Interaction of β-lactam carbenes with 3,6-diphenyltetrazines: a five-step cascade reaction for the direct construction of indeno[2,1-b]pyrrol-2-ones

A study of the nucleophilic addition of β-lactam carbenes to 3,6-diphenyltetrazines is reported. Instead of the formation of pyrazole derivatives like most reactions between nucleophilic or ambiphilic carbenes and 3,6-disubstituted tetrazines, β-lactam carbenes reacted with 3,6-diphenyltetrazines to produce indeno[2,1-b]pyrrol-2-ones in good yields. The reaction proceeds most probably through a five-step cascade process. This work has not only provided a one-pot operation for the efficient construction of mutisubstituted indeno[2,1-b]pyrrol-2-ones but also revealed the nucleophilicity of β-lactam carbenes.     

Gold Carbene or Carbenoid: Is There a Difference?

By reviewing the recent progress on the elucidation of the structure of gold carbenes and the definitions of metal carbenes and carbenoids, we recommend to use the term gold carbene to describe gold carbene‐like intermediates, regardless of whether the carbene or carbocation extreme resonance dominates. Gold carbenes, because of the weak metal‐to‐carbene π‐back‐donation and their strongly electrophilic reactivity, could be classified into the broader family of Fischer carbenes, although their behavior and properties are very specific.     

Stable non-N-heterocyclic carbenes (non-NHC): recent progress

This account summarizes the results that have been obtained by our group since 2000 in the area of stable singlet acyclic carbenes. It includes aryl- and alkyl-(phosphino)carbenes, aryl(amino)carbenes, and (amino)(phosphino)carbenes. Our most recent achievements, the transformation of stable carbenes into other stable carbenes are also discussed, along with preliminary results concerning the ligand properties of these species.     

Heterocyclic carbenes: synthesis and coordination chemistry

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of beta-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.     

Carbene-Stabilized Exceptional Silicon Halides

The isolation of stable carbenes, such as N-heterocyclic carbenes, cyclic alkyl amino carbenes, and abnormal N-heterocyclic carbenes, has encouraged synthetic chemists to use them as neutral ligands for stabilizing unusual compounds with main group elements. In this Minireview, an overview is provided of the recent developments in the chemistry of carbene-stabilized silicon halides, in which the silicon atoms are in low oxidation state. The structural diversities emanating from the differences in the HOMO–LUMO gaps of various carbenes will also be discussed.     

Stabilities of carbenes: independent measures for singlets and triplets

Thermodynamic stabilities of 92 carbenes, singlets and triplets, have been evaluated on the basis of hydrogenation enthalpies calculated at the G3MP2 level. The carbenes include alkyl-, aryl-, and heteroatom-substituted structures as well as cyclic 1,3-diheteroatom carbenes. Over a wide energy range, a good correlation is seen between the singlet-triplet gaps and the hydrogenation enthalpies of the singlets, but there are some clear outliers, which represent cases where the triplet has unusual stability or instability. By use of hydrogenation enthalpies, separate carbene stabilization enthalpy scales (CSEs) have been developed for singlets and triplets, and these highlight structural features that affect the stability of each. The treatment also allows estimates of aromaticity in cyclic carbenes. In this way, imidazol-2-ylidene is estimated to have an aromatic stabilization energy of about 20 kcal/mol.     

(Phosphino)(aryl)carbenes: effect of aryl substituents on their stabilization mode

A broad range of (phosphino)(aryl)carbenes, 1b-d, 10a,b, and 14a,b, were prepared by photolysis of their diazo precursors. The influence of the steric and electronic properties of the aryl ring on the structure and stability of these carbenes was studied both experimentally and theoretically. Among the different stabilization modes investigated, those featuring an acceptor as well as a spectator aryl substituent result in stable or at least persistent carbenes that could be completely characterized by classical spectroscopic methods. In marked contrast, the new substitution pattern featuring a donor aryl ring results in a very fleeting carbene.     

Carbenes and Nitrenes: Recent Developments in Fundamental Chemistry

Carbenes and nitrenes can exist in both singlet and triplet states, sometimes equally stable and interconverting either thermally or photochemically. Many carbene and nitrene reactions proceed via tunneling at low temperatures. Numerous singlet and triplet states have been characterized spectroscopically, and a detailed understanding of the chemical and physical properties of carbenes and nitrenes is emerging. There has been significant progress in the direct observation of carbenes, nitrenes, and many other reactive intermediates in recent years through the application of matrix photolysis and flash vacuum pyrolysis linked with matrix isolation at cryogenic temperatures. Our understanding of singlet and triplet states has improved through the interplay of spectroscopy and computations. Bistable carbenes and nitrenes as well as many examples of tunneling have been discovered and numerous rearrangements and fragmentations have been documented. The correlation of the zero‐field splitting parameter D with calculated spin densities on nitrenes and carbenes is discussed. This Minireview gives an overview of some of these developments.     

Photochemical,Metal-Free Sigmatropic Rearrangement Reactions of Sulfur Ylides

Sigmatropic rearrangement reactions constitute one of the most fundamental reactions of carbenes. While state-of-the-art synthetic methods require the use of expensive precious metal catalysts, the application of visible light for the photolysis of α-aryldiazoacetates is much less investigated and provides an operationally simple entry to carbenes under mild reaction conditions. Herein, we report on blue-light induced sigmatropic rearrangement reactions of sulfur compounds with α-aryldiazoacetates. This process, depending on the substitution pattern of the sulfide, opens up formal insertion reactions of carbenes into S−N, S−C, or C−H bonds.     

Are Copper(I) Carbenes Capable Intermediates for Cyclopropanations? The Case for Ylide Intermediates

A novel approach is used to synthesize a stable, ligated copper(I) carbene in the gas phase that is capable of typical metal carbenoid chemistry. However, it is shown that copper(I) carbenes generally undergo rapid unimolecular rearrangements including insertions into copper‐ligand bonds and Wolff rearrangements. The results indicate that most copper(I) carbenes are inherently unstable and would not be viable intermediates in condensed‐phase applications; an alternative intermediate that is less prone to rearrangements is required. Computational data suggest that ylides formed by the complexation of the carbene with solvent or other weak nucleophiles are viable intermediates in the reactions of copper(I) carbenes.     

Gold(I)-catalyzed intramolecular cyclopropanation of dienynes

Gold(I) complexes are the most active catalysts for the biscyclopropanation of dienynes to form tetracyclic compounds. PtII and ZnII are also able to promote the biscyclopropanation, although less efficiently. The configurations obtained in all cases with the use of gold(I) catalysts can be explained by the pathway proceeding through anti cyclopropyl gold carbenes. Similar intermediates are most probably involved in reactions catalyzed by RuII and PtII. Two different cyclopropanation pathways have been found; they depend on the structures of the cyclopropyl gold carbenes (anti or syn) and the relative arrangements of the metal carbenes and the alkenes.     

Incorporation of hexafluorobutyne into furans or phenols via reaction with iron(0) carbene or vinylketene complexes

Reaction of iron(0) carbene complexes with hexafluorobut-2-yne produces 3,4-bis(trifluoromethylated)furans in a process that is favored for electron-rich carbenes. No traces of furannulation or benzannulation are observed with Group 6 Fischer carbenes. Reaction of hexafluorobut-2-yne with a vinylketene iron(0) complex gives a 2,3-bis-trifluoromethylated phenol.     

Direct Observation of Aryl Gold(I) Carbenes that Undergo Cyclopropanation,C−H Insertion,and Dimerization Reactions

Mesityl gold(I) carbenes lacking heteroatom stabilization or shielding ancillary ligands have been generated and spectroscopically characterized from chloro(mesityl)methylgold(I) carbenoids bearing JohnPhos‐type ligands by chloride abstraction with GaCl₃. The aryl carbenes react with PPh₃ and alkenes to give stable phosphonium ylides and cyclopropanes, respectively. Oxidation with pyridine N‐oxide and intermolecular C?H insertion to cyclohexane have also been observed. In the absence of nucleophiles, a bimolecular reaction, similar to that observed for other metal carbenes, leads to a symmetrical alkene.     

Direct Observation of Aryl Gold(I) Carbenes that Undergo Cyclopropanation,C−H Insertion,and Dimerization Reactions

Mesityl gold(I) carbenes lacking heteroatom stabilization or shielding ancillary ligands have been generated and spectroscopically characterized from chloro(mesityl)methylgold(I) carbenoids bearing JohnPhos‐type ligands by chloride abstraction with GaCl₃. The aryl carbenes react with PPh₃ and alkenes to give stable phosphonium ylides and cyclopropanes, respectively. Oxidation with pyridine N‐oxide and intermolecular C?H insertion to cyclohexane have also been observed. In the absence of nucleophiles, a bimolecular reaction, similar to that observed for other metal carbenes, leads to a symmetrical alkene.     

CO2 laser induced reactions of chlorotrifluoroethene and 1,2-dichlorodifluoroethene with silane

Gas-phase reactions in C1FC:CF₂/SiH₄ and C1FC:CFCl/SiH₄ mixtures at total pressure arround 1 Torr induced by pulsed CO₂ laser radiation yield different products depending on whether the single laser irradiating wavelength is tuned to absorption band of olefin or silane. Channels assumed to explain variety of products are multiphoton dissociation of the olefin into carbenes, carbenes recombination and addition to parent olefin, substitution of halogen in transient carbenes by hydrogen of silane, and 1,2-migration of halogen in transient CFCl:CF^. radical.     

Carbenes in the synthesis of heterocycles (review)

The syntheses of heterocyclic compounds by reactions of carbenes with carbon-heteroatom multiple bonds, isomerization of unstable adducts of carbenes to carbon-carbon bonds, and the incorporation of carbenes in C-H, C-N, and C-O bonds are examined.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1443–1464, November, 1976.     

Geometrical isomerism in divalent-carbon intermediates

Three classes of monosubstituted carbenes (aryl-, vinyl-, carbonyl-) and one class of disubstituted carbenes (dicarbonyl-), all of them ground state triplets, can exist in more than one rotameric form with distinct zero field splitting parameters. Representative examples of these carbenes are discussed and selected aspects of triplet carbenes are evaluated, including their π spin density distribution, intramolecular H-abstraction, and cis-trans isomerization. Lastly, the structures assigned to two triplets generated by photolysis of dibenzoyldiazomethane are re-evaluated.     

Ambident Reactivity of Imidazolium Cations as Evidence of the Dynamic Nature of N-Heterocyclic Carbene-Mediated Organocatalysis

This work reveals ambident nucleophilic reactivity of imidazolium cations towards carbonyl compounds at the C2 or C4 carbene centers depending on the steric properties of the substrates and reaction conditions. Such an adaptive behavior indicates the dynamic nature of organocatalysis proceeding via a covalent interaction of imidazolium carbenes with carbonyl substrates and can be explained by generation of the H-bonded ditopic carbanionic carbenes.     

Hybrid ligands with N-heterocyclic carbene and chiral phosphaferrocene components

N-Heterocyclic carbenes (NHCs) possessing one or two 3,4-dimethylphosphaferrocenyl substituents and either methylene or ethylene alkyl bridges have been prepared. These carbenes turned out to be remarkably stable and were characterized by NMR methods and partly by mass spectrometry. Their molybdenum and ruthenium complexes were examined in order to determine the electronic properties and the coordination behaviour of these chiral PC- and PCP-chelate ligands, which combine a NHC unit as a strong sigma-donor with pi-accepting phosphaferrocene moieties. Crystal structures of one ligand precursor and of three complexes have been determined.     

The carbene reactivity surface: a classification

A new two-dimensional classification of singlet carbenes based on the difference in reactivity of their insertion reactions into the C-H bonds of acetonitrile and isobutane is presented. This classification combines the stability and the philicity of divalent species. Until now all of the experimentally based philicity scales are based on the addition to alkenes. Moreover, a new terminology for describing the reactivity of carbenes is introduced. Among the alkyl carbenes, acetyl carbene (2) and cyclopentadienylidene are shown as highly reactive electrophilic carbenes, whereas the other alkylidenes and alkenylidenes investigated are all less active than 2 and more nucleophilic. The stabilized-nucleophilic bicyclo[2.1.1]hex-2-en-5-ylidene (13) possesses a stability similar to that of cyclic alkyl amino carbene (CAAC) 18 and aminophosphoniocarbene 7. Strong hydrogen bridging is found between a C-H bond of acetonitrile and the nucleophilic carbenes 13 and 14.