Iridium-Catalyzed Diorganosilylene Insertion into C(sp²)-O Bond of Oxacycles Using Aryldiorganosilanes as Silylene...

Silylene transfer reactions, i.e., silylene cycloaddition and insertion reactions, allow single-step formation of two silicon-containing σ bonds. This has attracted much attention in synthetic organosilicon chemistry, whereas its development has been significantly hampered by the lack of synthetically viable, readily accessible silylene precursors. We herein report a new silylene insertion reaction using aryldiorganosilanes (ArR¹R²SiH) as silylene transfer reagents, in which a diorganosilylene unit is inserted into the O-C-(sp²) bond of unsaturated oxacycles, such as benzofurans, enabled by iridium catalysts with extrusion of Ar-H as the only byproduct. A variety of silylenes, including diaryl-, (aryl)-(alkyl)-, dialkyl-, and even (aryl)-(alkoxy)-silylene units, are transferred to give ring-enlarged oxasilacycles. This protocol is extended to the germylene insertion reaction using Ph₃GeH as a source of the diphenylgermylene group. The new silylene insertion is not limited to unsaturated oxacycles, including benzofurans and furans containing various functional groups, but is also applicable even to an acyclic enol ether, albeit in low yield. The complete silylene insertion process consists of two sequential silylene insertion processes, i.e., intermolecular silylene insertion into the C-(sp²)-H bond α to the oxygen atom, followed by intramolecular silylene insertion into the C-O bond. Formal silicon atom transfer is demonstrated using PhSiH₃ as a source of the silicon atom, which in the first step undergoes cyclative double C-H silylation, followed by the present Ir-catalyzed silylene insertion to benzofuran. Using 5-(2-diphenyl-hydro-silyl-ethyl)-benzofuran as a monomer, silylene-inserting polymerization has been demonstrated.