Asymmetric catalysis within chiral zeolitic nanospaces: Chiral host-guest match in GTM-3 zeolite

Abstract

In this work, the asymmetric catalytic activity of the recently discovered GTM-3 chiral zeolite is systematically explored for the ring-opening of trans-stilbene oxide using alcohols of different size as nucleophiles. Catalytic results show a volcano-type curve with a clear maximum enantioselective activity for a specific linear alcohol size of 4C (1-butanol); this is perfectly reproduced in opposite sign for GTM-3 catalysts obtained with the antipode of the structure-directing agent. Such trend evidences that a proper chiral host-guest size match must be established between the reactants and/or transition states and the chiral confined space provided by the zeolite framework in order for the zeolite catalyst to manifest its chirality. Moreover, secondary alcohols, bulkier in size, give higher enantioselectivities than their linear counterparts. On the other hand, variation of the reaction conditions enables to maximize the enantioselectivity of the reaction: in particular, variation of the concentration of the epoxide displays a strong influence, notably improving the enantiomeric excess obtained when the concentration of the epoxide is reduced. Modification of the reaction temperature also influences the enantiomeric excess, and especially, the selectivity to the different products. Our results suggest that if a proper chiral host-guest match is established within the confined chiral nanospace of the zeolite framework, then the enantioselectivity as well as the selectivity to the chiral products is maximized, which indeed can be manipulated through variations of the reaction conditions, leading to unprecedented enantiomeric excess values up to 51%.