Synthesis and structures of polysilacage compounds containing a silicon–silicon inter-element linkage

To explore the possibility of three-dimensional σ-conjugation originated from silicon–silicon inter-element linkages, 2,2,3,3,5,5,6,6,7,7,8,8-dodecamethyl-2,3,5,6,7,8-hexasilabicyclo[2.2.2]octane (1) was synthesized as a model compound. The molecular structure of 1 was shown to be slightly distorted from an ideal bicyclo[2.2.2]octane skeleton by X-ray analysis. Functionalization of 1 at bridgehead positions was achieved by treatment with BuLi–t-BuOK followed by a reaction with an electrophile. UV spectra of 1 and its derivatives demonstrated a bathochromic shift, particularly when dimensions of the molecular structure increased and a silyl or stannyl group was introduced at the bridgehead. This fact was understood in terms of three-dimensional σ-conjugation between silicon–silicon linkages. Computational study of model structures is also described.