Synthesis of Kr?hnke pyridines through iron-catalyzed oxidative condensation/double alkynylation/amination cascade strategy

Article abstract of DOI:10.1016/j.tet.2021.132429

An efficient protocol for the synthesis of symmetrical and unsymmetrical 2,4,6-trisubstituted pyridines via oxidative cascade annulation of arylacetylenes with benzylamines has been developed. The reaction proceeds smoothly utilizing iron(II) triflate as a catalyst and molecular oxygen as an oxidant with broad substrate scope. Mechanistic studies reveal that the reaction may be experiences an oxidative condensation followed by double alkynylation and amination process.

Full text of DOI:10.1016/j.tet.2021.132429

Tetrahedron xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
€
Synthesis of Krohnke pyridines through iron-catalyzed oxidative
condensation/double alkynylation/amination cascade strategy
Kovuru Gopalaiah*, Renu Choudhary
Department of Chemistry, University of Delhi, Delhi, 110007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient protocol for the synthesis of symmetrical and unsymmetrical 2,4,6-trisubstituted pyridines
via oxidative cascade annulation of arylacetylenes with benzylamines has been developed. The reaction
proceeds smoothly utilizing iron(II) triflate as a catalyst and molecular oxygen as an oxidant with broad
substrate scope. Mechanistic studies reveal that the reaction may be experiences an oxidative conden-
sation followed by double alkynylation and amination process.
Received 23 June 2021
Received in revised form
17 August 2021
Accepted 27 August 2021
Available online xxx
© 2021 Elsevier Ltd. All rights reserved.
Keywords:
Oxidative cascade annulation
2,4,6-Trisubstituted pyridines
Double alkynylation
Iron catalyst
Arylacetylenes
1. Introduction
of aldehydes with amino allenes followed by palladium-catalyzed
cyclization [12]. However, harsh reaction conditions or the utili-
The pyridines, especially the 2,4,6-trisubstituted pyridines
zation of stoichiometric amounts of reagents or expensive catalysts
are often involved in these reactions.
€
(Krohnke pyridines) are important class of nitrogen containing
heterocycles, shows a wide range of biological activities such as
antibacterial, antifungal, vasodilator, antidepressant, antitumor,
anticancer, etc. [1] These pyridines have been utilized in the syn-
thesis of photoluminescent polymers [2], polyimides [3], chemo-
sensors [4], ligands in catalysis [5], and also in photodynamic
cancer therapy because of structural resemblance of these pyri-
dines with symmetrical triarylthiopyrylium, telluropyrylium, and
selenopyrylium photosensitizers [6]. They are useful intermediates
in the preparation of drugs, agrochemicals, surfactants, and desic-
To address the above-mentioned drawbacks, one recent bur-
geoning strategy is to construct these pyridines via earth-abundant
first row transition-metal catalyzed oxidative coupling/annulation
reaction [13e16]. Indeed, such a strategy could complement the
classic methods in terms of reactivity, reaction parameters, selec-
tivity, functional group tolerance and substrate scope. Jiang and co-
workers, for example, have first developed the copper-catalyzed
oxidative coupling of ketones with aromatic methylamines for
the construction of 2,4,6-trisubstituted pyridines (Scheme 1, Eq. 1)
[13]. Subsequently, Chen's group reported the copper-catalyzed
oxidative sp3 CeH coupling of oxime acetates or acetophenones
with toluene derivatives to afford the 2,4,6-triarylpyridines
(Scheme 1, Eq. 2) [14]. Phan and co-workers disclosed an iron-
organic framework (VNU-22) catalyzed oxidative cascade reaction
of acetophenones with phenylacetic acids in the presence of
ammonium acetate as a nitrogen source for the preparation of
2,4,6-triarylpyridines (Scheme 1, Eq. 3) [15] Recently, our research
group has also demonstrated an iron-catalyzed oxidative process
for the preparation of 2,4,6-trisubstituted pyridines under aerobic
conditions using arylalkylketones and benzylamines as the
coupling partners (Scheme 1, Eq. 4) [16]. As a continuation of our
interest in the synthesis of 2,4,6-triarylpyridines [17] and in the
cants [7]. Due to their
p-stacking ability, coordination properties
and directional H-bonding, 2,4,6-triarylpyridines are used as sub-
strates for the preparation of therapeutic agents and supra-
molecules [8]. Classic methods for the synthesis of 2,4,6-
trisubstituted pyridines include (i)
a
multicomponent
Chichibabin-type pyiridine reaction employing an aldehyde, an
enolizable ketone, and ammonium acetate as a nitrogen source
using various catalysts [9], (ii) condensation of keto-oximes/oxime
acetates with aryl aldehydes [10] or oxiranes [11], and (iii) reaction
* Corresponding author.
E-mail address: gopal@chemistry.du.ac.in (K. Gopalaiah).
https://doi.org/10.1016/j.tet.2021.132429
0040-4020/© 2021 Elsevier Ltd. All rights reserved.
€
Please cite this article as: K. Gopalaiah and R. Choudhary, Synthesis of Krohnke pyridines through iron-catalyzed oxidative condensation/double
alkynylation/amination cascade strategy, Tetrahedron, https://doi.org/10.1016/j.tet.2021.132429

K. Gopalaiah and R. Choudhary
Tetrahedron xxx (xxxx) xxx
Feng and co-workers studied the cascade reaction of primary
amines and alkynes using CuBr2 as a catalyst and TBHP as an
oxidant leading to secondary propargylamines in moderate to good
yields [19]. The drawback of the Cu-catalyzed oxidative reactions is
the homocoupling of terminal alkynes [20], which may limit its
applications.
2. Results and discussion
We began our study by examining the reaction of phenyl-
acetylene (1a) and benzylamine (2a) at 100 ꢀC under molecular
oxygen atmosphere for 24 h. When 2 equiv of phenylacetylene
reacted with 1 equiv of benzylamine in the absence of any catalyst,
no desired product was obtained (Table 1, entry 1). To our delight,
pyridine 3a was obtained in the presence of FeCl3 and FeBr3, albeit
in low yields (entries 2 and 3). The product 3a was formed in 53%
yield when FeBr2 was used as a catalyst (entry 4). Interestingly, the
yield of 3a was increased to 71% when Fe(OTf)2 was employed
(entry 5). Low yields of 3a were observed in the reactions of
Fe(ClO4)2$xH2O and Fe(ClO4)3$xH2O (entries 6 and 7). Other iron
salts, such as Fe(OAc)2, Fe(acac)2, Fe(acac)3, FeSO4$7H2O,
Fe(NO3)3$9H2O, were not effective for the pyridine formation. We
next examined the effect of reaction solvents. The reactions pro-
ceeded with low yields in toluene and 1,4-dioxane, but ineffective
in polar solvents such as DMF and DMSO (entries 8e11). The effect
of air or molecular oxygen as oxidant for the reaction was also
examined. A lower yield of 3a was obtained when the reaction
conducted under air atmosphere, and only trace amount of pyri-
dine product was formed in the absence of molecular oxygen (en-
tries 12 and 13). Gratifying, the yield of the product 3a was further
improved to 84% when the reaction temperature increased to
120 ꢀC (entry 14). Therefore, the optimal conditions for the present
investigation are the following: 2.0 equiv of phenylacetylene, 1.0
Scheme 1. Oxidative methods for the synthesis of 2,4,6-trisubstituted pyridines.
development of iron-catalyzed oxidative processes [18], we report
herein a novel approach for the synthesis of 2,4,6-triarylpyridines
starting from readily available arylacetylenes and benzylamines
through iron-catalyzed oxidative condensation/double alkynyla-
tion and amination cascade strategy (Scheme 1, Eq. 5). Previously,
Table 1
Optimization of the oxidative cascade reactiona.
Entry
Iron catalyst
Solvent
Yield (%)b
1
e
e
0
2
FeCI3
e
19
3
FeBr3
e
30
4
FeBr2
e
53
5
6
7
8
Fe(OTf)2
Fe(CIO4)2.xH20
Fe(CIO4)3.xH20
Fe(OTf)2
Fe(OTf)2
Fe(OTf)2
Fe(OTf)2
Fe(OTf)2
Fe(OTf)2
Fe(OTf)2
e
71
45
26
65
e
e
toluene
1,4 dioxane
DMF
DMSO
e
9
52
10
11
12c
13d
14e
trace
trace
48
trace
84
e
e
a
Reaction parameters: 2.0 mmol of 1a, 1.0 mmol of 2a, 10 mol % of iron catalyst, 100 ꢀC, oxygen atmosphere (O2 balloon), 24 h.
Isolated yield.
The reaction was carried out under air atmosphere.
The reaction was carried out in a sealed tube.
The reaction was carried out at 120 ꢀC.
b
c
d
e
2

K. Gopalaiah and R. Choudhary
Tetrahedron xxx (xxxx) xxx
Table 2
Fe(OTf)2-catalyzed oxidative cascade reaction of arylacetylenes 1 with 2aa,b
.
Scheme 2. Reaction of two different alkynes with benzylamine.
3

K. Gopalaiah and R. Choudhary
Tetrahedron xxx (xxxx) xxx
equiv of benzylamine, 10 mol % Fe(OTf)2 at 120 ꢀC under O2 at-
mosphere for 24 h.
OLEDs [24], respectively.
To construct unsymmetrical 2,4,6-trisubstituted pyridines with
different groups at 2- and 6-positions, we carried out the reaction
of two different alkynes, namely, 1-chloro-4-ethynylbenzene and
1-ethynyl-4-methylbenzene, with benzylamine under the standard
reaction conditions (Scheme 2). It gave a mixture of three products
3q, 3c and 3j, which are separated and characterised by spectro-
scopic techniques.
Next, we examined the reactions of 1a with various substituted
benzylamines (Table 3). The benzylamines bearing electron-
donating groups (methyl, iso-propyl, methoxy, phenyl) and
electron-withdrawing substituents (fluoro, chloro, bromo) on the
aryl ring were proceeded well to give the desired products in good
yields (4a-4i). The substituents at different positions (para, meta or
ortho positions) on the arene ring of benzylamine did not much
affect the reaction efficiency (4a-4c). It is noteworthy that the
halogen-substituted benzylamines tolerated well, leading to
halogen-substituted 2,4,6-triarylpyridines (4g-4i), which could be
used further in various classical metal-catalyzed cross-coupling
reactions. Besides, the method was equally effective for strong
electron-withdrawing trifluoromethyl groups containing amine,
namely, 3,5-bis(trifluoromethyl)benzylamine to give the corre-
sponding product 4j in 71% yield under the standard reaction
conditions. Furthermore, piperonylamine also smoothly under-
went the oxidative reaction to furnish the desired product in good
With the established reaction conditions in hand, we first
evaluated the scope of alkynes bearing different aromatic and
heteroaromatic rings (Table 2). As illustrated, the phenylacetylenes
containing electron-withdrawing groups such as fluoro, chloro and
bromo at the para or meta positions of the phenyl ring were effi-
ciently reacted with benzylamine, affording the desired products
with good yields (3b-3e). The phenylacetylenes containing ester
group also gave the pyridine products 3f-3g albeit in low yields, but
nitro substrate namely 1-ethynyl-4-nitrobenzene failed to afford
the desired product. Phenylacetylenes bearing electron-donating
groups such as methyl, hydroxy, methoxy functionalities at the
ortho, meta or para positions also smoothly underwent the oxida-
tive cascade reaction to furnish the corresponding products in good
yields (3h-3l). Electron-rich disubstituted and trisubstituted phe-
nylacetylenes also furnished the desired products 3m and 3n in 89%
and 86% yields, respectively. Additionally, the heteroaryl-
substituted alkynes also served as suitable reacting partners to
offer good yields, as exemplified by 3o and 3p. Unfortunately, 1-
hexyne and 1-decyne failed to give the desired products. It is
noteworthy to mention that the synthesized products 3d, 3j, 3o and
3p are useful building blocks for the preparation of photo-
luminescence polymers [21], liquid crystalline polymers [22], 4-
arylpyridines [23], and conjugated polymer chemosensors and
Table 3
Fe(OTf)2-catalyzed oxidative cascade reaction of 1a with benzylamines 2a,b
.
4

K. Gopalaiah and R. Choudhary
Tetrahedron xxx (xxxx) xxx
Table 4
Fe(OTf)2-catalyzed oxidative cascade reaction of arylacetylenes 1 with benzylamines 2a,b
.
yield (4k, 66% yield). In addition to benzylamines, the naphthyl-
substituted amine viz., 1-(2-naphthyl)methanamine was also
compatible with the reaction conditions, affording the corre-
sponding product 4l in 73% yield. However, an aliphatic amine such
as n-octylamine did not give the desired product.
Subsequently, we turned our attention to synthesize pyridines
possessing identical aryl functionalities at 2, 4 and 6-positions with
the present synthetic protocol (Table 4). The reactions of phenyl-
acetylenes and benzylamines bearing electron-donating groups
such as methyl, methoxy at the para, meta or ortho positions of the
phenyl rings, afforded the pyridines 5a-5e in good to excellent
yields. Similar results were obtained from phenylacetylenes and
benzylamines substituted by electron-withdrawing groups (chloro,
fluoro) in para or ortho-positions of aryl rings (5f-5h, 78e89%
yields). 1-Ethynylnaphthalene and 1-naphthylmethylamine were
also found to be suitable starting materials for this oxidative
cascade reaction to furnish the product 5i in 57% yield. Further-
more, the heteroaryl substituted alkynes and amines were served
as good substrates for this reaction to provide good yields of 5j and
5k. It is also noteworthy to mention that the pyridine products 5a
5

K. Gopalaiah and R. Choudhary
Tetrahedron xxx (xxxx) xxx
Scheme 3. Control experiments.
Scheme 4. Proposed mechanism for the synthesis of 2,4,6-trisubstituted pyridines.
and 5k are valuable precursors for the preparation of luminescent
mesoporous metal-organic frameworks [25], multibranched or
the preparation of propargylamines from alkynes and benzyl-
amines under oxidative conditions was demonstrated by Feng and
co-workers [19]. Furthermore, it is believed that the chalcone 8
could be generated by hydrolysis of the imino-chalcone 10 (Scheme
4) during the purification process. Besides, phenylacetylene was
treated with iron(II) triflate in the absence of benzylamine under
the standard reaction conditions, but acetophenone product was
not formed (Scheme 3, Eq. 3). Thus, we contemplated a different
mechanistic pathway for the present cascade reaction as compared
to the previously reported oxidative annulation of arylalkylketones
and benzylamines [16].
On the basis of our observations and previous studies
[18e,19,29], a plausible mechanism for the formation of triarylpyr-
idines is outlined in Scheme 4. At first, benzylamine (2a) would
undergo the oxidative self-condensation in the presence of iron
catalyst and molecular oxygen to give N-benzylidenebenzylamine
star-shaped
p-conjugated materials [26], oligo(2,2-bithien-5-yl)-
substituted pyridine derivatives [27], and topoisomerase I in-
hibitors [28].
To understand the mechanism of the present oxidative annu-
lation, we conducted some control experiments (Scheme 3).
Treatment of benzylamine (2a) with iron(II) triflate in the presence
of molecular oxygen at 120 ꢀC for 4 h furnished the self-
condensation product N-benzylidenebenzylamine (6) in 92% yield
(Scheme 3, Eq. 1). Next, we carried out the reaction between phe-
nylacetylene (1a) and benzylamine (2a) in the presence of iron(II)
triflate and molecular oxygen at a lower temperature (100 ꢀC) and
interrupted the reaction after 10 h, resulting the formation of
propargylamine 7 and chalcone 8 in 46% and 27% yields, respec-
tively, along with the desired product 3a (Scheme 3, Eq. 2). Notably,
6

K. Gopalaiah and R. Choudhary
Tetrahedron xxx (xxxx) xxx
Bull. 66 (2011) 1191.
(6) with the liberation of ammonia. Addition of iron-acetylide,
which is generated in situ from phenylacetylene and iron catalyst,
to the imine 6 would produce the propargylamine 7. Coordination
of the triple bond in alkyne 7 to the iron catalyst enhances the
electrophilicity of the alkyne, and subsequent nucleophilic attack of
the ammonia would produce the imino-chalcone 10 through the
allenylamine intermediate 9. Next, the 1,4-addition of another iron-
acetylide to the chalcone 10 leads to the formation of aminoalkyne
11. The intermediate 11 would then undergo 6-endo-dig cyclization
to form dihydropyridine 12, which further experiences the oxida-
tion under molecular oxygen to give the 2,4,6-triarylpyridine 3a.
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In summary, we have developed a one-step strategy to access
2,4,6-trisubstituted pyridines from arylacetylenes and benzyl-
amines by Fe(II)-catalyzed aerobic oxidative cascade annulation.
The method is straightforward with simple reaction set up on the
benchtop. Alkynes and benzylamines with a variety of aryl and
heteroaryl substituents are effective coupling partners. The devel-
oped protocol has been found to be applicable for the synthesis of
pharmaceutically important compounds and building blocks for
functional materials. The mechanistic investigations disclosed that
this cascade annulation proceeded through imine formation by
self-condensation of benzylamine, double alkynylation and ami-
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Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
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Acknowledgements
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The authors sincerely thank Council of Scientific & Industrial
Research (CSIR), New Delhi (Grant 02(0332)/18/EMR-II) and Uni-
versity of Delhi (Grant IoE/FRP/PCMS/2020/27) for the financial
support. The authors are grateful to CIF, University of Delhi, for
providing instrumentation facilities. RC thanks UGC, Government
of India, for the research fellowship.
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