Iron-Catalyzed E-Selective Dehydrogenative Borylation of Vinylarenes with Pinacolborane

We report the dehydrogenative borylation of vinylarenes with pinacolborane (HBpin) catalyzed by an iron(0) complex "(PMe3)(4)Fe". A variety of monosubstituted and disubstituted vinylarenes underwent this iron-catalyzed transformation, affording E-vinyl boronate esters (VBEs) selectively in high yields. In addition, we coupled this iron-catalyzed dehydrogenative borylation with further transformations of the resulting vinyl boronate esters and developed various one-pot procedures for the functionalization of the vinylic C-H bonds in vinylarenes. Mechanistic studies reveal that this Fe-catalyzed reaction proceeds through syn-insertion of vinylarenes into a Fe-boryl species followed by beta-hydrogen elimination from a syn coplanar conformation of the borylalkyl iron intermediate.