Chromium-Catalyzed Selective Dimerization/Hydroboration of Allenes to Access Boryl-Functionalized Skipped (E,Z)-Dienes.

A chromium-catalyzed dimerization/hydroboration of allenes is developed to access synthetically versatile boryl-functionalized skipped dienes with a catalyst generated in situ from CrCl2 and a pyridine-2,6-diimine ligand mes PDI. A variety of allenes reacted with pinacolborane (HBpin) to afford the corresponding boryl-functionalized (E,Z)-1,4-dienes in high yields and with excellent selectivity. Electron paramagnetic resonance (EPR) spectroscopic studies suggest that this chromium-catalyzed reaction probably proceeds through a chromium(I) hydride intermediate.