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(sp2)–H
bond silylations, catalytic methods for aliphatic C­(sp3)–H bond silylations are relatively rare. Here we report a
pincer ruthenium catalyst for intramolecular silylations of various
primary C­(sp3)–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(sp3)–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 η2-silene ruthenium
hydride species is determined to be an off-cycle intermediate.