Catalytic Hydroboration of Esters by Versatile Thorium and Uranium Amide Complexes

Abstract

The challenging hydroboration of esters is achieved using simple uranium and thorium amides, U[N(SiMe3)2]3 and [(Me3Si)2N]2An[κ2-(N,C)-CH2Si(CH3)2N(SiMe3)] (An = Th or U) acting as precatalysts in the reaction with pinacolborane (HBpin). All three complexes showed impressive catalytic activities, reaching excellent yields. A large scope of esters was investigated including aliphatic, aromatic, and heterocyclic esters that were transformed cleanly to the corresponding hydroborated alcohols, which readily hydrolyzed to the free alcohols. Moreover, the actinide catalysts demonstrated unexpected high functional tolerance toward nitro, halide, cyano, and heteroaromatic functional groups. The reaction exhibited excellent selectivity toward the ester when additional double and triple unsaturated C-C bonds were present. Lactones and poly caprolactone have been successfully cleaved to the monomeric units, showing a great promise toward polymer degradation and recycling. Detailed kinetic studies are provided in order to determine the rate dependence on the concentration of catalyst, HBpin, and ester. A plausible mechanism is proposed based on stoichiometric reactions, DFT calculations, thermodynamic measurements, and deuterium-labeling studies.