Mild reductive rearrangement of oximes and oxime ethers to secondary amines with hydrosilanes catalyzed by B(C6F5)3

Abstract

The strong boron Lewis acid tris(pentafluorophenyl)borane, B(C₆F₅)₃, has been found to catalyze the reductive rearrangement of oximes and their ether derivatives at room temperature with hydrosilanes as the reducing agents. Cyclic substrates undergo ring enlargement, and the secondary amine products are generally formed in good yields. Control experiments combined with a DFT computational analysis of the reaction mechanism suggest that there are three energetically accessible reaction pathways (paths A–C), either or not involving hydroxylamine derivatives. Paths A and B proceed through the intermediacy of a common N,O-bissilylated hydroxylamine, and the ring-expanding rearrangement yields an iminium ion. With no intermediate at the hydroxylamine oxidation level (path C), the reaction mechanism resembles that of the Beckmann rearrangement where an O-silylated oxime converts into a nitrilium ion. The reduction–rearrangement sequence (paths A and B) is slightly preferred over the rearrangement–reduction order of events (path C), especially at ambient temperature.