Synthesis of triphenylpyridines: Via an oxidative cyclization reaction using Sr-doped LaCoO3 perovskite as a recyclable heterogeneous catalyst

Article abstract of DOI:10.1039/c9ra04096j

An La_0.6Sr_0.4CoO₃ strontium-doped lanthanum cobaltite perovskite was prepared via a gelation and calcination approach and used as a heterogeneous catalyst for the synthesis of triphenylpyridines via the cyclization reactio

Full text of DOI:10.1039/c9ra04096j

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PAPER
Synthesis of triphenylpyridines via an oxidative
cyclization reaction using Sr-doped LaCoO3
perovskite as a recyclable heterogeneous catalyst†
Cite this: RSC Adv., 2019, 9, 23876
Thu N. M. Le, Son H. Doan, Phuc H. Pham,
Tien T. T. Tran, Minh-Vien Le, Tung T. Nguyen
Khang H. Trinh, Tien V. Huynh,
*
*
and Nam T. S. Phan
An La0.6Sr0.4CoO3 strontium-doped lanthanum cobaltite perovskite was prepared via a gelation and
calcination approach and used as a heterogeneous catalyst for the synthesis of triphenylpyridines via the
cyclization reaction between ketoximes and phenylacetic acids. The transformation proceeded via the
oxidative functionalization of the sp3 C–H bond in phenylacetic acid. The La0.6Sr0.4CoO3 catalyst
Received 30th May 2019
Accepted 13th July 2019
demonstrated
a superior performance to that of the pristine LaCoCO3 as well as a series of
homogeneous and heterogeneous catalysts. Furthermore, the La0.6Sr0.4CoO3 catalyst was facilely
recovered and reused without considerable decline in its catalytic efficiency. To the best of our
knowledge, the combination of ketoximes with easily available phenylacetic acids is novel.
DOI: 10.1039/c9ra04096j
rsc.li/rsc-advances
valuable building blocks for numerous important organic
structures.13–17
1. Introduction
Perovskites have attracted extensive interest from
researchers worldwide owing to their signicant physicochem-
ical properties, such as redox behavior, superconductivity,
ferroelectricity, magnetoresistance, and ionic conductivity.18–20
Perovskites are mixed oxides, generally existing in the form of
Functionalized pyridines have emerged as a precious class of
privileged N-heterocyclic compounds, existing in a wide range
of biologically active natural products, medicinal chemicals,
agricultural chemicals, and functional organic materials.1–4
Among the numerous pyridines, 2,4,6-triphenylpyridines offer
important applications, and the development of effective
synthetic approaches for these structures has been considered
as a hot topic. The most popular method relies on the three-
component cyclocondensation transformation of acetophe-
nones, benzaldehydes, and ammonium acetate under diverse
conditions.5 Recently, new synthetic protocols have been
developed, in which the utilization of ketoximes as starting
materials has been explored for the synthesis of the above-
mentioned heterocyclic compounds. Ren et al. developed an
efficient strategy to achieve 2,4,6-triphenylpyridines via the
CuBr-catalyzed coupling of oxime acetates with aldehydes.6 Fu
reported the Cu(OTf)2-catalyzed oxidative sp3 C–H coupling of
oxime acetates with toluene, benzylamines, and p-toluene-
sulfonylhydrazones.7 Iron(III) chloride was used for the cycliza-
tion of ketoxime carboxylates with N,N-dialkylanilines or
aldehyde to afford densely substituted pyridines8,9 (Scheme 1a).
The coupling of aryl amines or benzyl halides and aromatic
ketones for the synthesis of triarylpyridines is also known.10–12
Certainly, ketoximes have recently gained signicant interest as
Faculty of Chemical Engineering, HCMC University of Technology, VNU-HCM, 268 Ly
Thuong Kiet, District 10, Ho Chi Minh City, Vietnam. E-mail: tungtn@hcmut.edu.vn;
ptsnam@hcmut.edu.vn; Fax: +84 8 38637504; Tel: +84 8 38647256 extn 5681
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra04096j
Scheme 1 The difference between previous works (a) and this work
(b).
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Table 1 Studies of reaction conditionsa
Entry
Solvent
Catalyst amount (mol%)
Oxidant
Yieldb (%)
1
2
Chlorobenzene
Heptane
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
5
5
5
5
0
1
3
7
5
5
5
5
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
K2S2O8
AgNO3
Oxygen
TBHP
54
22
83
17
4
21
51
87
8
3
4c
5
6
7
8
9
10
11
12d
11
2
55
a
Reaction conditions: acetophenone oxime acetate (0.6 mmol), phenylacetic acid (0.2 mmol), solvent (2 mL), oxidant (0.6 mmol), at 140 ꢀC under
air for 2 h and under argon for 6 h. b GC yield. c Reaction at 120 ꢀC. d TBHP in water. DTBP ¼ di-tert-butylperoxide, TBHP ¼ tert-butyl hydroperoxide.
ABO3, where A represents an alkali, alkaline or lanthanide for CO oxidation,24,32 NO oxidation,33 syngas conversion,34,35 and
metal, and B stands for a transition metal.21–23 The metal A is hydrogen production.36 Nonetheless, the application of perov-
responsible for the stability of the perovskite structure, and the skite catalysts in the organic synthesis eld is extremely rare in
substitution of A by other metals affects the valence state of the literature. Additionally, the utilization of easily available
metal B, leading to different catalytic activities.24 Certainly, phenylacetic acids as green starting materials for organic
perovskites have been extensively utilized as heterogeneous synthesis via the oxidative functionalization of sp3 C–H bonds
catalysts for numerous reduction–oxidation processes.25–31 has gained considerable attention.37–39 In this work, we report
LaCoO3-based perovskites are one of the most active catalysts the utilization of a strontium-doped lanthanum cobaltite
perovskite as a recyclable heterogeneous catalyst for the
synthesis of triphenylpyridines via oxidative cyclization reaction
between ketoximes and phenylacetic acids (Scheme 1b). Single
oxides or salts of lanthanum, strontium, and cobalt demon-
strated a low catalytic performance for the transformation,
indicating the importance of the synergistic effect in the
perovskite catalyst. To the best of our knowledge, the oxidative
cyclization between ketoximes and phenylacetic acids has not
been previously mentioned in any literature.
2. Experimental
The strontium-doped lanthanum cobaltite perovskite (La0.6
-
Sr0.4CoO3) powder was prepared from La(NO3)3, Sr(NO3)2, and
Co(NO3)2 via a gelation and calcination approach, as previously
reported in the literature.33 The perovskite was accordingly
characterized using several conventional analysis techniques
(Fig. S1–S4†). In a representative experiment, a mixture of ace-
tophenone oxime acetate (0.106 g, 0.6 mmol), phenylacetic acid
(0.027 g, 0.2 mmol), and diphenyl ether (16 mL, 0.1 mmol) as an
Fig. 1 Leaching experiment indicating that no additional 2,4,6-tri-
phenylpyridine was formed after the removal of the catalyst.
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Table 2 The oxidative cyclization between acetophenone oxime acetate and phenylacetic acid utilizing different catalystsa
Heterogeneous
catalyst
Entry
Homogeneous catalyst
Yieldb (%)
1
2
FeCl3
FeCl2
26
6
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Fe(NO3)3
FeSO4
CuSO4
Cu(NO3)2
NiSO4
Co(NO3)2
La(NO3)2
Sr(NO3)2
3
13
57
8
38
4
2
39
70
60
43
19
11
38
40
83
Cu(BDC)
ZIF-8A
UiO-66$TFA
La2O3
CoO
SrO
LaCoCo3
La0.6Sr0.4CoO3
a
Reaction conditions: acetophenone oxime acetate (0.6 mmol), phenylacetic acid (0.2 mmol), DTBP (0.6 mmol), toluene (2 mL), catalyst (5 mol%),
b
140 ꢀC, under air for 2 h and under argon for 6 h. GC yield.
internal standard was added to a 12 mL screw-cap pressurized pressure at room temperature overnight on a Schlenk line, and
vial containing toluene (2 mL). The reaction mixture was reutilized in the catalyst recycling studies.
magnetically stirred and heated for 5 min. The La0.6Sr0.4CoO3
perovskite catalyst (2.3 mg, 5 mol%) was then added to the vial.
The reaction mixture was magnetically stirred for 2 min to
completely disperse the catalyst in the liquid phase, followed by
3. Results and discussion
the addition of di-tert-butylperoxide (DTBP, 0.14 mL, 0.6 mmol).
The resulting mixture was stirred at 140 ꢀC for 2 h. Subse-
quently, the reactor tube was back-lled with argon and then
stirred at 140 ꢀC for 6 h. The reaction mixture was diluted with
ethyl acetate (5 mL) and washed with saturated NaHCO3 solu-
tion (5 mL). The organic layer was dried using anhydrous
Na2SO4. Reaction yields were recorded from the GC analysis
results based on the diphenyl ether internal standard. To isolate
the triphenylpyridine product, the ethyl acetate phase was
concentrated under reduced pressure, and puried by column
chromatography on silica gel with a hexane/ethyl acetate solvent
mixture. The triphenylpyridine was consequently conrmed by
GC-MS, 1H NMR, and 13C NMR. The strontium-doped
lanthanum cobaltite perovskite catalyst was collected from the
reaction mixture by centrifugation, washed carefully with ethyl
acetate, acetone, and diethyl ether, dried under reduced
The strontium-doped lanthanum cobaltite perovskite was used
as a heterogeneous catalyst for the synthesis of 2,4,6-triphe-
nylpyridine via the oxidative cyclization reaction between ace-
tophenone oxime acetate and phenylacetic acid (Scheme 1b).
The reaction proceeded in the presence of an oxidant, which is
required for the oxidative functionalization of the sp3 C–H bond
in phenylacetic acid. Initially, the reaction conditions were
optimized to improve the yield of the triphenylpyridine product
(Table 1). For the reactions conducted in the liquid phase, the
nature of the solvent may be critical, especially when a solid
catalyst is employed. Thus, the impact of solvent on the yield of
2,4,6-triphenylpyridine was studied. Running the reaction in
chlorobenzene gave 54% yield of the substituted pyridine (entry
1). Heptane was inferior to toluene (entries 2 and 3). It was
noted that polar aprotic solvents such as DMF, DMAc, DMSO,
and NMP were almost ineffective for the reaction, with less than
5% yield recorded (see the ESI† for details).
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Scheme 2 Control experiments.
With these results, the inuence of temperature on the product was detected when the temperature was lower than
reaction was explored. The reaction was conducted at different 120 ꢀC, and 17% yield was detected for the reaction performed
temperatures, ranging from room temperature to 140 ꢀC. No at 120 ꢀC (entry 4). Additionally, the effect of the catalyst amount
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on the yield of 2,4,6-triphenylpyridine was investigated. Only yield of 2,4,6-triphenylpyridine was addressed. The experiment
4% yield was detected in the absence of the strontium-doped was conducted under standard conditions, using a variety of
lanthanum cobaltite perovskite catalyst, indicating that the inorganic and organic oxidants. The reaction using K2S2O8 as
perovskite was critical for the oxidative cyclization reaction the oxidant gave a sluggish crude mixture, with only 8% yield
(entry 5). Using 1 mol% catalyst, the reaction afforded 21% yield (entry 9). Similarly, AgNO3 and oxygen were not suitable for this
(entry 6). As expected, increasing the perovskite quantity reaction (entries 10 and 11, respectively). tert-Butyl hydroper-
enhanced the yield of the triphenylpyridine product. Extending oxide in water was somewhat effective, affording the triphe-
the catalyst amount to 3 mol% led to 51% yield, while 87% yield nylpyridine product in 55% yield (entry 12).
was achieved in the presence of 7 mol% catalyst (entries 7 and 8,
respectively).
Since the synthesis of 2,4,6-triphenylpyridine via oxidative
cyclization between acetophenone oxime acetate and phenyl-
An oxidant should be present for the oxidative functionali- acetic acid was conducted in toluene, a critical issue is that the
zation of the sp3 C–H bond in phenylacetic acid, which facili- leached species from the strontium-doped lanthanum cobaltite
tates the cyclization.37 Accordingly, the impact of oxidant on the perovskite may be accountable a considerable part of the
Scheme 3 Proposed reaction mechanism.
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that almost no additional 2,4,6-triphenylpyridine was generated
in the absence of the strontium-doped lanthanum cobaltite
perovskite (Fig. 1). These results veried that the perovskite-
catalyzed oxidative cyclization between acetophenone oxime
acetate and phenylacetic acid proceeded under truly heteroge-
neous conditions.
To emphasize the advantages of using the strontium-doped
lanthanum cobaltite perovskite catalyst, its activity in the
oxidative cyclization reaction was compared to that of other
catalysts (Table 2). The reaction was conducted in toluene at
140 ꢀC for 8 h, using 3 equivalents of ketoxime and 3 equivalents
of DTBP, in the presence of 5 mol% catalyst. First, traditional
homogeneous catalysts were utilized for the reaction (entries 1–
10). The FeCl3-catalyzed transformation afforded 26% yield,
while 6% yield was observed for the reaction using the FeCl2
catalyst. Fe(NO3)3 and FeSO4 displayed a low catalytic perfor-
mance for the reaction, with 3% and 13% yields being detected,
respectively. In the case of copper salts, the use of CuSO4
resulted in 57% yield, while 8% yield was noted for the reaction
using a Cu(NO3)2 catalyst. NiSO4 demonstrated a low catalytic
performance, although 38% yield was recorded. The three
precursors used to synthesize the strontium-doped lanthanum
cobaltite perovskite catalyst, including La(NO3)3, Sr(NO3)2, and
Co(NO3)2, also exhibited low catalytic activity, forming the ex-
pected product in 2%, 39%, and 4% yields, respectively. In
a second series of experiments, several heterogeneous catalysts
were utilized for the reaction (entries 11–18). Cu(BDC),
a copper–organic framework, was active for the reaction,
producing the product in 70% yield. The reaction using a ZIF-8A
catalyst, a zinc–organic framework, progressed to 60% yield.
UiO-66$TFA, a zirconium–organic framework, was less reactive
toward the oxidative cyclization reaction, generating the desired
product in 40% yield. The single oxides La2O3, CoO, and SrO
exhibited low activity, though 38% yield was detected for the
case of SrO (entries 14–16). The pristine perovskite, LaCoCo3,
was also not very active towards the transformation, although
40% yield was detected (entry 17). Interestingly, using the
strontium-doped perovskite La0.6Sr0.4CoO3 as a catalyst for the
oxidative cyclization reaction led to 83% yield (entry 18).
To comprehend the mechanism of the oxidative cyclization
reaction, several control experiments were performed, as high-
lighted in Scheme 2. The presence of (2,2,6,6-
tetramethylpiperidin-1-yl)oxy (TEMPO) as a radical scavenger
had a signicant effect on the formation of benzaldehyde (5) in
the rst step, which implied the involvement of a radical
process in the oxidative decarboxylation of phenylacetic acid (1)
(Scheme 2a). In the next two experiments, high conversions to
benzaldehyde were achieved, which indicated that 2-hydroxy-2-
phenylacetic acid (3) and 2-oxo-2-phenylacetic acid (4) were the
likely intermediates of this process (Scheme 2b and c, respec-
tively). When the oxidative cyclization reaction between ketox-
ime (7) and 2-oxo-2-phenylacetic acid (4) was subsequently
carried out under standard conditions, 2,4,6-triphenylpyridine
(16) was obtained in 86% yield, suggesting that 2-oxo-2-
phenylacetic acid may be an important intermediate (Scheme
2d). The cyclization of the ketoxime (7) and benzaldehyde (5)
generated 2,4,6-triphenylpyridine (16) in high yield (92%),
Fig. 2 Reutilization of the La0.6Sr0.4CoO3 catalyst.
catalytic efficiency. Thus, to clarify if leaching was serious in
this reaction, a control experiment was performed. The reaction
was carried out in toluene under argon at 140 ꢀC for 8 h using 3
equivalents of ketoxime and 3 equivalents of DTBP in the
presence of 5 mol% catalyst. Aer the rst 160 min reaction
time with 47% yield of 2,4,6-triphenylpyridine recorded, the
perovskite catalyst was removed by centrifugation. The reaction
solution was transferred to a new 12 mL screw-cap pressurized
vial. The reaction vial was back-lled with argon and the
resulting mixture was subsequently stirred at 140 ꢀC for an
additional 320 min. Aliquots were taken at different time
intervals, and subsequently analyzed by GC. The GC data
provided quantitative information regarding the residual,
catalytically active sites in the toluene phase. It was observed
Fig. 3 XRD results for the fresh (a) and reutilized (b) La0.6Sr0.4CoO3
perovskite catalysts.
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which demonstrates that benzaldehyde (5) may be the key reutilization. In the best case, it should be possible to recover
intermediate in this reaction (Scheme 2e). The reaction between and reutilize the solid catalyst many times before it ultimately
the ketoxime (7) and benzaldehyde (5) did not proceed in the becomes totally deactivated. Accordingly, the reutilization of the
presence of the antioxidant TEMPO, indicating that a radical La0.6Sr0.4CoO3 catalyst was investigated for the synthesis of
process may be involved and that the interaction of TEMPO with 2,4,6-triphenylpyridine via the oxidative cyclization reaction
the radical species generated in the catalytic cycle may have between acetophenone oxime acetate and phenylacetic acid
cease the transformation (Scheme 2f). (Z)-3-Amino-1,3- under the standard conditions. The reaction was conducted in
diphenylprop-2-en-1-one (12) was synthesized and allowed to toluene at 140 ꢀC for 8 h using 3 equivalents of ketoxime and 3
undergo the cyclization with the ketoxime (7) under two equivalents of DTBP, in the presence of 5 mol% catalyst. At the
different conditions. The yield of 2,4,6-triphenylpyridine in the end of each catalytic run, the La0.6Sr0.4CoO3 catalyst was
cyclization without the La0.6Sr0.4CoO3 catalyst was similar to collected from the reaction mixture by centrifugation, washed
that of the cyclization using the La0.6Sr0.4CoO3 catalyst (Scheme carefully with ethyl acetate, acetone, and diethyl ether. The
2g). The reaction between (1), (7), and (7b) produced 3 products recovered catalyst was dried under reduced pressure at room
(16, 16b, and 16c) in similar yields under the standard condi- temperature overnight on a Schlenk line, and reutilized for the
tions (Scheme 2h). These results suggest that (Z)-3-amino-1,3- next experiment. The experimental results indicated that the
diphenylprop-2-en-1-one (12) may be the intermediate in this La0.6Sr0.4CoO3 catalyst could be reutilized several times in the
reaction, and that the transformation of (Z)-3-amino-1,3- oxidative cyclization reaction. At the h catalytic run, 82%
diphenylprop-2-en-1-one (12) to 2,4,6-triphenylpyridine (16) yield of 2,4,6-triphenylpyridine was recorded (Fig. 2). Moreover,
could proceed in the absence of the La0.6Sr0.4CoO3 catalyst.
the recovered La0.6Sr0.4CoO3 catalyst was characterized via XRD
Based on the aforementioned results and previous and FT-IR analyses, and the results were compared with that of
reports,8,37,40–43 a plausible mechanism is proposed in Scheme 3. the new strontium-doped perovskite. The XRD (Fig. 3) and FT-IR
The rst step is the aerobic oxidation of phenylacetic acid (1) to (Fig. 4) observations revealed that the La0.6Sr0.4CoO3 catalyst
benzaldehyde (5) via the formation of 2-hydroxy-2- phenylacetic was stable in the synthesis of 2,4,6-triphenylpyridine via the
acid (3) and 2-oxo-2-phenylacetic acid (4), followed by the single oxidative cyclization reaction under the standard conditions.
electron transfer (SET) process between benzaldehyde and Mn+1
With these achievements, we consequently extended the
to generate benzoyl radical (6). It should be noted that scope of this work to the synthesis of several 2,4,6-triphe-
a benzylic radical (2) is formed in this process. The next step is nylpyridines via oxidative cyclization reactions between ketox-
the reductive cleavage of the N–O bond of the ketoxime (7) by Mn imes and phenylacetic acids utilizing the La0.6Sr0.4CoO3
via a single electron transfer (SET) to achieve the Mn+1 species perovskite catalyst (Table 3). The reaction was conducted in
and imine radical (8), which undergo rapid coordination to Mn toluene at 140 ꢀC for 8 h using 3 equivalents of ketoxime and 3
to give the imino-Mn+1 intermediate (9). Tautomerization of the equivalents of DTBP, in the presence of 5 mol% catalyst. At the
imino-Mn+1 intermediate (9) forms the enamino-Mn+1 interme- end of the experiment, the triphenylpyridine product was
diate (10), which reacts with the benzoyl radical (6) to generate puried by column chromatography. First, different acetophe-
a new radical (11). This radical undergoes a single electron none oxime acetates were employed for the oxidative cyclization
transfer (SET) process, resulting in the formation of interme- with phenylacetic acid. Under these conditions, 2,4,6-
diate (12) upon regeneration of the Mn species. Subsequently,
the condensation of the intermediate (12) with a second mole-
cule of the ketoxime (7) produces the intermediate (13) and
releases NH2OAc. Tautomerization of intermediate (13) affords
intermediate (14), followed by the intramolecular cyclization of
intermediate (14), generating intermediate (15). Finally, the
dehydration of intermediate (15) produces 2,4,6-triphenylpyr-
idine (16). In this catalytic cycle, M is the cobalt species in the
La0.6Sr0.4CoO3 catalyst. It should be noted that using CoO as
a catalyst for the oxidative cyclization reaction resulted in a very
low yield (entry 15, Table 2). The pristine perovskite, LaCoCo3,
was also not very active towards the transformation (entry 17,
Table 2), while utilizing the strontium-doped perovskite, La0.6
-
Sr0.4CoO3 as the catalyst afforded 80% yield (entry 18, Table 2).
It was previously reported that substituting the La(III) site by
Sr(II) site in the perovskite will improve the mobility of lattice
oxygen, thus signicantly enhancing the redox activity of the
catalyst.24,44 Certainly, the synergistic effect in the La0.6Sr0.4CoO3
catalyst is critical for the oxidative cyclization reaction.
One critical aspect that must be addressed for the oxidative
cyclization reaction using the strontium-doped lanthanum
Fig. 4 FT-IR observations for the fresh (a) and reutilized (b) La0.6
-
cobaltite perovskite catalyst is the possibility of catalyst Sr0.4CoO3 perovskite catalysts.
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Table 3 Synthesis of 2,4,6-triphenylpyridines via oxidative cyclization reaction between ketoximes and phenylacetic acids utilizing the La0.6
-
Sr0.4CoO3 perovskite catalysta
Entry
Reactant 1
Reactant 2
Product
Yieldb (%)
1
79
2
3
4
62
74
67
5
55
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Table 3 (Contd.)
Entry
Reactant 1
Reactant 2
Product
Yieldb (%)
6
68
7
73
8
69
9
72
10
54
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Table 3 (Contd.)
Entry
Reactant 1
Reactant 2
Product
Yieldb (%)
11
53
12
72
13
53
14
59
15
50
a
Reaction conditions: phenylacetic acids (0.2 mmol), ketoximes (0.6 mmol), DTBP (0.6 mmol), toluene (2 mL), La0.6Sr0.4CoO3 catalyst (5 mol%), 8 h
b
at 140 ꢀC under air for 2 h and under argon for 6 h. Isolated yield.
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triphenylpyridine was obtained in 79% yield (entry 1). Aceto-
phenone oxime acetates possessing an electron-donating
substituent on the benzene ring were also reactive towards the
reaction. Under the standard conditions, 4-phenyl-2,6-di-p-tol-
ylpyridine (entry 2), 2,6-bis(3-methoxyphenyl)-4-phenylpyridine
(entry 3), and 2,6-bis(2-methoxyphenyl)-4-phenylpyridine
(entry 4) were generated in 62%, 74%, and 67% yields, respec-
tively. Similarly, acetophenone oxime acetates containing an
electron-withdrawing substituent on the benzene ring were
utilized, producing 2,6-bis(4-chlorophenyl)-4-phenylpyridine
(entry 5), 2,6-bis(3-chlorophenyl)-4-phenylpyridine (entry 6),
and 2,6-bis(4-bromophenyl)-4-phenylpyridine (entry 7) in 55%,
68%, and 73% yields, respectively. (E)-3,4-Dihydronaphthalen-
1(2H)-one O-acetyl oxime was reactive, and the reaction afforded
the corresponding product in 69% yield (entry 8). Using (E)-1-
(thiophen-2-yl)ethan-1-one O-acetyl oxime led to 72% yield of
the desired product (entry 9). Moving to phenylacetic acids
containing a substituent, the oxidative cyclization reactions
with acetophenone oxime acetate afforded the corresponding
triphenylpyridine products in reasonable yields (entries 10–12).
Heterocyclic acetic acids were also competent substrates,
affording the products in moderate yields (entries 13–15).
Acknowledgements
The Vietnam National University – Ho Chi Minh City (VNU-
HCM) is acknowledged for nancial support via project no.
NV2019-20-02.
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Conflicts of interest
There are no conicts to declare.
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