Development of a system for clean and regioselective mononitration of aromatic compounds involving a microporous solid, dinitrogen tetroxide and air{

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. The use of such acids causes plant corrosion and generates large volumes of spent reagents, which, given the current environmentally conscious climate, are increasingly unacceptable. Major efforts are therefore being made to develop processes with lower environmental impact. Inorganic solids can offer significant benefits by providing both effective catalysis and, in some cases, enhanced selectivity. For example, in our own research we have utilised zeolites to enhance the para selectivity in chlorination, bromination, acylation and methanesulfonylation reactions of simple aromatic substrates. In addition, such solids are easily removed from reaction mixtures and can often be recycled. Therefore, study of the use of inorganic solids as catalysts for electrophilic aromatic substitution reactions can make an important contribution to green chemistry. Aromatic nitration is a particularly important problem since nitro compounds represent versatile chemical feedstocks for a wide range of industrial products, including pharmaceuticals, agrochemicals, dyestuffs, and explosives. Traditional nitration with a mixture of nitric and sulfuric acids (the mixed acid method) is notoriously unselective for nitration of substituted compounds, and disposal of the spent liquors presents a serious environmental concern. Consequently, there is intense international effort to address these problems and several alternative methods for aromatic nitrations have been developed. None of these methods, however, is totally devoid of disadvantages. At present, the best combination of high yield, high para selectivity and low solvent use is achieved using a reagent system comprising nitric acid, zeolite Hß as catalyst, and acetic anhydride (for moderately active aromatics) or trifluoroacetic anhydride (for deactivated aromatics) as activator. Even these systems, however, produce at least two molar equivalents of carboxylic acid as by-product. Another approach towards clean nitration involves the use of dinitrogen tetroxide in combination with oxygen or ozone as an oxidant. The method where ozone is employed most likely involves dinitrogen pentoxide, as this is known to be a highly active nitrating agent. The method utilising oxygen is less clear cut and requires the use of tris(pentane-2,4-dionato)iron(III) (Fe(acac)3) as a catalyst in an organic solvent. 15 In principle, this could lead to a highly atom-efficient process (eqn. (1)), but it is not regioselective. Therefore, we decided to study the use of dinitrogen tetroxide as a nitrating agent in the presence of microporous solids, in order to determine if such solids were able to catalyse the process and impart para selectivity.