No abstract

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

No abstract

No abstract