Novel sila-2,4-cyclopentadienes via 1,1-ethylboration of Si- and C functionalised di(alkyn-1-yl)silanes

Di(alkyn-1-yl)dimethylsilanes and -methylsilanes of the type Me₂Si(CC-CH₂NMe₂)₂ (1a), Me(H)Si(CC-CH₂NMe₂)₂ (1b), Me₂Si(CC-CH₂NMe₂)CC-R¹ R¹=Bu (3a), SiMe₃ (4a)], and Me(H)Si(CC-CH₂NMe₂)CC-R¹ R¹=Bu (3b), SiMe₃ (4b)] were prepared, and their reactivity towards triethylborane, BEt₃, was studied. In all cases, the final products of 1,1-ethylboration were sila-2,4-cyclopentadienes (siloles) 7, 10-13, bearing a diethylboryl group in 3- or 4-position. If the CH₂NMe₂ and the boryl group are in neighboured positions, the Me₂N group is always coordinated to the boron atom. Side products 9b and 16b containing an electron-deficient Si-H-B bridge could be detected by NMR spectroscopy. The proposed structures of the siloles follow from a consistent set of multinuclear magnetic resonance data (¹H, ¹¹B, ¹³C and ²⁹Si NMR).