A method for fluorodecarboxylation

Abstract: The present invention is directed to a method for replacing a carboxylic acid group with a fluorine group in a halogenated aliphatic carboxylic acid compound having the formula, R-COOH, to prepare a fluorinated product having the formula, R-F, wherein R is a halogenated aliphatic group including straight- and branched-chain aliphatic groups selected from the group consisting of halogenated aliphatic and alkoxy-substituted halogenated aliphatic groups, wherein the method comprises the steps of (a) reacting the halogenated aliphatic carboxylic acid compound with bromine trifluoride, and (b) recovering the fluorinated product.

Description: The present invention is directed to a method for preparing fluorinated organic compounds. More particularly, the present invention is directed to a method for the fluorodecarboxylation of halogenated aliphatic carboxylic acid compounds to form fluorinated organic products which are useful as inhalation anesthetics. Anesthetics belong to a class of biochemical depressant drugs which affect the vital functions of all types of cells, especially nervous tissue cells. General anesthetics produce analgesia, loss of consciousness, diminished reflex activity, and muscular relaxation, with minimal depression of the vital functions. Anesthetics may be gaseous (volatile) or fixed (nonvolatile). Gaseous anesthetics are inhaled and enter the bloodstream through the lungs and fixed anesthetics are administered parenterally or through the alimentary canal. Many currently used gaseous anesthetics are halogenated compounds. These compounds tend to cause less metabolic disturbance and are less flammable than traditional gaseous anesthetic compounds such as ether and cyclopropane. Examples of halogenated anesthetic compounds include halothane (CF₃CHBrCl) and trichloroethylene (Cl₂C=CHCl) as well as halogenated ether compounds such as enflurane (CHF₂OCF₂CHClF), fluroxene (CF₃CH₂OCH=CH₂), methoxyflurane (Cl₂CHCF₂OCH₃), and isoflurane (CF₃CHClOCHF₂). A particularly useful halogenated ether anesthetic is sevoflurane, (CF₃)₂CHOCH₂F, also known as 2-(fluoromethoxy)-1,1,1,3,3,3-hexafluoropropane or fluoro-methyl-1,1,1,3,3,3-hexafluoro-2-propyl ether. Sevoflurane has a very low blood-gas solubility partition coefficient (0.6) which provides rapid equilibrium time, fast induction time and rapid recovery time. These properties make it especially useful for outpatient surgery, see D.F. Halpern , Chemtech , pp. 304-308 (May 1989). The preparation of fluorinated compounds such as sevoflurane tends to be difficult because of the limited number of selective fluorination reactions available. Direct fluorination of organic compounds to replace hydrogen is statistical, non-selective and generally accompanied by the formation of many side products. Hence fluorinated compounds are usually prepared by first synthesizing a substituted organic intermediate, wherein the substituent group is at the site to be fluorinated, and then displacing the substituent group with a fluoride ion. Metal fluorides, for example, have been used to displace chlorine substituent groups. United States patent no. 3,683,092, issued to Regan et al. , discloses a method for synthesizing sevoflurane which comprises methylation of hexafluoroisopropyl alcohol followed by fluorination with either (a) bromine trifluoride or (b) chlorine gas followed by potassium fluoride. United States patent no. 4,469,898, issued to Coon et al. , discloses a method for synthesizing sevoflurane which comprises mixing hexafluoroisopropyl alcohol, formaldehyde, hydrogen fluoride, and a protonating, dehydrating and fluoride ion generating agent. United States patent no. 4,874,901, issued to Halpern et al., discloses a method for fluorinating halogenated ether compounds. In particular, compounds such as sevoflurane can be prepared by reacting chloromethyl hexafluoroisopropyl ether with either potassium fluoride or sodium fluoride. Patrick et al. , J. Org. Chem. , 48 , 4158-4159 (1983), reports that alkyl carboxylic acids can be fluorodecarboxylated with xenon difluoride (XeF₂) in the presence of hydrogen fluoride. Although the use of xenon difluoride on a small scale can be effective, the cost of xenon difluoride makes its use on large scale impractical. Furthermore, when alkoxyacetic acids are fluorodecarboxylated with xenon difluoride, significant amounts of side products are formed. The use of xenon difluoride (XeF₂) is also described in the Canadian Journal of Chemistry, vol. 64, pages 138-141 (1986). United States patent no. 4,847,427, issued to Nappa , discloses a method for preparing fluorocarbon polyethers which comprises neutralizing a perfluorinated carboxylic acid by heating with fluorine in the presence of metal fluoride to replace the carboxylic acid group. While the above methods are useful for preparing certain fluorinated compounds, these methods are complex, expensive, and often provide fluorinated products in low yield together with considerable amounts of side products. Hence there is a need for improved methods for the preparation of fluorinated compounds. The present invention provides such an improved method for preparing fluorinated compounds from the corresponding carboxylic acids in high yield and purity which does not rely on xenon difluoride. More specifically, the present invention provides an improved procedure for the preparation of sevoflurane and other similar types of fluorinated anesthetics. The present invention is directed to a method for replacing a carboxylic acid group with a fluorine group in a halogenated aliphatic carboxylic acid compound having the formula, R-COOH, to prepare a fluorinated product having the formula, R-F, wherein R is a halogenated C₁-C₆-alkoxy C₁-C₆-alkyl group, wherein the method comprises the steps of (a) reacting the halogenated aliphatic carboxylic acid compound with bromine trifluoride, and (b) recovering the fluorinated product. The term "halogenated aliphatic carboxylic acid compound", as used herein, means an aliphatic carboxylic acid compound sufficiently halogenated such that the compound will not decompose upon contact with bromine trifluoride. Compounds which are insufficiently halogenated will ignite, burn, or otherwise decompose when contacted with bromine trifluoride and will not provide a fluorinated product. The term "halogenated C₁-C₆-alkoxy C₁-C₆-alkyl group" similarly means a C₁-C₆-alkoxy C₁-C₆-alkyl group, respectively, sufficiently halogenated such that, it will not decompose upon contact with bromine trifluoride. The exact degree of halogenation (the number of halogen atoms present in the molecule) and the exact type of halogenation (the type of halogen atoms present in the molecule) may be varied in the aliphatic carboxylic acid compound in order to obtain the desired final fluorinated product. Such variations are within the capabilities of those skilled in the art without the need for undue experimentation. In a most preferred embodiment, the fluorinated aliphatic carboxylic acid compound is a member selected from the group consisting of 1,1,1,3,3,3-hexafluoro-(2-propoxy)acetic acid and (2,2,2-trifluoroethoxy)acetic acid. In a most preferred embodiment, the fluorinated product is a member selected from the group consisting of 2-fluoromethoxy-1,1,1,3,3,3-hexafluoropropane and fluoromethoxy-2,2,2-trifluoroethane. The C₁-C₆-alkyl moiety is branched or unbranched and preferably contains from 1 to 4 carbon atoms. The C₁-C₆-alkoxy moiety is branched or unbranched and preferably contains from 1 to 4 carbon atoms. The term "halogen", as used herein, refers to the chemically related elements consisting of fluorine, chlorine, bromine and iodine, and is preferably fluorine. Bromine trifluoride (BrF₃) is a colorless liquid having a melting point of 8.77° C. In general, bromine trifluoride will be present in the reaction mixture in a stoichiometric excess compared to the total amount of halogenated aliphatic carboxylic acid compound. A 1:1 molar ratio of bromine trifluoride to halogenated aliphatic carboxylic acid compound, respectively, represents a 3:1 equivalent ratio with respect to fluorine. In a preferred embodiment, bromine trifluoride and the halogenated aliphatic carboxylic acid compound will be present in the reaction mixture in an equivalent ratio from about 2:3 to about 3:1, more preferably in an equivalent ratio from about 2:3 to about 1:1, and most preferably in an equivalent ratio of about 2:3, respectively. When the halogenated aliphatic carboxylic acid starting material compound is liquid, the fluorodecarboxylation reaction may be carried out without solvent. When the halogenated aliphatic carboxylic acid compound is not liquid, the fluorodecarboxylation reaction may be carried out in an inert solvent. Useful inert solvents include, but are not limited to, highly halogenated compounds such as dichloromethane (methylene chloride), chloroform, carbon tetrachloride, trifluoromethane, perfluorodecalin, and the like, and mixtures thereof. In a preferred embodiment, the inert solvent is carbon tetrachloride. The temperature of the fluorodecarboxylation reaction is not critical, but reaction usually takes place at room temperature. The temperature of the reaction mixture may be maintained at just above the boiling point of the fluorinated product to permit distillation of the product concurrent with its synthesis, thereby reducing degradation of the product when exposed to the harsh conditions of the fluorodecarboxylation reaction mixture. The fluorinated product may also be recovered from the reaction mixture in any other known fashion. For example, the reaction mixture may be warmed to a temperature sufficient to decompose excess bromine trifluoride by reaction with the solvent (a temperature of about 50°C. is generally sufficient in the presence of carbon tetrachloride). The reaction mixture may then be washed with sodium bisulfite (NaHSO₃) and the organic layer separated. The compounds which may be prepared by the fluorodecarboxylation method of the present invention include compounds which possess very desirable anesthesia activities. In particular, the anesthetic compounds which may be prepared have central nervous system depressant properties which include analgesia, hypnosis, sedation, increased pain threshold, and barbiturate and/or general anesthetic potentiation. Many of the compounds provide highly potent anesthesia with immediate onset and a short duration of action. These properties are highly desirable in circumstances where acute severe pain must be eliminated over a short period of time, such as in anesthesiology. The preferred compounds provide reduced rigidity at high doses, superior motor coordination recovery, or less respiratory depressive and/or cardiovascular depressive activity. The present invention is further illustrated by the following examples which are presented for the purpose of demonstrating, but not limiting, the method of this invention. EXAMPLE 1 1,1,1,3,3,3-Hexafluoro-(2-propoxy)acetic acid . This Example illustrates a method for preparing a halogenated aliphatic carboxylic acid starting material compound according to the present invention. A solution of 1,1,1,3,3,3-hexafluoro-2-propanol (42.1g, 0.25 mole, Aldrich) and bromoacetic acid (69.5g, 0.5 mole) in water (150ml) was brought to pH 12.5 by addition of 50% sodium hydroxide solution (47g) with stirring and maintained at pH 12.5 +/- 0.5 by addition of 50% sodium hydroxide solution, as needed. The reaction solution was heated to reflux for 3.5 hours, then cooled. A solution of 37% hydrochloric acid (61.3g) and 8% hydrochloric acid (27.1g) was added to the reaction solution to bring the pH to 1. The reaction solution was then distilled (azeotrope) under a Dean-Stark trap. The lower layer in the Dean-Stark trap was separated (53.2g) and sublimed (88° C., 7-9mm, dry ice/acetone trap) to yield 27.12g (48% yield) of 1,1,1,3,3,3-hexafluoro-(2-propoxy)acetic acid having mp. 60-62° C. When chloroacetic acid was substituted for bromoacetic acid in the above procedure, a 19.8% yield of 1,1,1,3,3,3-hexafluoro-(2-propoxy)acetic acid was obtained. EXAMPLE 2 2-Fluoromethoxy-1,1,1,3,3,3-hexafluoropropane. This Example illustrates the fluorodecarboxylation of a halogenated aliphatic carboxylic acid compound to a fluorinated product according to the method of the present invention. A solution of 1,1,1,3,3,3-hexafluoro-(2-propoxy)acetic acid (2.26g, 0.01 mole) from Example 1 in carbon tetrachloride (100g) was introduced into a 400ml Teflon vessel equipped with an air-tight cover, a thermometer, a Teflon stirring bar, a bromine trifluoride inlet and a gas outlet connected to a dry ice/acetone trap. Bromine trifluoride (1.05g, 0.42ml, 0.0067 mole) was slowly added to the solution. When addition of bromine trifluoride was complete, the reaction mixture was heated gently to just under reflux for 5 hours to remove volatiles and decompose excess bromine trifluoride (by reaction with carbon tetrachloride). The reaction mixture was then cooled and washed with dilute sodium bisulfite (NaHSO₃) solution. The organic layer was separated and analysis by gas chromatography showed a 75% yield of the product 2-fluoromethoxy-1,1,1,3,3,3-hexafluoropropane. EXAMPLE 3 (2,2,2-Trifluoroethoxy)acetic acid . This Example illustrates another method for preparing a halogenated aliphatic carboxylic acid starting material compound according to the present invention. A solution of 2,2,2-trifluoroethanol (84g, 1 mole, Aldrich) and chloroacetic acid (189g, 2 moles) in water (600ml) was brought to pH >13 by addition of 50% sodium hydroxide solution with stirring and maintained at pH >13 by addition of 50% sodium hydroxide solution, as needed. The reaction solution was heated to 84-93° C. for 3.5 hours, maintained at 90-100° C. overnight, then cooled. Concentrated hydrochloric acid was added to the reaction solution to bring the pH to about 1. The reaction solution was then heated until all 2,2,2-trifluoroethanol was distilled. The reaction solution was then cooled and extracted 12 times with dichloromethane (250ml portions). The combined dichloromethane extracts were concentrated under vacuum to yield 104.3g of residue which was taken up in acetone (200ml). The acetone mixture was filtered to remove inorganic salts, concentrated under vacuum, then distilled in two fractions (the first fraction distilled at 93-94.9° C. at 4.6mm/Hg and the second fraction distilled at 95.1-95.2° C. at 4.6mm/Hg). The two fractions were combined to afford 87.9g of the product (2,2,2-trifluoroethoxy)acetic acid. Anal. Calculated for C₄H₅F₃O₂: C, 30.38%; H, 3.16%. Found: C, 30.35%; H, 3.00%. EXAMPLE 4 Fluoromethoxy-2,2,2-trifluoroethane. This Example illustrates the fluorodecarboxylation of another halogenated aliphatic carboxylic acid compound to a fluorinated product according to the method of the present invention. A solution of (2,2,2-trifluoroethoxy)acetic acid (2.2g, 0.0014 mole) from Example 3 in carbon tetrachloride (80g) was introduced into a 400ml Teflon vessel equipped with an air-tight cover, a thermometer, a Teflon stirring bar, a bromine trifluoride inlet and a gas outlet connected to a dry ice/acetone trap. Bromine trifluoride (5.7g, 2ml, 0.041 mole) was slowly added to the solution. When addition of bromine trifluoride was complete, the reaction mixture was heated gently to just under reflux for 5 hours to remove volatiles and decompose excess bromine trifluoride (by reaction with carbon tetrachloride). The reaction mixture was then cooled and washed with dilute sodium bisulfite solution. The organic layer was separated and analysis by gas chromatography showed a 90% yield of the product fluoromethoxy-2,2,2-trifluoroethane. EXAMPLE 5 Fluoromethoxy-2,2,2-trifluoroethane. This Example illustrates the fluorodecarboxylation of a halogenated aliphatic carboxylic acid compound to a fluorinated product according to the method of the present invention using dichloromethane as the solvent. A solution of (2,2,2-trifluoroethoxy)acetic acid (20.0g, 0.127mole) from Example 3 in dichloromethane (229.3g) was introduced into a 400ml Teflon vessel equipped with an air-tight cover, a thermometer, a Teflon stirring bar, a bromine trifluoride inlet and a gas outlet connected to a dry ice/acetone trap. The vessel was placed in a water bath at room temperature and bromine trifluoride (11.4g, 4ml, 0.084 mole) was slowly added to the solution. When addition of bromine trifluoride was complete, the reaction mixture was stirred overnight, then washed with dilute sodium bisulfite solution. The organic layer was separated (194g) and analysis by gas chromatography showed a 100% yield of the product fluoromethoxy-2,2,2-trifluoroethane. COMPARATIVE EXAMPLE 6 Fluoromethoxy-2,2,2-trifluoroethane. This Example illustrates the fluorodecarboxylation of a halogenated aliphatic carboxylic acid compound to a fluorinated product using xenon difluoride. Xenon difluoride (2g, 0.012mole) was slowly added over a period of one hour to a solution of (2,2,2-trifluoroethoxy)acetic acid (1.9g, 0.012mole) from Example 3 in carbon tetrachloride (20g) in a glass vessel. When addition of xenon difluoride was complete, the reaction mixture was heated to 45° C. for 0.5 hour, then cooled. The reaction mixture was then distilled to provide 0.44g of product (25% yield). Gas chromatography analysis showed the major product to be fluoromethoxy-2,2,2-trifluoroethane contaminated with significant amounts of CF₃CH₂OCF₂H, CF₃CHFOCH₂F, CF₂Cl₂, and CFCl₃.

Claims: A method for replacing a carboxylic acid group with a fluorine group in a halogenated aliphatic carboxylic acid compound having the formula, R-COOH, to prepare a fluorinated product having the formula, R-F, wherein R is a halogenated C₁-C₆-alkoxy C₁-C₆-alkyl group wherein the method comprises the steps of: (a) reacting the halogenated aliphatic carboxylic acid compound with bromine trifluoride; and (b) recovering the fluorinated product. The method according to claim 1, wherein bromine trifluoride and the halogenated aliphatic carboxylic acid compound are present in an equivalent ratio from 2:3 to 3:1, respectively. The method according to claim 2, wherein bromine trifluoride and the halogenated aliphatic carboxylic acid compound are present in an equivalent ratio from 2:3 to 1:1, respectively. The method according to any one of the preceding claims, wherein the halogenated aliphatic carboxylic acid compound is reacted with bromine trifluoride in an inert solvent. The method according to claim 4, wherein the inert solvent is selected from the group consisting of dichloromethane, chloroform, carbon tetrachloride, and mixtures thereof. The method according to any one of the preceding claims, wherein the halogenated aliphatic carboxylic acid compound is a fluorinated aliphatic carboxylic acid compound. The method according to claim 6, wherein the fluorinated aliphatic carboxylic acid compound is 1,1,1,3,3,3-hexafluoro-(2-propoxy)acetic acid, whereby 2-fluoromethoxy-1,1,1,3,3,3-hexafluoropropane is produced as fluorinated product. The method according to claim 6, wherein the fluorinated aliphatic carboxylic acid compound is (2,2,2-trifluoroethoxy) acetic acid, whereby fluoromethoxy-2,2,2-trifluoroethane is produced as the fluorinated product.

Inventors: HALPERN DONALD F, ROBIN MARK L, HALPERN, DONALD F., ROBIN, MARK L.
Applicants: OHMEDA PHARMA PROD, OHMEDA PHARMACEUTICAL PRODUCTS DIVISION INC.
Application year: 1990
Main classification: C07C43
IPC codes: C07B39, C07C41, C07C17, C07C43, C07C19, C07C51, C07C59