On the Mechanism of the Palladium(II)-Catalyzed Decarboxylative Olefination of Arene Carboxylic Acids. Crystallographic Characterization of Non-Phosphine Palladium(II) Intermediates and Observation of Their Stepwise Transformation in Heck-like Processes

Mechanistic studies of a palladium-mediated decarboxylative olefination of arene carboxylic acids
are presented, providing spectroscopic and, in two instances, crystallographic evidence for intermediates
in a proposed stepwise process. Sequentially, the proposed pathway involves carboxyl exchange between
palladium(II) bis(trifluoroacetate) and an arene carboxylic acid substrate, rate-determining decarboxylation
to form an arylpalladium(II) trifluoroacetate intermediate (containing two trans-disposed S-bound dimethyl
sulfoxide ligands in a crystallographically characterized form), then olefin insertion and β-hydride elimination.
Because of the unique mode of generation of the arylpalladium(II) trifluoroacetate intermediate, a species
believed to be substantially electron-deficient relative to phosphine-containing arylpalladium(II) complexes
previously studied, it has been possible to gain new insights into those steps that are common to the Heck
reaction, namely, olefin insertion and β-hydride elimination. The present results show that there are notable
differences in reactivity between arylpalladium(II) intermediates generated by decarboxylative palladation
and those produced in conventional Heck reactions. Specifically, we have found that more electron-rich
alkenes react preferentially with an arylpalladium(II) trifluoroacetate intermediate formed by decarboxylative
palladation, whereas an opposite trend is found in conventional Heck reactions. In addition, we have found
that the aralkylpalladium(II) trifluoroacetate intermediates that are formed upon olefin insertion in the present
study are stabilized with respect to β-hydride elimination as compared to the corresponding phosphine-ligated aralkylpalladium(II) complexes. We have also crystallographically characterized an aralkylpalladium(II) trifluoroacetate intermediate derived from arylpalladium(II) insertion into norbornene, and this structure,
too, contains an S-bound dimethyl sulfoxide ligand; the ipso-carbon of the transferred aryl group and
trifluoroacetate function as the third and fourth ligands in the observed distorted square-planar palladium(II) complex.