Site-Fixed Hydroboration of Alkenes under Metal-Free Conditions: Scope and Mechanistic Studies

An unprecedented and general metal-free hydroboration of alkenes with BBr3 as the boration reagent in the presence of iPr2NEt is reported. The addition of iPr2NEt not only suppresses alkene oligomerization and bromoboration side reactions, but also provides a proton source for hydroboration. More importantly, the site-fixed installation of a boryl group at the original position of the internal double-bond is easily achieved using our strategy as compared with traditional transition-metal-catalyzed hydroboration processes. Preliminary studies on the mechanism revealed a distinct reaction pathway that involves radical species and may operate through frustrated-Lewispair-type single-electron transfer.