Pd-NHC (PEPPSI) Complexes: Synthetic Utility and Computational Studies into Their Reactivity

Organ, Michael G.; Chass, Gregory A.; Fang, De‐Cai; Hopkinson, Alan C.; Valente, Cory

doi:10.1055/s-2008-1067225

Cited by 213 publications

(96 citation statements)

References 123 publications

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“…According to the proposed general catalytic cycle for Pd-PEPPSI-IPr, this homocoupling product originates from the activation of the catalyst. 23 Formation of a black precipitate was observed, presumably palladium metal. …”

Section: Catalytic Applicabilitymentioning

confidence: 99%

Synthesis of an extremely sterically shielding N-heterocyclic carbene ligand

Weber¹,

Loos²,

Röminger³

et al. 2012

Arkivoc

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The N-heterocyclic carbene (NHC) ligand IPr** features substituents of unprecedented steric demand (IPr** = 1,3-bis[2,6-bis[(4-tert-butylphenyl)methyl]-4-methylphenyl]imidazol-2-ylidene). The NHC structure is an advanced derivative of the IPr system, and of the IPr* by Berthon-Gelloz and Markó. In the IPr** ancillary ligand, two para-methyl are introduced and eight methyl groups of IPr are formally replaced by a 4-tert-butylphenyl substituent, respectively, thereby sterically shielding both a coordinated metal and its second ligand. Favorable features of ligand and late transition metal complexes include high solubility, aesthetic NMR spectra, and a tendency towards crystallization.

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Section: Catalytic Applicabilitymentioning

confidence: 99%

Synthesis of an extremely sterically shielding N-heterocyclic carbene ligand

Weber¹,

Loos²,

Röminger³

et al. 2012

Arkivoc

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“…Pd-PEPPSI-IPr is activated by treatment with MeMgCl to form the active Pd(0) carbene complex, with dissociation of the labile 3-chloropyridine ligand. [18] Oxidative addition of the silylborane to this complex is facilitated by the good s-donor ability of the carbene ligand. Insertion of the alkyne must proceed via a three-coordinated complex, in order to allow coordination of the triple bond to the metal center.…”

Section: Mechanismmentioning

confidence: 99%

“…Reductive elimination, finally, is facilitated by bulky ligands, such as PEPPSI-IPr. [18] The success of the carbene ligand is probably the result of these elementary catalytic steps being favored.…”

Section: Mechanismmentioning

confidence: 99%

“…[16] PEPPSI (pyridine-enhanced precatalyst preparation stabilization and initiation) catalysts, [17] which are commercial air-and water-stable palladium complexes, have proven to exhibit excellent reactivity in a large number of palladium-catalyzed cross-coupling reactions. [18] The commercial Pd(II) complexes are activated with base to generate catalytically active Pd(0) carbene species. Encouraged by the fact that Nheterocyclic carbenes catalyzed the silastannative carbocyclization of 1,6-enynes, [6b,c] in moderate to good yields, we decided to switch to dichloro-N,N-bisA [19] as catalyst.…”

Section: Palladium-catalyzed Silaborationsmentioning

confidence: 99%

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Regioselective Preparation of Functionalized exo‐Methylene‐ cyclopentanes and exo‐Methylenepyrrolidines via Silaborative Carbocyclization of 1,6‐Enynes

et al. 2010

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Silaborative carbocyclization of 1,6-enynes catalyzed by Pd-PEPPSI-IPr {PEPPSI = pyridine-enhanced precatalyst preparation stabilization and initiation; IPr = N,N-bisA C H T U N G T R E N N U N G [2,6-(diisopropyl)phenyl]imidazolium} employing either (dimethylphenylsilyl)pinacolborane or (chlorodimethylsilyl)pinacolborane provides access to densely functionalized five-membered rings as single diastereomers in excellent yields. The vinylboronate functions were employed in palladium-catalyzed Suzuki cross-coupling reactions with a range of aryl bromides, containing electron-withdrawing as well as electron-donating substituents, furnishing arylated exo-methylenecyclopentanes or exo-methylenepyrrolidines in good yields. Subsequent oxidation of the isopropoxydimethylsilyl function generated via addition of (chlorodimethylsilyl)-pinacolborane provided access to hydroxymethyl derivatives of the arylated compounds. Use of a chiral ester, bismenthyl (2-propenyl)(2-propynyl)malonate, afforded two diastereomeric products which could be separated, thereby giving access to the cyclized compounds as single isomers, with opposite absolute configurations at the newly formed stereocenter.

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“…Examples of this type are rare as specially designed, highly active catalysts are typically required to couple unactivated alkyl fragments efficiently, [12] with the trade-off that discrimination between C alkyl ÀBr and C alkyl ÀCl bonds is now less chemoselective. For example, we have shown that NHC-Pd catalysts (NHC = N-heterocyclic carbene), generated in situ from either an imidazolium salt [13] or a pre-catalyst, [14] namely [Pd-PEPPSI-IPr] (1), [15] can effectively couple both unactivated primary C alkyl ÀBr and C alkyl ÀCl bonds in the Negishi reaction at room temperature. [16] During the course of these studies, however, we discovered a unique property that allowed us to chemoselectively couple C alkyl À Br bonds in the presence of C alkyl À Cl bonds with alkylzinc reagents: that is, solvent polarity.…”

mentioning

confidence: 99%