Two series of tert-butyldialkylamines have been prepared and examined for borane complexation. The complexing ability of each amine in the two series examined decreases in the order shown. First series: t-BuN(CH2CH2)2O 1a > t-BuNEt2 1b > t-BuNPr n 2 1c > t-BuN(CH2CH2OMe)2 1d ≫ t-BuNBu i 2 1e. Second series: t-BuNBu i Me 2a > t-BuNPr i Me 2b > t-BuNBu i Et 2c > t-BuNBu i Prn 2d ≫t-BuNPr i Et 2e. The reactivity of the corresponding borane adducts toward 1-octene increases in the reverse order. The following amines form highly reactive liquid borane adducts hydroborating 1-octene in tetrahydrofuran at room temperature in less than 1 h: t-BuN(CH2CH2OMe)2, t-BuNBu i Et, and t-BuNPr i Me. The limit of borane complexation among the amines examined is reached for t-BuNBu i 2 exchanging borane neither with BMS nor with BH3−THF. Among the various borane adducts prepared, the more promising borane adducts, t-Bu(CH3OCH2CH2)2N−BH3 (7), t-BuMePr i N−BH3 (8), and t-BuEtBu i N−BH3 (9), were selected for complete hydroboration and reduction studies. Hydroboration studies with the new, highly reactive trialkylamine−borane adducts 7−9 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 1−2 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 slow hydroboration. However, the more hindered olefins, α-pinene and 2,3-dimethyl-2-butene, give stable monoalkylboranes very rapidly, with further hydroboration proceeding relatively slowly. The hydroborations can also be carried out conveniently in other solvents, such as THF, tert-butyl methyl ether, and n-pentane. A significant rate retardation is observed in dichloromethane. Regioselectivity studies of 1-hexene and styrene using these amine−borane adducts show selectivities similar to that of BH3−THF. The rates and stoichiometry of the reaction of t-BuMePr i N−BH3 in tetrahydrofuran with selected organic compounds containing representative functional groups were also examined at room temperature. The reductions of esters, amides, and nitriles, which exhibit a sluggish reaction at room temperature, proceed readily under reflux conditions in tetrahydrofuran and dioxane and without solvent (at 85−90 °C). The carrier amines can be recovered by simple acid−base manipulations in good yield and readily recycled to make the borane adducts.
Borane adducts with selected N,N-dialkylanilines have been prepared and examined as hydroborating agents. The adduct H3B:NPhPr i 2 is a solid, considered less desirable than liquid adducts as hydroborating agents. Fortunately, the adducts H3B:NPhBu i Me, H3B:NPhPr i Me, H3B:NPhPr i Et, and H3B:NPhPr i Pr n are liquids above 0 °C, hydroborating 1-octene in tetrahydrofuran in less than 1 h at room temperature. Convenient procedures are described for generating gaseous diborane quantitatively, either by thermal dissociation of the borane−N,N-diisopropylaniline adduct or by the reaction of a 2.00 M solution of sodium borohydride in triglyme with a 5.40 M solution of boron trifluoride in triglyme.
Several N,N-diethyl-tert-alkylamines, such as N,N-diethyl-2-methyl-2-butylamine (1, t-PentNEt2), N,N-diethyl-2,3-dimethyl-2-butylamine (2, t-HexNEt2), N,N-diethyl-2,3,3-trimethyl-2-butylamine (3, t-HeptNEt2), and N,N-diethyl-1,1,3,3-tetramethylbutylamine (4, t-OctNEt2) with varying steric bulk around nitrogen (by changing the tert-alkyl group) have been prepared and examined as borane carriers. The complexing ability of these N,N-diethyl-tert-alkylamines with borane decreases in the order: t-BuNEt2 > t-PentNEt2 > t-HeptNEt2 > t-HexNEt2 > or = t-OctNEt2. From these preliminary studies, the more promising tert-octyldialkylamines were selected for detailed studies. The optimum steric bulk around the nitrogen atom was established by comparing various tert-octyldialkylamines containing variable steric requirements for both the alkyl groups. The complexing ability of these amines with borane decreases in the order shown: t-OctNMe2 (5) > t-OctNEtMe (6) > t-OctN-(CH2CH2)2O (7) > t-OctNEt2 (4) > t-OctNBuiMe (8) > t-OctNPr(n)2 (9). The reactivity of the corresponding borane adducts toward 1-octene increases in the reverse order. Among the various tert-octyldialkylamine-boranes prepared and examined, only t-OctNEt2 (4) forms a highly reactive liquid borane adduct, which hydroborates 1-octene in tetrahydrofuran rapidly at room temperature. Accordingly, detailed hydroboration studies with this new, highly reactive amine-borane adduct, t-OctEt2N:BH3 (10) and representative mono-, di-, tri-, and tetra-substituted olefins were carried out at room temperature (22 +/- 3 degrees C) in selected solvents, tetrahydrofuran, dioxane, tert-butyl methyl ether, n-pentane and dichloromethane. Simple unhindered olefins were hydroborated to the trialkylborane stage, whereas hindered olefins were partially hydroborated to the mono or dialkylborane stage. The hydroborations can be carried out conveniently in a variety of solvents. The amine-borane adduct showed enhanced reactivity in dioxane but low reactivity in dichloromethane. 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 back to the borane adduct.
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