New and general nitrogen heterocycle synthesis: Use of heteropoly acids as a heterogeneous recyclable catalyst

Article abstract of DOI:10.1080/00397910903161793

An efficient synthetic method of six-and five-member nitrogen heterocyclic compounds was developed. Nitrogen heterocyclic compounds were prepared by condensation of the-dicarbonyl compounds with the corresponding-or-amino alcohols, subsequent cyclization, and spontaneous aromatization in the presence of a catalytic amount of Keggin-type heteropoly acids under very mild conditions. Copyright

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New and General Nitrogen Heterocycle
Synthesis: Use of Heteropoly Acids as a
Heterogeneous Recyclable Catalyst
Rahim Hekmatshoar a , Sodeh Sadjadi a , Samaheh Sadjadi a , Majid
M. Heravi a , Yahya S. Beheshtiha a & Fatemeh F. Bamoharram b
a Department of Chemistry, School of Science, Alzahra University,
Vanak, Tehran, Iran
b Department of Chemistry, School of Sciences, Azad University,
Khorasan Branch, Mashad, Iran
Version of record first published: 06 May 2010.
To cite this article: Rahim Hekmatshoar, Sodeh Sadjadi, Samaheh Sadjadi, Majid M. Heravi, Yahya
S. Beheshtiha & Fatemeh F. Bamoharram (2010): New and General Nitrogen Heterocycle Synthesis:
Use of Heteropoly Acids as a Heterogeneous Recyclable Catalyst, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 40:11, 1708-1716
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Synthetic Communications1, 40: 1708–1716, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903161793
NEW AND GENERAL NITROGEN HETEROCYCLE
SYNTHESIS: USE OF HETEROPOLY ACIDS AS A
HETEROGENEOUS RECYCLABLE CATALYST
Rahim Hekmatshoar,1 Sodeh Sadjadi,1 Samaheh Sadjadi,1
Majid M. Heravi,1 Yahya S. Beheshtiha,1 and
Fatemeh F. Bamoharram2
1Department of Chemistry, School of Science, Alzahra University,
Vanak, Tehran, Iran
2Department of Chemistry, School of Sciences, Azad University, Khorasan
Branch, Mashad, Iran
An efficient synthetic method of six- and five-member nitrogen heterocyclic compounds was
developed. Nitrogen heterocyclic compounds were prepared by condensation of the b-dicar-
bonyl compounds with the corresponding b- or c-amino alcohols, subsequent cyclization,
and spontaneous aromatization in the presence of a catalytic amount of Keggin-type
heteropoly acids under very mild conditions.
Keywords: Amino alcohols; bifunctional (acid and redox) catalyst; b-dicarbonyl compounds; Keggin-type
heteropoly acid
INTRODUCTION
The biological activity of some pyrrole derivatives has prompted the develop-
ment of new synthetic pathways for 4,5,6,7-tetrahydroindoles with various substi-
tution patterns.[1] Different tetrahydroindoles are found in the structures of many
biologically active compounds, such as antibiotics, antipsychotic agents,[2] and blood
platelet aggregation inhibitors.[3] They can also be used as ligands for transition
metals.[4] Various multistep synthetic procedures have been described in the
literature, including reduction of indoles[5] or annulation reactions.[1]
Synthesis of the pyridine ring system and its derivatives[6] occupies an impor-
tant place in the realm of natural and synthetic organic chemistry because of the
therapeutic and pharmacological properties of these compounds. They have emerged
as integral backbones of more than 7,000 existing drugs.[6,7] The pyridine ring is also
an integral part of agrochemicals, preparative organic chemistry, and coordination
chemistry. In addition to these important biological applications, pyridine deriva-
tives are ideal scaffolds for making libraries of drug-like compounds and generating
Received April 21, 2009.
Address correspondence to Rahim Hekmatshoar, Department of Chemistry, School of Science,
Alzahra University, Vanak, Tehran, Iran. E-mail: rhekmatus@yahoo.com
1708

GENERAL NITROGEN HETEROCYCLE SYNTHESIS
1709
libraries of inhibitors of HIV-1 protease and factor Xa. Thus, the synthesis of these
heterocycles is of current importance.[7]
The catalytic function of heteropoly compounds[8] (heteropoly acids and salts)
has attracted much attention, and they are used in solution as well as in the solid
state. Because of their weak superacidic and redox properties, low toxicity, ease of
handling, low cost, stability, water tolerance, recoverability, and reusability, hetero-
poly compounds are useful as versatile catalysts in reactions requiring electrophilic
catalysis.[9]
The successful applications of Keggin-type heteropoly acid as acid, redox, and
bifunctional (acid and redox) catalyst for a wide variety of organic transformations
prompted us to explore the potential of Keggin-type heteropoly acids as catalysts for
a one-pot heterocyclocondensation process.
The present work was initiated to test the scope and practicality of a general
approach to the syntheses of unsaturated nitrogen heterocyclic compounds envi-
sioned as a cyclocondensation of amino alcohols and b-dicarbonyl compounds,
using Keggin-type heteropoly acids as the catalyst, as shown in Scheme 1. From a
synthetic point of view, the heteropoly acid is required for the aromatization of
the heterocycle products.
In previous papers,[9,10] the preparation of 4,5,6,7-tetrahydroindol-4-one
derivatives by applying the Knorr pyrrole synthetic methodology to several
5-substituted-1,3-cyclohexanediones has been reported. For the synthesis of
2,3-unsubstituted tetrahydroindoles, a modification of the Knorr method reported
by Bobbit et al.[11] was used, giving the desired compounds in poor yields.[9] The
synthesis of pyrroles and indoles via palladium-catalyzed oxidation of hydroxyena-
minones was reported by Ohta,[12] and this procedure was extended to the synthesis
of potential central nervous system agents by Ravina.[7]
In this communication, we describe the alternative efficient synthesis of
six-membered (tetrahydroquinolin-5-ones, pyridine) and five-membered (pyrrole
and tetrahydroindol-4-ones) nitrogen heterocyclic compounds, which were prepared
by condensation of the b-dicarbonyl compounds with the corresponding b- or
c-amino alcohols, and subsequent cyclization followed by spontaneous aromatiza-
tion (Scheme 2).
Our first efforts were directed to the simple, fully aromatic, five-membered
heterocycles.
Scheme 1. Nitrogen heterocycles synthesis.

1710
R. HEKMATSHOAR ET AL.
Scheme 2. Plausible reaction mechanism.
To determine the optimum conditions for the reaction, similar reactions were
conducted under different conditions, and the experimental results are shown in
Table 1.
To determine the appropriate ratio of the reagents, the model reaction
(reaction between cyclohexane-1,3-dione and 2-amino-ethanol) was carried out with
different molar ratios of two reactants. First, 2 mmol of cyclohexane-1,3-dione,
1 mmol of 2-amino-ethanol, and 0.01 mmol of Keggin-type heteropoly acid
(H3PW12O40) were added to 15 mL of water, and the solution was then refluxed
for 1 h; 24% of 1,5,6,7-tetrahydro-indol-4-one was observed (Table 1, entry 1).
Although it is possible to increase the yield of 1,5,6,7-tetrahydro-indol-4-one by
increasing the reaction time under similar conditions, mild reaction conditions would
be highly desirable. Only traces of product were formed when the reaction was con-
ducted at room temperature for 6 h in a water solution using cyclohexane-1,3-dione
reagent (Table 1, entry 2). To increase the yield of 1,5,6,7-tetrahydro-indol-4-one, the
reaction was carried out using a different solvent (tetrahydrofuran, THF) because of
the low solubility of the starting material in water. When the THF solution was
stirred at room temperature for 6 h, traces of 1,5,6,7-tetrahydro-indol-4-one were
generated (Table 1, entry 3). However, when the volume of THF was decreased to
1 mL, the yield of 1,5,6,7-tetrahydro-indol-4-one was increased to 39% (Table 1,
Table 1. Reaction of cyclohexane-1,3-dione with 2-amino-ethanol in the presence of H3PW12O40 under
different conditions to generate 1,5,6,7-tetrahydro-indol-4-one
Entry 1,3-Dicarbonyl compound Amino alcohol Solvent (ml) Reaction conditiona Product (%)
1
2
3
4
5
6
7
8
9
2
1
Water (15 ml)
Water (1 ml)
THF (10 ml)
THF (1 ml)
Solventless
Solventless
Solventless
Solventless
Solventless
Reflux, 1 h
rt, 6 h
rt, 6 h
rt, 6 h
rt, 6 h
rt, 6 h
rt, 6 h
rt, 6 h
rt, 6 h
24
Trace
Trace
39
2
1
2
1
2
1
2
1
47
65
1.5
1
1
1
84
90
1
1.5
2
1
79
aIn all entries, 0.01 mmol H3PW12O40 was added to the reaction solution.

GENERAL NITROGEN HETEROCYCLE SYNTHESIS
1711
entry 4). Comparing entry 4 to entry 3 (Table 1), an increase in the concentration of
the reactants led to an efficient increase in the yield of 1,5,6,7-tetrahydro-
indol-4-one. The same reaction was carried out under solventless conditions
(Table 1, entry 5). The results indicate that solvent-free system was found to be
the best choice for the reaction. Based on conditions used in entry 5 (Table 1), we
were also surprised to find that changing the limiting reagent from 2-amino-ethanol
to cyclohexane-1,3-dione also dramatically increases the yield of 1,5,6,7-tetrahydro-
indol-4-one to 90% (Table 1, entry 8). Since heteropoly acids were also used as a
catalyst in the reaction with carbonyl compounds, with excess amounts of 1,3-
dicarbonyl compound as expected, 1,5,6,7-tetrahydro-indol-4-one and the aldol
condensation product were observed in the solution.
Based on the results of Table 1, it appears that the experimental procedures
and conditions shown for entry 8 are the best choices for other similar reactions,
and all the experimental results shown in Table 2 were conducted under these
conditions.
Turning our attention to six-membered heterocycles, we allowed b-dicarbonyl
compounds to react with 3-amino-propan-1-ol under the optimized conditions to
form the simple adduct (Table 2).
In our studies, we investigated the activity of various Keggin-type heteropoly
acids. Representative results are shown in Table 3. The results indicate that the
nature of the catalyst plays an important role on their catalytic activities. The great-
est yield of products has been achieved in the presence of H5[PMo11VO40] as catalyst,
and H3[PW12O40] gave the lowest yields.
Because of the complicated nature of the reaction, it seemed rather difficult to
make an exact assessment of the catalyst role. However, we may make some assump-
tions that agree with the experimental data and literature.
The Keggin anion has an assembly of 12 corner-shared octahedral MoO6 from
trimetallic groups [Mo3O13] around a heteroatom tetrahedron PO4. The introduction
of vanadium(V) into the Keggin framework of [PMo12O40]3ꢀ is beneficial for the
catalytic reaction.[13] Because the metal substitution may modify the energy and
composition of the lowest unoccupied molecular orbital (LUMO) and redox proper-
ties, the energy and composition of the LUMOs have significant effects on the cata-
lytic activity heteropoly acids with different charges.[14] Substitution of vanadium
ions into the molybdenum framework stabilizes the LUMOs because these orbitals
derive in part from vanadium d-orbitals, which have been assumed to be more stable
than those of molybdenum and tungsten.[15] The abundance of different isomers may
also play an important role in catalytic performance. In addition, different positional
Mo atom(s) substituted by the V atom(s) in [PMo12O40]3ꢀ may create different
vanadium chemical environments, thus causing these catalysts to exhibit varying
catalytic performances.
By variation of the addenda atoms, the electrochemical character of them can
be widely changed. The addenda atoms can be ordered by decreasing oxidizing
ability in the following way: V(V) > Mo(VI) > W(VI).[16]
The liquid-phase oxidation of organic substances catalyzed by heteropoly acids
is usually carried out in homogenous or biphasic systems. Heterogeneous liquid–solid
systems are also used but less frequently. Among various oxidants that can be used
in these reactions, oxygen (air) and hydrogen peroxide are the most important.[17]

1712
R. HEKMATSHOAR ET AL.
Table 2. Reaction of 1,3-dicarbonyl compound with amino alcohol in the presence of Keggin-type
heteropoly acid
Entry 1,3-Dicarbonyl compound
Amino alcohol
Product
Yield (%)
1
94
2
3
4
91
89
98
5
93
6
87
aReaction conditions: Keggin-type heteropoly acids (0.01 mmol), 1,3-dicarbonyl compound (1 mmol),
amino alcohol (1.5 mmol), solvent-free system, rt, 6 h.
In our previous papers, we reported the oxidation of tertiary amines and primary
aromatic amines with aqueous hydrogen peroxide under the influence of heteropoly
acid catalyst,[18] N-oxidation of pyridine carboxylic acids using hydrogen peroxide
catalyzed by heteropoly anion,[19] and oxidation of benzylic, allylic, and aliphatic
alcohols to carbonyl compounds.[20]

GENERAL NITROGEN HETEROCYCLE SYNTHESIS
1713
Table 3. Reaction of cyclohexane-1,3-dione with 2-amino-ethanol in the presence
of heteropoly acids catalysts; effect of the type of heteropoly acids catalysts
Entry
Heteropoly acidsa
Product (%)b
1
2
3
4
H3[PW12O40
H3[PMo12O40
H5[PMo11VO40
H4[SiW12O40
]
90
96
98
93
]
]
]
aReaction conditions: Keggin-type heteropoly acids (0.01 mmol), 1,3-dicarbonyl
compound (1 mmol), amino alcohol (1.5 mmol), solvent-free system, rt, 6 h.
bYields analyzed by gas chromatography.
Selective oxidation with mixed oxides and oxide-like catalysts such as
heteropoly acids involves the activation of CꢀH or CꢀC bonds as well as that of
the oxidant on the catalyst surface and frequently occurs by a Mars–van Krevelen
redox mechanism,[17] which is shown in Scheme 3. The catalyst oxidizes the substrate
and then, in another step, is reoxidized by O2. The heteopoly acids maintained their
Keggin structure in the course of the reactions, which were confirmed by examining
their infrared (IR) spectra. Previous study indicated the reoxidation of the catalyst
occurred without losing the Keggin structure.[19]
To know whether the catalyst would succumb to poisoning and loss of catalytic
activity during the reaction, we investigate the reusability of the catalyst. For this
purpose, we first carried out the reaction in the presence of the catalyst. After com-
pletion of the reaction, the catalyst was removed, washed with methanol, dried at
80 ꢁC for 2 h, and subjected to a second run of the reaction process with the same
substrate. The results of the first experiment and subsequent experiments were
almost consistent in yields. We have found that Keggin catalyst can be reused several
times without any appreciable loss of activity. The several recoveries only slightly
decreased the catalytic activity, pointing to the stability and retention capability of
this useful polyanion. In Table 4, the comparison of efficiency of the catalyst after
five reuses is reported.
In summary, we describe a convenient and efficient protocol for the synthesis
of six- and five-membered nitrogen heterocyclic compounds using heteropoly acid as
a green, recyclable, and bifunctional (acid and redox) catalyst. The simple experi-
mental procedure combined with ease of recovery and reuse of this catalyst make this
procedure quite simple, more convenient, and environmentally benign.
Scheme 3. Mars–van Krevelen redox mechanism.

1714
R. HEKMATSHOAR ET AL.
Table 4. Comparison of efficiency of catalysts after five reactions of cyclohexane-1,3-dione with
2-amino-ethanol in the presence of heteropoly acid catalysts
Yield (%)a
Entry
Heteropolyacid
First
Second
Third
Fourth
Fifth
1
2
3
4
H3[PW12O40
H3[PMo12O40
H5[PMo11VO40
H4[SiW12O40
]
90
96
98
93
87
94
95
91
85
92
94
91
83
91
92
88
81
89
91
85
]
]
]
Notes. Reaction conditions: Keggin-type heteropoly acids (0.01 mmol), 1,3-dicarbonyl compound
(1 mmol), amino alcohol (1.5 mmol), solvent-free system, rt, 6 h.
aYields were analyzed by gas chromatography.
EXPERIMENTAL
Chemicals and Apparatus
All the chemicals were obtained from Merck Company and used as received.
Phosphotungstic acid and molybdophosphoric acid were purchased from Merck
Company; H4[PMo11VO40] and H4[SiW12O40] were prepared according to the
literature.[21,22]
Typical Procedure
A catalytic amount of Keggin-type heteropoly acid catalyst (H5[PMo11VO40])
(1 mol%) was added to a mixture of b-dicarbonyl compounds (1 mmol) and amino
alcohols (1.5 mmol), and the reaction mixture was stirred at room temperature for
6 h. The progress of the reaction was monitored by thin-layer chromatography
(TLC). After completion of the reaction, the mixture was diluted with chloroform
and the catalyst was removed by simple filtration (the catalyst is not soluble in
chloroform). The combined organic layer was dried over Na2SO4 and concentrated
to dryness in vacuo, and the residue was purified by column chromatography (eluted
with 2:8 EtOAc–petroleum ether) to afford the pure product.
The filtered catalyst was reactivated by heated at 80 ꢁC for 2 h and reused at
least five times. All products gave satisfactory spectral data in accord with the
assigned structures.[7,23]
ACKNOWLEDGMENT
The authors are thankful to Alzahra Research Council for the partial financial
support.
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