Process for the preparation of dimethylmaleic anhydride

The reaction of maleic acid, fumaric acid and/or maleic anhydride in the presence of N-acylated heterocyclic amidines and at elevated temperature affords dimethylmaleic anhydride in good yield. Catalytic amounts of the amidine employed are sufficient for said reaction.

The present invention relates to a process for the preparation of 
dimethylmaleic anhydride by reacting maleic acid, fumaric acid, maleic 
anhydride or mixtures thereof in the presence of N-acylated heterocyclic 
amidines or salts thereof and at elevated temperature. 
Processes for the preparation of dimethylmaleic anhydride from 2 mol of 
maleic anhydride, maleic acid and/or fumaric acid are known from German 
published applications Nos. 2 233 862 and 2 233 889, said reaction being 
carried out at elevated temperature and in the presence of at least 1 mol 
of amidine containing a primary or secondary N atom. Good yields can only 
be obtained if the reaction mixture is subjected to acid hydrolysis. 
Furthermore, the large amount of amidine used is considered 
disadvantageous. 
Accordingly, the present invention relates to a process for the preparation 
of dimethylmaleic anhydride by reacting 2 equivalents of maleic acid, 
fumaric acid and/or maleic anhydride in the presence of an amidine, an 
amidine salt or a mixture thereof and at a temperature of at least 
90.degree. C., in which process the amidine is of formula I and the 
amidine salt of formula II 
##STR1## 
in which formulae R.sup.1 is acyl and R.sup.2 is a hydrogen atom, an 
aliphatic or aromatic hydrocarbon radical or acyl or R.sup.1 and R.sup.2 
together are the diacyl radical of a 1,2-dicarboxylic acid, Y is the anion 
of an inorganic or organic protonic acid and n is an integer from 1 to 3, 
and X, together with the group 
##STR2## 
forms the radical of a substituted or unsubstituted 5- or 6-membered 
heterocyclic ring which may contain further hetero atoms. 
In the process of the present invention, it is preferred to employ maleic 
acid, maleic anhydride or 1:1 mixtures (molar ratio) thereof. 
Radicals of an unsubstituted or further substituted 5- or 6-membered 
heterocyclic ring which may contain further hetero atoms, which radicals 
are formed by X together with the group 
##STR3## 
are e.g. imidazolyl, pyrazolyl, triazolyl, thiazolyl, isothiazolyl, 
oxadiazolyl, thiadiazolyl, oxazolyl, pyridinyl, pyridazinyl, pyrimidinyl, 
pyrazinyl and triazinyl radicals. 
If these radicals are further substituted, they may contain for example 
halogens such as fluorine, chlorine or bromine, phenyl groups, alkyl or 
alkoxy groups having 1 to 4 carbon atoms, amino groups, monoalkylamino or 
dialkylamino groups having 1 to 4 carbon atoms in each alkyl moiety, or 
hydroxyl groups, or they may be condensed with further monocyclic or 
heterocyclic rings. Preferred substituents are halogen, C.sub.1 -C.sub.4 
-alkyl and C.sub.1 -C.sub.4 alkoxy. Examples of condensed 5- or 6-membered 
heterocyclic ring systems are: benzimidazole, benzothiazole, benzoxazole, 
pterin, purine, quinoline, isoquinoline, naphthyridine, phthalazine, 
cinnoline, quinazoline and quinoxaline. 
Radicals or a 5- of 6-membered heterocyclic ring which are formed by X 
together with the group 
##STR4## 
are preferably not further substituted. The heterocyclic ring is 
preferably a 2-thiazolyl radical, especially the 2-pyridinyl radical. 
The amidines of formula I are known or may be prepared in a manner known 
per se by acylating compounds of formula Ia 
##STR5## 
wherein X is as defined for formula I and R.sup.2' is a hydrogen atom or 
an aliphatic or aromatic hydrocarbon radical as defined for R.sup.2. 
Examples of suitable starting amidines of formula Ia are: 
2-aminoimidazole, 2-aminobenzimidazole, 3-aminopyrazole, 
3-amino-5-methylpyrazole, 3-amino-4-bromo-5-methylpyrazole, 
3-amino-1-phenylpyrazole, 3-amino-1,2,4-triazole, 
3,5-diamino-1,2,4-triazole, 4-amino-1,2,3-triazole, 2-amino-1,3-thiazole, 
3-aminoisothiazole, 2-amino-5-chlorothiazole, 2-amino-4-phenylthiazole, 
2-aminobenzothiazole, 2-amino-6-bromobenzothiazole, 
2-amino-4,6-dibromobenzothiazole, 3-amino-4-phenylfurazan, 
3-amino-4-methylfurazan, 3-aminoisoxazole, 2-aminooxazole, 
2-aminobenzoxazole, 2-aminopyridine, 2-amino-3-methylpyridine, 
2-amino-4-methylpyridine, 2-amino-6-methylpyridine, 
2-amino-5-bromopyridine, 2-amino-6-bromopyridine, 
2-amino-5-chloropyridine, 2-amino-3,5-dibromopyridine, 
2-amino-3,5-dichloropyridine, 2-amino-3-methylaminopyridine, 
2,6-diaminopyridine, 2,3-diaminopyridine, 2-aminopyrazine, 
2-aminopyrimidine, 6-amino-2-chloropyrimidine, 6-amino- 
2,4-dimethylpyrimidine, 2-amino-5-bromo-4,6-dimethylpyrimidine, 
2-amino-6-chloropyrimidine, 2-amino-4,6-dichloropyrimidine, 
6-amino-2,4-dichloropyrimidine, 2-amino-4,6-dimethylpyrimidine, 
4,6-diaminopyrimidine, 6-amino-4-methylpyrimidine, 3-aminopyridazine, 
2-amino-1,3,5-triazine, 2,4,6-triamino-1,3,5-triazine, 
2-amino-4,6-dichloro-1,3,5-triazine, 2-amino-4,6-dimethyl-1,3,5-triazine, 
4-amino-6-hydroxy-2-methyl-1,3,5-triazine, 
2,4-diamino-6-methyl-1,3,5-triazine, 8-aminopurine, 2-aminopurine, 
6-aminopurine (adenine), 2-amino-6-bromopurine, 2-amino-6-chloropurine, 
6-amino-2,8-dichloropurine, 8-amino-2,6-dichloropurine, 
6-amino-2-methylpurine, 2,8-diaminopurine, 6,8-diaminopurine, 
7-methyl-2,6,8-triaminopurine, 1-aminoisoquinoline, 2-aminoquinoline, 
2,4-diaminoquinoline, 2-amino-1,7-naphthyridine, 
2-amino-1,5-naphthylidine, 2-amino-6,7-dimethyl-1,8-naphthyridine, 
2-aminoquinoxaline, 2,3-diaminoquinoxaline, 4-aminoquinazoline. 
If amidine salts of formula II are employed in the process of the present 
invention, then n is an integer from 1 to 3 and Y is preferably the anion 
of formic acid, acetic acid, propionic acid, hydrochloric acid, 
hydrobromic acid, sulfuric acid or phosphoric acid. However, Y is most 
preferably the anion of an aliphatic monocarboxylic acid having 2 to 4 
carbon atoms, in particular acetic acid (n=1). These salts can be prepared 
in conventional manner by treating the amidine of formula I with the 
corresponding acid. This preparation can be effected direct in situ or the 
isolated salt can be used for the reaction. 
R.sup.2 is an aliphatic hydrocarbon radical is preferably branched or, in 
particular, linear C.sub.1 -C.sub.12 -alkyl, preferably C.sub.1 -C.sub.6 
alkyl, or C.sub.5 -C.sub.7 -cycloalkyl and as an aromatic hydrocarbon 
radical is C.sub.6 -C.sub.12 aryl, C.sub.7 -C.sub.16 aralkyl, C.sub.7 
-C.sub.16 alkaryl or C.sub.8 -C.sub.16 alkaralkyl. Examples are methyl, 
ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, pentyl, hexyl, 
octyl, decyl, dodecyl, cyclopentyl, cyclohexyl, cycloheptyl, phenyl, 
naphthyl, benzyl, 2-phenylethyl, methylphenyl, ethylphenyl and 
methylbenzyl. R.sup.2 is preferably a halogen atom. 
R.sup.1 and R.sup.2 as acyl preferably contain 1 to 12 carbon atoms. The 
acyl radical may be of the formula R.sup.3 CO, wherein R.sup.3 is linear 
or branched alkyl preferably containing 1 to 6 carbon atoms, cycloalkyl 
preferably containing 5 to 7 ring carbon atoms, aryl containing 6 to 12 
carbon atoms or aralkyl containing 7 to 12 carbon atoms. Examples of acyl 
are formyl, acetyl, propionyl and benzoyl, with acetyl and propionyl being 
particularly preferred. 
If R.sup.1 and R.sup.2 together are the diacyl radical of a 
1,2-dicarboxylic acid, said radical, together with the N atom to which 
R.sup.1 and R.sup.2 are attached, forms a 5-membered imide ring. The 
diacyl radical may for example be substituted by C.sub.1 -C.sub.12 alkyl, 
halogen, preferably chlorine, or by cyano or phenyl and may correspond to 
the formula --CO--R.sup.4 --CO--, wherein R.sup.4 is a divalent aliphatic 
or aromatic hydrocarbon radical to which the two carbonyl groups are 
attached in the 1,2-position. Examples of suitable diacyl radicals are 
1,2-cyclohexylene, 1,2-cyclopentylene, ethylene, ethenylene and 
1,2-phenylene. Examples of 1,2-dicarboxylic acids from which the diacyl 
radical is derived are 1,2-cyclopentanedicarboxylic and 
1,2-cyclohexanedicarboxylic acid, succinic acid, alkylated succinic acids 
such as methylsuccinic, dimethylsuccinic, ethylsuccinic, propylsuccinic, 
isopropylsuccinic, butylsuccinic, pentylsuccinic, hexylsuccinic, 
octylsuccinic, decylsuccinic, dodecylsuccinic, phenylsuccinic or 
diphenylsuccinic acid, maleic acids such as maleic, methylmaleic, 
dimethylmaleic, ethylmaleic, propylmaleic, phenylmaleic, diphenylmaleic, 
cyanomaleic, 1-phenyl-2-methylmaleic and 1-methyl-2-cyanomaleic acid, 
phthalic acid, chlorophthalic acid, dichlorophthalic acid, 
tetrachlorophthalic acid or tetrabromophthalic acid. Most preferably, 
R.sup.1 and R.sup.2 together as diacyl radical are derived from phthalic 
acids, maleic acids or succinic acids. 
The reaction of the present invention may be carried out in an organic 
solvent which is inert to the reactants. Examples of such solvents are 
unsubstituted or chlorinated aromatic hydrocarbons, e.g. benzene, toluene, 
xylenes, chlorobenzene or dichlorobenzenes, dialkyl sulfoxides, e.g. 
dimethyl sulfoxide, methyl cellosolve, hexamethylphosphoric triamide, 
N,N-dialkylamides of a lower monocarboxylic acid, e.g. dimethylformamide 
or dimethylacetamide, or lower dialkyl esters of carbonic acid, e.g. 
dimethyl carbonate or diethyl carbonate. Mixtures of such solvents may 
also be employed. If the amidine salt of the general formula II is 
prepared direct in situ, the acid used, e.g. an aliphatic C.sub.2 -C.sub.4 
carboxylic acid, in particular acetic acid, may also be employed as 
solvent. 
In accordance with a preferred embodiment, the reaction of the invention is 
carried out without addition of a solvent or, in particular, in anhydrous 
acetic acid. 
The reaction temperature is preferably in the range from 90.degree. to 
200.degree. C., most preferably from 110.degree. to 180.degree.0 C. The 
reaction may, if appropriate, be carried out under pressure. 
A buffer compound, e.g. an alkali metal acetate such a sodium acetate, may 
also be added to the reaction mixture. If maleic anhydride is employed 
alone, it is convenient to add water, advantageously in an amount of 0.5 
to 20% by weight, based on the amount of maleic anhydride employed. 
The compounds of formulae I and II may be employed in amounts of up to at 
least 1 mol per 2 mol of maleic acid, fumaric acid and/or maleic 
anhydride. Surprisingly, it has been found that also the use of catalytic 
amounts of amidine of formula I or amidine salt of formula II is 
sufficient and that also when employing preferably 1 to 20 mol%, 
especially 1 to 10 mol% and, most preferably, 5 to 10 mol%, based on the 
reactants, the reaction affords high yields. The use of catalytic amounts 
is therefore preferred. 
The isolation and purification of the reaction product are effected by 
conventional methods, e.g. distillation, steam distillation, extraction or 
crystallisation. It is a particular advantage of the process of the 
present invention that the reaction product can be isolated direct, 
without having to effect acid hydrolysis, so that high yields can be 
obtained. In this process, the amidine compounds can be recovered in 
quantitative yield. 
Dimethylmaleic anhydride is a valuable intermediate for the preparation of 
light-sensitive polymers containing dimethylmaleic imidyl groups (q.v. 
German published application No. 2 626 769).

The invention is illustrated in more detail by the following Examples. 
Percentages are by weight. 
EXAMPLE 1 
116 g (1.0 mol) of maleic acid and 20.2 g (0.1 mol) of 
N-(pyrid-2-yl)-3,4-dimethylmaleinimide are boiled under reflux for 48 
hours in 300 ml of glacial acetic acid. The residue is then steam 
distilled. The distillate is filtered and the filtrate is dried, affording 
42.8 g (48%) of dimethylmaleic anhydride with a melting point of 
91.degree.-93.degree. C. By extracting the residual aqueous phase with 
ether, a further 7.89 g (12.3%) of dimethylmaleic anhydride are isolated. 
EXAMPLE 2 
98.0 g (1.0 mol) of maleic anhydride are added in portions over 1 hour to a 
boiling solution of 20.2 g (0.1 mol) of 
N-(pyrid-2-yl)-3,4-dimethylmaleinimide and 9 ml (0.5 mol) of water in 400 
ml of glacial acetic acid. The mixture is subsequently boiled under reflux 
for 22 hours. Working up as indicated in Example 1 affords 52.4 g (63%) of 
dimethylmaleic anhydride with a melting point of 92.degree.-93.degree. C. 
EXAMPLE 3 
A solution of 98 g (1.0 mol) of maleic anhydride in 300 ml of glacial 
acetic acid is added dropwise over 20 minutes to a boiling solution of 
20.2 g (0.1 mol) of N-(pyrid-2-yl)-3,4-dimethylmaleinimide in 100 ml of 
glacial acetic acid. Subsequently, a solution of 9 ml (0.5 mol) of water 
in 50 ml of glacial acetic acid is added dropwise over 11/2 hours and the 
mixture is boiled further for 20 hours. Working up as indicated in Example 
1 affords 50.8 g (60.6%) of dimethylmaleic anhydride with a melting point 
of 91.degree.-93.degree. C. 
EXAMPLE 4 
A solution of 58.0 g (0.5 mol) of maleic acid, 49.0 g (0.5 mol) of maleic 
anhydride and 20.2 g (0.1 mol) of N-(pyrid-2-yl)-3,4-dimethylmaleinimide 
in 300 ml of glacial acetic acid is boiled under reflux for 6 hours. The 
glacial acetic acid is removed and the residue is then heated for 15 
minutes to 150.degree. C. After subsequent steam distillation of the 
residue, the resultant dimethylmaleic anhydride is isolated as indicated 
in Example 1. Yield: 37.1 g (39%). 
EXAMPLES 5-14 
A solution of 58 g (0.5 mol) of maleic acid, 49 g (0.5 mol) of maleic 
anhydride and the amidine or amidine salt are dissolved in 300 ml of 
glacial acetic acid and the mixture is heated under reflux for 48 hours. 
The glacial acetic acid is removed, the residue is then steam distilled 
and the resultant dimethylmaleic anhydride (DMA) is isolated in accordance 
with Example 1. Further details are given in the following Table. 
______________________________________ 
Yield 
Ex- DMA (Per- 
am- Amount centage 
ple Amidine or amidine salt 
(mol %) by weight) 
______________________________________ 
5 
##STR6## 10 18.7 
6 
##STR7## 10 22.8 
7 
##STR8## 10 49 
8 
##STR9## 10 46.7 
9 
##STR10## 50 (Test 1) 50 (Test 2) 
8.7 15.0 
10 
##STR11## 10 33 
11 
##STR12## 50 (Test 1) 50 (Test 2) 
7.3 30.5 
12 
##STR13## 10 14.6 
13 
##STR14## 10 9.8 
14 
##STR15## 10 23.8 
______________________________________