Visible-Light-Mediated Lewis Acid-Catalyzed Diradical Hydrogen Atom Transfer Reaction of Enamides.

β-Lactams are of great importance in organic chemistry and medicinal science due to their essential biologically active and unique architectures. Herein, the facile photochemical synthesis of highly functionalized β-lactams derivatives from easily accessible enamides, enabled by a Lewis acid-catalyzed diradical hydrogen atom transfer (DHAT), is described. Notably, trisubstituted enamides with tertiary C(sp3)-H bonds can efficiently deliver adjacent all-carbon quaternary β-lactams. This effective protocol is compatible with a variety of functional groups and can be applied to the modification of bioactive molecules. Based on mechanistic evidences and computational studies, it is suggested that Lewis acid catalysis can lower the energy of the triplet excited state of enamides and modulate the reactivity of the readily formed 1,2-diradical intermediate to unlock a carbon-to-carbon 1,5-DHAT and consequent cyclization of the 1,4-diradical intermediate that allows the functionalization of C(sp3)-H bonds to construct a variety of novel β-lactams. This distinctive activation mode of Lewis acid catalysis paves a new way of DHAT reactions for the transformation of C(sp3)-H bonds.