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 Si(2)Cl(6) and [nBu(4)N]Cl (7:2 equiv) as the silylation reagent. The corresponding butadiene dianion salt [nBu(4)N](2)[1] was isolated in 36 % yield after recrystallization. The negative charges of [1](2−) are mainly delocalized across its two carbanionic (Cl(3)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 AlCl(3), 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.