Rate and Mechanism of the Oxidative Addition of Phenyl Iodide to Pd0 Ligated by Triphenylarsine: Evidence for the Formation of a T-Shaped Complex [PhPdI(AsPh3)] and for the Decelerating Effect of CH2=CH−SnBu3 by Formation of [Pd0(η2-CH2=CH−SnBu3)(AsPh3)2]

Abstract

The oxidative addition of phenyl iodide to the palladium(o) generated from [Pd0(dba)2] and n equivalents of AsPh3 (the most efficient catalytic precursor in Stille reactions) proceeds from [(solv)Pd0(AsPh3)2] (solv= solvent). However, the latter is present only in trace concentrations because it is involved in an equilibrium with the major, but nonreactive, complex [Pd0(dba)(AsPh3)2]. As regards the phosphine ligands, dba has a decelerating effect on the rate of the oxidative addition by decreasing the concentration of the reactive species. Relative to PPh3, the effect of AsPh3 is to increase the rate of the oxidative addition of PhI by a factor ten in DMF and seven in THF, independent of the value of n, provided that n > or = 2. In contrast to PPh3, the addition of more than two equivalents of AsPh3 to [Pd0(dba)2] (dba= trans,trans-dibenzylideneacetone) does not affect the kinetics of the oxidative addition because of the very endergonic displacement of dba from [Pd0(dba)(AsPh3)2] to form [Pd0(AsPh3)3]. The complex trans-[PhPdI(AsPh3)2], formed in the oxidative addition, is involved in a slow equilibrium with the T-shaped complex [PhPdI(AsPh3)] after appreciable decomplexation of one AsPh3. Under catalytic conditions, that is, in the presence of a nucleophile, such as CH2=CH-SnBu3 which is able to coordinate to [Pd0(AsPh3)2], a new Pd0 complex is formed: [Pd0(eta2-CH2=CHSnBu3)(AsPh3)2]; however, this complex does not react with PhI. Consequently, CH2=CH-SnBu3 slows down the oxidative addition by decreasing the concentration of the reactive species [(solv)Pd0(AsPh3)2]. This demonstrates that a nucleophile may be not only involved in the transmetallation step, but may also interfere in the kinetics of the oxidative addition step by decreasing the concentration of reactive Pd0.

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

“…In that range, the H ortho proton of the metallated phenyl (near 7 ppm) and the H para and H meta protons (near 6.5 ppm, partially overlapped with each other), are observed. As reported in the literature,8 [PdPhI(AsPh 3 ) 2 ] ( 8 ) gives rise in CDCl 3 to two sets of signals corresponding to two different chemical species (Figure 3 a). Similar behavior (with a smaller proportion of the minor signal) is observed for [PdPhCl(AsPh 3 ) 2 ] ( 7 ) (Figure 3 b).…”

“…The major species in each spectrum is [PdPhX(AsPh 3 ) 2 ]. The minor species in the solution of 8 was attributed in the literature to the so‐called T‐shaped [PdPhI(AsPh 3 )(S)],8 but this is not correct. The signals of the minor species correspond to the only signals observed in the CDCl 3 solutions of the corresponding dimers [( μ ‐X) 2 Pd 2 Ph 2 (AsPh 3 ) 2 ] (X=Cl ( 9 ), I ( 10 )), and are in fact the unchanged dimers.…”

“…To our surprise the title of a recent paper announced “Evidence for the Formation of a T‐Shaped Complex” and claimed in the text “the first spectroscopic evidence for the formation of the so‐called T‐shaped complex” 8. The news became less striking to us when we observed that the T‐shaped complex was defined in several places in the text as [PdPhI(AsPh 3 )(S)] (S=CDCl 3 , THF, DMF), which is not a T‐shaped 14‐electron complexes but a square‐planar 16‐electron complex.…”

Mechanistic Aspects of Metal‐Catalyzed C—C and C—X Bond‐Forming Reactions

“…These studies were based on kinetic measurements of the palla- [50,149]. This study revealed that, under those conditions, the species preceding the transmetallation steps is complex 61, bearing a DMF as a ligand (Scheme 1-27).…”

Stille coupling reactions catalysed by a polymer supported palladium complex

“…This result is in contrast to the better transmetallation ability reported for vinyl respect to phenyl group [26]. However, it has been demonstrated that a nucleophile such as n Bu 3 SnCH@ CH 2 , which is able to coordinate Pd(0) species, causes in some catalytic systems the formation of new palladium complexes that do not react with the substrate [27]. Among the tested aryl bromides, bromobenzene was the less reactive: the reaction with n Bu 3 SnPh carried out at 120°C stopped at a 38% yield in biphenyl (entry 12-2).…”