Cyclization by metal-catalyzed hydrogen atom transfer/radical-polar crossover

Catalytic transformation of alkenes via the metal-hydride hydrogen atom transfer (MHAT) mechanism has notably advanced synthetic organic chemistry. This review focuses on MHAT/radical-polar crossover (MHAT/RPC) conditions, offering a novel perspective on generating electrophilic intermediates and facilitating various intramolecular reactions. Upon using cobalt hydrides, the MHAT mechanism displayed exceptional chemoselectivity and functional group tolerance, making it invaluable for the construction of complex biologically relevant molecules under mild conditions. Recent developments have enhanced regioselectivity and expanded the scope of MHAT-type reactions, enabling the formation of cyclic molecules via hydroalkoxylation, hydroacyloxylation, and hydroamination. Notably, the addition of an oxidant to traditional MHAT systems enables the synthesis of rare cationic alkylcobalt(IV) complexes, bridging radical mechanisms to ionic reaction systems. This review culminates with examples of natural product syntheses and exploration of asymmetric intramolecular hydroalkoxylation, highlighting the ongoing challenges and opportunities for future research to achieve higher enantioselectivity. This review revisits the historical evolution of the MHAT mechanism and provides the groundwork for further innovations in the synthesis of structurally diverse and complex natural products.