The Hydroboration of Silylacetylenes. The "Silyl-Markovnikov" Hydroboration Route to Pure (Z)-1-(2-Borylvinyl)silanes and β-Keto Silanes

Article abstract of DOI:10.1021/ja00195a046

The regiochemistry and stoichiometry of the hydroboration of simple 1-silylacetylenes with 9-borabicyclo<3.3.1>nonane (9-BBN) was examined in detail. (Trimethylsilyl)acetylene (1c) results in only β-mono- and dihydroboration products, behavior common to nonsilylated terminal acetylenes.It is less reactive than its alkyl counterparts toward 9-BBN.It was also demonstrated that (1-trimethylsilyl)alkynes form (α-borylvinyl)silanes, similar to 1-alkynes, but they are much less reactive.Clean monohydroboration of these silylacetylenes is achieved only with a bulky (e.g. t-Bu) group at the C-2 position.However, compared to 1-alkynes, their dihydroboration products differ markedly, with the former giving 1,2-diboryl compounds, while the latter results in 1,1-diboryl adducts. (Triethyl- and tert-butyldimethylsilyl)propynes exhibit similar reactivity to that of the analogous trimethylsilyl derivative.The triisopropylsilyl group was found to not only completely suppress dihydroboration but also gave exclusively (Z)-(β-borylvinyl)silane adducts (9β).Oxidation of 9β produces the corresponding β-keto silanes (12) in excellent yields (i.e. 83-90percent).The bulky (triethyl- and tert-butyldimethylsilyl)propyne hydroboration products were found to be thermally unstable with the initially formed (α-borylvinyl)silanes isomerizing to α/β mixtures through their 1,2-diboryl diadducts (10).Heating 10 produces (3,3-diborylpropyl)silanes (14).Oxidation of the triethylsilyl compound 14b affords the corresponding γ-silylpropanol (15b).The complete characterization of the intermediate organoboranes by 75-MHz 13C NMR was achieved.A detailed discussion of the phenomena involved is presented.The observed results are attributed primarily to the additional steric effects imposed on an unsaturated system by a trialkylsilyl group when compared to a hydrogen atom at that position.