmol), 2a (0.2 mmol), catalyst (20 mol%),
base (0.5 equiv.), oxidant (1.0 equiv.), solvent (2.0 mL), 150 1C, under an
b
c
oxygen atmosphere, 2 h. Isolated yield. NR = no reaction. For 6 h.
d
e
For 8 h. 12 h.
reaction (Table 1, entries 2–5). Subsequently, several bases such
as Cs2CO3, Na2CO3, K2CO3 and Et3N were explored, which
showed that NaHSO3 was a suitable base and other bases were
inferior (Table 1, entries 6–9). Screening of a series of oxidants
revealed that the oxidants such as DTBP, TBHP and PIDA
disfavored the transformation, and molecular oxygen was the
best choice (Table 1, entries 10–12). Moreover, only 15% of the
desired product was formed after 12 h (Table 1, entry 13), when
we carried out the reaction under an air atmosphere. This
might be due to the formation of acetophenone as a byproduct
via the hydrolysis of oxime acetate. We also performed the
reaction under nitrogen and argon atmospheres without
adding an oxidizing agent, but, unsatisfactory results were
found in both cases (Table 1, entries 14 and 15). Therefore,
an oxidizing agent is very important in this reaction to get the
product in high yields, and the oxidizing agent could accelerate the
reaction by the oxidation of phenyl acetic acid to benzaldehyde in
the presence of a copper catalyst. Replacing DMF with other
solvents, such as DMSO, CH3CN, DCE, dioxane or toluene, no
significant improvement in the yield of the desired product was
observed (Table 1, entries 16–20). Then, we tested the temperature;
the yield of 3a dropped to 40% when the reaction temperature was
reduced to 120 1C (Table 1, entry 21). Next, we focused on the
Results and discussion
Initially, we started our investigation by taking oxime acetate
(1a) and phenyl acetic acid (2a) as model substrates to optimize
the reaction conditions. Thus, in our first attempt, the reaction
was performed with CuI (10 mol%), and NaHSO3 (0.5 equiv.) in
DMF (2 mL) at 150 1C for 2 h under an oxygen atmosphere.
To our delight, the desired 2,4,6-triphenyl pyridine (3aa) was
formed in 40% yield (Table 1, entry 1). Other copper catalysts
including CuBr, CuCl, CuCl2 and Cu(OAC)2 were screened and
CuCl was found to be the most effective catalyst for this
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loading of the catalyst; fortunately, the yield of 3a was remarkably
Encouraged by the above results, we employed a series of
increased to 90%, when the catalyst loading was increased to phenyl acetic acids for further extension of the substrate scope
20 mol%. However, no significant improvement in the yield of 3a (Scheme 3). To our delight, a wide variety of commercially
was observed at 30 mol% of the catalyst (Table 1, entries 23 and 24). available phenyl acetic acids having electron-donating or
No product was observed in the absence of either catalyst or electron-withdrawing groups at different positions were accom-
a base, which suggests that a metal/base combination is modated and gave the corresponding pyridines in moderate to
required for the reaction to proceed (Table 1, entries 25 and 26). high yields. Phenyl acetic acids 2b and 2c bearing halogen
After having the optimized reaction conditions in our hands, groups on the C-4 or C-2 positions could react with various
the scope and limitations of the reaction were investigated. oxime acetates and gave the products in good yields. Notably,
As shown in Scheme 2, various oxime acetates (1) smoothly phenyl acetic acid 2d having a strong electron-withdrawing
reacted with phenylacetic acid (2a), and provided the corres- group like nitro on the C-4 position could proceed to provide
ponding products (3aa–3ai) in excellent yields under optimal the corresponding pyridine in 65% yield. Moreover, phenylacetic
reaction conditions. Unsubstituted oxime acetate reacted with acid 2e with an electron-donating group was also proven to be a
phenylacetic acid (2a) and gave the corresponding product in suitable substrate for this transformation, affording the corres-
90% yield (3aa). Oxime acetates having electron-donating ponding product in 67–88% yields.
groups such as 4-methyl and 4-methoxy groups on the aromatic
To probe the reaction mechanism, several control experiments
ring smoothly reacted with phenylacetic acid 2a and afforded have been performed (Scheme 4). Since the reaction involves
the respective pyridines in 92%, and 94% yields, respectively oxygen, radical trapping experiments were conducted by treating
(3ab and 3ac). Oxime acetates having electron-withdrawing TEMPO or BHT with phenyl acetic acid and oxime acetate under
groups such as 4-fluoro, 4-chloro and 2,4-dichloro could also the optimal conditions. The results showed that the reaction was
be converted into the desired products in good yields (3ad–3af). completely suppressed in the presence of either TEMPO or BHT
However, 4-nitro substituted oxime acetate could not serve as a (Scheme 4, eqn (1)). Based on previous reports9 and our control
viable substrate, as it could not provide the intended product. experiments (Scheme 4, eqn (2)), it could be concluded that
It might have occurred because of the strong electronic effect of aldehyde is the key intermediate in this reaction and it was
the nitro group. It is worth mentioning that aliphatic ketoxime
acetate such as isobutyl oxime acetate also reacted smoothly
and gave the desired product in good yields (3ah). Moreover,
hetero-aromatic ketoxime acetate, i.e. furan ketoxime acetate,
could efficiently get transferred to the desired product (3ai).
Scheme 2 Substrate scope of various oxime acetates. Reaction
conditions: 1 (0.2 mmol), 2a (0.2 mmol), CuCl (20 mol%), and NaHSO3
(0.5 equiv.) in DMF (2.0 mL) at 150 1C, under an oxygen atmosphere, 2–4 h.
Isolated yields.
Scheme 3 Substrate scope of various oxime acetates and phenyl acetic
acids. Reaction conditions: 1 (0.2 mmol), 2 (0.2 mmol), CuCl (20 mol%),
NaHSO3 (0.5 equiv.) in DMF (2.0 mL) at 150 1C, under an oxygen
atmosphere, 2–4 h. Isolated yields.
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decorboxylative coupling of C(sp3) arylacetic acids with oxime
acetates in the presence of CuCl as a catalyst in DMF at 150 1C
under an oxygen atmosphere. In this reaction various oxime
acetates and phenylacetic acids are well tolerated, and they
provided the 2,4,6-triphenyl pyridines in moderate to good
yields. This method not only used easily available starting
substrates but also avoided the usage of expensive ligands
and oxidants.
Conflicts of interest
There are no conflicts to declare.
Scheme 4 Control experiments.
Acknowledgements
The authors thank CSIR (Council of Scientific and Industrial
Research), New Delhi, India for the award of a fellowship
(No: 09/002(0513)/2017-EMR-I) to B. V. Prasad.
Notes and references
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Scheme 5 Plausible mechanism.
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a
plausible mechanism of the reaction is illustrated in Scheme 5.
First, the reductive cleavage of the N–O bond of 1a is initiated
by copper salts to generate an imino-copper(II) complex A that
undergoes tautomerization to afford copper enamide inter-
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from phenylacetic acid via oxidation in the presence of CuCl
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aldehyde C gives the imine intermediate D. Subsequently,
imine intermediate D reacts with another molecule of oxime
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In conclusion a simple and efficient protocol has been
disclosed for the synthesis of 2,4,6-triphenyl pyridines via the
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