164071-99-2Relevant articles and documents
The ubiquitous cross-coupling catalyst system 'Pd(OAc)2'/2PPh3 forms a unique dinuclear PdI complex: An important entry point into catalytically competent cyclic Pd3 clusters
Scott, Neil W. J.,Ford, Mark J.,Schotes, Christoph,Parker, Rachel R.,Whitwood, Adrian C.,Fairlamb, Ian J. S.
, p. 7898 - 7906 (2019/09/06)
Palladium(ii) acetate 'Pd(OAc)2'/nPPh3 is a ubiquitous precatalyst system for cross-coupling reactions. It is widely accepted that reduction of in situ generated trans-[Pd(OAc)2(PPh3)2] affords [Pd0(PPh3)n] and/or [Pd0(PPh3)2(OAc)]- species which undergo oxidative addition reactions with organohalides-the first committed step in cross-coupling catalytic cycles. In this paper we report for the first time that reaction of Pd3(OAc)6 with 6 equivalents of PPh3 (i.e. a Pd/PPh3 ratio of 1?:?2) affords a novel dinuclear PdI complex [Pd2(μ-PPh2)(μ2-OAc)(PPh3)2] as the major product, the elusive species resisting characterization until now. While unstable, the dinuclear PdI complex reacts with CH2Cl2, p-fluoroiodobenzene or 2-bromopyridine to afford Pd3 cluster complexes containing bridging halide ligands, i.e. [Pd3(X)(PPh2)2(PPh3)3]X, carrying an overall 4/3 oxidation state (at Pd). Use of 2-bromopyridine was critical in understanding that a putative 14-electron mononuclear 'PdII(R)(X)(PPh3)' is released on forming [Pd3(X)(PPh2)2(PPh3)3]X clusters from [Pd2(μ-PPh2)(μ2-OAc)(PPh3)2]. Altering the Pd/PPh3 ratio to 1?:?4 forms Pd0(PPh3)3 quantitatively. In an exemplar Suzuki-Miyaura cross-coupling reaction, the importance of the 'Pd(OAc)2'/nPPh3 ratio is demonstrated; catalytic efficacy is significantly enhanced when n = 2. Employing 'Pd(OAc)2'/PPh3 in a 1?:?2 ratio leads to the generation of [Pd2(μ-PPh2)(μ2-OAc)(PPh3)2] which upon reaction with organohalides (i.e. substrate) forms a reactive Pd3 cluster species. These higher nuclearity species are the cross-coupling catalyst species, when employing a 'Pd(OAc)2'/PPh3 of 1?:?2, for which there are profound implications for understanding downstream product selectivities and chemo-, regio- and stereoselectivities, particularly when employing PPh3 as the ligand.
Decelerating effect of alkenes in the oxidative addition of phenyl iodide to palladium(0) complexes in heck reactions
Amatore, Christian,Carré, Emmanuelle,Jutand, Anny,Medjour, Youcef
, p. 4540 - 4545 (2008/10/08)
In DMF, the oxidative addition of PhI to Pd0(PPh3)4 or to the anionic Pd0(PPh3)3(OAc)- is slower in the presence of an alkene (styrene, methyl acrylate). Indeed, the concentration of the reactive Pd0(PPh3)2 or Pd0(PPh3)2(OAc)- complex decreases because of its coordination to the alkene to form the unreactive (η2-CH2=CHR)Pd0(PPh3)2 (R = Ph, CO2Me) or (η2-CH2= CHPh)Pd0(PPh3)2(OAc)-, respectively. As already evidenced in palladium-catalyzed Stille reactions, this work establishes that, in palladium-catalyzed Heck reactions as well, the nucleophile plays a role in the kinetics of the oxidative addition (decelerating effect), as soon as it may coordinate Pd0 complexes. This is an essential observation, in view of the general belief that the nucleophile enters the catalytic cycle only at the stage of the attack on the aryl-PdII complex formed in the oxidative addition. Whenever the oxidative addition is not rate determining, the decelerating effect of the alkene on this reaction is in favor of a higher efficiency of the catalytic cycle.
