Molecular Addition Compounds. 13. N,N-Diisopropyl-N-isobutylamine−Borane: The First Highly Reactive Trialkylamine−Borane Reagent for Hydroborations

Abstract

N,N-Diisopropyl-N-isobutylamine (1) forms a stable liquid borane adduct (4.7 M in BH3), when diborane is bubbled into the neat amine. The adduct thus formed is stable indefinitely at room temperature under an inert atmosphere. Hydroboration studies with this new, highly reactive amine−borane adduct, H3B:NPri2Bui (2) and representative olefins, such as 1-hexene, styrene, β-pinene, cyclopentene, norbornene, cyclohexene, 2-methyl-2-butene, α-pinene, and 2,3-dimethyl-2-butene, were carried out at room temperature (22 ± 3 °C) in selected solvents, tetrahydrofuran, dioxane, tert-butyl methyl ether, n-pentane, and dichloromethane. The reactions are faster in dioxane and n-pentane, requiring ∼2 h for the hydroboration of simple, unhindered olefins to the trialkylborane stage. Moderately hindered olefins, such as cyclohexene and 2-methyl-2-butene, give the corresponding dialkylboranes rapidly, with further hydroboration to the trialkylborane stage slower. However, the hindered α-pinene and 2,3-dimethyl-2-butene structures give stable monoalkylboranes very rapidly, with further hydroboration proceeding relatively slowly. The hydroborations also proceed readily in other solvents, such as THF and tert-butyl methyl ether. However, a dramatic rate retardation in dichloromethane is observed. The hydroboration of less hindered olefins is not complete even after 48 h at room temperature, and starting amine−borane still persisted as observed by 11B NMR analysis of the reaction mixture. Surprisingly, more hindered olefins, such as α-pinene and 2,3-dimethyl-2-butene, underwent hydroboration relatively rapidly with no detectable starting amine−borane in 11B NMR analysis of the reaction mixture. The alkylboranes obtained after hydroboration were oxidized with hydrogen peroxide/sodium hydroxide, and the product alcohols were obtained in quantitative yields, as established by GC analysis. The carrier amine was recovered by simple acid−base manipulations in good yield and can be readily recycled to make the borane adduct. This new amine−borane adduct was also very efficient for the synthesis of important dialkylborane reagents. Various dialkylboranes, such as disiamylborane, dIpc2BH, 2-dIcr2BH, 9-BBN, and Chx2BH, were conveniently prepared in dioxane and THF. In the case of solid dialkylborane products, the amine can usually be separated by simple filtration. When the product is a liquid dialkylborane, such as disiamylborane, the presence of amine in the reaction medium usually does not interfere with further reactions.