Dual activation in asymmetric allylsilane addition to chiral N-acylhydrazones: Method development, mechanistic studies, and elaboration of homoallylic amine adducts

Abstract

Chiral N-acylhydrazones derived from commercially available 4-benzyl-2-oxazolidinone provide a rigid, conformationally restricted template to impart facial selectivity in additions to C=N bonds. In the presence of indium(III) trifluoromethanesulfonate [In(OTf)3], N-acylhydrazones undergo highly diastereoselective fluoride-initiated additions of allylsilanes (aza-Sakurai reaction). Mechanistic studies including control experiments and comparisons with allyltributylstannane, allylmagnesium bromide, and allylindium species implicate a dual activation mechanism involving addition of an allylfluorosilicate species to a chelate formed from In(OTf)3 and the chiral N-acylhydrazone. The N-N bonds of the adducts are readily cleaved in a two-step protocol to provide synthetically useful homoallylic N-trifluoroacetamides. Further elaboration of the latter compounds through Wacker oxidation and olefin metathesis provides diversely functionalized building blocks and expands the potential applications of this C-C bond construction approach to asymmetric amine synthesis. © 2006 American Chemical Society.