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

Abstract

AbstractZinc 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 (HL1) and the small NHC ligand, 2,3‐dimethyl‐3,4‐dichloroimidazol‐2‐ylidene (DDI) was synthesized and characterized. In particular, Zn(HL1)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 ZnH2(DDI) (Zn5) and 2 equiv. of dissociated (Bpin)2L1. Complex Zn5, generated in situ by this bifunctional catalyst activation step, was shown to be more active than the bis‐NHC analog ZnH2(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.