3,3′,4,4′-Tetraminobiphenyl (TAB) is a valuable intermediate and final product in various areas. For example, TAB is used as monomer in the preparation of polybenzimidazole (PBI) polymers, which are characterized by excellent thermal and mechanical stability. The PBI polymers are widely used as proton-conducting materials for fuel cell applications (compare U.S. Pat. Nos. 2,895,948, 3,174,947, 5,317,078 and 6,187,231). TAB is also used as an antioxidant and as an agent for stabilizing epoxide resins.
In the prior art, TAB was prepared by three known methods. One such known method is ammonolysis of 3,3′-dichlorobenzidine (DAB) in the presence of mainly Cu catalysts (both copper salts and elemental Cu) using aqueous NH3. For example, French Patent Specification No 1,475,631 describes such an ammonolysis of DCB, in the presence of a Cu—I salt and/or of Cu2O and CaCl2 at an elevated temperature preferably 150–210° C. and under an elevated inert gas pressure. The crude TAB thus obtained is purified via its salt formation with a strong acid (yield of TAB is about 70% of theory). Subsequently, various attempts were made to obtain TAB in highly pure form and in high yields from crude TAB as shown below.
The process of U.S. Pat. No. 3,865,876 describes the improvement on the result of the method in accordance with the above mentioned French Patent Specifications by using essentially only CuCl as a catalyst in the ammonolysis of DCB. The yield of TAB of theory having purity of about 75–82% is between about 85 and 87%. This product has a Cu content of about 3–6% by weight. The process of U.S. Pat. No. 3,943,175 (CuCl/Cu powder can also be used as catalyst, in addition to CuCl) describes the purification of TAB (converting it into its sulfate by means of sulfuric acid, isolation of the sulfate and liberation there from of TAB by means of a base). The TAB thus liberated is dissolved and reprecipitated from an aqueous solution advantageously with the addition of activated charcoal and diatomaceous earth. However, the Cu content present in TAB is about 0.6 to 0.9% and the yield at most 45.7% of theory, relative to DCB employed.
The German Patent (Ger. Offen. DE 3, 111, 470) discloses the purification of crude TAB (obtained by ammonolysis process) by boiling it with H2O containing activated carbon and sodium dithionate (yield of TAB is 75.9% with ≦0.0005% Cu content). The Japanese Patent (JP 60,158,146) also describes the purification of TAB by refluxing the crude TAB with activated charcoal, aq. FeCl3 solution and hydrazine hydrate (yield of TAB: 83.2% containing ≧10 ppm Cu). Three more patents (U.S. Pat. Nos. 4,433,168 and 5,235,105 and Eur. Pat. Appl. EP 522,577) describe the purification of crude TAB (obtained from ammonolysis of DCB with copper catalyst) by crystallizing it in water in presence of 0–5% by weight of activated carbon and about 1–2% by weight of a water-soluble reducing agent (alkali metal dithionate or alkali metal sulfite) at temperature of 100–140° C. under nitrogen2 atmosphere (yield of TAB: 88.2% of theory with only 10 ppm Cu).
In the second method for producing TAB, which has generated substantial interest, the starting material is benzidine which is acetylated with acetic anhydride, to form N,N-diacetylbenzidine. The latter compound is then nitrated with conc. HNO3 to form 3,3′-dinitro-N,N-diacetylbenzidine which is base hydrolyzed to form 3,3′-dinitrobenzidine. This is then reduced by any of various means to form TAB [H. Vogel and C. S. Marvel, J. Poly. Sci. Part Al, 1531(1963)].
The third method describes the production of TAB from biphenyl which comprises the following six steps: (1) acetylating the biphenyl in the presence of an appropriate Friedel-Crafts catalyst to obtain 4,4′-diacetylbiphenyl (DAcB); (2) oximating the DAcB to form DAcB dioxime; (3) subjecting the dioxime to a double Beckmann rearrangement to obtain N,N-diacetylbenzidine. (DiAcBz); (4) Nitrating the DiAcBz to obtain 3,3′-dinitro-N,N′-diacetylbenzidine (DNAcBz); (5) removing the acetyl groups of the DNAcBz by basic hydrolysis to form 3,3′-dinitrobenzidine (DNB) and (6) reducing the nitro groups of DNB to form TAB (U.S. Pat. No. 5,041,666).
All the foregoing methods suffer from the following disadvantages associated with them:    1. They utilize benzidine and DCB as the raw materials, which are known carcinogens.    2. Direct ammonolysis of DCB catalyzed by copper salts requires high temperature (200–300° C.) at a pressure of 900–1000 psig making the process more hazardous from safety. Our own experience with direct ammonolysis of DCB by following the patented procedures is very disappointing to obtain TAB as it is always accompanied by tarry material.    3. In the direct ammonolysis, copper form complexes probably tightly with aryl TAB, so that liberating TAB from the complex becomes tedious.    4. In the ammonolysis process, TAB is always contaminated with triaminobiphenyl and little amount of copper, so that separation of these impurities from TAB becomes tedious.    5. In the dioximation method, the number of steps involved is six starting from expensive biphenyl raw material.
All the above methods make use of relatively expensive starting materials and the reaction conditions are harsh to carry out. Thus, any method for producing TAB utilizing a cheaper raw material, which is both safer and easier to handle, would be very desirable.
Accordingly, it is an object of the present invention to provide a novel process for the synthesis of TAB, which is substantially free of one or more of the disadvantage prior process. Another object is to provide a novel process for the synthesis of TAB of greater purity than heretofore possible.