Preparation of 3,5-diaminobenzotrifluoride

Abstract: 3,5-Diaminobenzotrifluoride can be produced, in a single step, by reacting 4-chloro-3,5-dinitrobenzotrifluoride, in methanol, with hydrogen gas, in the presence of magnesium oxide, and in the presence of a catalyst comprising palladium on a carbon support.

Description: This invention relates to a process for the preparation of 3,5-diaminobenzotrifluoride by the reduction of 4-chloro-3,5-dinitrobenzotrifluoride using hydrogen as a reducing agent in the presence of palladium on carbon as a catalyst. Since 4-chloro-3,5-diaminobenzotrifluoride is difficult to separate from 3,5-diaminobenzotrifluoride itself, it is a particular object of the present invention to provide a process which produces 3,5-diaminobenzotrifluoride without significant production of 4-chloro-3,5-diaminobenzotrifluoride. The reduction of aromatic nitro compounds containing halogen on the aromatic ring is unpredictable. Hydrogenation using a palladium on carbon catalyst generally reduces the nitro group to an amine. However, the hydrogenation has been reported to fail in some cases. In addition, the effects of such hydrogenation upon a ring halogen are unpredictable. Occasionally the ring halogen is removed from the ring and replaced by a hydrogen. However, in many cases the hydrogenation leaves the ring halogen intact. EP-A-038465 discloses a method for preparing 2-trifluoromethylaniline by catalytic reduction and dechlorination of 2-trifluoromethyl-4-chloronitrobenzene with molecular hydrogen in a one-step reaction. W. B. Whalley in The Journal of the Chemical Society 1949, pages 3016 to 3020 discloses the preparation of dihydroxy and hydroxymethoxybenzotrifluorides. The scientific paper describes the reduction of 3:5-dinitrobenzotrifluoride to the respective diamine compound by hydrogenation and the reduction of 5-chloro-2-nitrobenzofluoride to the amine compound by reaction with tin and hydrochloric acid. Surprisingly, it has now been found that 3,5-diaminobenzotrifluoride can be produced, in a single step, by reacting 4-chloro-3,5-dinitrobenzotrifluoride, in methanol, with hydrogen gas and magnesium oxide, in the presence of a catalyst comprising palladium on a carbon support. 3,5-Diaminobenzotrifluoride is a valuable intermediate used in the synthesis of polyimide polymers. The present process is advantageous in that the starting material, 4-chloro-3,5-dinitrobenzotrifluoride is commercially available. It is an object of the present invention to provide a process that produces 3,5-diaminobenzotrifluoride (DABTF) containing a very low level of impurities and particularly very little, if any, 4-chloro-3,5-diaminobenzotrifluoride (CDABTF). It is desirable that the final product contain no more than 0.5% of the 4-chloro-3,5-diaminobenzotrifluoride side product. 4-Chloro-3,5-diaminobenzotrifluoride is a particularly troublesome impurity since it is very difficult to separate from the desired 3,5-diaminobenzotrifluoride. The vapor pressure difference between 4-chloro-3,5-diaminobenzotrifluoride and 3,5-diaminobenzotrifluoride is small, and therefore, separation by distillation is difficult. In addition, the chloro compound appears to be more volatile than the diamino compound itself, and accordingly, any attempt to distill crude 3,5-diaminobenzotrifluoride containing a small percentage of 4-chloro-3,5-diaminobenzotrifluoride produces a distillate richer in the 4-chloro product. Other processes that might be used include multi-step recrystallizations and multiple extraction. These latter two processes are expensive and therefore not preferred. It is a further object of the proposed invention to conduct the reaction in a solvent, such as methanol, that can be readily removed from the reaction mixture, and easily recycled. In addition, a solvent such as methanol will tend to keep water in solution rather than allowing it to separate. There are many advantages to having the reaction conducted in a homogeneous phase, rather than in a two-phase system. 3,5-Diaminobenzotrifluoride may be produced, in a single step, by reacting 4-chloro-3,5-dinitrobenzotrifluoride (CDNBTF) in methanol, with hydrogen gas and magnesium oxide, in the presence of a catalyst comprising palladium on a carbon support. It is also preferable to handle the catalyst wet. The preferred form for the catalyst is 2 to 10% on a carbon support. The most preferred catalyst is 5% palladium on carbon. A particular advantage of using a palladium on carbon catalyst is that the palladium can be easily recovered when the catalyst loses effectiveness. While the reaction can be conducted with widely varying amounts of catalyst, obviously, the reaction tends to be slow if insufficient catalyst is used. For the preferred catalyst (5% Pd on carbon), the reaction can be run at a wide range of catalyst levels. Concentration as low as about 0.5 g of catalyst (dry weight) per 100 g of 4-chloro-3,5-dinitrobenzotrifluoride can be used. The preferred range is 4 to 6 g of catalyst (dry weight) per 100 g of 4-chloro-3,5-dinitrobenzotrifluoride. The magnesium oxide base not only serves to speed up the reaction, but, as will be set forth more fully below, influences the actual course of the reaction. Although we do not wish to be bound by theory, it is possible that a base speeds up the reaction by reacting with the hydrogen chloride formed when the chlorine is removed from the ring by attack of the hydrogen. In the absence of a base, the hydrogen chloride produced tends to poison the palladium catalyst. This diminishes the activity of the catalyst, and leads to incomplete conversion of starting material to product. As shown in the Comparative Examples, the use of bases other than magnesium oxide leads to side reactions when methanol is the solvent. As set forth below, methanol is an advantageous solvent for this reaction. Thus, it is an advantage of magnesium oxide as a base that one can conduct the reaction in methanol. The use of methanol is advantageous since it has a rather low boiling point and can be readily removed when the reaction is completed. In addition, methanol can be readily recycled because it does not form an azeotrope with water and thus, can be separated from the water produced in the reaction by distillation. The reaction is conducted at moderate pressure; that is, about 3.1-6.3 bar (30 to 75 psig). At the lower end, the pressure is not critical. Although the reaction can be run at pressures less than 3.1 bar (30 psig), such reactions tend to be slow. Accordingly, it is preferred to run the reaction at about 4 bar (40 psig). A total pressure of 6.3 bar (75 psig) is not an upper limit on the useable pressure, but is rather the upper end of the preferred range of operation. To speed the rate of hydrodechlorination, the reaction should be run up to about 22 bar (300 psig). The hydrogen pressure can be approximated by subtracting the known vapor pressure of methanol from the total pressure. The reaction can be conducted at moderate temperatures between room temperature and about 125°C. If the reaction is run at a temperature much lower than room temperature, it tends to be too slow to be useful. In practice, we have found it convenient to mix the reactants at about room temperature and allow the heat of the reaction to warm the reaction mixture to a temperature of about 80-90°C. The temperature can be controlled by slowing down the feed rate of hydrogen. After the initial phase of the reaction, the temperature is held at about 80-90°C for a period of one to two hours. In larger reactors, depending upon the nature of the heat exchange available, a slower rate of hydrogen addition may be required to keep the temperature in the range of about 80-90°C. In such cases the reaction will take somewhat longer. It is important to watch the temperature carefully since methanol has an appreciable vapor pressure at temperatures above 125°C and this added pressure can create problems. The preferred method for isolating and purifying 3,5-diaminobenzotrifluoride is solvent evaporation followed by distillation. Methods such as recrystallization and other methods well-known to those skilled in the art can be used, but are not preferred. EXAMPLES EXAMPLE 1 A solution of 4-chloro-3,5-dinitrobenzotrifluoride (10 g) in methanol (100 mL) was charged in a Parr hydrogenator bottle and mixed with magnesium oxide (1.6 g - two equivalents based upon 4-chloro-3,5-dinitrobenzotrifluoride) and with a carbon supported palladium catalyst (5% Pd on carbon containing 50% water, 1.2 g wet weight). After purging with nitrogen, hydrogen was charged periodically in the reaction bottle to maintain its pressure at 3.8-4.5 bar (40-50 psig). The reaction temperature began at approximately 20°C and began to rise as soon as the agitation began. In eight minutes, the temperature rose to 87°C and was held at 80°C for 1.5 hours. The reaction mixture was then allowed to cool to room temperature over approximately 42 minutes. After the reaction, analysis of the reaction mixture by gas chromatography showed formation of 6.29 g of 3,5-diaminobenzotrifluoride and no 4-chloro-3,5-diaminobenzotrifluoride was detectable. The yield of 3,5-diaminobenzotrifluoride was 97% based upon starting material. EXAMPLE 2 Example 1 was repeated using 3.0 g magnesium oxide (four equivalents based upon 4-chloro-3,5-dinitrobenzotrifluoride). The temperature rose to 72°C in fourteen minutes. The reaction mixture was held at 75°C for twenty-two minutes and then allowed to cool to room temperature over a period of 28 minutes. After the reaction, analysis of the reaction mixture by gas chromatography showed formation of 6.1 g of 3,5-diaminobenzotrifluoride and 0.28 g of 4-chloro-3,5-diaminobenzotrifluoride. The yield of 3,5-diaminobenzotrifluoride was 94% based upon starting material. EXAMPLE 3 Example 1 was repeated using 20 g of 4-chloro-3,5-dinitrobenzotrifluoride in 200 mL methanol with 3.2 g magnesium oxide (two equivalents based upon 4-chloro-3,5-dinitrobenzotrifluoride). In eighteen minutes, the temperature rose to 94°C and was held at 80°C for 2 hours. The reaction mixture was then allowed to cool to room temperature over approximately 85 minutes. After the reaction, analysis of the reaction mixture by gas chromatography showed formation of 12.9 g of 3,5-diaminobenzotrifluoride and 0.06 g of 4-chloro-3,5-diaminobenzotrifluoride. The yield of 3,5-diaminobenzotrifluoride was 99% based upon starting material. EXAMPLE 4 Example 1 was repeated using 0.8 g magnesium oxide (one equivalent based upon 4-chloro-3,5-dinitrobenzotrifluoride). In seven minutes, the temperature rose to 84°C and was held at 80°C for 73 minutes. The reaction mixture was then allowed to cool to room temperature over approximately 30 minutes. After the reaction, analysis of the reaction mixture by gas chromatography showed formation of 5.81 g of 3,5-diaminobenzotrifluoride and 0.04 g of 4-chloro-3,5-diaminobenzotrifluoride. The yield of 3,5-diaminobenzotrifluoride was 89% based upon starting material. EXAMPLE 5 Example 1 was repeated using 60 mL methanol. In thirteen minutes, the temperature rose to 80°C and was held at 80°C for 1.5 hours. The reaction mixture was then allowed to cool to room temperature and removed in approximately 108 minutes. After the reaction, analysis of the reaction mixture by gas chromatography showed formation of 5.65 g of 3,5-diaminobenzotrifluoride and 0.08 g of 4-chloro-3,5-diaminobenzotrifluoride. The yield of 3,5-diaminobenzotrifluoride was 87% based upon starting material. EXAMPLE 6 Example 5 was repeated using 0.8 g wet weight of the carbon supported palladium catalyst. In six minutes, the temperature rose to 80°C and was held at 80°C for 1 hours. The reaction mixture was then allowed to cool to room temperature over approximately 30 minutes. After the reaction, analysis of the reaction mixture by gas chromatography showed formation of 5.84 g of 3,5-diaminobenzotrifluoride and 0.48 g of 4-chloro-3,5-diaminobenzotrifluoride. The yield of 3,5-diaminobenzotrifluoride was 90% based upon starting material. EXAMPLE 7 Example 1 was repeated using 0.8 g wet weight of the carbon supported palladium catalyst. In ten minutes, the temperature rose to 83°C and was held at 80°C for 105 minutes. The reaction mixture was then allowed to cool to room temperature over approximately 32 minutes. After the reaction, analysis of the reaction mixture by gas chromatography showed formation of 6.23 g of 3,5-diaminobenzotrifluoride and 0.03 g of 4-chloro-3,5-diaminobenzotrifluoride. The yield of 3,5-diaminobenzotrifluoride was 96% based upon starting material. Comparative Examples 8 and 9 Example 1 was repeated using ethanol and 1-propanol. The following table gives the results. Example Solvent DABTF (g) CDABTF (g) DABTF Mass Yield (%) 1 Methanol 6.29 0.00 97 8 Ethanol 5.91 0.44 91 9 1-Propanol 5.41 0.88 83 Comparative Examples 10 and 11 Example 2 was repeated using ethyl acetate and isopropyl alcohol. The following table gives the results. Example Solvent DABTF (g) CDABTF (g) DABTF Mass Yield (%) 2 Ethanol 6.1 0.28 94 10 Ethyl Acetate 3.90 2.62 60 11 Isopropyl Alcohol 4.02 2.53 62 Comparative Examples 12 to 16 Example 1 was repeated using different bases. The following table gives the results Example CDNBTF(g) Base DABTF (g) CDABTF (g) DABTF Mass Yield (%) 1 10 MgO 6.29 0.00 97 12 10 CaO 0.90 0.86 14 13 10 None 2.85 0.76 44 14 5 Na₂CO₃ 2.04 0.07 63 15 10 NaOH 1.99 0.11 31 16 10 NaOH 3.62 2.45 56

Claims: A process for the preparation of 3,5-diaminobenzotrifluoride which comprises treating 4-chloro-3,5-dinitrobenzotrifluoride, in a methanol solvent, with hydrogen gas in the presence of magnesium oxide and in the presence of a catalyst which comprises palladium on a carbon support. A process according to Claim 1 in which the catalyst is 5% palladium on carbon. A process according to Claim 2 in which the catalyst is present at a level of 4 g (dry weight) per 100 g of 4-chloro-3,5-dinitrobenzotrifluoride. A process according to Claim 2 in which the catalyst is present at a level of 6 g (dry weight) per 100 g of 4-chloro-3,5-dinitrobenzotrifluoride. A process according to Claim 2 in which the catalyst is present at a level of 1 g (dry weight) per 100 g of 4-chloro-3,5-dinitrobenzotrifluoride. A process according to Claim 2 in which the catalyst is present at a level of 0.5 g (dry weight) per 100 g of 4-chloro-3,5-dinitrobenzotrifluoride. A process according to any one of Claims 1 to 6 run at a temperature inn to 90°C. A process according to any one of Claims 1 to 6 run at a temperature up to 125°C. A process according to any one of Claims 1 to 8 including the additional last step of evaporating said methanol. A process according to any one of Claims 1 to 9 in which the concentration of said 4-chloro-3,5-dinitrobenzotrifluoride in said methanol is up to 17%. A process according to claim 1 which comprises converting at least 87% of 4-chloro-3,5-dinitrobenzotrifluoride into 3,5-diaminobenzotrifluoride and into less than 0.5% 4-chloro-3,5-diaminobenzotrifluoride comprising (A) preparing a mixture of a solution of up to 17% 4-chloro-3,5-dinitrobenzotrifluoride in methanol in the presence of hydrogen, a palladium on carbon catalyst, and at least one equivalent of magnesium oxide; and (B) heating said mixture to a temperature between room temperature and 125°C. A method according to Claim 11 including the additional last step of evaporating said solvent. A process according to Claim 11 in which the catalyst is 5% palladium on carbon. A method according to Claim 11 wherein the concentration of said catalyst is 4 to 6 g (dry weight) per 100 g of 4-chloro-3,5-dinitrobenzotrifluoride.

Inventors: ALBRIGHT DAVID O JR, ALBRIGHT, DAVID O., JR.
Applicants: OCCIDENTAL CHEM CO, OCCIDENTAL CHEMICAL CORPORATION
Application year: 1992
Main classification: C07C209
IPC codes: C07C211, B01J23, C07B61, C07C209