14-Electron T-Shaped [PdRXL] Complexes: Evidence or Illusion? Mechanistic Consequences for the Stille Reaction and Related Processes

Casares, Juan A.; Espinet, Pablo; Salas, Gorka

doi:10.1002/1521-3765(20021104)8:21<4843::aid-chem4843>3.0.co;2-i

Cited by 78 publications

(68 citation statements)

References 17 publications

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“…Alternatively, species C could form from a halogen‐bridged dimeric species (analogous to B), in particular, if bulky ligands are involved, which could lead to the isomeric form of C . This alternative Pd species C and its transition states with the aryl group positioned cis to the NHC ligand were also calculated by using DFT (Figure S31) and were demonstrated to be similar in energy (≈0.3 kcal lower in energy).…”

Section: Resultsmentioning

confidence: 97%

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: Resultsmentioning

confidence: 97%

Importance of the Reducing Agent in Direct Reductive Heck Reactions

et al. 2017

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“…1 These 14-electron complexes have been shown to be primary actors in the catalytic cycles of homogeneous catalysts [13,14] and have been suggested to participate in an dissociative binding mechanism of cisplatin to DNA [15]. Today, a general consensus exists in which these species are considered unstable and only transitory in nature.…”

Section: Introductionmentioning

confidence: 99%

Computational insights on the possibility of tri-coordinated cisplatinated adducts with protein models

Ortega‐Carrasco

Cossío

Lledós

et al. 2012

Journal of Inorganic Biochemistry

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“…The origin of this low reactivity could be due to slow transmetalation, [35-38] homo-coupling problems,[39] or Pd catalysed protodeboronation,[40] so we have carried out some mechanistic experiments to shed light on this. A competition experiment was set up (Scheme 3): a single vial containing 0.5 mmol of 3-fluoro-chlorobenzene, 0.75 mmol of phenyl boronic acid, 0.75 mmol of 2,3,6 trifluorophenyl boronic acid and 3 equivalents of base was heated under standard conditions and then directly analysed by 19 F NMR spectroscopy.…”

Section: Resultsmentioning

confidence: 99%