Microwave-assisted synthesis of pyrazoline derivatives on soluble polymer

Article abstract of DOI:10.1081/SCC-200030984

1-Phenyl-3-substituted-2-pyrazolinyl-5-carboxylates could be synthesized rapidly and regioselectively in good yield through the protocol of 1,3-dipolar cycloaddition of polyethylene glycol supported acrylic acid with aldehyde phenylhydrazones under microw

Full text of DOI:10.1081/SCC-200030984

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Microwave‐Assisted Synthesis
of Pyrazoline Derivatives on
Soluble Polymer
Min Xia a & Xue‐jie Pan a
a Department of Applied Chemistry , Zhejiang
Institute of Science and Technology , Hangzhou,
310033, P.R. China
Published online: 10 Jan 2011.
To cite this article: Min Xia & Xue‐jie Pan (2004) Microwave‐Assisted Synthesis
of Pyrazoline Derivatives on Soluble Polymer, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry,
34:19, 3521-3528, DOI: 10.1081/SCC-200030984
To link to this article: http://dx.doi.org/10.1081/SCC-200030984
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 19, pp. 3521–3528, 2004
Microwave-Assisted Synthesis of Pyrazoline
Derivatives on Soluble Polymer
*
Min Xia and Xue-jie Pan
Department of Applied Chemistry, Zhejiang Institute of Science and
Technology, Hangzhou 310033, P.R. China
ABSTRACT
1-Phenyl-3-substituted-2-pyrazolinyl-5-carboxylates could be synthe-
sized rapidly and regioselectively in good yield through the protocol of
1,3-dipolar cycloaddition of polyethylene glycol supported acrylic acid
with aldehyde phenylhydrazones under microwave irradiation.
Key Words: Liquid-phase synthesis; Polyethelene glycol; Pyrazoline
derivatives; Microwave.
It is known that pyrazoline derivatives are significant compounds, not
only as intermediates and agricultural pesticides, but as effective luminescent
and fluorescent substances as well.[1] There exist various approaches to their
*
Correspondence: Min Xia, Department of Applied Chemistry, Zhejiang Institute of
Science and Technology, Hangzhou 310033, P.R. China; E-mail: xiamin@hxcnc.com.
3521
DOI: 10.1081/SCC-200030984
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

3522
Xia and Pan
synthesis; however, 1,3-dipolar cycloaddition through nitrilimines generated
in situ is the most powerful and versatile route to construct the five-member
heterocycles.[2] A number of methods have been reported for the generation
in situ of nitrilimines, e.g., the base-induced dehydrochlorination of the hydra-
zonyl halides,[3] the thermal decomposition of 2,5-disubstituted tetrazoles[4] or
the sodium salt of 2-nitrohydrazones,[5] the photolysis of 3,4-disubstituted
sydones[6] or 2,5-disubstituted tetrazoles,[7] and the oxidation of aldehyde
arylhydrazones with lead tetraacetate.[8] Nevertheless, all of them have short-
comings such as troubles with operation, harsh conditions, difficulty in pre-
paration of starting substrates, low yields of nitrilimines, or the use of toxic
reagents. (Diacetoxy)iodobenzene is the most useful and promising reagent
among the hypervalent iodine compounds, which can be prepared readily
without any toxicity and is an effective oxidant, especially for the oxidation
of the compounds containing N atoms in good yields under mild conditions.[9]
In recent years, the application of microwave (MW) irradiation in organic
synthesis has been the focus of considerable attention and is becoming an
increasingly popular technology.[10] The prominent features of the microwave
approach are the rapid reactions, clean reaction conditions, and ease of
manipulation. Reactions in “dry media” or under solvent-free conditions are
especially appealing as they provide an opportunity to work with open
vessels, thus avoiding the risk of high pressure development. In the literature,
there exist many reports that 1,3-dipolar cycloadditions can be effectively
promoted through microwave irradiation,[11] thus affording the rapid and
convenient routines to the construction of five-member heterocycles.
Currently, organic synthesis of small molecular compounds on soluble
polymers, i.e., liquid-phase chemistry, has increasingly become an attractive
field.[12] It couples the advantages of homogeneous solution-phase chemistry
(high reactivity, lack of diffusion phenomena, and ease of analysis without the
cleavage-and-check procedure) with those of solid-phase chemistry (use of
excessive reagents, easy isolation and purification of products). Besides,
owing to the homogeneity of liquid-phase reactions, reaction conditions can
be readily shifted from solution-phase systems without many changes, and
the amount of excessive reagents is less than that in solid-phase reactions.
Among the various soluble polymers, polyethylene glycol (PEG) is the
most useful and promising. Due to the low melting point (mp) of PEG
(ꢀ518C), it can be readily melted to turn liquid under microwave heating.
Therefore, synthesis of small organic molecules on PEG by microwave
heating has the advantage in that PEG plays the roles of polymer support
and liquid solvent as well.[13] In connection with our work on the synthesis
of small molecular compounds on soluble polymer,[14] herein we report
the microwave-promoted synthesis of pyrazoline derivatives through 1,3-

Microwave-Assisted Synthesis of Pyrazoline Derivatives
3523
Scheme 1.
cycloaddition of aryldehyde phenylhydrazones with PEG-bound acrylate
(Sch. 1).
We selected PEG 4000 as the polymeric support due to its good compro-
mise of crystallization and loading capacity. The terminal hydroxyl groups
were active enough to be esterified by acryloyl chloride in the presence of
pyridine at room temperature overnight. The conversion of terminal hydroxyl
groups was quantitative, which could be determined by 1H NMR to check that
the signal of hydroxyl groups on PEG at 4.58 ppm disappeared. The PEG-
linked acrylate (1) was taken as the dipolarophile to react with the nitrilimines
generated in situ by the oxidation of aldehyde phenylhydrazones with (diace-
toxy)iodobenzene. The 1,3-dipolar cycloaddition was carried out smoothly
and rapidly under microwave heating within 4 minutes to form the PEG
loaded pyrazoline intermediates (2). During microwave irradiation, the
PEG-bound acrylate (1) was melted into a liquid, in which substrates were
ensured to react with each other in homogeneity. The PEG 4000 in our case
acted simultaneously as a polymeric support and as a solvent as well under
MW irradiation, transporting energy and diffusing the chemicals in the reac-
tion system. The target products of 1-phenyl-3-substituted-2-pyrazolinyl-
5-carboxylates (3) were completely released from the support [checked by
thin-layer chromatography (TLC)] in the media of 1 N NaOMe/MeOH solu-
tion at room temperature overnight. Owing to the excellent precipitation of
PEG in ether, the PEG-bound pyrazoline intermediates (2) could be obtained
in excellent purities after simple filtration and washing, avoiding the compli-
cated isolation and separation procedure. The target products could be
obtained without further chromatography.
The results are shown in Table 1, indicating that the 1,3-dipolar cyclo-
addition had obvious electronic effects. When the aldehydes had electron-
donating groups, the yields were excellent. However, when there existed

3524
Xia and Pan
Table 1. 1,3-Dipolar cycloaddition of nitrilimines with PEG-bound acrylate under
microwave irradition.a
Entry
3a
Ar
Yield (%)
84
Entry
3e
Ar
Yield (%)
88
3b
92
3f
94
3c
3d
86
74
3g
3h
61
0
aThe yields were determined by loading capacity of terminal hydroxyl groups on PEG
4000 with 0.5 mmol/g.
electron-withdrawing groups, the yields were low, especially for the strongly
electron-withdrawing NO2 group where the expected product could not be
obtained. Not only aromatic hydrazones but also heterocyclic and a,
b-unsaturated hydrazones were efficient substrates.
It was found that PEG-supported 1,3-dipolar cycloaddition was comple-
tely regioselective,[15] since only the 1-phenyl-3-substituted-2-pyrazolinyl-5-
1
carboxylates (3) were provided. The regioselectivity was determined by H
NMR that the chemical shifts of C5-H were at 4.8 ppm, at relatively low field.
In conclusion, we herein report the rapid and readily microwave-assisted
1,3-dipolar cycloaddition for pyrazoline derivatives on PEG support, utilizing
(diacetoxy)iodobenzene to generate nitrilimines in situ. The PEG-supported
synthesis of pyrazoline derivatives could give the products in good yields
and excellent purities with complete regioselectivity, avoiding the complicated
isolation and separation procedure of intermediates. Therefore, the protocol
described was an effective and easy approach to the pyrazoline derivatives.
EXPERIMENTAL SECTION
1. General procedure for the preparation of PEG 4000-supported acry-
late (1): At 08C, the acryoyl chloride (3 mmol) in anhydrous CH2Cl2
(10 mL) was added dropwise to the mixture of PEG 4000 (1 mmol,

Microwave-Assisted Synthesis of Pyrazoline Derivatives
3525
2 g) and pyridine (5 mmol) in anhydrous CH2Cl2 (15 mL) over 1
hour; the resultant mixture was stirred at room temperature overnight.
After removal of the solvent, the residue was dissolved in CH2Cl2
(10 mL), and cold ether (70 mL) was poured to precipitate the
white solid, which was washed by cold ether three times
(3 ꢀ 20 mL). After drying under reduced pressure, the PEG 4000-
bound acrylate (1) was obtained as a white powder. 1H NMR
(400 MHz, CDCl3): d (ppm) 3.53 ꢀ 3.84 (m, PEG backbone,
OCH2CH2O), 4.33 (t, J ¼ 4.8 Hz, 2 H, PEG-OCH2CH2OCO), 5.88
(dd, J ¼ 10 Hz, J ¼ 2 Hz, 1 H), 6.20 (dd, J ¼ 16.5 Hz, J ¼ 10 Hz,
1 H), 6.43 (dd, J ¼ 16.5 Hz, J ¼ 2 Hz, 1 H).
2. General procedure for the preparation of PEG 4000-supported
pyrazoline intermediates (2): At room temperature, the well-ground
mixture of PEG 4000-bound acrylate (1) (0.5 mmol, 1 g), (diacetoxy)
iodobenzene (1 mmol), and aldehyde phenylhydrazone (1 mmol) was
put in an open vessel and irradiated for 4 minutes at 450 w (irradiated
for 2 minutes and cooled for 1 minute, cycled for two times) in a
domestic microwave oven. After the mixture was cooled to room
temperature, CH2Cl2 (10 mL) was added and the resultant mixture
was filtrated. The filtrate was poured with ether (70 mL) and stirred
to precipitate the white solid, which was filtrated and washed with
ether three times (3 ꢀ 20 mL). After drying under reduced pressure,
the PEG 4000-bound pyrazoline intermediate (2) was obtained as a
white powder.
3. General procedure for the preparation of 1-phenyl-3-substituted-
2-pyrazolinyl-5-carboxylates (3): The PEG 4000-loaded pyrazoline
intermediate (2) was dissolved in a solution of 1 N NaOMe/MeOH
(15 mL). The resultant solution was stirred at room temperature over-
night. By pouring with water (30 mL), the solution was extracted with
ether (3 ꢀ 10 mL). After drying over anhydrous MgSO4, the organic
solvent was removed to give the target products without further
chromatography.
Compound 3a: mp 105 ꢀ 1068C (Lit.[15] 106 ꢀ 1078C); 1H NMR
(400 MHz, CDCl3): d (ppm) 3.42 (dd, J ¼ 17 Hz, J ¼ 6.5 Hz, 1 H), 3.68
(dd, J ¼ 17 Hz, J ¼ 12.5 Hz, 1 H), 3.75(s, 3 H), 4.83(dd, J ¼ 12.5 Hz,
J ¼ 6.5 Hz, 1 H), 6.88(t, J ¼ 7 Hz, 1 H), 7.12 (d, J ¼ 8 Hz, 2 H), 7.25 ꢀ 7.41
(m, 5 H), 7.72 (d, J ¼ 7 Hz, 2 H); FT-IR (KBr) y (cm21) 3029, 2952, 1737,
1596, 1504, 1493, 1397, 1336, 1263, 1135, 1016, 889, 745, 686; MS m/z
(%) 280 (Mþ, 33.34), 221 (100), 118 (14.79), 104 (18.45), 91 (28.05), 77
(53.97), 51 (30.91).

3526
Xia and Pan
1
Compound 3b: mp 1358C; H NMR (400 MHz, CDCl3): d (ppm) 3.38
(dd, J ¼ 7 Hz, J ¼ 17 Hz, 1 H), 3.62 (dd, J ¼ 13 Hz, J ¼ 17 Hz, 1 H), 3.75
(s, 3 H), 4.78 (dd, J ¼ 7 Hz, J ¼ 13 Hz, 1 H), 5.99 (s, 2 H), 6.80
(d, J ¼ 8 Hz, 1 H), 6.86 (t, J ¼ 7 Hz, 1 H), 7.02 (dd, J ¼ 1.5 Hz, J ¼ 8 Hz,
1 H), 7.09 (d, J ¼ 8 Hz, 2 H), 7.25 ꢀ 7.29 (m, 2 H) 7.38 (d, J ¼ 1.5 Hz, 1 H);
FT-IR (KBr) y (cm21) 3042, 2921, 1732, 1599, 1500, 1454, 1351, 1318,
1220, 1039, 936, 876, 812, 747, 694, 669, 619; MS m/z (%) 324 (Mþ,
51.86), 265 (100), 235 (10.85), 207 (24.05), 104 (9.44), 91 (9.86), 77
(34.16), 51 (16.19).
Compound 3c: mp 103 ꢀ 1048C; 1H NMR (400 MHz, CDCl3): d (ppm)
2.37 (s, 3 H), 3.40 (dd, J ¼ 17 Hz, J ¼ 7 Hz, 1 H), 3.67 (dd, J ¼ 13 Hz,
J ¼ 17 Hz, 1 H), 3.75 (s, 3 H), 4.81 (dd, J ¼ 13 Hz, J ¼ 7 Hz, 1 H), 6.87
(t, J ¼ 7.5 Hz, 1 H), 7.11 (d, J ¼ 8 Hz, 2 H), 7.20 (t, J ¼ 8 Hz, 2 H),
7.25 ꢀ 7.30 (m, 2 H), 7.60 (d, J ¼ 8 Hz, 2 H); FT-IR (KBr) y (cm21) 3027,
2950, 1740, 1597, 1497, 1378, 1321, 1268, 1200, 1122, 1031, 880, 821,
753, 692; MS m/z (%) 294 (Mþ, 38.29), 235 (100), 117 (10.44), 104 (9.56),
91 (22.54), 77 (28.18), 51 (13.71).
1
Compound 3d: mp 84 ꢀ 858C; H NMR (400 MHz, CDCl3) d (ppm)
3.39 (dd, J ¼ 6.5 Hz, J ¼ 17 Hz, 1 H), 3.62 (dd, J ¼ 13 Hz, J ¼ 17 Hz, 1 H),
3.73 (s, 3 H), 4.81 (dd, J ¼ 13 Hz, J ¼ 6.5 Hz, 1 H), 6.48 (dd, J ¼ 2 Hz,
J ¼ 3.5 Hz, 1 H), 6.63 (d, J ¼ 3.5 Hz, 1 H), 6.87 (t, J ¼ 7 Hz, 1 H), 7.09
(d, J ¼ 8 Hz, 2 H), 7.25 ꢀ 7.29 (m, 2 H), 7.49 (d, J ¼ 2 Hz, 1 H); FT-IR
(KBr) y (cm21) 3136, 2954, 1735, 1595, 1503, 1373, 1264, 1133, 1002,
922, 887, 804, 746, 690; MS m/z (%) 270 (Mþ, 58.45), 211 (100), 183
(18.37), 117 (8.83), 104 (8.89), 91 (13.05), 77 (42.28), 51 (27.00).
Compound 3e: mp 115 ꢀ 1178C (Lit.[15] 114 ꢀ 1158C); 1H NMR
(400 MHz, CDCl3): d (ppm) 3.38 (dd, J ¼ 7 Hz, J ¼ 17 Hz, 1 H), 3.64 (dd,
J ¼ 17 Hz, J ¼ 13 Hz, 1 H), 3.74 (s, 3 H), 3.83 (s, 3 H), 4.79 (dd, J ¼ 13 Hz,
J ¼ 7 Hz, 1 H), 6.86 (t, J ¼ 7.5 Hz, 1 H), 6.92 (d, J ¼ 9 Hz, 2 H), 7.10
(d, J ¼ 8 Hz, 2 H), 7.25 ꢀ 7.3 (m, 2 H), 7.66 (d, J ¼ 8.5 Hz, 2 H); FT-IR
(KBr) y (cm21) 3043, 2958, 1735, 1596, 1501, 1392, 1250, 1132, 1034,
879, 826, 743, 691; MS m/z (%) 310 (Mþ, 48.45), 251 (100), 162 (18.10),
135 (25.33), 117 (14.70), 104 (14.23), 91 (68.32), 77 (61.24), 57 (33.13), 51
(27.65), 43 (33.31).
1
Compound 3f: mp 123 ꢀ 1248C; H NMR (400 MHz, CDCl3): d (ppm)
3.32 (dd, J ¼ 6.5 Hz, J ¼ 17 Hz, 1 H), 3.56 (dd, J ¼ 13 Hz, J ¼ 17 Hz, 1 H),
3.75 (s, 3 H), 4.82 (dd, J ¼ 6.5 Hz, J ¼ 13 Hz, 1 H), 6.62 (d, J ¼ 16.5 Hz,
1 H), 6.88 (t, J ¼ 7.5 Hz, 1 H), 7.06 (d, J ¼ 7.5 Hz, 2 H), 7.19 (d, J ¼ 16.5 Hz,
1 H), 7.26 ꢀ 7.30 (m, 3 H), 7.36 (t, J ¼ 7.5 Hz, 2 H), 7.47 (d, J ¼ 7.5 Hz, 2 H);
FT-IR (KBr) y (cm21) 2950, 1741, 1599, 1501, 1324, 1200, 1122, 1038, 958,
882, 748, 691; MS m/z (%) 306 (Mþ, 58.42), 247 (100), 115 (15.11), 104
(17.99), 91 (19.36), 77 (76.95), 51 (33.04).

Microwave-Assisted Synthesis of Pyrazoline Derivatives
3527
Compound 3g: mp 143 ꢀ 1448C; 1H NMR (400 MHz, CDCl3): d (ppm)
3.63 (dd, J ¼ 6.5 Hz, J ¼ 17 Hz, 1 H), 3.74 (s, 3 H), 3.92 (dd, J ¼ 13 Hz,
J ¼ 17 Hz, 1 H), 4.84 (dd, J ¼ 6.5 Hz, J ¼ 13 Hz, 1 H), 6.89 (t, J ¼ 7 Hz,
1 H), 7.11 (d, J ¼ 8 Hz, 2 H), 7.25 ꢀ 7.30 (m, 4 H), 7.38 ꢀ 7.40 (m, 1 H),
7.84 ꢀ 7.86 (m, 1 H); FT-IR (KBr) y (cm21) 3064, 2952, 1739, 1599, 1502,
1435, 1389, 1322, 1265, 1204, 1141, 1036, 879, 750, 691, 666; MS m/z
(%) 314 (Mþ, 41.92), 255 (100), 117 (13.83), 104 (9.31), 91 (23.88),
77 (49.44), 51 (27.25).
ACKNOWLEDGMENT
We are grateful for the financial support of the Natural Science Fund of
Zhejiang Province (202031).
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Received in Japan April 22, 2004