Defective NiO Nanoparticles Supported on Nb ₂ O ₅ for Solid‐Catalyzed Markovnikov‐Selective Hydroboration of Styrenes

Markovnikov selective hydroboration of alkenes with an air‐ and moisture‐stable diboron reagent is a more challenging reaction than the anti ‐Markovnikov‐selective one. We report that nickel oxide (NiO) supported on niobium pentoxide (Nb 2 O 5 ) efficiently catalyzed the hydroboration of 4‐ tert ‐butylstyrene, affording a Markovnikov (branched) adduct with excellent selectivity. To the best of our knowledge, this is the first report of the Markovnikov‐selective hydroboration of styrenes with a diboron reagent, bis(pinacolato)diboron, using a solid catalyst. NiO on Nb 2 O 5 contains Ni 3+ , which is more catalytically active than Ni 2+ , as a major species in contrast to bulk NiO. However, the catalytic activity decreased upon recycling tests because the Ni species leached from NiO/Nb 2 O 5 . The catalytically active species were NiO on Nb 2 O 5 and leached Ni species. To improve the reusability of the catalyst system, NiO/Nb 2 O 5 , ZnO was introduced as a redeposition site for the leached Ni species. Ultimately, the desired Markovnikov adduct was obtained in high yield and selectivity using NiO/Nb 2 O 5 in the presence of ZnO (92% yield and 92% selectivity), and catalyst reusability was also improved.