Hydroboration. 56. Convenient and Regiospecific Route to Functionalized Organosilanes through the Hydroboration of Alkenylsilanes.

Article abstract of DOI:10.1021/jo01306a006

A systematic and detailed study of the hydroboration of several representative alkenylsilanes is reported.By use of a 1:1 molar stoichiometry, the hydroboration of vinyltrimethylsilane (1) at 0 deg C with BH3*THF gives a mixture of dialkylborane products exclusively with a ca. 60:40 distribution of a α- and β-silylethyl groups in these adducts.Redistribution of the reaction mixture at a somewhat higher temperature (50 deg C) gives the corresponding monoalkylboranes.The redistribution process is regiospecific, leaving the original 60:40 ratio of α- and β-silylethyl groups in the dialkylboranes unchanged in the monoalkylboranes produced.With a 3:1 ratio of 1 to BH3*THF, equimolar amounts of dialkyl- and trialkylborane products are formed in the hydroboration reaction.Under these conditions, the α to β ratio changes modestly to 50:50.The failure of the hydroboration to attain complete conversion to the trialkylborane stage is attributed largely to the low reactivity of meso-bis<1-(trimethylsilyl) ethyl>borane produced in the reaction in the further hydroboration of 1.The change in the regioselectivity from 60:40 to 50:50 with a 3:1 stoichiometry is attributed to the higher β selectivity of dialkylboranes in the hydroborationof 1.Thus, the hydroboration of 1 with 9-borabicyclo<3.3.1>nonane (9-BBN) is regiospecific, giving exclusively the B-<β-(trimethylsilyl)ethyl>-9-borabicyclo<3.3.1>nonane (3) product.Redistribution of 3 with BH3*THF gives essentially complete conversion to 9-BBN and pure bis<2-(trimethylsilyl)ethyl>borane.This compound hydroborates 1 rapidly at 0 deg C, giving predominantly (ca. 85percent) the β adduct, tris<2-(trimethylsilyl)ethyl>borane.Hydroboration of propen-2-yltrimethylsilane with BH3*THF gives primarily (91percent) β boron placement.However, use of 9-BBN eliminates the formation of the minor internal product and gives (2-Me3Si-1-Pr)-9-BBN exclusively.Hydroboration of cis-1-propenyltrimethylsilane (17) with BH3*THF proceeds to the dialkylborane stage, with no significant formation of the corresponding trialkylborane.The hydroboration is highly regioselective, giving the bis<1-(trimethylsilyl)-1-propyl>borane product in 95percent regioisomeric purity.This borane is unusual in that it is monomeric in solution, as revealed by IR and 11B NMR data.Hydroboration of 17 with 9-BBN is much less selective than that with BH3*THF, a rare occurrence, giving essentially equal amounts of regioisomeric adducts.Hydroboration of allyltrimethylsilane with either BH3*THF or 9-BBN gives the (γ-silylpropyl)borane product exclusively.Hydroboration of 3-buten-1-yltrimethylsilane with BH3*THF gives the (δ-silylbutyl)borane adduct in 94percent isomeric purity.With 9-BBN, the hydroboration is regiospecific, giving the (δ-silylbutyl)borane adduct exclusively.Thus, through hydroboration, the syntheses of α, β, γ, and δ boron-functionalized organosilanes are achieved, making such compounds available for use as synthetic intermediates.The 13C and 11B spectra...