Fischer Carbene Complexes Research Articles

Metal‐Mediated Synthesis of a Mixed Arduengo‐Fischer Carbodicarbene Ligand

Carbodicarbenes are strong C‐donor ligands, which have found numerous applications in organometallic and main group element chemistry. Herein, we report a structurally distinct carbodicarbene ligand, which is formed by dinitrogenative coupling of a Fischer carbene complex with an N‐heterocyclic diazoolefin. The resulting carbonyl complex serves as a stable source for the mixed Arduengo‐Fischer carbodicarbene ligand. Facile ligand transfer reactions were demonstrated to occur with gold(I), copper(I), palladium(II), and rhodium(I) complexes.

Metal-Mediated Synthesis of a Mixed Arduengo-Fischer Carbodicarbene Ligand.

Carbodicarbenes are strong C-donor ligands, which have found numerous applications in organometallic and main group element chemistry. Herein, we report a structurally distinct carbodicarbene ligand, which is formed by dinitrogenative coupling of a Fischer carbene complex with an N-heterocyclic diazoolefin. The resulting carbonyl complex serves as a stable source for the mixed Arduengo-Fischer carbodicarbene ligand. Facile ligand transfer reactions were demonstrated to occur with gold(I), copper(I), palladium(II), and rhodium(I) complexes.

N‐Tosylhydrazones as [N,N] Synthons in Heterocyclic Chemistry: Synthesis of 3,5‐Disubstituted N‐Alkenyl‐1H‐pyrazoles

AbstractA new mode of reactivity of N‐sulfonylhydrazones towards alkynes is described: ketone‐derived N‐tosylhydrazones (NTHs) bearing α‐H atoms behave as [NN] synthons in their reactions with alkoxy alkynyl Fischer carbene complexes (FCCs) and only the two N atoms are incorporated into the newly formed pyrazole ring. The scope of the reaction, leading to 5‐methoxy‐N‐alkenyl pyrazoles, proved to be wide regarding both partners; thus, a variety of aryl and primary, secondary, and tertiary alkyl substituents are tolerated within the FCC, while aryl alkyl NTHs, symmetric and asymmetric dialkyl NTHs and NTHs derived of cyclic ketones can be also employed. In addition, the reaction can be performed at large scale and applied for late‐stage diversification of natural products. A reasonable reaction mechanism is also proposed, which is supported by deuteration experiments and DFT calculations.

Catalytic synthesis of β-lactam derivatives by carbonylative cycloaddition of acylsilanes with imines via a palladium Fischer-carbene intermediate

AbstractFischer-type carbene complexes are characterized by the presence of a π-donating group, such as an alkoxy group on the carbene carbon. Despite the notable progress that has been made in synthetic methods that involve the use of Fischer-type carbene complexes, stoichiometric amounts of carbene complexes are still required for such reactions and catalytic variants remain elusive. This limitation primarily stems from the lack of suitable carbene precursors, which is in sharp contrast to the fact that carbene complexes bearing an electron-withdrawing group can be readily generated from the corresponding diazo esters. Here we report that acylsilanes can function as a precursor for a Fischer-carbene complex by the action of a palladium catalyst. This system can be used in catalytic carbonylative cycloaddition reactions with imines to form densely substituted β-lactam derivatives. A key siloxycarbene–palladium intermediate complex was isolated and successfully characterized by X-ray crystallography.

Synthesis of Fluorescent Pyrrolo[1,2-a]pyrimidines from Fischer Carbene Complexes as Building Blocks.

A novel synthetic methodology is reported for the synthesis of fluorescent pyrrolo[1,2-a]pyrimidines. Fischer carbene complexes served as the synthetic platform for (3+3) cyclization to form the heterocyclic moiety. The reaction process furnished two products, their ratio being modulated by the metal, base, and solvent. The selectivity exhibited was studied by analyzing the potential energy surface with density functional theory tools. The photophysical properties of absorption and emission were also evaluated. The dyes absorbed at wavelengths of 240-440 nm, depending on the substituents. The maximum emission wavelength was in the range of 470-513 nm, with quantum yields of 0.36-1.0 and a high Stokes shift range of 75-226 nm.

Electron‐Rich Oxycarbenes: New Synthetic and Catalytic Applications beyond Group 6 Fischer Carbene Complexes

AbstractOxycarbenes have emerged as useful intermediates in synthetic chemistry. Compared to the widely studied oxycarbene metal complexes bearing Group 6 metals, the synthetic and catalytic applications of oxycarbenes beyond Group 6 Fischer carbene complexes are less explored because of the difficulty in controlling their reactivity and the need to use a stoichiometric amount of a presynthesized Group 6 metal carbene complex as the starting material. This Minireview summarizes early synthetic and catalytic applications of late‐transition‐metal oxycarbene complexes and highlights recent advances in free oxycarbene reactions and transition‐metal‐catalyzed reactions involving oxycarbenes. We hope this Minireview will inspire further developments in this emerging area.

Electron-Rich Oxycarbenes: New Synthetic and Catalytic Applications beyond Group 6 Fischer Carbene Complexes.

Oxycarbenes have emerged as useful intermediates in synthetic chemistry. Compared to the widely studied oxycarbene metal complexes bearing Group 6 metals, the synthetic and catalytic applications of oxycarbenes beyond Group 6 Fischer carbene complexes are less explored because of the difficulty in controlling their reactivity and the need to use a stoichiometric amount of a presynthesized Group 6 metal carbene complex as the starting material. This Minireview summarizes early synthetic and catalytic applications of late-transition-metal oxycarbene complexes and highlights recent advances in free oxycarbene reactions and transition-metal-catalyzed reactions involving oxycarbenes. We hope this Minireview will inspire further developments in this emerging area.

Synthesis of Polysubstituted Symmetrical BODIPYs via Fischer Carbene Complexes: Theoretical, Photophysical and Electrochemical Evaluation.

A series of new symmetrical highly substituted BODIPYs 6 a-l was synthesized through a prefunctionalization approach in 35 %-89 % yields from the pyrrole core. This strategy allowed modulation of the substituents at the different positions based on the choice of Fischer's alkynyl carbenes, oxazolones and aldehydes used as precursors. The substituent variation at positions 2, 6, 3 and 5 had the greatest effect on the modulation of their photophysical properties such as absorption (λabs ) and emission (λem ) wavelengths, extinction coefficient (ϵ), quantum yields (ϕ), Stokes shifts (Δν), fluorescence decay, radiative (krad ) and non-radiative (knr ) constants and the CIE 1931 coordinates. Theoretical calculations allowed to corroborate the effect of the substituents of meso-position on the modification of the dihedral angles. Cyclic voltammetry studies revealed that the BODIPY series presents similar redox potential behavior, being electrochemically active even in successive cycles, which suggests that transport by diffusion is the dominant process.

Schrock‐TypeSilylidenesandGermylidenesFound Among the Silylene and Germylene Complexes of the Early and Mid‐Transition Metals

AbstractThe chemistry of Schrock‐type silylidene/germylidene complexes is the subject of this review. This is preceded by the general consideration of the prototypical transition metal carbene complexes and their classification as either Fischer or Schrock carbene complexes. Following this background, silylene/germylene complexes of the early (groups 3–4) and mid‐ (groups 5–7) transition metals are briefly discussed in terms of their synthesis and nature of the transition metal‐silylene/germylene bonding interaction. Major attention will be paid for the preparation, bonding nature, and particular reactivity of those complexes, which, on the basis of their spectroscopic, structural, and computational studies, are reliably categorized as the Schrock‐type silylene/germylene transition metal complexes, that is silylidenes/germylidenes.

Fischer and Schrock carbene complexes in the light of global and local electrophilicity‐based descriptors

AbstractThe carbon atom (carbene) of Fischer and Schrock complexes are electrophilic and nucleophilic, respectively. The reactivity index electrophilicity is a global reactivity parameter and can tell only about the total electrophilicity of the complexes. To differentiate between the reactivity patterns of these two carbenes, the philicity and multiphilic descriptor are calculated. In Fischer complexes, it is found that the philicity of the nucleophilic attack ( ) is higher than that of philicity of the electrophilic attack ( ) implying the electrophilic nature. A reverse order is found in the Schrock complex pointing nucleophilic character. The multiphilic descriptor (ΔωC = − ) is found to be positive in Fischer but negative in Schrock leading to the same conclusion. Fischer carbene complexes having general formula (CO)5Cr═CH‐R (R = CH3, Ph, CCH, CH═CH2, OCH3, OH, NHCH3, and NH2) the order of and ΔωC better describe the trend. The trend has been justified through energy decomposition in the purview natural orbital for chemical valence (EDA‐NOCV) analysis owing to the π contribution from the R group. The change in the reactivity patterns along the intrinsic reaction coordinate of two representative reactions is plotted. This way of understanding the reactivity parameters would help experimental chemists to predict the catalytic application of carbene complexes of transition metal without the classification of Fischer and Schrock type.

Design of a novel Fischer carbene complex which can facilitate thiol mediated site-specific protein immobilization

A novel Fischer carbene complex has been synthesized which has tuneable reactivity owing to its dual functionality to facilitate binding with peptides and proteins via thiol moiety in presence of the amine groups of abundant lysine residues. The Fischer carbene has been so designed that it can be made the reactive terminus of a self-assembled monolayer (SAM) via Click chemistry or cross-metathesis, with retention of its activity towards thiols. Hence, using such a Fischer carbene terminated SAM, proteins may be site-specifically immobilized on surfaces, under mild condition.

Fischer carbene complexes of cobalt(I): Synthesis and structure

Three different aryl substrates thiophene (ThH), ferrocene (FcH) and p-bromodimethylaniline (p-DMABr) were lithiated and reacted with [Co(CO)4SnPh3], according to the Fischer carbene protocol. The products [CoSnPh3(CO)3{C(OEt)R}] (R =Th, 1, p-DMA, 2 and R = Fc, 3) were analysed to unravel the role of the aryl carbene substituent in stabilizing intermediates and final products. The ethoxy carbene complexes were aminolysed by in situ generated HNMe2 to afford [CoSnPh3(CO)3{C(NMe2)R}] (R =Th, 4, p-DMA, 5) and with N,N-dimethylethylenediamine [CoSnPh3(CO)3{C(NHCH2CH2NMe2)R}] (R = p-DMA, 6) in high yields. The trigonal Co(CO)3 in the equatorial plane is very stable and efforts to displace a carbonyl in 6 was unsuccessful, both by heating and irradiation. The role of the aryl carbene substituents in stabilizing the electrophilic carbene carbon was investigated and studied by NMR spectroscopy in solution and single crystal X-ray structure determinations.

Roles played by carbene substituents during ligand transfer reactions between tungsten fischer carbene complexes and [Pt(COD)Cl2

Fischer carbene ligand transfer reactions from [W{C(X)(C6H4‐4‐R}(CO)5] (X = OEt: a series; X = NMe2: b series), containing remote tertiary amino substituents R = Rʹ2N at the phenyl ring, to platinum(II) of [Pt(COD)Cl2] precursors, are studied. The number of carbene ligands transferred per platinum ion in these cases are determined by the electronic and steric properties of the heteroatoms of the carbene ligand. Thus, neutral bis(carbene) complexes, [Pt{C(X)(C6H4‐4‐R)}2Cl2], (R=H (1a); R=NRʹ2 and Rʹ= Me (2a), Ph (3a), or 4-BrC6H4 (4a)), are formed from the ethoxycarbene precursors (X = OEt), while cationic tris(carbene) complexes [Pt{C(X)(C6H4‐4‐R)}3Cl]+ Z−, (R=H (1b) and R=NRʹ2 and Rʹ= Me (2b), Ph (3b), or 4-BrC6H4 (4b)) are obtained from the aminocarbene precursors (X = NMe2), the latter with different counter ions Z– = Cl–, [W(CO)5Cl]– or PF6–. Electro- and spectroelectrochemical studies indicate consecutive oxidations of the individual carbene ligands, but also a lack of electronic interactions across the (X)C=Pt=C(X) linkages.

Azacycle-Directed Formal Aromatic C(sp2)–H Insertion with Cr(0) Fischer Carbene Complex via Oxidative Hydrogen Migration

Azacycle-Directed Formal Aromatic C(sp²)–H Insertion with Cr(0) Fischer Carbene Complex via Oxidative Hydrogen Migration

Frontispiece: Fischer Carbene Complexes: A Glance at Two Decades of Research on Higher‐Order Cycloaddition Reactions

The frontispiece shows the transformation of Fischer Carbene Complexes in a wide variety of compounds through of Higher-order Cycloadditions. The flask, containing the carbene, pour on the paper the different products synthesized in the last 20 years. In the background, the blackboard draws traces of important contributions in the area, such as the orbital correlation diagrams for Higher-order Cycloadditions and the representation of molecular orbitals made by Woodward and Hoffmann, that established the bases in this research area. For more details see the Review by M. A. Vázquez et al. on 8233 ff.

Fischer Carbene Complexes: A Glance at Two Decades of Research on Higher-Order Cycloaddition Reactions.

The structure of Fischer carbene complexes (FCCs) is electron deficient. If bearing an α,β-unsaturated system, it can generate a wide variety of compounds by undergoing many different transformations, including higher-order cycloadditions. The latter are described as pericyclic reactions in which more than six electrons participate. These reactions have been employed in various areas of organic synthesis, resulting in highly selective compounds with a broad range of scaffolds. The first studies on higher-order cycloadditions involving FCCs frequently yielded competing byproducts. Many groups have attempted to increase selectivity by exploring distinct reaction conditions, reagents and co-catalysts (e. g., metal-mediated cycloadditions). The present review is the first to focus exclusively on using higher-order cycloadditions involving FCCs to synthesize carbocycles and heterocycles. Based on two decades of reports, an analysis is made of the main aspects of the mechanisms proposed for higher-order cycloadditions and the structural diversity obtained by the substituent effect.

The Effect of Remote Donor Substituents on the Properties of Alkoxy and Amino Fischer Carbene Complexes of Tungsten

AbstractTungsten Fischer ethoxy‐ and dimethylaminocarbene complexes [W{C(X)(C6H4‐4‐R}(CO)5] (X=OEt: series a; or X=NMe2: series b) are synthesized from phenyl substrates containing remote tertiary amino substituents R’2N with R’=Me (2), Ph (3) or C6H4Br‐4 (4). The π‐delocalization and carbene‐stabilizing effects of the distant tertiary amine donor substituent are investigated by NMR and IR spectroscopy, electrochemistry and quantum chemical calculations. A significant transfer of electron density from the remote 4‐R’2N substituent to the carbene C atom in the ethoxycarbene complexes is supported by NMR data and the solid‐state structure of 2 a. This is strongly attenuated in the amino‐substituted carbene complex 2 c and irrelevant in the dimethylaminocarbene complexes of series b, where the electron demand of the electrophilic carbene center is satisfied by the directly attached dimethylamino substituent. Quantum chemical calculations and IR spectroelectrochemistry on complexes 1 a‐‐4 a verify carbene‐centered reductions and a ligand based oxidation of complex 2 b.

Synthesis and structure of thienyl Fischer carbene complexes of PtII for application in alkyne hydrosilylation

Efficient alkyne hydrosilylation mediated by well-defined platinum(ii) bis(ethoxycarbene) complexes of the Fischer-type prepared by facile transmetallation.

The DFT study on pentannulation reaction of tungsten Fischer carbene complexes

One of the most valuable reactions in organometallic chemistry is the coupling of the Fischer carbene complexes with alkynes to furnish five or six membered rings among other products. However, the mechanism of the reaction is still controversial due to its complexity. In this work, a plausible mechanistic pathway of the annulation reaction between the [(CO)5W=C(OMe)Ph] and PhC≡CPh was investigated with the meta-GGA exchange-correlation functionals TPSS-D3(BJ) and a def2-TZVP basis set to examine two possible mechanistic pathways that lead to the five and six membered aromatic rings, respectively. It was found that the minimum energy pathway leads to the cyclopentannulation product rather than a phenol in accordance with the experiment.

Synthesis and structure of annulated dithieno[2,3-b;3ʹ,2ʹ-d]thienyl- and ring-opened 3,3ʹ-bithienyl Fischer carbene complexes

Nucleophilic attack on the central sulphur of dithieno[2,3-b;3ʹ,2ʹ-d]thiophene (DTT) by n-BuLi opens the central thiophene ring and afforded, after subsequent reaction with Cr(CO)6 and alkylation with [Et3O][BF4], a series of mono- and biscarbene complexes containing a 3,3ʹ-dithienyl backbone with a SBu substituent. Repeating the reaction with diisopropylamine as the nucleophile, leads to a dihydrodesulphurization reaction with ring-opening of the central thiophene ring of DTT and elimination of the sulphur atom. Subsequent reaction with n-BuLi or LDA, Cr(CO)6 and [Et3O][BF4] afforded 3,3ʹ-dithienyl mono- and biscarbene complexes. In both instances α,αʹ-dithienothiophene biscarbene complex was observed spectroscopically but not isolated. By using α,αʹ-dibromodithieno[2,3-b;3ʹ,2ʹ]thiophene as substrate, improved yields of the targeted mono- and biscarbene complexes of [2,3-b;3ʹ,2ʹ-d]-DTT (M=Cr, W) could be obtained. The biscarbene complexes were unstable in the reaction mixture but in the case of tungsten could be isolated after in situ aminolysis with dimethylamine. The use of KHMDS as base converted Cr(CO)6 to K[Cr(CO)5(CN)] and after reaction with DTT and subsequent alkylation with [Et3O][BF4], afforded the chromium tetracarbonyl carbene complex of DTT.