Mechanistic Aspects of Aryl-Halide Oxidative Addition, Coordination Chemistry, and Ring-Walking by Palladium

• Published in: Chemistry : a European journal 2015-11, Vol.21 (45), p.16113-16125
• Main Authors: Zenkina, Olena V., Gidron, Ori, Shimon, Linda J. W., Iron, Mark A., van der Boom, Milko E.
• Format: Article
• Language: English
• Subjects: Activation; Aromatic compounds; aryl halide activation; bond activation; Bonding; Carbon; Chemistry; coordination; Coordination compounds; Halides; Intermediates; Palladium; Phosphine; Phosphines; rearrangement; Substrates; Unsaturated; Walking
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201501580

This contribution describes the reactivity of a zero‐valent palladium phosphine complex with substrates that contain both an aryl halide moiety and an unsaturated carbon–carbon bond. Although η2‐coordination of the metal center to a CC or CC unit is kinetically favored, aryl halide bond activation is favored thermodynamically. These quantitative transformations proceed under mild reaction conditions in solution or in the solid state. Kinetic measurements indicate that formation of η2‐coordination complexes are not nonproductive side‐equilibria, but observable (and in several cases even isolated) intermediates en route to aryl halide bond cleavage. At the same time, DFT calculations show that the reaction with palladium may proceed through a dissociation–oxidative addition mechanism rather than through a haptotropic intramolecular process (i.e., ring walking). Furthermore, the transition state involves coordination of a third phosphine to the palladium center, which is lost during the oxidative addition as the Chalide bond is being broken. Interestingly, selective activation of aryl halides has been demonstrated by adding reactive aryl halides to the η2‐coordination complexes. The product distribution can be controlled by the concentration of the reactants and/or the presence of excess phosphine. To walk or not to walk: The initial reaction of palladium(0) with halide‐substituted stilbazole and tolan ligands is coordination to the bridging unsaturated CC bond (see figure). It was found experimentally that the palladium center may ring‐walk along the π‐system. A DFT investigation found that a dissociative process may occur prior to halide activation.