Abstract
C-H bond functionalization of arenes with boranes continues to be a challenge in catalysis, with late transition and rare earth metals shown to be catalytically active. In this study, group 4 metallocenes grafted onto acidic sulfated zirconia (SZO) are demonstrated to catalyze arene borylation with pinacolborane (HBpin). Catalysis studies at partial HBpin conversions (59-68%) using Cp(2)M(Me)/SZO (M = Ti, Zr, or Hf; Cp = cyclopentadienyl) catalysts reveal that Zr exhibits greater selectivity and activity than Ti and Hf. At 0.16 mol % of Zr, Cp(2)ZrMe/SZO achieves 332 turnovers at high HBpin conversion (86%), making this catalyst comparably active to previously reported Ir and Rh C-H borylation catalysts. At 160 °C, a maximum chemoselectivity of 82% for PhBpin was observed at 24% HBpin conversion. The superior activity of ionic Cp(2)ZrMe/SZO compared to neutral Cp(2)ZrMe/SiO(2) demonstrates the borylation mechanism relies on the highly electrophilic, coordinatively unsaturated cationic sites stabilized by the weakly coordinating sulfated support. Furthermore, both catalysts significantly outperform their molecular analogues, Cp(2)ZrMe(2) and [Cp(2)ZrMe][B(C(6)F(5))(4)], suggesting that the support enhances catalytic performance by stabilizing the active species.