Markovnikov-Selective, Activator-Free Iron-Catalyzed Vinylarene Hydroboration

Abstract

Two series of structurally related alkoxy-tethered NHC iron(II) complexes have been developed as catalysts for the regioselective hydroboration of alkenes. Significantly, Markonikov selective alkene hydroboration with HBpin has been controllably achieved using an iron catalyst (11 examples 35-90% isolated yield) with up to 37:1 branched:linear selectivity. anti-Markovnikov selective alkene hydroboration was also achieved using HBcat and modification of the ligand backbone (6 examples, 44-71% yields). In both cases, ligand design has enabled activator-free low oxidation-state iron catalysis.

Cobalt‐Catalyzed Regioselective Hydroboration of Terminal Alkenes

“…The vast majority of known nonprecious metal catalysts have been found to promote the hydroboration of terminal alkenes with anti‐Markovnikov selectivity. However, a few catalysts based on earth‐abundant metals were recently shown to promote the unusual Markovnikov alkene hydroboration . Following reports of Cu I catalysts, our group reported the first manganese‐catalyzed hydroboration of terminal alkenes with Markovnikov selectivity in 2016 .…”

“…In the same year, nickel and iron complexes with N‐heterocyclic carbene (NHC) ligands were reported to catalyze the Markovnikov hydroboration of a range of aromatic terminal alkenes ,. [10a] Very recently, Lu's group also reported an iron‐catalyzed Markovnikov hydroboration of alkenes using amide‐based tridentate ligands. [10b]…”

“…[10a] Very recently, Lu's group also reported an iron‐catalyzed Markovnikov hydroboration of alkenes using amide‐based tridentate ligands. [10b]…”

Iron N‐Heterocyclic Carbenes in Reduction Reactions

“…The group of Thomas has recently described the synthesis and full characterisation of a series of alkoxy‐tethered NHC Fe(II) complexes, which were catalytically active for the hydroboration of terminal alkenes with controlled and switchable regioselectivity . Ligand design allows to controlling the regioselectivity of the reaction.…”

Hydroboration of Alkenes and Alkynes Employing Earth‐Abundant Metal Catalysts

“…In 2013, Thomas and co‐workers deployed a bench‐stable iron(II) precatalyst, which, upon activation with EtMgBr, proved a viable system for the catalytic hydroboration of both alkenes and alkynes . Subsequently, Thomas and co‐workers reported a system in which regioselectivity of hydroboration could be switched by tuning the substituents on the catalyst and by using an appropriate borylating agent . An iron (II) polyhydride complex supported by a pincer ligand was utilized by Nishibayashi and co‐workers for the dimerization and borylation of alkynes .…”