Phosphotungstic acid: An efficient catalyst for synthesis of 2-substituted tetrahydroquinoline via imino diels-alder reaction and fluorescent studies

Article abstract of DOI:10.1080/00397910903531672

For the first time Phosphotungstic acid (PTA), is used as a some new catalyst for the synthesis of tetrahydroquinoline derivatives with high yields is described. PTA a heterogeneous catalyst is inexpensive, easily available, eco-friendly, water soluble, r

Full text of DOI:10.1080/00397910903531672

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Phosphotungstic Acid: An Efficient
Catalyst for Synthesis of 2-Substituted
Tetrahydroquinoline via Imino
Diels–Alder Reaction and Fluorescent
Studies
Kavitha C. S. Achar a , Kallappa M. Hosamani a & Harisha R.
Seetharamareddy a
a P.G. Department of Studies in Chemistry, Karnatak University,
Pavate Nagar, Karnataka, India
Version of record first published: 14 Dec 2010
To cite this article: Kavitha C. S. Achar, Kallappa M. Hosamani & Harisha R. Seetharamareddy (2010):
Phosphotungstic Acid: An Efficient Catalyst for Synthesis of 2-Substituted Tetrahydroquinoline via
Imino Diels–Alder Reaction and Fluorescent Studies, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 41:1, 33-40
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Synthetic Communications1, 41: 33–40, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903531672
PHOSPHOTUNGSTIC ACID: AN EFFICIENT CATALYST
FOR SYNTHESIS OF 2-SUBSTITUTED
TETRAHYDROQUINOLINE VIA IMINO DIELS–ALDER
REACTION AND FLUORESCENT STUDIES
Kavitha C. S. Achar, Kallappa M. Hosamani, and
Harisha R. Seetharamareddy
P.G. Department of Studies in Chemistry, Karnatak University, Pavate
Nagar, Karnataka, India
For the first time Phosphotungstic acid (PTA), is used as a some new catalyst for the syn-
thesis of tetrahydroquinoline derivatives with high yields is described. PTA a heterogeneous
catalyst is inexpensive, easily available, eco-friendly, water soluble, recoverable, stable
to aqueous reaction conditions and further more, these 2-methyl tetrahydroquinoline
derivatives shows a remarkable fluorescence property.
Keywords: Fluorescence; imino Diels–Alder reaction; phosphotungstic acid; reusable; 2-substituted-
1,2,3,4-tetrahydroquinoline
INTRODUCTION
We have interested in catalytic reaction to synthesize tetrahydroquinoline
derivatives by using p-substituted aniline, Schiff’s base and N-Vinyl pyrrolidinone
(NVP). The tetrahydroquinoline derivatives are the biologically most potential com-
pounds containing pyrrolidin-2-one ring. The functionalized quinolines have found
various applications in pharmaceutical industries, agrochemical industries and also
explored as synthetic blocks in the preparation of several alkaloids.[1–5] Hence, there
has been considerable interest in the development of new and efficient protocols for
the synthesis of tetrahydroquinoline derivatives.[6,7] The synthesis of 2-methylquinoline
derivatives[8,9] have usually synthesized on the traditional methods.
We describe herein the synthesis of 2-substituted tetrahydroquinoline using
novel, efficient and eco-friendly phosphotungstic acid as a catalyst.
RESULTS AND DISCUSSIONS
Our primary investigation was focused on the use of PTA as the catalyst in
the imino Diels–Alder reaction of p-substituted anilines (1), and NVP (3) stirred
Received August 3, 2009.
Address correspondence to Kallappa M. Hosamani, Department of Chemistry, Karnatak
University, Pavate Nagar, Dharwad-580 003, Karnataka, India. E-mail: dr_hosamani@yahoo.com
33

34
K. C. S. ACHAR, K. A. HOSAMANI, AND H. R. SEETHARAMAREDDY
Scheme 1. Synthesis of 2-methyl tetrahydroquinoline derivatives using PTA as catalyst.
Scheme 2. Synthesis of 2-aryl tetrahydroquinoline derivatives using PTA as catalyst.
at room temparature (Scheme 1, Scheme 2) respectively. The reaction was performed
in different solvents but acetonitrile was by far the most efficient with 0.2% of PTA.
Similarly, several aryl amines reacted smoothly with N-vinylpyrrollidinone (3),
to give corresponding tetrahydroquinolines (a–f, g–r) in 80–90% (Table 1) yields in
the presence of 2 mol% of PTA in acetonitrile at r.t (Scheme 1, Scheme 2). Their
1
structural elucidations are based on IR, H NMR, 13C NMR and mass spectral
data of the column-purified products.
Table 1. PTAa-catalyzed synthesis of 2-aryl substituted tetrahydroquinoline
derivatives
Entry
Compound
R
R0
Yieldb (%)
1
2
g
h
i
H
H
Cl
82
85
83
80
83
81
80
84
85
84
80
81
H
3
H
CH3
CH3O
H
4
j
H
Cl
5
k
l
6
Cl
Cl
Cl
7
m
n
o
p
q
r
CH3
CH3O
H
8
Cl
9
Me
Me
Me
Me
10
11
12
Cl
CH3
CH3O
a2 mol% PTA catalyst loaded.
bIsolated yield.

SYNTHESIS OF TETRAHYDROQUINOLINE CATALYZED BY PTA
35
Table 2. Recoverd PTAa catalysed for synthesis of 2-substituted
tetrahydroquinoline derivatives
Entry
Product
Yieldb
1
2
a
b
c
88
75
78
77
85
79
80
81
80
78
80
78
79
80
82
81
78
78
3
4
d
e
5
6
f
7
g
h
i
8
9
10
11
12
13
14
15
16
17
18
j
k
l
m
n
o
p
q
r
aRecoverd catalyst.
bIsolated yield.
Table 3. Fluorescence spectral data of compounds (a–f) in chloroform
Maximum wavelength (nm)
Stoke’s
Compound
Excitation
Emission
shift (nm)
Dn=cmꢀ1
a
b
c
d
e
f
400
400
406
419
418
497
473
497
500
512
499
564
73
97
94
93
81
67
3860
4880
4630
4330
3880
2390
In IR, the NH in all derivatives are observed at 3320–3360 cmꢀ1 and C O at
=
the region 1670.0 to 1678.3 cmꢀ1. In H NMR the NH peak was observed around
1
3–5 ppm and NH proton is disappeared by D2O treatment and all the methyl and
corresponding protons are observed at respective regions. Fluorescence study of
2-methyl substituted shows good emission spectra and data are summarized in
(Table 3).
Proposed Reaction Pathway
It is found that the most interesting catalyst phosphotungstic acid which
resulted tetrahydroquinoline derivatives (Scheme 3). Hence, it has opened new
directions for the synthesis of industrially important and biologically potential

36
K. C. S. ACHAR, K. A. HOSAMANI, AND H. R. SEETHARAMAREDDY
Scheme 3. Proposed plausible mechanistic pathway.
heterocyclic compounds. The functional groups generated in this reaction have the
potential for the further transformations which has opened new avenue for wide
range of applicability.
We tried one pot synthesis by using benzaldehyde, aniline and NVP catalyzed by
PTA according to the reports in literature.[10,11] However the reaction did not give the
expected 2-aryl tetrahydroquinolines (g–r), instead we got 2- alkyl tetrahydroquinolines
Scheme 4 (a–f). The result indicated that in presence of PTA, the masked aldehyde,
NVP reacts faster than benzaldehyde with aniline and undergoes Diels–Alder reaction
to yield 2-methyl tetrahydroquinoline. In this PTA catalyzed domino reaction, benzal-
dehyde remained unreacted. Encouraged by these results, we carried out the reaction of
various Schiff bases with NVP (Scheme 2) and the results are summarized in (Table 1).
The recovered catalyst which further used for reactions and results are presented in
(Table 2).
Scheme 4. Three components, one-pot synthesis, which affords 2-methyl tetrahydroquinoline derivatives.

SYNTHESIS OF TETRAHYDROQUINOLINE CATALYZED BY PTA
37
Scheme 5. Proposed plausible mechanistic pathway.
From these results, a plausible reaction pathway has been proposed
(Scheme 5)[12,13] based on the product formed and observed in this work. ‘W’ (Tungsten)
centre of PTA having higher oxidation state, strong electrophilic character and Lewis
acidity. Hence, Schiff base acts as nucleophile and attacks Tungsten which may initiate
the reaction with NVP. Hence, the reaction proceeds as in (Scheme 5) and formed due to
the nucleophilic attack of a lone pair of electrons of nitrogen in Schiff bases on the metal
leading to the cyclization of vinyl group in NVP with Schiff bases and later it undergoes
Diels–Alder reaction to favorable 2-aryl tetrahydroquinoline derivatives.
UV-Visible and Fluorescence Spectral Data Analysis
The experimental UV-visible spectra of 2-methyl tetrahydroquinoline in chloro-
form were obtained and are shown in (Table 3). The experimental results revealed
that remarkable photoluminescence properties. Both the absorption and fluorescence
maxima of the 2-methyl tetrahydroquinoline compounds showed good bathochromic
shift. The fluorescence spectral properties (Table 3) of compounds (a–f) are similar to
each other and the Stoke’s shift ranges from 67 to 97 nm. The synthesized compounds
(a–f) also emit in the blue green region of the visible spectrum.
EXPERIMENTAL
The melting points of the products were determined by open capillaries on a
Buchi apparatus and are uncorrected. The IR spectra were recorded on a Nicolet

38
K. C. S. ACHAR, K. A. HOSAMANI, AND H. R. SEETHARAMAREDDY
Impact-5700 FT-IR Spectrophotometer using KBr pellets. 1H NMR and 13C
NMR spectra were recorded on a Bruker Avance-300 F 300 MHz spectrometer in
CDCl3 using TMS as an internal standard with 1H resonant frequency of
300 MHz and 13C resonant frequency of 75 MHz. D2O exchange was applied to con-
firm the assignment of the signals of NH protons. The Mass spectra were recorded
on a GC-MS (Shimadzu QP2010S, Japan). The homogeneity of the compounds was
described by TLC detected by UV light and iodine vapors.
General Procedure for Synthesis of 2-Methyl and 2-Aryl Substituted
Tetrahydroquinoline Derivatives
PTA (0.1728 gm, 2 mol%) was added to a mixture of p-substituted anilines
(3 mmol) and NVP (0.80 ml, 7.5 mmol) in 15 cm3 of acetonitrile which afford
2-methyl tetrahydroquinoline derivatives (a–f) and PTA (0.1728 gm, 2 mol%) was
added to a mixture of Schiff base (3 mmol) and NVP (0.32 ml, 3 mmol) in 15 cm3
of acetonitrile afford 2-aryl tetrahydroquinoline derivatives (g–r). The reaction
mixture was stirred at room temperature and is monitored by TLC and reaction
completes in one hour. After completion, the reaction mixture was quenched with
25 cm3 saturated NaHCO3 aqueous solution and extracted with ethyl acetate
(30 ml). The combined organic layer was dried over anhydrous Na2SO4, concen-
trated and purified by column chromatography on SiO2 with an ethyl acetate and
petroleum ether mixture as eluent to afford the corresponding tetrahydroquinolines
(a–f)[13] and (g–r). After extraction, the water layer containing the catalyst could be
evaporated under reduced pressure to give a catalyst back with light yellow colored
solid. The recovered catalyst was washed with ether, dried at 85 ꢁC for 2 h under
high-pressure prior to use in the further reaction and it can be recyclable for 3 times.
The Spectral Data of Selected Compound
1-(2-Phenyl-1,2,3,4-tetrahydroquinolin-4-yl)pyrroꢀli1din-2-one) (g). Colorless
ꢀ1
solid, m.p. 152–154 ꢁC, IR (KBr): n ¼ 3326(NH) cm , 1669(C O) cm
;
1H
=
NMR (300 MHz, CDCl3) d ppm: 4.01 (s, 1H, NH), 3.21 (t, 1H, 5H, Ar ¼ CH),
2.25 (t, 2H, CH2), 6.58–7.09 (m, 5H, Ar-H), 7.32–7.45(m, Ar-H); 13C NMR
(75 MHz, CDCl3) d ppm 16.8, 37.0, 38.7, 44.8, 47.9, 57.8, 110.2, 118.2, 120.1,
128.7, 139.9, 145.1, 176.2.; MS: 292 (M þ 1).
1-[2-(4-Methoxyphenyl)-1,2,3,4-tetrahydroquinolin-4-yl]pyrrolidin-2-
one) (j). Colorless solid, m.p. 164–166 ꢁC, IR (KBr): n ¼ 3327(NH) cmꢀ1, 1673
1
(C O) cmꢀ1; H NMR (300 MHz, CDCl3) d ppm: 3.9 (s, 1H, NH), 2.5 (s, 3H,
=
CH3), 7.07–7.34 (m, 4H,Ar-H) 6.5–6.8 (m, 3H, Ar-H);13C NMR (75 MHz, CDCl3)
d ppm 16.5, 36.0, 37.7, 44.8, 47.9, 55.8, 57.6, 112.2, 117.9, 123.1, 126.8, 128.7,
131.9, 143.3, 160.1, 177.2.; MS: 322 (M þ 1).
1-(6-Chloro-2-phenyl-1,2,3,4-tetrahydroquinolin-4-yl)pyꢀrr1olidin-2-one
(k). Colorless solid, m.p. 158–160 ꢁC, IR (KBr): n ¼ 3326 (NH) cm , 1672 (C O)
=
cmꢀ1; H NMR (300 MHz, CDCl3) d ppm: 4.2 (s, 1H, NH), 7.06–7.21 (m, 4H,
1
Ar-H), 6.26–6.7 (m, 4H, Ar-H); 13C NMR (75 MHz, CDCl3) d ppm 16.8, 21.2,

SYNTHESIS OF TETRAHYDROQUINOLINE CATALYZED BY PTA
39
36.8, 38.1, 44.1, 47.4, 57.8, 112.5, 122.7, 126.5, 128.7, 131.7, 136.8, 177.2.; MS: 328
(M þ 1).
1-[6-Chloro-2-(4-methoxyphenyl)-1,2,3,4-tetrahydroquinolin-4-yl] pyrrolidin-
2-one (n). Colorless solid, m.p. 172–173 ꢁC, IR (KBr): n ¼ 3324 (NH) cmꢀ1, 1670
1
(C O) cmꢀ1; H NMR (300 MHz, CDCl3) d ppm: 4.2 (s, 1H, NH), 2.35 (s, 3H,
=
CH3), 3.73(s, 3H, OCH3), 6.72–67.01 (m, 4H, Ar- H), 6.2–6.7 (m, 3H, Ar-H); 13C
NMR (75 MHz, CDCl3) d ppm 16.2, 20.9, 36.4, 37.2, 43.2, 47.0, 57.0, 112.2,
118.5, 127.8, 129.9, 131.5, 156.8, 172.8.; MS: 358 (M þ 1).
1-(6-Methyl-2-phenyl-1,2,3,4-tetrahydroquinolin-4-yl)ꢀp1yrrolidin-2-one (ꢀo1).
Colorless solid, m.p. 155–158 ꢁC, IR (KBr): n ¼ 3360(NH) cm , 1685 (C O) cm
;
=
1
=
H NMR (300 MHz, CDCl3) d ppm: 2.70 (s, 3H, CH3), 6.52 (s, 1H, C CH),
7.20–7.7 (m, 4H, Ar-H); 13C NMR (75 MHz, CDCl3) d ppm 16.8, 36.5, 37.9, 43.8,
47.8, 57.7, 112.8, 114.8, 123.2, 129.5, 132.5, 136.8, 150.1, 176.1.; MS: 307 (M þ 1).
1-[2-(4-Methoxyphenyl)-6-methyl-1,2,3,4-tetrahydroquinolin-4-yl]
pyrrolidin-2-one (r). Colorless solid, m.p. 148–150 ꢁC, IR (KBr): n ¼ 3360 (NH),
1
1685 (C O) cmꢀ1; H NMR (300 MHz, CDCl3) d ppm: 2.70 (s, 3H, CH3), 6.52 (s,
=
1H, C CH), 7.20–7.7 (m, 4H, Ar-H); 13C NMR (75 MHz, CDCl3) d ppm 16.0,
36.2, 37.3, 44.2, 47.2, 56.7, 112.0, 113.8, 123.7, 131.8, 135.8, 159.1, 174.8.; MS: 337
=
(M þ 1).
CONCLUSIONS
This novel method provides a very interesting and a facile synthesis of
2-substituted tetrahydroquinoline derivatives using easily available and water soluble
Phosphotungstic acid (PTA) catalyzed imino Diels–Alder reaction in good yield are
described. The attractive features of this procedure are the mild reaction conditions,
short time, without heating, high conversions, cleaner reaction profiles, inexpensive.
Reusable, eco-friendly catalyst and all of which make a useful and attractive strategy
for the preparation of various tetrahydroquinoline. In support to these data, these
2-methyl substituted tetrahydroquinolines shows an excellent fluorescence properties
and the fluorescence properties of 2-aryl substituted tetrahydroquinolines is in
progress in our group.
ACKNOWLEDGMENTS
This research work is financially supported by Council of Scientific and
Industrial Research, New Delhi—110 012 (Ref. No: 01(2301)=09=EMR–II dated
19-03-2009). Ms. Kavitha C.S. Achar is grateful to UGC for the award of JRF
(Ref. No: KU=Sch=UGC=RFSM=2006-07=3236 dated 08-03-2007).
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