Hexa(1-alkynyl)disilanes. Synthesis, structure and 1,1-ethylboration

The reaction of hexachlorodisilane, Si2Cl6, with six equivalents of lithium alkynides, LiCCR (R=Bu, SiMe3) led to hexa(1-alkynyl)disilanes Si2(CCR)6 [R=Bu (1), Me3Si (2)], whereas the reaction with the propynide LiCCMe, under analogous conditions, afforded only tetra(1-propynyl)silane, Si(CCMe)43. The molecular structures of 2 and 3 were determined by X-ray analysis. Surprisingly, in the course of 1,1-ethylboration of the disilanes 1and 2 the SiSi bond is cleaved and finally monosilanes such as the substituted 5-silaspiro[4.4]nona-2,4,6,8-tetraenes 4 and 5, respectively, were obtained, compounds analogous to 6 which previously have been shown to result from 1,1-ethylboration of tetra(1-alkynyl)silanes Si(CCR)4 such as 3 (R=Me). In the case of the reaction of 2 with triethylborane, it proved possible to detect an intermediate 7, in which the disilane structure is still retained and, according to NMR data (1H, 11B, 13C, 29Si-NMR), only two of the six CCSiMe3 moieties have reacted.