Organopalladium

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.
 * 1894 - Phillips reports that palladium(II) chloride reduces to palladium metal by contact with ethylene.
 * 1907 - Autoclave technology introduced by W. Ignatieff makes it possible to carry out high pressure hydrogenation.
 * 1956 - In the Wacker process ethylene and oxygen react to acetaldehyde with catalyst PdCl2/CuCl2
 * 1972 - The Heck reaction  is a coupling reaction of a halogenide with an olefine. The catalyst is Pd(0).
 * 1973 - The Trost asymmetric allylic alkylation is a nucleophilic substitution.
 * 1975 - The Sonogashira coupling is a coupling reaction of terminal alkynes with aryl or vinyl halides.

Overview
In contrast to its next-door neighbors the group 11 elements, the element palladium in organic chemistry does not involve preparation of organopalladium compounds itself but rather organopalladium reactive intermediates. On top of that in many reactions only catalytical amounts of the metal are used.

Pd Alkene complexes
Palladium reacts with alkenes to form a pi complex which can react with a multitude of nucleophiles akin a oxymercuration reaction. The C-Pd bond is then removed by a reduction or an elimination. In the industrially important Wacker process, ethylene is converted to acetaldehyde with palladium chloride.

Pd Allyl complexes
Allyl compunds with suitable leaving groups react with palladium(II) salts to pi-allyl complexes having hapticity 3 such as the Allylpalladium chloride dimer. These intermediates too react with nucleophiles for example carbanions derived from malonates

Allylpalladium intermediates also feature in the Trost asymmetric allylic alkylation and the Carroll rearrangement and an oxo variation in the Saegusa oxidation.

Pd insertion compounds
Zerovalent Pd(0) compounds such as tris(dibenzylideneacetone)dipalladium(0) and tetrakis(triphenylphosphine)palladium(0) 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. Palladium(II) trifluoroacetate has been demonstrated to be effective in aromatic decarboxylation :


 * [[Image:AromaticDecarboxylationByPalladium.png|400px|Aromatic decarboxylation by Palladium(II) trifluoroacetate]]

In the proposed reaction mechanism Pd(II) replaces the carboxylic acid proton while losing a TFA group, carbon dioxide is lost in a first order reaction and TFA destroys the formed Ar-Pd-TFA complex without Pd changing its oxidation state.