Use of zeolites for greener and more para-selective electrophilic aromatic substitution reactions

Abstract

Electrophilic aromatic substitution is one of the most important reactions in synthetic organic chemistry. Such reactions are used for the synthesis of important intermediates that can be used as precursors for the production of pharmaceutical, agrochemical and industrial products. However, many commercial processes to produce such materials still rely on technology that was developed many years ago. Such processes commonly lead to mixtures of regioisomers and in recent years several new approaches have been developed to gain control over the regiochemistry of the reactions. Zeolites can act as heterogeneous catalysts, support reagents, entrain by-products, enhance product para-selectivities via shape-selectivity and avoid aqueous work-ups. For example, zeolites can have advantages in para-regioselective nitration, halogenation, alkylation, acylation and methanesulfonylation reactions under modest conditions. Moreover, usually they can be easily removed from reaction mixtures by simple filtration and regenerated by heating and can then be reused several times to give almost the same yield and selectivity as fresh samples. This review surveys the use of zeolites as para-selective catalysts for a whole range of aromatic substitution reactions, based largely on our own work in the area, but set in the wider context of other related work.