Heterogeneous Additive-Free Hydroboration of Alkenes Using Cu-Ni/Al2O3: Concerted Catalysis Assisted by Acid- Base Properties and Alloying Effects

This study describes our development of a Cu-based heterogeneous catalytic system for additive-free hydroboration of alkenes. Surveying monometallic Cu supported on various oxides revealed that Al2O3 provided the best catalytic support with respect to this reaction. A typical volcano-type relationship between the TOF and isoelectric point of the support was observed, which suggests that both acidic and basic properties were necessary to promote this reaction. The catalytic activity of Cu/Al2O3 was further enhanced by the addition of Ni, whereas the selectivity decreased as Ni content increased. The combination of X-ray diffraction, high-angle annular dark-field scanning transmission electron microscopy energy-dispersive system, and extended X-ray absorption fine structure analyses demonstrated the homogeneous formation of a Cu-Ni solid-solution alloy on Al2O3. The optimized Cu5Ni/Al2O3 catalyst exhibited high catalytic performance for styrene hydroboration (98% yield at 25 degrees C for 6 h). A mechanistic study revealed that (1) ethanol adsorbed on a Lewis acid site of Al2O3 released a proton to the alkene, generating a carbocation intermediate, (2) diboron was activated by a basic site in the Al2O3 structure, which was followed by a nucleophilic attack on the carbocation to form the final product, and (3) Ni acted as an efficient adsorption site for alkene, which facilitated the protonation of alkene as the rate-determining step. The catalysis was assisted by acid-base properties and an alloying effect, which resulted in a unique reaction mechanism and an efficient catalytic system for hydroboration.