A stable carbene: isolated 1,3-dimesitylimidazol-2-ylidene in a Schlenk flask

A persistent carbene (also known as a stable carbene) is a type of carbene demonstrating particular stability despite also being a reactive intermediate. A Schlenk flask, or Schlenk tube is a reaction vessel typically used in Air sensitive chemistry invented by Wilhelm Schlenk. In Chemistry, a carbene is a highly reactive Organic molecule containing a Carbon atom with six valence electrons and having the general formula A reaction intermediate or an intermediate is a Molecular entity that is formed from the reactants (or preceding intermediates and reacts further to give the directly The instability in these carbenes involves reactivity with substrates, or dimerisation (see Wanzlick equilibrium). A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces A persistent carbene (also known as a stable carbene or an Arduengo carbene is a type of Carbene demonstrating particular stability despite also being a Reactive Persistent carbenes can exist in the singlet state or the triplet state, with the singlet state carbenes being more stable. A diradical in Organic chemistry is a molecular species with two Electrons occupying two Degenerate Molecular orbitals (MO. In Physics, '''spin''' is the Angular momentum intrinsic to a body as opposed to Orbital angular momentum, which is the motion of its Center of mass

The field of stable carbene research was awakened in 1991 with a landmark discovery by the research group of Anthony J. Arduengo, III. Anthony J Arduengo III, PhD (born in 1952 is a Chemist perhaps most famous for his work in the field of Stable carbene research ^[1], which managed to isolate and obtain an X-ray structure of the stable carbene N,N'-diadamantyl-imidazol-2-ylidene:

Preparation of N,N'-diadamantyl-imidazol-2-ylidene

However, prior to this isolation, persistent carbenes had been proposed to exist by Ronald Breslow in 1957. Ronald C D Breslow (born 14 March 1931, Rahway, New Jersey) is an American Chemist. ^[2][3] The Hans-Werner Wanzlick group ^[4][5] were the first group to make (but not isolate) a stable carbene. In 1989 Guy Bertrand's group ^[6][7] were the first to make and isolate a stable carbene.

Typically, normal carbenes are very reactive short lived molecules that cannot be isolated, and are usually studied by observing the reactions they undergo. In Chemistry, a carbene is a highly reactive Organic molecule containing a Carbon atom with six valence electrons and having the general formula In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by However, persistent carbenes are much more stable and considerably longer lived. This means that in many cases these carbenes are thermodynamically stable in the absence of moisture and (in most cases) oxygen, and can be isolated and indefinitely stored. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Some persistent carbenes are not thermodynamically stable and dimerise slowly over days. A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces The less stable triplet state carbenes have half-lives measured in seconds, and cannot be stored but merely observed. In Physics, '''spin''' is the Angular momentum intrinsic to a body as opposed to Orbital angular momentum, which is the motion of its Center of mass Half-Life (computer-game page here It's already listed in the disambiguation page

Classes of stable carbenes

The following are examples of the classes of stable carbenes isolated to date:

Imidazol-2-ylidenes

Imidazol-2-ylidenes were the first (and the most stable) family of stable carbenes isolated, and hence are the most well studied and understood. A considerable range of imidazol-2-ylidenes have been synthesised, including those in which the 1,3-positions have been functionalised with alkyl, aryl,^[8] alkyloxy, alkylamino, alkylphosphino^[9] and even chiral substituents:^[9]

Stable imidazol-2-ylidenes. An alkyl is a Univalent radical consisting of Carbon and Hydrogen atoms arranged in a chain In the context of organic molecules aryl refers to any Functional group or Substituent derived from a Simple aromatic ring, may it be Phenyl The term chiral (pronounced /ˈkaɪɹ(əl̩/ is used to describe an object that is non- superimposable on its mirror image a) Low solubility b) No value given.

Arguably one of the more interesting functionalisations occurred with the 4,5-dichlorination of the imidazole moiety, resulting in an air-stable carbene. Imidazole is a Organic compound with the formula HC3H3N2 This Aromatic Heterocyclic is classified as an Alkaloid ^[10] Molecules containing two and even three imidazol-2-ylidenes have also been synthesised. ^[11][12]

Imidazol-2-ylidenes have been prepared by the deprotonation of imidazolium salts, and by the desulfurisation of thioureas with molten potassium. Imidazole is a Organic compound with the formula HC3H3N2 This Aromatic Heterocyclic is classified as an Alkaloid Desulfurisation is a chemical reaction involving the removal of Sulfur from a molecule ( e Thiourea is an Organic compound of Carbon, Nitrogen, Sulfur and Hydrogen, with the formula C[[Sulfur S]] Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 Imidazole-based carbenes are thermodynamically stable and generally have diagnostic ¹³C NMR chemical shift values between 210-230 ppm for the carbenic carbon. Typically, X-ray structures of these molecules show N-C-N bond angles of ca. 101-102°.

Triazol-5-ylidenes

The triazol-5-ylidenes pictured below were first prepared by Enders and co-workers^[13] by vacuum pyrolysis through loss of methanol from 2-methoxytriazoles. Pyrolysis is the Chemical decomposition of organic materials by heating in the absence of Oxygen or any other reagents except possibly Steam Only a limited range of these molecules have been reported, with the triphenyl substituted molecule being commercially available.

Triazol-5-ylidenes

Triazole-based carbenes are thermodynamically stable and have diagnostic ¹³C NMR chemical shift values between 210-220 ppm for the carbenic carbon. Triazole (Htrz refers to either one of a pair of Isomeric Chemical compounds with Molecular formula C2H3N3 having a five-membered The X-ray structure of the triphenyl substituted carbene above shows an N-C-N bond angle of ca. 101°. The 5-methoxytriazole precursor to this carbene was made by the treatment of a triazolium salt with sodium methoxide, which attacks as a nucleophile. In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to ^[13] This may indicate that these carbenes are less aromatic than imidazol-2-ylidenes, as the imidazolium precursors do not react with nucleophiles due to the resultant loss of aromaticity.

Cyclic and acyclic diaminocarbenes

A range of cyclic diaminocarbenes have been prepared principally by the Alder group in which the N-C-N unit is a member of a 5 or 6 membered ring,^[14][15][16] including a bicyclic example. The Alder group have prepared a range of acyclic diaminocarbenes. ^[17][18][19]

Synthesised cyclic and acyclic diaminocarbenes

Unlike the aromatic imidazol-2-ylidenes or triazol-5-ylidenes these carbenes appear not to be thermodynamically stable, as shown by the dimerisation of some unhindered cyclic and acyclic examples. See also Intramolecular forces ' Steric effects arise from the fact that each Atom within a Molecule occupies a certain ^[15][18] However more recent work by Alder^[20] suggests that these carbenes dimerise via acid catalysed dimerisation (see Wanzlick equilibrium). A persistent carbene (also known as a stable carbene or an Arduengo carbene is a type of Carbene demonstrating particular stability despite also being a Reactive Dihydroimidazole carbenes were prepared via the desulfurisation of thioureas with molten potassium^[15] deprotonation of the respective dihydroimidazolium salts. Deprotonation is a Chemistry term that refers to the removal of a Proton ( Hydrogen Cation H+ from a Molecule, forming The acyclic carbenes^[17][18] and the tetrahydropyrimidinyl^[16] based carbenes were prepared by deprotonation using strong homogeneous bases. Diaminocarbenes have diagnostic ¹³C NMR chemical shift values between 230-270 ppm for the carbenic carbon. The X-ray structure of dihydroimidazole carbene shows a N-C-N bond angle of ca. 106°, whilst the angle of the acyclic carbene is 121°, both greater than those seen for imidazol-2-ylidenes.

Heteroamino carbenes

Stable nucleophilic carbenes in which one of the nitrogen atoms adjacent to the carbene center has been replaced by an alternative heteroatom (e. In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to g. O, S or P)^{[21][22][6][7]} have been prepared, as well as a species in which both nitrogens have been replaced by phosphorus atoms. Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Phosphorus, (ˈfɒsfərəs is the Chemical element that has the symbol P and Atomic number 15 ^[23] However, these phosphorus substituted “carbenes” seem to exhibit some alkynic properties, and when published the exact carbenic nature of these red oils was in debate. ^[7]

Synthesised heteroamino carbenes

An aromatic heteroamino thiazole based carbene (analogous to the carbene postulated by Breslow) (R. Thiazole, or 13-thiazole, is a clear to pale yellow flammable liquid with a Pyridine -like odor and the molecular formula C3H3NS Breslow, J. Am. Chem. Soc. , 1957, 79, 1762. ) 5 has been prepared and characterised by X-ray crystallography. ^[21] Other formally aromatic α-heteroatom substituted carbenes have perhaps not been synthesised as they have the potential to dissociate into alkynes (i. Alkynes are Hydrocarbons that have at least one Triple bond between two Carbon atoms with the formula CnH2n-2. e. R1CCR2 as well as X=C=X,). The reaction of carbon disulfide with electron deficient acetylenes gives transient 1,3-dithiolium carbenes (i. e. where X = S) which then dimerise. Thus it is possible that the reverse of this process might be occurring in similar carbenes. ^[24]

A possible decomposition pathway for aromatic N-C-X (X = O, S) substituted carbenes

Acyclic non-aromatic carbenes with O, S and P atoms adjacent (i. e. alpha) to the carbene centre have been prepared, e. g. thio- and oxy-iminium based carbenes have been characterised by X-ray crystallography. ^[22]

Since oxygen and sulfur are divalent, steric protection of the carbenic centre is limited especially when the N-C-X unit is part of a ring. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the In chemistry divalent Anions are atoms or radicals with 2 additional Electrons when compared to their elemental state (that is with 2 more electrons than Protons See also Intramolecular forces ' Steric effects arise from the fact that each Atom within a Molecule occupies a certain These acyclic carbenes have diagnostic ¹³C NMR chemical shift values between 250-300 ppm for the carbenic carbon, further downfield than any other types of stable carbene. X-ray structures have show N-C-X bond angles of ca. 104 ° and 109 ° respectively.

Other nucleophilic carbenes

One stable N-heterocyclic carbene^[25] has a structure analogous to borazine with one boron atom replaced by methylene. Borazine is an Inorganic compound composed of the elements Boron, Nitrogen and Hydrogen. Boron (ˈbɔərɒn is a Chemical element with Atomic number 5 and the chemical symbol B. This results in a planar 6 electron compound.

In the second step of this reaction sequence the proton is abstracted by LiTMP, two cyclohexyl groups shield the carbene. Lithium tetramethylpiperidide or LiTMP is an organic base and a Non-nucleophilic base. Cyclohexane is a Cycloalkane with the Molecular formula C 6 H 12

Triplet state carbenes

In 2001, Hideo Tomioka and his associates were able to produce a comparatively stable triplet carbene, taking advantage of resonance. Resonance in Chemistry is a theory used to represent and model certain types of non-classical Molecular structures Resonance is a key component Triplet bis(9-anthryl)carbene has a half-life of 19 minutes. ^[26][27]

In 2006 the same group reported a triplet carbene with a half-life of 40 minutes. Half-Life (computer-game page here It's already listed in the disambiguation page ^[28] This carbene is prepared by a photochemical decomposition of a diazomethane with expulsion of nitrogen gas at a wavelength of 300 nanometers in benzene. Photochemistry, a sub-discipline of Chemistry, is the study of the interactions between Atoms, small Molecules, and light (or Electromagnetic radiation Chemical decomposition or analysis is the separation of a Chemical compound into elements or smaller compounds Diazomethane is the chemical compound CH2N2 In the pure form at room temperature it is a Yellow Gas, but it is almost universally used Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a As with the other carbenes this species contains large bulky substituents, in this molecule bromine and the trifluoromethyl groups, that shield the carbene and prevent or slow down the process of dimerisation to a 1,1,2,2-tetra(phenyl)alkene. In silico experiments show that the distance of the divalent carbon atom to its neighbours is 138 picometers with a bond angle of 158. In Silico is the second full-length studio album by Australian Drum and bass band Pendulum, released in Australia and In Molecular geometry, bond length or bond distance is the average distance between nuclei of two bonded Atoms in a Molecule. A picometre ( American spelling: picometer, symbol pm) is a unit of Length in the Metric system, equal to one trillionth Molecular geometry or molecular structure is the three- Dimensional arrangement of the Atoms that constitute a Molecule. 8°. The dihedral angle is 85. In Aerospace engineering, the Dihedral is the Angle between the two wings see Dihedral. 7° which makes the phenyl groups almost at right angles to each other. Exposure to oxygen (diradical) converts the carbene to the corresponding benzophenone and the diphenylmethane compound is formed when it is trapped by 1,4-cyclohexadiene. Benzophenone is the Organic compound with the formula ( C 6 H 52C O, generally abbreviated Ph 2CO

History of stable carbenes

1957: Breslow proposed that a thiazol-2-ylidene was involved in the catalytic cycle of vitamin B1. A catalytic cycle in Chemistry is a term for a multistep reaction mechanism that involves a Catalyst. For the similarly spelled pyrimidine see Thymine Thiamin or thiamine, also known as Vitamin B1 ^[2] This was the first example of a stable nucleophilic carbene being implicated in a reaction mechanism. In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to Chemistry, a reaction mechanism is the step by step Sequence of Elementary reactions by which overall Chemical change occurs. In the catalytic cycle shown below two molecules of furfural react to give furoin, via a thiazol-2-ylidene catalyst, generated in situ by C2-deprotonation of a thiazolium salt moiety:

Furoin formation from furfural, catalysed by thiamine

Evidence that the thiazol-2-ylidene was a stable intermediate in the above catalytic cycle was suggested by a deuterium exchange experiment. A catalytic cycle in Chemistry is a term for a multistep reaction mechanism that involves a Catalyst. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by The Chemical compound furfural is an industrial chemical derived from a variety of agricultural byproducts including corncobs, Oat and Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst Deprotonation is a Chemistry term that refers to the removal of a Proton ( Hydrogen Cation H+ from a Molecule, forming Thiazole, or 13-thiazole, is a clear to pale yellow flammable liquid with a Pyridine -like odor and the molecular formula C3H3NS Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth Breslow demonstrated that under standard reaction conditions (in deuterated water) the thiazolium C2-proton was rapidly exchanged for a deuteron in a statistical equilibrium. Heavy water is water which contains a higher proportion than normal of the Isotope Deuterium, as deuterium oxide, D2O or ²H2O In a Chemical process, chemical equilibrium is the state in which the chemical activities or Concentrations of the reactants and products have no net change ^[3]

Deuterium exchange of the C2-proton of thiazolium salt

This confirmed that the C2-proton of the salt was labile, and was proposed to be exchanged as a result of the generation of a stable thiazol-2-ylidene intermediate.

Wanzlick equilibrium

1960: Wanzlick et al. proposed that dihydroimidazol-2-ylidenes were generated from 2-trichloromethyl dihydroimidazoles, with the loss of chloroform by vacuum pyrolysis. Imidazoline is a Nitrogen -containing heterocycle derived from Imidazole. Chloroform, also known as trichloromethane and methyl trichloride, is a Chemical compound with formula C[[Hydrogen H]] Cl Pyrolysis is the Chemical decomposition of organic materials by heating in the absence of Oxygen or any other reagents except possibly Steam ^[4][5]

Wanzlick's mechanism for the reaction of dihydroimidazol-2-ylidene with electrophiles

Wanzlick et al. believed that once prepared these carbenes existed in an unfavourable equilibrium with its corresponding dimer. A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces This assertion was based on reactivity studies which they believed showed that the free carbene reacted with electrophiles (E-X). In Chemistry, an electrophile (literally electron-lover) is a Reagent attracted to Electrons that participates in a chemical reaction by accepting The dimer (tetraaminoethylene) was believed to be inactive to the electrophiles (E-X), and thought to merely act as a stable carbene reservoir. Tetraaminoethylene organic compounds of the generla chemical formula (R2N2C=C(NR22 ^[29]

Lemal^[30] and separately Winberg^[31] tested Wanzlick’s hypothesis of a carbene-dimer equilibrium by heating two differently N-aryl substituted tetraaminoethylenes together:

Dimer cross-over experiment

This reaction did not produce a mixed dimeric product, and accordingly indicated that a 'carbene-dimer equilibrium' did not exist for these dihydroimidazol-2-ylidenes. Tetraaminoethylene organic compounds of the generla chemical formula (R2N2C=C(NR22

Lemal^[30] proposed an alternative mechanism to account for the reactions observed by Wanzlick’s^[29] by considering the reactivity of the electron rich tetraaminoethylenes and not the then hypothetical stable carbenes. Tetraaminoethylene organic compounds of the generla chemical formula (R2N2C=C(NR22 ^[5]

Lemal's mechanism for the reaction of tetraaminoethylene with electrophiles. Tetraaminoethylene organic compounds of the generla chemical formula (R2N2C=C(NR22 In conditions of excess E-X, the salt (blue) is formed. In conditions of catalytic E-X, the dimer (purple) will be formed. However, this is based on the assumption that the dimer is more stable than the carbene, however, see Chen's work below. A persistent carbene (also known as a stable carbene or an Arduengo carbene is a type of Carbene demonstrating particular stability despite also being a Reactive E-X may be an acid or even a metal salt e. g. Li-Cl

Lemal believed that the tetraaminoethylene, not the carbene, reacted with the electrophile (E-X) to generate a transient cationic species. Tetraaminoethylene organic compounds of the generla chemical formula (R2N2C=C(NR22 An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge He proposed that this cation then dissociated into the free carbene plus the resultant salt. The free carbene could then either re-dimerise (regenerating the tetraaminoethylene starting material) or react with E-X (as Wanzlick originally predicted), with either route eventually giving the same reaction product, the dihydroimidazolium salt. Tetraaminoethylene organic compounds of the generla chemical formula (R2N2C=C(NR22 More recent work by Alder^[20] has shown that unhindered diaminocarbenes form dimers by acid catalysed dimerisation as shown in the Lemal reaction above. See also Intramolecular forces ' Steric effects arise from the fact that each Atom within a Molecule occupies a certain In excess acid conditions the dimer forms the salt.

1970: Wanzlick et al. prepared but did not isolate the first imidazol-2-ylidene by the deprotonation of imidazolium salt. ^[32] Wanzlick^[29] as well as Hoffmann^[33] believed that these imidazole-based carbenes, with a 4n+2 π-electron ring system, should be more stable than the 4,5-dihydro analogues, due to Hückel-type aromaticity. In Organic chemistry, Hückel's rule estimates whether a Planar ring Molecule will have Aromatic properties Unfortunately, perhaps believing that these carbenes were still too reactive to be isolated, they resorted to trapping these carbenes with reagents such as mercury and isothiocyanate:

Preparation and trapping of an imidazol-2-ylidene

1991: After nearly 30 years Arduengo et al. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum Isothiocyanate is the Chemical group - N = C = S, formed by substituting Sulfur for Oxygen in the Isocyanate revisited this area, and remarkably managed not only to isolate a stable carbene but also to acquire an X-ray structure of it. X-ray crystallography is a method of determining the arrangement of Atoms within a Crystal, in which a beam of X-rays strikes a crystal and scatters ^[1] Given the prevailing belief at that time that all carbenes existed only as highly reactive, transient species, it is understandable that few attempts had been made prior to this to isolate these species. Arduengo et al. found that simple deprotonation of an imidazolium chloride with a strong base gave the carbene:

Preparation of N,N'-diadamantyl-imidazol-2-ylidene

This carbene was found to be indefinitely stable at room temperature (in the absence of oxygen and moisture), and melted at 240-241 °C without decomposition. Deprotonation is a Chemistry term that refers to the removal of a Proton ( Hydrogen Cation H+ from a Molecule, forming Another interesting chemical property of this molecule was a characteristic resonance in the ¹³C NMR spectrum at 211 ppm for the deshielded carbenic carbon. The X-ray structure revealed longer N–C bond lengths in the ring of the carbene than in the parent imidazolium compound, indicating that there was very little double bond character to these bonds. X-ray scattering techniques are a family of non-destructive analytical techniques which reveal information about the crystallographic structure chemical composition In Molecular geometry, bond length or bond distance is the average distance between nuclei of two bonded Atoms in a Molecule.

1991: Hindered N,N'-diadamantyl-imidazol-2-ylidene. (external viewer)	1992: Less hindered tetramethyl-imidazol-2-ylidene	1995: Cyclic diaminocarbene 1,3-dimesityl-imidazol-4,5-dihydro-2-ylidene (external viewer)	1996: Acyclic diaminocarbene bis(diisopropylamino)carbene	1997: Air-stable 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene. (external viewer)

1992: Initially many researchers believed that this carbene's unique stability was due to the bulky N-adamantyl substituents, which prevented the carbene from dimerising due to steric hindrance. Adamantane (tricyclodecane is a colourless crystalline compound with a Camphor -like odour See also Intramolecular forces ' Steric effects arise from the fact that each Atom within a Molecule occupies a certain However, the Arduengo laboratory later also isolated and acquired an X-ray structure of an imidazol-2-ylidene in which the bulky N-adamantyl groups were replaced with smaller methyl groups. In Chemistry, a methyl group is a Hydrophobic Alkyl Functional group named after Methane (4 ^[8]

This showed that steric factors were not the predominant stabilising factors, and that imidazole-2-ylidenes were thermodynamically stable. See also Intramolecular forces ' Steric effects arise from the fact that each Atom within a Molecule occupies a certain Thermodynamic reaction control or kinetic reaction control in a Chemical reaction can decide the composition in a reaction product when competing reactions lead to

1995: Arduengo and co-workers also went on to obtain an X-ray structure of the first dihydroimidazol-2-ylidene, a cyclic diaminocarbene. ^[14] This hindered molecule demonstrated that the aromatic imidazolium ring system, with the 4-5 carbon double bond, was also not critical to the stability of these carbenes. Later work performed by Denk et al. suggested that these dihydroimidazole carbenes were in part reliant on steric protection to prevent dimerisation, and thus not thermodynamically stable, unlike their aromatic imidazol-2-ylidene analogues^[15]. However, in light of the work of Alder^[20] it would seem the dimerisation was acid (or metal) catalysed (see Wanzlick equilibrium). A persistent carbene (also known as a stable carbene or an Arduengo carbene is a type of Carbene demonstrating particular stability despite also being a Reactive

1996: Alder et al. isolated and acquired an X-ray structure of the first acyclic diaminocarbene. ^[17] This carbene showed that diaminocarbenes without a cyclic backbone could be prepared. However, the real virtue of this carbene was that it has the ability to rotate around the N-C carbene bonds. By measuring the barrier to rotation of these bonds, the extent of double bond character in these bonds could be measured. Alkane stereochemistry concerns the Stereochemistry of linear Alkanes and the linear alkane Conformers The existence of more than one conformation is due This allowed the ylidic nature of this carbene to be determined. An ylid or ylide (US is a neutral Molecule with a positive and a negative charge on adjacent Atoms They appear in Organic chemistry Like the cyclic diaminocarbenes, unhindered examples tend to dimerise. ^[18][19][20]

1997: Arduengo and co-workers reported the synthesis of the first air-stable carbene, 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene:^[10]

1997-1998: The preparation of a thiazol-2-ylidene by Arduengo et al. ^[21] and an aminothiocarbene and aminooxycarbene by Alder et al. ,^[22] demonstrated that at least one nitrogen adjacent to the carbene centre could be replaced by another heteroatom without destroying the stability of these molecules:

Heteroatomic (O, S) stabilised carbenes. Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 (External viewer)

However, these carbenes are not thermodynamically stable as decomposition and dimerisation have been observed for unhindered examples.

Some time before Arduengo’s initial discovery in 1988, Bertrand et al. had isolated a red oil, the molecular structure of which can be represented as either a λ³-phosphinocarbene or λ⁵-phosphaacetylene:^[6][7]

Alkyne and carbene resonances structures of Bertrand’s carbene. Chemistry, phosphaalkyne is a Molecule which has a Phosphorus - Carbon Triple bond.

These molecules exhibit both carbenic and alkynic reactivity. Alkynes are Hydrocarbons that have at least one Triple bond between two Carbon atoms with the formula CnH2n-2. An X-ray structure of this molecule has not been obtained and at the time of publication some doubt remained as to their exact carbenic nature. The carbene was made by the reaction of an imidazol-2-ylidene with carbon tetrachloride. Carbon tetrachloride, also known by many other names (see Table is the Organic compound with the formula CCl4 This extra stability probably results from the electron-withdrawing effect of the chlorine atoms, which must reduce the electron density on the carbon atom bearing the lone pair, via induction through the sigma-backbone. The Polar effect or electronic effect in Chemistry is the effect exerted by a Substituent on modifying Electrostatic forces operating on a nearby Chlorine (ˈklɔriːn from the Greek word 'χλωρóς' ( khlôros, meaning 'pale green' is the Chemical element with Atomic number 17 and Electron density is the measure of the Probability of an Electron being present at a specific location lone pair is a (valence electron pair without bonding or sharing with other Atoms They are found in the outermost Electron shell of an atom so lone pairs The inductive effect in Chemistry is an experimentally observable effect of the transmission of charge through a chain of Atoms in a Molecule

Arduengo’s initial publication has excited considerable interest in the field of stable carbenes. ^[1] Since publication, this paper has been cited many hundreds of times. Work in this field has included a diverse range of topics from theoretical calculations, to the practical application of these carbenes as metal ligands in catalysis, e. In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally g. the second generation Grubbs' catalyst:

Grubbs Catalyst 2nd Generation

With the establishment of some of the fundamental principles of this chemistry, it is clear that this subject is no longer a laboratory curiosity, but has established itself as a chemical research field in its own right, and is set to grow still further in the future. First generation catalyst The First Generation Catalyst is often used in Organic synthesis to achieve Olefin cross-metathesis (see below Ring-opening

General methods of preparing stable carbenes

Stable carbenes are very reactive molecules and so it is important to consider the reaction conditions carefully when attempting to prepare these molecules. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by Stable carbenes are strongly basic (the pKa value of the conjugate acid of an imidazol-2-ylidene was measured at ca. In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and Within the Brønsted - Lowry ( protonic) theory of acids and bases, a conjugate acid is the acid member HX of a pair of two compounds that transform 24)^[34] and react with oxygen. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Clearly these reactions must be performed under a dry, inert atmosphere, avoiding protic solvents or compounds of even moderate acidity. In Computer science, ACID ( Atomicity Consistency Isolation Durability) is a set of properties that guarantee that Database transactions are Furthermore, one must also consider the relative stability of the starting materials. Whilst imidazolium salts are stable to nucleophilic addition, other non-aromatic salts are not (i. In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to e. formamidinium salts)^[35]. Formamidiumium Cations are positively charged organic chemical compounds which can be represented by the chemical formula + Consequently in these cases, strong unhindered nucleophiles must be avoided whether they are generated in situ or are present as an impurity in other reagents (e. g. LiOH in BuLi).

Several approaches have been developed in order to prepare stable carbenes, these are outlined below.

Deprotonation

Deprotonation of carbene precursor salts with strong bases has proved a reliable route to almost all stable carbenes:

Deprotonation of precursor salts to give stable carbenes. Deprotonation is a Chemistry term that refers to the removal of a Proton ( Hydrogen Cation H+ from a Molecule, forming

Several bases and reaction conditions have been employed with varying success. The degree of success has been principally dependent on the nature of the precursor being deprotonated. The major drawback with this method of preparation is the problem of isolation of the free carbene from the metals ions used in their preparation.

Metal hydride bases

One might believe that sodium or potassium hydride^[14][21] would be the ideal base for deprotonating these precursor salts. Potassium hydride, KH is a Chemical compound of Potassium and Hydrogen. The hydride should react irreversibly with the loss of hydrogen to give the desired carbene, with the inorganic by-products and excess hydride being removed by filtration. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Traditionally inorganic compounds are considered to be of mineral not biological origin In practice this reaction is often too slow in suitable solvents (e. g. THF) due to the relative insolubility of the metal hydride and the salt.

The addition of soluble “catalysts” (DMSO, tBuOH)^[1][8] considerably improves the rate of reaction of this heterogeneous system, via the generation of tert-butoxide or dimsyl anion. Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst tert -Butanol, or 2-methyl-2-propanol (colourless liquid or white solid depending on the ambient temperature is the simplest Tertiary alcohol. However, these catalysts have proved ineffective for the preparation of non-imidazolium adducts as they tend to act as nucleophiles towards the precursor salts and in so doing are destroyed. The presence of hydroxide ions as an impurity in the metal hydride could also destroy non-aromatic salts. In Chemistry, hydroxide is the most common name for the diatomic Anion OH− consisting of Oxygen and Hydrogen

Deprotonation with sodium or potassium hydride in a mixture of liquid ammonia/THF at -40 °C has been reported to work well by Hermann et al. Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor ^[9] for imidazole based carbenes. Arduengo and co-workers^[21] managed to prepare a dihydroimidazol-2-ylidene using NaH. However, this method has not been applied to the preparation of diaminocarbenes.

Potassium tert-butoxide

Arduengo and co-workers^[8] have used potassium tert-butoxide without the addition of a metal hydride to deprotonate precursor salts. An alkoxide is the Conjugate base of an Alcohol and therefore consists of an organic group bonded to a negatively charged Oxygen atom

Alkyllithiums

The use of alkyllithiums as strong bases^[1] has not been extensively studied, and have been unreliable for deprotonation of precursor salts. An organolithium reagent is an Organometallic compound with a direct bond between a Carbon and a Lithium atom With non-aromatic salts, n-BuLi and PhLi can act as nucleophiles whilst t-BuLi can on occasion act as a source of hydride, reducing the salt with the generation of isobutene:

Reduction of formamidinium salts with tert-butyllithium

Lithium amides

Lithium amides like LDA and lithium tetramethylpiperidide (LiTMP)^[17][18] generally work well for the deprotonation of all types of salts, providing that not too much LiOH is present in the n-BuLi used to make the lithium amide. Isobutylene (or 2-methylpropene is a Hydrocarbon of significant industrial importance Formamidiumium Cations are positively charged organic chemical compounds which can be represented by the chemical formula + Lithium diisopropylamide is the Chemical compound with the formula 2NLi Lithium tetramethylpiperidide or LiTMP is an organic base and a Non-nucleophilic base. Lithium hydroxide (LiOH is a corrosive Alkali hydroxide. It is a white Hygroscopic Crystalline material n -Butyllithium (abbreviated BuLi is the most prominent Organolithium reagent. Titration of lithium amide can be used to determine the amount of hydroxide in solution.

Metal hexamethyldisilazides

The deprotonation of precursor salts with metal hexamethyldisilazides^[16] works very cleanly for the deprotonation of all types of salts, except for unhindered formamidinium salts, where this base can act as a nucleophile to give a triaminomethane adduct. Bis(trimethylsilylamine (also known as hexamethyldisilazane or HMDS is a chemical reagent with the Molecular formula (CH33Si-NH-Si(CH33

Metal free carbene preparation

The preparation of stable carbenes free from metal cations has been keenly sought to allow further study of the carbene species in isolation from these metals. Separating a carbene from a carbene-metal complex can be problematic due to the stability of the complex. Accordingly, it is preferable to make the carbene free from these metals in the first place. Indeed, some metal ions, rather than stabilising the carbene, have been implicated in the catalytic dimerisation of unhindered examples.

Shown right is an x-ray structure showing a complex between a diaminocarbene and potassium HMDS. This complex was formed when excess KHMDS was used as a strong base to deprotonate the formamidinium salt. Parameters work only in their own section When a parameter is not needed please leave it empty when a parameter is wrong just clear Formamidiumium Cations are positively charged organic chemical compounds which can be represented by the chemical formula + Removing lithium ions resulting from deprotonation with reagents such as LDA can be especially problematic. Potassium and sodium salt by-products tend to precipitate from solution and can be removed. Lithium ions may be chemically removed by binding to species such as kryptanes or crown ethers. Crown ethers are heterocyclic Chemical compounds that consist of a ring containing several Ether groups

Metal free carbenes have been prepared in several ways as outlined below:

Dechalcogenation

Another approach of preparing carbenes has relied on the desulfurisation of thioureas with molten potassium in boiling THF. Desulfurisation is a chemical reaction involving the removal of Sulfur from a molecule ( e Thiourea is an Organic compound of Carbon, Nitrogen, Sulfur and Hydrogen, with the formula C[[Sulfur S]] Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 "THF" redirects here For other uses see THF (disambiguation. ^[15][36] A contributing factor to the success of this reaction is that the byproduct, potassium sulfide, is insoluble in the solvent. Potassium sulfide is the Chemical compound with the formula K2S The elevated temperatures suggest that this method is not suitable for the preparation of unstable dimerising carbenes. A single example of the deoxygenation of a urea with a fluorene derived carbene to give the tetramethyldiaminocarbene and fluorenone has also been reported:^[37]

Preparation of carbenes by dechalcogenation. Deoxygenation is a Chemical reaction involving the removal of molecular oxygen (O2 from a reaction mixture or solvent or the removal of Oxygen atoms from Urea is an Organic compound with the Chemical formula ( N[[hydrogen H]]22 C[[oxygen O]] Fluorene, or 9H-fluorene, is a Polycyclic aromatic hydrocarbon.

The desulfurisation of thioureas with molten potassium to give imidazol-2-ylidenes or diaminocarbenes has not been widely used. Desulfurisation is a chemical reaction involving the removal of Sulfur from a molecule ( e Thiourea is an Organic compound of Carbon, Nitrogen, Sulfur and Hydrogen, with the formula C[[Sulfur S]] Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39

Vacuum pyrolysis

Vacuum pyrolysis, with the removal of neutral volatile by-products (CH₃OH, CHCl₃), has been used to prepare dihydroimidazole and triazole based carbenes:

Preparation of carbenes via vacuum pyrolysis.

Historically the removal of chloroform by vacuum pyrolysis of d adducts was used by Wanzlick^[5] in his early attempts to prepare dihydroimidazol-2-ylidenes but this method is not widely used. Pyrolysis is the Chemical decomposition of organic materials by heating in the absence of Oxygen or any other reagents except possibly Steam The Enders laboratory^[13] has used vacuum pyrolysis of a c adduct to generate a triazolium-5-ylidene c.

Bis(trimethylsilyl)mercury

Bis(trimethylsilyl)mercury (CH₃)₃Si-Hg-Si(CH₃)₃ reacts with chloro-iminium and chloro-amidinium salts to give a metal-free carbene and elemental mercury. Bis(trimethylsilylmercury a chemical reagent with formula (CH33-Si-Hg-Si-(CH33 An iminium Salt or Cation in Organic chemistry has the general structure + and is as such a protonated or substituted Amidinium Cations are positively charged organic chemical compounds which can be represented by the chemical formula H2N=CH-NH2+ Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum ^[38] e. g. : (CH₃)₃Si-Hg-Si(CH₃)₃ + R₂N=C(Cl)-NR₂⁺ Cl^- → R₂N-C:-NR₂ + Hg_(l) + (CH₃)₃Si-Cl

Photochemical decomposition

Persistent triplet state carbenes have been prepared by photochemical decomposition of a diazomethane product via the expulsion of nitrogen gas, at a wavelength of 300 nm in benzene. Photochemistry, a sub-discipline of Chemistry, is the study of the interactions between Atoms, small Molecules, and light (or Electromagnetic radiation Diazomethane is the chemical compound CH2N2 In the pure form at room temperature it is a Yellow Gas, but it is almost universally used Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14

Purification

Stable carbenes are very reactive, and so the minimum amount of handling is desirable using air-free techniques. Air-free techniques refer to a range of manipulations in the chemistry Laboratory for the handling of compounds that are Air-sensitive. However, provided rigorously dry, relatively non-acidic and air-free materials are used, stable carbenes are reasonably robust to handling per se. By way of example, a stable carbene prepared from potassium hydride can be filtered through a dry celite pad to remove excess KH (and resulting salts) from the reaction. On a relatively small scale, a suspension containing a stable carbene in solution can be allowed to settle and the supernatant solution pushed through a dried membrane syringe-filter. Stable carbenes are readily soluble in non-polar solvents such as hexane, and so typically recrystallisation of stable carbenes can be difficult, due to the unavailability of suitable non-acidic polar solvents. Recrystallization (see also Crystallization) is a physical process that has meanings in Chemistry, Metallurgy and Geology. Air-free sublimation as shown right can be an effective method of purification, although temperatures below 60 ^oC under high vacuum are preferable as these carbenes are relatively volatile and also could begin to decompose at these higher temperatures. Indeed, sublimation in some cases can give single crystals suitable for x-ray analysis. However, strong complexation to metal ions like lithium will in most cases prevent sublimation. Lithium (ˈlɪθiəm is a Chemical element with the symbol Li and Atomic number 3

Chemistry of stable carbenes

Basicity and nucleophilicity

The nucleophilicity and basicity of imidazol-2-ylidenes have been studied by Alder et al. ^[34] who revealed that these molecules are strong bases, having a pKa of ca. 24 for the conjugate acid in DMSO:

Measurement of the pKa value for the conjugate acid of an imidazol-2-ylidene.

However, further work by Alder has shown that diaminocarbenes will deprotonate the DMSO solvent, with the resulting anion reacting with the resulting amidinium salt.

Using D6-DMSO as an NMR solvent can have unexpected results.

Reaction of imidazol-2-ylidenes with 1-bromohexane gave 90% of the 2-substituted adduct, with only 10% of the corresponding alkene, indicating that these molecules are also reasonably nucleophilic. In Organic chemistry, an alkene, olefin, or olefine is an unsaturated Chemical compound containing at least one Carbon In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to

Dimerisation

Imidazol-2-ylidenes and triazol-5-ylidenes are thermodynamically stable and do not dimerise, and have been stored in solution in the absence of water and air for years. A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances This is presumably due to the aromatic nature of these carbenes, which is lost upon dimerisation. In fact imidazol-2-ylidenes are so thermodynamically stable that only in highly constrained conditions are these carbenes forced to dimerise.

Chen and Taton^[39] made a doubly-tethered diimidazol-2-ylidene by deprotonating the respective diimidazolium salt. Deprotonation is a Chemistry term that refers to the removal of a Proton ( Hydrogen Cation H+ from a Molecule, forming Only the deprotonation of the doubly-tethered diimidazolium salt with the shorter methylene (-CH₂-) linkage resulted in the dicarbene dimer:

Dimerisation of tethered diimidazol-2-ylidenes. Methylene is the chemical species R2C named after Methane, in which two of the carbon atom's valence electrons form no bonds

If this dimer existed as a dicarbene, the electron lone pairs on the carbenic carbon would be forced into close proximity. lone pair is a (valence electron pair without bonding or sharing with other Atoms They are found in the outermost Electron shell of an atom so lone pairs Presumably the resulting repulsive electrostatic interactions would have a significant destabilising effect. Electrostatics is the branch of Science that deals with the Phenomena arising from what seems to be stationary Electric charges Since Classical To avoid this electronic interaction, the carbene units dimerise. In Chemistry, a carbene is a highly reactive Organic molecule containing a Carbon atom with six valence electrons and having the general formula

On the other hand, heteroamino carbenes (e. g. R₂N-C:-OR or R₂N-C:-SR) and non-aromatic carbenes such as diaminocarbenes (e. g. R₂N-C:-NR₂) have been shown to dimerise,^[40] albeit quite slowly. This has been presumed to be due to the high barrier to singlet state dimerisation:

"Least motion" (path A - not allowed) and "non-least motion" (path B) routes of carbene dimerisation. In Theoretical physics, a singlet usually refers to a one-dimensional representation (e

However, more recent work by Alder ^[20] suggests that diaminocarbenes do not truly dimerise, but rather form the dimer by reaction via formamidinium salts, a protonated precursor species (see Wanzlick equilibrium). Formamidiumium Cations are positively charged organic chemical compounds which can be represented by the chemical formula + A persistent carbene (also known as a stable carbene or an Arduengo carbene is a type of Carbene demonstrating particular stability despite also being a Reactive Accordingly, this reaction can be acid catalysed. This reaction occurs because unlike imidazolium based carbenes, there is no loss of aromaticity in protonation of the carbene.

Unlike the dimerisation of triplet state carbenes, these singlet state carbenes do not approach head to head (“least motion”), but rather the carbene lone pair attacks the empty carbon p-orbital (“non-least motion”). In Physics, '''spin''' is the Angular momentum intrinsic to a body as opposed to Orbital angular momentum, which is the motion of its Center of mass In Theoretical physics, a singlet usually refers to a one-dimensional representation (e lone pair is a (valence electron pair without bonding or sharing with other Atoms They are found in the outermost Electron shell of an atom so lone pairs An atomic orbital is a Mathematical function that describes the wave-like behavior of an electron in an atom Carbene dimerisation can also be acid or metal catalysed, and so care must be taken when determining if the carbene is undergoing true dimerisation.

Reactivity of stable carbenes

The chemistry of stable carbenes has not been fully explored. However, Enders et al. ^[13][41][42] have performed a range of organic reactions involving a triazol-5-ylidene. These reactions are outlined below and may be considered as a model for other carbenes.

Reactions of triazol-5-ylidene. ^[42]

These carbenes tend to behave in a nucleophilic fashion (e and f), performing insertion reactions (b), addition reactions (c), [2+1] cycloadditions (d, g and h), [4+1] cycloadditions (a) as well as simple deprotonations. In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to An insertion reaction is a chemical reaction where one chemical entity interposes itself into an existing bond of typically a second chemical entity e An addition reaction, in Chemistry, is in its simplest terms an Organic reaction where two or more molecules combine to form a larger one A cycloaddition is a Pericyclic Chemical reaction, in which two π bonds are lost and two σ bonds are gained the resulting reaction is a A cycloaddition is a Pericyclic Chemical reaction, in which two π bonds are lost and two σ bonds are gained the resulting reaction is a Deprotonation is a Chemistry term that refers to the removal of a Proton ( Hydrogen Cation H+ from a Molecule, forming The insertion reactions (b) probably proceed via deprotonation, resulting in the generation of a nucleophile (^-XR) which can attack the generated salt giving the impression of a H-X insertion. An insertion reaction is a chemical reaction where one chemical entity interposes itself into an existing bond of typically a second chemical entity e Deprotonation is a Chemistry term that refers to the removal of a Proton ( Hydrogen Cation H+ from a Molecule, forming

Care must be taken to check that a stable carbene is truly stable. The discovery of a stable isothiazole carbene (2) from an isothiazolium perchlorate (1) by one research group ^[43] was questioned by another group ^[44] who were only able to isolate 2-imino-2H-thiete (4). An isothiazole is a type of Organic compound containing a five-membered Aromatic ring that consists of three carbon atoms one nitrogen atom and one sulfur atom The intermediate 3 was proposed through a rearrangement reaction. A rearrangement reaction is a broad class of Organic reactions where the carbon skeleton of a Molecule is rearranged to give a Structural isomer of the original This carbene is no longer considered stable ^[45].

Carbene complexation

Imidazol-2-ylidenes, triazol-5-ylidenes (and less so, diaminocarbenes) have been shown to co-ordinate to a plethora of elements, from alkali metals, main group elements, transition metals and even lanthanides and actinides. Trends The alkali metals show a number of trends when moving down the group - for instance decreasing electronegativity increasing reactivity and decreasing melting and boiling In Chemistry and atomic Physics, main group elements are elements in groups (periodic columns whose lightest members are represented by Hydrogen, In Chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings It commonly refers to any element in Terminology The Trivial name " Rare earths " is sometimes used to describe all the lanthanoids together with Scandium and Yttrium History of the actinoid series From the earlier known chemical properties of actinium (89 up to uranium (92 indicating a relation to the Transition metals it was generally A periodic table of elements gives some idea of the complexes which have been prepared, and in many cases these have been identified by single crystal X-ray crystallography. The periodic table of the chemical elements is a tabular method of displaying the Chemical elements Although precursors to this table exist its invention is X-ray crystallography is a method of determining the arrangement of Atoms within a Crystal, in which a beam of X-rays strikes a crystal and scatters ^{[46] [16] [47]}

Stable carbenes are believed to behave in a similar fashion to organophosphines in their co-ordination properties to metals. Organophosphorus compounds are Chemical compounds containing Carbon - Phosphorus bonds. These ligands are said to be good σ-donors through the carbenic lone pair, but poor π-acceptors due to internal ligand back-donation from the nitrogen atoms adjacent to the carbene centre, and so are able to co-ordinate to even relatively electron deficient metals. In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally lone pair is a (valence electron pair without bonding or sharing with other Atoms They are found in the outermost Electron shell of an atom so lone pairs In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Enders ^[48] and Hermann ^[49][50] have shown that these carbenes are suitable replacements for phosphine ligands in several catalytic cycles. Phosphine is the common name for phosphorus hydride (PH3 also known by the IUPAC name phosphane and occasionally phosphamine. In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally A catalytic cycle in Chemistry is a term for a multistep reaction mechanism that involves a Catalyst. Whilst they have found that these ligands do not activate the metal catalyst as much as phosphine ligands they often result in more robust catalysts. Several catalytic systems have been looked into by Hermann and Enders, using catalysts containing imidazole and triazole carbene ligands, with moderate success. ^{[46][48][49][50]}Grubbs ^[51] has reported replacing a phosphine ligand (PCy₃) with an imidazol-2-ylidene in the olefin metathesis catalyst RuCl₂(PCy₃)₂CHPh, and noted increased ring closing metathesis as well as exhibiting “a remarkable air and water stability”. Olefin metathesis or transalkylidenation is an Organic reaction that entails redistribution of alkylene fragments by the scission of carbon - carbon double bonds Molecules containing two and three carbene moieties have been prepared as potential bidentate and tridentate carbene ligands. In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally ^[11]. ^[12]

Carbenes in organometallic chemistry

Carbenes can be stabilised as organometallic species. Organometallic chemistry is the study of Chemical compounds containing bonds between Carbon and a Metal. These transition metal carbene complexes fall into two categories:

• Fischer carbenes in which carbenes are tethered to a metal and an electron-withdrawing group (usually a carbonyl),
• Schrock carbenes; in which carbenes are tethered to a metal and an electron-donating group. A transition metal carbene complex in Organometallic chemistry is a compound bearing a formal carbon-metal bond. Ernst Otto Fischer ( November 10, 1918 &ndash July 23, 2007) was a German Chemist who won the Nobel Prize for pioneering The Polar effect or electronic effect in Chemistry is the effect exerted by a Substituent on modifying Electrostatic forces operating on a nearby Richard Royce Schrock (born January 4, 1945) is an American chemist and Nobel laureate recognized for his contributions to the Metathesis reaction used The Polar effect or electronic effect in Chemistry is the effect exerted by a Substituent on modifying Electrostatic forces operating on a nearby The reactions that such carbenes participate in are very different from those in which organic carbenes participate.

Some persistent carbenes are used as ancillary ligand in organometallic chemistry. In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally Organometallic chemistry is the study of Chemical compounds containing bonds between Carbon and a Metal. One of the most notable examples is in the second generation Grubbs' catalysts. First generation catalyst The First Generation Catalyst is often used in Organic synthesis to achieve Olefin cross-metathesis (see below Ring-opening

Triplet state carbene chemistry

Persistent triplet state carbenes are likely to have very similar reactivity as other non-persistent triplet state carbenes. In Chemistry, a carbene is a highly reactive Organic molecule containing a Carbon atom with six valence electrons and having the general formula

Physical properties

Those carbenes that have been isolated to date tend to be white solids with low melting points.

These carbenes tend to sublime at low temperatures under high vacuum.

One of the more useful physical properties is the diagnostic chemical shift of the carbenic carbon atom in the 13C-NMR spectrum. Typically this peak is in the range between 200 and 300 ppm, where few other peaks appear in the 13C-NMR spectrum. An example is shown left for a cyclic diaminocarbene which has a carbenic peak at 238 ppm.

References

Further reading

Reviews on persistent carbenes:

• Carbene Chemistry: From Fleeting Intermediates to Powerful Reagents, (Chapter 4, Hideo Tomioka (triplet state); Chapter 5 (singlet state), Roger W. Alder) - ed. Guy Bertrand
• Reactive Intermediate Chemistry By Robert A. Moss, Matthew Platz, Maitland Jones (Chapter 8, Stable Singlet Carbenes, Guy Bertrand)
• Alder, R. W. ; Blake, M. E. ; Chaker, L. ; Harvey, J. N. ; Paolini, F. P. V. ; Schütz, J. (2004). "When and How Do Diaminocarbenes Dimerize?". Angew. Chem. Int. Ed. Engl. 43 (44): 5896–5911. doi:10.1002/anie.200400654. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
• R. W. Alder, in 'Diaminocarbenes: exploring structure and reactivity', ed. G. Bertrand, New York, 2002
• Herrmann, W. A. ; Köcher, C. (1997). "N-Heterocyclic Carbenes". Angew. Chem. , Int. Ed. Engl. 36 (20): 2162–2187. doi:10.1002/anie.199523711. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
• Regitz, M. (1996). "Stable Carbenes—Illusion or Reality?". Angew. Chem. , Int. Ed. Engl. 30 (6): 674–676. doi:10.1002/anie.199106741. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.