title: Mechanistic Aspects of Aryl-Halide Oxidative Addition, Coordination Chemistry, and Ring-Walking by Palladium.
abstract: 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 η(2) -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 η(2) -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 η(2) -coordination complexes. The product distribution can be controlled by the concentration of the reactants and/or the presence of excess phosphine.
authors.affiliation: Department of Organic Chemistry, The Weizmann Institute of Science, 76100 Rehovot (Israel).|Current address: Faculty of Science, University of Ontario Institute of Technology, Oshawa, ON (Canada).
authors.forename: Olena V
authors.initials: OV
authors.lastname: Zenkina
authors.affiliation: Department of Organic Chemistry, The Weizmann Institute of Science, 76100 Rehovot (Israel).
authors.forename: Ori
authors.initials: O
authors.lastname: Gidron
authors.affiliation: Department of Chemical Research Support, The Weizmann Institute of Science, 76100 Rehovot (Israel).
authors.forename: Linda J W
authors.initials: LJ
authors.lastname: Shimon
authors.affiliation: Department of Chemical Research Support, The Weizmann Institute of Science, 76100 Rehovot (Israel).
authors.forename: Mark A
authors.initials: MA
authors.lastname: Iron
authors.affiliation: Department of Organic Chemistry, The Weizmann Institute of Science, 76100 Rehovot (Israel). milko.vanderboom@weizmann.ac.il.
authors.forename: Milko E
authors.initials: ME
authors.lastname: van der Boom
keywords: aryl halide activation; bond activation; coordination; palladium; rearrangement