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 Si2 Cl6 and [nBu4 N]Cl (7:2 equiv) as the silylation reagent. The corresponding butadiene dianion salt [nBu4 N]2 [1] was isolated in 36 % yield after recrystallization. The negative charges of [1]2- are mainly delocalized across its two carbanionic (Cl3 Si)2 C 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.