Merrifield resin-supported quinone as an efficient biomimetic catalyst for metal-free, base-free, chemoselective synthesis of 2,4,6-trisubstituted pyridines

Article abstract of DOI:10.1039/c9gc02409c

Metal-free, base-free, biomimetic and chemoselective synthesis of 2,4,6-trisubstituted pyridines was developed under mild conditions for the first time. The heterogeneous biomimetic catalyst-recoverable Merrifield resin-supported quinone-was fully characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectrometry (XPS) and energy dispersive X-ray spectroscopy (EDX). This supported quinone catalyst exhibited excellent catalytic reactivity for chemoselective synthesis of 2,4,6-trisubstituted pyridines, providing an efficient and green method for the synthesis of pyridine derivatives under mild conditions. Mechanistic investigations were conducted to gain insights into the heterogeneous biomimetic catalyst as well as the resulting transformation. The successful capture of intermediates offered direct and clear evidence for the proposed mechanism.

Full text of DOI:10.1039/c9gc02409c

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Merrifield resin-supported quinone as an efficient
biomimetic catalyst for metal-free, base-free,
chemoselective synthesis of 2,4,6-trisubstituted
pyridines†
Cite this: Green Chem., 2019, 21,
5683
Qing Yang,a Yilin Zhang,b Wei Zeng,a,c Zheng-Chao Duan,a Xinxin Sanga and
a
Dawei Wang
*
Metal-free, base-free, biomimetic and chemoselective synthesis of 2,4,6-trisubstituted pyridines was
developed under mild conditions for the first time. The heterogeneous biomimetic catalyst – recoverable
Merrifield resin-supported quinone – was fully characterized by Fourier transform infrared spectroscopy
(FT-IR), X-ray photoelectron spectrometry (XPS) and energy dispersive X-ray spectroscopy (EDX). This
supported quinone catalyst exhibited excellent catalytic reactivity for chemoselective synthesis of 2,4,6-
trisubstituted pyridines, providing an efficient and green method for the synthesis of pyridine derivatives
under mild conditions. Mechanistic investigations were conducted to gain insights into the heterogeneous
biomimetic catalyst as well as the resulting transformation. The successful capture of intermediates
offered direct and clear evidence for the proposed mechanism.
Received 14th July 2019,
Accepted 17th September 2019
DOI: 10.1039/c9gc02409c
rsc.li/greenchem
ation of o-naphthoquinone catalysts in the presence of cocata-
Introduction
lysts such as acids and bases.6a Afterward, the same group
Quinones are useful building blocks and important precursors identified an o-naphthoquinone-catalyzed oxidative deamina-
for a wide range of biological compounds, such as pharmaceu- tion reaction where molecular oxygen and water serve as
ticals, antibiotics, anticancer drugs, pigments and dyes.1 the sole oxidant and nucleophile respectively.6b In 2017, Clift
Scientists have put in a lot of effort and achieved great progress and co-workers developed a sequential quinone-catalyzed oxi-
in this area.2 Notably, scientists have found the significant use dative decarboxylation/Mukaiyama–Mannich addition, which
of quinones in organic catalysis in recent years.3 For example, provided a highly efficient route for α-amino acid homologa-
Stahl et al. reported the quinone-catalyzed aerobic oxidative tion under mild reaction conditions.7 Luo et al. proved that a
coupling of primary benzylic amines to afford secondary simple ortho-quinone catalyst could effectively facilitate the
imines and the dynamic self-sorting of imine products in good oxidative synthesis of heterocycles such as quinoxalines, ben-
yields.4 Later, they developed a new method for the aerobic zoxazoles and benzimidazoles from primary amines in good
oxidation of secondary amines which employs 1,10-phenan- yields with oxygen as the oxidant.8a In 2019, Luo et al. identi-
thro-line-5,6-dione as a bifunctional o-quinone catalyst via a fied the mechanism of aerobic oxidation of α-branched cyclic
biological pathway involving a hemiaminal intermediate.5a In tertiary amines, secondary amines, and primary amines in a
2016, Kim and Oh reported a highly modular aerobic dehydro- bioinspired ortho-quinone catalytic system.8c From a sustain-
genation protocol for primary and secondary amines, and able-development and green chemistry standpoint, there is an
cross-couplings of amines were obtained through the utiliz- urgent need to develop a recoverable catalytic system to
achieve atom economical reactions with higher selectivity and
under more environmentally benign conditions, such as dehy-
drogenation reactions.9,10
aKey Laboratory of Synthetic and Biological Colloids, Ministry of Education,
Functionalized 2,4,6-trisubstituted pyridines are one of the
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122,
most important and useful heterocyclic compounds in
modern industry.11 These are important intermediates that
find wide applications and have been diversified into subunits
such as anti-oxidation materials, anti-bacterial materials and
anticancer drugs in pharmaceuticals and pesticides in agricul-
ture. The conventional method for the synthesis of 2,4,6-tri-
China. E-mail: wangdw@jiangnan.edu.cn
bC. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown,
West Virginia 26506, USA
cDalian Wondersun Biochemical Technology Co. LTD., Double D4 Street,
Development Zone, Dalian 116600, Liaoning Province, China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9gc02409c
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substituted pyridines is based on the modified multicompo-
nent pyridine transformation from an enolizable ketone and
aldehyde and uses ammonium salt as the nitrogen source in
the presence of a catalyst at high reaction temperature.12
Recently, several methodologies have been developed for the
synthesis of 2,4,6-triarylpyridines, such as the condensation of
keto-oximes with aryl aldehydes,13a,b the reaction of amino
acids with aryl ketones,13c taking Eosin Y13d as a photoredox
agent or using HOTf,13e,f copper,13g–i iron,13j palladium,13k
Iodine,13l B(C6F5)3,13m and 4,6-dihydroxysalicylic acid13n as cat-
alysts. Based on the previous dehydrogenation and borrowing
hydrogen reactions,14–16 we recently developed several ruthe-
nium and iridium catalysts and studied their catalytic
activity.17 However, these ruthenium and iridium complexes
are costly and lack reusability, resulting in huge waste for dehy-
drogenation reactions.18 As part of our continuing efforts in
the development of green methods for borrowing hydrogen
reactions and quinone transformations,19 herein, we report
that heterogeneous Merrifield resin-supported quinone can
effectively catalyze the synthesis of 2,4,6-trisubstituted pyri-
dines in high yields with good recovery performance.
Fig. 1 FT-IR spectra of (a) the Merrifield resin, (b) NQ-MR, and (c) NQ.
on the benzene ring. For NQ (Fig. 1c), the broad peak around
3261 cm−1 belonged to –OH and the strong peak at 1130 cm−1
(characteristic absorption peak of the C–O bond) reflected the
stable existence of the phenolic hydroxyl group in NQ com-
pounds. Delightfully, a peak at 1677 cm−1 was found for the
supported catalyst NQ-MR (Fig. 1b) which was similar to the
carbonyl (CvO) absorption in NQ, thereby indicating the suc-
cessful coupling of NQ on the Merrifield resin. Meanwhile, the
disappearance of the hydroxyl characteristic peak from the
Results and discussion
Preparation of the NQ-MR catalyst
First, 2,3-dichloro-1,4-naphthoquinone was added into KOH
aqueous solution. The reaction mixture was stirred at 70 °C for
3 hours and then acidified with a few drops of concentrated
hydrochloric acid until pH ≈ 2. After purification, a yellow
solid was obtained, which is the target product 2-chloro-3-
hydroxy-1,4-naphthoquinone (NQ, up to 82% yield). Then, an
appropriate amount of NQ was mixed with the Merrifield resin
(MR) swollen in DMF solution and processed for 18 hours at
60 °C in the presence of NaH according to Williamson’s syn-
thesis (Scheme 1).20 Finally, after ultrasonic washing and
drying, the desired catalyst Merrifield resin-supported
naphthoquinone (NQ-MR) was acquired (for the detailed
preparation process, please see the ESI†).
NQ-MR spectrum and the emergence of
a C–O bond
(1153 cm−1) in the fingerprint region can further confirm the
successful conversion of NQ to NQ-MR.
The energy-dispersive X-ray (EDX) spectra of the MR and
NQ-MR (Fig. 2) reveal the existence of the expected elements
(C, O, and Cl) in the structure. An increase in the oxygen
content could be attributed to the addition of carbonyl oxygen
in the quinones. Next, Fig. 3 clearly displays the uniform distri-
bution of Cl atoms on the surface of the carrier material (for
more details, see the ESI†).
Finally, the surface elemental chemical states and the con-
tents of the resin-loaded chloride quinone catalyst were investi-
gated by XPS. As shown in Fig. 4a, the characteristic peaks of
O 1s, C 1s and Cl 2p for NQ-MR appear at ∼532.8 eV, ∼284.8
Characterization of the NQ-MR catalyst
After NQ-MR synthesis, Fourier transform infrared spec-
troscopy (FT-IR), energy dispersive X-ray spectroscopy (EDX)
and X-ray photoelectron spectrometry (XPS) were used to
characterize the supported catalyst NQ-MR. The FT-IR spectra
of the Merrifield resin, NQ-MR and NQ are shown in Fig. 1.
The Merrifield resin (Fig. 1a) shows two characteristic peaks at
1265 and 676 cm−1, corresponding to the chloromethyl group
Scheme 1 The synthesis of supported NQ-MR.
5684 | Green Chem., 2019, 21, 5683–5690
Fig. 2 EDX pattern of the Merrifield resin and NQ-MR.
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confirmed the fact that a quinone relative resin was obtained,
which was grafted to the surface of a support at a weight per-
centage of 32% approximately (for more details, see the ESI†).
Catalytic activity
Initially, we investigated the synthesis of 2,4,6-triphenylpyri-
dine (3a) using simple acetophenone (1a) and benzylamine
(2a) in the presence of 10 mol% 2-chloro-1,4-naphthoquinone
at 80 °C in MeOH (Table 1, entry 1), resulting in 45% product
yield. Unfortunately, 1,4-benzoquinone exerted very poor
efficiency towards 3a (Table 1, entry 2). Compared with the
unsubstituted benzoquinone (Scheme 2), catalysts containing
hydroxyl groups and chlorine were able to achieve 3a in yields
of 25% and 38%, respectively (Table 1, entries 3 and 4 versus
entry 2). Further experiments demonstrated that monosubsti-
tuted naphthoquinones tend to exhibit good catalytic perform-
ance in this reaction. It was worth noting that the yield of 3a
was up to 72% when taking 2-Cl-3-OBn-NQ as the catalyst,
probably due to the co-effect of oxygen and chlorine in the
ortho position of the carbonyl group in the naphthoquinone
structure (Table 1, entry 7). Subsequently, with the loaded cata-
Fig. 3 SEM image of (a) the MR and EDX elemental maps of (b) MR, (c)
C atoms, and (d) Cl atoms.
Table 1 Screening of reaction conditionsa
Entry
Catalyst
Solvent
Ratio(3a/3a′/3″)
Yield(3a)b [%]
1
2
3
4
5
6
7
8
9
2-Cl-NQ
BQ
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeCN
DCE
EtOH
Toluene
H2O
EtOH
none
96/4/0
—
—
93/7/0
85/15/0
—
90/10/0
94/6/0
78/19/3
—
100/0/0
87/13/0
—
—
—
45
<5
25
38
31
20
72
74
62
23
78
66
7
Fig. 4 XPS spectra of NQ-MR (a) surface. C 1s (b), O 1s (c) and Cl 2p
(d) core-level spectra of catalyst surface.
2,5-DH-BQ
2,6-DCBQ
5-OH-NQ
2,3-DCNQ
2-Cl-3-OBn-NQ
NQ-MR
NQ-MR
NQ-MR
NQ-MR
NQ-MR
NQ-MR
—
eV and ∼200.8 eV, respectively, which is consistent with the
chemical composition of the sample. The Gaussian curve of
the C 1s core-level spectrum (Fig. 4b) after the initial fitting
was deconvoluted into three peaks of varying intensities at 10
11
12
286.6 eV (CvO), 285.8 eV (C–O/C–Cl) and 284.8 eV (C–C/C–
H/CvC), in agreement with original expectation. The O 1s
13
signal (Fig. 4c) shows two peaks at 532.0 eV and at 533.0 eV, 14
<5
<5
15
NQ-MR
corresponding to the CvO bond in chloride quinone and a C–
a Reagents and conditions: 1a (2.0 mmol), 2a (1.5 mmol), catalyst
(10 mol%), solvent (3.0 mL), 80 °C or reflux, air, 24 h, “—” = not deter-
mined; the ratio was determined by GC. b Yields of the isolated
product.
O–C bond produced in the resin and catalyst reaction. The Cl
2p region (Fig. 4(d)) shows four distinct peaks: the Cl 2p1/2
main peak at 201.8 eV and its satellite at 198.4 eV, as well as
the Cl 2p3/2 main peak at 200.2 eV and its satellite at 197.5 eV.
Therefore, it is certain that most of the chloromethyl groups
on the surface of the Merrifield resin were consumed in the
reaction, as can be seen from the two minor satellite peaks
belonging to free Cl−1 (198.4 eV and 197.5 eV). These results
well confirmed the previous speculations. Furthermore, com-
bining the elemental content and the resulting peak area, we Scheme 2 Several benzoquinones and naphthoquinones.
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lyst NQ-MR in hand, our efforts were focused on improving the para- and meta-substituted acetophenones, containing elec-
catalytic efficiency of NQ-MR. To our delight, the supported tron-donating and electron-withdrawing groups, could be con-
quinone achieved relatively high yield in MeOH, under the verted to the corresponding 2,4,6-triarylsubstituted symmetri-
same reaction conditions (Table 1, entry 8). Inspired by this cal pyridines in 58–91% yields (3a–3l). In particular, the ortho-
result, solvent impacts on this system were taken into con- substituted aromatic ethyl ketone reacts only with 4-fluoroace-
sideration and investigated next. Solvents including MeOH, tophenone (3m), possibly due to steric hindrance.
MeCN, DCE, EtOH, toluene and water were tested as the reac- Furthermore, 3,4-dichloro and 2,4-difluoro substituted aryl
tion media specifically, and EtOH was found to be the best ketones yielded the desired products 3n and 3o in 59% and
solvent for this reaction (Table 1, entries 8 to 13). Furthermore, 80% yields. In addition, the use of the heterocyclic ethyl
at 80 °C in EtOH, no 3a product was formed in the absence of ketone, naphthalene ketone and biphenyl ketone can also
a catalyst, suggesting that a catalyst was crucial to this reaction promote the synthesis of symmetric pyridines (3p–3s). For
(Table 1, entry 14). It should be noted that the reaction could amine substrates, both substitutions at the para- (3aa–3ad),
not take place under solvent-free conditions (entry 15).
meta- (3ae–3ag) and ortho- (3ah–3aj) positions were all well-tol-
Next, we studied the scope of this reaction with respect to erated. It was noteworthy that in the amine substrate part,
different functional group components (Table 2). Different polysubstituted benzylamine, heterocyclic methylamine and
alkyl amine provided the corresponding products (3ak, 3al and
3am) under the reaction conditions. However, it was observed
that only a mixture was obtained, when the reaction was set up
Table 2 Substrate expansion of reactionsa,b
with two different ketones in one reaction.
Mechanism exploration
Catalyst investigation
This metal-free and base-free catalytic system revealed high
activity in the synthesis of 2,4,6-triphenylpyridine derivatives
without photosensitizers and light. It was observed in the fol-
lowing control experiments that simple benzoquinones are
ineffective in catalyzing this reaction and all kinds of naphtho-
quinones are only able to produce a low-to-moderate yield of
2,4,6-triphenylpyridine. A further control experiment revealed
that the carrier Merrifield resin could not promote this reac-
tion. Meanwhile, the blank experiment disclosed that the reac-
tion could not take place without a catalyst (Scheme 3). When
supported NQ-MR was applied to the synthesis with no photo-
sensitizers or light implemented, a high catalytic activity was
obtained as anticipated, most likely due to the synergistic
effect of the 1,4-naphthoquinone and Merrifield resin.
Capture of intermediates
The detection, trapping, and even separation of key intermedi-
ates are considered as an efficient means to explore the poss-
ible pathway or the reaction mechanism. A control reaction of
benzyl amine without acetophenone under NQ-MR conditions
was conducted and the results showed an 83% yield for the
a Reagents and conditions: 1a (2.0 mmol), 2a (1.5 mmol), catalyst
(10 mol%), EtOH (3.0 mL), 80 °C, air, 24 h. b Yields of the isolated
product.
Scheme 3 The catalyst exploration experiments.
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Scheme 4 The capture of intermediates.
N-benzyl-1-phenylmethanimine intermediate (4). Further
investigation demonstrated that p-methyl-acetophenone (1b)
could continue to react with N-benzyl-1-phenylmethanimine to
produce 3b in a slightly higher yield (81%). Therefore,
N-benzyl-1-phenylmethanimine is believed to be the key reac-
tion intermediate to understand the reaction mechanism
(Scheme 4).
To gain insights into the reaction pathway, p-methyl-
benzaldehyde was selected as a partner to examine the reac-
tion. The mixture of acetophenone (1a), p-fluoro-benzylamine
(2b) and p-methylbenzaldehyde was stirred under the optimal
conditions with NQ-MR as the catalyst. The results showed
that both products 3aa and 3ab were obtained in 47% and
21% yields, respectively. This disclosed that an aldehyde was a
possible intermediate during the synthesis of 2,4,6-triphenyl-
pyridine derivatives (Scheme 5).
Further mechanism exploration was also performed using
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) (1 eq.), a useful
radical scavenger, to investigate the possibility of the radical
process in this reaction (Scheme 6). The experimental results
disclosed that TEMPO could not prevent the production of
Scheme 7 The possible reaction mechanism.
2,4,6-triphenylpyridine 3a, implying that the involvement of nisms for the synthesis of 2,4,6-triphenylpyridine derivatives,
free-radical intermediates should be ruled out during this as shown in Scheme 7. The first step of pathway I was the con-
NQ-MR catalyzed reaction.
densation of benzyl amine (2a) with NQ-MR to generate inter-
Based on the above explorations and previous mediate A. After isomerization, imine B was attacked by
references,13a,b,d we proposed two plausible reaction mecha- another benzyl amine to produce C. Subsequently, intermedi-
ate C could easily release imine D, which underwent isomeri-
zation and gave again the first intermediate (A). Next, imine D
reacted with enolate (G) to give compound H. After dehydro-
genation, intermediate I successively reacted with enolate (G)
and ammonia to form tetrahydropyridine (K), which under-
went the second dehydrogenation to complete the reaction.
For the second pathway, benzyl amine was easily converted
into an aldehyde (5) under NQ-MR conditions. Next, the unsa-
turated ketone (6) was generated through the condensation of
benzyl amine (2a) with acetophenone (1a). After isomerization,
Scheme 5 Mechanism exploration experiments.
enolate (G) could react with ammonia to form enamine (L),
which could attack the unsaturated ketone (6) to give inter-
mediate N. After oxidation in air, the desired product (3a) was
formed. In this methodology, N-benzyl-1-phenylmethanimine
(4), benzaldehyde (5) and chalcon (6), which are the most
important key intermediates, were all detected, indicating that
two pathways were both possible (Scheme 7). These results
were in line with previous results13h and supported by the
Scheme 6 Mechanism exploration experiments.
literature.13i
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Scheme 9 The synthesis of 2,4,6-triphenylpyridine on a gram scale.
was 3.12% (w/w). Therefore, the results revealed that the leach-
ing of NQ-MR was negligible, and NQ-MR showed good stabi-
lity for the synthesis of 2,4,6-trisubstituted pyridines.
Finally, the gram-scale synthesis of 2,4,6-triphenylpyridine
was carried out with NQ-MR as the catalyst. Over 10 g of
desired pyridine derivative can be separated in 73% yield,
suggesting that NQ-MR was a truly excellent catalyst for this
transformation under mild conditions (Scheme 9).
4. Conclusions
Fig. 5 Hammett plot of para-substituted ketones.
In conclusion, we developed a class of novel heterogeneous
chloride quinones, to be supported on the Merrifield resin as
an efficient catalyst. It was thoroughly characterized by Fourier
transform infrared spectroscopy (FT-IR), X-ray photoelectron
spectrometry (XPS) and energy dispersive X-ray spectroscopy
(EDX). This supported quinone catalyst exhibited good cata-
lytic efficiency for the chemoselective synthesis of 2,4,6-trisub-
stituted pyridines under mild conditions. Compared with
strong acid or metal catalysts, this NQ-MR system was a much
greener and milder catalyst for the synthesis of 2,4,6-trisubsti-
tuted pyridines, which provided a new idea for organic
catalysis.
To gain more insights into the possible reaction mecha-
nism, we employed the Hammett plot equation and used
several typical starting materials to build the Hammett plot
equation (Fig. 5). The results revealed that this reaction was
greatly impacted by the electronic effect, and a negative charge
on substrates might be favored during the supposed transition
state.
Reusability of the catalyst
In addition, the catalyst NG-MR was dealt and centrifuged
several times with water, dried and recycled for reuse in the
synthesis of 2,4,6-triphenylpyridine. We found that the reac-
tion yield of 3a gradually decreased with the number of cycles
increased, despite the fact that good yield was still attainable
even after five trials (Scheme 8). To better illustrate this
NQ-MR catalyst, the hot filtration experiments were set up. The
amount of the NQ-MR catalyst leaching into water for the
coupling of acetophenone and benzylamine at 80 °C was
detected by comparing the content of the carbonyl group
before and after each reaction cycle. The results revealed that
the amount of the carbonyl group leaching after the fifth run
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We gratefully acknowledge the financial support of this work
from the National Natural Science Foundation of China
(21776111 and 21861039), the Fundamental Research Funds
for the Central Universities (JUSRP 51627B) and MOE & SAFEA
for the 111 Project (B13025).
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