Density Functional Theory study on the formation of the active catalysts in palladium catalysed reaction (original) (raw)
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2022
We explored the formation of active palladium catalyst species by degradation of Pd-acetate dimer with the addition of phosphine ligands (PH3 and PPh3 ) with an automated reaction search employing Density Functional Theory calculations followed by kinetic studies with stochastic simulation analysis. Our reaction search starting from dimeric form, considered a resting state of the catalyst, produced similar monomeric species by sequential ligand addition as found in the experimental investigation of the active catalytic species in Heck reactions. We analyzed the bonding in the Pd-acetate dimer and the role of Pd in the stability of the dimeric species. We implemented the Gillespie Stochastic Simulation Algorithm and applied it to the degradation reaction path. This algorithm can give more insights into multi-channel reaction paths. The energetics of the degradation path is reasonably achievable in the experimental reaction conditions that make dimeric species a potential catalytic pr...
Mechanistic and kinetic studies of palladium catalytic systems
Journal of Organometallic Chemistry, 1999
It is established that new reactive anionic palladium(0) complexes species are formed in which palladium(0) is ligated by either chloride ions: Pd(0)(PPh3)2Cl− (when generated by reduction of PdCl2(PPh3)2) or by acetate ions: Pd(0)(PPh3)2(OAc)− (when generated in situ in mixtures of Pd(OAc)2 and PPh3). The reactivity of such anionic palladium(0) complexes in oxidative addition to aryl iodides strongly depends on the anion born by the palladium(0). The structure of the arylpalladium(II) complexes formed in the oxidative addition also depends on the anion. Indeed, intermediate anionic pentacoordinated arylpalladium(II) complexes are formed: ArPdI(Cl)(PPh3)2− and ArPdI(OAc)(PPh3)2−, respectively, whose stability depends on the chloride or acetate anion brought by the palladium(0). ArPdI(Cl)(PPh3)2− is rather stable and affords trans ArPdI(PPh3)2 at long times via a neutral pentacoordinated solvated species ArPdI(S)(PPh3)2 involved in an equilibrium with the chloride ion. ArPdI(OAc)(PPh3)2− is quite unstable and rapidly affords the stable trans ArPd(OAc)(PPh3)2 complex. Consequently, the mechanism of the PdCl2(PPh3)2-catalyzed cross-coupling of aryl halides and nucleophiles has been revisited. The nucleophilic attack does not proceed on the trans ArPdI(PPh3)2 complexes as usually postulated but on the intermediate neutral pentacoordinated species ArPdI(S)(PPh3)2 to afford a pentacoordinated anionic complex ArPdI(Nu)(PPh3)2− in which the aryl group and the nucleophile are adjacent, a favorable position for the reductive elimination which provides the coupling product Ar–Nu. The mechanism of the Heck reactions catalyzed by mixtures of Pd(OAc)2 and PPh3 has also been revisited. The nucleophilic attack of the olefin proceeds on ArPd(OAc)(PPh3)2 and not on the expected trans ArPdI(PPh3)2 complex which is never formed when the oxidative addition is performed from Pd(0)(PPh3)2(0Ac)−. This work emphasizes the crucial role played by the anions born by the precursors of palladium(0) complexes and rationalize empirical findings dispersed in literature concerning the specificity of palladium catalytic systems.
Monatshefte für Chemie - Chemical Monthly, 2008
The mechanism of the preactivation process of trans-dichlorobis(diethanolamine-N)palladium(II) complex is investigated using density functional theory. The role of diethanolamine (a solvent for the reaction in the absence of a strong base) and acetonitrile (solvent for the reaction in the presence of a strong base) is analyzed by using a discrete model. The Onsager model is applied to assess the effect of the bulk medium. Both models show that diethanolamine activates the complex and thus is a better suited solvent for the Heck reactions of the investigated complex. Keywords Reaction mechanisms Á Quantum chemical calculations Á Trans-[PdCl 2 (DEA) 2 ] Á Solvent effects
Pd(I) Phosphine Carbonyl and Hydride Complexes Implicated in the Palladium-Catalyzed Oxo Process
Journal of the American Chemical Society, 2008
Reduction of compound "Pd(bcope)(OTf)2" [bcope = (c-C8H14-1,5)PCH2CH2P(c-C8H14-1,5); OTf = O3SCF3], with H2/CO yields a mixture of Pd(I) compounds [Pd2(bcope)2(CO)2](OTf)2 (1) and [Pd2(bcope)2(-CO)(-H)](OTf) (2), whereas reduction with H2 or Ph3SiH in the absence of CO leads to [Pd3(bcope)3(-H)2](OTf)2 (3). Exposure of 3 to CO leads to 1 and 2. The structures of 1 and 3 have been determined by X-ray diffraction. Complex [Pd2(bcope)2-(CO)2] 2+ displays a metal-metal bonded structure with a square planar environment for the Pd atoms and terminally bonded CO ligands and is fluxional in solution. DFT calculations aid the interpretation of this fluxional behaviour as resulting from an intramolecular exchange of the two inequivalent P atom positions via a symmetric bis-CO-bridged intermediate. A cyclic voltammetric investigation reveals a very complex redox behaviour for the "Pd(bcope)(OTf)2"/CO system and suggests possible pathways leading to the formation of the various observed products, as well as their relationship with the active species of the PdL2 2+ /CO/H2-catalyzed oxo processes (L2 = diphosphine ligands).
Molecular Physics, 2010
Two different yet related topics are discussed: (i) the reduction of palladium (II) in Pd(OAc) 2 complexes reacting with phenyl phosphines and leading to Pd(0) phosphines complexes and (ii) the carbonylation reaction of allyl chlorides catalyzed by these Pd(0) species. The results show that the overall reduction is an exothermic process that can be accomplished along two different reaction paths: one of them is clearly favoured. Similarly, three different channels have been determined for the carbonylation reaction that primarily differ for the timing and the way with which the reacting species bind the metal. In the first path (σ-path), the allyl fragment interacts very weakly with the metal, whereas, successively the CO molecule strongly binds it and reacts with the allyl. A second channel (π-η 3 pathway) is characterized by a π-η 3 interaction between the allyl fragment and the palladium, to which the CO molecule binds, before the two units react affording the product. In both cases, two consecutive migrations of the chlorine "assist" the course of the reaction. In a third case (η 2 pathway) the allyl fragment initially enters the palladium coordination sphere, then the CO molecule simultaneously binds it and the phosphorous atom of one phosphine ligand. The first two paths are favoured.
Organometallics, 1998
For PPh 3 , mixtures of Pd(dba) 2 and nTFP (TFP) tri-2-furylphosphine, n g 2) in DMF and THF (S) lead to the formation of Pd(dba)(TFP) 2 , SPd(TFP) 3 in equilibrium with SPd-(TFP) 2. The substitution of dba by the phosphine in Pd(dba)L 2 to form SPdL 3 is easier for L) TPF than for L) PPh 3. The less ligated complex SPd(TFP) 2 is the reactive species in the oxidative addition with phenyl iodide. In THF, {Pd(dba) 2 + nTFP}, a mixture often used as a catalyst promoter in several synthetic organic reactions, is found to be less reactive than {Pd(dba) 2 + nPPh 3 } for small values of n (n) 2 or 4) whereas it is more reactive for higher values of n (n > 6). Conversely, in DMF, {Pd(dba) 2 + nTFP} is always found to be more reactive than {Pd(dba) 2 + nPPh 3 } whatever n (n g 2).