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.

Intramolecular Transfer of Pd Catalyst on Carbon–Carbon Triple Bond and Nitrogen–Nitrogen Double Bond in Suzuki–Miyaura Coupling Reaction

“…The values of stabilization energy of complexation of stilbene, tolan, and azobenzene with t-Bu 3 PPd were −27.9, −25.6, and −20.8 kcal/mol, respectively ( Figures S3-S5). These results support our interpretation, although calculations for model PdPH 3 systems with ethylene and acetylene indicated that the Pd-C bonds are stronger in the acetylene complex than in the ethylene complex [16,17].…”

Reactions of nickel(0) with organochlorides, organobromides, and organoiodides: mechanisms and structure/reactivity relationships

“…This selectivity was such that the normal order of reactivity of aryl halides (I > Br > Cl) could be overturned, and was attributed to a 'ring-walking' process where the nickel moves from the initial site of coordination across a π-system to the aryl halide. [46][47][48][49][50][51] This proposal was supported by DFT calculations.…”

Phenanthrene Synthesis by Palladium-Catalyzed Benzannulation with o-Bromobenzyl Alcohols through Multiple Carbon–Carbon Bond Formations

“…Alkoxopalladium G undergoes β-carbon elimination with the release of acetone, providing the arylpalladium H followed by reductive elimination to yield aryl bromide 8 and regenerate the initial palladium species. Subsequently, the smooth oxidative addition of 8 likely proceeds through a ring-walking pathway , to generate arylpalladium species I , which undergoes the 6- endo - trig cyclization to furnish benzylpalladium intermediate J . Finally, β-hydrogen elimination from J releases the desired phenanthrene 3a with the initial palladium catalyst regenerated.…”