Bifunctional Activation of NHC‐Zinc Pre‐Catalyst for Effective Hydroboration of Quinolines and Nitriles

Abstract

Abstract Zinc dihydride complexes supported by bulky N‐heterocyclic carbenes (NHC) are efficient catalysts in hydroboration reactions. Herein, a set of zinc complexes with an amino‐pyrrolide chelate (H L 1 ) and the small NHC ligand, 2,3‐dimethyl‐3,4‐dichloroimidazol‐2‐ylidene (DDI) was synthesized and characterized. In particular, Zn(H L 1 ) 2 (DDI) ( Zn4) displayed high activity in catalytic nitrile dihydroboration using 0.01 mol % loading, and quinoline hydroboration using 0.05 mol % loading at room temperature. Stoichiometric reaction of Zn4 with 4 equiv. of pinacolborane (HBpin) produced ZnH 2 (DDI) ( Zn5 ) and 2 equiv. of dissociated (Bpin) 2 L 1 . Complex Zn5 , generated in situ by this bifunctional catalyst activation step, was shown to be more active than the bis‐NHC analog ZnH 2 (DDI) 2 and previously reported examples with bulky NHC ligands. In addition, variable time normalization analysis (VTNA) showed first‐order dependence on [quinoline], [HBpin] and [ Zn4 ], revealing that a single Zn−H is involved in the catalytic cycle, as was also observed in the stoichiometric reaction of Zn5 with quinoline. VTNA also indicated that this catalyst faces neither deactivation nor product inhibition.