In the Heck reaction between aryl halides and n-butyl acrylate, the palladacycle {Pd[kappa(1)-C, kappa(1)-N-C=(C(6)H(5))C(Cl)CH(2)NMe(2)](mu-Cl)}(2), 1, is merely a reservoir of the catalytically active Pd(0) species [1](Pd colloids or highly active forms of low ligated Pd(0) species) that undergoes oxidative addition of the aryl halide on the surface with subsequent detachment, generating homogeneous Pd(II) species. The main catalytic cycle is initiated by oxidative addition of iodobenzene to [1], followed by the reversible coordination of the olefin to the oxidative addition product. All the unimolecular subsequent steps are indistinguishable kinetically and can be combined in a single step. This kinetic model predicts that a slight excess of alkene relative to iodobenzene leads to a rapid rise in the Pd(0) concentration while when using a slight excess of iodobenzene, relative to alkene, the oxidative addition product is the resting state of the catalytic cycle. Competitive experiments of various bromoarenes and iodoarenes with n-butyl acrylate catalyzed by 1 and CS, CP, and NCN palladacycles gave the same rho value (2.4-2.5 for Ar-Br and 1.7-1.8 for Ar-I) for all palladacycles employed, indicating that they generate the same species in the oxidative addition step. The excellent fit of the slope with the sigma(0) Hammett parameter and the entropy of activation of -43 +/- 8 J mol(-1) K(-1) are consistent with an associative process involving the development of only a partial charge in the transition state for the oxidative step of iodobenzene.

OPPh 2 and t-BuSCH 2 CH 2 C ¼ C(o-NC 5 H 4 ), respectively. The molecular structures of 1 and 3 have been ascertained by means of X-ray diffraction analysis. The catalytic properties of these mixed donor group pincer-type palladacycles have been evaluated in the arylation of olefins (Heck reaction). The pincer palladacycle 1 is highly active for the coupling of aryl iodides and aryl bromides with n-butyl acrylate. In contrast it is only moderately active for the coupling of aryl chlorides substituted with electron-withdrawing groups and inactive for the coupling of electron neutral and electron deactivated aryl chlorides.

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Palladium compounds containing a Pd‐C that are stabilized intramolecularly by at least one two‐electron donor group have numerous applications as pre‐catalysts. In particular, these compounds denominated palladacycles possess very attractive properties such as high thermal stability and low air sensitivity and they are ease to handle. These compounds are very active catalyst precursors in Heck coupling, Suzuki coupling, Buchwald–Hartwig amination and related coupling reactions under mild conditions, for instance at room temperature or in water. However, palladacycles serve as precursors to highly active palladium(0) species consist of soluble clusters or colloids, often stabilized by quaternary ammonium salts in highly dilute solutions or highly active low‐coordinate complexes of electron‐rich phosphines or carbenes. Moreover, chiral palladacycles are very important catalysts for enantioselective allylic rearrangements, allylic additions and aldol chemistry, since they act as Pd(II) Lewis acids and no redox process occurs

Pages 12059 and 12063. Equations 1, 3, and 4 were published incorrectly. The correct equations are given here.

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