Enantioselective Reactions Using Modified Microporous and Mesoporous Materials

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Homochiral molecules are important chemical intermediates for pharmaceuticals and agrochemicals and, at present, most are synthesised using either non-catalytic methods or using homogeneous catalysts. There is a significant interest in the design of immobilised heterogeneous catalysts that are capable of high enantioselection. An approach based on the immobilisation of cations within microporous (zeolite Y) and mesoporous (MCM-41) materials will be described. Three enantioselective reactions will be discussed: (a) the dehydration of butan-2-ol over sulfoxide modified zeolite H-Y; (b) the aziridination of alkenes over bis(oxazoline) modified Cu-exchanged zeolite H-Y and (c) the epoxidation of cis-stilbene using chiral salen modified Mn-exchanged AIMCM-41. The main feature of the approach used to design enantioselective heterogeneous catalysis concerns the modification of cations that are ion-exchanged with extra-framework sites of zeolites and mesoporous materials. These charge balancing cations can function as heterogeneous catalysts and, under appropriate conditions (e.g. non-aqueous solvents), are highly stable. Furthermore, these cations can be modified by the addition of chiral ligands and, consequently, can function as enantioselective catalysts. The zeolite or mesoporous framework can be considered as part of the coordination shell of the cation, which can enhance the enantioselection of these catalysts.Key wordsEnantioselective heterogeneous catalysiszeolitesmesoporous materialsalcohol dehydrationacid catalysisaziridination of alkenesepoxidation