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

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 (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)(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 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.