JPH04134063A - Production of n-arylmaleimides - Google Patents

Abstract

PURPOSE:To obtain the title high-purity compound in high yield by imidating through ring closure an N-substituted maleinamic acid in a mixture of sulfuric acid and an aromatic solvent subjected to azeotropic dehydration, in the presence of an alkaline (earth) metallic salt of maleic acid and an aprotic polar solvent. CONSTITUTION:A mixture of sulfuric acid and an aromatic solvent (preferably toluene or xylene) is subjected to azeotropic dehydration and then in the mixture, a compound shown by formula I (R<1> to R<5> are H, halogen, NO2, OH, COOH, 1-6C alkyl, alkoxy, phenyl or sulfone) is imidated through ring closure in the presence of an alkaline (earth) metallic salt of maleic acid and an aprotic polar solvent (preferably DMSO OR DMF) under heating at 80-160 deg.C for 1-6 hours, preferably at 90-140 deg.C for 1-4 hours to give a compound shown by formula II. The amount of the aprotic polar solvent used is properly 1-7.5mols based on 100mol compound shown by formula I in terms of effective suppression of formation of by-products.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides a high-yield N-arylmaleimide which is useful as a heat resistance improver for resins such as ABS, MMA, and PVC, and as an intermediate for pharmaceuticals and agricultural chemicals. Relating to a method of manufacturing at a high rate.

[Conventional technology]

Methods for producing maleimides have been studied for a long time. For example, in Japanese Patent Application Laid-open No. 53-68770,
Without isolating the maleamic acid produced by reacting maleic anhydride and an amine compound in an organic solvent,
A method of dehydration and ring closure in the presence of an aprotic polar solvent such as dimethylformamide or dimethyl sulfoxide and an acid catalyst is disclosed. However, this method requires a large amount of expensive and toxic aprotic polar solvents such as dimethylformamide, which increases the production cost of maleimides, and the acid catalyst makes it difficult to produce maleimides. Due to the deterioration of the solvent, there is a large loss, and the boiling point of these aprotic polar solvents is high.
There are problems such as difficulty in removing these solvents.

Japanese Patent Publication No. 51-40078 describes solvents such as toluene, xylene, and chlorobenzene having a boiling point of 80°C or higher as diluents, and chlorosulfonic acid, p-)luenesulfonic acid, benzenesulfonic acid, orthophosphoric acid, pyrophosphoric acid,
A method is disclosed in which thermal dehydration and ring closure are carried out together with a catalyst such as phosphorous acid, and the water produced at this time is distilled out of the yarn by azeotropy with a solvent. This method is superior to the above-mentioned methods in that it does not use an aprotic polar solvent, so the maleimide produced can be easily separated and recovered. However, although this method uses a relatively large amount of an expensive acid catalyst such as chlorosulfonic acid, the yield of maleimides is low and is not economically satisfactory as an industrial production method.

The reason for the decrease in the yield of maleimides is that an amine component is liberated by hydrolysis of maleimides and maleamic acids, and the acid catalyst is deactivated by this liberated amine. Furthermore, deactivation of this acid catalyst not only reduces the conversion rate of maleamic acids, but also decomposition reactions of maleimides and maleamic acids are likely to occur when the acid catalyst is deactivated. This causes a decrease in yield. Furthermore, the production of by-products also causes a decrease in purity and an increase in purification costs.

[Problem to be solved by the invention]

The purpose of the present invention is to produce high-purity N-arylmaleimides in high yields by using a solvent and an inexpensive acid catalyst that allow for easy separation and recovery of maleimide, and by preventing deactivation of the acid catalyst. The purpose is to provide a method.

[Means to solve the problem]

The present invention azeotropically dehydrates a mixture of sulfuric acid and an aromatic solvent,
Then, in this mixture, in the presence of an alkali metal salt or alkaline earth metal salt of maleic acid and an aprotic polar solvent, compounds of the general formula (1) (wherein R', R', R'', R'' and R5 are each independently a hydrogen atom, a halogen atom,
A general method that involves heating an N-substituted maleamic acid represented by a nitro group, a hydroxyl group, a carboxyl group, an alkyl group having 1 to 6 carbon atoms, an alkoxy group, a phenyl group, or a sulfone group to undergo ring-closing imidization. Formula (2) (wherein, R', R2, R3, R' and R5 are defined as
The present invention relates to a method for producing N-arylmaleimides represented by formula (same as the definition of general formula (1)).

The present invention will be explained in detail below.

In the method of the present invention, a mixture of sulfuric acid and an aromatic solvent is first azeotropically dehydrated.

Sulfuric acid means something containing H2S Oh, and specific examples include concentrated sulfuric acid, diluted sulfuric acid, fuming sulfuric acid, etc. Concentrated sulfuric acid is used because of its ease of handling and ease of azeotropic dehydration. It is preferable. In addition, sulfonic acids such as sulfuric anhydride, p-)luenesulfonic acid, methanesulfonic acid, benzenesulfonic acid, ethylsulfonic acid, and octylsulfonic acid may be used in combination with the sulfuric acid.

On the other hand, the aromatic solvent may be any aromatic solvent that is inert to N-arylmaleamidic acids and N-arylmaleimides and immiscible with water. For example, benzene, toluene, xylene, cumene, ethylbenzene,
Examples include chlorobenzene, nitrobenzene, and anisole, and toluene and xylene are preferably used because of ease of controlling the reaction temperature and removing the solvent.

In the above azeotropic dehydration, when concentrated sulfuric acid is used as the sulfuric acid, approximately 0
．． Subsequently (
This is suitable from the viewpoint of increasing the yield of maleimide in the ring-closing imidization reaction. When dilute sulfuric acid is used as the sulfuric acid, the amount of water distilled out is generally greater than the above amount.

Azeotropic dehydration is carried out at, for example, 70 to 180°C, preferably 80°C.
This can be carried out by heating at ~160°C, and can be carried out under normal pressure, increased pressure, or reduced pressure.

If the amount of sulfuric acid used is too small, the subsequent imidization reaction will not proceed sufficiently and the reaction time will tend to increase. In addition, if the amount is too large, a large amount of by-products due to hydrolysis, polymerization, and isomerization reactions tend to be produced. It is appropriate to do so.

On the other hand, if the ratio of N-arylmaleamide acids to the total charged weight (total weight of maleamic acid, solvent, catalyst, and maleate salt) at the start of the imidization reaction is low, side reactions such as isomerization reactions and polymerization reactions may occur. A large amount of product is produced, and if the temperature is too high, water produced by the reaction cannot be distilled out of the system. As a result, a hydrolysis reaction of N-arylmaleamidic acids and N-arylmaleimides occurs, which not only makes it easier to produce maleic acid and (N-aryl)-2-arylaminosuccimide as by-products, but also deactivates the acid catalyst. (.Therefore, N for the total preparation 100
- The weight ratio of the arylmaleamide acids is adjusted to be in the range of 10 to 50, and the amount of aromatic solvent used is determined by the weight of the N-arylmaleamide acids in order to perform the reaction smoothly and satisfy economic conditions. It is appropriate to set the amount to 1 to 7 times, preferably 1 to 4 times.

The N-substituted maleamic acid represented by the general formula (1) is
Preferably, at least two of R1 to R5 are hydrogen atoms, and the others are substituents other than hydrogen atoms.

Specific examples of N-arylmaleamidic acids represented by general formula (1) include N-phenylmaleamidic acid,
N-(2-chlorophenyl)maleamidic acid, N-(
3-chlorophenyl)maleamic acid, N-(4-chlorophenyl)maleamic acid, N-(2-nitrophenyl)maleamic acid, N-(2-hydroxyphenyl)maleamic acid, N-(2-aminophenyl) )
Maleamic acid, N-(2,6-dichlorophenyl)
Maleamic acid, N-(4-methoxyphenyl)maleamic acid, N-(4-ethoxyphenyl)maleamic acid, N-(4-methylphenyl)maleamic acid, N-(2,6-dimethylphenyl)malein Amic acid, N-(4-carboxyphenyl)maleamic acid, N-(2,4,6-dichlorophenyl)maleamic acid, N-(2,4,6-drimethylphenyl)maleamic acid, N -(4-sulfonylphenyl)maleamic acid, N-biphenylmaleamic acid, etc. can be mentioned.

- The maleamic acids of Tsuzuriyo (1) can be obtained almost stoichiometrically, for example, by the reaction of maleic anhydride and an aromatic primary amine. aromatic first
Examples of amines include aniline, nitroaniline, aminophenol, aminobenzoic acid, anisidine, ethoxyphenylamine, monochloroaniline, dichloroaniline, toluidine, xylidine, ethylaniline, 4-
Aminobiphenyl, metanilic acid, etc. can be mentioned. 0.1 mole of maleic anhydride to 0.0 mole of primary aromatic amine.
9 to 1.1 mol, preferably 0.95 to 1.05 mol may be reacted.

In the method of the present invention, an alkali metal salt (e.g. lithium salt, sodium salt, potassium salt) or an alkaline earth metal salt (e.g. magnesium salt, calcium salt,
In the presence of a strontium salt, barium salt) and an aprotic polar solvent, the N-substituted maleamic acid of the general formula (1) is heated to undergo ring-closing imidization to obtain the N-arylmaleimide of the general formula (2). . Alkali metal and alkaline earth metal salts of maleic acid and aprotic polar solvents can be added to the mixture of sulfuric acid and aromatic solvent before azeotropic dehydration; It is preferable to add it to the mixture of sulfuric acid and aromatic solvent after azeotropic dehydration, from the viewpoint of avoiding deterioration in quality.

Specific examples of alkali metal salts or alkaline earth metal salts of maleic acid include dilithium maleate, disodium maleate, dicalimula maleate, magnesium maleate, calcium maleate, strontium maleate, and barium maleate. be able to. The amount of alkali metal salt or alkaline earth metal salt of maleic acid to be used is determined based on 100 moles of maleamic acid as a raw material, from the viewpoint of effectively preventing deactivation of the acid catalyst and satisfying economic efficiency. , 0.1 to 10 mol, preferably 1 to 7.5 mol.

In the present invention, an aprotic polar solvent is a solvent that does not release protons and is highly polar.

High polarity means, for example, that the dielectric constant is preferably 20 or more. Examples of aprotic polar solvents are dimethylsulfoxide, N,N'-dimethylformamide, N,N''-dimethylacetamide, hexamethylphosphoric triamide, l.

3-dioxane, 1,4-dioxane, N-methyl-2
-Pyrrolidone, acetonitrile, etc. may be mentioned. Among these, dimethyl sulfoxide (DMSO) and N are preferred because of their ease of handling, resistance to modification by acid catalysts, and economic efficiency.

Preferably, No-dimethylformamide (DMF) is used.

The amount of aprotic polar solvent to be used is 0.5 mol per 100 mol of maleamic acid as a raw material, from the viewpoint of effectively suppressing the production of by-products and satisfying economic efficiency.
A suitable range is from 1 to 10 mol, preferably from 1 to 7.5 mol.

Regarding the reaction conditions, the reaction temperature is, for example, 80 to 160°C, preferably 90 to 140°C, and the reaction time varies depending on the reaction temperature, etc., but it is appropriate to set it to, for example, 1 to 6 hours, preferably 1 to 4 hours. be. Furthermore, this reaction can be carried out not only under normal pressure but also under increased pressure or reduced pressure within the above-mentioned temperature range. In addition, when the reaction temperature is high, known polymerization inhibitors (for example, alkyl monophenols, alkyl bisphenols, thiodipropionic acid esters, dithiocarbamates, salicylates,
(salicylic acid ester) may also be appropriately present in the reaction system. Furthermore, if necessary, an alkali metal salt or alkaline earth metal salt such as maleic acid, maleic anhydride, sodium acetate, sodium p=toluenesulfonate, sodium sulfate, calcium acetate, magnesium sulfate, etc. can be added to the reaction system. .

Furthermore, since alkali metal salts or alkaline earth metal salts of maleic acid are insoluble in aromatic solvents, they can be easily separated and recovered by filtration etc. after the imidization reaction, and the recovered ones can be used again for this purpose. It can be used in the method of invention.

N represented by general formula (2) obtained by the method of the present invention
-Arylmaleimides include, for example, N-phenylmaleimide, N-(2-chlorophenyl)maleimide, N-(3-chlorophenyl)maleimide, N-(4-
Chlorophenyl)maleimide, N-(4-nitrophenyl)maleimide, N-(2-hydroxyphenyl)maleimide', N-(2-aminophenyl)maleimide, N
-(26-dichlorophenyl)maleimide, N-(4-
methoxyphenyl)maleimide, N-(4-ethoxyphenyl)maleimide, N-(4-methylphenyl)maleimide, N-(2,6-dimethylphenyl)maleimide, N-(4-carboxyphenyl)maleimide, N-(
2,4,6-)dichlorophenyl)maleimi F, N-(
2,4,etrimethylphenyl)maleimide, N-(4
-sulfonylphenyl)maleimide, N-biphenylmaleimide, and the like.

〔Effect of the invention〕

According to the method of the present invention, maleimide can be easily separated and recovered using an aromatic solvent, and inexpensive sulfuric acid is used as an acid catalyst at a relatively low temperature to prevent deactivation of the acid catalyst while increasing the yield. Pure N-arylmaleimides can be produced.

Furthermore, the method of the present invention has the advantage that the reaction time is shorter than conventional methods for producing N-arylmaleimides, and the resulting N-arylmaleimides are less colored.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

Example 1 80 d of toluene and 2.0 ml of concentrated sulfuric acid were added to a 200 d four-way flask equipped with a thermometer, a water separator, a cooling tube, and a stirrer, and the mixture was stirred and refluxed for 25 minutes at a temperature range of 103 to 106°C under normal pressure. 0.47 nl of water produced was removed from the yarn by azeotropy. After heating to reflux and azeotropic dehydration,
Allow to cool to 80°C, and add N-phenylmaleamic acid 3.
8.2 g (0.2 mol), disodium maleate 1.604 g (0.01 mol) and dimethyl sulfoxide 0.72 d (0.01 mol) were added, and the resulting water 3.5- was added to the reaction system with toluene. The imidization reaction was carried out in a temperature range of 103 to 109° C. for 3 hours while distilling the mixture off.

The weight ratio of N-phenylmaleamic acid to the total amount of 100 charged at the start of the imidization reaction was 35.

After the reaction was completed, the mixture was allowed to cool to 60°C, and the suspended disodium maleate was filtered off by suction filtration, and then toluene was removed under reduced pressure to obtain 37.71 g of N-phenylmaleimide as a yellow solid. .

When analyzed by high performance liquid chromatography, the purity of the crystals was 84.6 wt%', and the yield was 94.5 mol% based on the raw material N-phenylmaleamic acid.

Example 2 The amount of disodium maleate added was 0.802 g (0.
.

The weight ratio of N-phenylmaleamic acid to the total amount of 100 charged at the start of the imidization reaction was 35.

The yield of N-phenylmaleimide was 40.08 g, which was analyzed by high performance liquid chromatography.
The purity of this crystal was 78.9 wt%, and the yield was 91.3 mol% based on the raw material N-phenylmaleamic acid.

Example 3 The reaction was carried out in the same manner as in Example 1 except that the amount of sulfuric acid used was 1.5 ml and the amount of disodium maleate added was 0.802 g (0,005 mo 1).

The weight ratio of N-phenylmaleamic acid to the total amount of 100 charged at the start of the imidization reaction was 35.

The yield of N-phenylmaleimide was 38.09 g, which was analyzed by high performance liquid chromatography.
The purity of this crystal was 84.7 wt%, and the yield was 93.2 mol% based on the raw material N-phenylmaleamic acid.

Example 4 The amount of disodium maleate added was 0.802 g (0.
,005 mol) and further 0.508 g of maleic acid.
The reaction was carried out in the same manner as in Example 1 except that (0,005 mo 1) was added.

The weight ratio of N-phenylmaleamic acid to the total amount of 100 charged at the start of the imidization reaction was 35.

The yield of N-phenylmaleimide was 40.52 g, which was analyzed by high performance liquid chromatography.
The purity of this crystal was 80.6 wt%, and the yield was 94.3 mol% based on the raw material N-phenylmaleamic acid.

Example 5 To a 200 d four-way flask equipped with a thermometer, a water separator, a condensing tube, and a stirrer, 95 toluene and 2.07 nl of concentrated sulfuric acid were added and stirred and refluxed for 25 minutes at a temperature range of 103 to 108°C under normal pressure. 0.3 - of the water produced was removed from the yarn by azeotropy. After heating under reflux and azeotropic dehydration, 80
℃, 45.13 g (0.2 mo 1) of N(4-chlorophenyl)maleamidic acid, 0.802 g (0.005 m 1) of disodium maleate, and 0.72 J (0.2 mo 1) of dimethyl sulfoxide. 01mo
1) was added, and the imidization reaction was carried out in a temperature range of 103 to 110° C. for 3 hours while distilling 3.3 J of the produced water out of the reaction system together with toluene.

N-(
The weight ratio of 4-chlorophenyl) maleamic acid is 3
It was 5.

After completion of the reaction, the suspended disodium maleate was allowed to cool to 60°C and filtered off by suction filtration, and then toluene was removed under reduced pressure to obtain N-(4-chlorophenyl)maleimide as a yellow solid. 93g was obtained.

When analyzed by high performance liquid chromatography, the purity of this crystal was 81.2 wt%, and the yield was 93.7 mol% based on the raw material N-(4-chlorophenyl)maleamic acid.

Example 6 92ml of toluene and 2.0ml of concentrated sulfuric acid were added to a 200d flask equipped with a thermometer, a cooling tube equipped with a water separator, and a stirrer, and stirred for 25 minutes at a temperature range of 103 to 108°C under normal pressure. 0.4 ml of water produced by refluxing was removed from the system by azeotropy. After heating to reflux and azeotropic dehydration, 8
Cool to 0°C, add 44°24 g (0.2 mo 1) of N(4-methoxyphenyl)maleamic acid,
Disodium maleate 0.802g (0,005m
o l) and dimethyl sulfoxide 0,72 gloss (0,O
The imidization reaction was carried out in a temperature range of 103 to 108° C. for 3 hours while distilling the produced water (3.3 mol) out of the reaction system together with toluene.

N-(
The weight ratio of 4-methoxyphenyl) maleamic acid is 3
It was 5.

After completion of the reaction, the suspended disodium maleate was allowed to cool to 60°C and filtered off by suction filtration, and then toluene was removed under reduced pressure to obtain N-(4-methoxyphenyl)maleimide 40 in the form of a yellow solid. .64g was obtained.

When analyzed by high performance liquid chromatography, the purity of this crystal was 80.3 wt%, and the yield was 92.7 mol% based on the raw material N-(4-methoxyphenyl)maleamic acid.

Example 7 To a 200-four-piece flask equipped with a thermometer, a water separator, a condensing tube, and a stirrer, 98% of toluene and 2.0% of concentrated sulfuric acid were added at a temperature range of 103 to 110°C under normal pressure. The mixture was stirred and refluxed for 25 minutes, and 0.4 - of water produced was removed from the system by azeotropy. After heating to reflux and azeotropic dehydration, 80°C
N(2,6-dimethylphenyl)maleamidic acid 46.85 g (0.2 mol), disodium maleate 0.802 g (0,005 mo 1)
and dimethyl sulfoxide 0.72 mJ (0.01 m
The imidization reaction was carried out in a temperature range of 103 to 109° C. for 3 hours while distilling 3.4 ml of the produced water out of the reaction system together with toluene.

N-(
The weight ratio of 4-methoxyphenyl) maleamic acid is 3
It was 5.

After completion of the reaction, the suspended disodium maleate was allowed to cool to 60°C and filtered off by suction filtration, and then toluene was removed under reduced pressure to obtain N-(2,6-dimethylphenyl)maleimide as a yellow solid. 44.10 g was obtained.

When analyzed by high performance liquid chromatography, the purity of this crystal was 82.4 wt%, and the yield was 90.3 mol% based on the raw material N-(2,6-dimethylphenyl)maleamic acid. .

Example 8 Toluene 80- and concentrated sulfuric acid 2,0- were added to a 200 ml four-way flask equipped with a thermometer, a water separator, a cooling tube, and a stirrer, and the mixture was heated in a temperature range of 103 to 106°C under normal pressure for 25 minutes. 0.3ml of water produced by stirring and refluxing! was removed from the system by azeotropy. After heating to reflux and azeotropic dehydration,
Cool to 80°C, and add N-phenylmaleamic acid 3.
8.2 g (0°2 mol), calcium maleate 1.
Add 540 g (0, Olmol) and 0.72 d (0, Olmol) of dimethyl sulfoxide, and the resulting water3.
7ml! The imidization reaction was carried out in a temperature range of 103 to 110° C. for 3 hours while distilling the mixture out of the reaction system together with toluene.

The weight ratio of N-phenylmaleamic acid to the total amount of 100 charged at the start of the imidization reaction was 35.

After the reaction was completed, the mixture was allowed to cool to 60°C, and the suspended calcium maleate was filtered off by suction filtration, and then toluene was removed under reduced pressure to obtain 40.06 g of N-phenylmaleimide in the form of a yellow solid.

When analyzed by high performance liquid chromatography, the purity of this crystal was 79.9 wt%, and the yield was 92.4 mol% based on the raw material N-phenylmaleamic acid.
Met.

Example 9 The reaction was carried out in the same manner as in Example 1 except that 0.77i (0.01 mol) of N,N'-dimethylformamide was used instead of dimethyl sulfoxide. However, the amount of water distilled out during pretreatment was 0.37 nl, and the amount of water distilled out during imidization was 3.5-nl.

The weight ratio of N-phenylmaleamic acid to the total amount of 100 charged at the start of the imidization reaction was 35.

The yield of N-phenylmaleimide was 37.94 g, which was analyzed by high performance liquid chromatography.
The purity of this crystal was 85.3 wt%, and the yield was 93.4 mol% based on the raw material N-phenylmaleamic acid.

Example 10 The amount of disodium maleate added was 2.406 g (0
The reaction was carried out in the same manner as in Example 1, except that the amount was changed to However, the amount of water distilled out during pretreatment was 0.3-, and the amount of water distilled out during imidization was 3.6-.

The weight ratio of N-phenylmaleamic acid to the total amount of 100 charged at the start of the imidization reaction was 35.

The yield of N-phenylmaleimide was 36.40 g, which was analyzed by high performance liquid chromatography.
The purity of this crystal was 92.1 wt%, and the yield was 96.8 mol% based on the raw material N-phenylmaleamic acid.

Comparative Example 1 Tornile 80-1 concentrated sulfuric acid
．． 0- and N-phenylmaleamidic acid 38.2
The imidization reaction was carried out in a temperature range of 103 to 108° C. for 3 hours while adding 0.2 mo 1 of g (0.2 mo 1) and distilling the water produced under normal pressure out of the thread by azeotropy.

The weight ratio of N-phenylmaleamic acid to the total amount of 100 charged at the start of the imidization reaction was 35.

After the reaction is completed, it is left to cool to 60°C, unreacted N-phenylmaleamide acid is filtered off by suction filtration, and toluene is removed under reduced pressure to obtain N-phenylmaleimide 1 in the form of a yellow solid.
0.53 g was obtained.

When analyzed by high performance liquid chromatography, the purity of this crystal was 70.0 wt%, and the yield was 21.3 mol% based on the raw material N-phenylmaleamic acid.
Met.

Comparative Example 2 The same operation as Comparative Example 1 was performed except that p-4 toluenesulfonic acid hydrate was used instead of concentrated sulfuric acid.

The weight ratio of N-phenylmaleamic acid to the total amount of 100 charged at the start of the imidization reaction was 35.

The obtained yellow solid N-phenylmaleimide was prepared in 14.
It was 89 g.

When analyzed by high performance liquid chromatography, the purity of this crystal was 60.5 wt%, and the yield was based on the raw material N-
The amount was 26.0 mol% based on phenylmaleamic acid.

Comparative Example 3 The same operation as in Example 1 was performed except that disodium maleate and dimethyl sulfoxide were not used.

As a result, the yellow solid N-phenylmaleimide was 42
．． 40 g was obtained, and analysis by high performance liquid chromatography showed that the purity of the crystals was 68.6 wt%, and the yield was 84.0 mol% based on the raw material N-phenylmaleamic acid.

Comparative Example 4 The same operation as in Example 1 was performed except that dimethyl sulfoxide was not used.

As a result, the yellow solid N-phenylmaleimide was 36
．． 87 g was obtained and analyzed by high performance liquid chromatography, the purity of this crystal was 82.1 wt.
%, and the yield was 89.3 mol% based on the raw material N-phenylmaleamic acid.

Comparative Example 5 Example 1 except that disodium maleate was not used
The same operation was performed.

As a result, the yellow solid N-phenylmaleimide was 42
．． 20 g was obtained and analyzed by high performance liquid chromatography, the purity of this crystal was 72.9 wt.
%, and the yield was 88.8 mol% based on the raw material N-phenylmaleamic acid.

The above results are summarized in Table 1.

Claims (2)

[Claims] (1) A mixture of sulfuric acid and an aromatic solvent is azeotropically dehydrated, and then, in this mixture, in the presence of an alkali metal salt or alkaline earth metal salt of maleic acid and an aprotic polar solvent, the general formula (1) ▲There are mathematical formulas, chemical formulas, tables, etc.▼(I) (In the formula, R^1, R^2, R^3, R^4 and R^5
each independently represents a hydrogen atom, a halogen atom, a nitro group, a hydroxyl group, a carboxyl group, an alkyl group having 1 to 6 carbon atoms, an alkoxy group, a phenyl group, or a sulfone group. ) General formula (2) that involves ring-closing imidization by heating the N-substituted maleamic acid represented by , R^3, R^4 and R^5
is the same as the definition of general formula (1)). (2) The production method according to claim 1, wherein the alkali metal salt or alkaline earth metal salt of maleic acid is recovered and reused after the completion of the ring-closing imidization reaction.