N-Heterocyclic carbene complexes enabling the α-arylation of carbonyl compounds

Ostrowska, Sylwia; Scattolin, Thomas; Nolan, Steven P.

doi:10.1039/d1cc00913c

Cited by 49 publications

(28 citation statements)

References 187 publications

(101 reference statements)

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“…Despite the fact that intramolecular version of α ‐arylation reactions in amides are well explored that are catalyzed using Pd II complexes, only a very few examples account for intermolecular α ‐arylation reaction of amides and relatively less explored utilizing the NHC palladium(II) complexes. [ 7a,8b,13 ] The intermolecular α ‐arylation reaction of amide also demands the involvement of slightly higher catalyst loading. [ 8b,13a ] Palladium(II) complexes [ 2 ]–[ 5 ] were employed as precatalysts in α ‐arylation of N ‐methyl oxindole (Table 1).…”

Section: Resultsmentioning

confidence: 99%

Dinuclear Pd^II complexes bearing mixed NHC/Py/PPh₃ donor set ligands: Catalytic applications and electrochemical investigations

Naskar

Mondal

Majumder

et al. 2022

Applied Organom Chemis

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Bimetallic palladium(II) complexes containing classical NHC donor ligands are becoming increasingly popular owing to their various catalytic applications. However, examples of the aforementioned complexes with mixed NHC/PPh3 ligands are still rare. Bimetallic palladium(II) complexes possessing these mixed ligands are described starting from a C2‐symmetric bis‐imidazolium salt containing 4,4′‐substituted central biphenyl ring. All the palladium(II) complexes have been tested as precatalysts in α‐arylation of oxindole and Suzuki–Miyaura coupling reactions. The complex composed of mixed NHC/PPh3 donor ligands shows superior catalytic activity compared with the corresponding PEPPSI type complexes when applied in α‐arylation of oxindole. The dinuclear complexes display better activity compared with the mononuclear complexes. The preliminary electrochemical measurements show the facile oxidation of PdII in the presence of combined NHC/PPh3 ligands compared with a combination of NHC/Py ligands.

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Section: Resultsmentioning

confidence: 99%

Dinuclear Pd^II complexes bearing mixed NHC/Py/PPh₃ donor set ligands: Catalytic applications and electrochemical investigations

Naskar

Mondal

Majumder

et al. 2022

Applied Organom Chemis

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“…The α-arylation of carbonyl compounds has become a widely practiced class of coupling reactions, and enantioselective versions of this process have been reported in both intermolecular and intramolecular forms with a range of carbonyl compounds and sp 2 -electrophiles. − The intermediates in the α-arylations that form the carbon–carbon bond are C-bound arylpalladium enolate complexes. Examples of arylpalladium enolate complexes have been prepared, and the rates of reductive elimination as a function of the steric and electronic properties of the enolate have been studied. ,, However, analogous enolate complexes that are putative intermediates in the enantioselective processes and that undergo reductive eliminations to form enantioenriched, chiral products have not been studied.…”

Section: Introductionmentioning

confidence: 99%

Direct Observation of Diastereomeric α-C-Bound Enolates during Enantioselective α-Arylations: Synthesis, Characterization, and Reactivity of Arylpalladium Fluorooxindole Complexes

Kalkman

Hartwig

2021

J. Am. Chem. Soc.

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The Pd-catalyzed asymmetric α-arylation of carbonyl compounds is a valuable strategy to form benzylic stereocenters. However, the origin of the stereoselectivity of these reactions is poorly understood, and little is known about the reactivity of the putative diastereomeric arylpalladium enolate intermediates. To this end, we report the synthesis and characterization of a series of diphosphine-ligated arylpalladium fluoroenolate complexes, including complexes bearing a metal-bound, stereogenic carbon and an enantioenriched chiral diphosphine ligand. These complexes reductively eliminate to form chiral α-aryl-α-fluorooxindoles with enantioselectivities and rates that are relevant to those of the catalytic process with SEGPHOS as the ancillary ligand. Kinetic studies showed that the rate of reductive elimination is slightly slower than the rate of epimerization of the intermediate, causing the reductive elimination step to impart the greatest influence on the enantioselectivity. DFT calculations of these processes are consistent with these experimental rates and suggest that the minor diastereomer forms the major enantiomer of the product. The rates of reductive elimination from complexes containing a variety of electronically varied aryl ligands revealed the unusual trend that complexes bearing more electron-rich aryl ligands react faster than those bearing more electron-poor aryl ligands. Noncovalent Interaction (NCI) and Natural Bond Orbital (NBO) analyses of the transition-state structures for reductive elimination from the SEGPHOS-ligated complexes revealed key donor–acceptor interactions between the Pd center and the fluoroenolate fragment. These interactions stabilize the pathway to the major product enantiomer more strongly than they stabilize that to the minor enantiomer.

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“…Transition-metal complexes of N -heterocyclic carbene have been often applied to catalytic reactions, as discussed in many reviews. − Because the knowledge of the ligand effects on the N–H σ-bond activation is indispensable for searching a better catalyst, we investigated here the N–H σ-bond activations of NHMe 2 and NH(Me)(Bs) using N -heterocyclic carbene IPr (Scheme ) as a ligand instead of PCy 3 . In Ni(IPr)(NHMe 2 ) R5 , the concerted oxidative addition of the N–H σ-bond occurs with a Δ G ° ‡ value of 15.0 kcal/mol and Δ G ° value of 2.1 kcal/mol, as shown in Figure A.…”

Section: Results and Discussionmentioning

confidence: 99%

Theoretical Study of N–H σ-Bond Activation by Nickel(0) Complex: Reaction Mechanism, Electronic Processes, and Prediction of Better Ligand

2022

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N−H σ-bond activation of alkylamine by Ni(PCy 3 ) was investigated using density functional theory (DFT) calculations. When simple alkylamine NHMe 2 is a reactant, both concerted oxidative addition in Ni(PCy 3 )(NHMe 2 ) and ligand-to-ligand H transfer reaction in Ni(PCy 3 )(C 2 H 4 )(NHMe 2 ) are endergonic and need a high activation energy. When NH(Me)(Bs) (Bs = SO 2 Ph, a model of tosyl group used in experiments) is a reactant, both reactions are exergonic and occur easily with a much smaller activation energy. The much larger reactivity of NH(Me)(Bs) than that of NHMe 2 results from the stronger Ni− N(Me)(Bs) bond than the Ni−NMe 2 bond and the presence of the Ni−O bonding interaction between the Bs group and the Ni atom in the product. N-Heterocyclic carbene, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), is computationally predicted to be better than PCy 3 because the Ni−NMe 2 and Ni−N(Me)(Bs) bonds in the IPr complex are stronger, respectively, than those of the PCy 3 complex. The introduction of the electronwithdrawing Bs group to the N atom of amine and the use of IPr as a ligand are recommended for the N−H σ-bond activation. The C−H σ-bond activations of benzene via the oxidative addition and the ligand-toligand H transfer reaction were also investigated here for comparison with the N−H σ-bond activation. The differences between the C−H σ-bond activation of benzene and the N−H σ-bond activation of these amines are discussed in terms of the N−H, C−H, Ni− Ph, and Ni−NMe 2 , and Ni−N(Me)(Bs) bond energies and back-donation to benzene from the Ni atom.

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N-Heterocyclic carbene complexes enabling the α-arylation of carbonyl compounds

Abstract: The considerable importance of α-arylated carbonyl compounds, which are widely used as final products or as key intermediates in the pharmaceutical industry, has prompted numerous research groups to develop efficient...

Cited by 49 publications

References 187 publications

Dinuclear Pd^II complexes bearing mixed NHC/Py/PPh₃ donor set ligands: Catalytic applications and electrochemical investigations

Dinuclear Pd^II complexes bearing mixed NHC/Py/PPh₃ donor set ligands: Catalytic applications and electrochemical investigations

Direct Observation of Diastereomeric α-C-Bound Enolates during Enantioselective α-Arylations: Synthesis, Characterization, and Reactivity of Arylpalladium Fluorooxindole Complexes

Theoretical Study of N–H σ-Bond Activation by Nickel(0) Complex: Reaction Mechanism, Electronic Processes, and Prediction of Better Ligand

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N-Heterocyclic carbene complexes enabling the α-arylation of carbonyl compounds

Abstract: The considerable importance of α-arylated carbonyl compounds, which are widely used as final products or as key intermediates in the pharmaceutical industry, has prompted numerous research groups to develop efficient...

Cited by 49 publications

References 187 publications

Dinuclear PdII complexes bearing mixed NHC/Py/PPh3 donor set ligands: Catalytic applications and electrochemical investigations

Dinuclear PdII complexes bearing mixed NHC/Py/PPh3 donor set ligands: Catalytic applications and electrochemical investigations

Direct Observation of Diastereomeric α-C-Bound Enolates during Enantioselective α-Arylations: Synthesis, Characterization, and Reactivity of Arylpalladium Fluorooxindole Complexes

Theoretical Study of N–H σ-Bond Activation by Nickel(0) Complex: Reaction Mechanism, Electronic Processes, and Prediction of Better Ligand

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Dinuclear Pd^II complexes bearing mixed NHC/Py/PPh₃ donor set ligands: Catalytic applications and electrochemical investigations

Dinuclear Pd^II complexes bearing mixed NHC/Py/PPh₃ donor set ligands: Catalytic applications and electrochemical investigations