PROCESS FOR PREPARING 4-SUBSTITUTED PHTHALIC ANHYDRIDES

Abstract: A process for preparing substituted phthalic anhy­ drides, e.g. 4-methylphthalic anhydride, in which the Diels-­ Alder addition product of a conjugated diene, e.g. isoprene, and maleic anhydride is reacted with bromine in the pre­ sence of an acid acceptor. The acid acceptor can be present in catalytic amounts. Typical acid acceptors include dime­ thylformamide and pyridine.

Description: PROCESS FOR PREPARING SUBSTITUTED PHTHALIC ANHYDRIDES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process for preparing substituted phthalic anhydrides. More particularly, it relates to a process for preparing 4-methylphthalic anhydride. 2. Related Art Phthalic anhydrides are valuable raw materials for making various useful products. The anhydrides are useful as intermediates in the chemical synthesis of herbicides and particularly in the synthesis of certain herbicides used to protect cereal crops. Other uses for these raw materials include polycyclic dyes, alkyd and epoxy resins, polyesters and plasticizers. Various processes are known for preparing substituted phthalic anhydrides. In two such processes 4-methyl 1,2,3, 6-tetrahydrophthalic anhydride is dehydrogenated either by sulfur or by bromine in acetic acid. Yields of 4methylphthalic anhydride of 59%-87% are claimed for the former method, while the latter gives only a 16% yield (see IZV. Akad Nauk SSSR, Ser Khim, 6, 1315(1973) English trans. Pg. 1271 and D. Craig, Journal of Am. Chem. Soc., Vol. 72, Pg. 3732(1950)). U.S. Patent No. 2,391,226 (Clifford et al., December 18, 1945) discloses addition products of chlormaleic anhydride and dichlormaleic anhydride prepared by the Diels Alder reaction and the dehydrochlorination of these products in the presence of a catalyst, such as a secondary or tertiary amine. However, the six-carbon ring is usually only partially dehydrogenated yielding a substituted dihydrophthalic anhydride. U.S. Patent No. 2,264,429 (Bergman, December 2, 1941) discloses a process for preparing substituted phthalic anhydride in a single reaction. This reaction involves the combination of the condensation reaction between a diene and maleic anhydride and the dehydrogenation reaction. This combination is achieved by carrying out the condensation reaction in nitrobenzene or another nitrated aromatic substance which not only acts as a diluent, but also as a dehydrogenating agent by reducing itself and giving the corresponding amine. This patent discloses o-nitrotoluene as another example of a nitrated aromatic substance useful in the disclosed process. SUMMARY OF THE INVENTION A process for preparing substituted phthalic anhydrides in good yields would be advantageous because of the various useful products that are prepared from these anhydrides. It is an object of the present invention to provide a unique, cost effective process for the preparation of substituted phthalic anhydrides. Other objects and advantages of the present invention are shown throughout the specification. In accordance with the present invention, it has now been discovered that substituted phthalic anhydrides can be prepared by a process which comprises reacting the Diels Alder addition product of a conjugated diene and maleic anhydride with bromine in the presence of an acid acceptor. The acid acceptor can be present in catalytic amounts. DETAILED DESCRIPTION OF THE INVENTION The present invention comprises a process for preparing substituted phthalic anhydrides by reacting the Diels-Alder addition product of a conjugated diene and maleic anhydride with bromine in the presence of an acid acceptor. The substituted phthalic anhydrides of this invention can include a substituent or the lack of a substituent at each of the four available sites on the benzene ring, i.e. the 3,4,5 and 6 carbon positions. These substituents can be selected from the group consisting of C1 to C10 alkyl, C6 to C14 aryl and C1 to C16 aralkyl and wherein the alkyl, aryl and aralkyl are optionally substituted with halogens, nitro groups, cyano groups and carboxylic groups. The process can use the Diels-Alder addition product as a starting material or can comprise a first step of actually preparing this addition product. The Diels-Alder addition product can be derived from other than the Diels Alder reaction. In addition, the double bond isomers of the Diels-Alder addition product can be used in the process of this invention. The Diels-Alder addition products of this invention are formed by reacting maleic anhydride with a conjugated diene. The conjugated diene can include butadiene, 2,3dimethylbutadiene, other substituted butadienes and preferably isoprene. The addition product can be prepared by reacting the maleic anhydride with the conjugated diene in a nitrogen atmosphere. The maleic anhydride is usually heated until it melts, then the conjugated diene is added slowly under the surface of the melt. When the addition of the diene is completed, the reactants can then be heated up to a reaction temperature of from about 550C up to about 1200C with temperatures in the upper end of the range from about 100 C up to about 12O0C being preferred. The reactants are kept within the reaction temperature range until the reaction is completed,usually for about 1 hour. The reaction can be exothermic, therefore external cooling can be required to maintain the reactants within the reaction temperature range. After the reaction to form the addition product is completed, excess diene can be stripped from the reaction zone under vacuum at a pressure which will minimize sublimation of the addition product. The reaction used to prepare the addition product can take place in the presence or absence of an appropriate solvent. However, when the acid acceptor will be added only in catalytic amounts, the absence of a solvent is preferred. An appropriate solvent can be dimethylformamide (DMF). When DMF is used, a solution of the addition product in DMF results and no solids are formed during the reaction. The bromination step can then be carried out directly upon this DMF/addition product solution after excess diene is stripped from the reaction zone. The DMF in this instance would act both as a solvent and as an acid acceptor reagent. The stoichiometry of this Diels-Alder addition reaction usually involves one mole of the maleic anhydride reacting with one mole of the conjugated diene to produce one mole of the addition product, therefore it is economically desirable to react equimolar quantities of the reactants. However, a fractional molar excess of the diene is usually used to ensure that all the maleic anhydride is consumed in the reaction. When the Diels-Alder addition product is used as a starting material and the acid acceptor is present in greater than catalytic amounts, the following procedure can be used in accordance with this invention. The addition product is made up into a solution with a suitable solvent and the acid acceptor reagent is added to this solution. Some examples of suitable solvents are chlorobenzene and DMF. When DMF is used as the solvent, a sufficient quantity is used so that DMF can also participate in the reaction as the acid acceptor reagent. Bromine can then be added slowly to the reactor. After the bromine addition begins, a hydrogen bromide/acid acceptor adduct will begin to form and may or may not precipitate from solution. The bromine addition can be mildly to moderately exothermic. During the bromine addition, the temperature in the reaction zone is maintained at from about 350C up to about 150 C. When a solvent is used in the reaction a temperature range of from about 350 C up to the boiling point of the solvent is preferred. After the bromine addition is completed, the temperature within the reaction zone can be slowly increased to a range from about 70 0C up to about 180 0C to ensure completion of the reaction. After the reaction is completed, a crude solution remains within the reactor. This crude solution can then be cooled to temperatures from about 0 0C up to about 60 0C and then water can be added. Two separate layers, an aqueous and an organic layer result, which can be separated. When DMF is used as both the solvent and the acid acceptor reagent, chloroform can be used to extract the substituted phthalic anhydride. Chloroform is then added prior to the addition of water and the aqueous layer can be washed with chloroform after the separation of layers. Chlorobenzene can also be used as an extraction and washing agent. The organic layers can then be combined, concentrated and distilled to form the desired end product, substituted phthalic anhydride. When the Diels-Alder addition product is used as a starting material and the acid acceptor is used in a catalytic amount, the following procedure can be used in accordance with this invention. The addition product is heated in a reactor having a nitrogen atmosphere until it melts. The acid acceptor catalyst is first added to the melt, then the temperature is raised to about 12O0C. Bromine can then be added slowly to the reactor under the surface of the melt. After the bromine addition begins, hydrogen bromide will slowly begin to evolve, then increase to a fairly constant rate. The bromine addition can be adjusted to such a rate that none of the characteristic bromine color is evident within the liberated hydrogen bromide. The bromine addition can be mildly to moderately exothermic. During the bromine addition, the temperature in the reaction zone is maintained at from about 100C up to about 1800C with from about 135 0C up to about 145 0C being preferred. After the bromine addition is completed, the temperature within the reaction zone can be slowly increased up to about 180 0C to ensure completion of the reaction. This is evidenced by the cessation of hydrogen bromide evolution. A bubbler containing mineral oil in the exit line can be used to monitor hydrogen bromide evolution. A crude melt remains within the reactor after the reaction is completed. This melt can be distilled directly, without an aqueous work-up, to produce a high purity substituted phthalic anhydride. This process can take place in the presence of an appropriate solvent, however the absence of a solvent is preferred. When a solvent is used an aqueous work-up can be required as an additional purification step, especially if the solvent reacts with hydrogen bromide. The acid acceptors that can be used in this process are varied, however pyridine and dimethylformamide are preferred, with DMF being most preferred. DMF is less costly than pyridine and no solids are formed during the reaction when DMF is used as both the solvent and the acid acceptor reagent. The stoichiometry of the dehydrogenation reaction usually involves one mole of the addition product reacting with two moles of bromine and four moles of the acid acceptor to produce one mole of the substituted phthalic anhydride. It is therefore economically desirable to react quantities of the reagents that are proportionate to this stoichiornet ry. The acid acceptors that can be used in this process as catalysts are varied, however pyridine and dimethylformamide are preferred, with DMF being most preferred. When DMF is the catalyst used, its residue can remain within the reactor with the residue that remains after the distillation of the substituted phthalic anhydride end product, whereas this may not be the case when pyridine is used as the catalyst. The reaction can occur without a catalyst, however the reaction rate and the purity and yield of the desired end product would be lower than when a catalyst is used. The concentration of the acid acceptor used in this process as a catalyst can range from about 0.1 to 10.0 weight percent with a range from about 1.0 to 5.0 weight percent being preferred. These weight percents are based on the weight of the Diels-Alder addition product used in the reaction. In a preferred embodiment of this invention, 4methylphthalic anhydride, (4 MPA), is prepared. This process comprises reacting 4-methyl-1,2,3,6tetrahydrophthalic anhydride, (4-MTPA), with bromine in the presence of an acid acceptor. The acid acceptors used can be dimethylformamide, (DMS), or pyridtne with DMF being most preferred. This process can be carried out iri the presence of an appropriate solvent such as chlorobenzene or even an excess of DMF. The 4-MPA resulting from this process can be distilled after an aqueous work-up/solvent extraction step. In another preferred embodiment, the acid acceptor is used only in a catalytic amount. In this embodiment, the preferred acid acceptor used can be as described above, however the absence of a solvent is preferred. The 4-MPA resulting from this process can be distilled directly from the reaction zone without the need for an aqueous work-up or other solvent extraction step. In another preferred embodiment 4-MPA can be prepared by a process which comprises a first step of reacting isoprene and maleic anhydride to form the Diels-Alder addition product, 4-MTPA, then proceeding to react the 4 MTPA with bromine in the presence of an acid acceptor whether in catalytic amount or reagent amount as described above. The following Examples describe various embodiments of the invention. Other embodiments will be apparent to one of ordinary skill in the art from a consideration of this specification or practice of the invention disclosed herein. It is intended that the specifications and Examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the Claims which follow the Examples. EXAMPLE 1 In a 1 litre 3-necked flask fitted with a dropping funnel, a mechanical stirrer, a pot thermometer and a condenser was placed 98.1 gms (1.0 mole) of maleic anhydride. The flask was heated in an oil bath until the maleic anhydride melted. Isoprene (69.5 gms, 1.02 moles) was then added dropwise to the flask below the surface of the maleic anhydride melt, using an extension tube of TEFLON fluorocarbon polymer attached to the dropping funnel, at such a rate as to minimize the reflux. The temperature of the reactants in the flask was controlled with intermittent cooling and kept between 550C and 1000C. After the addition of the isoprene was completed, the reactor flask was heated to 120 0C and maintained at that temperature for 60 minutes to ensure complete reaction. Excess isoprene was then stripped from the reactor at 100 mm/900C for 20 minutes. The faintly yellow, molten 4-methyl-1,2,3,6tetrahydrophthalic anhydride (4-MTPA, mp 580C-630C) remained in the reactor flask. EXAMPLE 2 A 2 liter 3-necked flask was fitted with a heating mantle, a pot thermometer, a 250 ml dropping funnel, a mechanical stirrer and a condenser. The flask was charged with 98.1 gms (1.0 mole) of maleic anhydride and 400 mls of dimethylformamide (DMF), then the resulting solution was heated to 550C. A total of 110 mls of isoprene (74.9 gms, 1.1 moles) was added dropwise to the flask while maintaining the temperature of the reactants within the flask between 550C to 900C by means of intermittent cooling in a cold water bath. After the addition of isoprene was completed, the solution in the reactor flask was heated to the 900C to 0 100 C range and maintained at that temperature for 60 minutes to ensure complete reaction. The solution was then cooled to 500C and then excess isoprene was stripped from the reactor at 50 mm Hg for 30 minutes. After stripping, the remaining solution was returned to atmospheric pressure, the temperature was adjusted to 600C, and bromine was added dropwise. The temperature of the solution began to increase with the bromine addition and was allowed to reach a temperature range of between 80 0C to 900C. This temperature range was maintained throughout the 2 hours bromine addition by gentle cooling. After the bromine addition was completed, the dark solution that resulted was heated to a temperature range of between 1100C to 1200C and maintained at that temperature for 1 hour. The solution was cooled to below 600C, diluted with 400 mls of chloroform, and then further cooled to 200C. The stirred solution was treated with 400 mls of water and the aqueous phase was separated and extracted again with 400 mls of chloroform. The combined chloroform layers were washed with 600 mls of water and then stripped to a brown crystalline residue via a rotary evaporator. The crude residue was vacuum distilled using a 3" Vigreaux column and a short, uncooled condenser to give 132.3 gms (82% yield based on maleic anhydride) of a yellow liquid, 4-MPA, which later solidified (bp 150 C-160 C/lO mm, mp 750C-890C). EXAMPLE 3 A solution of 16.6 gms (0.10 mole) of 4-MTPA in 60 mls of chlorobenzene and 32.7 mls (32 gms, 0.404 moles) of pyridine was prepared in a 3-necked flask. Bromine (10.3 mls, 32.1 gms, 0.202 moles) was added dropwise to this solution. The temperature of the reactants began to increase with the bromine addition and was allowed to reach a temperature range of between 300C to 42or. This temperature range was maintained until the bromine addition was completed, then the reactants were heated to 70 0C and maintained at this temperature for 30 minutes. The reactants were cooled to 20C and 100 mls of water added to the flask. Two phases formed which were separated, and the aqueous phase was further extracted with two 25 mls portions of chlorobenzene. The combined organic fractions were concentrated under high vacuum to leave 14.1 gms (87% yield) of a yellow solid. This yellow solid was virtually pure 4-MPA when analyzed by g.c. and NMR. EXAMPLE 4 The following example represents a generalized procedure. Molten 4-MTPA (166.1 gms, 1.0 mole) was prepared in a 3-necked flask without solvent as described above in Example 1. Dimethylformamide (DMF, 5.0 gms, 0.068 moles) was added to the flask. The content of the flask was then heated to 120 0C and stirred mechanically while bromine (103.5 mls, 322.8 gms, 2.02 miles) was added dropwise under the surface of the melt. The reactor temperature increased to approximately 1400C and was maintained at between 135 0C to 1450C throughout the 2.5 to 3.5 hours bromine addition by means of an oil bath termostatically controlled at 1400C to 145 0C. Hydrogen bromide began to evolve a few minutes after the bromine addition began and was neutralized in a trap trtiliing a sodium hydroxide solution. The bromine addition rate was adjusted so that no bromine color was observed in the hydrogen bromide liberated. After the addition of the bromine was completed, the dark brown melt was maintained at 14000 for 15 minutes, then slowly heated to 1800C to complete the liberation of hydrogen bromide. After 1 hour at 1800C, the crude brown melt remaining was distilled from the flask using a 3" Vigreaux column and a short, uncooled condenser. The pale yellow distillate was collected quickly at bp 153 C-157 C/ 7 mm. After cooling, a pale yellow to white solid product, 4-methylphthalic anhydride (4-MPA), resulted. The 4-MPA weighed between 124-130 g (76%-80% yield), with a melting point of 180C-890C, and an assay by hplc of 95% purity. EXAMPLE 5 Molten 4-MTPA (166.1 gms, 1.0 mole) was prepared as described in Example 1. Pyridine (8.5 ml, 8.3 gms, 0.105 mole) was added to the flask. The mechanically stirred cotltents of the flask were maintained at 1050C-1200C as bromine (103.5 ml, 322.8 gms, 2.02 moles) was added dropwise under the surface of the melt over 3.5 hrs. Hydrogen bromide evolved continuously. After the addition was completed, the reactor flask temperature was maintained at 1100C for 2 hrs. Gc analysis showed that the reaction was incomplete, so another 10 mls of bromine was added at 110 0C-1200C over 40 min. The dark brown melt was then heated to 15O0-1550C for 4 hrs to complete the liberation of hydrogen bromide. The crude melt was distilled under vacuum as in Ex < ,nple 4. After a small forerun, the main fraction was collected at bp 1290C-1310C/1.5 mm. The white solid, 4-MPA, obtained on cooling weighed 110.7 gms (62% yield) and was 90% pure by hplc assay.

Claims: What is Claimed is: 1. A process for preparing a substituted phthalic anhydride which comprises reacting the Diels-Alder addition product of a conjugated diene and maleic anhydride with bromine in the presence of an acid acceptor. 2. The process of Claim 1 wherein said conjugated diene is isoprene. 3. The process of Claim 1 wherein the acid acceptor is dimethylformamide. 4. The process of Claim 1 wherein the acid acceptor is pyridine. 5. The process of Claim 1 wherein the reaction is carried out in the presence of chlorobenzene as a solvent. 6. The process of Claim 1 wherein a catalytic amount f acid acceptor is used. 7. The process of Claim 6 wherein said conjugated diene is isoprene. 8. The process of Claim 6 wherein the acid acceptor is dimethylformamide. 9. The process of Claim 6 wherein the acid acceptor is pyridine. 10. The process of Claim 6 wherein the reaction is carried out in the absence of a solvent. 11. The process of Claim 10 wherein the substituted phthalic anhydride is distilled directly. 12.A process for preparing 4-methylphthalic anhydride which comprises reacting 4-methyl-1,2,3,6-tetrahydrophthalic anhydride with bromine in the presence of an acid acceptor. 13. The process of Claim 12 wherein the acid acceptor is dimethylformamide. 14. The process of Claim 12 wherein the acid acceptor is pyridine. 15. The process of Claim 12 wherein the reaction is carried out in the presence of chlorobenzene as a solvent. 16. The process of Claim 12 wherein a catalytic amount of acid acceptor is used. 17. The process of Claim 16 wherein the acid acceptor is dimethylformamide. 18. The process of Claim 16 wherein the acid acceptor is pyridine. 19. A process for preparing a substituted phthalic anhydride which comprises: a) reacting a conjugated diene and maleic anhydride to form a Diels-Alder addition product; b) reacting the addition product with bromine in the presence of an acid acceptor. 20. The process of Claim 19 wherein the diene is isoprene. 21. The process of Claim 19 wherein the acid acceptor is dimethylformamide. 22. The process of Claim 19 wherein the acid acceptor is pyridine. 23. The process of Claim 22 wherein step b) is carried out in the presence of chlorobenzene as a solvent. 24. The process of Claim 19 wherein steps a) and b) are carried out in the presence of a solvent. 25. The process of Claim 19 wherein a catalytic amount of acid acceptor is used. 26. The process of claim 25 wherein steps a) and b) are carried out in the absence of a solvent. 27. The process of Claim 26 wherein the substituted phthalic anhydride is distilled directly. 28. The process of Claim 25 wherein the diene is isoprene. 29. The process of Claim 25 wherein the acid acceptor is dimethylformamide. 30. The process of Claim 25 wherein the acid acceptor is pyridine. 31. A process for preparing 4-methylphthalic anhydride which comprises: a) reacting isoprene and maleic anhydride to form 4 methyl-1,2,3,6-tetrahydrophthalic anhydride; b) reacting the 4-methyl-1,2,3,6-tetrahydrophthalic anhydride with bromine in the presence of an acid acceptor. 32. The process of Claim 31 wherein the acid acceptor is dimethylformamide. 33. The process of Claim 31 wherein the acid acceptor is pyridine. 34. The process of Claim 33 wherein step b) is carried out in the presence of chlorobenzene as a solvent. 35. The process of Claim 31 wherein a catalytic amount of acid acceptor is used. 36. The process of Claim 35 wherein the acid acceptor is dimethylformamide. 37. The process of Claim 35 wherein the acid acceptor is pyridine. 38. The process of Claim 36 wherein steps a) and b) are carried out in the absence of a solvent. 39. The process of Claim 38 wherein the 4methylphthalic anhydride is distilled directly.

Inventors: TELSCHOW, JEFFREY E.
Applicants: STAUFFER CHEMICAL COMPANY
Application year: 1985
Main classification: C07D307
IPC codes: C07C51, C07D307