Zirconium-Catalyzed Imine Hydrogenation via a Frustrated Lewis Pair Mechanism

Zirconium-based frustrated Lewis pairs (FLPs) are active imine hydrogenation catalysts under mild conditions. Complexes of the type [Cp R 2 ZrOMes][B(C 6 F 5 ) 4 ] utilize the imine substrate itself as the Lewis base component of the FLP. Catalyst performance is a function of ligand structure; in general more bulky, more electron rich cyclopentadienyl derivatives give the best results. However, Cp* derivatives are not catalytically active, being stable after initial heterolytic cleavage of H 2; this allows experimental verification of the competence of the zirconocene–imine pair in FLP-type heterolytic H 2 cleavage. Enamines and protected nitriles are also hydrogenated if an additional internal phosphine base is used.