Ring Expansion of Silacyclobutanes with Allenoates to Selectively Construct 2- or 3-(E)-Enoate-Substituted Silacyclohexenes

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.