A novel synthesis of 5-perfluorophenyl 4,5-dihydro-1H-pyrazoles in THF or water

Article abstract of DOI:10.1016/j.jfluchem.2007.06.010

5-Perfluorophenyl 4,5-dihydro-1H-pyrazoles were synthesized from 1,3-dipolar cycloaddition reaction of perfluorobenyl 2,4,6-triisopropylbenzenesulfonylhydrazone and α,β-unsaturated carbonyl compounds or acrylonitrile in THF or water. It was worthy to note

Full text of DOI:10.1016/j.jfluchem.2007.06.010

Journal of Fluorine Chemistry 128 (2007) 1379–1384
www.elsevier.com/locate/fluor
A novel synthesis of 5-perfluorophenyl 4,5-dihydro-1H-pyrazoles
in THF or water
b,
Wan Pang a,b, Shifa Zhu b, Huanfeng Jiang a, , Shizheng Zhu
*
*
a College of Chemistry, South China University of Technology, Guangzhou 510640, China
b Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China
Received 20 May 2007; received in revised form 10 June 2007; accepted 24 June 2007
Available online 6 July 2007
Abstract
5-Perfluorophenyl 4,5-dihydro-1H-pyrazoles were synthesized from 1,3-dipolar cycloaddition reaction of perfluorobenyl 2,4,6-triisopropyl-
benzenesulfonylhydrazone and a,b-unsaturated carbonyl compounds or acrylonitrile in THF or water. It was worthy to note that better results were
obtained when water was employed as the solvent, which was considered as an effective, economic and environmentally friendly method to
synthesize these pyrazole derivatives.
# 2007 Elsevier B.V. All rights reserved.
Keywords: Perfluorobenyl; Pyrazoles; a,b-Unsaturated carbonyl compounds; Water
1. Introduction
The introduction of fluorine into organic molecules can
make a profound and unexpected influence on the physical
and biological properties. Recently, there has been growing
interest in fluorine-containing aromatic compounds due to the
unique physical properties of the fluorine atom [7b]. In our
previous work [7], we have reported a catalytic process for
the perfluorophenyl dizomethane with activated aryl alde-
hydes or tosyl imines to give perfluorophenyl-containing
epoxides, alkenes or aziridines exclusively with trans
structure (Scheme 1).
The Bamford–Stevens reaction has been widely used to
synthesize diazo compounds [1]. Usually, the reaction is carried
outinconventionalsolventsuchasbenzene,toluene, acetonitrile,
dioxane, N,N-dimethylformamide, tetrahydrofuran, hexane,
ethyl acetate, nitromethane, ether and methyl alcohol [2], which
greatly limited their attractiveness from the environmental point
of view. Water, being cheap, readily available, nontoxic and
environment concerns, results in a safer process and has clear
advantages as an environmentally friendly solvent alternative in
organic synthesis and is attracting more and more attention [3].
Furthermore, the use of water facilitates the catalyst-product
separation, which greatly interest chemists [4].
2. Results and discussions
As a part of our continuing study on the chemical
transformation of perfluorophenyl diazomethane [7], we
recently investigated the reactivity of dipolar cycloaddition
of perfluorophenyl diazomethane with a,b-unsaturated carbo-
nyl compounds or acrylonitrile. Initially, perfluorobenyl 2,4,
6-triisopropylbenzenesulfonylhydrazone 1 was treated with
1.2 equiv. quantity of methyl acrylate 2a in the presence of
triethylamine in THF at room temperature to give the
corresponding product 3a in 88% yield (Scheme 2). In the
reaction, the perfluorophenyl diazomethane was generated in
situ by Bamford–Stevens reaction using perfluorobenyl 2,4,6-
triisopropylbenzenesulfonylhydrazone in the presence of
1H-pyrazoles and 4,5-dihydro-1H-pyrazoles derivatives have
been found diverse applications in medicine and agriculture. In
particular, they are widely used as antitumor, antibacterial, anti-
fungal, anti-parasitic, anti-tubercular and insecticidal agents [5].
Some of these compounds also have anti-inflammatory, anti-
diabetic, anesthetic and analgesic properties [6]. Hence, various
mild methods capable of generating libraries of heterocycles of
this type are very attractive.
* Corresponding authors. Tel.: +86 21 54925184; fax: +86 21 64166128.
E-mail address: zhusz@mail.sioc.ac.cn (S. Zhu).
0022-1139/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2007.06.010

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W. Pang et al. / Journal of Fluorine Chemistry 128 (2007) 1379–1384
typical perfluorophenyl group at d = À141.5, À153.4 and
À160.8 ppm in the 2:1:2 integral. Its infrared spectrum has the
strong absorption of N–H at 3277 cmÀ1
.
Under the prompting of such good result mentioned above,
we examined a variety of alkenes under the same reaction
conditions. However, only the terminal alkenes bearing
electron-withdrawing group such as acrylate, acrylonitrile
and vinyl ketone could give the desired products in high yields.
Other terminal alkenes bearing electron-donating groups such
as ethyl vinyl ether and butyl vinyl ether, styrene and push-pull
alkenes did not gave the desired products. The results were
summarized in Table 1 (Method A).
Scheme 1.
Table 1
Reaction results of 1 with 2
Entry
Y in 2
Product
Yield (%)a
M.P. (8C)
Method Ab
Method Bc
The reaction mechanism is proposed as follows (Scheme 3)
[8]. Firstly, triethylamine was added to a colorless solution of 1
in THF, then there appeared an intense yellow coloration,
indicating the formation of a diazo compound A which reacted
with a,b-unsaturated carbonyl compounds or acrylonitrile 2 to
form 1,3-dipolar cycloaddition product 5-(perfluorophenyl)-
4,5-dihydro-3H-pyrazole-3-carboxylate I. The intermediate I
was unstable, which, on one hand, isomerized quickly to the
corresponding product 3, on the other hand decompose to diazo
compound and dipolarophile by two routes. The corresponding
products (3a-g) were colorless crystals. However, when they
were dissolved in organic solvents for a few hours, the colorless
solution gradually changed to primrose. It was worth to notice
that in the case of ethyl methacrylate 2h, because of the methyl
group, the product was ethyl 5-methyl-3-(perfluorophenyl)-4,5-
dihydro-1H-pyrazole-5-carboxylate, and it was buff liquid. Its
1H NMR spectra clearly conformed to its structure.
1
2
3
4
5
6
7
CO2Me (2a)
CO2Et (2b)
CO2Bui (2c)
CO2Bun (2d)
CO2But (2e)
CN (2f)
3a
3b
3c
3d
3e
3f
88
90
98
99
99
99
99
99
99
45
130–132
128–130
127–129
119–121
139–141
124–126
127–129
94
95
C(O)Me (2g)
3g
a
Isolate yield base on 1.
Method A: THF as the solvent.
Method B: water as the solvent.
b
c
triethylamine, which avoided the hazard of diazomethane in its
handling.
The configuration of product 3a was fully characterized by
various spectra. From its MS, it is clearly showed the molecular
ion peak at 294 (M+) and basic peak at 194 (C6F5CHN2 ) in EI.
The 1H NMR spectrum of 3a showed a broad peak at d = 6.38
(s, NH), and the no equal ring CH2 at 3.14 and 3.43,
respectively, they couple with each other (J = 17 Hz). Another
ring proton is 5.40 ppm. 19F NMR spectrum of 3a exhibited a
+
Recently, water as the green solvent for chemical reactions
has attracted considerable attention because of cost, safety and
environment concerns. This prompted us to investigate whether
Scheme 2.
Scheme 3.

W. Pang et al. / Journal of Fluorine Chemistry 128 (2007) 1379–1384
1381
Table 2
˚
Selected bond lengths (A) and angles (8) of compound 3a and 3f
Compound 3a
Compound 3f
˚
Bond lengths (A)
˚
Bond lengths (A)
Angles (8)
Angles (8)
C6–C7: 1.513(3)
C7–N1: 1.477(3)
N1–N2: 1.342(2)
N2–C9: 1.291(3)
C9–C8: 1.507(3)
C8–C7: 1.554(3)
C9–C10: 1.463(3)
C6–C7–N1: 109.48(17)
C7–N1–N2: 113.05(17)
N1–N2–C9: 109.75(16)
N2–C9–C8: 113.71(17)
C9–C8–C7: 101.19(17)
C8–C7–C6: 115.80(16)
C10–C9–C8: 128.27(18)
C1–C7: 1.507(4)
C7–N1: 1.473(3)
N1–N2: 1.334(3)
N2–C9: 1.292(3)
C9–C8: 1.494(4)
C8–C7: 1.542(4)
C9–C10: 1.423(4)
C1–C7–N1: 110.6(2)
C7–N1–N2: 114.0(2)
N1–N2–C9: 108.5(2)
N2–C9–C8: 114.5(2)
C9–C8–C7: 101.3(2)
C8–C7–C1: 116.5(2)
C10–C9–C8: 124.5(2)
C10–C9–N2: 120.6(3)
C
10–C9–N2: 118.02(17)
Angle of phenyl ring-five membered: 89.378
Angle of phenyl ring-five membered: 87.058
this reaction could occur in water. Subsequently, we attempted
the reaction of 1 with 1.2 equiv. quantity of methyl acrylate 2a
in the presence of triethylamine in water at room temperature.
Fortunately, the corresponding 1,3-dipolar cycloadditon pro-
duct was obtained in quantitative yield though the reaction
system was not homogenous. The results were summarized in
Table 1 (Method B).
The structure of 3a and 3f were further confirmed by the X-
ray crystal diffraction analysis (Figs. 1 and 2). It is noteworthy
that the torsion angle of phenyl and five membered ring was
almost 908. The selected bond lengths and angles of compound
3a and 3f were summarized in Table 2.
3. Conclusions
Excited by the above results, we would like to know whether
the reaction could also occur without triethylamine. Accord-
ingly, 1 was treated with 1.2 equiv. quantity of methyl acrylate
2a in water at room temperature for more than 1 week.
Unfortunately, the start materials were unchangeable (mon-
itored by TLC) and 1 was reclaimed (Table 1, entry 2), which
showed that the reaction could not occur absolutely without
base.
In summary, we described a novel method to synthesize the
5-perfluorophenyl 4,5-dihydro-1H-pyrazoles through the Bam-
ford–Stevens reaction and dipolar cycloaddition reaction in
THF or water. We were excited to find that the corresponding
reactions of 1 and 2 proceeds very efficiently in a water
suspension medium and that 4,5-dihydro-1H-pyrazole deriva-
tives 3 are obtained in quantitative, simple operation and easier
product work-up can be achieved. This methodology is superior
from the point of view of yield and is more environmentally
friendly than the reported methods.
4. Experimental
Melting points were measured in a SGW1X-4 micro-
melting point apparatus and were uncorrected. 1H and 19F
NMR spectra were recorded in CDCl3 (unless mentioned in
text), Bruker AM-300 spectrometer with Me4Si and CFCl3
(with upfield negative) as the internal and external standards,
respectively. IR spectra were obtained with a Nicolet AV-360
spectrophotometer. Lower resolution mass spectrum or high-
resolution mass spectra (HRMS) were obtained on a Finnigan
GC-MS 4021 or a Finnigan MAR-8430 instrument using the
electron impact ionization technique (70 eV), respectively. The
X-ray structural analysis was performed with a Rigaku/AFC 7R
Diffractometer. Elemental analyses were performed by this
institute.
Fig. 1. Crystal structure of 3a.
4.1. General procedure for the reaction in THF
To a mixture of 2 (1.2 equiv.) and Et3N (51 mg, 0.5 mmol,
1.0 equiv.) in THF (2 mL), a solution of 1 (238 mg, 0.5 mmol)
in THF (2 mL) was added dropwise over 2 h by the means of a
syringe pump at room temperature. After addition, the reaction
mixture was stirred for another 1 h until complete consumption
Fig. 2. Crystal structure of 3f.

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W. Pang et al. / Journal of Fluorine Chemistry 128 (2007) 1379–1384
of starting material as judged by TLC analysis. Removal of the
solution in vacuo, and the residue was purified by silica gel
chromatography using ethyl ester-hexane as eluent to afford the
desired products 3.
respectively. All calculations were performed using the
SHELX-97 program.
4.2.2. Ethyl 5-(perfluorophenyl)-4,5-dihydro-1H-pyrazole-
3-carboxylate 3b
4.2. General procedure for the reaction in water
To a suspension of a mixture of 1 (238 mg, 0.5 mmol) and 2
(1.2 equiv.) in water (2 mL), Et3N (51 mg, 0.5 mmol, 1.0 equiv.)
was added dropwise with vigorous stirring over 42 min. After
addition, the reaction mixture was stirred for another 1 h at room
temperature until complete consumption of starting material as
judged by TLC analysis. The reaction mixture was diluted with
water (10 mL) and extracted with ether (3 Â 8 mL) and the
combined organic phases were washed with brine and dried over
anhydrous Na2SO4, filtered and concentrated under reduced
pressure using a rotary evaporator to afford the desired products
3. The yield was almost quantitative. The crude crystals thus
obtained were recrystallized from ethyl ester and hexane to give
pure 3 as colorless crystal.
IR (nmax, cmÀ1): 3328, 2986, 1699, 1556, 1527, 1500,
1
1406, 1338, 1289, 1245, 1120, 1012, 985, 945, 872, 750. H
NMR d (ppm): 6.39 (s, 1H), 5.39 (dd, J1 = 10 Hz, J2 = 13 Hz,
1H), 4.33 (q, J = 4 Hz, 2H), 3.42 (dd, J1 = 13 Hz, J2 = 17 Hz,
1H), 3.13 (dd, J1 = 10 Hz, J2 = 17 Hz, 1H), 1.37 (t, J = 7 Hz,
3H). 19F NMR d (ppm): À141.8ꢀÀ141.9 (m, 2F), À153.9 (t,
J = 20 Hz, 1F), À161.3ꢀÀ161.4 (m, 2F). MS m/z (ion, %): 308
(M+, 56), 263 (27), 234 (35), 194 (100), 168 (9), 141 (21), 95
(48). Anal. Calcd for C12H9F5N2O2: C; 46.76, H; 2.94, N;
9.09%. Found: C; 46.82, H; 2.98; N; 9.07%.
4.2.1. Methyl 5-(perfluorophenyl)-4,5-dihydro-1H-
pyrazole-3-carboxylate 3a
4.2.3. Isobutyl 5-(perfluorophenyl)-4,5-dihydro-1H-
pyrazole-3-carboxylate 3c
IR (nmax, cmÀ1): 3277, 1689, 1551, 1501, 1444, 1358,
1
1254, 1131, 1012, 953, 874, 750. H NMR d (ppm): 6.38 (s,
IR (nmax, cmÀ1): 3298, 2969, 1734, 1655, 1506, 1386,
1310, 1255, 1124, 993, 826, 788, 753. 1H NMR d (ppm): 5.39
(dd, J1 = 10 Hz, J2 = 13 Hz, 1H), 4.28 (t, J = 7 Hz, 2H), 3.42
(dd, J1 = 13 Hz, J2 = 17 Hz, 1H), 3.14 (dd, J1 = 10 Hz,
J2 = 17 Hz, 1H), 1.77ꢀ1.68 (m, 2H), 1.47ꢀ1.39 (m, 2H),
0.96 (t, J = 7 Hz, 3H). 19F NMR d (ppm):À141.3 ꢀÀ141.4 (m,
2F), -153.4 (t, J = 20 Hz,1F), À160.8ꢀÀ161.0 (m, 2F). MS m/z
(ion, %): 336 (M+, 20), 280 (94), 263 (93), 234 (64), 194 (100),
168 (9), 141 (21), 57 (35), 41 (55). Anal. Calcd for
C14H13F5N2O2: C; 50.01, H; 3.90, N; 8.33%. Found: C;
49.98, H; 3.92; N; 8.23%.
1H), 5.40 (dd, J1 = 10 Hz, J2 = 13 Hz, 1H), 3.88 (s, 3H), 3.43
(dd, J1 = 13 Hz, J2 = 17 Hz, 1H), 3.14 (dd, J1 = 10 Hz,
J2 = 17 Hz, 1H). 19F NMR d (ppm): À141.5ꢀÀ141.6 (m,
2F), À153.4 (t, J = 21 Hz, 1F), À160.8ꢀÀ161.0 (m, 2F). 13C
NMR d (ppm): 162.6, 145.2 (dm, J = 253.7 Hz), 141.5, 141.1
(dm, J = 248.2 Hz), 137.7 (dm, J = 252.8 Hz), 114.3 (t,
J = 15 Hz), 54.0, 52.3, 37.3. MS m/z (ion, %): 294 (M+, 68),
234 (28), 194 (100), 168 (12), 127 (12), 95 (43). Anal. Calcd for
C11H7F5N2O2: C; 44.91, H; 2.40, N; 9.52%. Found: C; 44.89,
H; 2.39; N; 9.56%.
4.2.1.1. X-ray data of 3a. C11H7F5N2O2: MW = 294.19,
CCDC no. 632046, monoclinic, space group: P2(1)/c, a =
4.2.4. Butyl 5-(perfluorophenyl)-4,5-dihydro-1H-pyrazole-
3-carboxylate 3d
´
´
´
˚
˚
˚
12.4056(18) A, b = 7.5033(11) A, c = 13.628(2) A; a = 90.008,
´
3
˚
b = 115.458(3)8, g = 90.008; V = 1145.4(3) A , Z = 4, Dc =
1.706 g/cm3, F(000) = 592. Radiation, Mo Ka (l = 0.71073
´
˚
A). Crystal dimension, 0.501 mm  0.453 mm  0.307 mm.
Intensity data were collected at 293(2) K with a Bruker P4
four-circle diffractometer with graphite monochromator and
´
IR(nmax, cmÀ1): 3283, 2966, 1574, 1523, 1503, 1456, 1362,
1253, 1172, 1120, 1037, 1006, 950, 881, 756.1H NMR d (ppm):
5.40 (dd, J1 = 10 Hz, J2 = 13 Hz, 1H), 4.06 (d, J = 7 Hz, 2H),
3.43 (dd, J1 = 13 Hz, J2 = 17 Hz, 1H), 3.15 (dd, J1 = 10 Hz,
˚
Mo Ka radiation (l = 0.71073 A). A total of 2615 independent
reflection were measured in range 1.82 < u < 28.238. The
structure was solved by directed methods and expanded using
Fourier techniques. The nonhydrogen atoms were refined
anisotropically, hydrogen atoms were included but not refined.
The final cycle of fullmatrix least-square refinement was based
on F2. The final R and wR value were 0.0457 and 0.1084,
J2 = 17 Hz, 1H), 2.10ꢀ2.02 (m, 1H), 0.98 (d, J = 7 Hz, 6H). 19
F
NMR d (ppm): À141.4ꢀÀ141.5 (m, 2F), À153.5 (t, J = 21 Hz,
1F), À160.89ꢀÀ161.1 (m, 2F). MS m/z (ion, %): 336 (M+, 28),

W. Pang et al. / Journal of Fluorine Chemistry 128 (2007) 1379–1384
1383
280 (65), 263 (55), 234 (47), 194 (67), 168 (4), 57 (35), 41
(100). Anal. Calcd for C14H13F5N2O2: C; 50.01, H; 3.90, N;
8.33%. Found: C; 50.05, H; 3.88; N; 8.24%.
4.2.7. 1-(5-(Perfluorophenyl)-4,5-dihydro-1H-pyrazol-3-
yl)ethanone 3g
4.2.5. tert-butyl 5-(perfluorophenyl)-4,5-dihydro-1H-
pyrazole-3-carboxylate 3e
IR (nmax, cmÀ1): 3314, 1641, 1541, 1499, 1418, 1347,
1241, 1132, 1020, 994, 949, 874, 802, 767. 1H NMR d (ppm):
6.38 (s, 1H), 5.38 (dd, J1 = 10 Hz, J2 = 13 Hz, 1H), 3.36 (dd,
J1 = 13 Hz, J2 = 17 Hz, 1H), 3.05 (dd, J1 = 10 Hz, J2 = 17 Hz,
1H), 2.46 (s, 3H). 19F NMR d (ppm): À141.8ꢀÀ141.9 (m, 2F),
À153.5 (t, J = 20 Hz, 1F), À160.8ꢀÀ161.0 (m, 2F). MS m/z
(ion, %): 278 (M+, 9), 263 (5), 235 (8), 194 (9), 181 (2), 168 (1),
111 (17), 43 (100). Anal. Calcd for C10H7F5N2O2: C; 47.49, H;
2.54, N; 10.07%. Found: C; 47.53, H; 2.58; N; 10.01%.
IR(nmax, cmÀ1): 3292, 2991, 1665, 1562, 1524, 1452, 1436,
1393, 1368, 1256, 1139, 1036, 1008, 953, 750.1H NMR d (ppm):
5.35 (dd, J1 = 10 Hz, J2 = 13 Hz, 1H), 4.23 (s, 1H), 3.38 (dd,
J1 = 10 Hz, J2 = 17 Hz, 1H), 3.11 (dd, J1 = 10 Hz, J2 = 17 Hz,
1H), 1.56 (s, 9H). 19F NMR d (ppm):À141.7ꢀÀ141.8 (m, 2F),
À154.2 (t, J = 20 Hz, 1F), À161.5ꢀÀ161.7 (m, 2F). MS m/z
(ion, %): 336 (M+, 3), 280 (35), 263 (12), 234 (18), 194 (18), 168
(2), 57 (100), 41 (50). Anal. Calcd for C14H13F5N2O2: C; 50.01,
H; 3.90, N; 8.33%. Found: C; 50.19, H; 3.92; N; 8.24%.
4.2.8. Ethyl 5-methyl-3-(perfluorophenyl)-4,5-dihydro-1H-
pyrazole-5-carboxylate 3h
4.2.6. 5-(Perfluorophenyl)-4,5-dihydro-1H-pyrazole-3-
carbonitrile 3f
IR (nmax, cmÀ1): 3346, 2986, 1738, 1651, 1525, 1495,
1
1314, 1274, 1183, 1097, 990, 809, 765. H NMR d (ppm):
6.67 (s, 1H), 4.25 (q, J = 7 Hz, 2H), 3.66 (d, J = 17 Hz, 1H),
3.00 (d, J = 17 Hz, 1H), 1.61 (s, 3H), 1.31 (t, J = 7 Hz, 3H).
19F NMR d (ppm): À139.5ꢀÀ139.6 (m, 2F), À154.0 (t,
J = 20 Hz, 1F), À162.0ꢀÀ162.2 (m, 2F). MS m/z (ion, %):
323 (M+, 1), 249 (20), 195 (65), 167 (15), 73 (2), 43 (48).
HRMS-EI (70 eV, m/z): calcd for C13H11F5N2O2: 322.0741.
Found: 322.0756.
IR (nmax, cmÀ1): 3358, 2230, 1507, 1389, 1121, 1011, 952,
874.1H NMR d (ppm): 6.51 (s, 1H), 5.44 (dd, J1 = 10 Hz,
J2 = 13 Hz, 1H), 3.40 (dd, J1 = 13 Hz, J2 = 17 Hz, 1H), 3.09
(dd, J1 = 10 Hz, J2 = 17 Hz, 1H). 19F NMR d (ppm):
À141.4ꢀÀ141.5 (m, 2F), -152.3 (t, J = 21 Hz, 1F),
À160.1ꢀÀ160.3 (m, 2F). MS m/z (ion, %): 261 (M+, 90),
194 (100), 181 (55), 168 (21), 94 (60). Anal. Calcd for
C10H4F5N3: C; 45.99, H; 1.54, N; 16.09%. Found: C; 46.30, H;
1.48; N; 16.15%.
Acknowledgement
The authors thank the National Natural Science Foundation
of China (NNSFC) (Nos. 20625205 and 20532040) for
financial support.
4.2.6.1. X-ray data of 3f. C10H4F5N3: MW = 261.16, CCDC
no. 632045, monoclinic, space group: P2(1)/n, a = 15.2700(19)
´
´
´
˚
References
˚
˚
A, b = 5.2118(6) A, c = 39.706(5) A; a = 90.008, b =
´
3
˚
102.178(4)8, g = 90.008; V = 3088.9(7) A , Z = 12, Dc =
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