H2 Cleavage, Hydride Formation, and Catalytic Hydrogenation of Imines with Zinc Complexes of C5Me5 and N‐Heterocyclic Carbenes

Jochmann, Phillip; Stephan, Douglas W.

doi:10.1002/anie.201303968

Cited by 94 publications

(60 citation statements)

References 88 publications

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“…The aluminium hydride compound (iso-butyl) 2 AlH is an odd example of a main group metal catalyst that can be used, but needs harsh conditions (100 bar H 2 , 100 °C, 24 h) 27 . Recently, a zinc hydride catalyst was introduced but similarly high pressures need to be applied (70-100 bar) 28,29 . Figure 2b shows metal-free organocatalysts.…”

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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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Imine hydrogenation with simple alkaline earth metal catalysts

et al. 2018

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“…[8] Catalytic imine reduction with H 2 ,h owever,h as received much less attention than alkene or ketone hydrogenation and remains achallenge. [16,17] Metal-free frustrated Lewis pairs (FLPs) [18] operate under much milder conditions (5 bar H 2 ,8 0-140 8 8C). [9a] Since the platinum-group metals are scarce [12] and may on account of their high toxicity pose problems in pharmaceutical products,i ti si ncreasingly popular to use more abundant and biocompatible metals.…”

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LiAlH₄: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

et al. 2018

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Dedicated to Professor Dietmar Stalke on the occasion of his 60th birthday.Abstract: Imine-to-amine conversion with catalytic instead of stoichiometric quantities of LiAlH 4 is demonstrated (85 8 8C, catalyst loading ! 2.5 mol %, pressure ! 1bar). The effects of temperature,p ressure,s olvent, and catalyst modifications,a s well as the substrate scope are discussed. Experimental investigations and preliminary DFT calculations suggest that the catalytically active species is generated in situ:L iAlH 4 + Ph(H)C=NtBu!LiAlH 2 [N(tBu)CH 2 Ph] 2 .Acooperative mechanism in whichL ia nd Al both play ap rominent role is proposed.

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“…[4] Besides reversible hydrogen binding,a ctivation and transfer, the storage and chemical activation of carbon dioxide has also been of major interest in FLP chemistry.The first suitable FLPs in this context contained aboron atom as acidic and ap hosphorus atom as basic site (B/P). [6] Up to now most common systems are those based on Group 13 elements such as boron or aluminum [7] as Lewis acids.F ew exceptions include Si and P V and af ew transition and rare-earth elements as alternatives.E xamples are the zinc-based FLP systems,n amely those of the combinations Zn/P,Zn/NHC,and Zn/R 2 P=C=PR 2 , [8] zirconium-based compounds including [Cp 2 ZrOC 6 H 4 PtBu 2 ][B(C 6 F 5 ) 4 ], [9] or the P V -containing species; [10] more are described in ar ecent review. [6] Besides fundamental interest in new chemical transformations and catalytic processes,t his is also of current interest in the context of activation of greenhouse gases.…”

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A Neutral Silicon/Phosphorus Frustrated Lewis Pair

et al. 2015

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Frustrated Lewis pairs (FLPs) have a great potential for activation of small molecules. Most known FLP systems are based on boron or aluminum atoms as acid functions, few on zinc, and only two on boron-isoelectronic silicenium cation systems. The first FLP system based on a neutral silane, (C2F5)3SiCH2P(tBu)2 (1), was prepared from (C2F5)3SiCl with C2F5 groups of very high electronegativity and LiCH2P(tBu)2. 1 is capable of cleaving hydrogen, and adds CO2 and SO2. Hydrogen splitting was confirmed by H/D scrambling reactions. The structures of 1, its CO2 and SO2 adducts, and a decomposition product with CO2 were elucidated by X-ray diffraction.

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H₂ Cleavage, Hydride Formation, and Catalytic Hydrogenation of Imines with Zinc Complexes of C₅Me₅ and N‐Heterocyclic Carbenes

Cited by 94 publications

References 88 publications

Imine hydrogenation with simple alkaline earth metal catalysts

Imine hydrogenation with simple alkaline earth metal catalysts

LiAlH₄: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

A Neutral Silicon/Phosphorus Frustrated Lewis Pair

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H2 Cleavage, Hydride Formation, and Catalytic Hydrogenation of Imines with Zinc Complexes of C5Me5 and N‐Heterocyclic Carbenes

Cited by 94 publications

References 88 publications

Imine hydrogenation with simple alkaline earth metal catalysts

Imine hydrogenation with simple alkaline earth metal catalysts

LiAlH4: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

A Neutral Silicon/Phosphorus Frustrated Lewis Pair

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H₂ Cleavage, Hydride Formation, and Catalytic Hydrogenation of Imines with Zinc Complexes of C₅Me₅ and N‐Heterocyclic Carbenes

LiAlH₄: From Stoichiometric Reduction to Imine Hydrogenation Catalysis