Cobalt‐Catalyzed Intramolecular C─H Silylation of Arenes

Direct C-Si bond formation via C-H silylation offers an efficient atom-economical route to organosilanes, yet existing strategies remain largely restricted to noble-metal catalysis. Here, we disclose the first cobalt-catalyzed intramolecular C-H silylation of arenes, enabled by the well-defined hydride complex HCo(PMe₃)₄. This catalyst sequentially activates Si-H and arene C─H bonds, enabling either an hydrosilylation/cyclization sequence or direct cyclization. The method features broad functional-group tolerance and delivers diverse heterosilacycles and silafluorenes in high yields and is extended to germacycle. DFT studies support a two-electron mechanism involving oxidative addition of the Si─H bond, H₂ elimination facilitated by a hydrogen acceptor, intramolecular C─H activation to form a cobaltacycle, and final reductive elimination to furnish the silacycle. Overall, this work establishes cobalt hydrides as a sustainable and effective alternative to noble metals for the synthesis of silacyclic architectures.