Uncatalyzed Carboboration of Seven-Membered-Ring <i>trans</i>-Alkenes: Formation of Air-Stable Trialkylboranes

Seven-membered-ring <i>trans</i>-alkenes undergo rapid, uncatalyzed carboboration reactions to form trialkylboranes as single diastereomers. In contrast with other trialkylboranes, which can ignite in the presence of oxygen, these trialkylboranes are stable in air. Hindered trialkylboranes can undergo reverse hydroboration reactions to form allylic silanes or can be oxidized to afford highly substituted triols. This reaction sequence permits the construction of compounds with up to five consecutive stereocenters. Control experiments and computational studies support a concerted mechanism for the migratory insertion of the alkene into the carbon–boron bond, similar to the mechanism for hydroboration.