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

• Published in: Chemistry, an Asian journal Vol. 11; no. 20; pp. 2867 - 2875
• Main Authors: Zhang, Jin, Xu, Jin-Zhou, Zheng, Zhan-Jiang, Xu, Zheng, Cui, Yu-Ming, Cao, Jian, Xu, Li-Wen
• Format: Journal Article
• Language: English
• Published: Germany Blackwell Publishing Ltd 20.10.2016; Wiley Subscription Services, Inc
• Subjects: Chemistry; density functional calculations; desymmetrization; Ligands; reaction mechanisms; ring expansion; silylation; reaction mechanisms; density functional calculations; desymmetrization; silylation; ring expansion
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.201600709

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 Pd0 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 PdII intermediate was found to be the rate‐determining step, whereas the enantioselectivity‐determining step, oxidative addition of silacyclobutane to the three‐membered metallocyclic PdII intermediate, was found to be quite sensitive to the steric repulsion between the chiral ligand and silacyclobutane. Powerful palladium: The ring‐expansion step—which started from an intermediate through alkyne insertion into a Pd−Si bond and led to a seven‐membered metallocyclic PdII intermediate—was found to be the rate‐determining step. The selective cleavage of the silacyclobutane Si−C bond by the three‐membered metallocyclic PdII intermediate was found to be the stereo‐controlling step (see scheme).