Synthesis, Structure, and Reactivity of Stable Silabenzenes

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Since silicon is located immediately below carbon in the periodic table, organosilicon chemists have been interested in the synthesis of important classes of compounds where carbon atoms are replaced by silicon. Silaaromatic compounds have particularly been attracting the attension of organosilicon chemists because of great importance of aromatic compounds in organic chemistry. Silaaromatic compounds have eluded isolation although some compouns of such sort have been spectroscopically observed in low temperature matrices. In this study, we have succeeded in the synthesis of the first stable silabenzene by taking advantage of a very efficient steric protection group, 2, 4, 6-tris [bis (trimethylsilyl) methyl] phenyl (abbreviated to Tbt group) which was developed by us. The precursor of the silabenzene was prepared from a stanna-1, 4-diene via a reaction of a silicon hydride with TbtLi and dehydrochlorinated by lithium diisopropyl amide to give the silabezene. The silabenzene has been shown to be a delocalized aromatic system from ィイD129ィエD1Si NMR (92.5 ppm), ィイD11ィエD1JィイD2Si-CィエD2 (83 Hz), and UV (301, 323, 331 nm). The X-ray crystallographic data also corroborate this conclusion ; the Si=C bond lengths are almost identical to each other (1.765 and 1.770 Å) and in the middle between normal Si-C double and single bonds. The C-C bond lengths are also close to that of benzene. The silabenzene undergoes interesting photchemical valence isomerization to the corresponding silabenzvalene. This is the first stable silabenzvalene and reverts to the silabenzene upon heating.