Ruthenium-Catalyzed Site-Selective Intramolecular Silylation of Primary C-H Bonds for Synthesis of Sila-Heterocycles.

Incorporating the silicon element into bioactive organic molecules has received increasing attention in medicinal chemistry. Moreover, organosilanes are valuable synthetic intermediates for fine chemicals and materials. Transition metal-catalyzed C-H silylation has become an important strategy for C-Si bond formations. However, despite the great advances in aromatic C(sp)-H bond silylations, catalytic methods for aliphatic C(sp)-H bond silylations are relatively rare. Here we report a pincer ruthenium catalyst for intramolecular silylations of various primary C(sp)-H bonds adjacent to heteroatoms (O, N, Si, Ge), including the first intramolecular silylations of C-H bonds α to O, N, and Ge. This method provides a general, synthetically efficient approach to novel classes of Si-containing five-membered [1,3]-sila-heterocycles, including oxasilolanes, azasilolanes, disila-heterocycles, and germasilolane. The trend in the reactivity of five classes of C(sp)-H bonds toward the Ru-catalyzed silylation is elucidated. Mechanistic studies indicate that the rate-determining step is the C-H bond cleavage involving a ruthenium silyl complex as the key intermediate, while a η-silene ruthenium hydride species is determined to be an off-cycle intermediate.