Ligand-enabled stereodivergence in nickel-catalyzed regioselective hydroboration of internal allenes

Abstract

•Regio- and stereoselective hydroboration of internal allenes •Stereodivergent synthesis of trisubstituted alkenylboronates •Isolation and characterization of nickel-allene and nickel-olefin intermediates •In-depth mechanistic and kinetic studies Developing protocols for stereodivergent synthesis of multi-substituted alkenylboronates will simplify and unify the synthesis of Z- and E-stereoisomers of multi-substituted alkenes. Although tremendous efforts have been made to devise stereoselective approaches to access Z- or E-alkenylboronates, catalyst-controlled stereodivergence in their synthesis remains extremely rare. Herein, we demonstrate that such stereodivergence can be achieved for nickel-catalyzed hydroboration of internal allenes, which enables convenient synthesis of both stereoisomers of trisubstituted alkenylboronates from the same starting reagents. Mechanistic studies reveal that these nickel-catalyzed allene hydroboration reactions do not follow conventional hydrometallation or borylmetallation pathways with metal-hydride or metal-boryl intermediates. On the basis of the synthesis of potential nickel intermediates and their stoichiometric reactions, we propose new pathways for this nickel-catalyzed allene hydroboration, involving initial complexation of allenes to low-valent nickel catalysts followed by the stereo-determining reaction of bisphosphine-ligated nickel-allene intermediates with pinacolborane (HBpin) to form nickel-olefin compounds, which contain coordinated Z- or E-alkenylboronates. Developing protocols for stereodivergent synthesis of multi-substituted alkenylboronates will simplify and unify the synthesis of Z- and E-stereoisomers of multi-substituted alkenes. Although tremendous efforts have been made to devise stereoselective approaches to access Z- or E-alkenylboronates, catalyst-controlled stereodivergence in their synthesis remains extremely rare. Herein, we demonstrate that such stereodivergence can be achieved for nickel-catalyzed hydroboration of internal allenes, which enables convenient synthesis of both stereoisomers of trisubstituted alkenylboronates from the same starting reagents. Mechanistic studies reveal that these nickel-catalyzed allene hydroboration reactions do not follow conventional hydrometallation or borylmetallation pathways with metal-hydride or metal-boryl intermediates. On the basis of the synthesis of potential nickel intermediates and their stoichiometric reactions, we propose new pathways for this nickel-catalyzed allene hydroboration, involving initial complexation of allenes to low-valent nickel catalysts followed by the stereo-determining reaction of bisphosphine-ligated nickel-allene intermediates with pinacolborane (HBpin) to form nickel-olefin compounds, which contain coordinated Z- or E-alkenylboronates.