Molecular addition compounds.: 15.: Synthesis, hydroboration, and reduction studies of new, highly reactive tert-butyldialkylamine-borane adducts

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)(2)O 1a > t-BuNEt2 Ib > t-BuNPr(2)(n)1c > t-BuN(CH2CH2OMe)(2) Id much greater than t-BuNBu2i le. Second series: t-(BuNBuMe)-Me-i 2a > t-(BuNPrMe)-Me-i 2b > t-(BuNBuEt)-Et-i 2c > t-(BuNBuPrn)-Pr-i 2d much greater than t-(BuNPrEt)-Et-i 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-(BuNBuEt)-Et-i, and t-(BuNPrMe)-Me-i. The limit of borane complexation among the amines examined is reached for t-BuNBu2i exchanging borane neither with EMS nor with BH3-THF. Among the various borane adducts prepared, the more promising borane adducts, t-Bu(CH3OCH2CH2)(2)N-BH3 (7), t-(BuMePrN)-N-i-BH3 (8), and t-(BuEtBuN)-N-i-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 l-hexene, styrene, beta-pinene, cyclopentene, norbornene, cyclohexene, 2-methyl-2-butene, alpha-pinene, and 2,3-dimethyl-2-butene, in tetrahydrofuran, dioxane, tert-butyl methyl ether, n-pentane, and dichloromethane, at room temperature (22 +/- 3 degrees 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, alpha-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 l-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-BuMePriN-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 degrees C). The carrier amines can be recovered by simple acid-base manipulations in good yield and readily recycled to make the borane adducts.