Organotin perchlorates as gentle Lewis acid catalysts in Mukaiyama reaction

Abstract

Organotin perchlorates catalyze Mukaiyama reaction of ketene silyl acetal in highly chemoselective but uncommon manners. The competition reaction between aldehyde and acetal leads to exclusive formation of the aldehyde aldols leaving the acetal counterpart intact, an unusual outcome in terms of reaction under acidic conditions. α-Enals react with ketene silyl acetal in preference to the corresponding alkanal. In the competition between electronically different aldehydes, an electron-donating group increases the reactivity of aldehyde while the reverse is true with an electron-withdrawing group. These are opposite to the reactivity order in nucleophilic addition to free carbonyls. In contrast to ketene silyl acetal, enol silyl ethers derived from ketones are not activated by organotin perchlorates. Thus, these two enol silyl ethers can be discriminated from each other. A disilyl enol ether derived from a keto ester undergoes the electrophilic attack by aldehyde and α-enone exclusively on the ester function. The catalytic activities of TBSClO 4 which should be formed if organotin perchlorates underwent transmetallation with ketene silyl acetal or silyl ether of the aldolate, are totally different from those of organotin perchlorates, indicating that the organotin species work as real active species. The reaction is interpreted in terms of the S n2 mechanism where the initial coordination of carbonyl group with organotin perchlorates plays a key role. The remarkable selectivitities are ascribed to the weak acidity of the catalysts. The reactivities of the complexed carbonyls are completely different from those of free carbonyls.