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 alpha-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 alpha-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.