Influence of structural factors on the thermolysis of 2- alkoxytetrafluoropropionic acid salts

Article abstract of DOI:10.1007/s11172-010-0137-9

2,2,2-Trifluoroethyl and phenyl trifluorovinyl ethers were obtained by the thermolysis of 2-substituted potassium tetrafluoropropionates ROCF(CF ₃)COOK (R = CF₃CH₂; Ph). The reaction mechanism is analyzed.

Full text of DOI:10.1007/s11172-010-0137-9

Russian Chemical Bulletin, International Edition, Vol. 59, No. 3, pp. 657—659, March, 2010
657
Influence of structural factors on the thermolysis
of 2ꢀalkoxytetrafluoropropionic acid salts
Yu. V. Zeyfman and S. R. Sterlin
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 6549. Eꢀmail: lsg@ineos.ac.ru
2,2,2ꢀTrifluoroethyl and phenyl trifluorovinyl ethers were obtained by the thermolysis
of 2ꢀsubstituted potassium tetrafluoropropionates ROCF(CF3)COOK (R = CF3CH2; Ph). The
reaction mechanism is analyzed.
Key words: hexafluoropropylene oxide, 2ꢀalkoxytetrafluoropropionic acids, 2ꢀphenoxyꢀ
tetrafluoropropionic acid, alkyl trifluorovinyl ethers, phenyl trifluorovinyl ether.
Thermal decomposition of fluorocarboxylic acids is a
universal method for the synthesis of higher linear fluoꢀ
roolefins.1,2 A particular case of this reaction is the
thermolysis of 2ꢀalkoxytetrafluoropropionic acid salts
ROCF(CF3)COO–M+ (R = alkyl, α,αꢀdihydrofluoroꢀ
alkyl, perfluoroalkyl; M is metal cation) (1)3,4 resulting in
trifluorovinyl ethers.
of the RO group and (b) the ionic radius of the metal
cation that exerts an electrophilic assistance of the elimiꢀ
nation of F– within the frame of the fiveꢀ or sixꢀmembered
transition state.
In order to confirm the above assumption, we synꢀ
thesized 2ꢀalkoxytetrafluoropropionic acids 2 by the reꢀ
action of the corresponding alcohols and an excess of
alkali with hexafluoropropylene oxide (HFPO) in
an ether—water system in the presence of Bu4N+Br–;
the acidification of the reaction mixture affords acids 2
in 40—70% yield (this procedure is a simplified version
of the synthesis of 2ꢀalkoxytetrafluoropropionic acids
described earlier3,5) (Scheme 1). Potassium salts 1a—c
were obtained by the treatment of acids 2 with aqueꢀ
ous KOH.
However, in the case of perfluorinated salts 1, perfluoꢀ
roalkyl vinyl ethers that found practical use as comonoꢀ
mers in the synthesis of fluoropolymers are formed in alꢀ
most quantitative yields,4 whereas the thermolysis of comꢀ
pounds 1 with R = Me, Et, and CF3CH2 leads to a comꢀ
plicated mixture of products.3 It was postulated3 that the
composition of the reaction products reflects the compeꢀ
tition between the decarboxylation accompanied by the
βꢀelimination of the fluoride ion to form trifluorovinyl
ethers and the intramolecular nucleophilic substitution of
the αꢀfluorine atom with the intermediate formation of
2ꢀalkoxyꢀ2ꢀtrifluoromethylꢀαꢀlactones, whose rate is deꢀ
termined by the influence of the alkoxy group. It was shown
for the thermolysis of salts 1 (R = Me) as an example that
the contribution of the latter direction of the process inꢀ
creased from 75—80% in the case of the potassium and
cesium salts to 100% for the sodium salt. This is due to the
"higher energy of formation of the crystalline lattice of
NaF." 3 However, reasons for the preferential (in the case
of the sodium salts, exclusive) αꢀelimination of the fluoꢀ
ride ion during the thermolysis of salts 1 remained beyond
the discussion.
Scheme 1
R = Et (a), CF3CH2 (b), Ph (c)
We do not contest the proposed3 scheme of thermolyꢀ
sis of salts 1 but believe that the influence of the nature of
the alkoxy group and the counterion on the composition
of the reaction products is determined by other factors,
namely: (a) the anionoid mobility of the αꢀfluorine atom
that increases in parallel to the electronꢀreleasing ability
The choice of alcohol was based on the fact that the
electronꢀdonating character of the RO group increases
inversely to the acidity of the starting alcohol (the pKa
values for PhOH, CF3CH2OH, and EtOH are 9.98,6 12.3,7
and 18.0,6 respectively) and, hence, the anionoid mobility
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 642—644, March, 2010.
1066ꢀ5285/10/5903ꢀ0657 © 2010 Springer Science+Business Media, Inc.

658
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 3, March, 2010
Zeyfman and Sterlin
Table 1. Yields of ethers 3 obtained by the
thermolysis of salts 1
Experimental
F NMR spectra were recorded on a Bruker 200SY instruꢀ
ment (188.3 MHz) in a ССl4 solution (CF3COOH as external
standard).
19
Salt 1
Yield of comꢀ
pound 3 (%)
R
M
2ꢀ(2,2,2ꢀTrifluoroethoxy)tetrafluoropropionic acid (2b).
HFPO (16.6 g, 0.1 mol) was introduced gradually with stirring
into a mixture of trifluoroethanol (19.3 g, 0.19 mol), KOH (19 g,
0.34 mol), water (40 mL), ether (50 mL), and Bu4NBr (1.5 g),
maintaining the temperature of the reaction mixture <30 °C.
Then the mixture was stirred for 1 h more at ~20 °C, and 30%
HCl was added to acidic pH. The ethereal layer was separated
and dried with MgSO4, and the fraction with b.p. 82—92 °C
(35 Torr) was isolated by distillation. The repeated distillaꢀ
tion gave 16 g of compound 2b, b.p. 61—63 °C (10 Torr), 68%
yield (based on HFPO taken in the reaction) (cf. Ref. 3: b.p.
125—127 °С). 1F NMR, δ: –3.3 (3 F(1)); 4.0 (3 F(3)); 54.5 (1 F(2).
Et
Na
K
Na
K
0*
10
4*
67
63
CF3CH2
Ph
K
* See Ref. 3.
of the αꢀfluorine atom increases in the series of salts
1c ≈ 1b << 1a. As shown by the thermolysis of these salts
(see Scheme 1), the yields of trifluorovinyl ethers 3 deꢀ
crease in the same sequence (Table 1).
The concept about the influence of the ionic radius
of the cation on the yield of trifluorovinyl ethers (the
ionic radii for Na+, K+, and Cs+ at the coordination numꢀ
ber six are8 0.95, 1.33, and 1.69 Å, respectively) makes
it possible to explain the described3 increase in the yield
of ether 3d (R = Me) during the thermolysis of salts 1
with R = Me in the series of cations Na+, K+, and Cs+
(Scheme 2).
2ꢀPhenoxytetrafluoropropionic acid (2c) was obtained analoꢀ
gously from HFPO and phenol in 40% yield, b.p. 93—98 °C
(3 Torr). 1F NMR, δ: 5.2 (3 F); 42.5 (1 F). Without further
purification acid 2c was transformed into potassium salt 1c, pyꢀ
rolysis of which is described below.
Ethyl trifluorovinyl ether (3a) was synthesized by the pyrolyꢀ
sis of potassium salt 1a in 10% yield; b.p. and the 1F NMR
spectrum coincide with the published values9 (acid 2a was synꢀ
thesized by the earlier described method5).
2,2,2ꢀTrifluoroethyltrifluorovinyl ether (3b). A solution of acid
2b (16 g, 0.065 mol) in МеОН was neutralized with a solution of
KOH in МеОН (using phenolphthalein as indicator) and evapꢀ
orated in vacuo, and the residue was dried over Р2О5 at 110 °С
(2—3 Torr), divided, and mixed with dry sand (20 g), and subꢀ
jected to pyrolysis at 10—15 Torr heating with a burner (or in the
bath with Wood´s alloy at 225—280 °С). Volatiles were collected
in a trap (–78 °C), and the condensate was distilled to obtain
ether 3b (8.5 g, 67%), b.p. 41—44 °С (b.p. and the 1F NMR
spectrum are identical to the described values3,10).
Scheme 2
Phenyl trifluorovinyl ether (3c). A mixture of potassium salt
1c (10.4 g, 0.037 mol) (the salt was preꢀdried over Р2О5 at
110—115 °C and 3 Torr) and dry sand (13 g) was subjected to
thermolysis in an oily pump vacuum, collecting the pyrolyzate in
a trap (–78 °C). The distillation of the condensate gave ether 3c
(5.9 g, 63%), b.p. 132—134 °C (b.p. and the 1F NMR spectrum
are identical to the described values11,12).
In the case of the smallest Na+ ion, only the fiveꢀ
membered transition state is formed in which the nucꢀ
leophilic substitution of the αꢀfluorine atom occurs
with the electrophilic assistance of Na+; no ether 3d is
formed. The formation of ether 3d (26 and 20% yields)
upon the thermolysis of the potassium and cesium salts,
whose ionic radii are substantially larger, indicates a tenꢀ
dency to form the sixꢀmembered transition state, although
the low yields of trifluorovinyl ether show that the reꢀ
action course is determined, to a great extent, by the
anionoid mobility of the αꢀfluorine atom and the formaꢀ
tion of αꢀlactone remains the predominant direction
of thermolysis.
The authors are grateful to the DuPont Company for
financial support.
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3. M. Y. Pellerite, J. Fluor. Chem., 1990, 49, 43.
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Synthesis of trifluorovinyl ethers
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 3, March, 2010
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Received January 15, 2009;
in revised form May 13, 2009