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The hydroboration of bis(trimethylsilyl)ethyne, Me 3 Si-C≡C-SiMe 3  1, with 9-borabicyclo [3.3.1] nonane (9-BBN) gives first the expected (Z)-alkene 5 which rearranges, without UV irradiation, after several days quantitatively into the (E)-alkene 6. Heating of 6 in the presence of a further equivalent of 1 leads, again almost quantitatively, to an allene, the 1,1,4,4-tetrakis(trimethylsilyl)-4-[9-(9- borabicyclo[3.3.1]nonyl)]buta-1,2-diene 8. Reactive structures arising from π-σ delocalisation involving the boryl group are proposed to be responsible both for the cis/trans-isomerisation and the allene formation. It is suggested that analogous structures may explain the previously observed formation of allenes 9 and 10, tin-analogues of 8. The structures of 5, 6 and 8 in solution follow from a consistent set of NMR data ( 1 H,  11 B,  13 C,  29 Si NMR), and the solid-state structure of 8 was confirmed by X-ray analysis.

Hydroboration of Bis(Trimethylsilyl)Ethyne. New Aspects of Hydroboration