Old reagent, new results: Aromatization of hantzsch 1,4-dihydropyridines with supported bismuth nitrate under microwave irradiation in solventless system

Article abstract of DOI:10.1080/10426500307795

Hantzsch 1,4-dihydropyriines were rapidly and efficiently oxidized using bismuth nitrate supported onto silica gel under microwave irradiation.

Full text of DOI:10.1080/10426500307795

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Phosphorus, Sulfur, and Silicon and the Related
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Old Reagent, New Results: Aromatization of Hantzsch
1,4-Dihydropyridines with Supported Bismuth Nitrate
under Microwave Irradiation in Solventless System
Mahmood Tajbakhsh a , Majid M. Heravi b , Abolfazl Hosseini a & Ahmad Shahrezaiee a
a University of Mazandaran , Babolsar, Iran
b Azzahra University , Vanak, Tehran, Iran
Published online: 27 Oct 2010.
To cite this article: Mahmood Tajbakhsh , Majid M. Heravi , Abolfazl Hosseini & Ahmad Shahrezaiee (2003) Old Reagent,
New Results: Aromatization of Hantzsch 1,4-Dihydropyridines with Supported Bismuth Nitrate under Microwave Irradiation in
Solventless System, Phosphorus, Sulfur, and Silicon and the Related Elements, 178:4, 773-776, DOI: 10.1080/10426500307795
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Phosphorus, Sulfur and Silicon, 2003, Vol. 178:773–776
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Copyright 2003 Taylor & Francis
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DOI: 10.1080/10426500390198129
OLD REAGENT, NEW RESULTS: AROMATIZATION
OF HANTZSCH 1,4-DIHYDROPYRIDINES
WITH SUPPORTED BISMUTH NITRATE
UNDER MICROWAVE IRRADIATION
IN SOLVENTLESS SYSTEM
Mahmood Tajbakhsh,a Majid M. Heravi,b Abolfazl Hosseini,a
and Ahmad Shahrezaieea
University of Mazandaran, Babolsar, Irana and
Azzahra University, Vanak, Tehran, Iranb
(Received August 2, 2002)
Hantzsch 1,4-dihydropyriines were rapidly and efficiently oxidized
using bismuth nitrate supported onto silica gel under microwave
irradiation.
Keywords: Aromatization; bismuth nitrate; Hantzsch; microwave
irradiation; solventless system
One of the most widely used synthesis of pyridines is that discovered
by Hantzsch in 1882.1 In this reaction, an aldehyde, ammonia, and
, -keto ester are condensed to afford 1,4-dihydropyridine, which is
consequently oxidized to the pyridine. Since then, aromatization of
Hantzsch 1,4-dihydropyridines (1,4-DHP) has attracted considerable
attention, and several oxidizing agents and variety of methods have
been reported in the literature.2
Recently, bismuth compounds have become attractive candidates for
use as reagents in organic synthesis because most bismuth compounds
are relatively nontoxic, readily available, inexpensive, and fairly in-
sensitive to small amounts of water.3 The biochemistry,4 toxicology,5
and environmental effects of bismuth compounds have been well docu-
mented and show that they are attractive candidates for use in green
chemistry.
Address correspondence to M. M. Heravi, Department of Chemistry, School of
Sciences, Azzahra University, Vanak, Tehran, Iran.
773

774
M. Tajbakhsh et al.
Bismuth nitrate pentahydrate has been used recently as a conve-
nient reagent for the oxidation of Hantzsch 1,4-dihydropyridines.7 In
spite of all advantages of the bismuth compound, this method suffers
from one or the other disadvantages like long reaction time, that is 14 h
in cases of taking 1a (R = H), use of acetic acid as a solvent, moderate
yields, and difficulties in isolation of products.
The reagent impregnated on solid supports, especially that are ef-
ficient in dry media8 have gained popularity in organic synthesis be-
cause of their selectivity and ease of manipulation. During the course
of the study of organic reactions in solvent free condition,9 we have
observed the usefulness of microwave irradiation in enhancing of the
reaction.10 In connection with these observations, we became interested
in developing a rapid and environmentally friendly condition to effect
1,4-dihydropyridine to pyridine conversion under microwave irradia-
tion in solventless system. Our initial attempts to induce oxidation of
the simple Hantzsch 1,4-dihydropyridine as a test case with 1 (R = H)
and bismuth nitrate without solvent under microwave irradiation was
not very promising as considerable amounts of (R = H) was recovered
unchanged even after relatively long reaction periode, that is 10 min.
However, when the reagent was supported onto an equal amount of sil-
ica gel and mixed with 1 (R = H), the oxidation was performed rapidly
(in 2 min) and efficiently under microwave irradiation to afford 12
(Scheme 1). To establish the generality of the method a variety of 1,4-
dihydropyridines 1–11 were oxidized under similar reaction conditions
(Table I). It is noteworthy that the oxidation of 1,4-dihydropyridine with
secondary alkyl and benzylic groups (entries 3, 4) at 4 position was ac-
companied by expulsion of these groups to afford 12 while in other case
the substituted groups at 4 position were retained in the pyridine ring
to give 13–20 (Table I).
SCHEME 1
In conclusion, a new expeditous, rapid, mild, and eco-friendly method
for oxidation of 1,4-dihydropyridines has been developed that may find
wide application in organic synthesis.

Old Reagent, New Results
775
TABLE I Oxidation of 1,4-Dihydropyridines (1–11) with Bismuth Nitrate
Supported onto Silica Gel Under Microwave Irradiation
Reaction time
(sec)
m.p.
( C)
m.p.(Lit)
( C)
Yield
(%)
Entry
R
Product
1
2
3
4
5
6
7
8
9
H
120
120
120
120
120
120
120
120
120
120
120
12
13
12
12
14
15
16
17
18
19
20
68–69
oil
69–7011
oil11
88
55
89
92
93
91
85
88
89
92
88
CH3CH2CH2
(CH3)2CH
C6H5CH2
C6H5
4-Cl-C6H4
2-NO2-C6H4
3-NO2-C6H4
4-NO2-C6H4
4-CH3O-C6H4
2-furyl
68–69
68–69
61–62
66–67
74–75
62–63
114–115
49–50
oil
69–7011
69–7011
62–6311
66–6711
7512
61–6311
114–11611
5013
10
11
oil12
aYields refer to isolated products.
EXPERIMENTAL SECTION
All of the dihydropyridines were prepared in the same manner, using
the appropriate aldehyde, ammonia, and ethyl acetoacetate. All prod-
ucts were known and identified by comparison of their physical and
spectroscopic data with those of authentic samples. All of the oxida-
tions were carried out in the same manner. Silica gel 60 for column
chromatography purchased from Merck was used as a solid support.
Oxidation of Dihydropyridines
General Procedure
Bismuth nitrate pentahydrate (0.5 g, 1 mmol) was mixed with silica
gel (0.5 g). The appropriate Hantzsch 1,2-dihydropyridine was added
to the above supported reagent and mixed throughly. This mixture was
put in a beaker and placed in microwave oven and irradiated for 2 min.
After completion of the reaction (monitored by TLC), chloroform (5 mL)
was added, and the solid filtered. From the filtrate chloroform was evap-
orated and the residue was crystallized from aq. EtOH to afford the
corresponding pyridine (Table I).
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[2] H. R. Memarian, M. M. Sadeghi, and A. R. Momeni, Synth. Commun., 31, 2241
(2001).

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M. Tajbakhsh et al.
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