Catalytic Recycling of a Th–H Bond via Single or Double Hydroboration of Inactivated Imines or Nitriles

The catalytic activity
of the metallacycle thorium amide [(Me3Si)2N]2Th­[κ2–(N,C)–CH2Si­(CH3)2N­(SiMe3)] (Th1) is presented
for the selective dihydroboration of nitriles
(CN) with pinacolborane (HBpin). Using significantly
low catalyst loading (0.1 mol %), the dihydroborated amines were achieved
by the hydroboration of the CN triple bond attached
with aromatic, aliphatic, and heteroatom backbones with high turnover
frequency (TOF) as compared to all the reported homogeneous metal
catalysts in this reaction. In addition, for aldimines (CN),
the hydroboration precatalyst Th2 has been synthesized
by the protonolysis of a seven-membered N-heterocyclic iminato ligand
(LH) and Th1. The Th2 crystal structure
and its performance in the synthesis of hydroborated secondary amine
are also here presented. Detailed kinetic studies and thermodynamic
and stoichiometric experiments provided us with cumulative evidence
supporting the proposed mechanism for the aforementioned reactions.