METHOD FOR PRODUCTION OF MALEIMIDES

Abstract: A method for the production of a maleimide by dehydration and ring-closure of maleinamic acid obtained by the reaction of maleic anhydride with an amine. The method is characterised by effecting ring-­ closure imidation of the maleinamic acid by heating the maleinamic acid in a water-insoluble or water-immscible inert organic solvent in the presence of a catalyst. The catalyst is an amine salt produced from the amine and an acid, a mixture of the amine salt and the acid, or a supported catalyst on a solid carrier the catalyst or the acid itself.

Description: This invention relates to a method for the production of maleimides. More particularly, this invention relates to a method for the production of maleimides by ring-closure imidation of maleinamic acids. Description of the Prior Art: Maleimides are compounds useful as raw materials for synthetic resins, medicines, and agricultural chemicals. Researches after methods for their production have long been under way. The most popular method to date effects the production of maleimides by the dehydration cyclization of maleinamic acids with a dehydrating agent such as acetic anhydride. One version of this method is disclosed in U.S. Patent No. 2,444,536. This method effects the production of maleimides by causing maleic anhydride to react with amines thereby forming maleinamic acids and dehydration cyclizing and, at the same time, imidating the maleinamic acids in the presence of acetic anhydride and sodium acetate. This method, however, has the disadvantage that the imidation requires expensive acetic anhydride to be used in at least an equivalent relative to the maleinamic acid and the separation and recovery of the formed maleimide from the imidation reaction solution necessitates use of a large volume of water and, as the result, entails disposal of a large amount of an acetic acid-containing effluent at great expense. Thus, this method may well be considered too expensive for commercial production of maleimides. A method which does not need such a chemical dehydration agent as acetic anhydride is disclosed in British Patent No. l,04l,027 and U.S. Patent No. 3,43l,276. This method effects the production of maleimides by thermally dehydrating and cyclizing maleinamic acids in conjunction with a solvent such as, for example, toluene, xylene, or chlorobenzene having a boiling point exceeding 80°C and serving as a diluent and an acid catalyst such as sulphur trioxide, sulphuric acid, or ortho-phosphoric acid, and distilling the system thereby causing azeotropic expulsion of the consequently formed water in conjunction with the solvent. As compared with the method which uses acetic anhdyride, this method proves advantageous in that it does not require the use of a large amount of such an expensive dehydrating agent as acetic anhydride and further that the formed maleimides are separated and recovered with ease. This method nevertheless has the disadvantage that the yield of the imidation is low as compared with that obtainable by the method using acetic anhydride. This disadvantage is logically explained by a theory that compared with the method which effects the imidation by the use of acetic anhydride, the method which effects the imidation by performing thermal dehydration in the specific solvent as described above involves a high reaction temperature and, therefore, tends to induce side reactions and inevitably manages to produce maleimides abounding with impurities and further that since maleimides are thermally unstable, the maleimides produced can be degenerated during the course of the reaction. Further, as a commercial process, this method is not economically satisfactory, because it requires the use of an expensive acid catalyst in a relatively large amount and, moreover, produces the maleimides in a low yield. There is another method which, as disclosed in Japanese Patent Laid-Open SHO 53(l978)-68,700 and Japanese Patent Publication SHO 57(l982)-42,043, comprises causing maleic anhydride to react on amines in the presence of an organic solvent thereby forming maleinamic acids and subjecting the maleinamic acids as held in a state not isolated from the reaction system to dehydration and cyclization in the presence of such an aprotic polar solvent as dimethyl formamide or dimethyl sulphoxide and an acid catalyst. By this method, there is a recognisable improvement in yield as compared with the second method described above. This method, however, has three disadvantages, that the cost of production of maleimides is high because expensive and highly toxic aprotic polar solvent such as dimethyl formamide is used in a large amount, that the solvent such as dimethyl formamide is degenerated by the action of an acid catalyst used in the reaction and, therefore, much of the solvent is lost and that since the aprotic polar solvent used in the reaction has a high boiling point, the solvent is removed from the produced malimides only with great difficulty. Japanese Patent Laid-Open SHO 54(l979)-30,l55 discloses a method for producing an oligoimide by using, as a catalyst, a mixture of an inorganic or organic oxygen-containing acid with a quaternary ammonium salt of the acid. The quaternary ammonium salt which is used, mixed with an acid catalyst in this method, however, is an ammonium salt whose nitrogen atom has been at least disubstituted. Specifically, this is an expensive interphase catalyst such as dimethyldialkyl ammonium methane sulphonate or tetraoctyl ammonium methane sulphonate The method, thus necessitating use of such a compound as indicated above, is inevitably judged to be an expensive approach. For this method to maintain a highly satisfactory yield of imidation, however, it is essential that the reaction should be continued with the ratio of the acid catalyst to the quaternary ammonium salt rigidly controlled within a certain range. When the catalyst which has been used once in the reaction is used again, the imidation cannot be obtained in a highly satisfactory yield because the ratio varies in the presence of the used catalyst. An effort to attain efficient reuse of the used catalyst, therefore, entails as a problem the fact that the management for maintenance of catalytic activity by subjecting the used catalyst to purifying and readjusting treatments calls for considerable labour. Japanese Patent Laid-Open SHO 60(l985)-l09,562 discloses a method for the production of monomaleimide by the cyclizing imidation of maleinamic acid in a mixed solvent containing a nonpolar solvent such as toluene or xylene and a polar solvent such as dimethyl sulphoxide or N-methyl pyrrolidone in a specific ratio in the presence of an acid catalyst such as p-toluenesulphonic acid or m-toluenesulphonic acid and a mixed catalyst containing the acid catalyst and an ammonium salt such as, for example, the salt thereof with maleinamic acid. In this method, however, since the acid catalyst and the polar solvent in the mixed solvent react with each other to form a complicated complex (which is widely variable with the ratio of the amounts of the two compounds and with temperature, for example), it is the complicated catalyst system composed of this complex, the acid, and the salt,that substantially produces the catalytic activity. Thus, the yield of the imidation is affected to a great extent by the composition of the three components mentioned above. When the reaction is carried out batchwise, it does not entail any appreciable disadvantage. When the reaction is carried out in a system which requires the catalyst and the solvent to be recycled, however, it entails various drawbacks. To be specific, this method renders the selection of reaction conditions complicate because the amount of the complicated complex produced owing to the use of the polar solvent varies and the catalyst varies in quality from one batch to another. Effectively therefore, this method is unsuitable for a continuous reaction. An object of this invention is to provide an improved method for the production of maleimides. Another object of this invention is to provide a method for producing a maleimide of high purity in a high yield by a safe and simple procedure. Yet another object of this invention is to provide a method for enabling the production of a maleimide to be effected easily by a continuous reaction. It is known from DE-C-1269126, US-3431276 and EP-A 129125 to provide a method for the production of a maleimide which comprises reacting maleic anhydride with an amine to produce maleinamic acid and effecting a dehydration and ring-closure reaction of the maleinamic acid characterised in that the ring-closure imidation of the maleinamic acid is effected by heating the maleinamic acid in a water-insoluble or water-immiscible inert organic solvent in the presence of an acid catalyst. According to the present invention, there is provided a method for the production of a maleimide of formula II: wherein R is one of alkyl of 1-20 carbon atoms, phenyl, benzyl, cyclohexyl, pyridyl, quinolyl and these moieties when substituted by halogen, carboxyl or nitro, which comprises the sequential steps of: reacting maleic anhydride with a primary amine of the formula NH₂-R, wherein R is as defined above, to form the corresponding maleinamic acid; and effecting a ring closure reaction of the maleinamic acid by heating the maleinamic acid in a water insoluble or water immiscible inert non-polar organic solvent in the presence of a catalyst, characterised in that: the solvent is used in a ration by volume of from 1 to 20:1 with respect to the maleinamic acid; the catalyst is a primary amine-acid addition-salt catalyst formed from the primary amine and a strong inorganic or organic acid, the catalyst being present in an amount of 2 to 400 mol% of acid addition component based on the maleinamic acid, or the catalyst is a mixture of the salt with the free acid, the content of the free acid being less than 60 mol%; the heating step is carried out at a temperature of from 120° to 250°C; and the water thus formed is continuously distilled off as a mixture with the solvent. Preferably, the imidation of the maleinamic acid by dehydration and ring-closure is carried out in the presence of a metal-containing compound. The metal-containing compound is preferably a zinc-containing compound. Preferably the metal-containing compound is an acetate. Attention is drawn to the present 'Applicants' co-pending European Application No. divided from the present application which is concerned generally with supporting the catalyst on a carrier when the catalyst comprises the amine salt or a mixture of the amine salt and the acid. A lengthy study carried out by the present inventors on reactions for the synthesis of maleimides and particularly the development of a catalyst for use in a ring-closure imidation reaction, has resulted in a finding that an amine salt produced by the neutralisation of the amine used as the raw material for the maleimide with an acid or a mixture of the amine salt with an acid manifests a catalytic activity of unusually high selectivity on the cyclizing imidation reaction. The present invention has been the result of this discovery. In the light of the long cherished theory that the presence of an oxygen-containing acid is indispensable to the reaction of ring-closure imidation, it is particularly surprising that the use of the amine salt produced by the neutralisation brings about an unusually high catalytic activity in the reaction of ring-closure imidation. Thus, a preferred method for the production of an maleimide according to this invention resides in causing dehydration and ring-closure imidation of a maleinamic acid obtained by the reaction of maleic anhydride with an amine to be carried out in a water-insoluble or water-immiscible inactive organic solvent in the presence of (a) an amine salt obtained from the amine and an inorganic or organic acid, or (b) a mixture of the amine salt with the inorganic or organic acid The maleinamic acids to be used in this invention are easily obtained generally by the reaction of primary amines with maleic anhydride. They are desired to be compounds represented by the following general formula I. wherein R denotes a member selected from the class consisting of alkyl of 1 to 20 carbon atoms, phenyl, benzyl, cyclohexyl, pyridyl, and quinolyl groups, and the same groups as mentioned above and possessed of halogen, carboxyl, or nitro substituents; providing that the alkyl groups or phenyl groups are more desirable than the other groups mentioned. Examples of the primary amine particularly useful as the raw material for the maleinamic acid in this invention include methylamine, ethylamine, n-propylamine, isopropylamine, n-butylamine, sec-butylamine, isobutylamine, tert-butylamine, n-hexylamine, n-dodecylamine, allylamine, benzylamine, cyclohexylamine, aniline, nitroaniline, aminomonochloroaniline, dichloroaniline, toluidines, xylidines, and ethylanilines. Synthesis of a maleinamic acid proceeds virtually stoichiometrically. For example, the maleinamic acid can be synthesized by causing the amine in an amount of 0.8 to l.5 mols, preferably 0.9 to l.2 mols, to react upon each mol of maleic anhydride. The organic solvent to be used in the present invention is desired to be capable of permitting the water formed by the reaction of dehydration and cyclization to be expelled from the reaction system through azeotropic distillation therewith, insoluble or immiscible in water, inert, and incapable of participating in the reaction. Examples of organic solvents meeting these requirements are benzene, toluene, oil fractions boiling at temperatures in the range of 50° to l20° C, xylenes, ethyl benzene, isopropyl benzene, cumene, mesitylene, tert-butyl benzene, pseudo-cumene, trimethyl hexane, octane, tetrachloroethane, nonane, chlorobenzene, ethyl cyclohexane, oil fractions boiling at temperatures in the range of l20° to l70°C, m-dicyclobenzene, sec-butyl benzene, p-dichlorobenzene, decane, p-cymene, o-dichlorobenzene, butyl benzene, decahydronaphthalene, tetrahydronaphthalene, dodecane, naphthalene, cyclohexyl benzene, and oil fractions boiling at temperatures in the range of l70° to 250°C. From the standpoint of enabling this reaction to proceed smoothly under satisfactorily economic conditions, the amount of this solvent to be used in the reaction is in the range of l to 20 times, preferably 3 to 7 times (by volume), the amount of maleimic acid. Further, the solvent is selected on the condition that it should possess a boiling point suiting the prevalent reaction conditions in due consideration of the solubility of the maleimide, price, and ease of handling. When the separation of the maleimide and the solvent after completion of the reaction demands an important consideration, there are times when the reaction performed by the use of a solvent of a low boiling point under application of pressure may prove to be more advantageous. As a catalyst, an amine salt can be used which is obtained by subjecting an inorganic an acid such as sulphuric acid, or organic monobasic or polybasic acid such as p-toluenesulphonic acid, orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, benzene sulphonic acid, or trichloroacetic acid with an amine as a raw material for the production of maleimide. This amine salt is preferred to be such that at least one of the protons of the monobasic acid or polybasic acid is substituted with an amine. As the catalyst, a mixture of the amine salt with the inorganic or organic acid can also be used. This mixed catalyst produces desirable results when the amine salt content thereof at least exceeds 40 mol%, and preferably falls in the range of 50 to 80 mol%. The amount of the catalyst to be used is preferably 20 to 200 mol%, as an acid component contained in the catalyst, based on the amount of the maleinamic acid, wherein the acid component contained in the catalyst means both an acid component which constitutes the amine acid salt and free acid. The neutralisation reaction may be carried out in the presence of a metal-containing compound and a stabilizer when occasion demands. The amine salt produced as described above or a mixture of the amine salt with the acid is preferably insoluble in the organic solvent to be used in the present invention. In such a reaction system therefore, this liquid catalyst assumes a state separated into the two layers, an organic layer and an inorganic layer. This state remains during and after the reaction. Moreover, the amine salt or the mixture thereof with the acid serving as the catalyst remains substantially unchanged before and after the reaction. A catalyst system of this nature therefore can be used in situ in the next cycle of reaction without being recovered and refined in the meantime. When this catalyst layer is to be used in the next cycle of reaction, the organic layer and the catalyst layer existing at the end of the reaction may be separated one from the other at a temperature in the range of 120° to 250°C, and the catalyst layer consequently recovered may be put to use directly in the next cycle of reaction. Otherwise, for the sake of convenience of handling, the recovered catalyst layer may be diluted with 5 to 20 % by weight of water to lower both temperature and viscosity so to be added to the reaction system for the next cycle of reaction. There are instances where the reaction can be carried out, as disclosed in U.S. Patent No. 4,623,734, in the presence of a metal-containing compound and a stabilizer. The metal-containing compound to be used in this case may be selected from among oxides, acetates, maleates, succinates, nitrates, phosphates, chlorides, and sulphates of at least one metal selected from the group consisting of zinc, chromium, palladium, cobalt, nickel, iron, and aluminium. Of these compounds, zinc acetate appears to be particularly effective. The amount of the metal-containing compound to be used may be in the range of 0.005 to 0.5 mol%, preferably 0.01 to 0.1 mol%, as metal, based on 1 mol of the maleinamic acid. Suitable stabilizers include methoxy benzoquinone, p-methoxyphenol, phenothiazine, hydroquinone, alkylated diphenyl amines, methylene blue, tert-butyl catechol, tert-butyl hydroquinone, zinc dimethyldithiocarbamate, copper dimethyldithiocarbamate, copper dibutyldithiocarbamate, copper salicylate, thiodipropionic esters, mercaptobenzimidazole, triphenyl phosphite, alkylphenols, and alkylbisphenols. The stabilizer plays the part of enabling the maleimide which is produced by the reaction of imidation to remain stable during the course of the imidation without being degenerated at the elevated temperature of the reaction. Regarding the amount of the stabilizer to be added, the addition of the stabilizer in a minute amount is generally not sufficiently effective while its addition in an unduly large amount is undesirable because it entails the drawback that an excess of stabilizer finds its way into the final product. The amount of the stabilizer to be effectively used may therefore be in the range of 0.005 to 0.5 mol%, preferably 0.05 to 0.3 mol%, based on l mol of the maleinamic acid. In a preferred manner of working the present invention, first maleic anhydride prepared as a solution in an organic solvent and an amine compound added thereto are allowed to react with each other at a temperature not exceeding l50°C, preferably falling in the range of 30° to l20°C, for a period of l5 to l20 minutes, preferably 30 to 60 minutes to produce maleinamic acid. Then, the reaction system in which the maleinamic acid is left unisolated, the catalyst or the catalyst layer separated from the reaction system of the preceding cycle of reaction, and optionally the metal-containing compound and/or the stabilizer are combined and heated at a temperature in the range of l20° to 250°C, preferably l30° to 220°C, for a period in the range of one hour to l5 hours, preferably 3 to 7 hours to effect the reaction in a continuous operation with the formed water expelled from the system through azeotropic distillation or to effect the reaction in a batchwise pattern with the expulsion of the formed water carried out at the end of the reaction. As the result, the maleimide is produced in a high yield. The maleimide which is consequently obtained is a compound represented by the general formula II, for example. wherein R has the same meaning as defined above. Typical examples of the maleimides include N-methyl maleimide, N-ethyl maleimide, N-n-propyl maleimide, N-isopropyl maleimide, N-n-butyl maleimide, N-sec-butyl maleimide, N-tert-butyl maleimide, N-n-hexyl maleimide, N-n-dodecyl maleimide, N-allyl maleimide, N-benzyl maleimide, N-cyclohexyl maleimide, N-phenyl maleimide, N-nitrophenyl maleimide, N-hydroxyphenyl maleimide, N-methoxyphenyl maleimide, N-ethoxyphenyl maleimide, N-monochlorophenyl maleimide, N-dichlorophenyl maleimide, N-monomethylphenyl maleimide, N-dimethylphenyl maleimide, and N-ethylphenyl maleimide. Of course, the maleimides which this invention is intended to embrace are not limited to the examples cited above. This invention which has been described above brings about the following advantages. (l) The present invention permits a maleimide of high purity to be produced in a high yield, because the amine salt formed by the reaction of neutralization of the amine as the raw material for the production of the maleimide with an inorganic or organic monobasic or polybasic acid or the mixture of the amine salt with an acid possesses a catalytic activity of high selectivity on the reaction of cyclizing imidation. (2) The cost of the catalyst can be substantially disregarded because the amine salt or the mixture used as the catalyst is stable enough to be used time and time again in the successive cycles of reaction. (3) Since the catalyst is used repeatedly, there is substantially no need for the treatment otherwise required to be given to the used catalyst for the eventual prevention of environmental pollution. Since the reaction system, therefore, can be operated in a closed pattern, there is virtually no possibility of the environment being polluted with discarded catalyst. As described in (1) through (3) this invention permits a maleimide to be easily produced inexpensively and safely. Now, the present invention will be described more specifically below with reference to working examples. Example 1 In a Meyer's flask having an inner volume of 300 ml, 100 g of orthoxylene and 60 g of sulphuric acid were dispersed. Then the Meyer's flask was kept cooled in a water bath, 122 g of cyclohexylamine was added dropwise thereto to obtain a xylene slurry of white crystals of dicyclohexylamine salt of sulphuric acid. Separately, a flask provided with a condenser incorporating therein a water separator, a dropping funnel, and a stirrer was charged first with 100 g of orthoxylene and then with l00 g of maleic anhydride and heated until the inner temperature thereof reached l00°C to effect dissolution of maleic anhydride. Then, a solution of l00g of cyclohexylamine in 600 g of orthoxylene was added dropwise to the solution while in a stirred state over a period of one hour to synthesize a slurry of N-cyclohexyl maleinamic acid in the solvent. Thereafter, this slurry and the slurry of amine salt were wholly combined and then left reacting for two hours by being heated and stirred at l47°C, with the water formed by the reaction continuously expelled from the reaction system through distillation in combination with the solvent. After the reaction was completed, the catalyst layer separated in a lower layer from the reaction solution at l47°C was removed. Subsequently, the reaction solution was cooled to 60°C and washed by 30 minutes' stirring with l00g of water. The water layer consequently formed was separated from the washed reaction solution. This operation repeated twice. Then, the remaining organic layer was distilled under a vacuum of l0 mmHg (abs) to expel the solvent therefrom. Then, the contents of the flask and 0.3 g of copper dibutyl dithiocarbamate newly added thereto were kept under a vacuum of 5 mmHg (abs) at a flask interior temperature of l30° to l50°C for 30 minutes to effect distillation of N-cyclohexyl maleimide. As the result, l68 g of bright white crystals of N-cyclohexyl maleimide were obtained. The purity of this product was 99.9 % by weight. The yield of this product was 92.8 mol% based on cyclohexylamine used as the raw material. A reaction was carried out by faithfully following the procedure described above, excepting the catalyst layer separated from the reaction system of the preceding cycle of reaction was used instead. Consequently, there were obtained l69 g of bright white crystals of N-cyclohexyl maleimide. The purity of this product was 99.8 % by weight and the yield thereof was 93.3 mol% based on cyclohexylamine used as the raw material. Example 2 In a Meyer's flask having an inner volume of 300 ml, l00 g of orthoxylene and 60 g of orthophosphoric acid were dispersed. Then, the Meyer's flask was kept cooled in a water bath and 6l g of cyclohexylamine was added dropwise thereto obtain a xylene slurry solution of white crystals of monocyclohexylamine salt of orthophosphoric acid. Separately, a flask provided with a condenser incorporating therein a water separator, a dropping funnel and a stirrer was charged first with l00 g of orthoxylene and then with l00 g of maleic anhydride and heated until the inner temperature of the flask reached l00°C, to effect dissolution of maleic anhydride. Then, a solution of l00 g of cyclohexylamine in 600 g of orthoxylene was added dropwise while in a stirred state over a period of one hour to synthesize a slurry solution of N-cyclohexyl maleinamic acid in orthoxylene. Then, this slurry and the slurry of amine salt were combined. The resultant mixture and 0.l g of copper dibutyl dithiocarbamate added thereto were left reacting for seven hours by being heated and stirred at l43°C, with the water formed by the reaction expelled from the reaction system in combination with orthoxylene in the meantime. After the reaction was completed, the catalyst layer separated in a lower layer from the reaction solution at l43°C was removed. Subsequently, the reaction solution was cooled to 60°C and washed by 30 minutes' stirring with l00 g of water, to separate the water layer from the reaction solution. This operation was repeated twice. The organic phase consequently obtained was distilled under a vacuum of l0 mmHg (abs) to expel ortho-xylene. Then, the contents of the flask and 0.3 g of copper dibutyl dithiocarbamate newly added thereto were kept under a vacuum of 5 mmHg (abs) at an inner flask temperature of l30° to l50°C for 30 minutes to effect distillation of N-cyclohexyl maleimide. As the result, there were obtained l63 g of bright white crystals of N-cyclohexyl maleimide. The purity of this product was 99.8 % by weight and the yield thereof was 90.0 mol% based on cyclohexyl amine used as the raw material. A reaction was carried out by faithfully repeating the procedure described above, excepting the catalyst layer separated from the reaction system of the preceding cycle of reaction was used instead. Consequently, there were obtained l64 g of bright white crystal of N-cyclohexyl maleimide. The purity of this product was 99.8 % by weight and the yield thereof was 90.6 mol% based on cyclohexylamine used as the raw material. Example 3 In a Meyer's flask having an inner volume of 300 ml, l00 g of P-cymene and l0 g of orthophosphoric acid were dispersed. Then, the Meyer's flask was kept cooled in a water bath and 9.5 g of aniline was added dropwise thereto, to obtain a slurry of white crystals of monoaniline salt of orthophosphoric acid. Separately, a glass flask having an inner volume of l liter was fitted with a thermometer, a stirrer, and a water separator. Then, a solution of 53 g of powdered maleic anhydride in 50 g of P-cymene was placed in the glass flask. The flask was adjusted to an inner temperature of l30°C and a solution of 50 g aniline in 400 g of p-cymene was added peicemeal thereto over a period of 30 minutes, to synthesize a slurry solution of N-phenyl maleinamic acid. The slurry solution thus obtained and the whole of the amine salt slurry, 0.034 g of zinc acetate, and 0.065 g of p-methoxyphenol added thereto were left reacting at a temperature of 180°C for three hours, with the water formed therein expelled from the reaction solution through distillation in combination with p-cymene. After completion of the reaction, the catalyst layer was removed. Thereafter, the reaction solution was cooled to 30°C, washed with water, and distilled under a vacuum to expel p-cymene. Consequently, there were obtained 94.0 g of crystals of N-phenyl malemimide. The purity was found by liquid chromatography to be 93.7% by weight. The yield of the product was 94.7 mol% based on amine. A reaction was carried out by faithfully repeating the procedure described above, excepting the catalyst layer separated from the reaction system of the preceding cycle of reaction. As the result, there were obtained 95.0 g of crystals of N-phenyl maleimide. The purity of this product was found by liquid chromatography to be 93.2 % by weight. The yield thereof was 95.2 mol% based on amine. Control 1 To a solution of 98 g of maleic anhydride in 500 g of toluene, 99.2 g of cyclohexylamine was added dropwise at a temperature in the range of 50° to 70°C while in a stirred state. After the dropwise addition, the resultant mixture was stirred for two hours. Consequently N-cyclohexyl maleinamic acid was obtained. Then, the reaction solution and 14 g of methanesulphonic acid, 8.7 g of methanesulphonic acid-N-cyclohexyl maleinamate, and 15 g of N-methyl pyrrolidone added thereto were left reacting at 115°C for 3.5 hours in a refluxed state, with the formed water continuously expelled from the reaction system through azeotropic distillation with toluene. Then, the resultant reaction solution was caused to react with 15 g of acetic anhydride for 30 minutes, cooled to 60°C, and filtered to be deprived of insoluble impurities, the catalyst, and unaltered N-cyclohexyl maleinamic acid. Subsequently, the filtrate was washed by 30 minutes' stirring with 200 g of water to separate the water layer. This operation was repeated twice. The organic layer consequently obtained was distilled under a vacuum of 30 mmHg (abs) to expel toluene. As the result, there was obtained l72g of a brown solid containing cyclohexyl maleimide. By the GPC analysis, the N-cyclohexyl maleimide was found to be l0.0 % by weight and the yield of N-cyclohexyl maleimide was to be 9.6 mol%, based on cyclohexylamine. When the reaction solution was analysed by gas chromatography, the results showed the same yield. Control 2 A reaction was carried out by faithfully repeating the procedure of Control l, excepting 73 g of N-butylamine was used in the place of cyclohexylamine, l4 g of methanesulphonic acid and 8.4 g of tributylamine salt of methanesulphonic acid were used as a reaction catalyst, and dimethyl formamide was used in the place of N-methyl pyrrolidone as the polar solvent. Consequently, there was obtained l5l g of a brown oily liquid containg N-n-butyl maleimide. By the GPC analysis, the content of N-n-butyl lmaleimide was found to be l3.4 % by weight and the yield thereof to be l3.2 mol% based on N-butylamine used as the raw material. When the reaction solution was washed with water and analysed by gas chromatography, the results showed the same yield. Control 3 A solution of 43.l g of maleic anhydride in 33l.8 g of chlorobenzene and 39.7 g of cyclohexylamine added thereto were left reacting at 40°C for one hour. The slurry solution of N-cyclohexyl maleinamic acid consequently obtained and l2 ml of dimethyl acetamide and 2 g of trichloroacetic acid added thereto were left reacting at l35°C for six hours, with the formed water continuously expelled in combination with the solvent from the reaction system in the meantime. At the end of the reaction, there was obtained 402 g of reaction solution. This reaction solution, on analysis by gas chromatography, was found to contain 2.2 % by weight of N-cyclohexyl maleimide. The yield of this N-cyclohexyl maleimide was l2.3 mol%, based on cyclohexylamine used as the raw material. Example 4 In a Meyer's flask having an inner volume of 300 ml, l00g of orthoxylene and 60 g of orthophosphoric acid were dispersed. Then, the Meyer's flask was kept cooled in a water bath and 37 g of cyclohexylamine was added dropwise thereto to obtain a slurry solution of a white mixed catalyst of monocyclohexylamine salt of orthophosphoric acid and orthophosphoric acid in xylene. The amine salt content in the mixed catalyst was 60.9 mol%. Separately, a flask provided with a condenser incorporating therein a water separator, a dropping funnel, and a stirrer was charged first with l00 g of orthoxylene and then with l00 g of maleic anhydride and heated until the inner temperature thereof reached l00°C, to dissolve the maleic anhydride. Then, a solution of l00 g of cyclohexylamine in 600 g of orthoxylene was added dropwise while in a stirred state over a period of one hour, to synthesize a slurry solution of N-cyclohexyl maleinamic acid in orthoxylene. Thereafter, this slurry solution and the mixed slurry solution of amine salt and orthophosphoric acid (the amount of the mixed catalyst corresponding to 60.7 mol%, based on maleinamic acid) and 0.l g of copper dibutyl dithiocarbamate added thereto were left reacting for seven hours by being heated and stirred at l43°C, with the water formed by the reaction continuously expelled by distillation in combination with orthoxylene from the reaction system in the meantime. After the reaction was completed, the catalyst layer separated in a lower layer from the reaction solution at l43°C was removed. Subsequently, the reaction solution was cooled to 60°C and washed by being stirred with l00 g of water for 30 minutes, to separate the water layer. This operation was repeated twice. The organic layer consequently formed was distilled under a vacuum of l0 mmHg (abs) to expel orthoxylene. Then, the contents of the flask and 0.3 g of copper dibutyl dithiocarbamate newly added were left standing under a vacuum of 5 mmHg (abs) at a flask inner temperature of l30° to l50°C for 30 minutes, to effect distillation of N-cyclohexyl maleimide. As the result, there were obtained l7l g of bright white crystals of N-cyclohexyl maleimide. The purity of this product was 99.8 % by weight and the yield thereof was 94.4 mol% based on cyclohexylamine used as the raw material. Thereafter, a reaction was carried out by faithfully following the procedure described above, excepting the catalyst layer separated from the reaction system of the preceding cycle of reaction was used instead. Consequently, there were obtained l7l g of bright white crystals of N-cyclohexyl maleimide. The purity of this product was 99.8 % by weight and the yield thereof was 94.4 mol% based on cyclohexyl amine used as the raw material. Example 5 In a Meyer's flask having an inner volume of 300 ml, 100 g of p-cymene and 100 g of orthophosphoric acid were dispersed. Then, the Meyer flask was kept cooled in a water bath and 56 g of n-butyl amine was added drowpwise thereto, to obtain a white mixed slurry solution of n-butylamine salt of orthophosphoric acid and orthophosphoric acid. The amine content in this mixed catalyst was 75.0 mol%. Separately, a reactor was prepared by furnishing a glass flask having an inner volume of 1 litre with a thermometer, a stirrer, and a water separator. Then, a solution of 53 g of powdered maleic anhydride in 50 g of p-cymene was placed in the reactor. Subsequently, the inner temperature of the reactor was adjusted to 130°C and a solution of 40 g of n-butyl amine in 400 g of p-cymene was added thereto piecemeal in a dropwise manner over a period of 30 minutes, to synthesise a slurry solution of N(n-butyl)maleinamic acid. The slurry solution thus obtained and the slurry solution of mixed catalyst (the amount of mixed catalyst corresponding to 186.5 mol% based on the maleinamic acid), 0.034 g of zinc acetate, and 0.065 g of p-methoxy phenol added thereto were left reacting at a temperature of 180°C for three hours, with the formed water continuously expelled in combination with p-cymene from the reaction system in the meantime. After the reaction was completed, the catalyst layer was removed and the reaction solution weighed 620g. The concentration of N-(n-butyl) maleimide in this reaction solution was 11.5% by weight. The yield of the maleimide was 85.1 mol% based on n-butylamine used as the raw material. A reaction was carried out by faithfully repeating the procedure described above, excepting the catalyst layer spearated from the reaction system of the preceding cycle of reaction was used instead, to obtain 625 g of a reaction. solution. The concentration of N-(n-butyl) maleimide in this reaction solution was found by analysis to be ll.6 % by weight. The yield was 86.5 mol%. Example 6 In a Meyer's flask having an inner volume of 300 ml, 60 g of orthophosphoric acid was placed. Then, this Meyer's flask was kept cooled in a water bath and 46 g of cyclohexylamine was added dropwise thereto, to obtain a viscous slurry solution of mixed catalyst of cyclohexyl-monoamine of phosphoric acid and phosphoric acid. The content of amine salt in this mixed catalyst was 75.8 mol%. Separately, a glass autoclave having an inner volume of l litre and provided with a thermometer, a condenser, a dropping funnel, and a stirrer was charged first with l00 g of orthoxylene and then with l00 g of maleic anhydride and then heated until the inner temperature of the flask reached l00°C, to effect dissolution of the maleic anhydride. Subsequently, a solution of l00 g of cyclohexylamine in 400 g of orthoxylene was added dropwise thereto while in a stirred state over a period of 30 minutes, to synthesize a slurry solution of N-cyclohexyl maleinamic acid in the solvent. Then, the slurry solution and the slurry of mixed catalyst (the amount of the mixed catalyst corresponding to 60.7 mol% based on the maleinamic acid added thereto) were left reacting at l52°C for three hours in a closed system without expulsion of the formed water. In the reaction system, the inner pressure was l.2 atm at the outset of the reaction and was 7.0 atm after three hours of the reaction. After the reaction was completed, the inner temperature was adjusted to l30°C and then the inner pressure was allowed to fall to normal atmospheric pressure. The reaction product was removed from the autoclave and left standing at rest. Consequently, the reaction solution separated clearly into two layers, i.e. an organic layer and a catalyst layer. The organic layer weighed 660 g. The content of N-cyclohexyl maleimide in this organic layer, on analysis by gas chromatography, was found to be 24.6 % by weight. The yield of this maleimide was 89.8 mol%, based on cyclohexylamine used as the raw material. Subsequently, a reaction was carried out by faithfully repeating the procedure described above, excepting the mixed catalyst layer of amine salt and acid separated from the reaction system of the preceding cycle of reaction was used instead. Consequently, there was obtained 664 g of an organic layer. The content of N-cyclohexyl maleimide in this organic layer was 24.5 % by weight. The yield thereof was 90.0 mol% based on cyclohexylamine used as the raw material.

Claims: A method for the production of a maleimide of formula II: wherein R is one of alkyl of 1-20 carbon atoms, phenyl, benzyl, cyclohexyl, pyridyl, quinolyl and these moieties when substituted by halogen, carboxyl or nitro, which comprises the sequential steps of: reacting maleic anhydride with a primary amine of the formula NH₂-R, wherein R is as defined above, to form the corresponding maleinamic acid; and effecting a ring closure reaction of the maleinamic acid by heating the maleinamic acid in a water insoluble or water immiscible inert non-polar organic solvent in the presence of a catalyst, characterised in that: the solvent is used in a ration by volume of from 1 to 20:1 with respect to the maleinamic acid; the catalyst is a primary amine-acid addition-salt catalyst formed from the primary amine and a strong inorganic or organic acid, the catalyst being present in an amount of 2 to 400 mol% of acid addition component based on the maleinamic acid, or the catalyst is a mixture of the salt with the free acid, the content of the free acid being less than 60 mol%; the heating step is carried out at a temperature of from 120° to 250°C; and the water thus formed is continuously distilled off as a mixture with the solvent. A method as claimed in Claim 1, characterised in that the amount of catalyst to be used is in the range of 20 to 200 mol%, as an acid component contained in the catalyst, based on the maleinamic acid. A method as claimed in Claim 1 or Claim 2, characterised in that the acid is sulphuric acid, p-toluenesulphonic acid, orthophosphoric acid, methaphosphoric acid, pyrophosphoric acid, benzenesulfonic acid, and/or trichloro-acetic acid. A method as claimed in any preceding claim, characterised in that the content of the free inorganic or organic acid in the mixed catalyst is in the range of 20 to 50 mol%. A method as claimed in any preceding claim, characterised in that the imidation of the maleinamic acid by dehydration and ring-closure is carried out in the presence of a metal-containing compound. A method as claimed in Claim 5, characterised in that the metal-containing compound is an acetate.

Inventors: BABA, MASAO, FUKUI, AKIO, KITA, YUICHI, NAKAGAWA, KOICHI, NAKAGAWA, YOICHI, SAKAMOTO, KENTARO
Applicants: NIPPON SHOKUBAI KAGAKU KOGYO CO., LTD
Application year: 1987
Main classification: C07D207
IPC codes: C07D207