La[N(SiMe₃)₂]₃-Catalyzed Deoxygenative Reduction of Amides with Pinacolborane. Scope and Mechanism

Tris[N,N-bis(trimethylsilyl)amide]lanthanum (La^NTMS) is an efficient and selective homogeneous catalyst for the deoxygenative reduction of tertiary and secondary amides with pinacolborane (HBpin) at mild temperatures (25-60 °C). The reaction, which yields amines and O(Bpin)₂, tolerates nitro, halide, and amino functional groups well, and this amide reduction is completely selective, with the exclusion of both competing inter- and intramolecular alkene/alkyne hydroboration. Kinetic studies indicate that amide reduction obeys an unusual mixed-order rate law which is proposed to originate from saturation of the catalyst complex with HBpin. Kinetic and thermodynamic studies, isotopic labeling, and DFT calculations using energetic span analysis suggest the role of a [(Me₃Si)₂N]₂La-OCHR(NR'₂)[HBpin] active catalyst, and hydride transfer is proposed to be ligand-centered. These results add to the growing list of transformations that commercially available La^NTMS is competent to catalyze, further underscoring the value and versatility of lanthanide complexes in homogeneous catalysis.