Tuning the Reactivity of a Heterogeneous Catalyst using N‐Heterocyclic Carbene Ligands for C−H Activation Reactions

We report the dramatic impact of the addition of N‐heterocyclic carbenes (NHCs) on the reactivity and selectivity of heterogeneous Ru catalysts in the context of C−H activation reactions. Using a simple and robust method, we prepared a series of new air‐stable catalysts starting from commercially available Ru on carbon (Ru/C) and differently substituted NHCs. Associated with C−H deuteration processes, depending on Ru/C‐NHC ratios, the chemical outcome can be controlled to a large extent. Indeed, tuning the reactivity of the Ru catalyst with NHC enabled: 1) increased chemoselectivity and the regioselectivity for the deuteration of alcohols in organic media; 2) the synthesis of fragile pharmaceutically relevant deuterated heterocycles (azine, purine) that are otherwise completely reduced using unmodified commercial catalysts; 3) the discovery of a novel reactivity for such heterogeneous Ru catalysts, namely the selective C‐1 deuteration of aldehydes. The addition of N‐heterocyclic carbenes on Ru/C induces a catalytic activity switch to favor hydrogen isotope exchange over reductive deuteration. This catalyst modification led to an increase in regio‐ and chemoselectivity in the context of the C−H deuteration of pharmaceutically relevant substructures, as well as to the discovery of a novel reactivity for such heterogeneous Ru catalysts, namely selective C‐1 deuteration of aldehydes.