Aspartyl β-Turn-Based Dirhodium(II) Metallopeptides for Benzylic C(sp³)-H Amination: Enantioselectivity and X-ray...

Amination of C(sp³)-H bonds is a powerful tool to introduce nitrogen into complex organic frameworks in a direct manner. Despite significant advances in catalyst design, full site- and enantiocontrol in complex molecular regimes remain elusive using established catalyst systems. To address these challenges, we herein describe a new class of peptide-based dirhodium(II) complexes derived from aspartic acid-containing β-turn-forming tetramers. This highly modular system can serve as a platform for the rapid generation of new chiral dirhodium(II) catalyst libraries, as illustrated by the facile synthesis of a series of 38 catalysts. Critically, we present the first crystal structure of a dirhodium(II) tetra-aspartate complex, which unveils retention of the β-turn conformation of the peptidyl ligand; a well-defined hydrogen-bonding network is evident, along with a near-C₄ symmetry that renders the rhodium centers inequivalent. The utility of this catalyst platform is illustrated by the enantioselective amination of benzylic C(sp³)-H bonds, in which state-of-the-art levels of enantioselectivity up to 95.5:4.5 er are obtained, even for substrates that present challenges with previously reported catalyst systems. Additionally, we found these complexes to be competent catalysts for the intermolecular amination of N-alkylamides via insertion into the C(sp³)-H bond α to the amide nitrogen, yielding differentially protected 1,1-diamines. Of note, this type of insertion was also observed to occur on the amide functionalities of the catalyst itself in the absence of the substrate but did not appear to be detrimental to reaction outcomes when the substrate was present.