Chiral Dienes as “Ligands” for Borane-Catalyzed Metal-Free Asymmetric Hydrogenation of Imines

Liu, Yongbing; Du, Haifeng

doi:10.1021/ja4025808

Cited by 218 publications

(107 citation statements)

References 67 publications

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“…[68] Similarly, Oestreich et al employed a chiral silane as the reductant either with B(C 6 F 5 ) 3 or a binaphthyl derived axially chiral borane. [69] Liu and Du [70] have used a chiral bis-borane which was generated in situ by hydroboration of a substituted divinylbinaphthyl derivative using HB(C 6 F 5 ) 2 . The best system (81) resulted in asymmetric hydrogenation of a series of 19 imines with asymmetric inductions between 74 % and 88 % ee (Scheme 32).…”

Section: Asymmetric Hydrogenationmentioning

confidence: 99%

Frustrated Lewis Pair Chemistry: Development and Perspectives

2015

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Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small-molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal-free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.

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Section: Asymmetric Hydrogenationmentioning

confidence: 99%

Frustrated Lewis Pair Chemistry: Development and Perspectives

2015

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“…8 A recent development was achieved by introducing chirality through a binaphthyl backbone (2), enabling hydrogenation at room temperature and consequently increasing asymmetric induction. 9 In intramolecular FLP systems appropriate linking of the Lewis acid and base has proven to be crucial for dihydrogen activation as well as for the hydrogen transfer process. 10 In ansa-aminoboranes, the Lewis acid and base are in close vicinity, but bulky substituents hinder dative bond formation.…”

Section: Introductionmentioning

confidence: 99%

“…The catalyst (S)-9 yielded alkyl amines 14 and 15 in over 30% ee, which are noteworthy values compared to previous FLP results. 9,11 Remarkably, N,N-symmetric enamines were quantitatively converted to corresponding amines with extremely high optical purity (Table 2, 17 & 18). Alkyl amine 19 was also obtained in high optical purity, 85% ee.…”

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confidence: 99%

Chiral Molecular Tweezers: Synthesis and Reactivity in Asymmetric Hydrogenation

et al. 2015

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ABSTRACT:We report the synthesis and reactivity of a chiral aminoborane displaying both rapid and reversible hydrogen activation. The catalyst shows exceptional reactivity in asymmetric hydrogenation of enamines and unhindered imines with stereoselectivity up to 99% ee. The reaction mechanism is analyzed via DFT calculations, which point to the importance of both repulsive steric and stabilizing intermolecular noncovalent forces in the stereodetermining hydride transfer step of the catalytic cycle.One of the most efficient and atom-economic ways of preparing chiral amines is through transition metal catalyzed asymmetric hydrogenation of prochiral imines and enamines. 1 The products, having chiral α-carbons, are important in synthetic chemistry due to their application as ligands, resolving agents, chiral auxiliaries and building blocks. Additionally, the chiral information and tendency to form hydrogen bonds, are essential features in molecular recognition, thus making them potential pharmaceuticals. 2 Drug substance preparation through transition metal catalysis requires tedious product purification due to strict demands on heavy metal residuals in the products. 3 Recently, main-group systems combining sterically hindered Lewis acids and bases have been reported to cleave molecular hydrogen heterolytically under mild reaction conditions. 4 The field commonly referred to as the chemistry of "Frustrated Lewis pairs" (FLP) has attracted an increased scientific and practical interest, mainly owing to their applicability as catalysts in homogeneous metal-free hydrogenations of imines, enamines, 5 N-heterocycles, 6 and first results have been disclosed for the hydrogenation of carbonyl compounds 7 as well. In this regard it is surprising that reports of the corresponding asymmetric reactions are still few. Intermolecular FLP systems involving chiral boranes have been successfully utilized for enantioselective imine hydrogenation. Use of inherently chiral terpene groups (like α-pinene and camphor) on boron has proven to be effective for hydrogenation of acetophenone N-arylimines and ee's up to 83% were reported (Figure 1, 1). 8 A recent development was achieved by introducing chirality through a binaphthyl backbone (2), enabling hydrogenation at room temperature and consequently increasing asymmetric induction. 9 In intramolecular FLP systems appropriate linking of the Lewis acid and base has proven to be crucial for dihydrogen activation as well as for the hydrogen transfer process. 10 In ansa-aminoboranes, the Lewis acid and base are in close vicinity, but bulky substituents hinder dative bond formation. Such structures generate high FLP reactivity and the substrate scope could be expanded to enamines, N-alkyl and alkyl imines. 5d,11 However, previous attempts to incorporate high reactivity of ansa-aminoboranes and asymmetric hydrogenation, namely the introduction of chiral amines via straightforward synthetic approaches into linked systems (Figure 1, 3), have resulted in only moderate enantioselectivities. 11...

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“…Figure 9.21 Asymmetric hydrogenation of ketimines using a chiral bisborane [48]. In summary, these observations show that the asymmetric hydrogenation of imines by chiral FLPs is a tremendously challenging task.…”

Section: Hydrogenation Of Iminesmentioning

confidence: 95%

“…The same approach was followed for the synthesis of chiral bisboranes by the hydroboration of 1,1 ′ -binaphthyl derivatives [48], which allowed the quick optimization of the catalyst (Figure 9.21).…”

Section: Hydrogenation Of Iminesmentioning

confidence: 99%

Cooperative H ₂ Activation by Borane‐Derived Frustrated Lewis Pairs

Paradies¹

2015

Cooperative Catalysis

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Chiral Dienes as “Ligands” for Borane-Catalyzed Metal-Free Asymmetric Hydrogenation of Imines

Cited by 218 publications

References 67 publications

Frustrated Lewis Pair Chemistry: Development and Perspectives

Frustrated Lewis Pair Chemistry: Development and Perspectives

Chiral Molecular Tweezers: Synthesis and Reactivity in Asymmetric Hydrogenation

Cooperative H ₂ Activation by Borane‐Derived Frustrated Lewis Pairs

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Chiral Dienes as “Ligands” for Borane-Catalyzed Metal-Free Asymmetric Hydrogenation of Imines

Cited by 218 publications

References 67 publications

Frustrated Lewis Pair Chemistry: Development and Perspectives

Frustrated Lewis Pair Chemistry: Development and Perspectives

Chiral Molecular Tweezers: Synthesis and Reactivity in Asymmetric Hydrogenation

Cooperative H 2 Activation by Borane‐Derived Frustrated Lewis Pairs

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Cooperative H ₂ Activation by Borane‐Derived Frustrated Lewis Pairs