This invention relates to a process for preparing trifluoromethyl-nitrodiphenyl ethers having the formula ##STR4## wherein m in 1 to 3, n is 1 or 2 and R is hydrogen, carboxyl, carboxylate, formyl, keto, oxime, cyano, alkyl, alkoxy, chloro, bromo, or N, N-dialkylamino. The trifluoromethylphenyl-nitrophenylethers that may be prepared by the process of this invention are members of a class of compounds useful as herbicides and/or as intermediates for the preparation of various herbicides, pesticides, dyestuffs, and pharmaceuticals. In recent years, the development of commercial utility for trifluoromethylphenyl-nitrophenylethers in the agricultural and pharmaceutical fields has led to considerable activity in the investigation of methods of preparation. The following U.S. patents disclose the preparation and/or use of various trifluoromethylphenyl-nitrophenylethers: U.S. Pat. Nos. 4,262,152; 4,031,131; 4,259,510; 3,941,830; 3,798,276; 3,928,416; 3,784,635; 4,063,929; 4,087,272; 4,263,277; 4,263,041; 4,002,662; 4,001,005; and 3,979,437.
Various methods for the preparation of 2-chloro-4-trifluoromethyl-4.sup.1 -nitrodiphenyl ethers are known. One method, disclosed in U.S. Pat. No. 4,259,510 comprises reacting a 2-chloro-4-trifluoromethylphenolate with a p-nitro-halobenzene. However, the phenolate reactant is not readily available commercially and, as a result, this synthesis route involves a preliminary step to prepare the phenolate. In another method, suggested in U.S. Pat. No. 4,031,131, a 3-chloro-4-halobenzotrifluoride is reacted with a phenoxide and the resultant diphenyl ether is subsequently nitrated. In addition to requiring an additional step, the subsequent nitration is inefficient and results in the production of undesired isomers. To avoid the disadvantage of a subsequent nitration step, the 3-chloro-4-halobenzotrifluoride may be reacted with a p-nitrophenoxide. However, it has been found that the presence of the nitro group drastically reduces the nucleophilicity of the phenoxide oxygen that is the reactive site, so that reaction with compounds such as 3,4-dichlorobenzotrifluoride are found to be extremely inefficient.
Nevertheless, it has now been found that, despite the adverse effect of the nitro group on the nucleophilicity of the phenoxide oxygen when the chlorohalobenzotrifluoride reactant is a chloro-fluorobenzotrifluoride such as 3-chloro-4-fluorobenzotrifluoride, the reaction with nitrophenoxides, especially p-nitrophenoxides can be easily affected, since the reaction of the phenoxide oxygen at the fluorine site proceeds smoothly even without a catalyst to provide the trifluoromethylphenyl-nitrophenylether product in high yield. In this matter, the need for a subsequent nitration step is eliminated. Furthermore, the method of this invention, utilizing chloro-fluorobenzotrifluoride reactants is particularly suitable for reactions with nitrophenoxides carrying highly sensitive functional groups, such as aldehydes which might otherwise be further oxidized in a subsequent nitration step such as required by the prior art process described above.
Chloro-fluorobenzotrifluorides may be prepared by the vapor phase chloro-denitration reaction of the corresponding chloro-nitrobenzotrifluoride with a chlorinating agent. Thus, for example, 3-chloro-4-fluorobenzotrifluoride may be prepared by vapor phase chloro-denitration reaction of chlorine with 4-fluoro-3-nitrobenzotrifluoride. The chloro-denitration process is carried out under conditions of temperature and pressure appropriate for a vapor phase reaction, the exact conditions being dependent on the properties of the particular reactants employed. Typically, the process is carried out at atmospheric conditions and at a temperature in the range of about 250.degree. to about 450.degree. Celsius, preferably 300.degree. to 400.degree. C. Typical of the chloro-substituted benzotrifluorides that may be prepared in this manner and employed as reactants in the process of the present invention are 3-chloro-4-fluorobenzotrifluoride; 4-chloro-3-fluorobenzotrifluoride; 2-chloro-5-fluorobenzotrifluoride; 5-chloro-2-fluorobenzotrifluoride; 2-chloro-4-fluorobenzotrifluoride; 4-chloro-2-fluorobenzotrifluoride; 3-chloro-5-fluorobenzotrifluoride; 2,5-dichloro-4-fluorobenzotrifluoride; 4,5-dichloro-2-fluorobenzotrifluoride; 3,5-dichloro-4-fluorobenzotrifluoride; 3,4-difluoro-5-chlorobenzotrifluoride; 2,5-difluoro-3-chlorobenzotrifluoride; 3,5-difluoro-4-chlorobenzotrifluoride; and the like.