Syntheses and structures of silicon analogues of cyclopropabenzenes

The reactions of an overcrowded diaryldilithiosilane, Tbt(Dip)SiLi2 (Tbt=2,4,6-tris[bis(trimethylsilyl)methyl]phenyl; Dip=2,6-diisopropylphenyl), with o-dibromobenzene and 1,2,4,5-tetrabromobenzene resulted in the synthesis and isolation of the first stable silacyclopropabenzene and bis(silacyclopropa)benzenes (IUPAC name is 4,8-disilatricyclo[5.1.0.03,5]octa-1,3(5),6-triene), respectively. The crystallographic analyses and theoretical calculations revealed that the lengths of the juncture carboncarbon bonds of the mono- and bis(silacyclopropa)benzenes were marginally in the range of carboncarbon bond lengths of usual benzene rings. It is shown that this structural feature is attributable to a decrease in the strain energy of silacyclopropabenzenes relative to the corresponding cyclopropabenzenes, and the strain energy is a simple additive function of the number of the fused three-membered rings involving Si atom.