Additions of benzylsulfonium ylides to aldehydes and ketones: are they under kinetic or thermodynamic control?

Abstract

The reaction of benzylsulfonium ylides with aromatic aldehydes gives predominantly trans epoxides whilst aliphatic aldehydes give mixtures of cis and trans epoxides. In order to probe the origin of the diastereoselectivity, single diastereoisomers of betaine intermediates were prepared by independent methods and subjected to ring closure and cross-over experiments. It was found that anti betaines derived from aliphatic and aromatic aldehydes gave exclusively trans epoxides. syn Betaines derived from aromatic aldehydes also gave exclusively trans epoxides whilst syn betaines derived from aliphatic aldehydes gave mixtures of cis and trans epoxides. From ring closure experiments in the presence of a more reactive aldehyde (cross-over experiments) it was found that anti betaines derived from aliphatic and aromatic aldehydes gave epoxides without incorporation of the more reactive aldehyde. In contrast, syn betaines derived from aromatic aldehydes gave exclusively epoxides derived from the more reactive aldehyde whilst syn betaines derived from aliphatic aldehydes gave mixtures of epoxides. These experiments indicated that the high trans selectivity observed in epoxidation with aromatic aldehydes is a result of irreversible formation of the anti betaine and reversible formation of the syn betaine. With aliphatic aldehydes the lower selectivity results from only partial reversibility in formation of the syn betaine.