PROCESS FOR THE SYNTHESIS OF PERFLUOROALKANDIENES

Abstract: Perfluorobutadiene and higher alkandienes, with terminal double bonds, are obtained from α,ω-dibromo or α,ω-bromo iodo per­ fluoroalkanes by dehalofluorination carried out at limited reaction times by means of an organometallic compound in the presence of an aprotic solvent selected from the class of hyd­ rocarbons or a polar aprotic solvent selected from the class of ethers.

Description: The present invention relates to a process for the synthesis of perfluorobutadiene and higher perfluoroalkandienes with terminal double bonds, by dehalofluorination of α,ω-dibromo or α,ω-bromo iodo perfluoroalkanes, carried out in the pre­ sence of organometallic compounds. The perfluorobutadiene is a chemically interesting compound, because it has been proposed as a termonomer in small amount together with CF₂=CH₂ and C₃F₆ to produce fluoroelastomers vulcanizable with peroxides (Daikin JP-B-47752 published 7.7. 77). It has also been suggested as a termonomer together with C₂F₄ and trifluoronitrosomethane to obtain another vulcanizable fluorinated elastomer (see DE-C-2 304 650 and J.Chem.Soc.Perkin I 1973, page 1111). In fact, perfluorobutadiene reacts by poly­ merization at the 1,2 position, thus leaving a double bond unaltered and available for cross-linking reactions. Besides perfluorobutadiene polymer is used as binder in "fluoro­ oxidizer" systems (see US-A-3 980 509) wherein the polymer acts both as a binder and fuel, and a fluoride salt as NH₄BF₄ acts as oxidizing agent (see J. Appl. Polymer Sc. 19 (1975), 1359). Another patent (US-A-3 353 904) mentions perfluoropolyenes containing terminal double bonds, as water-repellent agents for cotton. Although perfluorobutadiene is considered a very useful com­ pound, the development of products which can be obtained star­ ting from this compound has been limited because a synthesis of the compound in industrial scale was not available. The known processes for synthesizing perfluorobutadiene and higher perfluorodienes are briefly mentioned hereinafter. In US-A-3 046 304 the starting materials are ICl and CClF=CF₂. By reacting these compounds, CClFI-CClF₂ is obtained, which then dimerizes in the presence of an equal volume of Hg thus giving, in a yield of 82 %, CClF₂-CFCl-CFCl-CClF₂ which sub­ sequently can be dechlorinated by means of Zn powder in ethanol to give perfluorobutadiene in a 98 % yield. This synthesis has the drawback to be carried out with diffi­ culty on industrial scale because a large amount of Hg has to be used for the dimerization and the reaction mixture has to be strongly stirred. Therefore, serious problems arise re­ garding pollution and apparatus. Furthermore, in the subsequent dechlorination step there is the problem of dispersing the Zn powder in the liquid reaction mixture and this involves considerable difficulties. Moreover, the first step of the process, that is the reaction between CClF=CF₂ and ICl requries very long reaction times (6 weeks) and yields are rather low (72.6 %), see C.A. 74 (1971) 126.097 h. Higher perfluorodienes, such as 1,5-perfluorohexadiene, are obtained starting from CF₂Cl-CFClI mentioned above by telo­ merization of C₂F₄ in the presence of γ rays and under high pressure, thus obtaining the CF₂ClCFCl(C₂F₄)₂I telomer which is then chlorinated with chlorine in the presence of U.V. rays and then dehalogenated in two steps. In the first step, in the presence of Zn powder mixed with acetic acid and acetic anhydride, a double bond is formed by dechlorination, whereas in the second step, in the presence of powdered Zn in diethy­ leneglycol, dechlorofluorination occurs and the second double bond is formed (see CA 74 (1971) 126097 h). This method is complicated due to the many reaction steps and the total yield is very low. Furthermore, the use of radiation in an industrial process is complicated and not recommendable. Finally, C.A. 98 (1983) 126788 e, describes a method for the preparation of perfluorobutadiene starting from BrClFC-CBrF₂ which is added (by telomerization) to ClCF=CF₂ in the presence of U.V. rays thus obtaining BrF₂C-CClF-CClF-CBrF₂. Said reac­ tion product is dehalogenated using Zn powder and acetic acid and acetic anhydride. The method is not applicable to higher perfluoroalkandienes. Also for this process the difficulties for an industrial realization are remarkable because of the use of U.V. rays and powdered Zn. Furthermore, the synthesis of the brominated intermediate containing 4 C atoms occurs with low yields based on the CClF=CF₂. In IT-A-20935 A/85 in the name of applicant a method for the preparation of perfluoroalkandienes of the general formula (I), as hereinafter indicated, was described said process com­ prising the deiodofluorination of α,ω-diiodoperfluoroalkanes. It has now been unexpectedly found that it is possible to use as starting products α,ω-dibromoperfluoroalkanes and α,ω-bro­ moiodoperfluoroalkanes which by subsequent dehalogenation give final products of formula (II). Therefore, an object of the present invention is a process for the preparation of perfluoroalkandienes of the general formula: wherein a is an integer from 0 to 6, preferably from 0 to 4; b is an integer from 0 to 2 and the sum of a + b is from of 0 to 6, and the units having index a or b may also be alterna­ ting, said process comprising dehalofluorination of , -dibromo or α,ω-bromoiodo, indicated as dihalo hereinafter, perfluoro­ alkanes of the general formula: wherein X and Y are Br or I, X being different from Y in case X or Y is I; with an organometallic compound, either in the presence of an aprotic solvent selected from the class of hydrocarbons or in the presence of an aprotic polar solvent selected from the class of ethers and cyclic ethers or mixtures thereof. Examples of suitable organometallic compounds are alkyl or aryl magnesium halides, dialkyl magnesium or diaryl magnesium, Zn and Cd-alkyls, alkyl lithium or aryl-lithium. When perfluo­ rohexadiene is prepared, it is preferable to use alkyl lithium. Generally, the organometallic compounds are used as solutions in ether solvents. When organometallic compounds of Li or Cd are used, the solutions of the same in ether or hydrocarbon solvents are used. Preferred solvents are dioxane, tetrahydrofurane, diethylen­ glycoldimethylether, dimethoxyethane, hexane, octane and pet­ roleum ether. The reaction temperature is generally from -80 to + 150°C. The reactants may be used in stoichiometric molar ratio or in an amount of the organo-metallic compound moderately above or below the stoichiometric amount. The starting dibromoperfluoroalkanes are known products, which can be obtained for instance directly as ω-products together with C₂F₄Br₂ in the bromination reaction of C₂F₄ at high tem­ perature or by telomerizing C₂F₄ or C₂F₄/C₃F₆ with BrCF₂CF₂Br or of C₃F₆Br₂ with C₂F₄ or mixtures of perfluoroolefins. The same products are also obtained by reacting Br₂ with C₂F₄ and subsequent coupling or by telomerizing C₂F₄ with CF₂Br₂. By analogous and parallel reaction, α,ω-bromo, iodoperfluoroal­ kanes are obtained. The dehalofluorination reaction must be carried out under spe­ cific working conditions in particular intended to avoid, if possible, the coexistence of end product, reactants and by-­ products formed in the reaction mixture. Besides keeping the reaction time short, if possible, in the order of 30 minutes or even less, if the release of the gaseous reaction products can be controlled, it is also suitable to remove the end pro­ duct from the reaction medium, as son as it is formed either by introducing an inert gas, or by distilling the reaction solvent at atmospheric pressure or at reduced pressure. In this last case the end product is dragged away by the solvent during the distillation. As mentioned above, in order to have a proper course of the reaction, it is important that the reacting dihaloperfluoro­ alkane is dissolved in a suitable solvent selected from the ones previously mentioned. In practice it is suitable to mix the solution of the organo­ metallic compound having a molar concentration from 0.2 to 2.5 and preferably from 0.5 to 1.5 molar in the above mentioned solvents, with a solution of the dihaloperfluoroalkane in a solvent of the same type or in a different solvent which is inert towards the organometallic compound, for instance hexane. The following examples are given to illustrate and not to limit the possible embodiments of the present invention. EXAMPLE 1 9.0 g of Br(C₂F₄)₂Br (0.025 mols) in 50 ml of tetrahydrofurane (THF) are introduced into a 150 ml round-bottomed flask pro­ vided with magnetic stirrer, dropping funnel, thermometer, reflux cooler joined with a trap at -80°C. The mixture is brought to boiling temperature and 50 ml of a 1 M solution of C₂H₅MgBr (0.05 moles) in tetrahydrofurane are introduced at such a velocity that the effervescence caused by the reaction can be controlled. The released gases are con­ densed in the trap at -80°C. 2.8 g of a colourless liquid are collected which according to the gas-chromatographic analysis (G.C.) shows a single peak and according to the NMR analysis, IR analysis and boiling point it is identified as CF₂ = CF - CF = CF₂ . In the reactor 1.1 g of perfluorobutadiene remains together with the reaction solvent. Therefore, the yield is 96 %. EXAMPLE 2 10.8 g of Br(C₂F₄)₂Br (0.03 moles) and 70 ml of anhydrous ethyl ether are introduced into a reactor equal to that of example 1. After cooling to -80°C a solution of butyl-lithium in hexane (1.6 M) (0.06 moles) is added, adjusting the addition rate so that the temperature of the reaction mixture does not ex­ ceed -70°C. The mixture is allowed to reach room temperature by flowing an inert gas, subsequently it is brought to boiling tempera­ ture. A gaseous product is released (indentified as perfluoro­ butadiene) which is collected at -80°C in an amount equal to 4.6 g. In addition to the solvent, the butyliodide corresponding to the butyllithium used remains in the reactor. The yield is 94.7 %. EXAMPLE 3 8.1 g of Br(C₂F₄)₂I (0.02 moles) in 40 ml of tetrahydrofurane (THF) are introduced into a 100 ml flask equipped with mag­ netic stirrer, dropping funnel, thermometer, reflux cooler joined with a trap at -80°C. The mixture is heated to boiling temperature and then 40 ml of a 1M solution of C₂H₅MgBr (0.04 moles) in THF are intro­ duced at such a velocity that the effervescence caused by the reation can be controlled. The released gases are condensed in the trap at -50°C. The product collected in the trap weighs 2.7 g and is identified as perfluorobutadiene. 0.3 g of per­ fluorobutadiene remain dissolved in the reaction solvent and the yield is therefore 92.6%. EXAMPLE 4 Into a reactor analogous to the one of the preceding examples, wherein, however, the cooler has been replaced by a Vigreux column, 9.6 g of Br(C₂F₄)₃Br (0.02 moles) in 45 ml of anhydrous tetrahydrofurane are introduced. The mixture is heated to boi­ ling temperature and 42 ml of a 1 M solution of C₂H₅MgBr (0.042 moles) in THF are introduced at such a rate to distill a suffi­ cient amount of the debromofluorination product from the reac­ tion mixture. The released gases are collected in a trap at -80°C. Thus, 4.7 g of a liquid are separated which upon gas-chromato­ graphic analysis shows the following peaks: a main peak corresponding to 71%; a shoulder corresponding to 25 % and other lower peaks. The ¹⁹F-NMR-analysis confirms that the main product is: CF₂ = CF - CF₂ - CF₂ - CF = CF₂ 0.8 g of fluorinated products remain in the boiler; the pro­ ducts consist of n-perfluoro-1,5-hexadiene in an amount of 60 %. The yield of hexadiene is 70 %. COMPARATIVE EXAMPLE Into the reactor of example 1, a sample of pure I(C₂F₄)₂I, weighing 11.3 g (0.025 moles) is introduced and heated to 50°C; then the heat source is removed. 52 ml of C₂H₅MgBr 1.1 M in THF are quickly dropped. Exotherm and effervescence are observed. The product collected into the cooled trap, purified from tra­ ces of solvent, weighs 2 g and consists of 83 % perfluorobuta­ diene and 17 % cyclobutadiene. This last product is identified by NMR and gaschromatography associated with mass spectophotometry. The perfluorobutadiene obtained is identified by comparing its IR and NMR spectra with those known from the literature. In the reactor the per­ fluorobutadiene is no more present. The perfluorobutadiene yield is 41 %.

Claims: 1. A process for preparing perfluoroalkardienes of the general formula: wherein a is an integer from 0 to 6, b is an integer from 0 to 2 and the sum a + b is from 0 to 6, the units having index a or b optionally being alternated, which comprises dehalofluorination of α,ω-dibromo or α,ω-bromo iodoperfluo­ roalkanes of the general formula: wherein X and Y are Br or I, X being different from Y if X or Y is I, with an organometallic compound of Mg, Zn, Cd or Li, either in the presence of an aprotic solvent se­ lected from the class of hydrocarbon or in the presence of an aprotic polar solvent selected from the class of ethers and cyclic ethers and mixtures thereof, at a reac­ tion temperature of from -80 to + 150°C. 2. A process according to claim 1 wherein the reaction product is removed from the reaction mixture while it is formed, by distilling it together with the reaction solvent or by flowing an inert gas. 3. A process according to claims 1 or 2, wherein the organo­ metallic compound is selected from alkyl or aryl magnesium, dialkylmagnesium, diarylmagnesium, Zn or Cd alkyl, lithium alkyl or lithium aryl. 4. A process according to any one of the preceding claims, wherein perfluorohexadiene is obtained and lithium alkyl is used as organometallic compound.

Inventors: BARGIGIA, GIANANGELO, DR., MODENA, SILVANA, DR., TONELLI, CLAUDIO, DR., TORTELLI, VITO, DR.
Applicants: AUSIMONT S.R.L.
Application year: 1987
Main classification: C07C21
IPC codes: C07C17, C07C67, C07C21