Cross-Electrophile Silylation of Aryl Carboxylic Esters with Hydrochlorosilanes by SiH-Directed and Cr-Catalyzed Couplings

The formation of silicon–carbon bonds through the selective coupling of two strong electrophilic bonds has been less developed. We report here silylation reactions that proceed by coupling of strong Si–Cl/C–O bonds in a process that is promoted by chromium catalysis via α-agostic SiH → Cr interactions, allowing the formation of Si–C bonds under ambient conditions. The reactions occur by coupling of inexpensive hydrochlorosilanes with unactivated aryl carboxylic esters to give arylated hydrosilanes, while suppressing the side hydride-mediated reduction of aryl carboxylic esters in achieving high chemoselectivity. In addition to monosilylation, both geminal Si–Cl bonds of hydrodichlorosilanes couple with aryl C–O bonds smoothly to afford two Si–C bonds and achieve double silylations. The formation of disilanes by coupling of hydrochlorosilanes with two C–O bonds of diesters is also described. Experimental and theoretical studies suggest that silylation initiates by reaction of hydrochlorosilane with Cr, in which the α-agostic SiH interaction of the SiH group to Cr stabilizes the related intermediate and transition state, leading to cleavage of the Si–Cl bond by Cr with a low barrier to give silachromate. Further breaking of an unactivated C–O bond with Cr occurs along with the agostic interaction, which may serve as the rate-determining step in silylation. The observation of a normal second-order kinetic isotope effect indicates the effect of the agostic SiH → Cr interaction on the rates of coupling. Because reactive SiH groups of hydrochlorosilanes are retained in silylation, the approach provides a viable strategy to access tetraorganosilane motifs through late-stage hydrofunctionalization.