La[N(SiMe₃)₂]₃-Catalyzed Ester Reductions with Pinacolborane: Scope and Mechanism of Ester Cleavage

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Abstract

Tris[N,N-bis(trimethylsilyl)amido]lanthanum (LaNTMS) is an efficient, highly active, and selective homogeneous catalyst for ester reduction with pinacolborane (HBpin). Alkyl and aryl esters are cleaved to the corresponding alkoxy- and aryloxy-boronic esters which can then be straightforwardly hydrolyzed to alcohols. Ester reduction is achieved with 1 mol % catalyst loading at 25–60 °C, and most substrates are quantitatively reduced in 1 h. Nitro, halide, and amino functional groups are well tolerated, and ester reduction is completely chemoselective over potentially competing intra- or intermolecular alkene or alkyne hydroboration. Kinetic studies, isotopic labeling, and density functional theory calculations with energetic span analysis argue that ester reduction proceeds through a rate-determining hydride-transfer step that is ligand-centered (hydride is transferred directly from bound HBpin to bound ester) and not through a metal hydride-based intermediate that is often observed in organolanthanide catalysis. The active catalyst is proposed to be a La-hemiacetal, [(Me3Si)2N]2La-OCHR(OR)[HBpin], generated in situ from LaNTMS via hydroboronolysis of a single La–N(SiMe3)2 bond. These results add to the growing compendium of selective oxygenate transformations that LaNTMS is competent to catalyze, further underscoring the value and versatility of homoleptic lanthanide complexes in homogeneous catalytic organic synthesis.

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