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, a ubiquitous motif in organic chemistry, and exhibit high chemoselective control. 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) to avoid overreduction to amine products and utilises (EtO)3SiH as a mild stoichiometric reductant to enable catalyst turnover. The reaction generally proceeds with high yields (13 examples, 70 to 95% yield) and tolerates a variety of functional groups (alkene, ether, nitro, etc.). Stoichiometric mechanistic investigations suggest the regeneration of the active [Zr]–H catalyst is achieved through the σ-bond metathesis of Si–H and Zr–OR.