A Fischer carbene is a divalent organic ligand in an organometallic compound that is named for Ernst Otto Fischer. In a Fischer carbene, the carbene ligand is a σ-donor π-acceptor ligand. Because π-back donation from the metal centre is generally weak, the carbene carbon is electrophilic.
Structure and bonding
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.[1]
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 Fischer carbene is reaction between metal carbonyl complex with organolithium compounds. The corresponding lithium enolate-like structure is highly stabilized, and thus, needs to be quenched by a highly electrophilic alkylating reagent such as Meerwein’s salt.[2] Alkylation with MeI could be done with phase transfer system.[3] Alternatively, 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-like Fischer carbene which could undergo nucleophilic substitution with alcohol. [4]
Fischer carbenes with α-hydrogen are prepared by reaction of a metal carbonyl anion with a formamide. Treating the intermediate with excess amount of trimethylsilyl chloride yields this particular group of Fischer carbene complex.[5]
Elaboration of Fischer carbenes
With a suitable hydride abstracting reagent, such as trityl cation, the hydride on alkyl ligand of a metal complex could be abstracted to form a Fischer carbene.[6]
Decarbonylation from an unstabilized metal carbenoid
Recently, researchers in Spain reported that Fischer carbene could be effectively prepared from a decarbonylative process of a metal carbenoid derived from a stabilized diazo compound.[7]
Reactivity
Carbonyl-like reactivity
The carbene carbon of Fischer carbene is electrophilic in nature. Thus, Fischer carbene exhibits similar reactivity compared to carbonyl compound. Many of the reactions can be understood by using the carboxylic equivalent structure such as transesterification, Michael addition, and aldol reaction. 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 (1:1 volume ratio).[8][9] For comparison, methyl acetate has a pKa of 25.6, demonstrating the strong electron withdrawing nature of the Cr(CO)5 moeity.
The strong electron withdrawing nature of Fischer carbene is also reflected in many reactions. For instance, the Diels - Alder reaction between methyl acrylate and isoprene 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.[10]
Urotropin, a weak nucleophile, could participate in a Michael addtion to an alkynyl Fischer carbene, giving an interesting double addition product.[11]
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.[12][13]
Aldol condensation of Fischer carbenes could be achieved by using much weaker bases compared to its carbonyl counterpart, such as triethylamine.[14] [15]
Demetallation
Fischer carbenes could be oxidized to the corresponding carbonyl compounds using mild oxidants such as ceric ammonium nitrate (CAN).[16]
If the side chain of Fischer carbene bears an α-proton, it could be reversibly deprotonated with a weak base such as pyridine. This facilitates formation of chromium hydride species, which can undergo reductive elimination to give cis-enol ether.[17]
Dötz reaction
Exposing Fischer carbene with alkenyl side chain to an alkyne gives a highly substituted phenol. The phenolic carbon is originated from the CO ligand. The α,β-unsaturated part could also be from an electron rich aryl system, yielding a polycyclic aromatic system. This reaction was first discovered by Karl Dötz and was extensively developed by his group, thus giving the name Dötz reaction. It is sometimes called Wuff-Dötz reaction because William Wuff's group at Michigan State University also extensively contributed to the development of this reaction.[18]
The half-sandwich complex in the Dötz reaction could be demetallated to give correpsonding aryl product, or it could be further employed for a nucleophilic addition to aromatic system strategy for synthesis of fully-substituted benzene ring.[19]
The Dötz reaction has been employed as the key strategy in many syntheses of natural products. Below is a few showcases for the versatility of this reaction.[20] [21]
Interrupted Dötz reaction
In several cases, if the reactivity of the reagent does not meet or the conditions for Dotz mechanism to operate are not fulfilled, products derived from the interrupted Dotz reaction could be dominant. For instance, if the substituents on alkyne are too bulky, cyclobutene product would be observed instead.[22]
If the alkyne partner bearing a ketone substituent and both R and R’ are not bulky enough, a favored conformation for an 8e pi cyclization could be dominant leading to a fused bicyclic lactone system.[23] [24] [25]
Alkene or nucleophilic moiety on the side chain of alkyne partner could trap the resulting ketene through a [2+2] cycloaddition or nucleophilic addition respectively. This strategy was applied for the syntheses of blastmycinone and antimycinone.[26][27]
Fischer carbenes with an α-hydrogen could form could give cyclopentenone product similar to Pauson-Khand reaction. This is presumably because of a β-hydride elimination and reinsertion process.[28]
If the alkene moiety is present in Fischer carbene, but not in conjugation, cyclopropanation could be observed. The strategy was employed in a formal synthesis of carabrone.[29][30]
Photochemical properties of Fischer carbene
UV-VIS spectrum of a Fischer carbene shown a metal-to-ligand charge transfer band in the near ultraviolet. On one hand, this excitation promotes an electron from a metal centered orbital to a ligand centered orbital, making the carbene carbone more electron rich. On another hand, the metal centre, already electron poor because of the carbonyl ligands, becomes more electron poor, facilitating the migratory insertion to CO ligand. This migratory insertion gives a chromium metallacyclopropanone, which is a resonance form of metallated ketene. With ketene reactivity, the species could be trapped by several nucleophiles such as alcohols and amines, or could react in [2+2] cycloaddition with alkenes, imines, or aldehyde yielding the corresponding cyclobutane, β-lactam, and β-lactone adducts.[31]
See also
References
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