Dissociation of the Disilatricyclic Diallylic Dianion [(C₄Ph₄SiMe)₂]⁻² to the Silole Anion [MeSiC₄Ph₄]⁻ by Halide Ion Coordination or Halide Ion Nucleophilic Substitution at the Silicon Atom

Abstract: The reductive cleavage of the Si-Si bond in 1,1-bis(1-methyl-2,3,4,5-tetraphenyl-1-silacyclopentadiene) [(C₄Ph₄SiMe)₂] (1) with either Li or Na in THF gives the silole anion [MeSiC₄Ph₄]⁻ (2). The head-to-tail dimerization of the silole anion 2 gives crystals of the disilatricyclic diallylic dianion [(C₄Ph₄SiMe)₂]⁻² (3). The derivatization of 3 (crystals) with bromoethane (gas) under reduced pressure provides [(MeSiC₄Ph₄Et)₂] (4) quantitatively. The reverse addition of 3 in THF to trimethylsilyl chloride, hydrogen chloride, and bromoethane in THF gives 1-methyl-1-trimethylsilyl-1-silole [Me₃SiMeSiC₄Ph₄] (6), 1-methyl-2,3,4,5-tetraphenyl-1-silacyclo-3-pentenyl-1-methyl-1-silole [C₄Ph₄H₂SiMe-MeSiC₄Ph₄] (7), and 1-methyl-2,5-diethyl-2,3,4,5-tetraphenyl-1-silacyclo-3-pentenyl-1-methyl-1-silole [C₄Ph₄Et₂SiMe-MeSiC₄Ph₄] (8), respectively. The reaction products unambiguously suggest that the silole anion [MeSiC₄Ph₄]⁻ is generated by coordination of the chloride ion at the silicon atom in 3 or by the nucleophilic substitution of either chloride or bromide ion at one of two silicon atoms in 3. The quenching reaction of 3 dissolved in THF with water gives 1,2,3,4-tetraphenyl-2-butene, the disiloxane of 1-methyl-2,3,4,5-tetraphenyl-1-silacyclo-3-pentenyl [O(MeSiC₄Ph₄)₂] (10) and methyl silicate.