Terminal allenylsilanes

Hydroboration of terminal allenylsilanes with 9-BBN occurs from the less hindered side to afford [1] the kinetic products with Z geometry, which undergo fast 1,3-sigmatropic rearrangements to thermodynamically more stable ( )-2-(trimethylsilyl) allylboranes. Condensation of isomer with aldehydes proceeds through six-membered transition state and on treatment with 2-aminoethanol [2] gives the desired silated homoallylic alcohols with high diastereomeric purity (Scheme 6.15). 

Aromatic hydrocarbons are alkenylated with allenylsilane in the presence of GaCl3 at — 90 °C. A modest level of 0r// 0-seleetivity is observed. Organogallium electrophiles generated from allenes and GaCl3 are active intermediates in this reaction. While the allenylsilane reacts exclusively at the central carbon, 1,2-alkadiene reacts at the terminal carbon predominantly (Scheme 151).446... 

Allenylsilanes lacking a C-1 alkyl substituent do not function efficiently as three-carbon synthons in the [3 + 2] annulation. This phenomenon is attributable to the relative instability of the terminal vinyl cation intermediate required according to the proposed mechanism for the annulations (eq 6). Fully substituted five-membered rings result from annulations emplo3fing allenylsilanes substituted at both C-1 and C-S. ... 

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