Reactivity of Triethylborane towards Di(alkyn-1-yl)(chloro)silanes. Competition between 1,1-Organoboration and 1,2-Hydroboration

Reactions of di(alkyn-1-yl)(chloro)silanes, HSi(Cl)(C≡C-R)2, RSi(Cl)(C≡C-R)2 or HSi(Cl)(C≡C-R)C≡C-R′, with an excess of triethylborane, BEt3, proceed slowly (several days) at 100 – 120 ◦C. Twofold 1,1-organoboration of HSi(Cl)(C≡C-R)2 or HSi(Cl)(C≡C-R)C≡C-R′ leads to siloles, independent of R = nBu, tBu, SiMe3. This provides the most straightforward way to siloles bearing both a hydrogen and a chlorine at the silicon atom. However, in the cases of R = Ph, BEt3 acts as 1,2-hydroborating reagent in the intermolecular first step of the reaction, leading to 1-silacyclobutene derivatives. All siloles and 1-silacyclobutene derivatives were characterized by multinuclear NMR spectroscopy (1H, 11B, 13C and 29Si). Comparable 1-silacyclobutene derivatives were formed using 9-borabicyclo[3.3.1]nonane, 9-BBN, as a well established 1,2hydroborating reagent.