Reduction of secondary amides to imines catalysed by Schwartz’s reagent
Abstract
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 (Cp$_2$Zr(H)Cl) to avoid overreduction to amine products and utilises (EtO)$_3$SiH 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.
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