Abstract
The first experimental observations on the transmetalation step in the Negishi coupling were recently reported by Espinet et al. for the transmetalation reaction between trans-[Pd(Rf)(Cl)(PPh\(_3\))\(_2\)] (Rf = 3,5-dichloro-2,4,6-trifluorophenyl) and the organozinc reagents ZnMe\(_2\) and ZnMeCl, in THF. Interestingly, this study revealed that each organozinc reagent affords a different isomer (trans or cis, respectively) of the coupling intermediate [Pd(Rf)(Me)(PPh\(_3\))\(_2\)], and warned on the existence of secondary undesired transmetalations (e.g., methyl by aryl exchanges). Motivated by these exciting results, we decided to investigate theoretically the transmetalation reaction between the complex trans-[Pd(Me)(Cl)(PMePh\(_2\))\(_2\)] and the organozinc reagents ZnMeCl and ZnMe\(_2\). This mechanistic study was carried out in collaboration with the experimental group of Prof. Pablo Espinet and Prof. Juan Casares at the University of Valladolid. Importantly, this study represented the first experimental determination of reaction and activation Gibbs energies for a Negishi transmetalation reaction. Furthermore, these results were contrasted with DFT calculations that provided a detailed picture of the reaction pathway and whose calculated energy barriers resulted to be in good agreement with the experimental parameters. All these results are presented in this chapter.
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