The Synthesis of Trifluorovinyl Ethers

Yu.V.Zeifman, S.R.Sterlin

The establishment of Russian Academy of Sciences A.N.Nesmeyanov Institute of Organoelement Compounds RAS Russian Federation,
119991 Moscow, Vaviliva st. 28 Fax: (499) 135 6549,
E-mail: lsg@ineos.ac.ru

The thermolysis of alkali metals perfluoro-2-alkoxytetrafluoropropionates is a common method of preparing perfluoroalkylvinyl ethers, including such important industrial monomers as perfluoromethyl- and propylvinyl ethers, that are obtained in high yields (<90%)[1]. At the same time in the case of 2-alkoxytetrafluoropropionic acids 1 the thermal degradation of their salts ROCF(CF₃)COOM (2) (R = Me, Et, CF₃CH₂; M = alkali metal cation) leads to the formation of complex mixture of products, containing minute quantities of trifluorovinyl ethers. Thus the thermolysis of 1 (R = CF3CH2; M = Na) afforded ether 3a in 4% yield [2].

We have shown that ether 3a can be obtained by thermolysis of salt 2 (R = CF₃CH₂; M = K) (2a) in 67% yield. Similarly ether 3b was prepared from 2 (R = Ph; M = K) (2b) in 63% yield. The starting acids ROCF(CF₃)COOH (1) [R = CF₃CH₂ (1a); Ph (1b)] were obtained by the reaction of the corresponding alcohols with hexafluoropropene oxide (HFPO) and alkali in a system H₂O-ether in the presence of phase-transfer catalyst Bu₄NBr and subsequent acidification of the reaction mixture, that simplified the method of synthesis 1 described earlier [3].

The results obtained confirm the conclusion [2] that chemical behavior of salts 2 under the conditions of thermolysis depends to a considerable extent on the nature of both cation and alkoxy substituent.

2-(2,2,2-Trifluoroethoxy)tetrafluoropropionic acid 1a
Hexafluoropropene oxide (16,6 g, 0,1 mol) was gradually introduced into the mixture of 19,3g (0,19 mol) trifluoroethanol, 19g (0,34 mol) KOH, 40 ml of H₂O, 50 ml of ether and 1,5g of Bu₄NBr at <30^oC, then the mixture was stirred 1h at room temperature, acidified with 30% hydrochloric acid, ethereal layer was separated, dried with MgSO₄ and distilled to give a fraction with b.p. 82-92^oC/35 Torr. The following rectification afforded 16g (68% based on HFPO taken into reaction) of 1a, b.p. 61-63^oC/10 Torr. (lit. data: b.p. 125-127^oC [2]). NMR ¹⁹F spectrum (δ, p.p.m.): -3.3 (3F¹); 4.0 (3F³); 54,5 (1F²).

2-Phenoxytetrafluoropropionic acid 1b
was obtained similarly from HFPO and phenol in 40% yield, b.p. 93-98^oC/3 Torr; NMR ¹⁹F spectrum (δ, p.p.m.): 5,2 (3F); 42,5 (1F). The acid was transformed into its K-salt 2b without further purification.

2,2,2-Trifluoroethyltrifluorovinyl ether 3a.
A solution of 16g (0,065 mol) 1a in MeOH was neutralized with solution of KOH in MeOH (phenolphthalein as indicator), evaporated under reduced pressure, the residue was dried over P₂O₅ at 110^oC/2-3 Torr., then pulverized, mixed with 20g of dry sand and subjected to thermolysis at 10-15 Torr by heating in the flame of Bunsen burner (or in Wood alloy bath at 225-280^oC). The volatile products were collected in a trap (-78^oC), the condensate was distilled to give 8.5g (67%) of ether 3a, b.p. 41-44^oC (b.p. and NMR ¹⁹F spectrum were identical to that described in [2,4]).

Phenyltrifluorovinyl ether 3b.
A mixture of 10.4g (0.037 mol) salt 2b (dried over P₂O₅ at 110-115^oC/3 Torr.) and 13g of dry sand was subjected to thermolysis under vacuum of oil pump, collecting the volatile products in a trap (-78^oC). The condensate was distilled to give 5.9g (63%) of ether 3b, b.p. 132-134^oC (b.p. and NMR ¹⁹F spectrum were identical to that described in [5,6]).

Acknowledgement
Support of the given study by DuPont de Nemours Co. is gratefully acknowledged.

References

1. P.R.Resnick, US Pat. 3692843 (1972)
2. M.Y.Pellerite, J.Fluor/Chem., 49 (1990), 43
3. D.Sianesi, A.Pasetti, F.Tarii, J.Org.Chem., 31 (1966), 2312
4. M.-H. Hang, S.Rozen, UA Pat. 5350497 (1994)
5. J.Plummer, L.A.Wall, Trans., 3 (1963), 220
6. A.E.Feiring, S.Rozen, E.R.Wonchoba, J.Fluor.Chem., 89 (1998), 31