Aluminum-catalyzed enantio- and diastereoselective carbonyl addition of propargylsilanes. A new approach to enantioenriched vinyl epoxides.

The current realm of Lewis acid-promoted propargylsilane additions to carbonyl groups2 may be subdivided into two mechanistically distinct categories: (i) additions proceeding via a desilylation pathway;3 (ii) additions involving a [1,2]-silyl shift.4 Process (i) generates allenic alcohols and is most common when nonbulky silyl groups such as TMS are employed,3 whereas the latter scenario has been shown to furnish mixtures of dienyl alcohols5 and five-membered carbocyles.4 The chemodiversity of these two pathways can be explained in terms of the steric bulk of the silyl species: the less encumbering silyl group favors desilylation via nucleophilic addition to silicon forming an “ate” complex; the larger silyl group discourages addition, enabling a migration to occur. Nonstereoselective examples of each of the above transformations have been reported;2-5 however, this study not only constitutes the first intermolecular stereoselective propargylsilanecarbonyl addition reaction, but it also represents a new approach to the formation of vinyl epoxides.