CC 24 2521-2529..7971adisk chapter .. Page2521

Electrophilic aromatic substitution reactions are of considerable importance in the production of fine chemicals. However, many traditional processes suffer serious disadvantages, including low selectivity for the desired product and the requirement for large quantities of mineral or Lewis acids as activators. Such acids cause corrosion and generate large volumes of spent reagents. Major efforts are therefore being made to develop processes with lower environmental impact. Inorganic solids offer significant benefits by providing effective catalysis and, in some cases, enhanced selectivity.1 For example, in our own research we have utilised zeolites to enhance the para-selectivity in chlorination,2 bromination,3 acylation4 and methanesulfonylation5 reactions of simple aromatic substrates. In addition, such solids are easily recycled. Aromatic nitration is particularly important since nitro compounds are versatile feedstocks for a range of industrial products, including pharmaceuticals, agrochemicals, dyestuffs, and explosives. Traditional nitration with a mixture of nitric and sulfuric acids6 is notoriously unselective for nitration of substituted compounds, and disposal of the spent liquors presents a serious environmental concern.7 Consequently, several alternative methods for aromatic nitrations have been developed.8–15 At present, the best combination of high yield, high paraselectivity and low solvent use is nitric acid, zeolite HBEA (Hb) and acetic anhydride (for moderately active aromatics)12 or trifluoroacetic anhydride (for deactivated aromatics).13 Even these systems, however, produce carboxylic acid as byproduct. An alternative approach to clean nitration, pioneered by Suzuki et al., involves dinitrogen tetroxide and ozone,16 or dinitrogen tetroxide with oxygen and a catalyst.17 The use of oxygen rather than ozone was appealing and we demonstrated that use of zeolite Hß as catalyst provided improved paraselectivity and easier catalyst recovery.18 However, the method still employed a large excess of dinitrogen tetroxide, a halogenated solvent, cooling and a reaction time of two days. Therefore, we continue to study the reaction in order to find ways of overcoming the remaining disadvantages. A recent publication from Suzuki et al. reveals that his group is also trying to perfect this zeolite-catalysed approach,19 and we now therefore disclose our own further findings. Our initial objective was to improve our previous procedure18 by avoiding the need for cooling, decreasing the excess of dinitrogen tetroxide used and eliminating the solvent. These features were dictated by the volatility of dinitrogen tetroxide (bp 21 °C). Simply using an autoclave pressurised to 200 psi with oxygen allowed a reaction with the desirable features to proceed quickly and in high yield. Furthermore, when oxygen was replaced by air the reaction was still efficient. With air it required about 12 h to go to completion, still quicker than under the previous conditions.18 By contrast, when the pressurising gas was nitrogen, virtually no nitration occurred, demonstrating the involvement of oxygen in the reaction. It was not known whether the by-product was water, nitric acid or a mixture of the two. Eqn. (1) (n = 0–6) covers the various possible stoichiometries.