Preparation of fluorinated alkenes in ionic liquids

Article abstract of DOI:10.1016/S0022-1139(00)00331-6

The utility of ionic liquids (8-ethyl-1,8-diazabicyclo[5,4,0]-7-undecene trifluoromethanesulfonate and 8-methyl-1,8-diazabicyclo[5,4,0]-7-undecene trifluoromethanesulfonate) as a safe recyclable reaction medium for the synthesis of α-fluoro-α,β-unsaturate

Full text of DOI:10.1016/S0022-1139(00)00331-6

Journal of Fluorine Chemistry 106 (2000) 211±215
Preparation of ¯uorinated alkenes in ionic liquids
*
Tomoya Kitazume , Genji Tanaka
Department of Bioengineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan
Received 24 April 2000; received in revised form 25 July 2000; accepted 10 August 2000
Abstract
The utility of ionic liquids (8-ethyl-1,8-diazabicyclo[5,4,0]-7-undecene tri¯uoromethanesulfonate and 8-methyl-1,8-diazabicyclo[5,4,0]-
-undecene tri¯uoromethanesulfonate) as a safe recyclable reaction medium for the synthesis of a-¯uoro-a,b-unsaturated esters by the
7
Horner±Wadsworth±Emmons reaction with aldehydes in the presence of base, is described. # 2000 Elsevier Science S.A. All rights
reserved.
Keywords: Fluorinated alkenes; Ionic liquid; Clean technology
1
. Introduction
cene tri¯uoromethanesulfonate) for the preparation of a-
uoro-a,b-unsaturated esters.
¯
The challenge in chemistry to develop practical processes,
reaction media, conditions and/or utility of materials based
on the idea of green chemistry is having one of the most
important issues in scienti®c society [1±3]. In our study of
the greener chemistry, we have reported the utilities of
2. Results and discussion
Recently, we have revealed that ¯uorous (per¯uorinated)
¯uids have been recognized to have extremly stable, non-
polar/inert characters in organic reactions, and they are
suitable for reaction containing unstable reagents [26]. In
our continuous studies for the utility of ¯uorinated materials
as organic media, we have examined the preparation and
utility of ¯uorine-containing molten salts (ionic liquids)
[27,28] with unique properties. New ¯uorine-containing
molten salts (such as 8-ethyl-1,8-diazabicyclo[5,4,0]-7-
undecene tri¯uoromethanesulfonate 1 and 8-methyl-1,8-dia-
zabicyclo[5,4,0]-7-undecene tri¯uoromethanesulfonate 2)
are prepared directly from the reaction of 1,8-diazabicy-
clo[5,4,0]-7-undecene with ethyl or methyl tri¯uorometha-
nesulfonate in the same manner of 1-ethyl-3-
methylimidazolium tri¯uoromethansulfonate 3 [27]. The
structures were con®rmed by spectral data. Before the use
and reuse of ionic liquids (8-ethyl-1,8-diazabicyclo[5,4,0]-
7-undecene tri¯uoromethanesulfonate 1 and 8-methyl-1,8-
diazabicyclo[5,4,0]-7-undecene tri¯uoromethanesulfonate
2), ionic liquids were puri®ed under dynamic vacuum at
70±808C for 2 h, and then we checked the purity and
¯
uorous ¯uids as an alternative reaction media [4,5]. More-
over, our recent studies support the ease of handling and re-
use of ionic liquids, which in some cases can serve as a
solvent and a catalyst [6,7]. In the synthetic chemistry,
studies of reusable media such as ionic liquids [8±11] are
having an important impact on organic reactions such as
Friedel±Crafts reactions [12,13], alkylation reactions [14±
1
7], Diels±Alder reaction [18,19], palladium catalyzed ally-
lation [20] and asymmetric hydrogenation [21]. While the
Horner±Wadsworth±Emmons [22,23] reactions are one of
the important synthetic reactions especially in the synthesis
of ¯uorinated alkenes. Usually, ¯uorinated alkenes are pre-
pared from the addition±elimination of poly¯uoroethenes,
chloro¯uorocarbene, ethyl phenylsul®nyl¯uoroacetate,
triethyl 2-¯uoro-2-phosphonoacetate and organometallics
in polar organic solvents [24,25], and after quenching with
water, the products are extracted with organic solvents.
These processes generate the waste containing solvent
media and water. As part of our reserach aimed at greener
chemistry [4±7], we would like to describe the preparation
and utility of new ionic liquid as a reusable solvent (such as
1
19
structure by the H and F NMR spectra (no other peaks
except an ionic liquids 1 and 2) (Scheme 1).
8-ethyl-1,8-diazabicyclo[5,4,0]-7-undecene tri¯uorometha-
nesulfonate and 8-methyl-1,8-diazabicyclo[5,4,0]-7-unde-
While the synthesis of a-¯uoro-a,b-unsaturated esters
from the reaction of triethyl 2-¯uoro-2-phosphonoacetate
with aldehydes was reported by several groups [21], their
procedures required NaH as a base at diethyl ether re¯ux
*
Corresponding author. Tel.: ‡81-459245754; fax: ‡81-459245780.
E-mail address: tkitazum@bio.titech.ac.jp (T. Kitazume).
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 0 0 ) 0 0 3 3 1 - 6

2
12
T. Kitazume, G. Tanaka / Journal of Fluorine Chemistry 106 (2000) 211±215
ethyl-3-methyl-1H-imidazolium hexa¯uorophosphate 5 as
an ionic liquid decreased the yield (34 and 27%, respec-
tively) and recovered yield (75 and 81%).
1
The stereochemistry of products was con®rmed by H
NMR coupling constants (JH±F) and chemical shifts of the
ole®nic protons. It is well-known that the coupling constant
JH±F) of the Z-isomer is 25±45 Hz and that JH±F of the E-
(
isomer is 10±25 Hz. Further, the signal of the ole®nic proton
for the each of the carbonyl group is the area d 6±7 ppm. On
the basis of the coupling constant (JH� F ˆꢀ 35 Hz) and
chemical shift of produced materials, the stereochemistry of
the obtained main materials is that of the Z-isomer in the use
of 8-methyl-1,8-diazabicyclo[5,4,0]-7-undecene (DBU) as a
base. In contrast, in the use of K CO as a base, the obtained
Scheme 1. Preparation of ionic liquids.
temperature or n-BuLi as a base at � 788C in tetrahydro-
furan. In this paper, we have examined the recyclable ionic
liquids for the synthesis of a-¯uoro-a,b-unsaturated esters
from the reaction of triethyl 2-¯uoro-2-phosphonoacetate
with aldehydes in the presence of K CO or 1,8-diazabicy-
2
3
main materials is that of the E-isomer.
In this system, it is important to note that two kinds of
products, such as E- or Z-isomer, are obtained. This obser-
vation might be explained by the formation of intermediates
to produce the E- or Z-isomer. The selectivity of the synth-
esis of alkenes described above was attributed to the bulki-
2
3
clo-[5,4,0]-7-undecene (DBU) as a base at room tempera-
ture. The present reaction summarized in Table 1 smoothly
proceeded at room temperature in an ionic liquid. The
produced a-¯uoro-a,b-unsaturated ester was separated by
the extraction with diethyl ether (4 Â 10 ml), and ionic
liquid 1 was recovered more than 98%. When we used 1-
ethyl-3-methyl-1H-imidazolium tetra¯uoroborate 4 or 1-
‡
ness of sustituent groups ([DBUH ] is bulkiness more than
‡
K ), which led us to conclude that Z-isomer was produced
via TS-B.
Table 1
Preparation of a-fluoro-a,b-unsaturated esters
a
b
19
Entry
RCHO
Ionic liquid no.
Base
Yield/%
E/Z ratio
F NMR data d/ppm (J/Hz)
E-isomer
Z-isomer
1
2
3
4
5
6
7
8
9
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
1
2
3
1
2
3
1
1
1
1
1
1
K
K
K
2
2
2
CO
CO
CO
3
3
3
74
68
65
81
70
79
35
77
37
69
56
79
73:27
69:31
70:30
35:65
39:61
35:65
67:33
33:67
71:29
30:70
74:26
36:64
44.3 (22.3)
36.4 (35.2)
DBU
DBU
DBU
C
C
16
H
H
13CHO
13CHO
K
2
CO
DBU
CO
DBU
CO
DBU
3
3
3
38.8 (22.0)
38.3 (21.4)
46.4 (20.7)
30.5 (33.5)
30.5 (48.4)
40.8 (33.3)
16
PhCH(CH
PhCH(CH
3
)CHO
)CHO
K
2
1
1
1
0
1
2
3
2-pyridineCHO
2-pyridineCHO
K
2
a
Ionic liquid: 8-ethyl,8-diazabicyclo[5,4,0]-7-undecene trifluoromethanesulfonate 1; 8-ethyl-1,8diazabicyclo[5,4,0]-7-undecene trifluoromethanesulfo-
nate 2;1-ethyl-3-methyl-1H-imidazolium trifluoromethanesulfonate 3.
b
Yield was determined by 19F NMR intergral intensity using C
H CF as a internal standard.
6 5 3

T. Kitazume, G. Tanaka / Journal of Fluorine Chemistry 106 (2000) 211±215
213
methanesulfonate (71.2 g, 0.4 mol) was added slowly. After
the mixture was stirred for 2 h at room temperature, the
produced materials were heated at 708C for 1 h to remove
the volatile materials, giving 8-ethyl-1,8-diazabicy-
clo[5,4,0]-7-undecene tri¯uoromethanesulfonate
1
in
1
9
1
8%yield. H NMR (CDCl ): d 1.28 (3H, t, J ˆ 7:32 Hz),
3
.75±1.84 (8H, m), 2.15 (2H, m), 2.87 (2H, m), 3.55±3.59
(
(
2H, m), 3.60 (2H, q, J ˆ 7:32 Hz), 3.68 (2H, m). 13C NMR
CDCl ): d 13.554, 19.950, 22.841, 25.937, 28.069, 28.475,
3
4
1
6.210, 48.855, 48.968. 55.080, 120.584 (q, J ˆ 320:4 Hz),
65.966. 19F NMR (CDCl ): d 83.37 ppm from internal
3
�
1
C F . IR: 2928, 2932, 2296, 1626 cm . Anal. Calc. for
C H F N SO : C, 43.63; H, 6.41; N, 8.48. Found: C,
6
6
12
21
3
2
3
4
3.43; H, 6.41; N, 8.50.
Fig. 1. Reaction pathway.
3.3. 8-Methyl-1,8-diazabicyclo[5,4,0]-7-undecene
trifluoromethanesulfonate 2
Furthermore, recycling of the ionic liquid 1 is under
investigation from both environmental and economical
points of view. Successive reuse of the recovered ionic
liquid 1 in the same reaction yielded amounts of product
as high as in the ®rst cycle shown in Fig. 1.
Into the ¯ask containing 1,8-diazabicyclo[5,4,0]-7-unde-
cene (30.4 g, 0.2 mol) cooling with ice-bath, methyl tri¯uoro-
methanesulfonate (32.8 g, 0.2 mol) was added slowly. After
the mixture was stirred for 2 h at room temperature, the
produced materials were heated at 708C for 1 h to remove
the volatile materials, giving 8-methyl-1,8-diazabicy-
clo[5,4,0]-7-undecene tri¯uoromethanesulfonate 1 in 95%
Cycle
no.
Base
Yield/%
Recovered
ionic liquid/%
E/Z
ratio
1
2
1
2
K CO
2
74
71
81
88
98
97
98
96
73:27
74:26
35:65
32:68
3
3
K CO
2
yield.
1
DBU
DBU
H NMR (CDCl ): d 1.29 (3H, t, J ˆ 7:14 Hz), 2.13±
3
2.81(4H, m), 3.07 (2H, t, J ˆ 7:96 Hz), 3.48 (6H, m), 3.79
13
(
2
1
2H, t, J ˆ 7:69 Hz). C NMR (CDCl ): d 19.856, 22.132,
3
6.165, 28.649, 28.718, 41.203, 48.707, 48.847, 55.243,
20.459 (q, J ˆ 320:4 Hz), 166.516. F NMR (CDCl3):
19
3
. Experimental
.1. General procedure
All commercially available reagents were used without
d 83.37 ppm from internal C F . IR: 2939, 2294,
6
6
�
1
1
632 cm
.
3
3
.4. Ethyl 3-phenyl-2-fluoropropenate
1
further puri®cation. Chemical shifts of H (500 MHz) and
C NMR spectra were recorded in ppm (d) down®eld from
the following internal standard (Me Si, d 0.00, or CHCl , d
13
3.4.1. 8-Ethyl-1,8-diazabicyclo[5,4,0]-7-undecene
trifluoromethanesulfonate 1
An amount of 1.0 g of 1 was dissolved by adding ben-
zaldehyde (540 mg, 5 mmol) and triethyl 2-¯uoro-2-phos-
phoniacetate (726 g, 3 mmol) at room temperature. Into the
4
3
.24). The 1 F (470 MHz) NMR spectra were recorded in
ppm down®eld from the external tri¯uoroacetic acid (TFA)
9
7
in CDCl using a VXR 500 instrument. Yields quoted are
3
mixture solution, K CO (670 mg, 3 mmol) was stirred at
2
those of the products actually isolated.
3
room temperature. After stirring of 5 h at that temperature,
organic materials were extracted with diethyl ether
(10 Â 5 ml), and molten salt 1 was recovered more than
98% yield. The organic layer was dried over anhydrous
3.2. 8-Ethyl-1,8-diazabicyclo[5,4,0]-7-undecene
trifluoromethanesulfonate 1
MgSO , and then the solvent was removed. The yield (74%)
4
Into the ¯ask containing 1,8-diazabicyclo[5,4,0] 7-unde-
cene (60.8 g, 0.4 mol) cooling with ice-bath, ethyl tri¯uoro-
and E/Z ratio (73:27) were determined by the 19F NMR
integral intensities using benzotri¯uoride as an internal

2
14
T. Kitazume, G. Tanaka / Journal of Fluorine Chemistry 106 (2000) 211±215
standard. The resultant crude product was puri®ed by chro-
matography on silica gel, giving ethyl 3-phenyl-2-¯uoro-
propenate [22,23] in 63% yield.
7.69 Hz). 19F NMR (CDCl ): d 38.8 (d, J ˆ 33:52 Hz) from
3
C F .
6
6
1
E-isomer: H NMR (CDCl ): d 1.24 (3H, t, J ˆ 7:14 Hz),
3.6. Ethyl 3-(2-phenylethyl)-2-fluoropropenate
3
4
.25 (2H, q), 6.92 (1H, d, J ˆ 22:25 Hz), 7.34±7.48 (Ar±H).
F NMR (CDCl ): d 44.3 (d, J ˆ 22:35 Hz) from C F .
1
9
In the above reaction, 2-phenylpropanal (402 mg,
3 mmol) and DBU (or K CO ) were used, and then
worked-up similarly.
3
6 6
1
Z-isomer: H NMR (CDCl ): d 1.39 (3H, t, J ˆ 7:14 Hz),
3
2
3
4
.36 (2H, q), 6.93 (1H, d, J ˆ 35:16 Hz), 7.34±7.48 (Ar±H).
19
1
F NMR (CDCl ): d 38.4 (d, J ˆ 35:16 Hz) from internal
E-isomer: H NMR (CDCl ): d 1.36 (3H, t, J ˆ 7:14 Hz),
3
3
C F .
6
2.17 (3H, dd, J ˆ 6:32, 1.65 Hz), 4.20 (2H, qd, J ˆ 7:14,
1.65 Hz), 5.67 (1H, ddq, J ˆ 14:55, 9.89, 1.64 Hz), 6.04
6
Into the recovered ionic liquid from the above reaction,
benzaldehyde (540 mg, 5 mmol), triethyl 2-¯uoro-2-phos-
phoniacetate (726 g, 3 mmol) and K CO (670 mg, 3 mmol)
1
9
(1H, dd, J ˆ 21:42, 10.98 Hz), 7.28±7.44 (Ar±H). F NMR
(CDCl ): d 38.3 (d, J ˆ 21:4 Hz) from C F .
2
3
3
6 6
1
were added, and then the mixture was stirred at room
temperature. After 5 h of stirring, organic materials were
extracted with diethyl ether (10 Â 5 ml), and ionic liquid 1
was recovered at more than 97% yield. The organic layer
Z-isomer: H NMR (CDCl ): d 1.32 (3 H, t, J ˆ 7:14 Hz),
3
2.23 (3H, dd, J ˆ 4:12, 1.10 Hz), 4.28 (2H, qd, J ˆ 7:14,
1.65 Hz), 5.69 (1H, dd, J ˆ 48:35, 8.79 Hz), 5.89 (1H, ddq,
19
J ˆ 14:28, 8.79, 1.38 Hz), 7.28±7.44 (Ar±H). F NMR
(CDCl ): d 30.5 (d, J ˆ 48:4 Hz) from C F .
was dried over anhydrous MgSO , and then the solvent was
4
3
6 6
removed. The yield (71%) and E/Z ratio (74:26) were
determined by the 19F NMR integral intensities using ben-
zotri¯uoride as an internal standard.
3.7. Ethyl 3-(2-pyridinyl)-2-fluoropropenate
The mixture solution of benzaldehyde (540 mg, 5 mmol),
triethyl 2-¯uoro-2-phosphoniacetate (726 g, 3 mmol) and
In the above reaction, 2-formylpyridine (321 mg,
CO ) were used, and then
3
3 mmol) and DBU (or K
worked-up similarly.
2
1
3
¯
,8-diazabicyclo[5,4,0]-7-undecene
(DBU,
mmol) in 8-ethyl-1,8-diazabicyclo[5,4,0]-7-undecene tri-
456 mg,
1
E-isomer: H NMR (CDCl
4.26 (2H, q), 6.92 (1H, d, J ˆ 20:33 Hz), 7.23±7.90.
NMR (CDCl
): d 46.4 (d, J ˆ 20:33 Hz) from C
): d 1.24 (3H, t, J ˆ 7:14 Hz),
19
3
uoromethanesulfonate 1 (1.0 g) was stirred at room tem-
F
perature. After 3 h of stirring, organic materials were
extracted with diethyl ether (10 Â 5 ml), and ionic liquid
3
1
F .
6 6
Z-isomer: H NMR (CDCl
4.37 (2H, q), 7.15 (1H, d, J ˆ 35:16 Hz), 7.23±7.90, 7.28-
7.44 (Ar±H). 19F NMR (CDCl
): d 40.87 (d, J ˆ 35:1 Hz)
from C
): d 1.39 (3H, t, J ˆ 7:14 Hz),
3
1
was recovered more than 98% yield. The organic layer was
dried over anhydrous MgSO , and then the solvent was
4
3
removed. The yield (81%) and E/Z ratio (35:65) were
determined by the 19F NMR integral intensities using ben-
zotri¯uoride as an internal standard.
F .
6 6
Into the recovered ionic liquid from the above reaction,
benzaldehyde (540 mg, 5 mmol), triethyl 2-¯uoro-2-phos-
phoniacetate (726 g, 3 mmol) and 1,8-diazabicyclo[5,4,0]-
References
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7
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[
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(
10 Â 5 ml), and ionic liquid 1 was recovered more than
[
4] H. Nakano, T. Kitazume, Green Chem. 1 (1999) 21.
9
MgSO , and then the solvent was removed. The yield (88%)
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4
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[
(
3
.5. Ethyl 3-hexyl-2-fluoropropenate
[
[
[
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In the above reaction, heptanal (342 mg, 3 mmol) and
DBU (or K CO ) were used, and then worked-up similarly.
2
3
1
E-isomer: H NMR (CDCl ): d 0.90±1.50 (12H, m), 2.23
[14] V.R. Koch, L.L. Miller, R.A. Osteryoung, J. Am. Chem. Soc. 98
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3
(
(
2H, m), 4.30 (2H, q, J ˆ 7:14 Hz), 5.92 (1H, dt, J ˆ 21:98,
.24 Hz). 19F NMR (CDCl ): d 31.1 (d, J ˆ 21:98 Hz) from
8
C F .
3
[
[
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6
6
1
Z-isomer: H NMR (CDCl ): d 0.90±1.50 (12H, m), 2.23
3
(
2H, m), 4.28 (2H, q, J ˆ 7:15 Hz), 6.10 (1H, dt, J ˆ 33:52,
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T. Kitazume, G. Tanaka / Journal of Fluorine Chemistry 106 (2000) 211±215
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[
[
[
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2
1
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