1,1-ORGANOBORATION OF STERICALLY HINDERED TETRA-1-ALKYNYLSILANES AND THE CRYSTAL-STRUCTURE OF TETRAKIS[1-(3,3-DIMETHYL)BUTYNYL]SILANE

Tetra-1-alkynylsilanes, Si(C=CR(1))(4) with R(1) = (t)Bu (1) and SiMe( 3) (2) react with triethylborane, Et(3)B, by intermolecular 1,1-ethylo boration followed by intramolecular 1,1-vinyloboration, to give the su bstituted siloles 3 and 5 with two R(1)C=C groups linked to silicon. T he formation of 5 (R(1) = SiMe(3)) requires exactly one equivalent of Et(3)B, whereas 3 (R(1) = (t)Bu) is formed after approximate to 20 d i n boiling toluene in the presence of a large excess of Et(3)B, and the remaining (t)Bu-C=C groups in 3 do not react with Et(3)B. The 1,1-org anoboration of 2 using one equivalent of 9-ethyl-9-borabicyclo[3.3.1]n onane, Et-9-BBN, proceeds similar to the Et(3)B-reaction, to give main ly the silole derivative 6, in which the bicyclic ring system is enlar ged by two carbon atoms. This product is the result of a kinetically c ontrolled reaction. The single crystal X-ray analysis of 1 has been ca rried 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 correspo nding tin compound 1 (Sn).