Palladium-Catalyzed Desymmetrization of Silacyclobutanes with Alkynes to Silicon-Stereogenic Silanes: A Density Functional Theory Study.

The palladium-catalyzed desymmetrization of silacyclobutanes using electron-deficient alkynes proceeds with high enantioselectivity in the presence of a chiral P ligand; this provides a facile approach for the synthesis of novel silicon-stereogenic silanes. In this work, we used hybrid density functional theory (DFT) to elucidate the mechanism of the palladium-catalyzed desymmetrization of silacyclobutanes with dimethyl acetylenedicarboxylate. Full catalytic cycle including two different initiation modes that were proposed to be a possible initial step to the formation of the 1-pallada-2-silacyclopentane/alkyne intermediate-the oxidative addition of the palladium complex to the silacyclobutane Si-C bond (cycle MA) or coordination of the Pd complex with the alkyne C≡C bond (cycle MB)-have been studied. It was found that the ring-expansion reaction began with cycle MB is energetically more favorable. The formation of a seven-membered metallocyclic Pd intermediate was found to be the rate-determining step, whereas the enantioselectivity-determining step, oxidative addition of silacyclobutane to the three-membered metallocyclic Pd intermediate, was found to be quite sensitive to the steric repulsion between the chiral ligand and silacyclobutane.