Regioselectivity of the rhodium-catalyzed hydroboration of vinyl arenes: Electronic twists and mechanistic shifts

Any substituent does it: The hydroboration of vinyl arenes with pinacol borane (HBPin) and cationic rhodium complexes selectively placed the boron proximal to the aryl rather than phenyl ring, regardless of whether this ring bears electron-donating or electron-withdrawing substituents. In competition experiments between styrene and various vinyl arenes, preferential hydroboration also occured at the substituted arene (see scheme). Hammett plots indicate a break in the mechanism. (Chemical Equation Presented). © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.