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

Abstract

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, 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 LaNTMS is competent to catalyze, further underscoring the value and versatility of lanthanide complexes in homogeneous catalysis.

Manganese‐Catalyzed Hydroborations with Broad Scope

“…The catalytic deoxygenative hydroboration of amides to amines is a safe alternative to hydrogenations or the use of LiAlH 4 . Rare earth metal catalysts were recently reported;[ 7f , 7g , 17 ] an abnormal N ‐heterocyclic carbene‐potassium catalyst was active at 40 °C; [20] the combination KO t Bu/BEt 3 enabled amide reductions at 25–60 °C with pinacolborane; [21] 2,6‐di‐ tert ‐butyl‐phenolate lithium‐THF was catalytically active at 60 °C. [22] The simple pre‐catalyst Mn(hmds) 2 proved active in the hydroboration of alkyl and aryl carboxamides with pinacolborane to primary amines in very good yields at 50 °C (Scheme 3 , center).…”

The Hidden Role of Boranes and Borohydrides in Hydroboration Catalysis

“…64 This final case may also have implications on the veracity of other metal amide-catalyzed hydroboration reactions. [65][66][67][68][69] The hydroboration of imines with HBpin can be mediated by the addition of alcohol or water. 70 Whilst the exact mechanism is as yet unclear, protic impurities should also be considered when devising new carbonyl hydroboration reactions.…”

Manganese‐Catalyzed Hydroborations with Broad Scope

“…The catalytic deoxygenative hydroboration of amides to amines is a safe alternative to hydrogenations or the use of LiAlH 4 . Rare earth metal catalysts were recently reported; [7f,g, 17] an abnormal N ‐heterocyclic carbene‐potassium catalyst was active at 40 °C; [20] the combination KO t Bu/BEt 3 enabled amide reductions at 25–60 °C with pinacolborane; [21] 2,6‐di‐ tert ‐butyl‐phenolate lithium‐THF was catalytically active at 60 °C [22] . The simple pre‐catalyst Mn(hmds) 2 proved active in the hydroboration of alkyl and aryl carboxamides with pinacolborane to primary amines in very good yields at 50 °C (Scheme 3, center).…”