Microwave assisted efficient synthesis of flavone using ZnO nanoparticles as promoter under solvent-free conditions

Article abstract of DOI:10.14233/ajchem.2019.21873

A simple and highly efficient protocol for synthesis of flavones from 1-(2-hydroxyphenyl)-3-aryl-1,3-propanediones in presence of ZnO nanoparticles as a promoter in thermal as well as microwave irradiation under solvent-free conditions have been demonstrated. The catalyst is inexpensive, stable, can be easily recycled/reused for several cycles with consistent activity and observed almost same yield confirming the stability of the catalyst. It is believed that the present approach will become an alternative route for the conventional reactions. Because in this protocol, yield is quite high, short reaction time, simple work up, catalyst can be recycled as well as it is free of any hazardous by-products formation during workup.

Full text of DOI:10.14233/ajchem.2019.21873

Asian Journal of Chemistry; Vol. 31, No. 5 (2019), 1133-1136
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2019.21873
Microwave Assisted Efficient Synthesis of Flavone using ZnO
Nanoparticles as Promoter under Solvent-Free Conditions
*
*
PRADIP J. UNDE, NITIN M. THORAT and LIMBRAJ R. PATIL
Post graduate Department of Chemistry, Maharaja Jivajirao Shinde Mahavidyalaya, Shrigonda-413701, India
*Corresponding authors: E-mail: nitin12.thorat@gmail.com; limbrajp@gmail.com
Received: 12 December 2018;
Accepted: 24 January 2019;
Published online: 28 March 2019;
AJC-19343
A simple and highly efficient protocol for synthesis of flavones from 1-(2-hydroxyphenyl)-3-aryl-1,3-propanediones in presence of ZnO
nanoparticles as a promoter in thermal as well as microwave irradiation under solvent-free conditions have been demonstrated. The
catalyst is inexpensive, stable, can be easily recycled/reused for several cycles with consistent activity and observed almost same yield
confirming the stability of the catalyst. It is believed that the present approach will become an alternative route for the conventional
reactions. Because in this protocol, yield is quite high, short reaction time, simple work up, catalyst can be recycled as well as it is free of
any hazardous by-products formation during workup.
Keywords: Flavone, Nanocrystalline ZnO, Heterogeneous, Green Synthesis, Solvent-free synthesis.
is reported by Robinson andVenkataraman [12]. Several reported
methods are available for synthesis of a flavone skeleton but
one of the most commonly used methods are acid-catalyzed
cyclization of various 1,3-diketone derived from 2-hydroxy-
acetophenones [13,14].Various types of catalysts used for cycli-
zation of 1-(2-hydroxyphenyl)-3-aryl-1,3-propanedione. But
most of the reported ways of synthesis of flavones suffer from
considerable restrictions such as use of harsh reaction conditions,
prolonged reaction time, low yields, toxic and expensive catalysts/
reagents and solvents. In view of these restrictions, there is
still a need for exploration of effortless, efficient, rapid, eco-
compatible, and widely applicable routes for the synthesis of
flavone owing to their immense synthetic and medicinal rele-
vance. Also, due to the severe and burgeoning environmental
regulations, organic chemists are call for the development of
environmentally benevolent synthetic methodologies. Organic
reactions under solvent-free conditions using microwave irradi-
ation and conventional heating techniques has become a center
of attraction for researchers over the years [15-17].
INTRODUCTION
Flavones are class of the flavonoids composing a 2-phenyl-
1-benzopyran-4-one backbone structure, which is present in
fruits and vegetables ubiquitously. We consume flavanoids in
our daily diet by inadvertently which have a beneficial impact
on our health [1]. The flavones exhibited various biological
activities based on the position and nature of the various substi-
tuent on the flavone structure. The flavones show a broad spectrum
of biological activities due to their unique capacity to modulate
different enzyme systems [2].
The flavones are active against infectious and metabolic
diseases such as antiestrogenic, antimicrobial [3], anti-inflam-
matory, antioxidant [4], antiallergic, antitumor, vascular and
cytotoxic [5] activities. In many natural products, flavone structure
is frequently found such as luteolin and kaempferol. Due to
their fascinating structural features, they exhibit array of biol-
ogical activities, especially, anti-inflammatory, antimicrobial,
antitumor and cytotoxic [6-11].
A verity of methods are used for synthesis of different
flavones, which broadly can be classified into two groups viz.
chalcones and 1,3-diketones as intermediates synthesized from
o-hydroxyacetophenone. Most of the recently used methods
for the synthesis of flavones depends on 1,3-diketones which
Recently, zinc oxide nanoparticles have received a signi-
ficant attention because of its affordability, non-toxicity and
has environmental advantages i.e. low corrosion, minimum
completion time, easy transport, reuse of the catalyst, low waste
and demolition of the catalyst [18]. Because of low coordi-
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1134 Unde et al.
Asian J. Chem.
nation sites and higher surface area, ZnO nanoparticles are
considered to be more reactive.As the size decreases, the surface
area of catalyst increases immensely which is useful for the
greater catalytic activity. Some researchers have synthesized
flavones from chalcones by using ZnO nanoparticles [19-21].
Due to huge number of advantages correlated with this
eco-friendly nature, it has been used as a powerful catalyst for
various organic transformations [22,23]. Hence, in present
work in green synthesis of flavones by various methods, here
we report an efficient and eco-friendly catalytic application
of zinc oxide nanoparticles for synthesis of flavone from
cyclodehydration of various 1,3-propanedione in microwave
or conventional heating under solvent free conditions.
(300 MHz, CDCl3): δ 6.81 (s, 1 H), 7.39 (t, J = 7.8 Hz, 1H),
7.46-7.56 (m, 4H), 7.65-7.70 (m, 1H), 7.88-7.91 (m, 2H), 8.21
(d, J = 7.2 Hz, 1H); 13C NMR (75 MHz, CDCl3): δ 107.3, 117.9,
123.7, 125.1, 125.5, 126.1, 128.9, 131.5, 131.6, 133.7, 156.1,
163.3, 178.3 ; LC-MS m/z: 223 (M+H)+.
2-(4-Methoxyphenyl)chromen-4-one (4b): m.p. 157-
158 ºC [lit. 21]; IR (KBr, νmax, cm-1): 767, 1133, 1465, 1608,
1649; 1H NMR (300 MHz, CDCl3): δ 3.87 (s, 3H), 6.80 (s, 1H),
7.01 (m, 2H), 7.40 (t, J = 7.2 Hz), 7.68 (m, 1H), 7.53 (d, J = 8.1
Hz), 7.88 (d, 2H, J = 7 Hz), 8.21 (dd, J = 8.2 & 2.1 Hz, 1H);
13C NMR (75 MHz, CDCl3): δ 55.4, 105.9, 114.4, 117.9, 123.7,
123.9, 125.1, 125.6, 127.9, 133.6, 156.1, 162.4, 163.5, 178.5 ;
LC-MS m/z: 253 (M+H)+.
2-(4-Fluorophenyl)chromen-4-one (4c): m.p. 148-150
ºC [lit. 21]; IR (KBr, νmax, cm-1): 755, 806, 869, 1134, 1234,
1467, 1574, 1608, 1663; 1H NMR (300 MHz, CDCl3): δ 6.79
(s, 1H), 7.23 (m, 2H), 7.47 (t, J = 7.1 Hz, 1H), 7.58 (d, J = 7.1
Hz, 1H), 7.72 (m, 1H), 7.94 (m, 2H), 8.24 (d, J = 7.2 Hz, 1H);
13C NMR (75 MHz, CDCl3): δ 107.1, 116.6, 117.0, 123.9, 125.5,
126.0, 127.1, 128.1, 134.2, 156.0, 161.2, 162.6, 178.1; LC-
MS m/z: 241 (M+H)+.
7-Methoxy-2-phenyl-chromen-4-one (4d): m.p. 109-110
ºC [lit. 24]; IR (KBr, νmax, cm-1): 767, 908, 1163, 1450, 1606,
1626, 1647; 1H NMR (300 MHz, CDCl3): δ 3.91 (s, 3H), 6.77
(s, 1H), 6.95 (m, 2H), 7.51 (m, 3H), 7.88 (m, 2H), 8.11 (d, J = 8.7
Hz, 1H); 13C NMR (75 MHz, CDCl3): δ55.8, 100.3, 107.3, 114.9,
117.6, 126.0, 126.9, 128.9, 131.4, 131.6, 157.8, 162.9, 164.1,
177.7; LC-MS m/z: 253 (M+H)+.
7-Methoxy-2-(4-fluorophenyl)-chromen-4-one (4d):
m.p. 172-173 ºC [lit. 21]; IR (KBr, νmax, cm-1): 781, 841, 1022,
1294, 1456, 1514, 1608, 1660; 1H NMR (300 MHz, CDCl3):
δ 3.84 (s, 3H), 6.41-6.48 (m, 2H), 6.64 (s, 1H), 7.15 (t, J = 8.7
Hz, 2H), 7.89 (m, 1H), 7.90 (m, 2H); 13C NMR (75 MHz, CDCl3):
δ 55.6, 101.3, 108.1, 110.1, 115.7, 116.1, 128.9, 128.9, 130.1,
163.4, 165.3, 165.9, 166.7, 174.8; LC-MS m/z: 271 (M+H)+.
7-Methoxy-2-(4-methoxyphenyl)chromen-4-one (4f):
m.p. 194-195 ºC [lit. 24]; IR (KBr, νmax, cm-1): 862, 977, 1186,
1260, 1379, 1516, 1593, 1629; 1H NMR (300 MHz, CDCl3):
δ 3.82 (s, 3H), 3.86 (s, 3H), 6.86 (m, 2H), 6.60 (s, 1H), 6.93 (d,
J = 8.4 Hz, 2H), 7.77 (d, J = 8.4 Hz, 2H), 8.05 (d, J = 8.4 Hz,
1H); 13C NMR (75 MHz, CDCl3): δ 55.3, 55.7, 100.2, 105.9,
114.0, 114.2, 117.6, 126.7, 127.6, 157.7, 162.1, 162.8, 163.9,
177.6; LC-MS m/z: 283 (M+H)+.
6-Methoxy-2-phenyl-chromen-4-one (4g): m.p. 160-161
ºC [lit. 24]; IR (KBr, νmax, cm-1): 658, 846, 1030, 1255, 1361,
1488, 1618, 1641; 1H NMR (300 MHz, CDCl3): δ 3.89 (s, 3H),
6.79 (s, 1H), 7.28 (dd, J = 6.6 & J = 3.5 Hz, 1H), 7.46-7.50 (m,
4H), 7.57 (d, J = 2.7 Hz, 1H), 7.88-7.91 (m, 2H); 13C NMR
(75 MHz, CDCl3): δ 55.8, 104.7, 106.7, 119.4, 123.6, 124.4,
126.1, 128.8, 131.4, 131.7, 150.8, 156.9, 163.0, 178.2; LC-
MS m/z: 253 (M+H)+.
EXPERIMENTAL
All the chemical reactions were performed in corning glass-
ware which is oven-dried. All chemicals and solvents were
obtained from professional suppliers and used as such for
further reactions. Thin-layer chromatography (TLC) was carried
out on Merck silica gel plates, visualized with a UV lamp and
stained in DNP solution and KMnO4. The standard methods are
used for synthesis of 1-(2-hydroxyphenyl)-3-aryl-1,3-propane-
diones and their purities were established before utilization
by melting point. Melting points were determined by open
capillary tubes in paraffin oil bath. 1H and 13C NMR spectra
were obtained as solutions in CDCl3 solvents. The standard
1H NMR (300 MHz) and 13C NMR (75 MHz) were recorded on
a Varian Mercury spectrometer in CDCl3 solution. The tetra-
methylsilane was used as an internal standard. Perkin Elmer
Model 1600 series FTIR instrument was used to recorded IR
spectra. LC/MS (ES-API) instrument were used record Mass
spectra. The QTOF Micromass Mass Spectrometer (electro
spray ionization mode) was used for high-resolution mass spectra
(HRMS). All the compounds synthesized in this article were
previously reported. The spectroscopic and physical data is in
matching with reported values.
Synthesis of flavones using ZnO nanoparticles
By conventional method: A mixture of substituted 1,3-
propanedione (0.5 g, 2.08 mmol, 1.0 eq.) and ZnO nanoparticles
(1.0 eq.) was warmed in an oil bath at 100 ºC for 30 min.After
the finishing of reaction (TLC check), the product of reaction
was allowed to cool at room temperature. To this reaction mixture
10 mL of water and 10 mL of ethyl acetate were added. Organic
layer was isolated and the water layer was extracted in (2 × 10
mL) ethyl acetate. Mixed organic extract was dried over anhydrous
sodium sulfate and concentrated in vacuum. Catalyst recycles
by filtration of aqueous layer. The column chromatography is
used for purification of crude product by using silica gel in
hexane-ethylacetate solvent system to give the corresponding
flavones (4a-l) in high yields.
2-(4-Fluorophenyl)-6-methoxy-chromen-4-one (4h):
m.p. 152-153 ºC [lit. 24]; IR (KBr, νmax, cm-1): 719, 910, 1024,
1168, 1488, 1620, 1661, 1727; 1H NMR (300 MHz, CDCl3):
δ 3.90 (s, 3H), 6.76 (s, 1H), 7.18-7.30 (m, 3H), 7.48 (d, J = 9.3
Hz, 1H), 7.58 (d, J = 2.4 Hz, 1H), 7.92 (d, J = 9 Hz, 2H); 13C
NMR (75 MHz, CDCl3): δ55.9, 104.8, 116.1, 116.4, 119.4, 123.8,
124.4, 128.3, 128.4, 150.9, 157.0, 162.1, 166.3, 178.1; LC-MS
Microwave method:A mixture of substituted 1,3-propane-
dione (0.5 g, 2.08 mmol, 1.0 eq.) and ZnO nanoparticles (1.0
eq.) was heated in a microwave oven for 15 min. Same procedure
used for purification of flavones as mentioned in the conven-
tional heating method.
2-Phenyl-chromen-4-one (4a): m.p. 96-98 ºC [lit. 21];
IR (KBr, νmax, cm-1): 756, 1130, 1568, 1604, 1645; 1H NMR

Vol. 31, No. 5 (2019)
Microwave Assisted Efficient Synthesis of Flavone using ZnO Nanoparticles as Promoter 1135
TABLE-1
m/z: 271 (M+H)+; HRMS m/z [M+H]+: calcd for C16H11O3F:
271.0692.
EFFECT USE OF DIFFERENT AMOUNT OF
CATALYST ON THE SYNTHESIS OF FLAVONESa
6-Methoxy-2-(4-methoxyphenyl)chromen-4-one (4i):
m.p. 195-196 ºC [lit. 24]; IR (KBr, νmax, cm-1): 558, 817, 1014,
1196, 1268, 1454, 1584, 1607, 1647; 1H NMR (300 MHz, CDCl3):
δ 3.88 (s, 3H), 3.90 (s, 3H), 6.73 (s, 1H), 7.01 (d, J = 9.0 Hz,
2H), 7.27 (m, 1H), 7.58 (d, J = 3.0 Hz, 1H), 7.60 (d, J = 9.0
Hz, 1H), 7.86 (d, J = 9.0 Hz, 2H); 13C NMR (75 MHz, CDCl3):
δ 55.4, 55.9, 104.8, 105.4, 114.4, 119.3, 123.5, 124.1, 124.4,
127.8, 150.9, 156.8, 162.3, 163.1, 178.2; LC-MS m/z: 283
(M+H)+.
6-Fluoro-2-phenyl-chromen-4-one (4j): m.p. 128-129 ºC
[lit. 21]; IR (KBr, νmax, cm-1): 767, 835, 1176, 1359, 1570, 1624,
1660; 1H NMR (300 MHz, CDCl3): δ 6.82 (s, 1H), 7.39-7.46
(m, 1H), 7.50-7.61 (m, 4H), 7.85-7.93 (m, 3H); 13C NMR (75 MHz,
CDCl3): δ106.7, 110.7, 120.2, 121.7, 122.0, 126.2, 129.0, 131.3,
131.7, 152.3, 161.1, 163.1, 177.5 ; LC-MS m/z: 241 (M+H)+.
6-Bromo-2-phenyl-4H-chromen-4-one (4k): m.p. 176 ºC;
IR (KBr, νmax, cm-1): 756, 1130, 1568, 1604, 1645; 1H NMR
(300 MHz, CDCl3): δ 7.12 (s, 1H), 7.63 (m, 3H), 7.83 (d, J = 3.2
Hz, 1H), 8.11 (dd, J = 10.8 & 2.4 Hz, 1H), 8.13 (m, 3H); 13C
NMR (75 MHz, CDCl3): δ 107.4, 118.3, 121.7, 125.3, 126.9,
127.3, 129.6, 131.3, 132.5, 137.3, 155.1, 163.4, 176.3; HRMS
m/z [M+H]+: calcd for C15H9O2Br: 300.9864, found: 300.9867.
6-Chloro-2-phenyl-chromen-4-one (4l): m.p. 183-184
ºC [lit. 24]; IR (KBr, νmax, cm-1): 682, 908, 1132, 1307, 1438,
1457, 1567, 1601, 1651; 1H NMR (300 MHz, CDCl3): δ 6.82
(s, 1H), 7.50-7.56 (m, 4H), 7.60-7.65 (m, 1H), 7.88-7.90 (m, 2H),
8.17 (d, J = 2.3 Hz, 1H); 13C NMR (75 MHz, CDCl3): δ 107.3,
119.7, 124.7, 125.1, 126.3, 129.1, 131.1, 131.2, 131.8, 133.0,
154.4, 163.7, 177.1; LC-MS m/z: 257 (M+H)+.
O
O
O
ZnO
Heating, Solvent free
O
OH
3a
4a
Heating in oil bath
Heating in microwave
Entry
Catalyst
Time
(min)
Yield
(%)
Time
(min)
Yield
(%)
1
2
3
4
5
6
0.1 eq.
0.2 eq.
0.4 eq.
0.6 eq.
0.8 eq.
1.0 eq.
60
30
30
30
30
30
20
32
44
66
74
80
60
15
15
15
15
15
22
38
48
70
78
88
aReagent: 1-(2-hydroxyphenyl)-3-phenylpropane-1, 3-dione (0.5 g)
yields of the product remained comparable to subsequent experi-
ments (Fig. 1). Thus, it is finally inferred that the reusability
and recyclability of the catalyst remain constant without signi-
ficant loss of activity.
100
Yield
Run
50
0
1
2
3
4
5
Fig. 1. Recyclability of ZnO nano catalyst
RESULTS AND DISCUSSION
Optimization of reaction conditions:To study the catalytic
activity in synthesis of flavones from 1-(2-hydroxyphenyl)-3-
aryl-1,3-propanedione (3a) was chosen as a representation
reaction. For this, we first examined the synthesis of 2-phenyl-
4H-chromen-4-one (4a) in the presence of 0.1 equivalent of
ZnO nanoparticles (0.208 mmol, 0.012 g). While performing
conventional heating even after 60 min, the desired product
4a was produced in a low yield (entry 1, Table-1). Using 0.2
(0.442 mmol, 0.034 g) and 0.4 (0.832 mmol, 0.068 g)
equivalents of ZnO nanoparticles gives the desired amount of
product in moderate yields (entries 2 and 3, Table-1,), while
using 0.6 (0.10 g, 1.25 mmol) and 0.8 (0.166 mmol, 0.14 g)
equivalents of ZnO nanoparticles, there was insignificant raise
in the yield of the desired product (entry 4 and 5, Table-1).
Finally, it was observed that 1.0 (0.16 g, 2.08 mmol) equi-
valent of ZnO nanoparticles was the most suitable amount of
catalyst for achieving the desired product (Table-1, entry 6).
By applying the same approach under microwave heating, it
was also observed that the time taken for completion of this
reaction was now reduced to half of the time taken by conven-
tion heating (entry 1- 6, Table-1).
Next, to explore the diversity of this methodology, various
1-(2-hydroxyphenyl)-3-aryl-1,3-propanediones were tested
under the above reaction condition. Results showed that cycli-
zation of various 1-(2-hydroxyphenyl)-3-aryl-1,3-propanedi-
ones proceeded smoothly to completion within a short reaction
time to give the corresponding flavones in good to excellent
yields under both heating conditions (entry 1-10, Table-2).
Conclusion
In summary, a simple, rapid and environmentally benign
protocol is developed in which ZnO nanoparticles as an effective
promoter for synthesis of various flavones under solvent free-
conditions. Microwave assisted reactions are appeared to be
slightly more energy efficient as it requires less time for comp-
letion as compared to the traditionally heated reaction. This
protocol is more straight-forward because of mild reaction
conditions, economically viable and eco-friendly, thus providing
a very efficient alternative to traditional processes.
ACKNOWLEDGEMENTS
The authors are thankful to the Council of BCUD, Savitribai
Phule Pune University, Pune, India for financial assistance and
The Principal, Dr. D.K. Mhaske, Maharaja Jivajirao Shinde
Mahavidyalaya, Shrigonda, India for additional support for
this research project.
Regeneration of ZnO nanoparticles: The ZnO nanopar-
ticles catalyst reused and recovered by filtration of reaction
mixture. It was used for subsequent experiments (up to five times)
under the same reaction conditions. It was also observed that

1136 Unde et al.
Asian J. Chem.
TABLE-2
ZnO NANOPARTICLE PROMOTED SYNTHESIS OF FLAVONES FROM 1-(2-HYDROXYPHENYL)-3-ARYL-1,3-PROPANEDIONEa
O
O
O
ZnO, 100 °C
R2
R1
R3
Heating, Solvent free
O
R3
OH
4
3
Conventional heatingb
Microwave heatingc
Entry
Compd.
R1
R2
R3
Yieldd (%)
Yieldd (%)
1
2
3
4
5
6
7
8
9
H
H
H
OMe
OMe
OMe
H
H
H
H
H
H
H
H
H
H
H
OMe
OMe
OMe
F
Ph
4-MeOC6H4
4-FC6H4
Ph
4-FC6H4
4-MeOC6H4
Ph
4-FC6H4
4-MeOC6H4
Ph
80
81
78
79
80
80
81
80
81
78
79
80
88
87
88
86
88
87
88
86
89
86
85
87
4a
4b
4c
4d
4e
4f
4g
4h
4i
10
11
12
4j
4k
4l
Br
Cl
Ph
Ph
H
aReagents: (1) (0.5 g.) various 1, 3-propanediones (3a-3m, 1.0 eq.), (2) ZnO nanoparticle (1.0 eq.). b30 min in conventional heating. c15 min in
microwave. dIsolated yields.
12. J. Allan and R. Robinson, J. Chem. Soc., 125, 2192 (1924);
CONFLICT OF INTEREST
https://doi.org/10.1039/CT9242502192.
13. W. Baker, J. Chem. Soc., 1381 (1933);
https://doi.org/10.1039/jr9330001381.
14. H.S. Mahal and K. Venkataraman, J. Chem. Soc., 1767 (1934);
The authors declare that there is no conflict of interests
regarding the publication of this article.
https://doi.org/10.1039/jr9340001767.
15. N.M. Thorat, S.R. Kote and S.R. Thopate, Lett. Org. Chem., 11, 601 (2014);
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