Chiral β‐Lactams by Ruthenium‐Catalyzed Enantioselective CH Carbene Insertion

A ruthenium‐catalyzed intramolecular carbenoid insertion into a C(sp 3 )  H bond leads to the enantioselective formation of chiral β‐lactams. The reaction is catalyzed by a ruthenium complex featuring a chiral pyridine‐2,6‐bis(oxazoline) ligand, combined with a cyclometalated phenylimidazo[1,5‐ a ]pyridine ligand. This complex belongs to a recently introduced new class of ruthenium catalysts ( Angew. Chem. Int. Ed. 2020 , 59 , 12393). Starting from N ‐benzyl α‐diazoacetamides, chiral β‐lactams are provided in up to 82% isolated yield with up to 98% enantiomeric excess, without any occurrence of a Buchner side reaction. Mechanistic experiments reveal that β‐lactam (4‐membered ring) formation is inherently favored over the more common γ‐lactam (5‐membered ring) formation.