Enantioselective Dehydration of Butan-2-ol Using Zeolite Y Modified with Dithiane Oxides

Abstract

Modification of zeolite H–Y by dithiane oxides (2-R-1,3-dithiane 1-oxide;R=H, CH3, C6H5) is shown to enhance significantly its activity for the acid catalysed gas phase dehydration of butan-2-ol. The rate enhancement is observed for catalysts that are prepared by adding the dithiane oxide to the zeolite synthesis gel or by adsorption of the dithiane oxide onto commercial samples of zeolite H–Y. The origin of the rate enhancement is considered to result from a specific interaction between the dithiane oxide modifier with both the extra-framework and framework aluminium in the zeolite. Modification of zeolite H–Y with (R)-1,3-dithiane 1-oxide enhances the conversion of (S)-butan-2-ol compared to (R)-butan-2-ol in the temperature range 110–150°C when the two enantiomers are reacted separately. Modification with (S)-2-phenyl-1,3-dithiane 1-oxide gives a catalyst for which (R)-butan-2-ol is the most reactive of the two enantiomers. Reaction of racemic butan-2-ol over these chirally modified H–Y zeolites demonstrates that this modification procedure makes the zeolite enantiomerically discriminating and one enantiomer preferentially reacts, although both are present in the micropores under the reaction conditions. This effect is considered to be due to enantioselective rate enhancement, since, although the rate of dehydration of both enantiomers is enhanced in the chiral environment, the dehydration rate of one enantiomer is accelerated relative to the other. It is suggested that the effect is due to preferential adsorption at the chiral active site.