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).