Synthesis of 1-silacyclopent-2-ene derivatives using 1,2-hydroboration, 1,1-organoboration and protodeborylation

Article abstract of DOI:10.1002/aoc.3121

The reaction of di(alkyn-1-yl)vinylsilanes R¹(H ₂Ca?-CH)Si(C≡C - R)₂ (R¹ = Me (1), Ph (2); R = Bu (a), Ph (b), Me₂HSi (c)) at 25°C with 1 equiv. of 9-borabicyclo[3.3.1]nonane (9-BBN) affords 1-silacyclopent-2-ene derivatives (3a, 3b, 3c, 4a, 4b), bearing one Si - C≡C - R function readily available for further transformations. These compounds are formed by consecutive 1,2-hydroboration followed by intramolecular 1,1-carboboration. Treated with a further equivalent of 9-BBN in benzene they are converted at relatively high temperature (80-100°C) into 1-alkenyl-1-silacyclopent-2-ene derivatives (5a, 5b 6a, 6b) as a result of 1,2-hydroboration of the Si - C≡C - R function. Protodeborylation of the 9-BBN-substituted 1-silacyclopent-2-ene derivatives 3, 4, 5, 6, using acetic acid in excess, proceeds smoothly to give the novel 1-silacyclopent-2-ene (7, 8, 9, 10). The solution-state structural assignment of all new compounds, i.e. di(alkyn-1-yl)vinylsilanes and 1-silacyclopent-2-ene derivatives, was carried out using multinuclear magnetic resonance techniques (¹H, ¹³C, ¹¹B, ²⁹Si NMR). The gas phase structures of some examples were calculated and optimized by density functional theory methods (B3LYP/6-311+G/(d,p) level of theory), and ²⁹Si NMR parameters were calculated (chemical shifts δ²⁹Si and coupling constants ⁿJ(²⁹Si,¹³C)). Copyright