Fischer Carbene
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A Fischer carbene is a type of
transition metal carbene complex A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene. Carbene complexes have been synthesized from most transition metals and f-block metals, using many different synt ...
, which is an
organometallic compound 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 ...
containing a
divalent In chemistry, the valence (US spelling) or valency (British spelling) of an atom is a measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Valence is generally understood to be the number of chemica ...
organic
ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
. In a Fischer carbene, the
carbene In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a Valence (chemistry), valence of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms. Th ...
ligand is a σ-donor π-acceptor ligand. Because π-backdonation from the metal centre is generally weak, the carbene carbon is
electrophilic In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carr ...
. Fischer carbenes are named for Ernst Otto Fischer.


Structure

A metal carbene complex could be considered a Fischer carbene when the carbene is in singlet state. Delocalization of the lone pair from the substituent on carbene carbon raises the energy of pz orbital, thus forcing the two of electrons of carbene stay as an electron pair. Bonding between carbene and the metal centre involves a strong σ donation from sp2 orbital to an empty d orbital on metal centre and a weak π back donation from the metal centre to the empty pz orbital. Because the π donation is weak, the carbene carbon is electrophilic in nature. Because of this bonding property, Fischer carbenes often feature: # low oxidation state metal center # middle and late transition metals Fe(0), Mo(0), Cr(0) # π-acceptor metal ligands # π-donor substituents on the carbene atom such as alkoxy and alkylated amino groups.


Preparation

The most common strategy to prepare a Fischer carbene is reaction between a metal carbonyl complex with organolithium compounds. The corresponding lithium
enolate In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl () compounds. Rarely isolated, they are widely used as reagents in the Organic synthesis, synthesis of organic compounds. Bonding and structure Enolate ...
-like structure is highly stabilized, and thus, needs to be quenched by a highly electrophilic alkylating reagent such as Meerwein's salt. Alkylation with MeI could be done with a phase transfer system. Alternatively, the lithium cation could be exchanged with a tetraalkylammonium cation to give a more reactive enolate. This tetraalkylammonium salt could be acylated to give a highly electrophilic mixed
anhydride An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid (chemistry), acid. In organic chemistry, organic acid anhydrides contain the functional group . Organic acid anhydrides often form when one ...
-like Fischer carbene which could undergo
nucleophilic substitution In chemistry, a nucleophilic substitution (SN) 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) ...
with alcohol. : : Fischer carbenes with an α-hydrogen are prepared by reaction of a metal carbonyl anion with a
formamide Formamide is an amide derived from formic acid. It is a colorless liquid which is miscible with water and has an ammonia-like odor. It is chemical feedstock for the manufacture of sulfa drugs and other pharmaceuticals, herbicides and pesticides, ...
. Treating the intermediate with excess amount of
trimethylsilyl chloride Trimethylsilyl chloride, also known as chlorotrimethylsilane is an organosilicon compound ( silyl halide), with the formula , often abbreviated or TMSCl. It is a colourless volatile liquid that is stable in the absence of water. It is widely u ...
yields this particular group of Fischer carbene complex.


Elaboration of Fischer carbenes

With a suitable hydride abstracting reagent, such as the trityl cation, the hydride on alkyl ligand of a metal complex could be abstracted to form a Fischer carbene.


Decarbonylation from an unstabilized metal carbenoid

In 2021, Alvarez et al reported that a Fischer carbene could be effectively prepared from a decarbonylative process of a metal carbenoid derived from a stabilized
diazo compound In organic chemistry, the diazo group is an organic moiety consisting of two linked nitrogen atoms at the terminal position. Overall charge-neutral organic compounds containing the diazo group bound to a carbon atom are called diazo compounds ...
.


Reactivity


Carbonyl-like reactivity

The carbene carbon of a Fischer carbene is electrophilic in nature. Thus, Fischer carbenes exhibit similar reactivity compared to
carbonyl compounds In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such as aldehydes ...
. Many of the reactions can be understood by using the carboxylic equivalent structure such as
transesterification Transesterification is the process of exchanging the organic functional group R″ of an ester with the organic group R' of an alcohol. These reactions are often catalyzed by the addition of an acid or base catalyst. Strong acids catalyze the r ...
,
Michael addition In organic chemistry, the Michael reaction or Michael 1,4 addition is a reaction between a Michael donor (an enolate or other nucleophile) and a Michael acceptor (usually an α,β-unsaturated carbonyl) to produce a Michael adduct by creating a c ...
, and
aldol reaction The aldol reaction (aldol addition) is a Chemical reaction, reaction in organic chemistry that combines two Carbonyl group, carbonyl compounds (e.g. aldehydes or ketones) to form a new β-hydroxy carbonyl compound. Its simplest form might invol ...
. The Cr(CO)5 moiety is a strong electron withdrawing group making the α-proton highly acidic. A methoxy chromium carbene with a methyl side chain has a pKa of 12.5 in aqueous
acetonitrile Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula and structure . This colourless liquid is the simplest organic nitrile (hydrogen cyanide is a simpler nitrile, but the cyanide anion is not class ...
(1:1 volume ratio). For comparison,
methyl acetate Methyl acetate, also known as MeOAc, acetic acid methyl ester or methyl ethanoate, is a carboxylate ester with the formula CH3COOCH3. It is a flammable liquid with a characteristically pleasant smell reminiscent of some glues and nail polish remo ...
has a pKa of 25.6, demonstrating the strong electron withdrawing nature of the Cr(CO)5 moiety. The strong electron withdrawing nature of Fischer carbenes is also reflected in many reactions. For instance, the
Diels–Alder reaction In organic chemistry, the Diels–Alder reaction is a chemical reaction between a Conjugated system, conjugated diene and a substituted alkene, commonly termed the Diels–Alder reaction#The dienophile, dienophile, to form a substituted cyclohexe ...
between
methyl acrylate Methyl acrylate is an organic compound, more accurately the methyl ester of acrylic acid. It is a colourless liquid with a characteristic acrid odor. It is mainly produced to make acrylate fiber, which is used to weave synthetic carpets. It is a ...
and
isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. It is produced by many plants and animals (including humans) and its polymers ar ...
is completed in 7 days at room temperature with low para-meta selectivity. On the other hand, the Fischer carbene counterpart finished in 3 hours at room temperature with much higher para-meta selectivity. Urotropin, a weak nucleophile, could participate in a Michael addition to an alkynyl Fischer carbene, giving an interesting double addition product. The enolate-like structure, obtaining by deprotonation of Fischer carbene, could be alkylated. However, because the carbanion is highly stabilized, a reactive alkylating reagent, such as methyl fluorosulfonate ("magic methyl" reagent) or methyl bromoacetate is needed. Aldol condensation of Fischer carbenes could be achieved by using much weaker bases compared to its carbonyl counterpart, such as
triethylamine Triethylamine is the chemical compound with the formula N(CH2CH3)3, commonly abbreviated Et3N. Like triethanolamine and the tetraethylammonium ion, it is often abbreviated TEA. It is a colourless volatile liquid with a strong fishy odor remini ...
.


Demetallation

Fischer carbenes could be oxidized to the corresponding carbonyl compounds using mild oxidants such as
ceric ammonium nitrate Ceric ammonium nitrate (CAN) is the inorganic compound with the formula . This orange-red, water-soluble cerium salt is a specialised oxidizing agent in organic synthesis and a standard oxidant in quantitative analysis. Preparation, properties, ...
(CAN). If the side chain of Fischer carbene bears an α-proton, it could be reversibly deprotonated with a weak base such as
pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom . It is a highly flammable, weak ...
. This facilitates formation of chromium hydride species, which can undergo reductive elimination to give a cis-
enol ether In organic chemistry an enol ether is an alkene with an alkoxy substituent. The general structure is R2C=CR-OR where R = H, alkyl or aryl. A common subfamily of enol ethers are vinyl ethers, with the formula ROCH=CH2. Important enol ethers incl ...
.


Photochemical properties of Fischer carbenes

The UV-Vis spectrum of a Fischer carbene shows a metal-to-ligand charge transfer band in the near ultraviolet. On the one hand, this excitation promotes an electron from a metal centered orbital to a ligand centered orbital, making the carbene carbon more electron rich. On the other hand, the metal centre, already electron poor because of the carbonyl ligands, becomes more electron poor, facilitating migratory insertion to the CO ligand. This migratory insertion gives a chromium metallacyclopropanone, which is a resonance form of the metallated
ketene In organic chemistry, a ketene is an organic compound of the form , where R and R' are two arbitrary valence (chemistry), monovalent functional group, chemical groups (or two separate Substituent, substitution sites in the same molecule). The na ...
. With ketene reactivity, the species could be trapped by several nucleophiles such as alcohols and amines, or could react in +2
cycloaddition In organic chemistry, a cycloaddition is a chemical reaction in which "two or more Unsaturated hydrocarbon, unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of th ...
with alkenes, imines, or aldehyde yielding the corresponding cyclobutane, β-lactam, and β-lactone adducts.


See also

* Wulff–Dötz reaction


References

{{reflist Carbenes Organometallic compounds