Photocatalytic Synthesis of Medium-Sized Lactams via Cascade Ring Expansion Reaction of Unstrained Cyclic Amines with NHC-Borane

Medium-ring lactams are important structural motifs in many scientific fields but are difficult to synthesize due to unfavorable entropic/enthalpic effects and transannular interactions. Herein, an efficient strategy for accessing highly functionalized borylated 9- to 11-membered lactams is established, relying on photocatalytic cascade ring expansion of unstrained cyclic amines with N-heterocyclic carbene (NHC)-borane. Notably, this methodology features mild conditions, excellent functional group tolerance, and great downstream transformations, providing a powerful and practical platform for constructing diverse medium-ring lactams. Mechanistic studies support a radical mechanism involving nucleophilic NHC-boryl radical formation, radical ipso-cyclization, and subsequent 1,4-aryl migration from N-to-C atom to release the desired ring-expanded lactams.