Catalytic method for producing fluoroaromatic compounds using branched alkyl pyridinium salts
BACKGROUND OF THE INVENTION 
     This application is a continuation-in-part of application Ser. No. 816,479, filed January 6, 1986. The present invention relates to an improved method for preparing fluoroaromatic compounds (such as fluoronitrobenzene compounds) wherein N-(2-branched alkyl)-diorganoaminopyridinium salts particularly described herein are used as phase transfer catalysts. 
     Fluoronitrobenzene compounds such as 2-fluoronitrobenzene, 4-fluoronitrobenzene, and 2,4-difluoronitrobenzene, are useful as intermediates for the synthesis of various herbicidal compounds, dyes, and the like. Such compounds have been prepared from corresponding chloronitrobenzene compounds by so-called halogen exchange reactions, illustrated as follows: ##STR1## wherein MF represents an alkali metal fluoride salt. The reaction is generally conducted in an aprotic, polar, organic solvent, such as dimethylsulfoxide, dimethylformamide, tetramethylenesulfone (sulfolane), and the like. 
     Alkali metal fluoride salts are not soluble in such solvents. Therefore, the reaction mixtures usually contain two phases, i.e., solid and liquid phases or two immiscible liquid phases. Finger, et al., J. Am. Chem Soc., 78, 6034 (1956) and Duesel, et al., U.S. Pat. No. 3,064,058 (1962), describe the reaction of chloronitrobenzene compounds with finely-divided, solid potassium fluoride in aprotic polar solvents to produce corresponding fluoronitrobenzene compounds. Boudakian, et al., U.S. Pat. No. 3,240,824 (1966), describe the reaction of o-chloronitrobenzene with solid potassium fluoride at elevated temperatures, without any solvent or diluents, to produce o-fluoronitrobenzene. Napier and Starks, U.S. Pat. No. 3,992,432 (1976), describe a reaction involving two liquid phases. In the Napier and Starks reaction, the inorganic fluoride salt is dissolved in an aqueous phase, and the chloronitrobenzene compound is dissolved in a water-immiscible, organic phase. The reaction is catalyzed by a quaternary salt, which reportedly transfers ions across the phase interface. 
     Use of quaternary salt phase-transfer catalysts in solid-liquid, two phase reactions also has been known. For instance, Kunz, U.S. Pat. No. 4,069,262 (1978), describes the production of 2-fluoronitrobenzene by reacting 2-chloronitrobenzene with ultrafine particulate potassium fluoride in tetramethylenesulfone solvent using a macrocyclic ether (crown ether) or a quaternary ammonium halide (such as tetrabutylammonium chloride, benzyltrimethylammonium chloride, benzyltrimethylammonium fluoride or benzyltriethylammonium chloride) as the catalyst. 
     Tull, et al., U.S. Pat. No. 4,140,719 (1979), describes the production of 2,4-difluoro-5-chloronitrobenzene by reacting 2,4,5-trichloronitrobenzene with a solid fluorinating agent selected from NaF, KF, CsF, and C 1-4  alkyl quaternary ammonium fluoride, and mixtures thereof under substantially anhydrous conditions in the presence of a quaternary compound solid-liquid phase transfer catalyst. The liquid phase comprises an organic solvent in which the trichloro compound is soluble and the fluorinating agent is essentially insoluble. 
     Starks, &#34;Selecting a Phase Transfer Catalyst,&#34; Chemtech (February 1980), pages 110-117, describes patterns that purportedly enable prediction of catalysts for anion transfer from aqueous or solid inorganic phases to organic phases. 
     North, U.S. Pat. No. 4,287,374 (1981) discloses a process for the production of a monofluoronitrobenzene in which a monochloronitrobenzene is heated with an alkali metal fluoride and a phase transfer catalyst at a temperature of no more than 200° C., preferably 125°-170° C., especially 140°-150° C. North discloses, as examples of such catalysts which may be used, long chain alkylammonium halides. 
     In general, halide-exchange reactions for preparing fluoronitrobenzene compounds by reacting chloronitrobenzene compounds with fluoride salts in aprotic, polar organic solvents in the presence of quaternary ammonium salt phase-transfer catalysts proceed at faster rates when conducted at elevated temperature relative to rates obtainable at lower temperature. However, quaternary ammonium phase-transfer catalysts employed in heretofore known methods are less stable at higher temperature and have been found to decompose or lose their catalytic activity at elevated reaction temperatures. Moreover, U.S. Pat. No. 4,418,229 (to White), incorporated herein by reference, discloses that lower molecular weight catalysts, i.e., those having a total number of carbon atoms less than about 16, are less stable under the conditions (including elevated temperature) of the method of the invention disclosed therein than the therein preferred catalysts of higher molecular weight having about 16 or more carbon atoms. 
     The above cited White patent discloses the finding that in the conversion of chloronitrobenzene (&#34;CNB&#34;) compounds to corresponding fluoronitrobenzene (&#34;FNB&#34;) compounds using a quaternary ammonium salt phase-transfer catalyst at elevated temperatures, a high level of catalystic activity can be maintained by adding the catalyst to the reaction mixture incrementally during the course of the reaction. 
     However, there remains a substantial need in the art for new and improved processes for preparing fluoroaromatic compounds such as fluoronitrobenzenes. The present invention substantially fulfills such need. 
     DESCRIPTION OF THE INVENTION 
     Generally stated, the present invention provides an improved method for producing fluoroaromatic compounds (e.g., fluoronitrobenzene compounds) which includes reacting a chloroaromatic compound (e.g., a chloronitrobenzene compound) with a fluoride salt in a substantially anhydrous reaction mixture under halide-exchange conditions in the presence of a catalyzing amount of a phase-transfer catalyst, wherein the improvement comprises using a branched alkyldiorganoaminopyridinium salt (hereinafter sometimes referred to as PYR salt) as the phase-transfer catalyst. 
     The use of PYR salt as the catalyst allows use of an elevated reaction temperature with substantial freedom from inactivation of the catalyst, resulting in good reaction rates and yielts. The improved method is also effective for the conservation of catalyst at such elevated temperatures. 
     Alkyldiorganoaminopyridinium salts (or simply, PYR salts) which can be employed as the phase-transfer catalyst in the improved method are represented by the formula ##STR2## where: R 1  and R 2  are monovalent or divalent organo radicals independently selected from substituted and unsubstituted hydrocarbon radicals having from 1 to 13 carbon atoms and divalent alkylene radicals which together can be a part of a cyclic structure forming a ring having from 4 to 8 carbon atoms; 
     R 3  is a monovalent organo radical having from 4 to 18 carbon atoms and the formula ##STR3## where R 4  and R 5  are independently selected alkyl radicals each having from 1 to 8 carbon atoms; and X -   is an anion selected from the group consisting of fluoride, bromide and chloride. 
     DETAILED DESCRIPTION OF THE INVENTION AND OF THE MANNER AND PROCESS OF MAKING AND USING IT 
     Included within the N-alkyl-diorganoaminopyridinium salts of formula (1) are for example 
     (i) N-2-ethylhexyl-4-(N&#39;,N&#39;-dimethylamino)pyridinium chloride which has the formula ##STR4## 
     (ii) N-2-ethylhexyl-4-(4&#39;-methylpiperidinylamino)pyridinium chloride which has the formula ##STR5## 
     (iii) N-2-ethylhexyl-4-(N&#39;,N&#39;-di-n-butylamino)pyridinium chloride which has the formula ##STR6## i.e., the 4-(N&#39;,N&#39;-di-n-butylamino) analog of formula (i). 
     The N-alkyldiorganoaminopyridinium salts of formula (1) can be made by alkylating diorganoaminopyridines which can be made by reacting 4-hydroxypyridine with phosphorous pentoxide and a diorganoamine at 200° C. to 300° C., for example, 250° C. as shown by the following equation: ##STR7## where R 1  and R 2  are as defined above. 
     The alkylation of the above diorganoaminopyridines can be achieved in a straight forward manner utillizing such reagents as 2-ethylhexyl bromide, etc. at ambient temperatures in an inert organic solvent, for example, chloroform or toluene. Reaction with such alkyl compounds, as well as the corresponding tosylate or mesylate can be facilitated by using higher temperatures such as refluxing toluene, or temperatures up to about 150° C. 
     A preferred salt for use as a phase transfer catalyst in preparing fluoroaromatic (e.g. FNB) compounds from corresponding chloroaromatic (e.g. CNB) compounds is the salt of formula (i). The best salt contemplated for such use is the salt of formula (iii). 
     The halide-exchange conditions generally include elevated reaction temperatures, which are high enough to provide sufficient energy of activation for the reaction. Although such reaction temperatures might cause some catalyst inactivation, the temperature is preferably not so high as to cause rapid decay of catalytic activity or substantial decomposition of the reactants, the products, or the solvent. Although the reaction temperature may vary, depending upon the particular catalyst, solvent, and reactants used, generally it may be, for example, from about 120° C. to about 220° C., preferably from about 160° C. to about 215° C., and more preferably from about 205° C. to about 215° C. 
     Those skilled in the art will appreciate that a variety of equipment and techniques may be utilized in the method of the present invention, and the invention is not limited to any particular equipment or technique. The method is generally conducted by charging the reactants, solvent and PYR salt catalyst into a reaction vessel which is equipped with agitating and heating means. Advantageously, the entire amount of the reactants, solvent and PYR salt catalyst to be employed can be added initially. The reaction vessel may also advantageously include a reflux condenser or other means of recovering solvent vapors and means for blanketing the reaction mixture with a dry inert gas, e.g., nitrogen. The reaction mixture is heated to the desired reaction temperature and agitated. 
     The halide-exchange reaction conditions employed in the present invention advantageously include substantially anhydrous reaction conditions. The presence of water in the reaction can diminish yields and result in undesirable by-products. Various techniques may be used for dehydrating the reactants and solvent, such as vacuum drying, azeotropic distillation, chemical drying and the like. Azeotropic distillation, for example with benzene, can be used for drying all of the reactants and solvents; however, any convenient and operable technique may be employed. Due to the deleterious effect of water, the reaction mixture is preferably substantially devoid of water. Small amounts of water may be tolerated; however, a corresponding reduction in yield is generally experienced. Advantageously, the concentration of water in the reaction mixture is below about 5 wt. % and is preferably below about 1 wt. %, based on the weight of the reaction mixture. 
     The solvent for the catalyst, chloroaromatic (e.g., CNB) compound, and fluoroaromatic (e.g. FNB) compound is an aprotic, polar, organic solvent, which preferably has a relatively high boiling point, e.g., a boiling point above about 190° C. Lower boiling solvents may be used; however, pressure reactors may be required for their containment. Solvents having boiling points below a desired reaction temperature may be employed by conducting the reaction under superatmospheric pressure in such reactors. Examples of reaction solvents include dimethylsulfoxide, sulfolane, bis(2-methoxyethyl)ether, bis 2-(2-methoxyethoxy) ethyl ether, hexamethylphosphoramide, N-methylpyrolidinone, and dimethylformamide. Dimethylformamide and sulfolane are preferred solvents. Sulfolane is most preferred from the standpoint of commercial attractiveness. 
     The phase-transfer catalyst employed in the present method is soluble in the reaction solvent in an amount sufficient to catalyze the reaction. The PYR salt may be employed in any catalyzing amount, i.e., in any amount effective for catalyzing the conversion of the chloroaromatic (e.g., CNB) compound to the corresponding fluoroaromatic (e.g., FNB) compound. In general, the amount may correspond, for example, to a molar ratio of PYR salt to chloroaromatic (e.g., CNB) compound of from about 0.005:1 to about 0.5:1, preferably from about 0.04:1 to about 0.15:1, most preferably about 0.08:1. In general, amounts of PYR salt corresponding to molar ratios of less than about 0.005:1 may not provide sufficient catalytic activity, while amounts corresponding to molar ratios of more than 0.5:1 may result in insufficient additional benefit to justify the additional cost. As indicated above, the entire amount of PYR salt to be used is preferably added initially. However, if desired, a portion may be added initially with incremental addition of the remainder during the course of the reaction. Incremental addition may be, for example, substantially in accordance with the invention disclosed in the above-cited White patent. 
     The fluoride ion is provided by an alkali metal fluoride salt which is generally present in an amount at least substantially stoichiometric to the chloroaromatic (e.g. CNB) reactant. Preferred fluoride salts are potassium fluoride, rubidium fluoride, and cesium fluoride, and potassium fluoride is particularly preferred. The fluoride salt is advantageously finely-divided, to provide a greater superficial surface area which is accessible to the catalyst and the chloroaromatic (e.g. CNB) compound. Preferred concentrations of the fluoride salt range from about 1 to about 2 times the stoichiometric amount, most preferably from about 1.2 to about 1.6 times such amount. For example, in a method for producing a monofluoronitrobenzene compound, a preferred molar ratio of fluoride salt to chloronitrobenzene compound is about 1.5:1. Lower concentrations of fluoride salts can result in diminished reaction rates, and, although higher concentrations can be used, no appreciable benefit is generally realized therefrom. 
     In the chloronitrobenzene compound used as a starting material in the present invention, the relative positions of the nitro and chloro substituents, and the presence of other substituents on the ring can affect the reactivity of the starting compound. Generally, halogen exchange reactions involve compounds in which the chloride is in the ortho or para position with respect to the nitro group, and reactivity may increase when other electron-withdrawing groups are present on the ring. Compounds having chloro substituents in the meta as well as ortho and/or para positions may be used as starting materials, but usually only the chloro groups in the ortho and para positions will undergo halogen exchange. Accordingly, the method of this invention may be used for example for the synthesis of compounds such as 2-fluoronitrobenzene, 2-fluoro-3-chloronitrobenzene, 4-fluoronitrobenzene, 2,4-difluoronitrobenzene, 5-chloro-2,4-difluoronitrobenzene, and the like, from corresponding chloronitrobenzene compounds. The present method is particularly useful for the preparation of 4-fluoronitrobenzene from 4-chloronitrobenzene and 2-fluoronitrobenzene from 2-chloronitrobenzene. 
     The reaction is generally allowed to proceed until substantially all the chloroaromatic (e.g. CNB) compound has been converted to the corresponding fluoroaromatic (e.g. FNB) compound. A reaction time of from about 10 minutes to about 20 hours may typically be used, and the reaction will often be substantially complete after about 1 to about 6 hours. After the reaction is completed, the product can be recovered by any suitable procedure, such as extraction, distillation, steam distillation and the like. For some purposes, the purity of the crude reaction product, recovered as an organic phase after addition of water to the reaction mixture, will be satisfactory. 
     The method of this invention has been found to produce fluoronitrobenzene compounds in good yields with little formation of by-products. 
     Although the above description of the improved halide-exchange process of this invention has been given principally with reference to effecting conversion of chloroaromatic (e.g. CNB) compounds to corresponding fluoroaromatic (e.g. FNB) compounds in the presence of a solvent, the halide-exchange reaction can be carried out as a neat (i.e., solventless) reaction with the chloroaromatic (e.g. CNB) compound in &#34;the melt&#34; (i.e. in a molten state). 
     Chloroaromatic compounds suitable for use in the process of this invention include an aromatic ring, at least one chlorine atom as a substituent on the aromatic ring and at least one activating substituent located in an activating position on the ring for facilitating the desired nucleophilic substitution, i.e., exchange of fluorine for the chlorine. The activating substituent is an electron-withdrawing agent, including for example: nitro; cyano; trifuloromethyl; chlorocarbonyl; fluorocarbonyl; phenylcarbonyl wherein the phenyl moiety is unsubstituted or substituted with a substituent other than agents which are at least moderately strong electron-releasing agents (i.e., substituted with, for example, chloro, nitro, cyano, trifjloromethyl, and linear or branched alkyl groups, preferably having from 1 to about 12 carbon atoms); phenylsulfonyl wherein the phenyl moiety is unsubstitued or substituted with a substituent other than agents which are at least moderately strong electron-releasing agents (i.e., substituted with, for example, chloro, nitro, cyano, trifluoromethyl and linear or branched alkyl groups, preferably having from 1 to about 12 carbon atoms); and a combination of three, four or five additional chlorine atoms. In general, the chlorine atoms to be exchanged are in the ortho and/or para positions relative to the activating agent. 
     The aromatic ring may be, for example, a benzene ring or a pyridine ring. 
     Suitable chloroaromatic compounds for use in this invention include, for example: 
     (a) 2-chlorobenzonitrile 
     (b) 4-chlorobenzonitrile 
     (c) 2-chlorobenzoyl chloride 
     (d) 4-chlorobenzoyl chloride 
     (e) 2-chlorobenzoyl fluoride 
     (f) 4-chlorobenzoyl fluoride 
     (g) 2-chloro-benzotrifluoride 
     (h) 4-chloro-benzotrifluoride 
     (i) tetrachlorobenzene 
     (j) pentachlorobenzene 
     (k) hexachlorobenzene 
     (l) (4,4&#39;-dichloro-)diphenylsulfone 
     (m) 4,4&#39;-dichlorobenzophenone 
     (n) 3-chloro-phthalic anhydride 
     (o) 4-chloro-phthalic anhydride 
     (p) 3-chloro-phthaloyl dichloride 
     (q) 4-chloro-phthaloyl dichloride 
     (r) 1,4-dichloroanthracene-9,10-dione 
     (s) 3-chloro-4-(trifluoromethyl)pyridine 
     (t) 3-chloro-4-cyanopyridine 
     (u) 3-chloro-4-nitropyridine 
     (v) 2-chloronitrobenzene 
     (w) 2,3-dichloronitrobenzene 
     (x) 4-chloronitrobenzene 
     (y) 2,4-dichloronitrobenzene and 
     (z) 2,4,5-trichloronitrobenzene. 
     The fluoroaromatic compounds resulting from carrying out the process of this invention using the illustrative chloroaromatic compounds (a) through (z) above are set forth below, followed by utilities for such resulting compounds: 
     (a) 2-fluorobenzonitrile 
     (b) 4-fluorobenzonitrile 
     (c) 2-fluorobenzoyl chloride 
     (d) 4-fluorobenzoyl chloride 
     (e) 2-fluorobenzoyl fluoride 
     (f) 4-fluorobenzoyl fluoride 
     (g) 2-fluorobenzotrifluoride 
     (h) 4-fluorobenzotrifluoride 
     (i) tetrafluorobenzene 
     (j) pentafluorobenzene 
     (k) hexafluorobenzene 
     (l) (4,4&#39;-difluoro-)diphenylsulfone 
     (m) 4,4&#39;-difluorobenzophenone 
     (n) 3-fluoro-phthalic anhydride 
     (o) 4-fluoro-phthalic anhydride 
     (p) 3-fluoro-phthaloyl dichloride 
     (q) 4-fluoro-phthaloyl dichloride 
     (r) 1,4-difluoroanthracene-9,10-dione 
     (s) 3-fluoro-4-(trifluoromethyl)pyridine 
     (t) 3-fluoro-4-cyanopyridine 
     (u) 3-fluoro-4-nitropyridine 
     (v) 2-fluoronitrobenzene 
     (w) 2-fluoro-3-chloronitrobenzene 
     (x) 4-fluoronitrobenzene 
     (y) 2,4-difluoronitrobenzene and 
     (z) 5-chloro-2,4-difluoronitrobenzene. 
     Fluoroaromatic compounds (a) through (f) can be hydrolyzed to the corresponding fluorobenzoic acids, which are useful as intermediates for preparing herbicides and liquid crystals. The trifuoromethyl products (g) and (h) are useful as intermediates for perparing herbicides and medicinal agents such as flumetramide. The polyfluorobenzene compounds (i), (j) and (k) are useful as intermediates for preparing plant growth regulators. The fluorosulfone compound (1) is useful as a crosslinking agent and as a monomer which can be polymerized to provide engineering plastics. The fluoroketone (m) is useful as a monomer which can be polymerized to provide engineering plastics. The fluoroanhydrides (n) and (o) and the corresponding acid chlorides (p) and (q) are useful for esterifying with polyols to prepare fluorinated polyesters which are useful as wear-resistant fibers. The fluorinated anthracene dione (r) can be converted by well known methods to 1,4-bis[(aminoalkyl)amino]-- anthracene-9,10-diones which are useful as antineoplastics. The fluoropyridine compounds (s), (t) and (u) and the fluoronitrobenzene compounds (v), (w), (x), (y) and (z) are useful as intermediates for making herbicides and dyes. 
     Practice of this invention is further illustrated by the following non-limiting examples. All parts, percents and other amounts throughout this disclosure are by weight unless otherwise indicated. 
     The N-(2-ethylhexyl)-4-(N&#39;,N&#39;-dimethylamino)pyridinium chloride salt employed in the following examples can be prepared in accordance with the procedure described by Brunelle in U.S. Pat. No. 4,460,778, incorporated herein by reference. As described therein, a mixture of 12.217 grams of dimethylaminopyridine, 20.833 grams of 2-ethylhexylmethane sulfonate was stirred and heated at 110° C. for 1 hour. There was added to the resulting mixture 25 grams of toluene and the solution was refluxed for an additional hour. Toluene was then removed from the mixture under reduced pressure and the resulting crude mesylate salts were washed with hexane. The mesylate salts were then dissolved in methylene chloride and washed twice with a saturated sodium chloride solution. Methylene chloride was then removed under reduced pressure from the resulting product. There was obtained 31.728 grams of a product having a melting point of 189°-190° C. Based on method of preparation, the product was N-2-ethylhexyldimethylaminopyridine chloride (EHDMAPC). 
    
    
     EXAMPLE I 
     A reaction mixture of 4-chloronitrobenzene (CNB, 100 grams, 0.635 mole), potassium fluoride (52.40 grams, 0.902 mole) and EHDMAPC (1 gram) in 100 grams of sulfolane was heated with stirring for 5 hours at 210° C. At the end of this period, HPLC assay of a sample of the reaction mixture showed 47.544 area percent of 4-fluoronitrobenzene (FNB) and 44.946 area percent of CNB, corresponding to about 45 mole % conversion of CNB to FNB. The HPLC analysis was made using a Waters Bondapak™ C18 reverse phase column, a 2-microliter sample loop, a UV detector at 254 nanometers and a dual pump. The solvent system was a 1:1 mixture of the flows from pump A (1:1 methanol:water) and pump B (methanol). The combined flow rate was 1.5 microliters per minute. The peak areas of the resulting FNB (and residual CNB) were measured using a digital integrator. 
     The resulting fluoronitrobenzene product can be recovered in good yield by discontinuing heating and removing the product as distillate by simple distillation under increasingly lower pressure (to about 100 mm Hg absolute), while heating as necessary to maintain an acceptable distillation rate (e.g. at a temperature of the mixture of approximately 135°-150° C.) 
     EXAMPLE II 
     A neat reaction mixture (i.e. no solvent added) of 100 g of 4-chloronitrobenzene (CNB), 58.9 g of potassium fluoride, and 1 g of N-(2-ethylhexyl)-4-(N&#39;,N&#39;-di-methylamino)pyridinium chloride (EHDMAPC) was heated with stirring at 215° C. for an overnight period (approximately 19 hours). During this period conversion of CNB to 4-fluoronitrobenzene (FNB) proceeded to such an extent that HPLC analysis of a sample of the neat reaction mixture at the end of the period showed 42.5% FNB and 50.9% CNB. 
     BEST MODE CONTEMPLATED 
     The best most contemplated for carrying out this invention has been set forth in the above description, for example, by way of setting forth preferred materials and operating conditions, including but not limited to preferred ranges and values of amounts and other non-obvious variables material to successfully practicing the invention in the best way contemplated at the time of executing this patent application. 
     It is understood that the foregoing detailed description is given merely by way of illustration and that many modifications may be made therein without departing from the spirit or scope of the present invention.