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 or with amines in allylic amination as depicted below

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

One 2007 study reports an allylic C-H Amination also involving a Pd allyl complex but without a leaving group

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:

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.

Organopalladium(IV)

The first organopalladium(IV) complex, described in 1986 by reaction of methyl iodide with Me₂Pd(II)bpy (Me = methyl) is the complex Me₂Pd(IV)Ibpy with bpy a bidentate 2,2'-bipyridine ligand

Palladium compounds owe their reactivity to the ease of interconversion between Pd(0) and palladium(II) intermediates. There is no conclusive evidence however for the involvement of Pd(II) to Pd(IV) conversions in palladium mediated organometallic reactions . One reaction invoking such mechanism was described in 2000 and concerned a Heck reaction that was accompanied by a 1,5-hydrogen shift in the presence of amines :

The hydride shift was envisaged as taking place through a Pd(IV) metallocycle:

In related work the intermediate associated with the hydride shift remains Pd(II) :

and in other work (a novel synthesis of indoles with two Pd migrations) equilibria are postulated between different palladacycles :

and in certain intramolecular couplings synthetic value was demonstrated regardless of oxidation state :

See also

• Compounds of carbon with other elements in the periodic table:

References