Organopalladium

Organopalladium chemistry timeline

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.

Palladium 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. Fullerene ligands also bind with palladium in similar ways as ethylene.

Palladium allyl complexes

Allyl compounds 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.

Palladium-carbon sigma-bonded complexes

Various organic groups can bound to palladium and form stable sigma-bonded complexes. Currently, the alkyl, vinyl, aryl, and alkynyl complexes with Pd-C(sp3), Pd-C(sp2), Pd-C(sp) bonds are equally well-known. 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 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 (see palladium-catalyzed coupling reactions). An example is the Sonogashira reaction:

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) compound was described in 1986. This complex is Me₃Pd(IV)Ibpy with bpy as a bidentate 2,2'-bipyridine ligand It was synthesized by reaction of methyl iodide with Me₂Pd(II)bpy.

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. 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(IV) metallacycle:

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: