Dediazoniation reactions as potential routes to fluoroaromatics
The reactions of arenediazoniumio ns have been studied intensely by organic chemists. Besides the azo chemistry (of interest to the dye industry), dediazoniation reactions have also been investigated. In these processes, a carbon nitrogen bond cleaves to give an aryl cation and a nitrogen molecule. The aryl cation is a short lived species which is captured by a nucleophile or a solvent molecule (Figure 1). + -N, ) Ar-N2 - , Alý Nu »- Ar-Nu Figure 1 The objective of our investigation was to attempt to identify reaction conditions that would allow trapping of the aryl cation by fluoride. By using either fluoride donor solvents, fluoride or fluoride containing anions such as BF4- or PF6- as the solute in a very weakly nucleophilic solvent, we hoped to develop a viable industrial process for the production of fluoroaromatic compounds. At present, their method of production includes the use of liquid hydrogen fluoride which is an extremely poisonous and corrosive material. Our work mainly involved a kinetic study of the heterolytic dediazoniation of several substituted arenediazonium salts in a variety of possible fluoride donor solvents and an analytical study of the product distribution from reactions of arenediazonium salts with possible fluoride donor solvents and added fluoride ions. This study also looked at the photolytic decomposition of arenediazonium salts to see if this route was a viable industrial altemative to thermal dediazoniation. The choice of solvent to facilitate heterolytic dediazoniation is important. It has been shown that the solvents in which the heterolytic dediazoniation mechanism is predominant have low nucleophilicities (but are highly ionising) whereas, in solvents of high nucleophilicity, homolysis of arenediazoniums.a lts is favoured. Hence trifluoroethanol, hexafluoroisopropanol, trifluoroacetic acid, ethanol and water were chosen as solvents in which to study the reaction. The use of trifluoromethoxybenzene and difluoromethyl 2,2,2-trifluoroethyl ether as co-solvents in trifluoroethanol were investigated as well as the use of inorganic fluorides in trifluoroethanol and water. Finally, a short study of the reduction of m-nitrobenzenediazoniumte trafluoroborate in ethanol was undertaken to investigate the reduction mechanism.