Lactam Hydrogen Bonds as Control Elements in Enantioselective Transition-Metal-Catalyzed and Photochemical Reactions

In the last two decades, hydrogen bonds have been established as useful interactions to control the selectivity of various chemical transformations. In this Perspective, the contributions by our group to this growing field of research are summarized and analyzed. In the first section, a chiral template is presented which displays a 1,5,7-trimethyl-3-azabicyclo[3.3.1]nonan-2-one skeleton with a lactam binding site and that has been used in superstoichiometric quantities in a variety of photochemical and radical reactions. Chiral catalysts with a related architecture evolved from the template by introducing a suitable chromophore for harvesting photons in the ultraviolet (benzophenone, xanthone) or visible region (thioxanthone). They act mainly by sensitization and allow for a high catalytic turnover in enantioselective [2 + 2] photocycloadditions and in deracemization reactions. Eventually, the concept of lactam hydrogen bonding was transferred to transition-metal catalysis, and catalysts have been developed which combine, in an enzyme-like fashion, a site for substrate binding and a catalytically active site. Substrate binding has been mainly achieved by a V-shaped ligand based on a tricyclic octahydro-1H-4,7-methanoisoindol-1-one scaffold with a lactam hydrogen-bonding site. The catalytically active metal (ruthenium, manganese, rhodium) is perfectly positioned to the substrate for a site- and enantioselective transfer of an oxygen atom (oxidation, oxygenation) or a nitrogen-based fragment (aziridination, amination).