Molecular Addition Compounds. 15. Synthesis, Hydroboration, and Reduction Studies of New, Highly Reactive tert-Butyldialkylamine−Borane Adducts

Abstract

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.

Ultrasound Mediated Functional Group Reduction and Chemoselective Studies by DMAB

“…DMAB conveniently reduces amide to amine in very good yields within 22 minutes under ultrasound irradiation. According to literature reports 26,32,33 , the reduction of benzamide, acetanilide with N,N-diethylaniline borane requires 7-13 hours under conventional methods. In these reduction 1:2 stoichiometry ratios was followed, because the reduced amine forms a complex with borane hence excess borane needed to make the reduction convenient.…”

“…Kanth et al, prepared set of amine borane complexes from N,N-dialkyl anilines and N,N-dialkylamines, which is significantly more reactive than most other amine boranes [27][28][29] , this can be explained based on steric effects and electronic property of the groups attached to nitrogen atom which influence their reducing property 27,30,31 . Preparation of these amine borane complexes were cumbersome, which involves alkylation of aniline or mono alkyl aniline, which is delicate and time consuming 31,33 .…”

Lewis Acid‐Mediated Diastereoselective Reduction ofN‐Protected β‐Amino Ketones: Influence of the Nature of the Metal Atom and of the Nitrogen Protecting Group

“…This situation did not change when the reduction of β-amino ketones was performed with borane-dimethyl sulfide reagent (Table 1, entry 12), a reductive system with lower basicity than amine boranes. [30] We also examined the reduction of α-substituted N-protected β-amino ketones 1aϪg with hydrides in the presence of dry cerium trichloride as the Lewis acid. We first tested the reduction of 1a in CH 2 Cl 2 with BH 3 ·py complex, a convenient reducing agent because of its solubility in many organic solvents, in the presence of dry CeCl 3 .…”

Redução de amidas por boranos

“…(22) (23) Em muitos casos, apesar de ser inicialmente muito rápida, tem sido verificada também que essa evolução de hidrogênio termina bem antes de dois equivalentes de hidreto serem consumidos, variando normalmente entre 1,10 e 1,40 equivalentes. Posteriormente, o consumo restante de hidreto para evolução de hidrogênio torna-se muito lento 86 . Apesar de os complexos contendo ligações B-N serem moderadamente estáveis, mesmo na presença de oxigênio molecular e umidade 87,88 , a evolução incompleta de hidrogênio pode ser atribuída também ao ataque competitivo do redutor ao oxigê-nio 35,36 , conforme Equação 24.…”