SYNTHESES, STRUCTURES, AND REACTIONS OF SULFUR AND SELENIUM INSERTION PRODUCTS OF 1,1-DI-TERT-BUTYLSILIRANES

One-atom and two-atom sulfur and selenium ring expansion products of trans-1,1-di-tert-butyl-2,3-dimethylsilirane (1), 6,6-di-tert-butyl-6-silabicyclo[3.1.0]hexane (4), and 7,7-di-tert-butyl-7-silabicyclo[4.1.0]heptane (7) are prepared by stirring the silirane and the chalcogen in the presence of tetrabutylammonium halide salts and/or alkali-metal halide salts with crown ethers. Potassium fluoride, 18-crown-6, and selenium in benzene react with 1 to give the novel ring system trans-2,2-di-tert-butyl-3,4-dimethyl-1,2-selenasiletane (17) in 85% yield. When tetrabutylammonium chloride (TBAC) is stirred with selenium and 1 or 17 in benzene, trans-3,3-di-tert-butyl-4,5-dimethyl-1,2-diselena-3-silacyclopentane (18) is formed in 87% and 82% yields, respectively. Similar results are observed for analogous reactions with sulfur. The mechanism responsible for the differences in product distributions seems to involve chalcogen activation by salts and crown ethers. Control of chalcogen insertions is demonstrated by reaction of 17 and sulfur in the presence of TBAC, which results in a new ring system, trans-3,3-di-tert-butyl-4,5-dimethyl-1-thia-2-selena-3-silacyclopentane (21) in 64% yield. trans-2,2-Di-tert-butyl-3,4-dimethyl-1,2 (2) and cis-7,7-di-tert-butyl-6-thia-7-silabicyclo[3.2.0]heptane (5), prepared from reactions of sulfur with 1 and 4, respectively, are thermolyzed and photolyzed. Products consistent with the generation of a di-tert-butylsilathione fragment are observed, but bimolecular processes are not excluded. Three-dimensional structures of the bicyclic insertion products 7-9, 15, and 27 are elucidated using 1D and 2D NMR techniques. Experimental data are correlated with geometries calculated by molecular mechanics methods. The stereochemistries of 4,4-di-tert-butyl-2,3-dithia-4-silabicyclo[4.3.0]-nonane (8) and of 4,4-di-tert-butyl-2,3-diselena-4-silabicyclo[4.3.0]-nonane (15), prepared from reactions with 7 and sulfur and selenium, respectively, are further confirmed by single-crystal X-ray crystallography.