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.

ABSTRACT

Several less volatile oxygen-containing Lewis bases, such as tert-butyl methyl ether, dioxane, anisole, ethyl acetate, beta-chloroethyl ether, and monoglyme, were examined as prospective mono- and dichloroborane carriers. Dioxane, ethyl acetate, and beta-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 beta-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 beta-pinene, when compared to simple terminal and hindered olefins, giving a unique tool for selective hydroborations. Dichloroborane adducts of monoglyme and beta-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.