title: Catalyst activation, deactivation, and degradation in palladium-mediated Negishi cross-coupling reactions.
abstract: Pd-mediated Negishi cross-coupling reactions were studied by a combination of kinetic measurements, electrospray-ionization (ESI) mass spectrometry, (31)P NMR and UV/Vis spectroscopy. The kinetic measurements point to a rate-determining oxidative addition. Surprisingly, this step seems to involve not only the Pd catalyst and the aryl halide substrate, but also the organozinc reagent. In this context, the ESI-mass spectrometric observation of heterobimetallic Pd-Zn complexes [L2 PdZnR](+) (L=S-PHOS, R=Bu, Ph, Bn) is particularly revealing. The inferred presence of these and related neutral complexes with a direct Pd-Zn interaction in solution explains how the organozinc reagent can modulate the reactivity of the Pd catalyst. Previous theoretical calculations by González-Pérez et al. (Organometallics- 2012, 31, 2053) suggest that the complexation by the organozinc reagent lowers the activity of the Pd catalyst. Presumably, a similar effect also causes the rate decrease observed upon addition of ZnBr2 . In contrast, added LiBr apparently counteracts the formation of Pd-Zn complexes and restores the high activity of the Pd catalyst. At longer reaction times, deactivation processes due to degradation of the S-PHOS ligand and aggregation of the Pd catalyst come into play, thus further contributing to the appreciable complexity of the title reaction.
authors.affiliation: Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, 81377 München (Germany).
authors.forename: Katharina
authors.initials: K
authors.lastname: Böck
authors.forename: Julia E
authors.initials: JE
authors.lastname: Feil
authors.forename: Konstantin
authors.initials: K
authors.lastname: Karaghiosoff
authors.forename: Konrad
authors.initials: K
authors.lastname: Koszinowski
keywords: cross-coupling; electrospray-ionization mass spectrometry; homogeneous catalysis; kinetics; palladium