Ring Expansion of Silacyclobutanes with Allenoates to Selectively Construct 2- or 3-(<i>E</i>)-Enoate-Substituted Silacyclohexenes
Xiaoxiao Tang, Yan Zhang, Yulang Tang, Yi Li, Jiajing Zhou, Duyang Wang, Lu Gao, Zhishan Su, Zhenlei Song
Abstract
Silacycle is one of the most essential core frameworks in silicon-containing functional molecules. However, the structural diversity of silacycles has been largely limited due to the lack of general synthetic methods. Here, we report an efficient synthesis of exo-cyclic enoate-substituted silacyclohexenes by the ring expansion of silacyclobutanes with allenoates. The reaction proceeds with two regioselectivities during Si–C bond insertion. In the presence of the Pd/PR3 catalyst, unsubstituted allenoates undergo β, γ-insertion to form a Si–Cβ bond, giving 2-(E)-enoate-substituted silacyclohexenes. In this pathway, a chiral phosphoramidite ligand is used to construct the stereogenic silicon center enantioselectively. In the second pathway, in the presence of the PtCl2 catalyst, α-substituted allenoates undergo γ, β-insertion to form a Si–Cγ bond, leading to 3-(E)-enoate-substituted silacyclohexenes. The control experiments and density functional theory calculations have been performed to understand the regio- and stereochemical outcome of both Pd- and Pt-catalyzed ring expansion reactions.