Imine hydrogenation by alkylaluminum catalysts

Hatnean, Jillian A.; Thomson, Jordan W.; Chase, Preston A.; Stephan, Douglas W.

doi:10.1039/c3cc47889k

Cited by 50 publications

(48 citation statements)

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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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confidence: 99%

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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confidence: 99%

LiAlH₄: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

et al. 2018

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“…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 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 .…”

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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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“…Both DIBAL and TIBAL have been found to hydrogenate a number of imines through a hydroalumination of the imine double bond, followed by hydrogenolysis of the resulting Al-N bond (Scheme 10, bottom) [44]. Simple aluminum halides are sufficiently Lewis acidic to participate in FLP chemistry.…”

Section: Alternative Lewis Acids and Bases: Boronmentioning

confidence: 97%

“…To the end, our first efforts targeted intramolecular FLPs which incorporated both a phosphonium Lewis acid and an amide Lewis base for the sequestration of CO 2 [97]. The phosphinoaniline, Ph 2 P(o-C 6 H 4 N(H)Me), was oxidized with XeF 2 to give the difluorophosphorane (44). Fluoride ion abstraction with trimethylsilyl triflate resulted in the formation of the corresponding fluorophosphonium triflate salt.…”

Section: Alternative Lewis Acids: Phosphorusmentioning

confidence: 99%

Non-conventional Lewis Acids and Bases in Frustrated Lewis Pair Chemistry

Caputo

Stephan

2015

The Chemical Bond III

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The discovery of frustrated Lewis pairs (FLPs) was based on the reactions of combinations of sterically demanding phosphine donors and electrophilic boranes with dihydrogen. Since these early findings, the range of species that exhibit FLP chemistry has broadened dramatically to include a series of Lewis acids and bases. In this review, we describe a variety of such systems and the corresponding reactivity of FLPs derived from group 13-16 species.

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Imine hydrogenation by alkylaluminum catalysts

Abstract: Di-isobutylaluminum hydride and tri-iso-butylaluminum (DIBAL 1, TIBAL 2) are shown to be efficient hydrogenation catalysts for a variety of imines at 100 °C and 100 atm of H2, operating via a hydroalumination/hydrogenolysis mechanism.

Cited by 50 publications

References 53 publications

LiAlH₄: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

LiAlH₄: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

Imine hydrogenation with simple alkaline earth metal catalysts

Non-conventional Lewis Acids and Bases in Frustrated Lewis Pair Chemistry

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Imine hydrogenation by alkylaluminum catalysts

Abstract: Di-isobutylaluminum hydride and tri-iso-butylaluminum (DIBAL 1, TIBAL 2) are shown to be efficient hydrogenation catalysts for a variety of imines at 100 °C and 100 atm of H2, operating via a hydroalumination/hydrogenolysis mechanism.

Cited by 50 publications

References 53 publications

LiAlH4: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

LiAlH4: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

Imine hydrogenation with simple alkaline earth metal catalysts

Non-conventional Lewis Acids and Bases in Frustrated Lewis Pair Chemistry

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LiAlH₄: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

LiAlH₄: From Stoichiometric Reduction to Imine Hydrogenation Catalysis