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.
