para-Selective nitration of halogenobenzenes using a nitrogen dioxide–oxygen–zeolite system

Electrophilic aromatic substitution reactions are of considerable importance in the production of fine chemicals. However, the traditional processes suffer a number of disadvantages, such as low selectivity towards the desired product and the requirement for large quantities of mineral or Lewis acids as activators. In turn these acids are responsible for corrosion problems within the plant and the generation of large volumes of spent reagents, which, given the current environmentally conscious climate, are increasingly unacceptable. Major efforts are therefore being made towards developing processes that can reduce the volumes of spent liquors produced. Inorganic solids can offer significant benefits for these processes by providing both effective catalysis and, in some cases, enhanced selectivity. Additionally they are easily removed from reaction mixtures and in some cases recycling is possible. For example, we have utilised zeolites to enhance the para-selectivity in chlorination,1 bromination,2 acylation3 and methanesulfonylation4 reactions of simple aromatic substrates. Aromatic nitro compounds represent particularly versatile chemical feedstocks for a wide range of industrial products, such as pharmaceuticals, agrochemicals, dyestuffs and explosives. Traditionally, nitration has been performed by a mixture of nitric and sulfuric acids (mixed acid method).5 However, the method is notoriously unselective for nitration of substituted aromatic compounds and the disposal of the spent acid reagents presents a serious environmental issue. In order to address these problems several alternative methods for aromatic nitration have been developed recently. For example, lanthanide triflates have been used to catalyse nitration with nitric acid, which avoids the use of large volumes of sulfuric acid in the process.6 However, this provides no enhancement of selectivity. Selectivity of the nitration process can be enhanced by solid catalysts such as clays, but primarily zeolites, which can improve selectivity using alkyl nitrates,7 acyl nitrates,8 or even nitric acid itself.9,10 We have reported the use of zeolite Hb in conjunction with a mixture of acetic anhydride and nitric acid, which currently offers the best combination of yield and paraselectivity for nitration of simple aromatic compounds.11 None of these methods, however, are totally devoid of disadvantages. Another approach towards clean nitration involves the use of dinitrogen tetroxide in combination with oxygen or ozone as an oxidant.12 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.13 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 zeolites, in order to determine if they were able to catalyse the process and impart para-selectivity.