Kinetics and Mechanistic Aspects of the Heck Reaction Promoted by a CN−Palladacycle

Abstract

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.

Estratégias para estudo das correlações de energia livre em acoplamento aril-aril de Suzuki: elucidando ciclos catalíticos através da equação de Hammett

“…4 Um belo trabalho de investigação sobre aspectos mecanísticos do acoplamento de Heck, promovido por ciclopaladatos, foi apresentado por Dupont. 11 Nele foi avaliada a influência do grupo substituinte em brometos de arila na reação com acrilato de n-butila através de quatro ciclopaladatos. Os resultados mostraram uma lineariedade satisfatória (R 2 = 0,98) quando σ o era aplicado para todas as quatro fontes de paládio, fornecendo ρ = 2,5.…”

Ligand-free copper(i) oxide nanoparticle-catalysed amination of aryl halides in ionic liquids

“…Analogous to several other publications dealing with Pd-or Ni-NPs, it is possible that the Cu 2 O-NPs act as a catalyst reservoir for Cu(I), and a molecular species might act as the active species. 32,51 There are different ways of how a molecular species can theoretically be leached from the particle surface as Beletskaya and co-workers have revealed. Usually there is an equilibrium between leaching and the Ostwald-ripening of the particles.…”

“…28 Ionic liquids (ILs) are widely known as so-called "designer solvents". They can act as promoters and activators, 29,30 as protecting agents and stabilisers, 28,[30][31][32][33][34][35] as reducing agents 11,30,35 and certainly as (co-)solvents themselves for molecular and nanoscale species. [36][37][38][39] Only limited reports about the use and influence of ionic liquids as solvents for nanocatalytic C-N cross coupling reactions are known.…”

Poly(4‐vinylpyridine) and Quadrapure TU as Selective Poisons for Soluble Catalytic Species in Palladium‐Catalyzed Coupling Reactions – Application to Leaching from Polymer‐Entrapped Palladium

“…Thus, in many contributions, authors have identified catalysis by soluble species but could not rule out some catalysis by supported sites. The Hg(0) test is capable of extinguishing catalysis by free Pd(0), [8,[22][23][24]26,40,41] although it is not believed that it can discriminate between heterogeneous, macroscopic palladium particles, soluble palladium nanoparticles, or homeopathic [35,36,42,43] palladium (vide supra). [8] What is needed is a poison that is selective for leached, homogeneous species.…”