Molecular Addition Compounds. 11. N-Ethyl-N-isopropylaniline−Borane, A Superior Reagent for Hydroborations and Reductions

Abstract

Hydroboration studies with a new, highly reactive amine−borane adduct, H3B−NPhEtPri, 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, in tetrahydrofuran, dioxane, tert-butyl methyl ether, n-pentane, and dichloromethane, at room temperature (22 ± 3 °C) were carried out. The reactions are faster in dioxane, requiring 0.5−1 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 slow. 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 can also be carried out in other solvents, such as THF, tert-butyl methyl ether, and n-pentane. A significant rate retardation is observed in dichloromethane. Regioselectivity studies in the hydroboration of 1-hexene, styrene, and allyl chloride with H3B−NPhEtPri in selected solvents were made. The selectivities are similar to those reported for BH3−THF with 1-hexene and styrene, whereas some differences were noted for allyl chloride. 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 rates and stoichiometry of the reaction of H3B−NPhEtPri in tetrahydrofuran with selected organic compounds containing representative functional groups were examined at room temperature. Simple aldehydes, ketones, carboxylic acids, and aliphatic esters were reduced to the alcohol stage. Acid chlorides, anhydrides, and aromatic carboxylic esters were unreactive under similar conditions. Imines, tertiary amides, and nitriles were reduced to the corresponding amines. However, primary and secondary amides and nitro compounds were not reduced under these conditions. The reduction of esters, amides, and nitriles, which exhibit a sluggish reaction at room temperature, proceeds readily under reflux conditions in tetrahydrofuran and dioxane and also without solvent (at 85−90 °C). The carrier amine was recovered by simple acid−base manipulations in good yield and can be readily recycled to make the borane adduct.