The deoxygenative reduction of amides to amines is an important transformation in academic, pharmaceutical, and industrial synthetic chemistry, and the natural prevalence and synthetic accessibility of amides makes them valuable precursors to amines. However, the relative inertness of the amide C=O bond typically necessitates the use of harsh, difficult to handle, and non-selective reductants. He we report that tris[N,N-bis(trimethylsilyl)amide]lanthanum (LaNTMS) 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 over intermolecular 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 LaNTMS is competent to catalyze (e.g., aldehydes,1 ketones,1 esters2), further underscoring the value and versatility of lanthanide complexes in homogeneous catalysis.

Rapid and selective lanthanide-mediated deoxygenative reduction of amides with pinacolborane: Scope and mechanism

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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