PROCESS FOR THE PREPARATION OF N-SUBSTITUTED AMINOMETHYLPHOSPHONIC ACIDS

Abstract: A process for the preparation of an N-substituted aminomethylphosphonic acid comprising reacting a substituted amine, urea or carbamate substrate compound with phosphorous acid and formaldehyde in an acidic medium.

Description: This invention relates to the synthesis of N-substituted aminomethylphosphonic acids, and more particularly to an improved process in which an amide compound is reacted with phosphorous acid and formaldehyde in an acidic medium to produce an N-substituted aminomethylphosphonic acid. N-substituted aminomethylphosphonic acids are useful intermediates in the preparation of various products, including sequestering agents and herbicides. Thus, for example, an N-alkyl-N-phosphonomethylamino acid, such as N-isopropyl-N-phosphonomethylglycine, can be dealkylated under alkaline conditions to the corresponding N-phosphonomethylamino acid using the method of the application of Miller and Balthazor, EP-A-0 187 633. N-phosphonomethylglycine, known also by its common name glyphosate, is a highly effective and commercially important phytotoxicant useful in controlling a large variety of weeds. It is applied to the foliage of a very broad spectrum of annual and perennial grasses and broadleaf plants. Industrial uses include control of weeds along roadsides, waterways, transmission lines and in storage areas and other non-agricultural areas. Usually glyphosate is formulated into herbicidal compositions in the form of its various salts in solution, preferably water. Because of its commercial importance, many processes for making glyphosate have been published. Processes are also known for the preparation of other phosphonomethylated amine compounds. In the former category, for example, is Gaertner U.S. Patent 3,927,080 which describes the preparation of N-t-butyl-N-phosphonomethylglycine by reacting t-butylamine with a bromoacetate ester to produce an ester of N-t-butylglycine, and thereafter reacting the N-t-butylglycine ester with formaldehyde and a dialkyl phosphite to produce esters of N-t-butyl-N-phosphonomethylglycine. The latter product is hydrolyzed under acidic conditions to produce glyphosate. European patent No. 00 55 695 discloses a process for splitting a l-arylmethyl group from an N-l-arylalkyl-N-phosphonomethylglycine by hydrogenolytic cleavage. The glyphosate precursor is prepared by reaction of an N-l-arylalkylglycine with phosphorous acid and formaldehyde in an aqueous hydrochloric acid medium. Pfliegel et al U.S. Patent 4,065,491 describes the preparation of glyphosate directly by condensation of glycine, formaldehyde, and a dialkyl phosphite in an aqueous alkaline medium comprising sodium hydroxide. Ehrat U.S. Patent 4,237,065 describes a synthesis substantially similar to that disclosed in Pfliegel et al. However, Ehrat carries out the reaction using a tertiary amine base in an alcohol medium rather than the sodium hydroxide solution utilized by Pfliegel et al. Irani and Moedritzer U.S. Patent 3,288,846 also describes the reaction of other nitrogen compounds such as ammonia, or a primary or secondary amine, with an aldehyde or ketone and phosphorous acid to form an aminoalkylenephosphonic acid. However, unlike the processes disclosed by Pfliegel et al and Ehrat, the Irani process is carried out in an aqueous medium having a pH below about 4. Shin et al U.S. Patent 3,567,768 describes the preparation of an aminoalkylenephosphonic acid compound by reaction of a reactive nitrogenous material (i.e., a nitrogen containing or nitrogenous compound such as ammonia, a primary amine, or secondary amine), an aldehyde or ketone, and an excess of phosphorous acid. Where the nitrogenous reactant is ammonia or an ammonium salt, the product is the same as that prepared in accordance with the Krueger patent, discussed below. The exemplary disclosure of Shin describes a preparation in which phosphorous acid is premixed with ammonium chloride and water, and the resultant mixture is heated to reflux while formaldehyde is added thereto. Japanese patent Sho 47[1972]-112 describes a method for the treatment of cellulose fibers with a solution which is prepared by the reaction of a nitrogen compound, phosphorous acid, and formalin. The nitrogen compound is one which contains two or more amino groups, such as for example, urea, thiourea, guanidine, or an alkyldiamide. However, the reference is concerned with enhancing the characteristics of the treated fiber and contains no disclosure of the structure of any product that may be formed by reaction of the aforesaid materials. Nor does the reference report any analytical work which might provide an indication of the structure of such product. Krueger et al. U.S. Patent 4,009,204 describes the preparation of nitrilo tris(methylenephosphonic acid) by reaction of an aliphatic amide with formaldehyde and a phosphorus trihalide. In the Krueger process, the amide substrate is preferably premixed with the aldehyde and the phosphorus trihalide added dropwise thereto. Alternatively, the aldehyde and phosphorus trihalide are premixed, and the acid amide slowly added to the latter premixture. Various processes are known in the art for the preparation of amides, carbamates, and ureas. French patent 2,523,576 describes a method for preparing N-acyliminodiacetic acid by reaction of an unsubstituted amide with formaldehyde and carbon monoxide in the presence of a carbonylation catalyst. Provision of a method for conversion of the N-acyliminodiacetic acid to N-phosphonomethyliminodiacetic acid would establish an advantageous route to glyphosate, since Hershman U.S. Patent 3,969,398 describes a process for preparation of glyphosate by oxidation of N-phosphonomethyliminodiacetic acid in the presence of activated carbon, and Franz U.S. Patent 3,950,402 describes a similar process using a noble metal on carbon as a catalyst. The disclosure of Miller and Balthazor, EP-A-0 187 633 describes the phosphonomethylation of N-isopropylglycine to N-isopropylglyphosate and conversion of the latter intermediate to glyphosate by dealkylation in an alkaline medium. Other processes are available for the preparation of a wide variety of substituted and unsubstituted amides, carbamates, and ureas corresponding to the formula: where typically R is hydrogen, alkyl, cycloalkyl, aryl, carboxyalkyl, alkoxy, aryloxy, or amino, R¹ and R² are independently selected from among hydrogen, alkyl, cycloalkyl, aryl, arylalkyl, and carboxyalkyl, and X is oxygen or sulfur. The application of Reitz, EP-A-0 188 188, describes a novel and advantageous method for the preparation of various amides. US-A-4,491,548 discloses a process for preparing phosphonomethylated amino acids by phosphonomethylating an N-carbamylated amino acid derivative with formaldehyde and a phosphorous source to obtain a phosphonomethylated N-carbamylated amino acid derivative which is then in a subsequent step hydrolyzed to the corresponding phosphonomethylated amino acid, i.e., N-phosphonomethylglycine, or a derivative thereof. In the method of preparing an N-carbamylated amino acid derivative, an appropriate primary amino acid derivative is suspended in a solvent, and thereafter phosgene gas is then bubbled through the primary amino acid derivatives suspended in the solvent. DE 2,423,881, which corresponds to US-A-4,079,006, discloses that the precipitation of scale forming salts in an aqueous system is inhibited by adding either stoichiometric or sub-stoichiometric, including threshold, amounts to the aqueous system of a phosphonomethylaminocarboxylate. These same compounds function as sequestering or chelating agents. The compounds of interest may be prepared by reacting an aqueous mixture of a cyclic amide, formaldehyde and orthophosphorous acid or phosphorous trichloride and water. The carboxylate group on the molecule may contain from 3 to 15 carbon atoms between the nitrogen and the carboxylate group. Among the several objects of the present invention, therefore, may be noted the provision of an improved process for the preparation of N-substituted aminomethylphosphonic acid compounds; the provision of such a process which can be carried out without isolation of intermediates; the provision of such a process which produces products suitable for conversion to glyphosate or glyphosate derivatives, for example, by the process of EP-A-0 187 633 or U.S. 3,950,402 or U.S. 3,969,398; and the provision of a process by which N-substituted aminomethylphosphonic acid compounds may be prepared from a variety of substituted amide compounds; and, in particular, the provision of a process for producing N-substituted aminomethylphosphonic acids from substituted amides of the type produced in accordance with Reitz, EP-A-0 188 188, and for the preparation of N-phosphonomethyliminodiacetic acid from N-acyliminodiacetic acids. Briefly, therefore, the present invention is directed to a novel process for the preparation of an N-substituted aminomethylphosphonic acid comprising preparing a mixture of an aqueous acidic medium comprising phosphorous acid; an acid selected from the group consisting of sulfuric acid, hydrochloric acid and hydrobromic acid; and a substrate compound represented by the formula: wherein R¹ is alkyl or carboxyalkyl and R² is carboxyalkyl, Y is oxygen or sulfur, and R³ is selected from the group consisting of alkyl and carboxyalkyl; heating the mixture to a temperature between 70°C and 120°C; and thereafter slowly adding formaldehyde to the mixture. Other objects and features will be in part apparent and in part pointed out hereinafter. In accordance with the invention, a novel process has been discovered by which substituted amides for example, amides of the type prepared in accordance with the aforesaid Reitz disclosure, can be phosphonomethylated to produce an N-substituted aminomethylphosphonic acid which can be converted to glyphosate by the process of Miller et al, EP-A-0 187 633 By reaction of the amide with formaldehyde and phosphorous acid in an acidic medium, a phosphonomethyl group is substituted on the amine nitrogen in place of the carbonyl substituent. In the process of the invention, at least one of the substituents on the amine nitrogen is not subject to hydrolysis under the reaction conditions. In the context of this disclosure "not subject to hydrolysis" or "not hydrolyzable" more precisely means that the group in question is not subject to hydrolytic cleavage from the amine nitrogen under the reaction conditions but remains combined with the nitrogen in the reaction product. If the product of the reaction is a glyphosate precursor, that non-hydrolyzable substituent is preferably an alkyl group, such as, for example, isopropyl, which is subject to subsequent removal under alkaline conditions in accordance with the process of EP-A-0 187 633. In another preferred application of the process of the invention, the substrate is an N-acyliminodiacetic acid, and the product is N-phosphonomethyliminodiacetic acid. As indicated above, the later compound is converted to glyphosate by contacting it with a molecular oxygen-containing gas in the presence of an activated carbon, as described in Hershman U.S. Patent 3,969,398. Generally, the substrate utilized in the process of the invention can be represented by the formula: In this formula, R¹ is an alkyl or carboxyalkyl group and R² is carboxyalkyl. Preferably, the substituent comprising R¹ may contain up to about 8 carbon atoms. Among the alkyl groups which may comprise R¹ are methyl, ethyl, n-propyl, isopropyl, n-butyl, secondary butyl, tertiary butyl, octyl, cyclohexyl, ethylcyclopentyl, and the like. The carboxyalkyl groups which may comprise R¹ and/or R² include, for example, carboxymethyl, carboxyethyl, carboxypropyl, carboxylbutyl, carboxypentyl, and carboxyheptyl. Substituent R³ in the substrate compound is selected from alkyl, and carboxyalkyl. Generally, R³ may contain up to about 8 carbon atoms. Because the substituent of which it is a part is removed hydrolytically, it is feasible for R³ to contain significantly more than 8 carbon atoms, but normally this would serve no useful purpose. Where R³ is an alkyl or carboxyalkyl group, it may typically comprise any of the substituents which may comprise R¹ and/or R² as listed above. In the structural formula for the substrate compound, Y comprises either oxygen or sulfur, but is preferably oxygen. In carrying out the process of the invention, it is preferred that the substrate be initially dissolved or dispersed in an aqueous acidic medium, comprising phosphorous acid and an acid selected from hydrochloric acid, hydrobromic acid or sulfuric acid, the acid typically having a strength of 10-25% by weight. Hydrochloric acid at a strength of between about 15% and about 25% by weight is a particularly effective medium for carrying out the reaction. After the substrate has been dissolved or dispersed in the aqueous acidic medium, the resultant mixture is heated to a temperature of 70°C to 120°C, and thereafter formaldehyde is slowly added to the mixture. Advantageously, the system may be heated to reflux temperature, which in the case of 20% hydrochloric acid is typically in the range of 105°C-110°C. As noted, reaction with phosphorous acid and formaldehyde may then proceed without isolation of any intermediate. The process may be initiated by heating a mixture of substrate and phosphorous acid in a hydrochloric, hydrobromic or sulfuric acid medium. In an alternative procedure, the substrate may be mixed with a phosphorus trihalide, such as phosphorus trichloride or phosphorus tribromide, and water, the phosphorus trihalide reacting with the water to produce a mixture of phosphorous acid and either hydrochloric or hydrobromic acid. Whenever and in whatever form it is introduced into the reaction system, phosphorous acid should be charged in a proportion of at least about one equivalent per equivalent of substrate. About 10% excess of phosphorous acid is preferably charged. The temperature of the system is preferably maintained at at least about 90°C, more preferably at just below atmospheric reflux while formaldehyde is added slowly thereto. Formaldehyde may be added in any of its various monomeric or oligomeric forms, but is most conveniently added as formalin. Typically, addition of formaldehyde may be carried out over a period of at least 0.5 hour, after which the temperature is preferably maintained at reflux for at least 2 hours. Formaldehyde should be added in an amount of at least one mole per mole of substrate. Formaldehyde stoichiometry is governed by the same considerations discussed above with respect to phosphorous acid. Complete conversion of the substrate is promoted by introducing formaldehyde into the reaction zone in an approximate 20% excess based on the stoichiometric requirement. After the reaction is completed, the product may be recovered if desired by conventional techniques such as, for example, crystallization. However, where the N-substituted aminomethylphosphonic acid is prepared as an intermediate for glyphosate, such conversion to glyphosate can be carried out directly, without recovery of the N-substituted N-phosphonomethylglycine from the reaction medium. Where the process of the invention is used in the synthesis of glyphosate, the initial substrate is preferably an amide in which R¹ is an alkyl substituent and R² is carboxymethyl. Most preferably, the alkyl substituent is isopropyl and the R³ substituent is methyl. In this instance, the product of the reaction is N-isopropyl-N-phosphonomethylglycine, which is converted in very high yield to glyphosate in accordance with the alkaline medium dealkylation process described in the aforesaid application of Miller et al, EP-A-0 187 633. The following examples illustrate the invention: Example 1 A 3-necked 50 ml flask was fitted with a condenser, addition funnel, thermometer, and magnetic stir bar. N-Acetyl-N-isopropylglycine (2.55 g; 16.0 mmol), phosphorous acid (1.38 g; 16.8 mmol) and aqueous 20% by weight hydrochloric acid solution (10 ml) were introduced into the flask. The resulting mixture dissolved quickly, and was heated to a temperature of 106°C over a period of 5 minutes. When this temperature range was reached, addition of a 37% by weight formaldehyde solution in water was begun. Over a period of about 0.5 hour, 1.56 g (19.2 mmol) of formaldehyde was added to the reacting mixture. After addition of the formaldehyde was complete, the reaction mixture was maintained at 106°C-108°C for an additional 3.5 hour. All volatiles were removed at reduced pressure and the residue was purified by ion-exchange chromatography (Dowex 50X8-400). The isolated product was a glassy solid (2.85 g; 84.4%) which was determined by NMR analysis to be substantially pure N-isopropyl-N-phosphonomethylglycine. Example 2 A crude reaction mixture of N-acetyliminodiacetic acid was prepared from acetamide (5.90 g; 0.10 mol)) by the process described in French patent 2,523,576. This crude product was dissolved in a 20% aqueous hydrochloric acid solution and the resulting mixture was added to a 250 ml 3-necked flask equipped with an addition funnel and thermometer. Phosphorous acid (8.61 g; 0.105 mol) was added and the solution was heated to a temperature of 106°C-107°C. When this temperature had been reached, formaldehyde (8.23 g of a 44% aqueous solution; 0.212 mole) was added to the mixture in the flask over a period of 30 minutes. After addition of formaldehyde was complete, the temperature was maintained at 106°-107°C for an additional period of 3.5 hour. The reaction mixture was concentrated under reduced pressure and a precipitate formed. The product was collected by filtration to yield 14.4 g of N-phosphonomethyliminodiacetic acid (63% yield based on the starting amount of acetamide).

Claims: A process for the preparation of an N-substituted aminomethylphosphonic acid represented by the formula: wherein R¹ is alkyl or carboxyalkyl, and R² is carboxyalkyl, characterized by: preparing a mixture of an aqueous acidic medium comprising phosphorous acid; an acid selected from sulfuric acid, hydrochloric acid and hydrobromic acid; and a substrate compound represented by the formula: wherein R¹ and R² are as defined above, Y is oxygen or sulfur, and R³ is selected from the group consisting of alkyl and carboxyalkyl; heating the mixture to a temperature between 70°C and 120°C; and thereafter slowly adding formaldehyde to the mixture. A process as set forth in Claim 1 wherein said mixture is maintained at a temperature of approximately atmospheric reflux temperature during the addition of formaldehyde. A process as set forth in Claim 1 or 2 where R¹ is isopropyl or carboxymethyl. A process as set forth in any of Claims 1 to 3 where R¹ and R² are carboxymethyl. A process as set forth in any of Claims 1 to 4 where R³ is methyl. A process as set forth in any of Claims 1, 2, 3 and 5 where the reaction product is N-(isopropyl)-N-phosphonomethylglycine. A process as set forth in any of Claims 1 to 5 where the reaction product is (N-phosphonomethyl)-iminodiacetic acid. A process for the preparation of glyphosate or a salt thereof which comprises the conversion according to known methods of the reaction product obtained by the process of either of Claims 6 and 7. A process as set forth in Claim 8 where the conversion is carried out without recovery from the reaction medium of the N-substituted-N-phosphonomethylglycine reaction product obtained by the process of either Claim 6 or 7. A process as set forth in any of Claims 8 or 9 comprising conversion of N-isopropyl-N-phosphonomethylglycine into a salt of glyphosate by dealkylation in alkaline medium. A process as set forth in any of Claims 8 or 9 comprising contacting (N-phosphonomethyl)-iminodiacetic acid with an oxygen-bearing gas in the presence of a catalyst.

Inventors: MILLER, WILLIAM HAROLD, PULWER, MITCHELL JOEL, REITZ, DAVID BRUCE
Applicants: MONSANTO COMPANY
Application year: 1986
Main classification: C07F9
IPC codes: C07F9