Efficient one-pot synthesis of 2-chloro-1,1, 1-trifluoro-2-alkenes under solvent-free conditions

Article abstract of DOI:10.1080/00397910802431123

The improved one-pot Wittig reaction had been used to prepare trifluoromethyl-containing olefins under solvent-free conditions. Treatment of aldehydes with PPh3 and CF3CCl3 in the presence of K2CO3 at 100°C afforded 2-chloro-1,1,1-trifluoro-2-alkenes in g

Full text of DOI:10.1080/00397910802431123

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Efficient One-Pot Synthesis of
2-Chloro-1,1, 1-trifluoro-2-
alkenes Under Solvent-Free
Conditions
Mu-Wang Chen a , Xing-Guo Zhang a , Ping Zhong a &
Mao-Lin Hu a
a College of Chemistry and Materials Engineering,
Wenzhou University , Wenzhou, Zhejiang, China
Published online: 29 Jan 2009.
To cite this article: Mu-Wang Chen , Xing-Guo Zhang , Ping Zhong & Mao-Lin Hu
(2009) Efficient One-Pot Synthesis of 2-Chloro-1,1, 1-trifluoro-2-alkenes Under
Solvent-Free Conditions, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 39:5, 756-763, DOI:
10.1080/00397910802431123
To link to this article: http://dx.doi.org/10.1080/00397910802431123
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Synthetic Communications1, 39: 756–763, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802431123
Efficient One-Pot Synthesis of 2-Chloro-1,1,
1-trifluoro-2-alkenes Under Solvent-Free
Conditions
Mu-Wang Chen, Xing-Guo Zhang, Ping Zhong, and Mao-Lin Hu
College of Chemistry and Materials Engineering, Wenzhou University,
Wenzhou, Zhejiang, China
Abstract: The improved one-pot Wittig reaction had been used to prepare
trifluoromethyl-containing olefins under solvent-free conditions. Treatment of
aldehydes with PPh3 and CF3CCl3 in the presence of K2CO3 at 100 ꢀC afforded
2-chloro-1,1,1-trifluoro-2-alkenes in good to moderate yields.
Keywords: Olefins, one pot, solvent free, trifluoromethyl, Wittig reaction
INTRODUCTION
Recently, trifluoromethylated compounds have received considerable
attention and have found diverse applications in the areas of materials
science,[1] agrochemistry, and biomedical chemistry[2] because of their
unique chemical, physical, and biological properties. Accordingly, the
development of new reagents and improved techniques for the synthesis
of the trifluoromethyl-containing organic compounds is an important
area of research in fluorooganic chemistry.[3] Extensive studies have been
made to develop cheaper and more efficient synthetic methodology for
the introduction of the trifluoromethyl group into organic compounds.[4]
The –CH¼C(Cl)CF3 moiety is an important structural unit for
synthetic pyrethroids, which are photostable and highly effective
Received May 4, 2008.
Address correspondence to Xing-Guo Zhang, College of Chemistry and Mate-
rials Engineering, Wenzhou University, Xueyuan Road, Wenzhou, Zhejiang
325027, China. E-mail: xgzhang99@yahoo.com.cn
756

2-Chloro-1,1,1-trifluoro-2-alkenes
757
Scheme 1. Synthesis of 2-chloro-1,1,1-trifluoro-2-alkenes.
insecticides.[5] Compounds containing this moiety are also widely used as
synthetic precursors of trifluoromethyl alkynes and heterocycles.[6] This
trifluoromethyl C2-synthon is usually prepared from 1,1,1-trihalo-2,2,2-
trifuloroethanes because of their commercial availability. Fujita and
Hiyama reported the CF3CCl3=Zn=Ac2O reagent system, which trans-
formed aldehydes to 2-chloro-1,1,1-trifluoro-2-alkenes by expending 5
equiv. of zinc powder.[7] Korotchenko et al. developed a new catalytic
redox reaction between hydrazones and 5 equiv. of CF3CCl3 in the
presence of CuCl and aqueous ammonia,[8] which transformed aromatic
aldehydes to fluorinated olefins in moderate yield and stereoselectivity. Li
et al. reported a one-pot Wittig reaction of aldehydes with CF3CCl3 and
PPh3 in acetonitrile.[9] However, acetonitrile is a poisonous solvent, and
reaction yields decreased dramatically in unpurified acetonitrile.
One-pot reactions and solvent-free reactions have many advantages
including cost savings, reduced energy consumption, decreased reaction
times, and a considerable reduction in reactor size. These attributes have
inspired substantial research effort directed toward the development of
solvent-free reactions.[10] In our earlier work,[11] potassium carbonate
was successfully used to promote a one-pot Wittig reaction. Herein, we
report a convenient and efficient method to prepare 2-chloro-1,1,1-
trifluoro-2-alkenes via improved one-pot Wittig reaction under solvent-
free conditions (Scheme 1).
DISCUSSION
An initial experiment treated 4-chlorobenzaldehyde (1a) with 1,1,1-
trichloro-2,2,2-trifuloroethane and PPh3 at different temperatures. The
results are summarized in Table 1. These results suggested that reaction
temperature has dramatic effects on the yields. In optimizing the reaction
conditions, only a trace amount of 2a was isolated under room tempera-
ture (Table 1, entries 1 and 2), and yield increased to 18% when reaction
temperature increased upto 50 ꢀC (Table 1, entry 4). We were happy to
find that the yield of 2a was enhanced to 70% when this one-pot reaction

758
M.-W. Chen et al.
a
Table 1. Optimization of Wittig reaction of 4-chlorobenzaldehyde with CCl3CF3
Entry
PPh3 (equiv.)
K2CO3 (equiv.)
Temperature (ꢀC)
Yield 2a (%)b
1
2
3
4
5
6
7
8
9
1
2
1
2
2
1
2
2
2
—
—
—
—
0.1
—
—
0.1
1
25
25
50
50
Trace
Trace
7
18
30
39
70
86
85
50
100
100
100
100
aReaction conditions were 1a (5 mmol), CF3CCl3 (6 mmol), PPh3 (5=10 mmol),
and K2CO3 (0.5=5 mmol) at 90 ꢀC for 8 h.
bIsolated yield.
was carried out in a sealed tube at 100 ꢀC (Table 1, entry 7). It is
noteworthy that 2 equiv. of triphenylphosphine are necessary because
the initially formed (dichlorotrifluoroethyl) phosphonium salt was de-
chlorinated by the second equivalent of triphenylphosphine (Scheme 2).
In Burton et al.’s review[12] and Li et al.’s research,[13] activated zinc
powder and Lewis acids were often used to promote the Wittig reaction.
However, we found they had only a small effect on the reaction. By
screening metal, acid, and base, we were delighted to find that the yield
of 2a increased to 86% in the presence of 10% mol of potassium
carbonate (Table 1, entry 8). The role of potassium carbonate is not clear.
Increasing the amount of potassium carbonate (1 equiv.) had no obvious
influence on yields (Table 1, entry 9).
Under the optimized reaction conditions, various aldehydes were
converted to corresponding fluorinated olefins in good to moderate
yields. The results are summarized in Table 2. All the aliphatic and
Scheme 2. Synthesis of trifluoromethyl ylide.

2-Chloro-1,1,1-trifluoro-2-alkenes
759
Table 2. Solvent-free Wittig reaction of aldehydes with CCl3CF3
Entry
Aldehyde 1, R¼
Olefin 2
Yield (%)a
2Z=2Eb
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
4-Cl-C6H4
4-CH3-C6H4
4-Br-C6H4
4-NO2-C6H4
4-CH3O-C6H4
4-CN-C6H4
4-F-C6H4
2-NO2-C6H4
3-Cl-C6H4
2-Cl-C6H4
3-Br-C6H4
C6H5
3,4(OCH2O)-C6H4
PhCH2CH2
PhCH(CH3)
PhCH ¼ CH
1-Naphthyl
2a
2b
2c
2d
2e
2f
86
77
97
93
73
68
84
75
84
76
94
77
65
48
55
49
32
75:25
75:25
100:0
54:46
100:0
100:0
97:3
100:0
75:25
85:15
98:2
68:32
86:14
65:35
57:43
100:0
100:0
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
aIsolated yield.
bZ=E ratios were determined by H NMR.
1
aromatic aldehydes ran smoothly in the procedure and Z-isomers were
major products. The structure and configuration of products was
1
confirmed by H NMR and 19F NMR spectra. In this one-pot reaction,
aromatic aldehydes afforded obviously higher yields than aliphatic
aldehydes. Significant influence of electronic effects on yields and stereo-
selectivity had not been observed when using aromatic aldehydes as
substrate. However, the yields of olefins decrease by the influence of
steric effects when aromatic aldehydes contain a substituted group in
the ortho position, such as when the R was 1-naphthyl. The yield of 2q
was 32% (Table 2, entry 17).
In conclusion, we have successfully developed a method to synthesize
2-chloro-1,1,1-trifluoro-2-alkenes via K2CO3-promoted Wittig reaction
of aldethydes with CF3CCl3=PPh3 under solvent-free conditions.

760
M.-W. Chen et al.
EXPERIMENTAL
1H NMR were determined in a CDCl3 solution with a Bruker Avance 300
(300 MHz) spectrometer using tetramethylsilane as the internal standard.
19F NMR spectra also were obtained in a CDCl3 solution on Bruker
Avance 300 (282.2 MHz) spectrometer using CFCl3 as external standard.
All chemical shifts (d) were expressed in parts per million, and coupling
constants (J) were given in hertz. Column chromatography was
performed using EM silica gel 60 (300–400 mesh).
General Procedure
Under an atmosphere of N2, a sealed tube was charged with aldehydes
(5 mmol), 1,1,1-trichlorin-2,2,2-trifuloroethanes (1.12 g, 6 mmol), PPh3
(2.62 g, 10 mmol), and K2CO3 (69 mg, 0.5 mmol). After the mixture was
heated at 100 ꢀC for 8 h, it was dissolved in ethyl acetate. The solvent
was evaporated under reduced pressure, and the residue was purified
by flash-column chromatography on a silica gel, eluting with hexanes=
ethyl acetate to give 2-chloro-1,1,1-trifluoro-2-alkenes 2.[7–9]
Data
(Z=E)-2a: 75:25; colorless oil. Yield 86%. 1H NMR (300 MHz, CDCl3) d:
7.68 (d, J ¼ 8.5 Hz, 0.5H), 7.53 (d, J ¼ 8.5 Hz, 1.5H), 7.44 (d, J ¼ 8.5 Hz,
0.5H), 7.41 (s, 0.25H), 7.38 (d, J ¼ 8.5 Hz, 1.5H), 6.71 (s, 0.75H). 19F
NMR (282 MHz, CDCl3) d: ꢁ61.54 (s, 0.75 F), ꢁ68.79 (s, 2.25 F).
(Z=E)-2b: 75:25; colorless oil. Yield 77%. 1H NMR (300 MHz,
CDCl3) d: 7.81 (d, J ¼ 8.1 Hz, 0.5H), 7.63 (d, J ¼ 8.1 Hz, 1.5H), 7.42
(d, J ¼ 8.1 Hz, 0.5H), 7.39 (s, 0.25H), 7.35 (d, J ¼ 8.1 Hz, 1.5H), 6.83
(s, 0.75H), 2.56 (s, 0.75H), 2.53 (s, 2.25H). 19F NMR (282 MHz, CDCl3)
d: ꢁ61.09 (s, 0.75 F), ꢁ68.34 (s, 2.25 F).
1
(Z)-2c: colorless oil. Yield 97%. H NMR (300 MHz, CDCl3) d: 7.60
(d, J ¼ 8.6 Hz, 2H), 7.46 (d, J ¼ 8.6 Hz, 2H), 6.68 (s, 1H). 19F NMR
(282 MHz, CDCl3) d: ꢁ68.76 (s, 3F).
(Z=E)-2d: 54:46; yellow solid. Yield 93%. 1H NMR (300MHz, CDCl3)
d: 8.27 (d, J¼ 8.7 Hz, 0.46H), 8.23 (d, J ¼ 8.7 Hz, 0.46H), 7.88 (d, J ¼ 8.7 Hz,
0.54H), 7.78 (d, J¼ 8.7 Hz, 0.54H), 7.44 (s, 0.46H), 6.83 (s, 0.54H). 19F
NMR (282 MHz, CDCl3) d: ꢁ61.62 (s, 1.38F), ꢁ68.91 (s, 1.62 F).
1
(Z)-2e: colorless oil. Yield 73%. H NMR (300 MHz, CDCl3) d: 7.75
(d, J ¼ 8.8 Hz, 2H), 7.24 (s, 1H), 6.96 (d, J ¼ 8.8 Hz, 2H), 3.79 (s, 3H). 19
NMR (282 MHz, CDCl3) d: ꢁ68.08 (s, 3 F).
F

2-Chloro-1,1,1-trifluoro-2-alkenes
761
1
(Z)-2f: yellow solid. Yield 68%. H NMR (300 MHz, CDCl3) d: 7.81
(d, J ¼ 8.3 Hz, 2H), 7.73 (d, J ¼ 8.3 Hz, 2H), 7.33 (s, 1H). 19F NMR
(282 MHz, CDCl3) d: ꢁ68.59 (s, 3 F).
(Z=E)-2g: 97:3; colorless oil. Yield 84%. 1H NMR (300 MHz, CDCl3)
d: 7.75 (m, 2H), 7.25 (s, 0.97H), 7.22 (s, 0.03 H), 7.12 (d, J ¼ 8.7 Hz,
1.94H), 7.09 (d, J ¼ 8.7 Hz, 0.06H). 19F NMR (282 MHz, CDCl3)
d: ꢁ61.86 (s, 0.09F), ꢁ68.11 (s, 2.91F).
(Z)-2h: colorless oil. Yield 75%. 1H NMR (300 MHz, CDCl3) d: 8.20
(m, 1H), 7.97 (d, J ¼ 8.2 Hz, 0.5H), 7.77 (m, 1.5H), 7.65 (m, 1.5H), 7.60 (s,
0.15H), 7.57 (s, 0.85H). 19F NMR (282 MHz, CDCl3), d: ꢁ68.98 (s, 3F).
(Z=E)-2i: 75:25; colorless oil. Yield 84%. 1H NMR (300 MHz,
CDCl3) d: 7.73 (s, 0.25H), 7.59 (s, 0.75H), 7.34 (m, 2H), 7.25 (s, 0.25H),
6.66 (s, 0.75H). 19F NMR (282 MHz, CDCl3) E: d: ꢁ61.66 (s, 0.75 F),
ꢁ68.81 (s, 2.75 F).
(Z=E)-2j: 85:15; colorless oil. Yield 76%. 1H NMR (300 MHz,
CDCl3) d: 7.97 (m, 1H), 7.59 (s, 0.15H), 7.36 (m, 3H), 7.20 (s, 0.85H).
19F NMR (282 MHz, CDCl3) d: ꢁ61.70 (s, 0.45 F), Z; ꢁ68.83 (s, 2.55F).
(Z=E)-2k: 98:2; colorless oil. Yield 94%. 1H NMR (300 MHz, CDCl3)
d: 8.01 (s, 0.02H), 7.86 (s, 0.98H), 7.58 (m, 1H), 7.53 (m, 1H), 7.40
(s, 0.02H), 7.32 (m, 1H), 7.23 (S, 0.98H). 19F NMR (282 MHz, CDCl3)
d: ꢁ61.59 (s, 0.06F), ꢁ68.86 (s, 2.94F).
(Z=E)-2l: 68:32; colorless oil. Yield 77%. 1H NMR (300 MHz, CDCl3)
d: 7.59 (m, 1.6H), 7.41 (m, 3.4H), 7.24 (s, 0.32H), 6.73 (s, 0.68H). 19F NMR
(282 MHz, CDCl3) d: ꢁ61.55 (s, 0.96 F), ꢁ68.80 (s, 2.04F).
(Z=E)-2m: 86:14; colorless oil. Yield 65%. 1H NMR (300 MHz,
CDCl3) d: 7.44 (s, 1H), 7.19 (m, 2H), 6.87 (s, 0.86H), 6.80 (s, 0.14H),
6.04 (s, 1.72H), 6.01 (s, 0.28H). 19F NMR (282 MHz, CDCl3) d: ꢁ58.96
(s, 0.42F), ꢁ66.21 (s, 2.58F).
(Z=E)-2n: 65:35; colorless oil. Yield 48%. 1H NMR (300 MHz,
CDCl3) d: 7.35–7.20 (m, 5H), 6.01 (t, J ¼ 6.1 Hz, 0.35H), 5.67
(t, J ¼ 6.1 Hz, 0.65H), 2.89 (t, J ¼ 6.1 Hz, 1.3H), 2.78 (m, 0.7H), 2.55
(t, J ¼ 6.1 Hz, 0.7H), 2.50 (m, 1.3H). 19F NMR (282 MHz, CDCl3)
d: ꢁ62.30 (s, 1.05F), ꢁ68.96 (s, 1.95F).
(Z=E)-2o: 57:43; colorless oil. Yield 55%. 1H NMR (300 MHz, CDCl3)
d: 7.35 (m, 5H), 6.62 (d, J ¼ 8.8 Hz, 0.57H), 6.35 (d, J ¼ 8.8 Hz, 0.43H),
4.06 (m, 1H), 1.47 (d, J ¼ 6.9 Hz, 3H). 19F NMR (282 MHz, CDCl3)
d: ꢁ62.10 (s, 1.29F), ꢁ68.19 (s, 1.71F).
(Z)-2p: colorless oil. Yield 49%. 1H NMR (300 MHz, CDCl3) d: 7.54
(d, J ¼ 7.5 Hz, 1H), 7.50 (m, 5H), 7.05 (m, 1H), 6.92 (m, 1H). 19F NMR
(282 MHz, CDCl3) d: ꢁ59.97 (s, 3F).
(Z)-2q: colorless oil. Yield 32%. 1H NMR (300 MHz, CDCl3) d: 7.65
(m, 4H), 7.40 (m, 3H), 6.90 (s, 1H). 19F NMR (282 MHz, CDCl3)
d: ꢁ68.61 (s, 3F).

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M.-W. Chen et al.
ACKNOWLEDGMENT
We thank the Office of Education of Zhejiang Province (20070535) and
Wenzhou Municipal Science and Technology Bureau (G20060075) for
financial support.
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