Enhancing Substrate-Metal Catalyst Affinity via Hydrogen Bonding: Pd(II)-Catalyzed β-C(sp 3 )-H Bromination of Free Carboxylic Acids.

The achievement of sufficient substrate-metal catalyst affinity is a fundamental challenge for the development of synthetically useful C-H activation reactions of weakly coordinating native substrates. While hydrogen bonding has been harnessed to bias site selectivity in existing C(sp 2 )-H activation reactions, the potential for designing catalysts with hydrogen bond donors (HBDs) to enhance catalyst-substrate affinity and, thereby, facilitate otherwise unreactive C(sp 3 )-H activation remains to be demonstrated. Herein, we report the discovery of a ligand scaffold containing a remote amide motif that can form a favorable meta -macrocyclic hydrogen bonding interaction with the aliphatic acid substrate. The utility of this ligand scaffold is demonstrated through the development of an unprecedented C(sp 3 )-H bromination of α-tertiary and α-quaternary free carboxylic acids, which proceeds in exceedingly high mono -selectivity. The geometric relationship between the NHAc hydrogen bond donor and the coordinating quinoline ligand is crucial for forming the meta -macrocyclophane-like hydrogen bonding interaction, which provides a guideline for the future design of catalysts employing secondary interactions.