Oxidative Addition of Palladium(0) Complexes Generated from [Pd(dba)2] and P-N Ligands: A Kinetic Investigation

Amatore, Christian; Fuxa, Alain; Jutand, Anny

doi:10.1002/(sici)1521-3765(20000417)6:8<1474::aid-chem1474>3.3.co;2-f

Cited by 19 publications

(21 citation statements)

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“…As reported previously for similar reactions, activated alkenes slow down the oxidative addition step by coordinating to the Pd 0 species, thereby stabilizing the Pd species through the delocalization of the electron density from the electron‐rich Pd 0 . The resulting species C undergoes migratory insertion into benzylideneacetone to form species D, which is in equilibrium with the energetically less favored palladium‐ O ‐enolate species D′ (≈2.7 kcal mol −1 higher in energy).…”

Section: Resultssupporting

confidence: 65%

Importance of the Reducing Agent in Direct Reductive Heck Reactions

et al. 2017

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The role of the reductant in the palladium N‐heterocyclic carbene (NHC) catalyzed reductive Heck reaction and its effect on the mechanism of the reaction is reported. For the first time in this type of transformation, the palladium‐NHC‐catalyzed reductive Heck reaction was shown to proceed in the presence of LiOMe and iPrOH even at 10 °C to give the products very efficiently in excellent yields and with exceptional chemoselectivities. This study shows that the reaction proceeds through two distinct mechanisms that depend on the nature of the reducing agent. In the presence of a protic solvent or acidic medium the reaction undergoes protonation to yield the reduced product, whereas in the absence of proton source, it proceeds through the insertion of the reductant followed by reductive elimination. The kinetic data reveal that the oxidative addition is the rate‐determining step in the reaction. The reaction profiles show first‐order kinetics in aryl iodide and Pd and zero‐order kinetics in LiOMe, benzylideneacetone, and the excess amount of NHC ligand. In addition, the reaction progress kinetic analysis shows that neither catalyst decomposition nor product inhibition occurs during the reaction. DFT calculations of the key steps confirm that the oxidative addition step is the rate‐determining step in the reaction. Deuterium‐labeling experiments indicate that the product is formed by the protonation of the Pd−Calkyl bond of the intermediate formed after enone insertion into the Pd−CAr bond. Application of chiral NHC ligands in the asymmetric reductive Heck reaction only results in poor enantioselectivities (enantiomeric excess up to 20 %) and is also substrate specific. DFT calculations suggest that the migration of the aryl group to the alkene of the substrate is the enantioselectivity‐determining step of the reaction. It is further shown that if the steric bulk at the enone is small (a methyl group), the two transition state barriers from [PdII(L2)(ArI)(enone)] species Cre and Csi, which have the re and si face of the enone substrate coordinated to Pd, are very similar, in line with the experimental results. With a slightly larger group (an isopropyl substituent) a significant difference in energy barriers is calculated (2.6 kcal mol−1), and in the experiment this product is formed with a modest enantiomeric excess (up to 20 %).

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Section: Resultssupporting

confidence: 65%

Importance of the Reducing Agent in Direct Reductive Heck Reactions

et al. 2017

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“…Coordination of substrate 1 to species A to form species A 2 stabilizes the electron‐rich d 10 Pd 0 center by removing the electron density from the metal center, and it consequently diminishes the concentration of the active catalyst species A in the oxidative addition step. A similar effect was observed for dba and other olefinic π‐acids in cross‐coupling reactions 17b. 21, 23 The coordination of benzylideneacetone 1 to the Pd 0 complex was also confirmed by 2D‐DOSY NMR spectroscopy, in which 25 equivalents of substrate 1 was introduced at ambient temperature (see the Supporting Information, Figure S14) 24…”

Section: Resultssupporting

confidence: 52%

Palladium(0)/NHC‐Catalyzed Reductive Heck Reaction of Enones: A Detailed Mechanistic Study

Raoufmoghaddam

Mannathan

Minnaard

et al. 2015

Chemistry A European J

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We have studied the mechanism of the palladium-catalyzed reductive Heck reaction of para-substituted enones with 4-iodoanisole by using N,N-diisopropylethylamine (DIPEA) as the reductant. Kinetic studies and in situ spectroscopic analysis have provided a detailed insight into the reaction. Progress kinetic analysis demonstrated that neither catalyst decomposition nor product inhibition occurred during the catalysis. The reaction is first order in the palladium and aryl iodide, and zero order in the activated alkene, N-heterocyclic carbene (NHC) ligand, and DIPEA. The experiments with deuterated solvent ([D7]DMF) and deuterated base ([D15]Et3N) supported the role of the amine as a reductant in the reaction. The palladium complex [Pd(0)(NHC)(1)] has been identified as the resting state. The kinetic experiments by stopped-flow UV/Vis also revealed that the presence of the second substrate, benzylideneacetone 1, slows down the oxidative addition of 4-iodoanisole through its competing coordination to the palladium center. The kinetic and mechanistic studies indicated that the oxidative addition of the aryl iodide is the rate-determining step. Various scenarios for the oxidative addition step have been analyzed by using DFT calculations (bp86/def2-TZVP) that supported the inhibiting effect of substrate 1 by formation of resting state [Pd(0)(NHC)(1)] species at the cost of further increase in the energy barrier of the oxidative addition step.

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“…Ethylene is bound more strongly by ≈70 kJ mol −1 , implying that reaction temperatures in the experiment have to be high enough to ensure reversibility of the ethylene coordination and the formation of 3 (0‐II) , which is—according to our calculations—the entry channel for C−I oxidative addition. The importance of ligand dissociation for the kinetics of this reaction step has been demonstrated by experimental studies by Amatore and Jutand on the oxidative addition of Ph−I to palladium( 0 ) complexes with various ligands 36, 37…”

Section: Resultsmentioning

confidence: 98%

Computational Study of a New Heck Reaction Mechanism Catalyzed by Palladium(II/IV) Species

2001

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In this theoretical study on the Heck reaction we explore the feasibility of an alternative pathway that involves a PdII/PdIV redox system. Usually, the catalytic cycle is formulated based on a Pd0/PdII mechanism. We performed quantum chemical calculations using density functional theory on a model system that consisted of diphosphinoethane (DPE) as a bidentate ligand and the substrates ethylene and phenyl iodide to compare both mechanisms. Accordingly, the PdII/PdIV mechanism is most likely to occur in the equatorial plane of an octahedral PdIV complex. The energy profiles of both reaction pathways under consideration are largely parallel. A major difference is found for the oxidative addition of the C-I bond to the palladium centre. This is a rate-determining step of the PdII/PdIV mechanism, while it is facile for a Pd0 catalyst. The calculations show that intermediate ligand detachment and reattachment is necessary in the course of the oxidative addition to PdII. Therefore, we expect the PdII/PdIV mechanism to be only feasible if a weakly coordinating ligand is present.

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Oxidative Addition of Palladium(0) Complexes Generated from [Pd(dba)2] and P-N Ligands: A Kinetic Investigation

Cited by 19 publications

References 0 publications

Importance of the Reducing Agent in Direct Reductive Heck Reactions

Importance of the Reducing Agent in Direct Reductive Heck Reactions

Palladium(0)/NHC‐Catalyzed Reductive Heck Reaction of Enones: A Detailed Mechanistic Study

Computational Study of a New Heck Reaction Mechanism Catalyzed by Palladium(II/IV) Species

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