BACKGROUND 5 1. Field

The present disclosure relates to a process for the preparation of a perfluorinated cis-alkene, and more specifically the preparation of cis- 1,1, 1,4 ,4,4- hexafluoro-2-butene.

10 2. Discussion of the Background Art

Fluorocarbon based fluids have found widespread use in industry in a number of applications, including as refrigerants, aerosol propellants, blowing agents, heat transfer media, and gaseous dielectrics. Because of the suspected environmental problems associated with the use of some of these fluids, including

I ₅ the relatively high global warming potentials (GWP) associated therewith, it is desirable to use fluids having the lowest possible greenhouse warming potential in addition to zero ozone depletion potential (ODP). Thus there is considerable interest in developing environmentally friendlier materials for the applications mentioned above.

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Fluorinated butenes having zero ozone depletion and low global warming potential have been identified as potentially filling this need. However, the toxicity, boiling point, and other physical properties in this class of chemicals vary greatly from isomer to isomer. One fluorobutene having valuable properties is cis-

25 1,1,1 ,4,4,4-hexafluorobutene. Thus, there is a need for new manufacturing processes for the production of hexafluorobutenes and in particular cis- 1,1, 1,4,4 ,4- hexafluorobutene.

There are several methods for producing hexafluoro-2-butene, but such 30 processes may give exclusively the trans-isomer (for example, the zinc reduction of 1,1,1, 4,4 ,4-hexafluoro-2-iodobutene; K. Leedham and R. N. Hazeldine, J. Chem. Soc, 1954, 1634). Processes that give a mixture of cis- and trans-isomers are likewise undesirable if a substantial proportion of the trans-isomer is formed. One reason is that the difference in boiling points for the two isomers is large (the trans- 35 isomer boiling at about 9°C and the cis-isomer boiling at about 32°C). For applications that depend in large part on the boiling point of the fluorocarbon, the large difference in boiling points may mean that only one isomer is suitable and the other isomer therefore represents a yield loss. Another reason such a mixture is undesirable is that a good means for recycling the undesired trans-isomer is lacking. Ideally, a suitable process will provide the cis:traπs isomers in a ratio of 10: 1 or better.

Still other processes for cis-olefins suffer from co-production of the corresponding alkane. In the present case, this means the co-production of 1,1,1,4,4,4-hexafluorobutane. This is likewise undesirable because it does not posses the low GWP that the corresponding butene does. Furthermore, like the trans-isomer, there is no convenient way to recycle this by-product. The sole prior art method for making cis-l,l,l,4,4,4-hexafluorobutene (J. Am. Chem. Soc, 71 (1949) 298) involves reduction of hexafluoro-2-butyne with hydrogen (100 atmospheres) using Raney nickel catalyst at room temperature. Not only does this pressure require specialized equipment, but the conversion was only 82 % and the product was a mixture of cis-hexafluoro-2-butene (41 % yield) and hexafluorobutane (25 % yield). Ideally the amount of over-reduced material should be less than 10 %. Still more preferably, the total amount of trans-isomer and butane are together less than 10 %.

R.N. Hazeldine, J. Chem. Soc, 1952, pp. 2504, also reported the reduction of hexafluorobutyne with Raney nickel at 60 ⁰ C and 15 atmospheres of hydrogen pressure to give cis-hexafluorobutene. Although some over-reduction to hexafluorobutane was mentioned, the yield of 91% is substantially better than the yield given in the reference cited above.

A few methods exist for the exclusive preparation of non-fluorinated cis- olefins to the exclusion of the corresponding trans-isomer. The most common of these is the catalytic reduction of alkynes. A number of catalysts may be employed for this transformation but they can, unfortunately, give a wide range of results and undesirable side reactions such as over-reduction to alkanes, formation of trans- olefins, and isomerization of cis to trans olefins. In addition, a wide range of variables can alter the results, such as temperature, mixing rate, solvent, and added reagents which may intentionally or unintentionally alter the reactivity of the catalyst (for a general discussion see P. N. Rylander, Catalytic Hydrogenation over Platinum Metals, Chapter 4, Academic Press, 1967). For example, depending on the temperature, the reduction of acetylene dicarboxylic acid using Pd on BaSCU can give either succinic acid (no double bond) at -18 ⁰ C or maleic acid (cis double bond) at 100 ⁰ C, while the ratios of cis to trans products for the reduction of p- methoxyphenylacetylene carboxylic acid with the same catalyst were similar (20 + 5 % trans isomer) over a wide temperature range (S. Takei and M. Ono, Nippon Nogei Kagaku Kaisi 18 (1942b) 119).

Catalysts that have been used for the selective reduction of non-fluorinated

5 alkynes to alkenes include Pd/C, Pd/BaSO ₄ , Pd/BaCO ₃ , and Pd/CaCO ₃ . In order to achieve high selectivity, however, the use of quinoline as a catalyst modifier has been recommended whether the catalyst is Pd/C, Pd/BaSO ₄ , or Lindlar's catalyst, Pd/CaCO ₃ /Pb (M. Hudlicky, Reductions in Organic Chemistry, 2 ^nd Ed., ACS Monograph 188, 1996, p 8).

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The Lindlar catalyst is probably the most common one used for the reduction of hydrocarbon alkynes to cis-alkenes, modified further by the addition of an aromatic amine such as quinoline or pyridine. The amines, while often useful in improving reaction selectivity, are not desirable from the standpoint of their

I ₅ toxicity. The quality of the quinoline used may also affect the outcome. The PdVCaCO ₃ ZPb catalyst, modified with pyridine, was successfully used in the reduction of an alkyne bearing a single fluorine on the carbon adjacent to the triple bond to give the corresponding cis-alkene (M. Prakesch, D. Gree, and R. Gree, J. Org. Chem., 66 (2001) 3146).

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As is well known in the art, however, fluorocarbons often behave quite differently compared to non-fluorinated alkanes, and perfluorinated compounds may behave quite differently than even partially fluorinated compounds of similar structure.

2 ₅

The present disclosure also provides many additional advantages, which shall become apparent as described below.

SUMMARY 30 A process for the preparation of perfluorinated cis-alkenes comprising: reducing a perfluorinated alkyne with hydrogen over a palladium catalyst in the presence of a non-aromatic amine catalyst modifier to form a product comprising the perfluorinated cis-alkene, wherein the perfluorinated alkyne has the general structure: 3S RfC≡CRf wherein Rf is a perfluorinated alkyl group having a carbon number in the range between about 1 to 6. Preferably, the process is for the preparation of cis- 1,1,1, 4,4,4-hexafluoro-2- butene comprising: reducing hexafluoro-2-butyne with hydrogen over a palladium catalyst in the presence of a non-aromatic amine catalyst modifier to form a product comprising the cis- 1,1,1 ,4,4,4,-hexafluoro-2-butene. Preferably, the over reduction to CF ₃ CH ₂ CH ₂ CF ₃ is less than 10 mole %, and wherein the cis- 1 , 1 , 1 ,4,4,4,- hexafluoro-2-butene product has less than about 10 mole % of trans- 1,1, 1,4,4,4- hexafluoro-2-butene.

The reduction step is carried out at a temperature in the range between about O ⁰ C to about 15O ⁰ C. Preferably, the reduction step is carried out at a temperature in the range between about 25°C to about 75°C.

The reduction step is preferably carried out in the presence of a reaction solvent. The reaction solvent is at least one solvent selected from the group consisting of: alkanes, aryls, alcohols, acids and esters. The reaction solvent is at least one solvent selected from the group consisting of: heptane, toluene, methanol, ethanol, acetic acid and ethyl acetate.

The reduction step is conducted at a pressure in the range between about 10 to 350 psig (72 to 2532 kPa), more preferably in the range between about 20 to 100 psig (145 to 723 kPa).

Preferably, hydrogen is added as needed to during the reduction step to avoid over-reduction.

The palladium catalyst includes palladium and a catalyst support, wherein the catalyst support is at least one material selected from the group consisting of: calcium carbonate, barium carbonate and sulfate, charcoal, activated carbon, and alumina. Preferred catalyst supports are calcium and barium carbonate, while calcium carbonate is most preferred.

The non-aromatic amine catalyst modifier is at least one selected from the group consisting of: alkali metal hydroxides, metals and metal salts. The alkali metal hydroxide is preferably KOH. The metal is at least one metal selected from the group consisting of: lead, zinc, ruthenium, copper, iron and tin. The preferred catalyst modifier is lead. The process further comprises the step of distilling the cis- 1,1, 1,4 ,4,4,- hexafluoro-2-butene product such that it has a cis- 1,1,1 ,4,4,4,-hexafluoro-2-butene concentration in the range between about 90 to 99.9%.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The starting material for this disclosure can be obtained by a variety of methods. One method is the dechlorination of CF ₃ CC1=CC1CF ₃ with zinc (A. L. Henne and W. G. Finnegan, J. Am. Chem. Soc, 71 (1949) 298). The latter in turn can be prepared by the reductive dimerization of CF ₃ CCI ₃ (S Tomioka et al, Chemistry Letters, 1991, 1825).

The results of Comparative Examples A and B show that neither Pd/C with quinoline nor Pd/BaSθ ₄ were satisfactory catalysts for the selective reduction of hexafluoro-2-butene to cis-l,l,l,4,4,4-hexafluoro-2-butene, even though these catalyst systems have been successfully employed for the selective reduction of non-fluorinated alkynes. Both were unacceptable due to over-reduction, even where less than one equivalent of hydrogen had been consumed Surprisingly, however, Pd/CaCOs/Pb worked exceeding well (Examples 1 and 2), even without an aromatic amine modifier such as quinoline.

The temperature used for the reduction can be varied over a wide range from about 0 ⁰ C to about 150 ⁰ C, but the higher temperatures reported earlier in order to achieve a high percentage of cis-olefin are not necessary according to the process of this disclosure. Reactions near room temperature are the most convenient, but slightly higher temperatures may be desirable in order to achieve improved productivity. Especially preferred temperatures are in the range between about 25 to 75 ⁰ C.

The reaction solvent can be any non-reactive solvent. These include alkanes, aryls, alcohols, acids and esters. Specific examples of these include heptane, toluene, methanol, acetic acid and ethyl acetate. Lower molecular weight alcohols are preferred and ethanol is most preferred.

The ratio of perfluorinated alkyne to catalyst can vary from about 1000 or more on a mole basis to about 1 , but is typically 2-100. For batch operations, a higher ratio (e.g. low catalyst loading) may be preferred to lower cost, while in semi-batch mode, in which the product is distilled out after one batch, followed by adding more perfluorinated alkyne and hydrogen for the next batch, lower ratios (higher catalyst loading) may be preferred to improve productivity.

The pressure can vary considerably within the range of equipment capability. High pressures tend to speed up the reaction but may lead to overreduction. Hence the preferred pressures are 10 to 350 psig (72 to 2532 kPa) and more preferably 20-100 psig (145 to 723 kPa). In order to maximize the utilization of hydrogen, less than one equivalent may be added initially and more added as needed to complete the reaction or to maintain a desired hydrogen pressure.

EXAMPLE 1

A 1 -liter autoclave was charged with 2.0 g of catalyst (5 % palladium on calcium carbonate poisoned with 3.5 % lead) and 160 mL ethanol. The autoclave contents were then cooled to -78 C. Air was removed by pressurizing to 60 psi (434 kPa) with nitrogen followed by evacuating. The sequence was repeated twice more. Hexafluoro-2-butyne (32 g) was then added and the contents warmed to 25°C.

Hydrogen gas was added to a pressure 90 psig (651 kPa) and was maintained at this pressure for approximately 20 hours at a reaction temperature of 25-28 ⁰ C. The autoclave contents were again cooled with the aid of a -78°C bath prior to venting hydrogen gas. The material in the autoclave was distilled to give 31.6 g of 97 % pure cis-hexafluoro-2-butene (97.5 % yield). Three such preparations were made and the combined materials redistilled to give the desired butene, bp 30-32 ⁰ C, in greater than 99.9 % purity (H NMR: 6.56 ppm; F NMR: -60.17 ppm).

EXAMPLE 2 Example 2 was run in a manner similar to that of Example 1 except that the ratio of hexafluoro-2-butyne to catalyst was doubled, and the hydrogen pressure reduced to a maximum of 60 psig (434 kPa). Similar results were obtained.

COMPARATIVE EXAMPLE A A pressure reactor was charged with 0.20 grams of 5 % palladium on carbon, 0.042 grams of reagent grade quinoline as catalyst poison and 25 mL of ethanol. After removing air as described in Example 1, 2.0 grams of hexafluoro-2- butyne was added. Hydrogenation was conducted at 20 psig (145 kPa) hydrogen pressure at room temperature. Workup and distillation as before gave 1.5 grams of CF ₃ CH ₂ CH ₂ CF ₃ ( ¹ H NMR: 2.5 ppm; ¹⁹ F NMR: -66.8 ppm). Thus, even though the operating pressure was lower, over-reduction readily occurred to give the undesired butane.

COMPARATIVE EXAMPLE B Hexafluoro-2-butyne was reduced in a similar manner to that of Example 2, except that 5 % palladium on barium sulfate was used as the catalyst. Analysis of the vapor phase prior to consumption of one equivalent of hydrogen showed starting material (76.3 %) and 1,1,1,4,4,4-hexafluorobutane (19.4 %), but no cis-hexafluoro- 2-butene. Analysis of the liquid phase likewise showed only starting material and 1,1,1 ,4,4,4-hexafluorobutane.

While we have shown and described several embodiments in accordance with our disclosure, it is to be clearly understood that the same may be susceptible to numerous changes apparent to one skilled in the art. Therefore, we do not wish to be limited to the details shown and described but intend to show all changes and modifications that come within the scope of the appended claims.