Hydroboration. 97. Synthesis of New Exceptional Chloroborane−Lewis Base Adducts for Hydroboration. Dioxane−Monochloroborane as a Superior Reagent for the Selective Hydroboration of Terminal Alkenes†

  • 10.1021/jo015527o
  • The Journal of Organic Chemistry
  • p 5359-5365, Volume 66, Issue 16,
  • journal-article
Several less volatile oxygen-containing Lewis bases, such as tert-butyl methyl ether, dioxane, anisole, ethyl acetate, β-chloroethyl ether, and monoglyme, were examined as prospective mono- and dichloroborane carriers. Dioxane, ethyl acetate, and β-chloroethyl ether form relatively stable boron trichloride adducts, but the boron trichloride adduct of monoglyme is not very stable and must be used immediately. On the other hand, tert-butyl methyl ether and anisole fail to form stable boron trichloride adducts and the corresponding ether-cleaved products are obtained. Among the selected oxygen-containing Lewis bases, only dioxane forms stable and reactive mono- and dichloroborane adducts. Monoglyme and β-chloroethyl ether give stable dichloroborane adducts requiring excess of diborane. Convenient methods for the preparation of mono- and dichloroborane adducts of dioxane from dioxane−BCl 3 and NaBH 4 in the presence of catalytic amounts of tri- or tetraglyme were developed. The dioxane−monochloroborane adduct hydroborates representative olefins cleanly and rapidly. The corresponding alcohols were obtained in quantitative yields after oxidation. Also, the hydroboration of several terminal olefins with dioxane−monochloroborane were highly regioselective and the primary alcohols were obtained almost exclusively (>99.5%), after oxidation. Accordingly, dioxane−monochloroborane should serve as a reagent of choice for such hydroborations. The dioxane−dichloroborane adduct showed remarkable selectivity toward 2-substituted terminal olefins, such as 2-methyl-1-butene and β-pinene, when compared to simple terminal and hindered olefins, giving a unique tool for selective hydroborations. Dichloroborane adducts of monoglyme and β-chloroethyl ether also showed high reactivity, even at room temperature, toward simple unhindered olefins. However, hydroboration of hindered olefins is slow and requires either higher temperatures or the addition of 1 equiv of boron trichloride to liberate free dichloroborane, as in the case of the previously known dichloroborane adducts of methyl sulfide and diethyl ether.


Equivalency of reagents key for successful synthesis

Review: Synthesized the same compound of Dioxane-Monochloroborane by the reaction of Sodium Borohydride with Dioxane-borontrichloride (experimental section, Page 5364). I found the equivalency of the reagents is very important for this reaction. The reaction between 10% excess...