Evidence for the Ligation of Palladium(0) Complexes by Acetate Ions: Consequences on the Mechanism of Their Oxidative Addition with Phenyl Iodide and PhPd(OAc)(PPh3)2 as Intermediate in the Heck Reaction

Amatore, Christian; Carré, Emmanuelle; Jutand, Anny; M’barki, Mohamed Amine; Meyer, Gilbert

doi:10.1021/om00012a029

Cited by 288 publications

(239 citation statements)

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“…10 These results definitively rule out the textbook mechanism (Scheme 1) and establish that PhPd(OAc)-(PPh 3 ) 2 is the key reactive intermediate in Heck reactions. Its reaction with olefins is the rate-determining step of the catalytic cycle (Scheme 5).…”

Section: Methodssupporting

confidence: 53%

“…Its oxidative addition to PhI affords not PhPdI(PPh 3 ) 2 , as postulated (Scheme 1), but a new complex, trans-PhPd II (OAc)(PPh 3 ) 2 . 10 The mechanism of its formation was investigated by amperometry at a rotating disk electrode polarized on the plateau of the oxidation wave of Pd 0 (PPh 3 ) 2 (OAc) -. Addition of PhI results in a fast decay of the oxidation current ( Figure 1) and provides kinetic data on the oxidative addition rate (Scheme 4).…”

Section: Methodsmentioning

confidence: 99%

“…This indicates that PhPd-(OAc)(PPh 3 ) 2 is formed ultimately and that the reaction proceeds in two separate steps via an intermediate shortlived anionic pentacoordinated complex, PhPdI(OAc)-(PPh 3 ) 2 -, in which both acetate and iodide ions remain coordinated to the palladium(II) center (Scheme 4, [Pd] ) 2 mmol dm -3 ). 10 Its short half-life (30 s) precludes 31 P NMR characterization. PhPd(OAc)(PPh 3 ) 2 is also involved in a slow endergonic equilibrium with a cationic complex, PhPd(PPh 3 ) 2 + , and AcO -(eq 8).…”

Section: Methodsmentioning

confidence: 99%

“…PhPd(OAc)(PPh 3 ) 2 is also involved in a slow endergonic equilibrium with a cationic complex, PhPd(PPh 3 ) 2 + , and AcO -(eq 8). 10 Therefore, the acetate ion borne by the palladium(0) remains ligated to two different phenylpalladium(II) complexes, one anionic and the other neutral. Moreover, through equilibrium (8) (Table 2), 10 probably because the solvated cationic trans-PhPd(PPh 3 ) 2 (DMF) + gives a transadduct which requires an endergonic trans-cis isomerization to allow syn-insertion of the olefin into the Ph-Pd bond (eq 9).…”

Section: Methodsmentioning

confidence: 99%

“…10 Even in the presence of excess PPh 3 , the intramolecular reduction of Pd(OAc) 2 (PPh 3 ) 2 affords anionic palladium(0) complexes ligated by acetate and involved in an equilibrium with PPh 3 (eqs 5 and 6). Noticeably, the overall reaction (eq 7) affords protons resulting from the hydrolysis to (O)PPh 3 of the phosphonium salt formed with the palladium(0) (eqs 3 and 4).…”

Section: Anionic Pd Intermediates In Pd-catalyzed Reactionsmentioning

confidence: 99%

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Anionic Pd(0) and Pd(II) Intermediates in Palladium-Catalyzed Heck and Cross-Coupling Reactions

2000

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The anions of PdCl2L2 and Pd(OAc)2, precursors of palladium(0) used in cross-coupling and Heck reactions, play a crucial role in these reactions. Tricoordinated anionic complexes Pd 0 L2Cl -and Pd 0 L2(OAc) -are the effective catalysts instead of the usually postulated Pd 0 L2 complex. The anion ligated to the palladium(0) affects the kinetics of the oxidative addition to ArI as well as the structure and reactivity of the arylpalladium(II) complexes produced in this reaction. Thus, pentacoordinated anionic complexes are formed, ArPdI(Cl)L2 -or ArPdI(OAc)L2 -, the precursor of neutral trans-ArPd(OAc)L2, instead of the usually postulated trans-ArPdIL2Palladium-catalyzed Heck reactions (eq 1) 1a,b,d,e or crosscoupling reactions between aryl halides and nucleophiles (eq 2) 1b-h have been intensively developed for their important synthetic applications.In both cases, the first step of the catalytic cycle is an oxidative addition of the aryl halide to the usually supposed 14-electron complex Pd 0 L 2 (L ) monodentate phosphine ligand) to afford a σ-arylpalladium(II) complex, trans-ArPdXL 2 (Schemes 1 and 2). The second step is a nucleophilic attack on trans-ArPdXL 2 , either a syn-addition (Heck reactions, Scheme 1) or a transmetalation (crosscoupling reactions, Scheme 2). The final product is generated by either -hydride elimination (Heck) or reductive elimination (cross-coupling).In the very first mechanistic approaches, these elemental steps have been investigated separately and under stoichiometric conditions, starting from isolated stable complexes: either palladium(0) complexes (for the oxidative addition) 2 or trans-ArPdXL 2 complexes (for the synaddition of olefins 3 or transmetalation of nucleophiles 4 ) or mixed complexes RPdR′L 2 (for the reductive elimination).5 Thanks to these studies, fundamental results are available on these elemental steps, e.g., influence of the ligand (basicity, cone angle) on the rate of oxidative addition or reductive elimination as well as influence of the nucleophile in the transmetalation or in the synaddition step. However, in most studies, the mechanistic investigations have not been performed in the context of a real catalytic cycle, i.e., starting from the precursor of the palladium(0) or from the real arylpalladium(II) complex involved in the nucleophilic attack. Working on isolated putative catalytic cycle segments and thus with stable complexes is risky because the real elemental

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Section: Methodssupporting

confidence: 53%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Anionic Pd Intermediates In Pd-catalyzed Reactionsmentioning

confidence: 99%

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