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)
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C
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C
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C(O)OMe (
1), and tetracycanoethylene, (NC)
2C
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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)
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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
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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)).