Process for the production of fluorinated nitriles from corresponding amides

Abstract: The invention relates to a process for reacting fluoro-amide of the formula CF 2 =CF-R f -CONH 2 , wherein R f is perfluoroalkyl or perfluoroalkoxy containing 2-20 carbon atoms and R is alkyl having 1-6 carbon atoms, with pyridine and trifluoroacetic anhydride to form fluoro-nitrile of the formula CF 2 =CF-R f -CN, the improvement comprising carrying out the reaction in polar solvent having the formula R-(CO)-NR'R", wherein R is H or alkyl having 1-6 carbon atoms, and R' and R" are independently alkyl having 1-6 carbon atoms, to form said fluoro-nitrile.

Description: FIELD OF THE INVENTION This invention is in the field of processes for synthesizing nitrile compounds having olefinic functionality. Such nitrile compounds can be used as cure site monomers in fluoroelastomers. BACKGROUND OF THE INVENTION Compounds of the general formula CF 2 =CF-R f CN wherein R f is perfluoroalkyl or perfluoroalkoxy containing 2-20 carbon atoms are useful as cure site monomers in fluoroelastomers. See, for example. U.S. Patent 3.467,638 (Gladding & Sullivan) or U.S. Patent 4,281,092 (Breazeale). A traditional method for making such compounds is by (a) first reacting the corresponding ester starting material (or a derivative thereof) with ammonia gas at low cooling temperature to form an amide compound, and (b) then converting the amide to the desired nitrite through a dehydration process. The traditional synthetic approach suffers from low yield due to the problems of a non-homogeneous reaction medium and from difficulties in product separation. Since the solubilities of fluorinated compounds are usually quite low in common organic solvents, a fluorinated solvent such as 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) is often used in these reactions as a product extraction solvent. CFCs are now undesirable for environmental reasons, and their manufacture is being phased out. Formation of significant quantities of side-products due to sensitivity of olefin functionality toward the reaction reagents also occurs. See U.S. Patent 4,138,426 (England). The yield from the first step of England Example 8, conducted in ether (ethyl ether), was only 9%, and CFC-113 was employed in separating the reaction product from the reaction mass. In the sec. and step of England, the first step amide was dehydrated to the nitrile in tetrahydrofuran solvent using pyridine and trifluoroacetic anhydride as dehydrating agent. Again. CFC-113 was used to aid separation of the product from the reaction mass. An improved method of synthesizing these compounds is desired. SUMMARY OF THE INVENTION This invention provides an improved process for synthesizing fluoro-nitrile compound of the formula CF 2 =CF-R f -CN from fluoro-amide CF 2 =CF-R f -CONH 2 by reacting fluoro-amide of the formula CF 2 =CF-R f -CONH 2 , wherein R f is perfluoroalkyl or perfluoroalkoxy containing 2-20 carbon atoms and R is alkyl having 1-6 carbon atoms, with pyridine and trifluoroacetic anhydride to form fluoro-nitrile of the formula CF 2 =CF-R f -CN, the improvement comprising carrying out the reaction in polar solvent having the formula R-(CO)-NR'R", wherein R is H or alkyl having 1-6 carbon atoms, and R' and R" are independently alkyl having 1-6 carbon atoms, to form said fluoro-nitrile. Preferred solvents for the reaction include N,N-dimethylformamide and N,N-dimethylacetamide. DETAILED DESCRIPTION OF THE INVENTION It has been discovered that the reaction of fluoro-amide with pyridine and trifluoroacetic anhydride to form fluoro-nitrile results in solvent and fluoro-nitrile product phases that can be easily separated without the aid of halogenated solvent if the reaction is carried out in polar solvent having a carbonyl group. The fluoro-amide can be prepared from fluoro-ester compounds which can have the formula CF 2 =CF-R f -COOR, wherein R is alkyl having 1-6 carbon atoms, and R f is perfluoroalkyl or perfluoroalkoxy having 2-20 carbon atoms. Preferably, R is methyl or ethyl. Preferred R f is perfluoroalkoxy, including -[OCF 2 CF(CF 3 )] n OCF 2 CF 2 - wherein n = 1-5, most preferably n = 1. and -O(CF 2 ) k - wherein k = 2-20, most preferably k = 2-4. When R f is -(CF 2 ) m -, m = 2-12. Preferably, m = 2.8. Such fluoro-ester compounds, or precursor acids that are readily esterified. are known. See. for example, U.S. Patents 3,546,186; 4,138,486; 4,275,226: and 4,281,092; and Zh. Org. Kim. 16 , 540 (1980). The reaction of fluoro-ester with ammonia or aqueous ammonium hydroxide to form fluoro-amide of the formula CF 2 =CF-R f -CONH 2 , wherein R f is as defined above, is carried out neat, i.e., in the absence of solvent, or in a solvent selected from a hydrogen-containing halocarbon or a solvent in which the only oxygen present is ether oxygen bonded to perfluoroalkyl. Preferred solvents are saturated halogenated compounds that are not chlorofluorocarbon (CFC), i.e., are hydrogen-containing halocarbons. Desirably, such solvents have boiling points no higher than 100°C. Methylene chloride is an especially preferred solvent. The ratio of equivalents of ammonia, or ammonium hydroxide, to fluoro-ester can be in the range 1.0 to 2.5, preferably 1.1 to 2.0. The reaction of fluoro-ester with ammonia can be carried out at such temperatures as from -80° to 0°C, preferably at -60° to -5°C, most preferably at -20° to -5°C. For reaction of fluoro-ester with aqueous ammonium hydroxide, the reaction can be carried out at such temperatures as from -20 ° to +25°C, preferably -5° to +20°C, most preferably +10° to +15°C. When reaction is started with fluoro-ester, desired fluoro-amide is easily separated from the reaction mass, and can be obtained in very pure form by distillation. Molar yields to fluoroamide from fluoro-ester are high, more than 70%, and usually more than 80%. The reaction of fluoro-amide with pyridine and trifluoroacetic anhydride to form fluoro-nitrile of the formula CF 2 =CF-R f -CN, wherein R f is as defined above, is carried out in anhydrous polar solvents having the formula R-(CO)-NR'R", wherein R is H or alkyl having 1-6 carbon atoms, and R' and R" are independently alkyl having 1-6 carbon atoms. Preferably, R is H or methyl, and R' and R" are both methyl or ethyl. Alternatively. R and either R' or R" can be connected to form a cyclic structure. Preferred solvents include N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone. A preferred dehydrating agent is the combination of pyridine and trifluoroacetic anhydride. In carrying out the reaction with fluoro-amide in anhydrous polar solvent. the ratio in equivalents of pyridine and trifluoroacetic anhydride to fluoro-amide is usually in the range of 1.0 to 2.0, preferably 1.1 to 1.5 for trifluoroacetic anhydride, and 1.0 to 2.5, preferably 1.0 to 2.0, and most preferably 1.1 to 1.5 for pyridine. The reaction of fluoro-amide with pyridine and trifluoroacetic anhyride can be carried out at temperatures such as from -25° to +25°C, preferably from -15 ° to 0°C. When reaction with fluoro-amide is carried out by the process of this invention, desired fluoro-nitrile predominantly separates from the reaction mass as a distinct liquid layer, with the other layer being solvent containing dehydrating agent reaction residues. Fluoronitrile can then be obtained in high yield by separating and washing that phase with water. followed by distillation. Molar yields to fluoro-nitrile from fluoro-amide are high, usually more than 75%, and often more than 80%. The reaction of fluoro-ester with ammonia or ammonium hydroxide to form fluoro-amide and the reaction of fluoro-amide with dehydrating agent in polar solvent to form fluoro-nitrile, together, are a process for synthesizing fluoro-nitrile from fluoro-ester, This sequence can be carried out with isolation and purification of intermediate fluoro-amide. In another embodiment of this invention, the sequence can be carried out consecutively without purifying the intermediate fluoroamide. in the latter sequence, for example, fluoro-ester can be reacted with anhydrous ammonia gas, preferably in the absence of solvent, followed by removal of excess ammonia and side product methanol under reduced pressure, but without costly and time consuming distillation of the crude fluoro-amide. Crude fluoro-amide can then be dissolved in the polar solvent and reacted with dehydrating agent to form fluoro-nitrile. which can be separated, washed, and purified by distillation as described above. This sequence has the significant advantage of eliminating a purification step. EXAMPLES The following code is used in the examples below: EVE = Methyl perfluoro-(5-methyl-4,7-dioxa-8-nonenoate) [CF 2 =CFOCF 2 CF(CF 3 )OCF 2 CF 2 -COOCH 3 ] AVE = Perfluoro-(5-methyl-4,7-dioxa-8-non-enoyl)amide [CF 2 =CFOCF 2 CF(CF 3 )OCF 2 CF 2 -CONH 2 ] 8-CNVE = Perfluoro-(8-cyano-5-methyl-3,6-dioxa-1-octene) [CF 2 =CFOCF 2 CF(CF 3 )OCF 2 CF 2 -CN] Example 1 Synthesis of 8-CNVE from AVE AVE (40.7 g, 0.1 mol) was dissolved in 42.5 mL of anhydrous N.N-dimethytformamide (DMF) and cooled to -20°C. Pyridine (9.48 g, 0.12 mol) was added slowly, followed by 25.2 g (0.12 mol) of trifluoroacetic anhydride. The reaction was slightly exothermic and the pot was kept at -10° to -15°C by external cooling. After addition of trifluoroacetic anhydride was complete, the reaction mixture was warmed to ambient temperature. At this time, a two-layer product mixture was observed. The top layer was mainly DMF solvent. while the bottom layer was essentially the fluoro-nitrile product with very small amount of DMF. The bottom layer was separated. washed with, water, and distilled to obtain 33.5 g of the desired 8-CNVE, confirmed by comparison of 19 F NMR, GC. and infrared measurements with those for a reference sample, as a clear, colorless liquid having a boiling point of 102°-104°C. Yield was 86%. Control A Experiments were carried out in the same way as Example 1 except that THF was used as the solvent instead of DMF. The reaction solutions developed a yet-low color, and more side products were detected by GC analysis. Yields to isolated 8-CNVE product were in the range of 40-50% at best. Control B This experiment was carried out in the same way as Example 1 except that methylene chloride was use as the solvent instead of DMF. The yield was reasonable as judged from GC analysis. However, the 8-CNVE product could not be recovered in pure form since 8-CNVE forms a distillation azeotrope with CH 2 Cl 2 . Example 2 Synthesis of 8-CNVE from EVE EVE (1698 g, 3.9 mol) was cooled to -15°C in a 5-liter glass pot. Anhydrous ammonia (76.4 g, 4.49 mol) was bubbled slowly through the EVE while controlling the temperature in the range of -15° to -5°C. After additon of ammonia was complete, the reaction mixture was stirred for 1 hr and allowed to warm up to 10°-15°C. Vacuum was then applied to remove excess ammonia and the methanol formed, leaving crude fluoro-amide (AVE) in the pot. DMF (1500 mL) was added to the crude AVE, and the solution was stirred and cooled to -15°C. Pyridine (354 g, 4.49 mol) was added to the solution, followed by slow addition of trifluoroacetic anhydride (TFAA, 943 g, 4.49 mol). Reaction temperature was maintained in the range of from -15°C to -5°C during the addition of TFAA. Alter TFAA addition was complete, the reaction mixture was stirred for 1 hr while allowing the mixture to warm up to room temperature. Cold water (0°-2°C, 500 mL) was added to the mixture with stirring during 10 min. When agitation was stopped, the mixture of wash water and DMF solvent separated as an upper layer, and fluoro-nitrile (8-CNVE) separated as an Insoluble lower layer which was removed and distilled at atmospheric pressure to obtain 1138 g of product boiling in the range 101°-103°C. GC analysis of this distillate showed it to be 98.5% pure 8-CNVE. Yield to 8-CNVE from EVE was 75% (molar basis).

Claims: In the process for reacting fluoro-amide of the formula CF 2 =CF-R f -CONH 2 , wherein R f is perfluoroalkyl or perfluoroalkoxy containing 2-20 carbon atoms and R is alkyl having 1-6 carbon atoms, with pyridine and trifluoroacetic anhydride to form fluoro-nitrile of the formula CF 2 =CF-R f -CN, the improvement comprising carrying out the reaction in polar solvent having the formula R-(CO)-NR'R", wherein R is H or alkyl having 1-6 carbon atoms, and R' and R" are independently alkyl having 1-6 carbon atoms, to form said fluoro-nitrile. The process of Claim 1, wherein said R f is -[OCF 2 CF(CF 3 )] n OCF 2 CF 2 - and n = 1-5. The process of Claim 2, wherein n = 1. The process of Claim 1, wherein R is H, and R' and R" are methyl or ethyl. The process of Claim 1, wherein R, R' and R" are methyl.

Inventors: HUNG MING-HONG, SUBRAMANYAM VINAYAKAM, HUNG, MING-HONG, SUBRAMANYAM , VINAYAKAM
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1996
Main classification: C07C231
IPC codes: C07C255, C07C253