Research: Mechanistic investigation of Pd-catalyzed reactions
In parallel to our
experimental work, we also investigate the mechanism of palladium-catalyzed
coupling reactions in general, and of our new reactions in particular, by means
of in situ spectroscopy and DFT calculations. This work is performed in
close cooperation with Prof. W. Thiel.
Oxidative addition of aryl halides to palladium catalysts
The oxidative addition of
aryl halides to palladium(0) complexes under formation of aryl-palladium(II)
species is the initiating step in numerous widely applied catalytic
cross-coupling reactions, e.g. Suzuki reactions, Heck
olefinations and Stille couplings. Unfortunately, the “textbook mechanism”
provides no explanation for the pronounced influence that counter-ions of the
Pd(II) pre-catalysts and added metal salts have on catalytic activities.
Based on DFT calculations,
we propose a different mechanism for the oxidative addition of aryl halides to
Pd-catalysts. Key intermediate is an anionic Pd-species in which the aryl halide
coordinates to the palladium via the halide atom.
This simple
mechanism offers an alternative explanation for the complex experimental
observations in cross-coupling reactions, without involving five-coordinate
Pd-intermediates.
The figure shows our calculated reaction path for the oxidative addition of
iodobenzene to [Pd(PMe3)2OAc]-
The Palladium-Catalyzed Cross-Coupling Reaction of Carboxylic Anhydrides with Arylboronic Acids
The mechanism of the cross-coupling of phenylboronic acid with acetic anhydride, a viable model of the widely used Suzuki reaction, has been studied by DFT calculations at the BP86/6-31G* level of theory. Two alternative catalytic cycles have been investigated, one starting from a neutral Pd(0)L2 complex, the other from an anionic “Jutand-type” [Pd(0)L2X]– species. The reaction profiles are in good agreement with the experimental findings, as both pathways require only moderate activation energies. Both pathways are dominated by cis-configured square planar palladium(II)diphosphine intermediates.
While the “textbook mechanism” mainly proceeds via trans-configured palladium(II)diphosphine complexes, cis-configured intermediates dominate in the calculated catalytic cycles. Moreover, according to the proposition of Amatore and Jutand, the anionic pathway involves five-coordinate species, whereas we find only four-coordinate intermediates, in qualitative agreement with related calculations on the oxidative addition of aryl halides to anionic palladium(0) complexes.