Cycloaddition reactions of furan derivatives with trifluoroethene

Article abstract of DOI:10.1016/S0022-1139(00)00250-5

The cycloaddition reaction of trifluoroethene with furan and a number of derivatives to give the unreported 1,1,2-trifluoro-7-oxa-bicyclo[2.2.1]hept-4-ene, and derivatives has been investigated. Reactions were carried out in stainless steel autoclaves at 200°C under autogenous pressure using an excess of trifluoroethene. Reactions were found to proceed with furan and derivatives with methyl groups in the 2 and/or 5 positions. Electron-withdrawing substituents in these positions prevented reaction however. Attempted dehydrofluorination of 1,1,2-trifluoro-7-oxa-bicyclo[2.2.1]hept-4-ene using a variety of bases gave no reaction, however use of LDA led to the preparation in 25% yield of 2,3-difluoro phenol.

Full text of DOI:10.1016/S0022-1139(00)00250-5

Journal of Fluorine Chemistry 104 (2000) 233±237
Cycloaddition reactions of furan derivatives with tri¯uoroethene
Richard D. Chambersa,*, Alan F. Gilberta, Richard L. Powellb
aDepartment of Chemistry, University of Durham Science Laboratories, South Road, Durham DH1 3LE, UK
bI.C.I. Chemicals and Polymers Limited, Research and Technology, P.O. Box 8, The Heath, Runcorn, Cheshire WA7 4QD, UK
Received 12 January 2000; accepted 27 January 2000
Abstract
The cycloaddition reaction of tri¯uoroethene with furan and a number of derivatives to give the unreported 1,1,2-tri¯uoro-7-oxa-
bicyclo[2.2.1]hept-4-ene, and derivatives has been investigated. Reactions were carried out in stainless steel autoclaves at 2008C under
autogenous pressure using an excess of tri¯uoroethene. Reactions were found to proceed with furan and derivatives with methyl groups in
the 2 and/or 5 positions. Electron-withdrawing substituents in these positions prevented reaction however. Attempted dehydro¯uorination of
1,1,2-tri¯uoro-7-oxa-bicyclo[2.2.1]hept-4-ene using a variety of bases gave no reaction, however use of LDA led to the preparation in 25%
yield of 2,3-di¯uoro phenol. # 2000 Elsevier Science S.A. All rights reserved.
Keywords: Tri¯uoroethene; Furan; Cycloaddition
1. Introduction
of isomers 4a and 4b, (Scheme 2). The use of tetrahydro-
furan as solvent, led to a cleaner product without signi®cant
reduction in conversion. Product characterisation was
achieved by elemental analysis and NMR, while mass-
spectrometry led essentially to retro Diels±Alder reaction
in the instrument, with parent peaks arising from the two
original reactants. The isomers 4a and 4b were identi®ed by
With the advent of `HFCs', particularly 134a, i.e.
CF3CH2F (1), there is an increasing interest in tri¯uor-
oethene (3FE, 2), which can be obtained from 1 by elimina-
tion of hydrogen ¯uoride, Scheme 1 [1].
Various cycloaddition reactions of tri¯uoroethene have
been described previously, including 2‡2 thermal dimerisa-
tion [2], addition to a ¯uorinated oxaziridine [3], and to
tri¯uoronitrosomethane [4]. However, Diels±Alder (4‡2)
processes are limited to minor components in the reactions
of 3FE with butadiene [5], and with isoprene, 2,3-dimethyl
butadiene, cis-1,3-pentadiene and trans-1,3-pentadiene [6].
It is only in the reaction with cyclopentadiene [7] that the
4‡2 process predominates. It seemed reasonable that furan
and derivatives, as cyclic, p-electron rich systems, could also
give Diels±Alder products and we have now demonstrated
that this is indeed the case.
1
observing the JHH and JFH values in the H and 19F NMR
spectra of the exo- and endo-hydrogen in 4a and 4b respec-
tively, as indicated in Scheme 2. The exo-hydrogen on C2 in
4a and the exo-¯uorine in 4b couple to the hydrogen on C3
and give rise to coupling constants of 4.4 and 7.7 Hz,
respectively. Coupling between the hydrogen on C3 and
endo-hydrogen or endo-¯uorine on C2 is too small to be
observed.
We attempted to convert the product to the diene (8) by
elimination of hydrogen ¯uoride but, using a variety of
procedures, e.g. heating to higher temperatures and treating
with the bases (25 M KOH, DBU, sodium methoxide,
sodium hydride, and potassium t-butoxide) no reaction
was obtained. However, with LDA the mixture of 4a and
4b reacted to give 2,3-di¯uorophenol (7), which was char-
acterised as the tosylate, together with substantial amounts
of an uncharacterised polymer. The latter material is soluble
in solvents such as acetone and ether but the resonance in the
1H and 19F NMR spectra, subsequently obtained from these
solutions, were extremely broad and yielded no useful
structural information. The most likely explanation for these
observations is that the endoxides 4a, 4b, react with base to
2. Results and discussion
A mixture containing furan and excess 3FE, sealed under
vacuum in an autoclave, was heated to ca. 2008C and, under
these conditions, we obtained 36% conversion to a mixture
* Corresponding author. Tel.: ‡44-191-374-3120;
fax: ‡44-191-384-4737.
E-mail address: r.d.chambers@durham.ac.uk (R.D. Chambers)
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 2 5 0 - 5

234
R.D. Chambers et al. / Journal of Fluorine Chemistry 104 (2000) 233±237
Scheme 1
form anion (5), which could lose ¯uoride ion but, more
likely, ring-opening occurs to give 6 which loses ¯uoride to
yield the phenol (7) (Scheme 3).
Electron-donating groups in the furan clearly favour the
Diels±Alder process because both mono- and di-methyl
compounds 8 and 9 gave the corresponding cyclo-adducts
(10) and (11) respectively, in good yield, (Schemes 4 and 5).
However, reactions of each of these adducts with base gave
polymer and no identi®able product. We also explored the
ring-opening of 8 and 9 with acid and this occurred readily,
but led to a very complex range of products that we have not
attempted to identify.
Scheme 3
Other furan derivatives, e.g. (R1ˆCH2OH; CH2O-
COCH3; COOH, R2ˆH) gave only traces of cyclo-adducts.
Mass spectra of solid samples were recorded on a VG 7070E
spectrometer. GLC mass spectra were recorded on the VG
7070E spectrometer linked to the Hewlett Packard 5790A
gas chromatograph ®tted with a 25 m cross-linked methyl
silicone capillary column.
Fractional distillation of product mixtures was carried out
using a Fischer Spahltrohr MMS255 small concentric tube
apparatus. Melting points were carried out at atmospheric
pressure and are uncorrected. Unless otherwise stated,
reagents were used as supplied. Solvents were dried by
standard methods and stored over a molecular sieve (type
4A).
3. Experimental
Gas liquid chromatographic (GLC) Analyses were car-
ried out using a Hewlett Packard 5890A gas liquid chro-
matograph equipped with a 25 m cross-linked methyl
silicone capillary column. Preparative scale GLC was car-
ried out using a Varian Aerograph Model 920 (catharometer
detector) gas liquid chromatograph with a packed column
(3 m 10% S.E. 30).
Elemental Analyses were obtained using a Perkin-Elmer
240 Elemental Analyser or a Carlo Erba Strumentazione
1106 Elemental Analyser.
NMR spectra were recorded on a Bruker AC 250
(250 MHz) or a Varian VXR400S (400 MHz). Chemical
shifts were recorded in ppm from TMS or CFCl3 in CDCl3
solution.
Infrared spectra were recorded on a Perkin-Elmer 457 or
577 Grating spectrometer using KBr discs (for solid sam-
ples) or thin ®lms between two KBr plates (for liquid
samples). Gaseous samples were condensed into a cylind-
rical cell ®tted with KBr plates.
3.1. Preparation of 1,1,2-trifluoro-7-oxabicyclo[2.2.1]
hept-4-ene (4) without solvent
An autoclave (140 ml) was charged with tri¯uoroethene
(36 g, 0.443 mol), furan (6.2 g, 91.6 mmol) and dipentene
(0.5 ml) and then heated in a rocking furnace at 2008C for
24 h. After cooling, volatile material was transferred from
the tube under reduced pressure. Distillation gave furan
(3.9g, 37% conversion) and 1,1,2-tri¯uoro-7-oxabicy-
clo[2.2.1]hept-4-ene (4) (nc) (4.4 g, 87% yield), as a mix-
Scheme 2

R.D. Chambers et al. / Journal of Fluorine Chemistry 104 (2000) 233±237
235
Scheme 4
Scheme 5
ture of exo-H 4a and endo-H 4b isomers. (Found: C, 48.1; H,
3.2. Preparation of 1,1,2-trifluoro-7-oxabicyclo[2.2.1]
hept-4-ene (4) with solvent
3.2. Calc. for C6F3H5O: C, 48.0; H, 3.3%). Preparative scale
GLC (508C) allowed separation of the isomers. The exo-H
isomer 4a gave: dH (400 MHz) 4.8 (1H, m, 3H), 4.9 (1H,
An autoclave (500 ml) was charged with tri¯uoroethene
(126.8 g, 1.547 mol), furan (20.9 g, 0.307 mol), THF
(50 ml) and dipentene (0.5 ml) and heated in a rocking
furnace at 2008C for 24 h. Volatile material was transferred
from the tube under reduced pressure. Distillation gave
tri¯uoroethene, furan (5.2 g, 75% conversion), THF and
product fractions including a fraction (14.3 g) containing
mostly endo-(H)-1,1,2-tri¯uoro-7-oxabicyclo[2.2.1]hept-4-
ene 4b, together with some of the exo-(H) isomer 4a and
a fraction (1.52 g) containing only the exo-(H) isomer
4a. Total isolated yield of 4a, b was 45%, (Found: C,
48.0; H, 3.5; Calc. for C6F3H5O: C, 48.0; H, 3.3%); 19F,
1H and 13C NMR spectra were identical with those of
the material prepared previously without added THF (see
Section 3.1).
2
3
3
ddd, JH F ˆ54.7, JH FBˆ15.6, JH H ˆ4.4, 2H), 5.0 (1H,
2
2
2
2
3
m, 6H), 6.57 (1H, JABˆ5.8, 4H or 5H), 6.62 (1H, JABˆ5.8,
4H or 5H); dF (400 MHz) 108.8 (1F, JABˆ232.0 dd
3JFBH ˆ15.6, JFBH ˆ5.8, 1BF), and 112.9 (1F, JAB
ˆ
3
2
6
3
3
232.0 dd, JFAF ˆ6.3, JFAF ˆ1.8, 1A-F), 192.1 (1F,
2
2
2
3
dd, JF H ˆ54.2, JF FBˆ6.4, 2F); dC (400 MHz) 78.27
2
2
2
2
3
3
(ddd, JCFˆ22.7, JCFˆ3.6, JCFˆ1.7, 3C), 80.9 (td,
2JCFˆ27.1, JCFˆ1.7, 6C), 87.2 (ddd, JCFˆ204.7, JCF
ˆ
3
1
2
33.2, 2JCFˆ15.1, 2C), 121.1 (ddd, 1JCFˆ265.1, 1JCFˆ257.2,
2JCFˆ15.1, 1C), 133.1 (dd, 3JCFˆ4.9, 3JCFˆ1.9, 5C), 136.1
(m, 4C). The endo-H isomer 4b: dH (400 MHz) 4.6 (1H, dd,
2JH F ˆ 54.2, 3JH FAˆ7.4, 2H), 4.8 (1H, m, 3H), 5.07 (1H,
2
2
2
m, 6H), 6.50 (1H, JABˆ6.0, d, Jˆ2.0, 4H or 5H), 6.57 (1H,
JABˆ6.0, 4H or 5H); dF (400 MHz) 106.2 (1F, JABˆ233.1,
3
3
dd, JFAH ˆ7.4, JFAH ˆ1.5, 1A-F) and
120.3 (1F,
JABˆ233.1, ddd, JFBF ˆ12.3, JFBH ˆ5.1, JFBH ˆ2.5,
2
6
3
3
3
3.3. Preparation of 6-methyl-1,1,2-trifluoro-7-oxabicyclo
[2.2.1]hept-4-ene and 3-methyl-1,1,2-trifluoro-7-
oxabicyclo[2.2.1]hept-4-ene (10a±d)
2
6
2
2
3
1B-F),
198.1 (1F, ddd, JF H ˆ54.4, JF FB 12.2,
2
2
2
3JF H ˆ7.7, 2F); dC (250 MHz) 79.1 (t, JCFˆ27.6, 6C),
2
2
3
83.2 (dd, 2JCFˆ22.5, 3JCF 7.4, 3C), 88.9 (ddd, 1JCFˆ209.0,
2JCFˆ29.4, JCFˆ15.5, 2C), 120.9 (ddd, JCFˆ270.2, JCF
An autoclave (140 ml) was charged with tri¯uoroethene
(32.6 g, 0.4 mol), 2-methyl furan (7.1 g, 91.6 mmol) and
dipentene (0.5 ml) and heated in a rocking furnace at 2008C
2
1
1
258.3, 2JCFˆ14.3, 1C), 134.4 (d, 3JCFˆ4.6, 4C or 5C), 134.7
3
(d, JCFˆ5.8, 4C or 5C).

236
R.D. Chambers et al. / Journal of Fluorine Chemistry 104 (2000) 233±237
2
3
3
for 24 h. Volatile material was transferred from the tube
under reduced pressure. Distillation gave 2-methyl furan
(0.35 g, 95% conversion) and adducts (10a±d) (nc) (9.19 g,
64%). (Found: C, 51.0; H, 4.1; Calc for C7H7F3O C, 51.2; H,
4.3%). Preparative scale GLC (608C) allowed separation
into two fractions each containing two of the 4 isomers.
Fraction 1 consisted of the exo-H isomers 10a and 10c,
while fraction 2 consisted of the endo-H isomers 10b and
10d. exo-H 6-methyl-1,1,2-tri¯uoro-7-oxabicyclo[2.2.1]-
hept-4-ene (10a) (nc) gave dH (400 MHz) 1.57 (3H, s, 6-
Me) 4.95 (1H, ddd, 2JH F ˆ55.1, 3JH FBˆ15.3, 3JH H ˆ4.5,
Me), 85.5 (ddd, JCF ˆ21.9, JCF ˆ4.1, JCF ˆ1.8, 3C),
2
1
1
2
3
1
87.2 (td, JCF ˆ25.6, JCF ˆ1.1, 6C), 93.0 (ddd, JCF
ˆ
ˆ
1
2
2
2
2
1
207.3, JCF ˆ33.1, JCF ˆ16.7, 2C), 122.0 (ddd, JCF
1
1
1
267.6, 1JCF ˆ262.3, 2JCF ˆ15.0, 1C), 137.1 (m, 4C or 5C),
1
2
139.3 (m, 4C or 5C). endo-H isomer 11b (nc) gave
(400 MHz) 1.52 (3H, s, Me), 1.60 (3H, s, Me), 4.56 (1H,
dd, 2JH F ˆ54.6, 2JH FAˆ7.4, 2H), 6.3 (2H, m, 4H and 5H);
2
2
2
dF (400 MHz) 107.5 (1F, JABˆ229.2, d, 3JFAH ˆ7.9, 1A-
2
3
F) and 127.8 (1F, JABˆ229.2, d, JFBF ˆ11.5, 1B-F),
2
2
3
203.4 (1F, dd, JF H ˆ54.6, JF HB ˆ11.6, 2F); dC
2
2
2
2
2
(400 MHz) 11.3 (s, Me), 13.6 (s, Me), 85.7 (dd, JCF
1
ˆ
2
2
2
2
2
2
2
3
2H), 4.97 (1H, m, 3H), 6.36 (1H, JABˆ5.6, 4H or 5H), 6.59
(1H, JABˆ5.6, 4H or 5H); dF (400 MHz) 114.4 (1F,
27.3, JCF ˆ25.4, 6C), 88.3 (dd, JCF ˆ25.4, JCF ˆ7.4,
1
2
1
3C), 91.1 (ddd, 1JCF ˆ213.6, 2JCF ˆ28.3, 2JCF ˆ16.0, 2C),
2 1 1
1
JABˆ228.8, dd, 3JFAF ˆ6.8, 1A-F) 117.9 (1F, JABˆ228.8,
121.2 (ddd, JCF ˆ273.7, JCF ˆ259.4, JCF ˆ14.5, 1C),
1
2
2
1
1
2
3
3
d, JFBH ˆ15.3, 1B-F) 190.6 (1F, dd, JF H ˆ55.6,
139.0 (m, 4C or 5C), 139.4 (m, 4C or 5C).
2
2
2
3JF FAˆ6.8, 2F); exo-H 3-methyl-1,1,2-tri¯uoro-7-oxabicy-
2
clo[2.2.1]hept-4-ene (10c) (nc) gave dH (400 MHz) 1.7 (3H,
s, 3-Me), 4.56 (1H, dd, 2JH F ˆ54.9, 3JH FBˆ16.0, 2H), 4.76
3.5. Preparation of 2,3-difluorophenol (7)
2
2
2
(1H, m, 6H), 6.41 (1H, JABˆ6.0, 4H or 5H), 6.53 (1H,
JABˆ6.0, 4H or 5H); dF (400 MHz) 108.1 (1F, JABˆ230.8,
Lithium diisopropylamide monoTHF (1.5 M solution in
hexane) (50 ml, 0.075 mol) was added dropwise over
75 min, under a dry nitrogen atmosphere, to a stirred solu-
tion of 1,1,2-tri¯uoro-7-oxa bicyclo[2.2.1]hept-4-ene (4a,
b) (5 g, 33.3 mmol) in dry THF (140 ml) at 708C. On
addition, the solution turned orange/brown. The solution
was maintained at 708C for 90 min and then allowed to
warm to room temperature over 2 h with stirring. The dark
brown solution was ®ltered, then stirred with water (50 ml).
Separation of the organic layer and then extraction of the
aqueous layer with diethyl ether removed some of the tarry
products. The pH of the aqueous layer was adjusted to 1
using conc. HCl and then extracted with diethyl
ether. Volatile material was distilled from these extracts
under reduced pressure to leave a tarry residue. Ether
was removed from the distillate to leave a straw coloured,
higher boiling liquid containing 2,3-di¯uorophenol (1.1 g;
25% yield), identi®ed by comparison of its NMR and IR
spectra with those of an authentic sample obtained from
Aldrich.
2
3
dd, JFBH ˆ15.9, JFBH ˆ6.3, 1B-F), 112.1 (1F, JAB
ˆ
ˆ
2
6
3
2
231.0, d, JF FAˆ6.8, 1A-F), 191.8 (1F, dd, JF H
2
2
2
2
54.7, JF FA6.8, 2F). endo-H 6-methyl-1,1,2-tri¯uoro-7-
2
oxabicyclo[2.2.1]hept-4-ene (10b) (nc) gave dH (400 MHz)
1.58 (3H, s, 6-Me) 4.60 (1H, dd, 2JH F ˆ53.8, 3JH FAˆ7.2,
2
2
2
2H), 4.97 (1H, m, 3H), 6.37 (1H, JABˆ5.6, 4H or 5H), 6.47
(1H, JABˆ5.6, 4H or 5H); dF (400 Mhz) 107.9 (1F,
JABˆ230.0, d, 3JFAH ˆ7.5, 1A-F), 128.7 (1F, JABˆ230.0,
2
3
2
d, JFBF ˆ11.6, 1B-F), 195.7 (1F, ddd, JF H ˆ54.4,
2
2
2
3JF FBˆ11.9, JF H ˆ8.1, 2F); endo-H 3-methyl-1,1,2-tri-
3
2
2
3
¯uoro-7-oxabicyclo[2.2.1]hept-4-ene (10d) (nc) gave dH
2
(400 MHz) 1.64 (3H, s, 3-Me), 4.42 (1H, dd, JH F
3
ˆ
2
2
54.9, JH FAˆ7.6, 2H), 4.68 (1H, m, 6H), 6.30 (1H,
2
JABˆ5.8, 4H or 5H), 6.53 (1H, JAB 5.8, 4H or 5H); dF
(400 MHz) 105.5 (1F, JABˆ231.6, d, 3JFAH ˆ7.4, 1A-F),
2
119.4 (1F, JABˆ231.2, dd, 3JFBF ˆ11.9, 3JFBH ˆ5.5, 1B-
2
6
2
3
F), 205.8 (1F, dd, JF H ˆ54.9, JF FBˆ12.0, 2F).
2
2
2
3.4. Preparation of 3,6-dimethyl-1,1,2-trifluoro-7-
oxabicyclo{2.2.1]hept-4-ene (11a, b)
3.6. Preparation of Tosylate of 2,3-difluorophenol
An autoclave (140 ml) was charged with tri¯uoroethene
(27.3 g, 0.33 mol), 2,5-dimethyl furan (6.54 g, 68.03 mmol)
and dipentene (0.5 ml) and heated in a rocking furnace
at 2008C for 24 h. Volatile material was transferred
from the vessel under reduced pressure. Distillation gave
tri¯uoroethene, 2,5-dimethyl furan (2.4 g) and a mixture of
exo- and endo-(H)-3,6-dimethyl-1,1,2-tri¯uoro-7-oxabicy-
clo[2.2.1]\hept-4-ene (11a, b) (nc), (7.6 g) (Found: C, 54.2;
H, 5.1. Calc. for C8H9F3O, C, 53.9; H, 5.05%); exo-H
isomer 11a (nc) gave (400 MHz) 1.52 (3H, s, Me), 1.60
(3H, s, Me), 4.56 (1H, dd, 2JH F ˆ54.6, 3JH FBˆ15.4, 2H),
Pyridine (1.6 g, 20.3 mmol) was added dropwise to a
stirred solution of para-toluenesulfonyl chloride (3.76 g,
19.2 mmol) and the product of the earlier reaction (Section
3.5) containing 1,2-di¯uorophenol (1.1 g 8.3 mmol), in
dichloromethane (10 ml) at room temperature; after stand-
ing overnight (16 h) a white solid appeared. After washing
with water and removal of the solvent, the remaining oil
contained p-toluenesulphonyl chloride. The oil was there-
fore dissolved in pyridine (20 ml) and this solution was then
dropped into vigorously stirred water (20 ml). A solid
precipitate formed which was isolated by ®ltration and then
recrystalised (ethanol/water) to give the p-tosylate of 1,2-
di¯uorophenol (1.3 g, 55%), m.p. 49±508C, (Found: C,
55.0; H, 3.45. Calc. for C13H10F2O3S, C, 54.9; H, 3.5%);
dH (400 MHz) 2.47 (3H, s, Me), 7.05 (3H, m, 4H, 5H and
2
2
2
6.3 (2H, m, 4H and 5H); dF (400 MHz) 113.9 (1F,
JABˆ228.0, d, 3JFAF ˆ7.4, 1A-F), 117.3 (1F, JABˆ228.0,
2
3
2
d, JFBH ˆ15.6, 1B-F), 190.5 (1F, dd, JF H ˆ54.8,
2
2
2
3JF FAˆ7.4, 2F); dC (400 MHz) 11.45 (s, Me), 13.5 (s,
2

R.D. Chambers et al. / Journal of Fluorine Chemistry 104 (2000) 233±237
237
6H), 7.35 (2H, JABˆ8.2, 30H and 50H), 7.77 (2H, JABˆ8.2,
References
20H and 60H); dF (400 MHz) 134.5 (1F, m, 2F), 151.0
(1F, m, 3F); EI m/z 284 (M , 4.5%); CI(NH3): m/z 302
‡
[1] R.L. Powell, A.P. Sharrat, GB Appln. No. 94,16009 (1994).
[2] G. Fuller, J.C. Tatlow, J. Chem. Soc. (1961) 3198.
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Acknowledgements
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We thank ICI Chemicals and Polymers Limited for
®nancial support and John Hutchinson for valuable discus-
sions.
[6] L.E. Walker, P.D. Bartlett, J. Am. Chem. Soc. 95 (1973) 150.
[7] B.M. Jacobson, P.D. Bartlett, J. Org. Chem. 38 (1973) 1030.