Organopalladium chemistry is a branch of organometallic chemistry that deals with organic palladium compounds and their reactions. Palladium is often used as a catalyst in the reduction of alkenes and alkynes with hydrogen. This process involves the formation of a palladium-carbon covalent bond. Palladium is also prominent in carbon-carbon coupling reactions, as demonstrated in tandem reactions.
Organopalladium chemistry timeline
1873 - A. N. Zaitsev reports reduction of benzophenone over palladium with hydrogen.
Unlike Ni, but similar to Pt, Pd halides form a variety of alkene complexes. The premier example is dichloropalladium. In this complex, the diene is easily displaced, which makes it a favored precursor to catalysts. In the industrially important Wacker process, ethylene is converted to acetaldehyde via nucleophilic attack of hydroxide on a Pd-ethylene intermediate followed by formation of a vinyl alcohol complex. Fullerene ligands also bind with palladium. Palladium acetate and related compounds are common reagents because the carboxylates are good leaving groups with basic properties. For example palladium trifluoroacetate has been demonstrated to be effective in aromatic decarboxylation:
Various organic groups can bound to palladium and form stable sigma-bonded complexes. The stability of the bonds in terms of bond dissociation energy follows the trend: Pd-Alkynyl > Pd-Vinyl ≈ Pd-Aryl > Pd-Alkyl and the metal-carbon bond length changes in the opposite direction: Pd-Alkynyl < Pd-Vinyl ≈ Pd-Aryl < Pd-Alkyl.
Palladium(0) compounds
Pd compounds include trisdipalladium and tetrakispalladium. These complexes react with halocarbon R-X in oxidative addition to R-Pd-X intermediates with covalent Pd-C bonds. This chemistry forms the basis of a large class of organic reactions called coupling reactions. An example is the Sonogashira reaction:
Organopalladium(IV)
The first organopalladium compound was described in 1986. This complex is Me3Pdbpy It was synthesized by oxidative addition of methyl iodide to Me2Pdbpy. Palladium compounds owe their reactivity to the ease of interconversion between Pd and palladium intermediates. There is no conclusive evidence however for the involvement of Pd to Pd conversions in palladium mediated organometallic reactions. One reaction invoking such mechanism was described in 2000 and concerned a Heck reaction. This reaction was accompanied by a 1,5-hydrogen shift in the presence of amines: The hydride shift was envisaged as taking place through a Pd metallacycle: In related work the intermediate associated with the hydride shift remains Pd: and in other work equilibria are postulated between different palladacycles: and in certain intramolecular couplings synthetic value was demonstrated regardless of oxidation state:
