1,1-Organoboration of Sterically Hindered Tetra-l-alkynylsilanes and the Crystal Structure of Tetrakis[l-(3,3-dimethyl)butynyl]silane

B ernd W rackm eyer* , H eid i E. M aisei, Jü rgen Süß, W olfgang M ilius Laboratorium für Anorganische Chemie der Universität Bayreuth. D-95440 Bayreuth Z. Naturforsch. 51b, 1320-1324 (1996); received March 14, 1996 Tetra-l-alkynylsilanes, 1,1-Organoboration, Siloles, X-Ray Tetra-l-alkynylsilanes, Si(CsCR*)4 with R 1 = 'Bu (1) and SiMe3 (2) react with triethylborane, E t3B. by intermolecular 1,1-ethyloboration followed by intramolecular 1.1-vinyloboration. to give the substituted siloles 3 and 5 with two R 'C = C groups linked to silicon. The formation of 5 (R 1 = SiMe3) requires exactly one equivalent of E t3B, whereas 3 (R 1 = 'Bu) is formed after = 20 d in boiling toluene in the presence of a large excess of E t3B, and the remaining 'B u -C = C groups in 3 do not react with E t3B. The 1,1-organoboration of 2 using one equivalent of 9-ethyl-9-borabicyclo[3.3.1]nonane, Et-9-BBN, proceeds similar to the E t3B-reaction, to give mainly the silole derivative 6, in which the bicyclic ring system is enlarged by two carbon atoms. This product is the result of a kinetically controlled reaction. The single crystal X-ray analysis of 1 has been carried out (space group Pnma; a = 990.0(2), b = 1668.0(3), c = 1479.0(3) pm), and its crystal structure is compared with that of the corre­ sponding tin compound 1 (Sn).