[4+2]Cycloadditions of organometallic-substituted siloles with dimethyl acetylenedicarboxylate and tetracyanoethylene

1-Sila-2,4-cyclopentadienes (siloles) bearing five organyl groups and a diethylboryl group in 3-position (3), four organyl groups, a trimethylstannyl and a diethylboryl group in 2,4-positions (4), four organyl groups, a diethylboryl group in 3-position and a hydrido function at the silicon atom (5) react by [4+2]cycloaddition with dimethyl acetylenedicarboxylate, MeOC(O)CCC(O)OMe (1), and tetracycanoethylene, (NC)2CC(CN)2 (2), to give 7-silanorbornadienes (6–8) and 7-silanorbornenes (10–12), respectively. The silole 5 is converted into isomers 8 and 8′, and 12 and 12′, in which the SiMe group in each major isomer (8 and 12) occupies the syn-position with respect to the C(2)C(3) bond. The molecular structure of 10a was determined by X-ray analysis. The 7-silanorbornadiene (7) rearranges into a benzene derivative by formation of an SiO bond and 1,3-migration of the trimethylstannyl group. All products were characterised in solution by multinuclear magnetic resonance (1H-, 11B-, 13C-, 29Si-, 119Sn-NMR spectroscopy). The geometries of 1,4,7,7-tetramethyl-7-silanorbornadiene, -7-silanobornene, and -7-silanorbornane were optimised by ab initio MO calculations (RHF/6-311+G(d,p) and chemical shifts δ29Si were calculated (GIAO-RHF/6-311+G(d,p)).