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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

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

Daisuke Tanaka, Stuart P Romeril, Andrew G Myers

J Am Chem Soc. 2005 Jul 27;127(29):10323-33.

PMID: 16028944

Abstract:

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 beta-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 beta-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 beta-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.

Chemicals Related in the Paper:

Catalog Number	Product Name	Structure	CAS Number	Price
AP42196316	Palladium(II) trifluoroacetate		42196-31-6	Price

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