Neutral photogenerated N-centred radicals as a general, catalytic direct hydrogen atom transfer platform for aliphatic C–H functionalization

Abstract

The introduction of diverse functionalities into omnipresent aliphatic C–H bonds is of notable value yet challenging in catalysis. While hydrogen atom transfer has emerged as a premier strategy, existing protocols frequently rely on O-centred radicals either generated from hazardous stoichiometric peroxides or within photocatalysts that are promiscuous in their excited state reactivity and non-conducive to structural tuning around the reactive centre. Here we disclose a general platform of readily prepared 9-arylacridine catalysts—a class of neutral, N-centred photocatalysts—that mediate aliphatic C–H bond hydrogen atom transfer under mild conditions. This strategy enables efficient generation of alkyl radicals, allowing various functionalizations ranging from activated substrates to gaseous ethane. The platform features broad substrate scope, scalability and compatibility with transition metal catalysis as well as multicomponent reactions. Its utility is demonstrated in the streamlined synthesis of pharmaceutical synthons and late-stage functionalization of bioactive molecules. Detailed mechanistic studies highlight the crucial role of ortho-substituents on the 9-aryl moiety of catalysts, providing a rational basis for catalyst design. Direct hydrogen atom transfer (d-HAT) for C(sp3)–H functionalization classically relies on O-centred photocatalysts. Now, neutral N-centred acridine photocatalysts have been added to the toolbox, enabling mild, scalable d-HAT that is compatible with a broad substrate scope and transition metal catalysis.