Cr-catalyzed allylic C(sp3)-H addition to aldehydes enabled by photoinduced ligand-to-metal charge transfer

Photoinduced ligand-to-metal charge transfer (LMCT) has emerged as an effective strategy for synthesizing organic molecules in a sustainable manner. However, the majority of existing reports on selective C(sp(3))-H bond functionalization via photoinduced LMCT focus on the use of late transition metals or rare-earth metals for radical additions or cross-couplings. In contrast, the utilization of photoinduced LMCT with 3d early transition metals poses a significant challenge. Herein, we describe an unprecedented approach to allylic C(sp(3))-H addition to aldehydes, employing chromium (Cr) complexes as catalysts through visible-light-induced LMCT. By investigating the reaction pathway through various mechanistic studies, including radical trapping, kinetic isotope effect (KIE) analysis, and transient absorption spectroscopy, valuable insights have been gained. The proposed mechanism suggests the intermediacy of bromine radicals through homolysis of the Cr-Br bond. Notably, this protocol expands our understanding of the photochemical properties of earth-abundant Cr complexes.