PROCESS FOR THE PREPARATION OF N-SUBSTITUTED AMINOMETHYLPHOSPHONIC ACIDS

Abstract: A process for the preparation of an N-substituted aminomethylphosphonic acid comprising reacting a 2,5-diketopiperazine compound with phosphorous acid and formaldehyde in an acidic medium.

Description: This invention relates to the synthesis of N-phosphonomethylglycine and more particularly to an improved process in which an N-substituted 2,5-diketopiperazine is reacted with phosphorous acid and formaldehyde in an acidic medium to produce an N-substituted aminomethylphosphonic acid intermediate which is subsequently transformed by dealkylation into N-phosphonomethylglycine. 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 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 phosphorous 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. A variety of 1,4-disubstituted 2,5-diketopiperazines are known to the art and are recognized to be useful for various purposes. Thus, for example, Chan et al U.S. patent 4,140,791 discloses the use of 1-4-di(2,6-dimethylphenyl)-2,5-diketopiperazine for control of various fungal diseases. Sut et al "N-Monoalkylation of Some 2-Oxo and 2,5-Dioxopiperazines" Chimie Therapeutique , Vol. 4 (3) pgs. 167-173 (1969), describes the synthesis of a series of 2-oxopiperazines and 2,5-dioxopiperazines which were found to have analgesic and anesthetic activities. Among the specific compounds disclosed by Sut et al are 2,5-diketopiperazines and 3-substituted 2,5-diketopiperazines which are mono- or dialkylated in an N-position, or N,N'-positions, with ethyl, benzyl, hydroxyethyl or acetoxyethyl. Other references contain specific disclosure of 1,4-dimethyl-2, 5-diketopiperazine, 1,4-diethyl-2,5-diketopiperazine, 1,4-diphenyl-2,5-diketopiperazine, and 1,4-dibenzyl-2, 5-diketopiperazine. However, it is believed that none of these references disclose the use of any such compounds in the preparation of any N-substituted aminomethylphosphonic acid. Okawara et al "Convenient Syntheses of Piperazine-2,5-diones and Lactams from Halocarboxamides Using Phase Transfer Catalysts" Chemistry Letters, 1981 , pgs. 185-189, shows the syntheses of various 1,4-disubstituted-2,5-diketopiperazines. Among the compounds whose syntheses are reported in Okawara et al are 1,4-dibenzylpiperazine-2, 5-dione, 1,4-diphenylpiperazine-2,5-dione and 1,4-diphenyl-3,6-dimethylpiperazine-2,5-dione. The reference does not report any use for the products synthesized. Cavicchioni et al "Base Promoted Reactions of α-Halogenoalkylanilides", J. Chem Soc., Perkin Trans. I, pgs. 2969-2972 (1982), reports the preparation of both N-N'-dialkyldiketopiperazines and 2-amino-2-haloalkyloxazolidones by intermolecular condensations of the same reactants used in the synthesis described by Okawara et al. Cavicchioni et al do not give much detail on the reaction system utilized and do not describe any uses for the N-N'-dialkylpiperazines obtained. Wong et al U.S. patent 4,400,330 describes the preparation of bis(phosphonomethyl)-2,5-diketopiperazine by phosphonomethylation of 2,5-diketopiperazine, followed by isolation of the bis(phosphonomethyl)-2,5-diketopiperazine and alkaline hydrolysis thereof to produce the trisodium salt of glyphosate. In the phosphonomethylation reaction, formaldehyde and glacial acetic acid are added to 2,5-diketopiperazine to produce a suspension which is refluxed. Thereafter, phosphorus trichloride is added to the reaction mixture which is then maintained at reflux until all hydrogen chloride byproduct has been driven off. After additional refluxing of the reaction slurry, the product is dried in vacuo, dissolved in water, and treated sequentially with caustic solution and then mineral acid to effect hydrolysis and produce glyphosate. EP-A-0 216 744 describes the novel compound 1,4-diisopropyl-2,5-diketopiperazine and its use in the preparation of N-isopropylglycine. EP-A-0 216 744 further describes the phosphonomethylation of N-isopropylglycine to N-isopropylglyphosate and conversion of the latter intermediate to glyphosate by dealkylation in an alkaline medium in accordance with EP-A-0 187 633. EP-A-0 216 746 describes novel processes for the preparation of various 1,4-disubstituted-2,5-diketopiperazines and the conversion of the latter to glyphosate via the N-substituted glycine derivatives. Certain of the subject matter disclosed therein relates to the present invention. Summary of the Invention Among the several objects of the present invention, therefore, may be noted the provision of an improved process for the preparation of N-phosphonomethylglycine; the provision of such a process which can be carried out without isolation of intermediates; the provision of such a process which produces intermediates suitable for conversion, without isolation, to glyphosate or a glyphosate salt, for example, by the process of EP-A-0 187 633, and the provision of a process by which N-phosphonomethylglycine may be prepared from 2,5-diketopiperazines of the type whose preparation is described in EP-A-0 216 744 and EP-A-0 216 746. Briefly, therefore, the present invention is directed to a novel process for the preparation of N-phosphonomethylglycine comprising reacting a 2,5-diketopiperazine compound with phosphorous acid and formaldehyde in an acidic medium, under the conditions specified in claim 1, said diketopiperazine compound represented by the formula wherein R¹ and R² are independently selected from benzyl and an alkyl group selected from methyl, ethyl, n-propyl iso-propyl, n-butyl, secundary butyl and tertiary butyl. The intermediate product can be represented by the formula provided that R¹ and R² are identical, otherwise the intermediate product comprises a mixture comprising and The intermediate is then, without isolation, dealkylated in the presence of a base to produce N-phosphonomethylglycine in the form of a salt. In carrying out the process of the invention, it is preferred that the substrate be initially dissolved or dispersed in an aqueous acidic medium selected from, hydrochloric acid, hydrobromic acid and sulfuric acid, the acid typically having a strength of 10% to 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 70-120°C to effect hydrolytic cleavage of the ring at the two amide bonds. Advantageously, the system may be heated to reflux temperature, which in the case of 20% hydrochloric acid is typically in the range of 105-110°C. In the acidic medium the reaction with phosphorous acid and formaldehyde may be carried out to produce the above described intermediate products 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 stoichiometric proportion of at least two moles per mole of substrate. Preferably, about a 10% excess of phosphorous acid is charged. With the phosphorous acid incorporated in the mixture, the temperature of the system is maintained at at least about 90°C, preferably at just below reflux, while formaldehyde is added slowly thereto. The formaldehyde may be added in any of its various monomeric or oligomeric forms, but is most conveniently added as formalin. Typically, the 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 a stoichiometric proportion of at least two moles per mole of substrate. Complete conversion of the substrate is promoted by introducing an approximately 20% excess of formaldehyde into the reaction zone. After the reaction is completed, the N-substituted-N-phosphonomethylglycine intermediate product may be recovered if desired by conventional techniques such as, for example, crystallization. However, where an N-substituted aminomethylphosphonic acid product is prepared as an intermediate for glyphosate, conversion to glyphosate is carried out directly without recovery of the intermediate from the phosphonomethylation reaction medium. In the process of the invention R¹ and R² are preferably alkyl substituents. Most preferably, both R¹ and R² are isopropyl. In such instance the intermediate 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 EP-A-0 187 633. The following examples of preparation of N-substituted-N-phosphonomethylglycine intermediates illustrate the invention. Example 1 A 100 ml 3-necked flask was equipped with a magnetic stir bar, thermometer, condenser and an addition funnel. To this flask were added 1,4-diisopropyl-2,5-diketopiperazine (1.98 g; 10.0 mmol), water (25 ml), concentrated hydrochloric acid (10 ml), and phosphorous acid (1.72 g; 21 mmol). The resulting mixture was heated rapidly to 105°C. At this point, a solution of 37% formaldehyde in water (1.95 g; 24 mmol) was added slowly drop-wise to the mixture. Upon completion of the addition of formaldehyde, the reaction mixture was heated at 104-105°C for 17 hours. Analysis of the reaction mixture by HPLC indicated the presence of 1.49 g (70.7% yield) of N-isopropyl-N-phosphonomethylglycine Example 2 Utilizing the procedure described in Example 1, 1,4-dimethyl-2,5-diketopiperazine (1.42 g; 10.0 mmol) was converted to N-methyl-N-phosphonomethylglycine. HPLC analysis indicated a yield of 97%. Upon isolation of the product by ion exchange chromatography, a yield of 2.93 g (80%) of the product was obtained. Example 3 Using the procedure described in Example 1, 1,4-dibenzyl-2,5-diketopiperazine (2.94 g; 10.0 mmol) was converted to N-benzyl-N-phosphonomethylglycine. After isolation by ion exchange chromatography, a yield of 1.83 g (35.3%) of product was obtained. In this reaction 50% of the starting 1,4-dibenzyl-2, 5-diketopiperazine was recovered unchanged.

Claims: A process for the preparation of N-phosphonomethylglycine comprising introducing a 2,5-diketopiperazine compound represented by the formula : wherein R¹ and R² are independently selected from benzyl and an alkyl group selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, secondary butyl and tertiary butyl, in an aqueous medium containing an acid selected from sulfuric acid, hydrochloric acid and hydrobromic acid ; heating the mixture to a temperature between about 70°C and about 120°C ; adding slowly formaldehyde to the mixture in the presence of phosphorous acid while the temperature of the mixture is maintained at least at about 90°C to provide an N-substituted-N-phosphonomethylglycine intermediate represented by the formula : provided that R¹ and R² are identical, the intermediate otherwise comprising a mixture comprising and dealkylating the N-substituted-N-phosphonomethylglycine intermediate without isolation in the presence of a base to produce N-phosphonomethylglycine in the form of a salt. A process as set forth in Claim 1 wherein R¹ and R² are the same. A process as set forth in Claim 2 wherein R¹ and R² are benzyl. A process as set forth in Claim 2 wherein R¹ and R² are isopropyl. A process as set forth in any of Claims 1 to 4 wherein the dealkylation is carried out in alkaline medium. A process as set forth in any of Claims 1 to 5 wherein said mixture is maintained at a temperature of approximately atmospheric reflux temperature during the addition of formaldehyde.

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