Carbene Species Research Articles

Grubbs Metathesis Enabled by a Light‐Driven gem‐Hydrogenation of Internal Alkynes

Abstract[(NHC)(cymene)RuCl2] (NHC=N‐heterocyclic carbene) complexes instigate a light‐driven gem‐hydrogenation of internal alkynes with concomitant formation of discrete Grubbs‐type ruthenium carbene species. This unorthodox reactivity mode is harnessed in the form of a “hydrogenative metathesis” reaction, which converts an enyne substrate into a cyclic alkene. The intervention of ruthenium carbenes formed in the actual gem‐hydrogenation step was proven by the isolation and crystallographic characterization of a rather unusual representative of this series carrying an unconfined alkyl group on a disubstituted carbene center.

Grubbs Metathesis Enabled by a Light-Driven gem-Hydrogenation of Internal Alkynes.

[(NHC)(cymene)RuCl2] (NHC=N‐heterocyclic carbene) complexes instigate a light‐driven gem‐hydrogenation of internal alkynes with concomitant formation of discrete Grubbs‐type ruthenium carbene species. This unorthodox reactivity mode is harnessed in the form of a “hydrogenative metathesis” reaction, which converts an enyne substrate into a cyclic alkene. The intervention of ruthenium carbenes formed in the actual gem‐hydrogenation step was proven by the isolation and crystallographic characterization of a rather unusual representative of this series carrying an unconfined alkyl group on a disubstituted carbene center.

Highly Reactive Cyclic Monoaryl Iodoniums Tuned as Carbene Generators Couple with Nucleophiles under Metal-Free Conditions.

SummaryCross-coupling reactions between aryl iodide and nucleophiles have been well developed. Iodoniums equipped with a reactive C-I(III) bond accelerate cross-coupling reactions of aryl iodide. Among them, cyclic diaryliodoniums are more atom economical; however; they are often in the trap of metal reliance and encounter regioselectivity issues. Now, we have developed a series of highly reactive cyclic monoaryl-vinyl iodoniums that can be tuned to construct C-N, C-O, and C-C bonds without metal catalysis. Under promotion of triethylamine, coupling reactions with aniline, phenol, aromatic acid, and indole proceed rapidly and regioselectively at room temperature. The carbene species is conceptualized as a key intermediate in our mechanism model. Furthermore, the coupling products enable diversity-oriented synthesis strategy to further build up a chemical library of diverse heterocyclic fragments that are in demand in the drug discovery field. Our current work provides a deep insight into the synthetic application of these highly reactive cyclic iodoniums.

Sila-Peterson Reaction of Cyclic Silanides.

Sila-Peterson type reactions of the 1,4,4-tris(trimethylsilyl)-1-metallooctamethylcyclohexasilanes (Me3Si)2Si6Me8(SiMe3)M (2a, M = Li; 2b, M = K) with various ketones were investigated. The obtained products strongly depend on the nature of the ketone component. With 2-adamantanone 2a,b afforded the moderately stable silene 3. 3 is the first example of an Apeloig–Ishikawa–Oehme-type silene with the tricoordinate silicon atom incorporated into a cyclopolysilane framework and could be characterized by NMR and UV spectroscopy as well as by trapping reactions with water, methanol, and MeLi. The reaction of 2b with aromatic ketones also follows a sila-Peterson type mechanism with formation of carbanionic species. With 1,2-diphenylcyclopropenone 2b reacted by conjugate 1,4-addition to give a spirocyclic carbanion. In most cases the underlying reaction mechanism could be elucidated by the isolation and characterization of unstable intermediates and final products after proper derivatization.

Ruthenium-Catalyzed Diastereoselective Synthesis of Fully Substituted Pyrrolidines from Anilines and Diazo Pyruvates.

An unprecedented synthesis of polysubstituted pyrrolidines from anilines and diazo pyruvates by ruthenium catalysis under mild reaction conditions is reported. An enol intermediate derived from the N-H insertion of aniline toward the ruthenium carbene species as well as an imine ester intermediate generated by SET-mediated oxidation of enol were proposed as the key intermediates. This strategy provides various pyrrolidines containing four contiguous stereocenters in good efficiency with high diastereoselectivities.

Dialkyl imidazolium acetate ionosilica as efficient and recyclable organocatalyst for cyanosilylation reactions of ketones

We report new heterogeneous organocatalyst based on silica hybrid supported N-heterocyclic carbene (NHC-) species. The organocatalyst is formed from an imidazolium iodide based ionosilica material, followed by iodide/acetate anion exchange. The imidazolium acetate generates the organocatalytic carbene via partial deprotonation of the imidazolium ring in situ. As monitored via EDX, solid state NMR and ion chromatography measurements, the iodide/acetate exchange involving the imidazolium ionosilica material took place only in small extent. Despite the fact that the exchanged material contains only a very small amount of acetate, we observed good catalytic activity and recyclability in cyanosilylation reactions of ketones with trimethylsilyl cyanide. The versatility of the catalyst was highlighted via reaction with several substrates, yielding the corresponding cyanohydrins in good yields. In recycling experiments, the material showed decreasing catalytic activity starting from the third reaction cycle, but high catalytic activity can be regenerated via another acetate treatment. Our work is important as it highlights the possibility to combine carbene chemistry and silica, which are antagonistic at a first glance. We show that imidazolium acetate based ionosilicas are therefore heterogeneous ‘proto-carbenes’, and that there is no need to form strongly basic silica supported NHCs to obtain heterogeneous NHC-organocatalysts. This work therefore opens the route towards heterogeneous and re-usable NHC-organocatalysts from supported ionic liquid imidazolium acetates.

Dinitrogen Activation: Dinitrogen Activation by Tricoordinated Boron Species: A Systematic Design (Adv. Theory Simul. 3/2020)

Dinitrogen activation is particularly challenging due to the inert nitrogen–nitrogen triple bond. In article number 1900205, Jun Zhu and co-workers report that dinitrogen activation can be made thermodynamically and kinetically favorable through through the use of a metal-free frustrated Lewis pair designed systematically from tricoordinated boron and N-heterocyclic carbene species.

How CuCl and CuCl2 Insert into C-N Bonds of Diazo Compounds: An Electronic Structure and Mechanistic Study.

The transition-metal Cu catalysts CuCl and CuCl2 have been widely employed to catalyze a series of chemical reactions with diazo compounds because of their high efficiency and selectivity. However, how to yield the active Cu carbene species from the Cu catalysts and diazo compounds still remains unclear. In this work, we performed a comprehensive theoretical investigation on the electronic structures of CuCl and CuCl2 in solution. The results indicate that the most stable structures for CuCl and CuCl2 are dimer and monomer, respectively. The C-N bond insertion of aryldiazoacetate by CuCl yields a stable bimetallic carbene species, which differs from the monometallic carbene generated from CuCl2.

Rh(III)-Catalyzed Relay Double Carbenoid Insertion and Diannulation of Sulfoximine Benzamides with α-Diazo Carbonyl Compounds: Access to Furo[2,3-c]isochromenes.

An efficient rhodium-catalyzed construction of furo[2,3-c]isochromene scaffolds through tandem double carbenoid insertion and diannulation of sulfoximine benzamides with α-diazo carbonyl compounds has been developed. Mechanistic studies revealed that the alkyl-rhodium intermediate generated by carbenoid insertion was directly trapped with another molecule of carbene species, followed by subsequent intramolecular cyclization reactions. Sulfoximine was released in situ, featuring a traceless directing fashion. The reactions proceeded smoothly under mild conditions with wide functional group tolerance.

Electrochemical-Induced Ring Transformation of Cyclic α-(ortho-Iodophenyl)-β-oxoesters.

Cyclic α‐(ortho‐iodophenyl)‐β‐oxoesters were converted in a ring‐expanding transformation to furnish benzannulated cycloalkanone carboxylic esters. The reaction sequence started by electrochemical reduction of the iodoarene moiety. In a mechanistic rationale, the resulting carbanionic species was adding to the carbonyl group under formation of a strained, tricyclic benzocyclobutene intermediate, which underwent carbon–carbon bond cleavage and rearrangement of the carbon skeleton by retro‐aldol reaction. The scope of the reaction sequence was investigated by converting cyclic oxoesters with different ring sizes yielding benzocycloheptanone, ‐nonanone and ‐decanone derivatives in moderate to good yields. Furthermore, acyclic starting materials and cyclic compounds carrying additional substituents on the iodophenyl ring were submitted to this reaction sequence. The starting materials for this transformation are straightforwardly obtained by conversion of β‐oxoesters with phenyliodobis(trifluoroacetate).

Coupling Reaction between Aldehydes and Non-Activated Hydrocarbons via the Reductive Radical-Polar Crossover Pathway.

Herein, we describe the generation of an organochromium-type carbanion species from a non-activated C-H bond and its nucleophilic addition to aldehydes. The catalytic carbanion generation occurred through formal deprotonation of a non-activated C-H bond under mild conditions and did not need the prefunctionalization or anion stabilizing group. Carbon radical intermediates generated by decatungstate photocatalyst-mediated hydrogen abstraction were captured by a chromium salt with the reductive radical-polar crossover reaction to produce organochromium carbanions.

Synthesis of α-cyano sulfoneviathermal rearrangement of 1,4-disubstituted triazole mediated by carbene and radical species

α-Cyano sulfone is produced conveniently from 1,4-disubstituted triazole, which involves carbene, ketenimine and radical species as the key intermediates.

Comparative Study of the Electronic Structures of μ-Oxo, μ-Nitrido, and μ-Carbido Diiron Octapropylporphyrazine Complexes and Their Catalytic Activity in Cyclopropanation of Olefins.

The electronic structure of three single-atom bridged diiron octapropylporphyrazine complexes (FePzPr8)2X having Fe(III)-O-Fe(III), Fe(III)-N-Fe(IV) and Fe(IV)-C-Fe(IV) structural units was investigated by Mössbauer spectroscopy and density functional theory (DFT) calculations. In this series, the isomer shift values decrease, whereas the values of quadrupole splitting become progressively greater indicating the increase of covalency of Fe-X bond in the μ-oxo, μ-nitrido, μ-carbido row. The Mössbauer data point to low-spin systems for the three complexes, and calculated data with B3LYP-D3 show a singlet state for μ-oxo and μ-carbido and a doublet state for μ-nitrido complexes. An excellent agreement was obtained between B3LYP-D3 optimized geometries and X-ray structural data. Among (FePzPr8)2X complexes, μ-oxo diiron species showed a higher reactivity in the cyclopropanation of styrene by ethyl diazoacetate to afford a 95% product yield with 0.1 mol % catalyst loading. A detailed DFT study allowed to get insight into electronic structure of binuclear carbene species and to confirm their involvement into carbene transfer reactions.

Nucleophilicity versus Brønsted Basicity Controlled Chemoselectivity: Mechanistic Insight into Silver- or Scandium-Catalyzed Diazo Functionalization

Diazo compounds are popular carbenoid precursors that can react with various transition metals to afford carbene species for further transformations. In this study, density functional theory calcul...

Ethylene Conversion into Propylene and Aromatics on HZSM-5: Insights on Reaction Routes and Water Influence

Ethanol is an alternative for producing petrochemicals, especially propylene and aromatics (benzene, toluene, and xylenes). To understand ethanol processing routes into olefins and aromatics, it is interesting to use ethylene that is the major primary product of ethanol reaction into hydrocarbons and the intermediate for the formation of olefins and aromatics. In this work, the influence of the operating conditions (ethylene partial pressure, reaction temperature and contact time) in the ethylene conversion into propylene and aromatics, and in the product yield was investigated using HZSM-5 zeolite as catalyst. Lower contact time and ethylene partial pressure, and higher reaction temperature favored propylene yield. Olefin production was based on the formation of carbene species from ethylene that reacts with ethylene to produce propylene and on ethylene dimerization to form butenes. On the other hand, intermediate reaction temperatures and contact times, and higher ethylene partial pressure promote the formation of aromatics, where the dehydrocyclization reaction is favored over hydrogen transfer. The presence of water vapor in long-term reactions deactivated the catalyst. For propylene production, the decrease of ethylene conversion was due to zeolite framework dealumination, while for aromatic formation the reaction mechanism was changed.

N-Heterocyclic Carbenes as Key Intermediates in the Synthesis of Fused, Mesoionic, Tricyclic Heterocycles.

Coupling between 5-bromoimidazo[1,5-a]pyridinium salts and malonate or arylacetate esters leads to a facile and straightforward access to the new mesoionic, fused, tricyclic system of imidazo[2,1,5-cd]indolizinium-3-olate. Mechanistic studies show that the reaction pathway consists of nucleophilic aromatic substitution on the cationic, bicyclic heterocycle by an enolate-type moiety and in the nucleophilic attack of a transient free N-heterocyclic carbene (NHC) species on the ester group; the relative order of these two steps depends on the nature of the starting ester. This work highlights the valuable implementation of free NHC species as key intermediates in synthetic chemistry, beyond their classical use as stabilizing ligands or organocatalysts.

Polycondensation of Hexamethylenetetramine in Anhydrous Acid Media as a New Approach to Carbyne-Like Materials and Its Application as Dispersant of Carbon Materials

Aminocumulene (AC), a technical name that results from the hexamethylenetetramine polycondensation in anhydrous sulfuric acid, was successfully performed toward the synthesis of oligomeric cumulenic compounds made up of carbyne-like (=C=C)x fragments as a main moiety together with amino endcapping groups. The tentative mechanism for the synthesis of the soluble and insoluble AC likely included the participation of carbene species as an intermediate. Spectral properties obtained from UV-Vis, infrared, and Raman spectroscopies, and surface chemistry analysis through X-ray photoelectron spectroscopy of the soluble AC confirmed the presence of a mixture of linear and branched aminocumulene-based oligomers. AC displayed high performance as a dispersant and stabilizer agent of both multi-walled carbon nanotubes and graphene in aqueous solutions after powerful insonation treatment under controlled temperature compared to the most commonly used dispersant agents. Thereby, AC is vitally important for the preparation of carbonaceous materials based on nanoinks in a wide variety of fields.

Unexpected Rearrangement of Dilithiated Isoindoline-1,3-diols into 3-Aminoindan-1-ones via N-Lithioaminoarylcarbenes: A Combined Synthetic and Computational Study.

The reaction of 2-aryl-3-hydroxyisoindolin-1-ones with the s-BuLi·TMEDA system in THF at -78 °C, affording a series of diastereomeric 3-aminoindan-1-ones via a novel rearrangement of the isoindolinone scaffold, is reported. It is proposed that α-elimination of LiOH from the transient N,O-dilithiated hemiaminal carbenoids leads to the formation of singlet carbenes followed by their trapping via an intramolecular C-H insertion. An alternative explanation based on an intramolecular Mannich reaction seem much less probable. A mechanistic-type study that combines spectroscopic data of the products and calculation results, with a special focus on the diverse lithiated intermediates that are most likely to engage in the title process (particularly those with internal Li bonds), is presented. The MP2 approach, including the NPA and QTAIM data, provided insight into structures and properties of all these species. Two reaction routes A and B appeared to be possible for the postulated carbene mechanism. An unusual metamorphosis of the CCN atom triad, from a near sp 1-azaallene-type in more stable noncarbene Li enolates to a roughly sp2 type in their carbene keto tautomers, is recognized in one of these pathways (route B). Dominant forms of resonance structures for the aforementioned tautomeric systems that have seven-membered quasi rings stabilized by Li+ ions bridging the N and carbonyl O atoms are indicated. Large computational difficulties arising from a huge impact of internal Li+ complexation on conformational preferences and electronic properties of carbonyl group-bearing lithium derivatives are also discussed. The new γ-keto carbene species under study belong to a subclass of acyclic aminoarylcarbenes.

Rhenium-Catalyzed Cyclization via 1,2-Iodine and 1,5-Hydrogen Migration for the Synthesis of 2-Iodo-1H-indenes.

A rhenium complex catalyzed the formation of 2-iodo-1H-indene derivatives through iodine and hydrogen migration of 3-iodopropargyl ethers. The reaction proceeded via generation of 1-iodoalkenylrhenium carbene species by sequential 1,2-iodine and 1,5-hydrogen shifts with readily available precursors under neutral conditions. The reaction mechanism and the reactivity of the generated alkenylcarbene species were also investigated.

Cu(I)-Catalyzed Three-Component Reaction of α-Diazo Amide with Terminal Alkyne and Isatin Ketimine via Electrophilic Trapping of Active Alkynoate-Copper Intermediate.

A three-component reaction between terminal alkyne, α-diazo amide, and isatin ketimine is realized by utilizing copper catalysis. A series of alkynyl-containing 3,3-disubstituted oxindoles were synthesized with high efficiency and diastereoselectivity under mild reaction conditions. This transformation is proposed to proceed through a Mannich-type trapping of an alkynoate copper intermediate derived from terminal alkyne and copper carbene species.