Ionic-Liquid Hydrogen-Bonding Promoted Alcohols Amination over Cobalt Catalyst via Dihydrogen Autotransfer Mechanism

Abstract

Higher amines are important high-valuable chemicals with wide applications, and amination of alcohols is a green route to them, which however generally suffers from harsh reaction conditions and use of equivalent base. Herein, we report an ionic-liquid (IL) hydrogen-bonding promoted dihydrogen autotransfer strategy for amination of alcohols to higher amines over cobalt catalyst under base-free conditions. Co(BF₄)₂ ⋅ 6 H₂O complexed with triphos and IL (e. g., tetrabutylphosphonium tetrafluoroborate, [P₄₄₄₄][BF₄]) shows high performances for the reaction and is tolerant of a wide scope of amines and alcohols, affording higher amines in good to excellent yields. Mechanism investigation indicates that the [BF₄]⁻ anion activates the alcohol via hydrogen bonding, promoting transfer of both hydroxyl H and α-H atoms of alcohol to the cobalt catalyst to form an aldehyde intermediate and cobalt dihydride complex, which are involved in the subsequent reductive amination. This strategy provides a green and effective route for alcohol amination, which may have promising applications in alcohol-involved alkylation reactions.