Photoexcited Pd() auxiliaries enable light-induced control in C(sp)-H bond functionalisation

Herein we report the photophysical and photochemical properties of palladacycle complexes derived from 8-aminoquinoline ligands, commonly used auxiliaries in C-H activation. Spectroscopic, electrochemical and computational studies reveal that visible light irradiation induces a mixed LLCT/MLCT charge transfer providing access to synthetically relevant Pd( iii )/Pd( iv ) redox couples. The Pd( ii ) complex undergoes photoinduced electron transfer with alkyl halides generating C(sp 3 )-H halogenation products rather than C-C bond adducts. Online photochemical ESI-MS analysis implicates participation of a mononuclear Pd( iii ) species which promotes C-X bond formation via a distinct Pd( iii )/Pd( iv ) pathway. To demonstrate the synthetic utility, we developed a general method for inert C(sp 3 )-H bond bromination, chlorination and iodination with alkyl halides. This new strategy in auxiliary-directed C-H activation provides predictable and controllable access to distinct reactivity pathways proceeding via Pd( iii )/Pd( iv ) redox couples induced by visible light irradiation. Visible light irradiation of 8-aminoquinoline Pd( ii ) complexes initiates photoinduced electron transfer with alkyl halides, affording C-H halogenation over C-C bond adducts. A method for inert C(sp 3 )-H bond halogenation (Br, Cl and I) is reported.