Organopalladium chemistry is a branch of
organometallic chemistry
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
that deals with organic
palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
compounds and their reactions. Palladium is often used as a catalyst in the reduction of
alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s and
alkyne
\ce
\ce
Acetylene
\ce
\ce
\ce
Propyne
\ce
\ce
\ce
\ce
1-Butyne
In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...
s with
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
. This process involves the formation of a palladium-carbon
covalent bond
A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
. Palladium is also prominent in carbon-carbon
coupling reaction A coupling reaction in organic chemistry is a general term for a variety of reactions where two fragments are joined together with the aid of a metal catalyst. In one important reaction type, a main group organometallic compound of the type R-M (R = ...
s, as demonstrated in
tandem reaction
A cascade reaction, also known as a domino reaction or tandem reaction, is a chemical process that comprises at least two consecutive reactions such that each subsequent reaction occurs only in virtue of the chemical functionality formed in the p ...
s.
Organopalladium chemistry timeline
* 1873 -
A. N. Zaitsev reports reduction of
benzophenone
Benzophenone is the organic compound with the formula (C6H5)2CO, generally abbreviated Ph2CO. It is a white solid that is soluble in organic solvents. Benzophenone is a widely used building block in organic chemistry, being the parent diarylket ...
over palladium with hydrogen.
* 1894 - Phillips reports that
palladium(II) chloride
Palladium(II) chloride, also known as palladium dichloride and palladous chloride, are the chemical compounds with the formula PdCl2. PdCl2 is a common starting material in palladium chemistry – palladium-based catalysts are of particular value ...
reduces to palladium metal by contact with
ethylene
Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds).
Ethylene i ...
.
* 1907 -
Autoclave
An autoclave is a machine used to carry out industrial and scientific processes requiring elevated temperature and pressure in relation to ambient pressure and/or temperature. Autoclaves are used before surgical procedures to perform sterilizat ...
technology introduced by
Vladimir Ipatieff
Vladimir Nikolayevich Ipatieff (also Ipatyev; russian: Владимир Николаевич Ипатьев); (November 21, 1867 (November 9 OS) – November 29, 1952) was a Russian and American chemist. His most important contributions are in the ...
makes it possible to carry out high pressure hydrogenation.
* 1956 - In the
Wacker process
The Wacker process or the Hoechst-Wacker process (named after the chemical companies of the same name) refers to the oxidation of ethylene to acetaldehyde in the presence of palladium(II) chloride as the catalyst. This chemical reaction was one of ...
ethylene
Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds).
Ethylene i ...
and
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
react to
acetaldehyde
Acetaldehyde (IUPAC systematic name ethanal) is an organic chemical compound with the formula CH3 CHO, sometimes abbreviated by chemists as MeCHO (Me = methyl). It is a colorless liquid or gas, boiling near room temperature. It is one of the mos ...
with catalyst PdCl
2/CuCl
2
* 1957 - Tetrakis(triphenylphosphine)palladium(0) is reported by Malatesta and Angoletta.
* 1972 - The
Heck reaction
The Heck reaction (also called the Mizoroki–Heck reaction) is the chemical reaction of an unsaturated halide (or triflate) with an alkene in the presence of a base and a palladium catalyst (or palladium nanomaterial-based catalyst) to form a s ...
is a
coupling reaction A coupling reaction in organic chemistry is a general term for a variety of reactions where two fragments are joined together with the aid of a metal catalyst. In one important reaction type, a main group organometallic compound of the type R-M (R = ...
of a halogenide with an olefin. Pd(0) intermediates are implicated.
* 1973 - The
Trost asymmetric allylic alkylation
Trost may refer to:
People
* Al Trost, United States soccer midfielder
* Barry Trost, American chemist
* Brad Trost, Canadian Member of Parliament
* Carlisle Trost, United States Navy officer
* Dolfi Trost, Romanian surrealist
* Katharina Trost, G ...
is a
nucleophilic substitution
In chemistry, a nucleophilic substitution is a class of chemical reactions in which an electron-rich chemical species (known as a nucleophile) replaces a functional group within another electron-deficient molecule (known as the electrophile). The ...
.
* 1975 - The
Sonogashira coupling
The Sonogashira reaction is a cross-coupling reaction used in organic synthesis to form carbon–carbon bonds. It employs a palladium catalyst as well as copper co-catalyst to form a carbon–carbon bond between a terminal alkyne and an aryl or v ...
is a coupling reaction of terminal alkynes with aryl or vinyl halides.
* 1994 - The Pd-catalyzed
Buchwald-Hartwig amination for C-N bond-forming reactions.
Palladium(II)
Alkene complexes
Unlike Ni(II), but similar to Pt(II), Pd(II) halides form a variety of alkene complexes. The premier example is
dichloro(1,5‐cyclooctadiene)palladium
Dichloro(1,5-cyclooctadiene)palladium is the organopalladium compound with the formula PdCl2(C8H12) where C8H12 is cycloocta-1,5-diene (cod) or abbreviated PdCl2(cod). It is a yellow solid that is soluble in chloroform. According to X-ray crystal ...
. In this complex, the diene is easily displaced, which makes it a favored precursor to catalysts. In the industrially important
Wacker process
The Wacker process or the Hoechst-Wacker process (named after the chemical companies of the same name) refers to the oxidation of ethylene to acetaldehyde in the presence of palladium(II) chloride as the catalyst. This chemical reaction was one of ...
, ethylene is converted to acetaldehyde via nucleophilic attack of hydroxide on a Pd(II)-ethylene intermediate followed by formation of a vinyl alcohol complex.
Fullerene ligands also bind with palladium(II).
![WackerCycleKeith&HenryImp](https://upload.wikimedia.org/wikipedia/commons/3/3b/WackerCycleKeith%26HenryImp.png)
Palladium(II) 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
Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is t ...
:
Allyl complexes
The iconic complex in this series is
allylpalladium chloride dimer
Allylpalladium(II) chloride dimer (APC) is a chemical compound with the chemical formula, formula hapticity, η3-C3H5)PdCl. This yellow air-stable compound is an important catalyst used in organic synthesis.Tatsuno, Y.; Yoshida, T.; Otsuka, S. ...
(APC).
Allyl
In organic chemistry, an allyl group is a substituent with the structural formula , where R is the rest of the molecule. It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, ...
compounds with suitable
leaving group In chemistry, a leaving group is defined by the IUPAC as an atom or group of atoms that detaches from the main or residual part of a substrate during a reaction or elementary step of a reaction. However, in common usage, the term is often limited t ...
s react with palladium(II) salts to
pi-allyl complexes having
hapticity
In coordination chemistry, hapticity is the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms. The hapticity of a ligand is described with the Greek letter η ('eta'). For example, η2 describes a ...
3. These intermediates too react with nucleophiles for example
carbanion
In organic chemistry, a carbanion is an anion in which carbon is trivalent (forms three bonds) and bears a formal negative charge (in at least one significant resonance form).
Formally, a carbanion is the conjugate base of a carbon acid:
:R3C ...
s derived from
malonate esters or with
amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituen ...
s in allylic amination as depicted below
:
![AllylicAmination](https://upload.wikimedia.org/wikipedia/commons/b/b6/AllylicAmination.svg)
Allylpalladium intermediates also feature in the
Trost asymmetric allylic alkylation
Trost may refer to:
People
* Al Trost, United States soccer midfielder
* Barry Trost, American chemist
* Brad Trost, Canadian Member of Parliament
* Carlisle Trost, United States Navy officer
* Dolfi Trost, Romanian surrealist
* Katharina Trost, G ...
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. 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
Zerovalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules.
Description
The combining capacity, or affinity of an ...
Pd(0) compounds include
tris(dibenzylideneacetone)dipalladium(0)
Tris(dibenzylideneacetone)dipalladium(0) or d2(dba)3is an organopalladium compound. The compound is a complex of palladium(0) with dibenzylideneacetone (dba). It is a dark-purple/brown solid, which is modestly soluble in organic solvents. Becau ...
and
tetrakis(triphenylphosphine)palladium(0)
Tetrakis(triphenylphosphine)palladium(0) (sometimes called quatrotriphenylphosphine palladium) is the chemical compound d(P(C6H5)3)4 often abbreviated Pd( PPh3)4, or rarely PdP4. It is a bright yellow crystalline solid that becomes brown upon de ...
. These complexes react with
halocarbon
Halocarbon compounds are chemicals in which one or more carbon atoms are linked by covalent bonds with one or more halogen atoms (fluorine, chlorine, bromine or iodine – ) resulting in the formation of organofluorine compounds, organochlori ...
R-X in
oxidative addition
Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. Oxidat ...
to R-Pd-X intermediates with
covalent
A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
Pd-C bonds. This chemistry forms the basis of a large class of organic reactions called
coupling reaction A coupling reaction in organic chemistry is a general term for a variety of reactions where two fragments are joined together with the aid of a metal catalyst. In one important reaction type, a main group organometallic compound of the type R-M (R = ...
s (see
palladium-catalyzed coupling reactions
In organic chemistry, a cross-coupling reaction is a reaction where two fragments are joined together with the aid of a metal catalyst. In one important reaction type, a main group organometallic compound of the type R-M (R = organic fragment, M = ...
). An example is the
Sonogashira reaction
The Sonogashira reaction is a cross-coupling reaction used in organic synthesis to form carbon–carbon bonds. It employs a palladium catalyst as well as copper co-catalyst to form a carbon–carbon bond between a terminal alkyne and an aryl or vin ...
:
:
Organopalladium(IV)
The first organopalladium(IV) compound was described in 1986. This complex is Me
3Pd(IV)(I)bpy (bpy = bidentate
2,2'-bipyridine
The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe or single closing quotation mark () in many typefaces, but it differs from them in being placed on the baseline o ...
ligand) It was synthesized by oxidative addition of
methyl iodide
Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one h ...
to Me
2Pd(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
The Heck reaction (also called the Mizoroki–Heck reaction) is the chemical reaction of an unsaturated halide (or triflate) with an alkene in the presence of a base and a palladium catalyst (or palladium nanomaterial-based catalyst) to form a s ...
. This reaction was accompanied by a 1,5-hydrogen shift in the presence of amines:
:
![HeckReactionWang2000](https://upload.wikimedia.org/wikipedia/commons/1/14/HeckReactionWang2000.svg)
The hydride shift was envisaged as taking place through a Pd(IV)
metallacycle
In organometallic chemistry, a metallacycle is a derivative of a carbocyclic compound wherein a metal has replaced at least one carbon center; this is to some extent similar to heterocycles. Metallacycles appear frequently as reactive intermediates ...
:
:
![HeckReactionWang2000Mechanism](https://upload.wikimedia.org/wikipedia/commons/9/92/HeckReactionWang2000Mechanism.svg)
In related work the intermediate associated with the hydride shift remains Pd(II):
:
![OrganopalladiumShiftKarig2002](https://upload.wikimedia.org/wikipedia/commons/c/c8/OrganopalladiumShiftKarig2002.svg)
and in other work (a novel synthesis of
indole
Indole is an aromatic heterocyclic organic compound with the formula C8 H7 N. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered pyrrole ring. Indole is widely distributed in the natural environmen ...
s with two Pd migrations)
equilibria are postulated between different palladacycles:
:
![CPd shift Larock 2004 rev](https://upload.wikimedia.org/wikipedia/commons/b/b3/CPd_shift_Larock_2004_rev.svg)
and in certain intramolecular couplings synthetic value was demonstrated regardless of oxidation state:
[''Pd-Catalyzed Alkyl to Aryl Migration and Cyclization: An Efficient Synthesis of Fused Polycycles via Multiple C-H Activation'' Qinhua Huang, Alessia Fazio, Guangxiu Dai, Marino A. Campo, and Richard C. Larock '']J. Am. Chem. Soc.
The ''Journal of the American Chemical Society'' is a weekly peer-reviewed scientific journal that was established in 1879 by the American Chemical Society. The journal has absorbed two other publications in its history, the ''Journal of Analytic ...
'' 2004, 126, 7460-7461
:
References
{{ChemicalBondsToCarbon