Process for preparing α-keto-carboxylic acids from acyl halides
BACKGROUND 
     The present invention relates to a process for the carbonylation of an acyl halide to form an α-keto-carboxylic acid as the product. 
     The practical value of such α-keto-carboxylic acids is that they can be used to prepare the corresponding α-amino-acids or α-hydroxy acids which play an important role in biochemistry. For example, Saegusa et al, Journal of Organic Chemistry, Vol. 42, No. 16, 2797-2798 (1977) teach the selective reduction of α-keto acids to α-hydroxy acids by the use of certain phosphites as reducing agents. And, Tachibana et al, Chemistry Letters, pp. 1765-1768 (1982) disclose the transformation of α-keto acids to the corresponding α-amino acids by the reaction with chiral pyridoxamine analog, (R)- or (S)-15-aminomethyl-14-hydroxy-5,5-dimethyl-2,8-dithia[9](2,5)pyrid inophane (4), and Zn 2+   in the molar ratio of 2:1, in methanol. 
     The preparation of α-keto-carboxylic acids and their derivatives has been the subject of a large number of investigations. According to Rodd, The Chemistry of Carbon Compounds (1952 edition), Vol. 1, pages 227-229, the following methods of preparation are available: 
     gentle oxidation of α-hydroxyacids containing a secondary hydroxyl group, or by the enzymatic deamination of α-amino-acids; 
     hydrolysis of an acyl cyanide; 
     hydrolysis of α-oximino-esters; 
     from glycidic acid esters on treatment with benzene saturated with boron trifluoride; 
     from α,β-dibromocarboxylic acids by forming a piperidine addition compound followed by hydrolysis; 
     from α-keto-acetals by ultraviolet irradiation in the presence of N-bromosuccinimide; 
     from α-bromomethylketones by boiling with selenium dioxide in absolute methanol or ethanol; 
     from carboxylic acid esters by oxidation with selenium dioxide; 
     permanganate oxidation of vinyl ketones; 
     from carboxylic acid esters by condensation with oxalic ester followed by decarboxylation; 
     from aldehydes via 5-alkylidine-2-thio-oxazolid-4-ones or by reaction with methyl methoxyacetate; 
     hydrolysis of azalactones or acetamido-acrylic acids; hydrolysis of the reaction product of Grignard reagents on diethyl-oxamic ester; 
     oxidation of α-hydroxyacid esters containing two β-hydrogen atoms by N-bromosuccinimide in carbon tetrachloride to β-bromo-α-keto-acid esters; and by the action of alpha on the dimethanesulphonates and ditoluene-p-sulphonates of α,β-dihydroxycarboxylic acids. 
     In pending U.S. Ser. No. 353,473, entitled &#34;Process for Preparing Alkyl Alpha-Keto-Carboxylic Acids from Alkyl Halides,&#34; filed on Mar. 1, 1982, there is disclosed a method of preparing a alkyl α-keto-carboxylic acid by reacting a primary alkyl halide in a liquid solvent medium with carbon monoxide at elevated temperature and elevated pressure in the presence of a catalytic amount of a metal carbonyl compound, such as dicobalt-octacarbonyl and an alkali metal base or an alkaline earth metal base. 
     Methods also are known for preparing arylpyruvic acids. For example, U.S. Pat. No. 4,152,352 discloses the preparation of an arylpyruvic acid by reacting an arylmethyl halide in a liquid solvent medium with carbon monoxide at pressures of 5 to 200 bars in the presence of a catalytic amount of a metal carbonyl compound and an alkaline earth metal inorganic base. Further, U.K. patent application No. 2,026,478A discloses that alkali metal salts of an arylpyruvic acid can be prepared by reacting an arylmethyl halide, carbon monoxide and an alkali metal base in the presence of a metal carbonyl compound as catalyst and in the presence of an alcohol or cyclic ether as solvent. 
     Finally, it is known in the art to react an acyl halide or alkyl halide in the presence of carbon monoxide, with a nucleophilic transition metal anionic complex to produce the corresponding acyl metal carbonyl derivative. See, for example, Heck, Organotransition Metal Chemistry, (Academic Press, New York, N.Y., 1974) pp. 200-209. 
     SUMMARY 
     It has now been found that α-keto-carboxylic acids of the general formula: ##STR4## in which R 1  and R 2  are the same or different and represent hydrogen, a hydrocarbyl radical, a substituted hydrocarbyl radical or a hydrocarbyloxy radical, can be prepared by carbonylating an acyl halide of the formula: ##STR5## wherein R 1  and R 2  are as defined above and X represents halogen, in a liquid solvent medium, with an alkali metal tetracarbonyl cobaltate complex of the formula: 
     
         MCo(CO).sub.4 
    
     wherein M is an alkali metal, to form the corresponding acylcobaltcarbonyl complex of the formula: ##STR6## wherein R 1  and R 2  are as defined above, reacting the acylcobaltcarbonyl complex thus formed with carbon monoxide and an alkali metal hydroxide or an alkaline earth metal hydroxide at elevated temperature and elevated pressure in a liquid solvent medium to form the corresponding alkali metal salt or alkaline earth metal salt of the product α-keto-carboxylic acid and thereafter acidifying the salt of the product α-keto-carboxylic acid to form the resultant α-keto-carboxylic acid. By this method, acyl halides are readily and inexpensively converted to α-keto-carboxylic acids. 
    
    
     THE INVENTION 
     The α-keto-carboxylic acids of the present invention are compounds having the general formula: ##STR7## wherein R 1  and R 2  are the same or different and are selected from hydrogen, hydrocarbyl radicals, substituted hydrocarbyl radicals and hydrocarbyloxy radicals. Preferably, the hydrocarbyl radicals are those that contain up to about 20 carbon atoms. For purposes of this invention a hydrocarbyl radical can be defined as an organic group solely composed of hydrogen and carbon atoms. Some non-limiting representative examples of hydrocarbyl radicals are alkyl, cycloalkyl, alkenyl, aralkyl, alkaryl, and aryl. 
     Examples of suitable alkyl groups are methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-amyl, and the various positional isomers thereof, and likewise the corresponding straight and branched chain isomers of hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, and the like. 
     Some examples of cycloalkyl groups are cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl, cycloundecyl, cyclododecyl, and the like. They may also be such cycloaliphatic groups as α-cyclopropyl-ethyl, α-cyclobutyl-propyl, β-cyclobutyl-propyl, and similar alkyl derivatives of the higher cycloalkyls. 
     Some examples of alkenyl groups are ethenyl, 1-propenyl, 2-propenyl, isopropenyl, 1-butenyl, 2-butenyl, 3-butenyl, and the corresponding branched-chain isomers thereof as for example, 1-isobutenyl, 2-isobutenyl, 2-sec-butenyl, including 1-methylene-2-propenyl, and the various isomers of pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, and dodecenyl, including 3,3-dimethyl-1-butenyl, 2,3-dimethyl-1-butenyl, 2,3-dimethyl-2-butenyl, 2,3-dimethyl-3-butenyl, 1-methyl-1-ethyl-2-propenyl, and the like. 
     Examples of alkaryl groups are tolyl, 2,3-xylyl, 2,4-xylyl, 2,5-xylyl, 2,6-xylyl, 3,4-xylyl, 3,5-xylyl; o, m, and p-cumenyl, mesityl, o, m, and p-ethylphenyl, 2-methyl-1-naphthyl, 3-methyl-1-naphthyl, 4-methyl-1-naphthyl, 5-methyl-2-naphthyl, 6-methyl-3-naphthyl, 7-methyl-1-naphthyl, 8-methyl-4-naphthyl, 1-ethyl-2-naphthyl, and its various positional isomers and the like. 
     Examples of aryl groups which may be present in the above general formula are phenyl, naphthyl, and the like. 
     Examples of aralkyl groups are benzyl, phenylethyl, 1-phenylpropyl, 2-phenylpropyl, 3-phenylpropyl, 1- and 2-isomers of phenylisopropyl, 1-, 2-, and 3-isomers of phenylbutyl, and the like. 
     The substituted hydrocarbyl radicals are hydrocarbyl radicals which contain substituents such as halogen, carboxyl, or amide radicals. 
     As mentioned above, the hydrocarbyl groups may be halogen substituted. Thus, chlorine, bromine, iodine, and fluorine may be substituted on the alkyl, cycloalkyl, alkenyl, alkaryl, aryl, and aralkyl groups which are present. Non-limiting examples of such substituted groups are chloromethyl, chloroethyl, bromoethyl, 2-fluoro-1,2-dibromoethyl, 1-iodopropyl, 2-fluoropropyl, 1-chlorobutyl, 2-bromobutyl, 2-iodo-2-methylpropyl, 1-chloropentyl, 3-fluoro-2-methylbutyl, 3-iodo-2-methylbutyl, 1-chloro-2,2-dimethylpropyl, 2-chloroheptyl, 3-fluorononyl, 1-chlorododecyl, and the like. Examples of halogenated cycloalkyl groups are chlorocyclopropyl, chlorocyclohexyl, 1,2-dichlorohexyl, bromocyclobutyl, iodocyclohexyl, and the like. 
     Examples of halogen-substituted alkenyl groups are bromoethenyl, chloroethenyl, iodoethenyl, 1-bromododecenyl, and the like. 
     Examples of halogenated alkaryl groups are chloro-o-tolyl, chloro-p-tolyl, chloro-m-tolyl, 2-bromo-3,4-xylyl, 4-bromo-2,3-xylyl, 5-bromo-2,4-xylyl, 2-bromo-4,5-xylyl, o-, m-, and p-tolyl, 3-bromomesityl, chloro(methyl)-1-naphthyl, iodo(ethyl)-1-naphthyl, all positional isomers of the above, and the like. 
     Examples of halogen substituted aryl groups are bromophenyl, 2-bromo-1-naphthyl, 3-bromo-1-naphthyl and all positional isomers thereof, 2,4-dibromophenyl, 2,3-dibromophenyl, 2,5-dibromophenyl, 2,3,4,5-tetrabromophenyl, 2,3,5,6-tetrabromophenyl, pentabromophenyl, all isomers of chlorophenyl, and all isomers of multichlorophenyl: 2-chloro-1-naphthyl and the remaining isomers thereof: 2,3-dichloro-1-naphthyl, 2,4-dichloro-1-naphthyl and the remaining positional isomers of dichloronaphthyl, 2,3,4,5-tetrachloro-1-naphthyl. 
     The hydrocarbyl groups may contain amide groups which may be illustrated by such non-limiting examples as: carbamoylmethyl, 2-carbamoylethyl, 4-carbamoylbutyl, 8-carbamoyl-2-ethyloctyl, 1,4-dicarbamoylbutyl, carbamoylcyclopentyl, carbamoylcyclohexyl, 2-carbamoyl-o-tolyl, 2-carbamoyl-m-tolyl, 3-carbamoyl-p-tolyl, (carbamoylmethyl)phenyl, (2-carbamoylethyl)-benzyl: o-, m-, and p-(carbamoylethyl)phenyl, and the like. 
     Representative examples of α-keto-carboxylic acids which can be made by the process of the present invention include: 
     pyruvic acid, 
     2-oxo-butanoic acid 
     2-oxo-3-methylbutyric acid, 
     2-oxo-dodecanoic acid, 
     cyclohexylglyoxylic acid, 
     2-oxo-hept-6-enoic acid, 
     phenylpyruvic acid, 
     p-tolylpyruvic acid, 
     benzylpyruvic acid, 
     2-chloro-2-oxopropinoic acid, 
     p-chlorophenylpyruvic acid, and the like. 
     Acyl halides suitable for use in the present process are those halides having the structural formula: ##STR8## wherein R 1  and R 2  are the same or different and are hydrogen, hydrocarbyl radicals, substituted hydrocarbyl radicals or hydrocarbyloxy radicals, as defined above, and X is halogen. Primarily, acyl halides having from 1 to about 20 atoms are preferred. Acylfluorides and acyliodides may be used, but it is preferable to use acylhalides of the &#34;middle halogens&#34;--i.e., acylchlorides and acylbromides. 
     Methods for preparing acyl halides are well known. For example, acyl halides are prepared by the substitution of a halide radical for the hydroxyl radical of a carboxylic acid. Three reagents are commonly used for this purpose: thionyl chloride, SOCl 2  ; phosphorus trichloride, PCl 3  ; and phosphorous pentachloride, PCl 5 . Thionyl chloride is particularly convenient, since the products formed besides the acid chloride are gases and thus are easily separated from the acid chloride. See, for example, Morrison and Boyd, Organic Chemistry (Allyn and Bacon, Inc., Boston, Mass., 1970), p. 601. 
     Representative examples of acyl halide reactants which can be used in the instant process include: 
     acetyl chloride, 
     acetyl bromide, 
     propanoyl chloride, 
     propanoyl bromide, 
     isobutyryl chloride, 
     isobutyryl bromide, 
     undecanoyl chloride 
     undecanoyl bromide, 
     cyclohexanecarbonyl chloride, 
     cyclohexanecarbonyl bromide, 
     hex-5-enoylchloride, 
     hex-5-enoylbromide, 
     phenylacetyl chloride, 
     phenylacetyl bromide, 
     p-tolylacetyl chloride, 
     p-tolylacetyl bromide, 
     3-phenylpropanoyl chloride, 
     3-phenylpropanoyl bromide, 
     chloroacetyl chloride 
     p-chlorophenylacetyl chloride, and the like. 
     The alkali metal tetracarbonyl cobaltate of the invention is characterized by the following formula: 
     
         MCo(CO).sub.4 
    
     wherein M is an alkali metal preferably selected from lithium, sodium, and potassium. The alkali metal tetracarbonyl cobaltate reactant is readily prepared by reducing dicobalt-octacarbonyl with a suitable reducing agent such as potassium hydride, sodium hydride or sodium amalgam in a solvent such as tetrahydrofuran to form the alkali metal tetracarbonyl cobaltate. 
     The reaction between the acyl halide reactant of the invention and the alkali metal tetracarbonyl cobaltate reactant is conveniently carried out at a reaction temperature of from about 0° C. up to about 50° C., with a temperature range of from about 10° C. up to about 30° C. being preferred, at ambient pressure, in the presence of a suitable reaction solvent. 
     Solvents which may be used in the reacton are those solvents which are inert under the reaction conditions. That is, the reaction is carried out in the presence of a solvent which does not enter into the reaction. Aprotic solvents which include ethers such as diethyl ether, dibutyl ether, 1-ethoxyhexane, tetrahydrofuran, 1,4-dioxane, 1,3-dioxolane, diglyme, 1,1-diethoxyethane, and tertiary amines such as pyridine, N-ethylpiperidine, triethylamine, tributylamine, N,N-diphenyl-N-methylamine, N,N-dimethylalanine, etc. can be used in the practice of the process. Also, dipolar aprotic solvents such as dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfone, tetramethylene sulfone, N-methylpyrrolidine, acetonitrile and like materials can be used in the process. Other solvents which are inert under the reaction conditions may be used, for example, low boiling hydrocarbons, halogenated hydrocarbons, examples of which are benzene, toluene, tetrachloroethane, the chlorinated benzenes, the chlorinated toluenes and the like. 
     In general, equimolar amounts of the alkali metal tetracarbonyl cobaltate complex and the acyl halide reactant are used in the reaction. 
     The reaction is readily conducted by placing the acyl halide and the cobaltate complex in a suitable reaction vessel having agitation means. The process is preferably conducted in a substantially anhydrous reaction system and, accordingly, the components of the reaction system should be brought together and maintained under a substantially dry, inert atmosphere. Thus, while it is possible to conduct this process in the presence of moisture, it is desirable to maintain the reaction system under an atmosphere of dry nitrogen or the like. 
     The mode of addition is not particularly critical. Accordingly, it is convenient to add the acyl halide reactant to a mixture of the other materials, add the alkali metal tetracarbonyl cobaltate complex to a mixture of the other materials, introduce all ingredients simultaneously into the reaction zone or the like. 
     The reaction of the acyl halide component and the cobaltate complex yields an acylcobaltcarbonyl complex of the formula: ##STR9## wherein the R 1  and R 2  substituents are derived from and are the same as those substituents contained in the acyl halide reactant. 
     The acylcobaltcarbonyl complex once formed is subsequently reacted with carbon monoxide and an alkali metal hydroxide or an alkaline earth metal hydroxide at elevated temperature and elevated pressure to form the corresponding alkali metal salt or alkaline earth metal salt of the product α-keto-carboxylic acid. An absence of basic reagent in the process prevents the formation of the desired α-keto-carboxylic acid. 
     Specific examples of alkali metal hydroxides and alkaline earth metal hydroxides which can be used in the practice of the process include: LiOH, NaOH, KOH, RbOH, Ca(OH) 2 , Ba(OH) 2  and Mg(OH) 2 . Calcium hydroxide is particularly preferred. 
     The amount of basic agent used in the process can vary within wide limits. In general, the molar ratio of the alkali metal hydroxide or alkaline earth metal hydroxide to the acylcobaltcarbonyl complex is preferably 1:1 to 10:1. 
     The reacton of the acylcobaltcarbonyl complex, basic reagent and carbon monoxide is carried out in the presence of a mixture of water and a suitable reaction solvent which is inert under the reaction conditions. Suitable solvents include the aforementioned aprotic solvents such as ethers, including diethyl ether, dibutyl ether, 1-ethoxyhexane, tetrahydrofuran, 1,4-dioxane, 1,3-dioxolane, diglyme, 1,1-diethoxyethane, tertiary amines such as pyridine, N-ethylpiperidine, triethylamine, tributylamine, N,N-diphenyl-N-methylamine, N,N-dimethylalanine, and the like. Also, the aforementioned dipolar aprotic solvents such as dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfone, tetramethylene sulfone, N-methylpyrrolidine, acetonitrile and like materials can be used in the reaction. Other solvents, for example, low boiling hydrocarbons, halogenated hydrocarbons, examples of which are benzene, toluene, tetrachloroethane, the chlorinated benzenes, the chlorinated toluenes and the like also may be used in the reaction. A particularly preferred solvent is tetrahydrofuran and a particularly preferred solvent system is water and tetrahydrofuran. Mixtures containing from about 10% to 90% by weight of water and from about 90% to 10% by weight tetrahydrofuran can be used. Typically, a solvent mixture containing about 90% by weight tetrahydrofuran and 10% by weight water is used as the solvent system. 
     The reaction between the acylcobaltcarbonyl complex, the alkali metal hydroxide or alkaline earth metal hydroxide reactant and carbon monoxide is advantageously carried out by bringing the mixture consisting of the acylcobaltcarbonyl complex and basic agent suspended in a mixture of water and solvent into contact, under nitrogen, in a suitable pressure-resistant reactor equipped with a stirrer, with a large excess of carbon monoxide (i.e., amounts greater than 2 moles of carbon monoxide per mole of the acylcobaltcarbonyl complex) introduced at the desired pressure and temperature, in accordance with techniques suitable for bringing about the reaction between a liquid phase and a gas phase. 
     The carbonylation reaction is carried out at a temperature in the range of from about 30° C. to about 150° C., preferably from about 50° C. to about 100° C., over a period of time of from about 1 to 20 hours. 
     In general, the reaction takes place at elevated carbon monoxide pressures which may range from about 300 psig to about 3000 psig. Preferably, the reaction takes place at a pressure in the range of about 800 psig to 1000 psig. The carbon monoxide may contain or be mixed with an inert gas, such as nitrogen. 
     Upon completion of the reaction, the product mixture is filtered, if necessary, as when Ca(OH) 2  is used as the basic reagent, resulting in the alkali metal salt or the alkaline earth metal salt of the desired α-keto-carboxylic acid being separated from the liquid reaction component as the main solid component which is thereafter acidified to form the product α-keto-carboxylic acid. The filtrate contains the remainder of the alkali metal salt or alkaline earth metal salt of the α-keto-carboxylic acid in addition to unreacted acylcobaltcarbonyl complex and the corresponding carboxylic acid. In some instances, a solid alkali metal salt or an alkaline earth metal salt does not form and instead remains in solution when certain basic reagents other than calcium hydroxide are used. In such cases, the salt is acidified in solution. 
     The metal salt of the α-keto-carboxylic acid is acidified with a dilute acid, such as hydrochloric acid, sulfuric acid, or nitric acid, so as to displace the α-keto-carboxylic acid from its alkali metal salt or its alkaline earth metal salt. The solution obtained is extracted with a suitable solvent, for example, an ether such as diethyl ether, and the organic extract thus obtained is purified by conventional acid-base workup. The final residue consists of the product α-keto-carboxylic acid and the corresponding carboxylic acid derived from the acyl halide reactant. The carboxylic acid can be recycled and transformed into starting acyl halide reactant by substitution of a halide radical for the hydroxyl radical of the carboxylic acid by the use of thionyl halide, phosphorous trihalide or phosphorous pentahalide as previously discussed. 
     If desired, esters of the α-keto-carboxylic acid products of the present invention can be prepared by esterifying the α-keto-carboxylic acid product according to conventional esterification techniques employing alcohols and acid catalysts such as, for example, BF 3 , BF 3 .HCl, BF 3 .MeOH, BF 3 .Et 2  O or diazomethane at suitable reaction conditions. 
     The following examples will illustrate the invention. 
     EXAMPLE I 
     Preparation of Potassium Tetracarbonyl Cobaltate 
     A 250 mL, three-neck round bottom flask in a dry box was charged with 4.0 g of 25% potassium hydride/oil dispersion. The flask was transferred to the hood and the potassium hydride was freed of oil by washing with dry tetrahydrofuran (3×5 mLs). The potassium hydride was suspended in 75 mLs of tetrahydrofuran and 3.0 g of dicobalt octacarbonyl was added slowly as a solution in 25 mLs of tetrahydrofuran. The solution was stirred at 25° C. for 1 hour and then filtered under nitrogen into a volumetric storage burette. Analysis by IR indicated a concentration of 0.164 mmoles of potassium tetracarbonyl cobaltate per mL of tetrahydrofuran. 
     EXAMPLE II 
     Preparation of Phenylpyruvic Acid 
     A 50 mL three-neck round bottom flask containing a solution of 3.64 mmoles of potassium tetracarbonyl cobaltate in 20 mLs tetrahydrofuran was cooled to 10° C. under a nitrogen atmosphere. Phenylacetyl chloride (0.563 g; 0.48 mLs) was added to the flask over a period of time of two minutes and the resultant reaction mixture was stirred at 10°-15° C. for three hours. The reaction mixture was then transferred into a 300 mL autoclave contaning a suspension of 1.07 g of calcium hydroxide in 25 mLs tetrahydrofuran and 5 mLs water, under nitrogen, using a transfer needle. The autoclave was sealed, flushed with carbon monoxide and pressurized to 900 psig with carbon monoxide and heated to 90° C. and stirred for nine (9) hours. The carbon monoxide was vented from the reaction mixture yielding a light yellow suspension (pH 12-13). The solution was acidified with 10% hydrochloric acid and treated with an excess of iodine to destroy residual cobaltate anion. The reacton mixture was diluted with 100 mLs diethyl ether and then treated with saturated sodium thiosulfate until the iodine color was dispelled. The layers were separated and the aqueous layer extracted with a second 100  mL volume of diethyl ether. The combined diethyl ether layers were dried over MgSO 4  and evaporated to give 810 mg of a yellow solid which was found to contain a small amount of sulfur. The product was then taken up in 15 mLs diethyl ether, filtered and extracted with 5% sodium hydroxide (3×25 mLs). Acidification and re-extraction gave 0.33 mmoles (9% yield) phenylpyruvic acid and 2.84 mmoles (78% yield) of phenylacetic acid. 
     Having described the process which Applicant regards as his invention, it should be recognized that changes and variations within the scope and spirit of the invention can be made by one skilled in the art and it is accordingly to be understood that the present description of the invention is illustrative only. It is desired that the invention be limited only by the lawful scope of the following claims.