Ruthenium-Catalyzed Transfer Hydrogenation of Imines by Propan-2-ol in Benzene

Samec, Joseph S. M.; Bäckvall, Jan‐E.

doi:10.1002/1521-3765(20020703)8:13<2955::aid-chem2955>3.0.co;2-q

Cited by 206 publications

(131 citation statements)

References 22 publications

(38 reference statements)

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“…Of particular interest is the dinuclear Ru complex 34, the so-called Shvo catalyst [55,56]. It has been established that, under the reaction conditions, this complex is in equilibrium with two monometal complexes (35 and 36) [57][58][59].…”

Section: Transition Metal-catalyzed Reductionsmentioning

confidence: 99%

“…The itaconic acid (51) is a good chelating ligand for the catalyst, and when the 16-electron Rh I active species 54 is formed, an oxidative addition of formic acid (49) takes place (a). Decarboxylation (b) of 55 liberates CO 2 (50), forming a Rh III -dihydride (56). A hydride transfer (c) leading to a pentacoordinated metal (57) and subsequent reductive elimination (d), in which the product (58) is liberated and a new substrate (51) is coordinated, closes the cycle.…”

Section: Transfer Hydrogenation Catalysts For Reduction Of C-c Doublementioning

confidence: 99%

See 1 more Smart Citation

Transfer Hydrogenation Including the Meerwein‐Ponndorf‐Verley Reduction

Klomp¹,

Hanefeld²,

Peters³

2006

The Handbook of Homogeneous Hydrogenation

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Section: Transition Metal-catalyzed Reductionsmentioning

confidence: 99%

Section: Transfer Hydrogenation Catalysts For Reduction Of C-c Doublementioning

confidence: 99%

Transfer Hydrogenation Including the Meerwein‐Ponndorf‐Verley Reduction

Klomp¹,

Hanefeld²,

Peters³

2006

The Handbook of Homogeneous Hydrogenation

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“…2c, these simultaneously deliver protic H δ+ and hydridic H δˉ building blocks and are considered to be intermediate to the catalysts shown in Fig. 2a and b 36 . And finally, highly Lewis-acidic borane catalysts, shown in Fig.…”

mentioning

confidence: 99%

Imine hydrogenation with simple alkaline earth metal catalysts

et al. 2018

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W ith molecular hydrogen being one of the cleanest reducing agents, catalytic hydrogenation using the more noble transition metals is among the most studied of all chemical processes 1 . Increasing social pressure towards a sustainable society, however, dictates replacement of costly, and often harmful, precious metals by more abundant first-row transition metals or even biocompatible redox inactive main group metals [2][3][4][5][6] . The alkaline earth metal calcium does not possess partially filled d orbitals for substrate activation, but has recently shown catalytic activities in the hydrogenation of C= C double bonds with molecular H 2 (ref. 7 ). Although restricted to conjugated C= C bonds, this example strikingly broke the dogma that transition metals are needed for alkene hydrogenation. This was followed by the development of metal-free frustrated Lewis pair (FLP) catalysts [8][9][10][11] and, most recently, cationic calcium hydride catalysts that are also able to hydrogenate unactivated alkenes 12 . Figure 1a shows a working hypothesis for styrene hydrogenation with a dibenzylcalcium catalyst (CaBn 2 ) 7 . The first step is the generation of a calcium hydride species, for which ample precedence exists [13][14][15][16][17] . Further reaction with H 2 may cause precipitation of insoluble (CaH 2 ) n , but catalyst loss is partly prevented by aggregation to soluble but undefined Ca x Bn y H z species. Despite a lack of d orbitals, alkene activation proceeds through a weak electrostatic calciumalkene interaction, recently shown to be of importance in calcium catalysis 18 . The benzylic calcium intermediate formed after insertion may, after successive styrene insertions, form polystyrene 19 , but high H 2 pressure (20-100 bar) can prevent this side reaction by promoting σ-bond metathesis. The latter step in the cycle is, like the initiation reaction, formally a deprotonation of H 2 by a resonance-stabilized benzylic carbanion. Considering the high pK a of H 2 (≈ 49) 20 , this reaction seemed questionable. Stoichiometric conversions of model systems, however, underscored the feasibility of this pathway 7 . Independent theoretical calculations illustrate that the final σ-bond metathesis step is indeed highly endergonic: Gibbs free energy of activation Δ G ‡ (60 °C, 20 bar) = 25.7 kcal mol −1 (ref. 21 ).As the highly atom-efficient catalytic reduction of imines by H 2 received much less attention than alkene or ketone hydrogenation [22][23][24] , it remained an important question whether calcium-catalysed hydrogenation can be extended to imine reduction. Current stateof-the-art imine hydrogenation catalysts can be divided into four categories that vary in terms of substrate activation and nucleophilic power ( Fig. 2a-d). Figure 2a shows organometallic metal hydrides that rely on hydride nucleophilicity. Apart from few early transition metal catalysts (Ti 25 , lanthanides 26 ), these are generally based on late transition metals (Rh, Ir) 22 . The aluminium hydride compound (iso-butyl) 2 AlH is an odd example of a ...

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“…This complex catalyzes both the racemization of enantiomerically pure amines [16] and the reduction of imines using isopropanol as the hydrogen source [8] . There is now convincing evidence that the mechanism of the reaction involves coordination of the imine to the metal followed by hydride transfer, that is, the monohydride mechanism of Figure 11.1 .…”

Section: Organometallic Catalysts Based On Ru Rh and Irmentioning

confidence: 99%

Imino Reductions by Transfer Hydrogenation

Wills

2008

Modern Reduction Methods

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11 History and BackgroundIn contrast to ketones, and in particular with respect to asymmetric reductions, the reduction of compounds containing C = N bonds has remained relatively underdeveloped. Some signifi cant breakthroughs have, however, been reported in the last ten years. This chapter will review reductions through metal hydride complexes, " Meerwein -Ponndorf -Verley ( MPV ) type " reductions, and other methods including transfer hydrogenation with organocatalysts and asymmetric LeuckartWallach aminations of ketones. Although there will be discussion of mechanistic aspects, the main focus of this review will be on more recent synthetic applications. The reader ' s attention is also directed to a number of recent useful reviews of this area [1] . Mechanisms of C= N Bond Reduction by Transfer HydrogenationUsing either organic or organometallic catalysts, transfer hydrogenation of imines and other C = N bond -containing substrates, is most commonly carried out using isopropanol, formic acid or a formate salt as the hydrogen source. The most commonly, but not exclusively, used metals are ruthenium, rhodium and iridium.The mechanism of the transfer hydrogenation of imines has been the subject of some extended synthetic, computational and mechanistic studies. The tutorial review by B ä ckvall [1f] provides a very good summary of this. An excellent review by Morris [1c] also contains an incisive overview of hydrogenation and transfer hydrogenation and provides a very logical nomenclature for the possible reduction reaction pathway.The mechanisms that may be considered for imine reduction are the same as for ketone reductions and may be divided into two broad classes (Figure 11.1 ). In the fi rst of these, the MPV reduction or its reverse (Oppenauer oxidation), a metal

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Ruthenium-Catalyzed Transfer Hydrogenation of Imines by Propan-2-ol in Benzene

Cited by 206 publications

References 22 publications

Transfer Hydrogenation Including the Meerwein‐Ponndorf‐Verley Reduction

Transfer Hydrogenation Including the Meerwein‐Ponndorf‐Verley Reduction

Imine hydrogenation with simple alkaline earth metal catalysts

Imino Reductions by Transfer Hydrogenation

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