Organolanthanide-Catalyzed Imine Hydrogenation. Scope, Selectivity, Mechanistic Observations, and Unusual Byproducts

Obora, Yasushi; Ohta, Tetsuo; Stern, Charlotte L.; Marks, Tobin J.

doi:10.1021/ja963775+

Cited by 102 publications

(89 citation statements)

References 51 publications

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“…To date, a comparable coupling of the cyclic imine 7 has only been reported for alkylsamarium complexes. [29] Instead of the C 5 H 5 ligand in 8, Marks et al used the more electron-rich and sterically more demanding C 5 Me 5 ligand in Cp*SmCH(SiMe 3 ) 2 (9). [29] For 9, the deprotonation of the methyl group in 7 is discussed as the key step in the formation of 10 (Scheme 5).…”

Section: Coupling Reactions Of Iminesmentioning

confidence: 99%

“…Synthesis of the samarium complex 10 by Marks. [29] The titanium(III) complex 8 crystallizes in the orthorhombic space group Pnma. The molecular structure (Figure 3) shows that the bonds between the titanium center and the nitrogen atoms are clearly different.…”

Section: Coupling Reactions Of Iminesmentioning

confidence: 99%

“…In such a way, a single bond is localized between N1 and C6, whereas a double bond is observed between N2 and C11. Compared to those of the literature-known complex 10, [29] the N-C bond lengths as well as the newly formed C-C bond of 8 [C15- After the successful synthesis of titanaaziridines by the reductive complexation of the imines 2a-d starting with 1a as well as the permethylated precursor 1b and the aldimine 2e, we decided to try monocyclopentadienyltitanium(IV) systems as starting materials. Against our expectations, the reduction of CpTiCl 3 (1c) or Cp*TiCl 3 (1d) with 1 equiv.…”

Section: Coupling Reactions Of Iminesmentioning

confidence: 99%

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Imines in the Titanium Coordination Sphere: Highly Reactive Titanaaziridines and Larger Titanacycles Formed by Subsequent C–C Coupling Reactions

et al. 2015

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The reductive complexation of aldimines is one of the promising synthetic tools to obtain titanaaziridines by reaction of the corresponding titanium halides and magnesium as a reducing agent. The effects of the substitution pattern of the imine ligand and the nature of the titanium fragment on the degree of activation of the η 2 -coordinated N-C double bond were investigated. With Cp # TiCl 3 (Cp # = cyclopentadienyl or pentamethylcyclopentadienyl) as the precursor for the reductive complexation of the imines N-(4-methylbenzylidene)aniline (2e) and 2-methyl-1-pyrroline (7), the formation of diazatitanacyclopentanes by McMurry-like C-C coupling is found. A six-membered titanacycle 8 is obtained by a Michael-like coupling reaction of 7. By these insights, it is

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Section: Coupling Reactions Of Iminesmentioning

confidence: 99%

Section: Coupling Reactions Of Iminesmentioning

confidence: 99%

Section: Coupling Reactions Of Iminesmentioning

confidence: 99%

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Imines in the Titanium Coordination Sphere: Highly Reactive Titanaaziridines and Larger Titanacycles Formed by Subsequent C–C Coupling Reactions

et al. 2015

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“…(ratio n/iso) (ratio n/iso) [31] The hydrogenation [46] and hydrosilylation [47] of imines had been reported previously, but not in sequential reactions. As substrates, six different phenyl-and alkyl-A C H T U N G T R E N N U N G alkynes and PhSiH 3 were used.…”

Section: P{mentioning

confidence: 99%

Bis(phosphinimino)methanide Rare Earth Amides: Synthesis, Structure, and Catalysis of Hydroamination/Cyclization, Hydrosilylation, and Sequential Hydroamination/Hydrosilylation

Rastätter

Zulys

Roesky

2007

Chemistry A European J

150

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A series of yttrium and lanthanide amido complexes [Ln{N(SiHMe(2))2}2{CH(PPh(2)NSiMe(3))2}] (Ln=Y, La, Sm, Ho, Lu) were synthesized by three different pathways. The title compounds can be obtained either from [Ln{N(SiHMe(2))2}3(thf)2] and [CH(2)(PPh(2)NSiMe(3))2] or from KN(SiHMe(2))2 and [Ln{CH(PPh(2)NSiMe(3))2}Cl(2)]2, while in a third approach the lanthanum compound was synthesized in a one-pot reaction starting from K{CH(PPh(2)NSiMe(3))2}, LaCl3, and KN(SiHMe(2))2. All the complexes have been characterized by single-crystal X-ray diffraction. The new complexes, [Ln{N(SiHMe(2))2}2{CH(PPh(2)NSiMe(3))2}], were used as catalysts for hydroamination/cyclization and hydrosilylation reactions. A clear dependence of the reaction rate on the ionic radius of the center metal was observed, showing the lanthanum compound to be the most active one in both reactions. Furthermore, a combination of both reactions--a sequential hydroamination/hydrosilylation reaction--was also investigated.

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“…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 main group metal catalyst that can be used, but needs harsh conditions (100 bar H 2 , 100 °C, 24 h) 27 .…”

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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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Organolanthanide-Catalyzed Imine Hydrogenation. Scope, Selectivity, Mechanistic Observations, and Unusual Byproducts

Cited by 102 publications

References 51 publications

Imines in the Titanium Coordination Sphere: Highly Reactive Titanaaziridines and Larger Titanacycles Formed by Subsequent C–C Coupling Reactions

Imines in the Titanium Coordination Sphere: Highly Reactive Titanaaziridines and Larger Titanacycles Formed by Subsequent C–C Coupling Reactions

Bis(phosphinimino)methanide Rare Earth Amides: Synthesis, Structure, and Catalysis of Hydroamination/Cyclization, Hydrosilylation, and Sequential Hydroamination/Hydrosilylation

Imine hydrogenation with simple alkaline earth metal catalysts

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