Organogold chemistry
Encyclopedia
Organogold chemistry is the study of compounds containing carbon
-gold
bonds. They are studied in academic research, but have not received widespread use otherwise. The dominant oxidation state
s for organogold compounds are I with coordination number
2 and a linear molecular geometry
and III with CN = 2 and a planar molecular geometry. The gold carbide Au2C2 was discovered in 1900.
) whereby the cation is usually fitted with a complexing agent to improve stability. The AuR2- anion is also linear just as other M(d10) species such as Hg(Me)2 and Pd(Me)22+. Gold is known to form acetylides (capable of forming polymeric structures), carbene
s and carbyne
s. The classic method for the preparation of LAuR compounds is by reaction of a Grignard reagent with a gold(I) halide. A subsequent reaction with a organolithium R-Li forms the ate complex.
In a special group of compounds, an aryl
carbon atom acts as a bridge between two gold atoms. One such compound, MesAu5,is formed in a reaction between Au(CO)Cl and the mesityl Grigard. Carbon can be coordinated with gold up to a value to 6. Compounds of the type C(AuL)4 are isolobal
with methane and those of type C(AuL)5+ isolobal with the methanium ion.
Gold cyanide compounds (MAu(CN)2) are of some importance to gold cyanidation
, a process for the extraction of gold from low-grade ore. The carbon to metal bond in metal cyanides is usually ionic but evidence exists that the C-Au bonding in the gold cyanide ion is covalent.
including catalysts by gold nanoparticles and thiol-monolayer gold surfaces, and homogeneous catalysis
that occurs with gold(I) or gold(III) compounds and is used for organic synthesis. Common commercially available gold catalysts are gold(I) chloride, gold(III) chloride
, chloroauric acid
and a range of gold phosphines such as chloro(triphenylphosphine)gold(I) . To increase the electrophilicity at the gold center, a more dissociated complex can be created through halide abstraction with an additive like silver triflate (AgOTf).
Gold coordinates preferentially to alkene
or alkyne
bonds, and bonds following the Dewar-Chatt-Duncanson model
. Gold is certainly not the only metal showing this type of bonding and reactivity, several metal ions isolobal with the simple hydrogen proton (cationic: emptied s-orbital) do as well: mercury(II) and platinum(II). In 2007 Furstner & Davies proposed the term pi-acid as a cognomen
for this type of ion (See also: cation-pi interaction
).
The resulting metal-multiple bond complex is electrophilic in nature and activated for nucleophilic attack. In oxymercuration the resultant organomercurial species is generated stoichiometrically, and requires an additional step to liberate the product. Facile protonolysis of the Au-C bond enables true catalysis. Other advantages of gold(I) catalysis include: air stability (high oxidation potential of Au(I) to Au(III) is a barrier to oxidation), lowered oxophilicity and thus tolerance towards water and alcohols, C-Au bonds that prefer protodeauration over beta-hydride elimination, and relative non-toxicity.
An early example of catalysis by gold was described by Ito in 1986 who reacted benzaldehyde
and methyl isocyanoacetate with a chiral
ferrocenylphosphine ligand and cationic gold(I) compound (Au(cHexCN)2)BF4 to form a chiral oxazoline
. This reaction was a first in more than one way: asymmetric Aldol reactions already existed but this was also the first catalytic one.
In a simple mechanistic picture, gold(I) simultaneously coordinates to two phosphine ligands and the carbon isocyanate group which is then attacked by the carbonyl group. Further studies on the bonding mode of Au(I) indicate that this simple picture may have to be revised.
In 1976, Thomas and coworkers reported conversion of phenylacetylene
to acetophenone
using tetrachloroauric acid in a 37% yield. In this reaction gold(III) was used as a homogeneous catalyst replacing mercury in oxymercuration. Interestingly, this same study lists a published yield >150%, indicating catalysis that perhaps was not acknowledged by the chemists.
Utimoto reacted gold(III)(NaAuCl4) with alkynes and water. Teles identified a major drawback of this method as Au(III) was rapidly reduced to catalytically dead metallic gold and returned to the theme of ligand supported Au(I) for the same transformation :
Another example of an Au(III) catalysed reaction (Hashmi et al. 2000) is in a forced alkyne / furan
Diels-Alder reaction
- these do not normally occur - ultimately forming a phenol
:
Further mechanistic studies conclude that this s not a concerted transformation, but rather an initial alkyne hydroarylation followed by a series of non-obvious intramolecular rearrangements, concluded with a 6-pi electrocyclization and rearomatization.
Heterogeneous gold catalysis is an older science. Gold is an attractive metal to use because of its stability against oxidation and its variety in morphology for instance gold cluster
materials. Gold has been shown to be effective in low-temperature CO oxidation and acetylene hydrochlorination to vinyl chlorides. The exact nature of the catalytic site in this type of process is debated. The notion that gold can catalyse a reaction does not imply it is the only way. However, other metals can do the same job inexpensively, notably in recent years iron (see organoiron chemistry
).
Other reactions are the use of gold in C-H bond activation
and Itoh Aldol reactions (1990's). Gold also catalyses coupling reaction
s .
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
-gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
bonds. They are studied in academic research, but have not received widespread use otherwise. The dominant oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
s for organogold compounds are I with coordination number
Coordination number
In chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....
2 and a linear molecular geometry
Linear molecular geometry
In chemistry, the Linear molecular geometry describes the arrangement of three or more atoms placed at an expected bond angle of 180º. Linear organic molecules, e.g. acetylene, are often described by invoking sp orbital hybridization for the carbon centers. Many linear molecules exist, prominent...
and III with CN = 2 and a planar molecular geometry. The gold carbide Au2C2 was discovered in 1900.
Gold(I)
Gold(I) complexes are 2-coordinate, linear, diamagnetic, 14 electron species. They exist as adducts LAuR with as ligand L for instance a triphenylphosphine or an isocyanide. The ligand prevents reduction of Au(I) to metallic Au(0) with dimerization of the organic residue. Gold(I) can also exist as the aurate MAuR2 (the ate complexAte complex
An ate complex in chemistry is a salt formed by reaction of a Lewis acid with a base whereby the central atom increases its valence . Often in chemical nomenclature the phrase ate is suffixed to the element in question. For example, the ate complex of a boron compound is called a borate...
) whereby the cation is usually fitted with a complexing agent to improve stability. The AuR2- anion is also linear just as other M(d10) species such as Hg(Me)2 and Pd(Me)22+. Gold is known to form acetylides (capable of forming polymeric structures), carbene
Carbene
In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is RR'C:, but the carbon can instead be double-bonded to one group. The term "carbene" may also merely refer to the compound H2C:, also called...
s and carbyne
Carbyne
In chemistry, a carbyne is a monovalent carbon radical species containing an electrically neutral univalent carbon atom with three non-bonded electrons.- Gas phase/reactive intermediate :...
s. The classic method for the preparation of LAuR compounds is by reaction of a Grignard reagent with a gold(I) halide. A subsequent reaction with a organolithium R-Li forms the ate complex.
In a special group of compounds, an aryl
Aryl
In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....
carbon atom acts as a bridge between two gold atoms. One such compound, MesAu5,is formed in a reaction between Au(CO)Cl and the mesityl Grigard. Carbon can be coordinated with gold up to a value to 6. Compounds of the type C(AuL)4 are isolobal
Isolobal principle
The isolobal principle is a strategy used in organometallic chemistry to relate the structure of organic and inorganic molecular fragments in order to predict bonding properties of organometallic compounds...
with methane and those of type C(AuL)5+ isolobal with the methanium ion.
Gold cyanide compounds (MAu(CN)2) are of some importance to gold cyanidation
Gold cyanidation
Gold cyanidation is a metallurgical technique for extracting gold from low-grade ore by converting the gold to a water soluble coordination complex. It is the most commonly used process for gold extraction...
, a process for the extraction of gold from low-grade ore. The carbon to metal bond in metal cyanides is usually ionic but evidence exists that the C-Au bonding in the gold cyanide ion is covalent.
Gold(III)
Gold(III) complexes are 4 coordinate, square planar, diamagnetic, toxic, 16 electron species. When the formal coordination number is less than 4, ligands such as chlorine can make up for it by forming a bridging ligand. Intramolecular chelation is another strategy. In general gold(III) compounds are toxic and therefore less studied than gold(I). For all practical purposes the chemistry is confined to monoarylgold(III) complexes.Gold catalysis
Gold-catalyzed reactions fall into two major categories: heterogeneous catalysisHeterogeneous catalysis
In chemistry, heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid, vs gas, but also immiscible liquids, e.g. oil and water. The great majority of practical heterogeneous catalysts...
including catalysts by gold nanoparticles and thiol-monolayer gold surfaces, and homogeneous catalysis
Homogeneous catalysis
In chemistry, homogeneous catalysis is a sequence of reactions that involve a catalyst in the same phase as the reactants. Most commonly, a homogeneous catalyst is codissolved in a solvent with the reactants.-Acid catalysis:...
that occurs with gold(I) or gold(III) compounds and is used for organic synthesis. Common commercially available gold catalysts are gold(I) chloride, gold(III) chloride
Gold(III) chloride
Gold chloride, traditionally called auric chloride, is a chemical compound of gold and chlorine. With the molecular formula Au2Cl6, the name gold trichloride is a simplification, referring to the empirical formula. The Roman numerals in the name indicate that the gold has an oxidation state of +3,...
, chloroauric acid
Chloroauric acid
Chloroauric acid is a inorganic compound with the formula HAuCl4. This pale yellow compound is a common precursor to gold in a variety of purposes. The term chloroauric acid is also sometimes used to describe other gold chlorides....
and a range of gold phosphines such as chloro(triphenylphosphine)gold(I) . To increase the electrophilicity at the gold center, a more dissociated complex can be created through halide abstraction with an additive like silver triflate (AgOTf).
Gold coordinates preferentially to alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
or alkyne
Alkyne
Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature...
bonds, and bonds following the Dewar-Chatt-Duncanson model
Dewar-Chatt-Duncanson model
The Dewar–Chatt–Duncanson model is a model in organometallic chemistry which explains the type of chemical bonding between an alkene and a metal in certain organometallic compounds. The model is named after Michael J. S. Dewar, Joseph Chatt and L. A...
. Gold is certainly not the only metal showing this type of bonding and reactivity, several metal ions isolobal with the simple hydrogen proton (cationic: emptied s-orbital) do as well: mercury(II) and platinum(II). In 2007 Furstner & Davies proposed the term pi-acid as a cognomen
Cognomen
The cognomen nōmen "name") was the third name of a citizen of Ancient Rome, under Roman naming conventions. The cognomen started as a nickname, but lost that purpose when it became hereditary. Hereditary cognomina were used to augment the second name in order to identify a particular branch within...
for this type of ion (See also: cation-pi interaction
Cation-pi interaction
Cation–π interaction, also known as the Dougherty effect, is a noncovalent molecular interaction between the face of an electron-rich π system with an adjacent cation . This unusual interaction is an example of noncovalent bonding between a monopole and a quadrupole...
).
The resulting metal-multiple bond complex is electrophilic in nature and activated for nucleophilic attack. In oxymercuration the resultant organomercurial species is generated stoichiometrically, and requires an additional step to liberate the product. Facile protonolysis of the Au-C bond enables true catalysis. Other advantages of gold(I) catalysis include: air stability (high oxidation potential of Au(I) to Au(III) is a barrier to oxidation), lowered oxophilicity and thus tolerance towards water and alcohols, C-Au bonds that prefer protodeauration over beta-hydride elimination, and relative non-toxicity.
An early example of catalysis by gold was described by Ito in 1986 who reacted benzaldehyde
Benzaldehyde
Benzaldehyde is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. This colorless liquid has a characteristic pleasant almond-like odor...
and methyl isocyanoacetate with a chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
ferrocenylphosphine ligand and cationic gold(I) compound (Au(cHexCN)2)BF4 to form a chiral oxazoline
Oxazoline
Oxazoline is both the five-membered ring heterocyclic chemical compound with the formula C3H5NO and the class of compounds containing this ring.- See also :* Desoxazoline * Oxazole* Oxazolidine* Oxazolidinedione...
. This reaction was a first in more than one way: asymmetric Aldol reactions already existed but this was also the first catalytic one.
In a simple mechanistic picture, gold(I) simultaneously coordinates to two phosphine ligands and the carbon isocyanate group which is then attacked by the carbonyl group. Further studies on the bonding mode of Au(I) indicate that this simple picture may have to be revised.
In 1976, Thomas and coworkers reported conversion of phenylacetylene
Phenylacetylene
Phenylacetylene is an alkyne hydrocarbon containing a phenyl group. It exists as a colorless, viscous liquid. In research, it is sometimes used as an analog for acetylene; being a liquid, it is easier to handle than acetylene gas.-Preparation:...
to acetophenone
Acetophenone
Acetophenone is the organic compound with the formula C6H5CCH3. It is the simplest aromatic ketone. This colourless, viscous liquid is a precursor to useful resins and fragrances.-Production:Acetophenone can be obtained by a variety of methods...
using tetrachloroauric acid in a 37% yield. In this reaction gold(III) was used as a homogeneous catalyst replacing mercury in oxymercuration. Interestingly, this same study lists a published yield >150%, indicating catalysis that perhaps was not acknowledged by the chemists.
Utimoto reacted gold(III)(NaAuCl4) with alkynes and water. Teles identified a major drawback of this method as Au(III) was rapidly reduced to catalytically dead metallic gold and returned to the theme of ligand supported Au(I) for the same transformation :
Another example of an Au(III) catalysed reaction (Hashmi et al. 2000) is in a forced alkyne / furan
Furan
Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen. The class of compounds containing such rings are also referred to as furans....
Diels-Alder reaction
Diels-Alder reaction
The Diels–Alder reaction is an organic chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon...
- these do not normally occur - ultimately forming a phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
:
Further mechanistic studies conclude that this s not a concerted transformation, but rather an initial alkyne hydroarylation followed by a series of non-obvious intramolecular rearrangements, concluded with a 6-pi electrocyclization and rearomatization.
Heterogeneous gold catalysis is an older science. Gold is an attractive metal to use because of its stability against oxidation and its variety in morphology for instance gold cluster
Gold cluster
Gold clusters in cluster chemistry are gold-derived materials that can either be discrete molecules or larger colloidal particles. Both types are describes as nanoparticles, with diameters of less than one micrometer.-Discrete gold clusters:...
materials. Gold has been shown to be effective in low-temperature CO oxidation and acetylene hydrochlorination to vinyl chlorides. The exact nature of the catalytic site in this type of process is debated. The notion that gold can catalyse a reaction does not imply it is the only way. However, other metals can do the same job inexpensively, notably in recent years iron (see organoiron chemistry
Organoiron chemistry
Organoiron chemistry is the chemistry of organometallic compounds containing a carbon-to-iron chemical bond. Organoiron compounds are relevant in organic synthesis as reagents such as iron pentacarbonyl, diiron nonacarbonyl and disodium tetracarbonylferrate. Iron adopts oxidation states from Fe...
).
Gold catalyzed reactions
Gold catalyzes many organic transformations, usually carbon-carbon bond formation from Au(I), and C-X (X = O, N) bond formation from the Au(III) state, due to that ion's harder Lewis acidity. Hong C. Shen summarized homogeneous reactions forming cyclic compounds into 4 main categories :- heteroatom nucleophilic additionNucleophilic additionIn organic chemistry, a nucleophilic addition reaction is an addition reaction where in a chemical compound a π bond is removed by the creation of two new covalent bonds by the addition of a nucleophile....
to unsaturated C-C bonds, especially to form small heterocycles (furans, pyrroles, thiophenes) - Hydroarylation: basically a Friedel-Crafts reactionFriedel-Crafts reactionThe Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877. There are two main types of Friedel–Crafts reactions: alkylation reactions and acylation reactions. This reaction type is a form of electrophilic aromatic substitution...
using metal-alkyne complexes. Example, the reaction of mesityleneMesityleneMesitylene or 1,3,5-trimethylbenzene is an aromatic hydrocarbon with three methyl substituents attached to the benzene ring. It is prepared by distillation of acetone with sulfuric acid or by trimerization of propyne in sulfuric acid, which, in both cases, acts as a catalyst and dehydrating agent....
with phenylacetylenePhenylacetylenePhenylacetylene is an alkyne hydrocarbon containing a phenyl group. It exists as a colorless, viscous liquid. In research, it is sometimes used as an analog for acetylene; being a liquid, it is easier to handle than acetylene gas.-Preparation:...
:
- Enyne cyclization, in particular cycloisomerization, one early example being an 5-exo-dig 1,6 enyne cycloisomerization:
- The soft gold(I) ion coordinates exclusively to the more pi-basic alkyne, leaving the alkene free to attack as the nucleophile.
- cycloaddition reactions with early example the cycloaddition of an nitrile oxide with an alkyne.
Other reactions are the use of gold in C-H bond activation
C-H bond activation
Carbon–hydrogen bond activation or C−H activation may be defined as a reaction that cleaves a carbon–hydrogen bond. Often the term is restricted to reactions involving organometallic complexes and proceeding by coordination of a hydrocarbon to the inner-sphere of metal, either via an...
and Itoh Aldol reactions (1990's). Gold also catalyses coupling reaction
Coupling reaction
A coupling reaction in organic chemistry is a catch-all term for a variety of reactions where two hydrocarbon fragments are coupled with the aid of a metal catalyst...
s .
See also
- Other chemistries of carbon with other elements in the periodic table.