Selective reduction of secondary amides to imines catalysed by Schwartz's reagent

The partial reduction of amides is a challenging transformation that must overcome the intrinsic stability of the amide bond and exhibit high chemoselective control to avoid overreduction to amine products. To address this challenge, we describe a zirconium-catalysed synthesis of imines by the reductive deoxygenation of secondary amides. This reaction exploits the excellent chemoselectivity of Schwartz's reagent (Cp2Zr(H)Cl) and utilises (EtO)3SiH as a mild stoichiometric reductant to enable catalyst turnover. The reaction generally proceeds with high yields (19 examples, 51 to 95 % yield) and tolerates a variety of functional groups (alkene, ester, nitro, etc.). Stoichiometric mechanistic investigations suggest the regeneration of the active [Zr]−H catalyst is achieved through the metathesis of Si−H and Zr−OR σ-bonds.