Building up Strain in One Step: Synthesis of an Edge-Fused Double Silacyclobutene from an Extensively Trichlorosilylated Butadiene Dianion

The exhaustive trichlorosilylation of hexachloro-1,3-butadiene was achieved in one step by using a mixture of Si₂Cl₆ and nBu₄N]Cl (7:2 equiv) as the silylation reagent. The corresponding butadiene dianion salt nBu₄N]₂ 1 ] was isolated in 36 % yield after recrystallization. The negative charges of 1 ]^2? are mainly delocalized across its two carbanionic (Cl₃Si)₂C termini (α-effect of silicon) such that the central bond possesses largely C=C double-bond character. Upon treatment with 4 equiv of HCl, 1 ]^2? is converted into neutral 1,2,3,4-tetrakis(trichlorosilyl)but-2-ene, 3 . The Cl^? acceptor AlCl₃, induces a twofold ring-closure reaction of 1 ]^2? to form a six-membered bicycle 4 in which two silacyclobutene rings are fused along a shared C=C double bond (84 %). Compound 4 , which was structurally characterized by X-ray crystallography, undergoes partial ring opening to a monocyclic silacyclobutene 2 in the presence of HCl, but is thermally stable up to at least 180 °C.