An Efficient Synthesis of 2,4,6-Triarylpyridines by Use of Benzyl Halides under Neat Conditions

Article abstract of DOI:10.1055/s-0035-1560365

An efficient synthesis of 2,4,6-triarylpyridines is described. Heating a mixture of an acetophenone, a benzyl halide, and ammonium acetate under neat conditions afforded the corresponding Kr?hnke pyridines in excellent yields.

Full text of DOI:10.1055/s-0035-1560365

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©
Georg Thieme Verlag Stuttgart · New York
2016, 27, 417–421
417
letter
Synlett
M. Adib et al.
Letter
An Efficient Synthesis of 2,4,6-Triarylpyridines by Use of Benzyl
Halides under Neat Conditions
Mehdi Adib*a
Neda Ayashia
2
Ar Me
Peiman Mirzaeib
free radical
halogenations
Ar2
O
a
School of Chemistry, College of Science, University of Tehran,
P. O. Box 14155-6455 Tehran, Iran
madib@khayam.ut.ac.ir
neat
Ar1
+ Ar2
X = Cl, Br
X
+ NH4OAc
1
50 °C, 8 h
Ar1
Ar1
N
b
Department of Chemistry, Shahid Beheshti University, Tehran,
Iran
Received: 14.08.2015
Accepted after revision: 04.10.2015
Published online: 24.11.2015
Several synthetic approaches have been reported for the
preparation of 2,4,6-triarylpyridines (TAP). The most com-
mon synthetic route to TAP involves reaction of acetophe-
DOI: 10.1055/s-0035-1560365; Art ID: st-2015-d0638-l
nones, benzaldehydes, and NH OAc or another nitrogen
source in the presence of various catalysts. Other synthet-
4
Abstract An efficient synthesis of 2,4,6-triarylpyridines is described.
Heating a mixture of an acetophenone, a benzyl halide, and ammonium
acetate under neat conditions afforded the corresponding Kröhnke pyr-
idines in excellent yields.
12
ic approaches include reaction between N-phenacylpyri-
dinium salts and α,β-unsaturated ketones in the presence
13a
of NH OAc, reaction between N-(diphenylphosphinyl)-1-
4
Key words 2,4,6-triarylpyridines, Kröhnke pyridines, acetophenones,
benzyl halides, heterocycles, cyclizations
aza-allyl anions and α,β-unsaturated carbonyl com-
13b
pounds, reaction between in situ generated α,β-unsatu-
13c
rated imines and CH nucleophiles, copper-catalyzed cou-
3d
Pyridines are used as precursors of agrochemicals,
pharmaceuticals, dyes, additives, and also in qualitative and
pling of oxime acetates with aldehydes,1 nickel(0)-cata-
lyzed intermolecular dehydrogenative [4+2] cycloaddition
1
13e
quantitative analysis. The pyridine ring can be found in
of 1,3-butadienes with nitriles,
reaction between
13f
many naturally occurring compounds such as vitamin B6
and a number of alkaloids.1,2 Furthermore, some pyridine
derivatives have been shown to possess antiviral, antican-
cer, antioxidant, antibacterial, antidote, and antifungal ac-
phenacylidenedimethylsulfurane, chalcones, and NH OAc,
4
and reaction of α-benzotriazolyl ketones with α,β-unsatu-
13g
rated ketones and NH OAc. However, most of these syn-
4
theses are multistep or low- to moderate-yielding process-
es.
1–3
tivities.
Pyridines with the 2,4,6-triaryl substitution pattern
commonly known as Kröhnke pyridines) are important
In continuation of our studies on the development of ef-
ficient methods for the synthesis of biologically active het-
erocyclic compounds from readily available starting materi-
(
heterocyclic compounds which, due to their excellent ther-
mal stability, have been used in the synthesis of thin films
14
als, we have described several syntheses of TAP including
4
and organometallic polymers. These pyridines have been
the solvent-free reaction between chalcones and
5
15a
utilized in preparation of polyimides, photoluminescent
NH OAc, solvent-free reaction between guanidine hydro-
4
polymers, chemosensors, in asymmetric catalysis,8 and
also in photodynamic cell-specific cancer therapy because
of structural similarity of these pyridines with symmetrical
triarylthiopyrylium, selenopyrylium, and telluropyrylium
6
7
chloride and chalcones under microwave irradiation, and
15b
reaction between acetophenoneoximes and aldehydes un-
der solvent-free conditions.1
5c
To the best of our knowledge there is no report in the
literature for the preparation of TAP starting with benzyl
halides. In accordance with the increasing need to develop
new and simple methods for the preparation of these sig-
nificant heterocyclic compounds, we herein wish to report
the possibility of replacing the aldehyde component with a
benzyl halide in a one-pot multicomponent reaction with
9
photosensitizers. They are useful intermediates in the syn-
thesis of drugs, herbicides, insecticides, desiccants, and sur-
10
factants. Due to their π-stacking ability, directional H-
bonding and coordination properties, polyarylpyridines are
used as substrates for the preparation of supramolecules
11
and therapeutic agents.
acetophenones and NH OAc. Benzyl halides are readily ac-
4
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Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, 417–421

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M. Adib et al.
Letter
cessible via radical halogenation of substituted methylben-
zenes. Therefore use of benzyl halides would increase the
flexibility of the reaction.
Table1 Optimization of the Reaction Conditions for the Synthesis of
16
2
,4,6-Triphenylpyridine (3a) from Acetophenone (1a), Benzyl Chloride
(
2a), and NH OAc
4
Reaction between acetophenone (1a), benzyl chloride
Entry
1a/2a/
NH OAc
Solvent
Temp
(°C)
Time
(h)
Yield of 3a
(
(
2a), and NH OAc was chosen as the model reaction
4
(%)a
4
Scheme 1), for which the reaction conditions, including the
effects of molar ratio of the reactants, reaction temperature,
reaction time, and suitable solvent would be optimized.
1
2
3
4
5
6
7
8
9
2:1:1
2:1:2
2:1:4
1:1:2
1:2:2
1:2:3
1:2:3
1:2:3
1:2:3b
1:2:3
1:2:3
1:2:3
1:2:3
1:2:3
1:2:3
1:2:3
1:2:3
1:2:3
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
EtOH
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
150
8
8
8
8
8
4
6
8
8
8
8
8
8
8
8
8
4
6
8
8
8
9
12
15
18
23
18
49
62
7
O
Ph
Cl
+
+
NH4OAc
Ph
N
3a
Ph
1
a
2a
Scheme 1 Synthesis of 2,4,6-triphenylpyridine (3a)
Initially, refluxing toluene was selected as the reaction
medium, and the reactions were carried out with different
equivalents of the three reactants (Table 1, entries 1–8). The
best result was obtained when the reaction was performed
with a 1:2:3 molar ratio of the three reactants 1a, 2a, and
NH OAc, respectively, with a reaction time of eight hours,
which led to an improved yield of 62% of 3a (Table 1, entry
8
1
0
1
2
17
44
42
13
trace
73
79
68
85
97
6
1
1
PhNO
PhNO
2
2
200
13
14
DMSO
DMSO
neat
neat
neat
neat
neat
neat
neat
150
4
200
15
16
17
18
100
). Next, different solvents such as EtOH, PhNO , and DMSO
2
120
were examined, but the results were not satisfactory (Table
, entries 10–14). The effect of neat conditions at different
150
1
150
temperatures was also tested. It could be seen that the best
result was again attained with a 1:2:3 molar ratio of the
three reactants, a reaction temperature of 150 °C, and a re-
action time of eight hours, in which 3a was obtained in 97%
yield (Table 1, entry 19, with 1a as the limiting reagent). In-
creasing the temperature beyond 150 °C led to a decrease in
yield due to the formation of side products (Table 1, entry
19
1:2:3
150
1:2:3b
2
0
1
150
2
1:2:3
200
85
a
Isolated yields.
Under nitrogen atmosphere.
b
21). We explored the effect of carrying out the reaction un-
der nitrogen in toluene and also under neat conditions,
which showed that the yields decreased to 7% and 6%, re-
spectively (Table 1, entries 9 and 20).
benzylamine or benzyl acetate. The imine or aldehyde 4
may be attacked by the enamine intermediate 5, generated
from condensation between acetophenone 1 and ammoni-
um acetate, to give the β-amino imino ketone intermediate
6. Elimination of ammonia from 6 would form the 2,4-dia-
ryl-1-azadiene intermediate 7, which may undergo nucleo-
philic addition of 5 or the enol form of acetophenone to
give the adduct 8. Adduct 8 may undergo cyclization and
After optimization of the reaction conditions, a series of
2,4,6-triarylpyridines 3 was prepared. Thus, a mixture of an
acetophenone 1, a benzyl chloride 2, and NH OAc were
4
stirred at 150 °C for eight hours to afford the corresponding
2,4,6-triarylpyridines 3a–r in 86–97% yields. TLC and NMR
spectroscopic analysis of the reaction mixtures clearly indi-
eliminate ammonia or H O to give the 3,4-dihydropyridine
2
cated formation of the corresponding TAP 3 in excellent
intermediate 9. This dihydropyridine is then oxidized under
the reaction conditions employed to afford TAP 3.
In conclusion, we have developed an efficient synthesis
of 2,4,6-triarylpyridines via a one-pot, three-component re-
action between acetophenones, benzyl halides, and ammo-
nium acetate. The use of benzyl halides in place of alde-
hydes and the excellent yields of the products are notable
features of this method. To the best of our knowledge this is
the first report for the preparation of TAP by use of benzyl
halides.
17
yields. The results are summarized in Table 2.
1
All the resulting products were characterized by H NMR
and 13C NMR spectroscopy, and their melting points were
compared with those of authentic samples reported in the
17
literature.
A mechanistic rationalization for this reaction is depict-
ed in Scheme 2. At first, benzyl halide 2 may undergo nucleo-
philic attack by ammonium acetate followed by an aerial
oxidation to form an imine intermediate or aldehyde 4 via
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, 417–421

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Synlett
M. Adib et al.
Letter
Table 2 Synthesis of 2,4,6-Triarylpyridines 3a–r under Neat Conditions
Ar2
O
neat
Ar1
Ar2
+
X
+
NH4OAc
1
50 °C, 8 h
Ar1
N
Ar1
1
2
3
Ar1
Ar CH
2
X
Product 3a
Mp (°C)
Yield (%)b
97
Entry
1
2
Cl
3a
135–136 (136–137)18a
Cl
2
3
4
5
6
7
8
9
3b
3c
3d
3e
3f
123–125 (124.5–125)13b
100–101 (99–100)18a
126–128 (127–128)12a
199–201 (202–203)13b
156–158 (157–158)15a
179–181 (178–180)18b
156–157 (155–157)15a
201–202 (200.6–202)19
124–125 (124–125)20
128 (128–129)20
89
87
95
96
93
91
89
96
92
92
87
94
90
88
Cl
MeO
Cl
Cl
Cl
O2N
Cl
Cl
Cl
Cl
Cl
3g
3h
3i
MeO
Cl
Cl
10
11
12
13
14
15
MeO
3j
Cl
Cl
MeO
MeO
MeO
Cl
3k
3l
135–136 (136–137)19
114–115 (113.8–115)19
175–176 (176–178)21
219–221 (221–223)20
MeO
Cl
Cl
3m
3n
3o
Cl
Cl
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M. Adib et al.
Letter
Table 2 (continued)
2
Product 3a
Yield (%)b
86
Entry
Ar1
2
Ar CH X
Mp (°C)
Cl
16
17
18
19
20
21
Cl
3p
190–192 (193–194)20
MeO
Cl
Cl
3q
3r
3a
3f
3j
268–270 (269–270)20
208–210 (209.4–210.1)19
135–136 (136–137)18a
156–158 (157–158)15a
124–125 (124–125)20
91
89
96
95
95
Cl
F
Cl
Cl
Br
Br
Br
MeO
a
TAP 3b,c,k–m were purified by column chromatography and other TAP by crystallization.17
Isolated yields based on the acetophenone 1.
b
N. Bioorg. Med. Chem. Lett. 2009, 19, 5565. (d) Suksrichavalit, T.;
Prachayasittikul, S.; Nantasenamat, C.; Isarankura-Na-Ayudhya,
S.; Prachayasittikul, V. Eur. J. Med. Chem. 2009, 44, 3259.
Acknowledgement
This research was supported by the Research Council of University of
Tehran.
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(
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H
NH4OAc
aerial oxidation
Ar2
Cl
Ar2
NH2 or Ar2 OAc
Ar2
X
2
4 X = NH or O
Ar2
Ar2
NH2
Ar2
Ar2
Ar2
H
O
NH2
Ar2
NH
Ar1
NH4OAc
[O]
Ar1
Ar1
Ar1
4
– NH3
Ar1
– XH2
Ar1
NH
NH Ar1
XH Ar1
Ar1
Ar1
N
Ar1
NH2 X
N
1
5
6
7 X = NH or O
8
9
3
Scheme 2 Proposed mechanism
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M. Adib et al.
Letter
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(17) General Procedure for the Preparation of 2,4,6-Triarylpyri-
dines 3a–r, Exemplified with Compound 3a
A mixture of acetophenone (1 mmol), benzyl chloride (2 mmol),
and NH OAc (3 mmol) in a 5 mL round-bottomed flask was
4
magnetically stirred at 150 °C for 8 h. Progress of the reaction
was followed by TLC monitoring. The reaction mixture was
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2
the product was extracted into CH Cl (2  2.5 mL). The com-
2
2
bined organic phases were dried over Na SO , filtered, the
2
4
(
d) Smith, N. M.; Raston, C. L.; Smith, C. B.; Sobolev, A. N. Green
solvent was removed, and the residue was purified by crystalli-
zation from absolute EtOH.
TAP 3b,c,k–m were purified by column chromatography using
n-hexane–EtOAc (6:1) as eluent. The solvent was removed, and
the solid residue was recrystallized from absolute EtOH.
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Yield 0.116 g (97%, in respect to acetophenone); colorless crys-
18a 1
tals; mp 135–136 °C (136–137 °C).
H NMR (500.1 MHz,
1
CDCl ): δ = 7.40–7.60 (m, 9 H, 9  CH), 7.78 (d, J = 8.0 Hz, 2 H, 2 
3
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1
3
C NMR (125.8 MHz, CDCl ): δ = 117.2 (CH), 127.2 (CH), 127.3
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(
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