Fischer-type Carbene Complexes Research Articles

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.

Transformation of Acetylene to Ethenylidene, Carbene, Acetylide, Vinyl, and Olefin Groups with Cp*Fe(1,2-Cy2PC6H4S).

Tautomerization of C2H2 at half-sandwich compound Cp*Fe(1,2-Cy2PC6H4S) exclusively produces an iron ethenylidene, Cp*Fe(=C=CH2)(1,2-Cy2PC6H4S) (2). Protonation of the ethenylidene causes nucleophilic attack of the Cα by sulfur, affording a sulfur-tethered carbene complex, [Cp*Fe=C(CH3)SC6H4PCy2]+ (3+). This Fischer-type carbene complex undergoes an unusual isomerization by migrating a hydrogen atom from the β-CH3 group to the α-C, leading to the formation of an olefin complex [Cp*Fe(η4-CH=CH2SC6H4PCy2]+ (4+). Compound 2 also displays diverse redox reactivities. It transforms to a neutral acetylide ferric complex (5) when reacting with free radical scavengers and to a cationic vinyl complex [Cp*Fe(η3-C(=CH2)SC6H4PCy2]+ (6+) upon 1e- oxidation. The interconversion between the vinyl and acetylide complexes can be realized through protonation/deprotonation reactions.

Understanding exo-selective Diels-Alder reactions involving Fischer-type carbene complexes.

The factors controlling the selectivity of the Diels-Alder cycloaddition reactions involving Fischer-type carbene complexes and cyclopentadiene have been explored computationally by means of density functional theory calculations. To this end, the influence of the substituents directly attached to the carbene ligand on the endo : exo ratio has been compared to the available experimental data and quantitatively analysed in detail by means of the combination of the activation strain model of reactivity and energy decomposition analysis methods. The insight gained in this computational study may be important for the rational design of exo-selective Diels-Alder reactions.

Dative and electron-sharing bonding in transition metal compounds.

Quantum chemical calculations using density functional theory at the BP86-D3(BJ)/def2-TZVPP level of theory are reported for transition metal compounds [TM]-L in high and low oxidation states involving carbene, carbyne, alkene, and alkyne ligands L. The nature of the [TM]-L bond is analyzed with the energy decomposition analysis - natural orbitals for chemical valence (EDA-NOCV) method. The calculations reveal that transition metal compounds with ligands, that are typically classified as donor-acceptor complexes possessing dative bonds (Fischer-type carbenes and carbynes, alkene, and alkyne complexes) or as TM compounds with electron-sharing bonds (Schrock-type carbenes and carbynes, metallacyclopropanes, and metallacyclopropenes), exhibit significant differences between the orbital interactions when closed-shell or open shell fragments are used. Fischer-type carbene complexes have much lower orbital interaction (ΔEorb ) values when singlet fragments are employed compared to triplet fragments. In contrast, singlet and triplet fragments of Schrock-type carbene complexes give similar ΔEorb values. The best description for Fischer-type carbyne complexes is found for neutral fragments in their electronic doublet state, which engage in a mixture of dative bonding (σ donation and π backdonation) and one electron-sharing π bond. The EDA-NOCV calculations of Schrock-type carbynes using open-shell species in their quartet electronic state give similar ΔEorb values as neutral fragments in their electronic doublet state. Alkene and alkyne complexes, but also metallacyclic species, are best described with singlet fragments, but the difference between the ΔEorb values for dative bonding and electron-sharing bonding using triplet fragments becomes much smaller for molecules that are considered as metallacycles. © 2018 Wiley Periodicals, Inc.

José Barluenga (1940–2016)

José Barluenga, emeritus professor at the University of Oviedo, passed away on September 7, 2016. Barluenga was one of the leading Spanish chemists of his generation, with research interests including metal-mediated activation of alkenes and alkynes, β-functionalized organolithium compounds, iodination reactions, asymmetric Diels–Alder reactions, carbene complexes, and cross-coupling reactions. Early studies from Barluenga's laboratory pioneered the field of metal-mediated activation of alkenes and alkynes towards nucleophiles. He also documented the relevant role of azadienes as precursors for nitrogen-containing organic molecules. His fascination with this class of compounds derived from his postdoctoral work and remained a central theme throughout his initial scientific career. His studies on β-functionalized organolithium compounds brought this elusive and highly reactive class of reagents into the mainstream of organic synthesis. In the mid-1980s, Barluenga prepared bis(pyridine)iodonium(I) tetrafluoroborate IPy2BF4, which is nowadays known as Barluenga's reagent. In addition to establishing the synthetic merit and potency of this reagent in different iodination strategies, he initiated incursions into iodination as a tool for “green” carbon–carbon and carbon–heteroatom coupling reactions. His design of new enantiopure 2-aminodienes culminated in the demonstration that asymmetric Diels–Alder reactions can be accomplished by locating chiral information at the diene. Following his characteristic desire to constantly push the existing boundaries of reactivity, Barluenga then moved on to the application of Fischer-type carbene complexes of Group 6 metals, including the use of nonstabilized carbene complexes to produce precursors for molecular materials. In his final years, he became interested in new concepts for cross-coupling transformations. Here, he explored the potential of palladium-catalyzed C–N coupling reactions to trigger complex synthetic cascades, often directed to the preparation of privileged heterocyclic scaffolds. Finally, he introduced N-tosylhydrazones as a safe alternative to diazo compounds for enabling unconventional metal-free coupling reactions. Barluenga received many national and international awards and distinctions. He was enthusiastically involved in the education of a total of 120 graduate students and a large number of postdoctoral workers, and proudly followed their careers. More than fifteen of his former co-workers have become full professors at various Spanish universities. Barluenga advanced the development of the chemistry community in Spain, and was a strong public advocate of academic research. His highly productive and prominent career comprises more than 600 publications and took place concurrently with the advent of Spanish science during the latter part of the 20th century. He was largely responsible for the reputation gained by the Spanish chemical community, while acting as a charismatic ambassador for the new generation of young Spanish scientists in his frequent visits to top universities, industrial research laboratories, and at conferences worldwide. Above all, he was always at home in his laboratories, where he displayed his never-ending enthusiasm for research, which he liked to refer to as his particular “journey to Ithaca”. José Barluenga was a proud family man and always treasured the free time he spent with his wife and children. Only his devotion to his family could partially interrupt the exhaustive dedication to science by this uniquely passionate, generous, and gifted chemist. His physical presence will be sadly missed, but his scientific legacy and his personality, commitment, and talent will continue to be remembered by all who had the privilege to interact with him.

Photoinduced Reactions of Diruthenium Tetrahydride Complexes: Carbon–Hydrogen Bond Cleavage of Tetrahydrofuran Leading to Bridging Cyclic Fischer-Type Carbene Complexes

Photochemical reactions of diruthenium tetrahydride complexes containing cyclopentadienyls as auxiliary ligands, CpsRu(μ-H)4RuCps (1) (a: Cps = C5Me5 (Cp*), b: C5EtMe4 (CpEt), and c: 1,2,4-C5(t-Bu)3H2 (Cp‡)), with tetrahydrofuran were studied for elucidation of the reaction mode of dinuclear complexes containing multiple bridging hydrides. Complexes 1a–c reacted with tetrahydrofuran under UV irradiation (365 nm) to produce bridging cyclic Fischer-type carbene complexes, {CpsRu(μ-H)}2(μ-cyclo-CCH2CH2CH2O−)(μ-H)2 (5), via consecutive C–H bond cleavage at the α-position of the oxygen atom. The characteristic cyclic structures of the bridging carbene ligands were unambiguously confirmed by X-ray diffraction studies. Line-shape analysis of the variable-temperature 1H NMR spectra of 5c demonstrated the rotation of the bridging carbene ligand.

ChemInform Abstract: Fischer Carbene Complexes Remain Favourite Targets, and Vehicles for New Discoveries

AbstractReview: 117 refs.

Ring-opening and double-metallation reactions of the N-Heterocyclic carbene ligand in Cp∗(IXy)Ru (IXy = 1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene) complexes. Access to an anionic fischer-type carbene complex of ruthenium

An N-heterocylic carbene (NHC) ring-opening reaction was observed upon treatment of the silyl complex Cp∗(IXy-H)(H)RuSiH2Mes [1; IXy=1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene; IXy-H=1-(2-CH2C6H3-6-methyl)-3-(2,6-dimethylphenyl)imidazol-2-ylidene-1-yl (the deprotonated form of IXy); Cp∗=η5-C5Me5] with LiCH2SiMe3. This reaction results in formation of a novel, anionic Fischer-type carbene complex Cp∗(H)Ru{κ2-CHN(Xyl)CH[Si(CH2SiMe3)Mes]N(Xyl)Li} (4). This ring-opening reaction demonstrates a new pathway for NHC degradation via the cooperation of ruthenium, silicon, and lithium. Additionally, treatment of the dinitrogen complex Cp∗(IXy-H)Ru(N2) with LiCH2SiMe3 led to the doubly-metallated complex [Cp∗(IXy-2H)Ru]Li (5), which exhibits a solid-state polymeric structure and metallation of both xylyl groups of the IXy ligand. Finally, removal of a hydride from 4, achieved with two equiv of B(C6F5)3, led to C–H activation of the Cp∗ ligand and formation of an unusual, formally dianionic η5-Me4C5CH2B(C6F5)3 ligand in a ruthenium carbene complex [η5-Me5C5CH2B(C6F5)3](H)Ru{κ3-CHN(Xyl)CH[SiH(CH2SiMe3)Mes]N(Xyl)} (7).

Fischer carbene complexes remain favourite targets, and vehicles for new discoveries.

Exciting new variations in Fischer-type carbene complex composition and reactivity have been realised by following or modifying well-established synthetic approaches such as metal carbonyl functionalization and modification of existing carbene ligands. The formation of targeted complexes for organic synthesis, carbene-containing chelates, and polynuclear carbene complexes, by employing 'click chemistry', warrants discussion. Transmetallation and α,α-dehydrogenation of ethers and amines have come into their own as viable synthetic methods to access carbene complexes with unique properties and activities. Successful mediation of carbene complex formation with pincer ligands has proved its worth. Quantum chemistry has become essential for supporting or initiating mechanistic proposals, but heuristic approaches such as invoking the vinylology principle to describe substituted phenylcarbene complexes are still valuable in the interpretation of bonding properties and the classification of complex types. Electrochemical studies now also constitute a powerful part of the experimental characterization tool kit.

Synthesis and characterization of novel nitroxide organometallic Fischer-type carbene complexes

Synthesis and characterization of novel nitroxide organometallic Fischer-type carbene complexes

Synthesis and characterization of novel nitroxide organometallic Fischer-type carbene complexes

Reactions of 4-amino TEMPO with alkynylmethoxy Fischer type carbene complexes lead to unknown transition organometallic nitroxide spin labels. According to the temperature and the nature of the aromatic group of the initial alkynyl carbene, Michael adducts and aminolysis products were obtained in various proportions. The reaction was extended to ferrocenyl carbene complexes to obtain novel organometallic spin systems containing two kinds of redox units. Their constitution is in agreement with IR, elemental analysis and mass spectrometry. Single X-ray diffraction has confirmed the molecular structure of the complexes and has showed the existence of intra and intermolecular hydrogen bonds. These air-stable paramagnetic carbenes have interesting redox behaviours and paramagnetic properties.

Theoretical characterization of a Ru N-Heterocyclic Carbene derivative of a polyoxometalate. Enhanced π-interaction in oxide supported TM-organic linkages

The electronic structure of the N-heterocyclic carbene (NHC) functionalized polyoxometalate [K(PW(9)O(34))(2)(cis-WO(2))(cis-RuLMe(2))](12-) (1) is studied computationally; and its Ru-NHC bonding situation is compared with other monometallic Ru-NHC complexes, as well as other representative carbene complexes. The DFT analysis further supports the low spin configuration, S = ½, as the ground state, as the unoccupied d ruthenium orbitals are high in energy and mixed with the d-type orbitals of W. We gauge the TM-carbene bonding nature by means of atoms in molecules (AIM) quantum theory and a modified version of energy decomposition analysis (EDA) based on orbital deletion. According to the calculations, the NCH ligand in POM derivative 1 is more tightly bonded than in its analogous neutral monometallic Ru-NHC complexes. This is manifested in a higher electron density ρ(r) at the bond critical point (bcp) between Ru and the carbene carbon atoms. The energetic contribution of σ-donation is similar for all Ru-NHC complexes, and significantly higher than for Fischer-type carbene complex, in agreement with the description of the N-heterocyclic carbene ligand as a strong σ-donor. Among the studied Ru-NHC complexes, the POM derivative 1 shows the largest π-back-donation ability, explaining the observed short Ru-C(carbene) bond distance and the calculated high interaction energy. The POM framework acting as ligand makes the ruthenium d electrons more loose via orbital mixing and promotes the back-donation to the ligand.

ChemInform Abstract: Carbene‐Transition Metal Complexes Formed by Double C—H Bond Activation

AbstractReview: [65 refs.]

ChemInform Abstract: A Protective Strategy in Carbene Complex Chemistry. Synthesis of Functionalized Fischer Carbene Complexes via Dianion Formation.

Anionic derivatives of Fischer-type carbene complexes (A and B) are stable toward strong bases and nucleophiles, and their organic moieties R and R 2 can be elaborated under strongly basic conditions to provide access to heteroaromatic and aliphatic carbene complexes bearing organic functional groups. The concept of anionic protection has been illustrated by the reactions of dianions 9 and 14 and the preparation of carbene complexes possessing up to four chiral centers. Some attempts to effect ortho lithiation of aromatic carbene complexes are also described

ChemInform Abstract: Stereoselective Synthesis of (E) and (Z)-Vinylsilane by the Addition Reaction of Organometallic Reagent to Silyl-Substituted Fischer-Type Carbene Complex.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Thiocarbarmoyl-Assisted Formation of a New Type of α- and β-Isomer Paddlewheel Pd24+ Compounds. Synthesis, Characterization, and Crystal Structures of [Pd(η1-SCNMe2)(η2-Tp)(PPh3)], [Pd2(μ-Hdppa)2(μ-SCNMe2)2][Cl]2, [Pd2(μ-dppa)2(μ-SCNMe2)2], and [Pd{η2-S2P(OEt)2}]2(μ-dppa)(μ-SCNMe2)

The reaction of the doubly thiocarbamoyl-bridged boat-form complex [Pd(PPh3)(Cl)]2(μ-SCNMe2)2, 1, with dithio (SS) and tris(pyrazoyl-1-yl)borate (Tp) ligands produces the monomer η1-thiocarbamoyl complexes [Pd(PPh3)(η1-CSNMe2)(η2-SS)] (SS = Et2NCS2, 2a; (EtO)2PS2, 2b) and [Pd(PPh3)(η1-CSNMe2)(η2-Tp)], 3, respectively. Treatment of 1 with dialkyldithiocarbonate, ROCS2−, yields Fischer-type carbene complexes [Pd(PPh3){η1-C(SR)(NMe2)}(η2-S2CO)] (R = Me, 4a; Et, 4b). The carbene complexes 4a and 4b are formed via alkyl migration of the alkyldithiocarbonate ligand to the thiocarbamoyl ligand. The organometallic α-isomer paddlewheel-type Pd24+ dipalladium complex without axial ligation, [Pd2(μ-Hdppa)2(μ-SCNMe2)2][Cl]2, 5, is prepared by the reaction of Hdppa (Hdppa = bis(diphenylphosphino)amine) with 1. Treatment of 5 with anionic oxygen, nitrogen, or carbon reagents (KOH, NaN3, NaNH2, NaCCH), instead of forming axial ligation complex, yields the deprotonation of the Hdppa ligand of 5 to obtain the α-isomer pad...

Reactivity of Cationic Dinitrosyl Bisphosphine Rhenium Complexes toward Acetylene: Base-Controlled Product Formation

Reactions of the dinitrosyl bisphosphine rhenium cations [Re(NO)2(PR3)2][BArF4] (R = Cy 1a; R = iPr 1b; [BArF4]− = tetrakis[(3,5-bis(trifluoromethyl)phenyl)borate]) with acetylene yielded the alkynyl (o-vinyl)hydroxylamido nitrosyl bisphosphine complexes [Re(CH═C(H)ONH)(C≡CH)(NO)(PR3)2][BArF4] (3a and 3b) in the absence of base, while in the presence of 2,6-di(tert-butyl)pyridine formation of the neutral alkynyl complexes [Re(C≡CH)(NO)2(PR3)2] (4a and 4b) was observed. A plausible mechanism is presented and supported by various NMR techniques, IR spectroscopy, and DFT calculations. Treatment of 4a and 4b with [H(OEt2)2][BArF4] gave the corresponding vinylidene complexes [Re(C═CH2)(NO)2(PR3)2][BArF4] (5a and 5b) in high yields, which only slowly convert into the alkynyl(o-vinyl)hydroxylamido nitrosyl bisphosphine complexes 3a and 3b when treated with a large excess of acetylene. Thiophenol or benzeneselenol underwent 1,2-additions onto the vinylidene ligands of 5a and 5b to yield the cationic Fischer-type carbene complexes [Re(C(XPh)CH3)(NO)2(PR3)2][BArF4] (X = S 6a,b; X = Se 7a,b) quantitatively. X-ray diffraction studies were carried out on 4b, 5b, and 7a.

Ligating properties of anionic Fischer-type carbene complexes, [(CO)5MC(X)Y

With the simplest of anionic Fischer-type carbene complexes acting as ligands, Cp(2)Zr(Cl){OCMe}M(CO)(5) compounds (M = Cr or W) promote alpha-olefin oligomerization and polymerization in the presence of MAO. Attaching an N-heterocyclic ring to the carbene carbon atom in similar precursors, allows a variety of hard metal ions and fragments to be captured by external bidentate coordination. The outcome of the attachment of a phosphorus or sulfur functionality to an alpha-carbon of an O-anionic carbene is formation of a bidentate ligand and then internal four-membered carbene-heteroatom chelate formation. alpha-Deprotonated carbene complexes are also precursors for remote, one-N, six-membered carbene complexes of various metals whereas alpha-C-, alpha-N- or alpha-O-deprotonated as well as beta-deprotonated Fischer-type carbene complexes display unique synthon properties towards Ph(3)PAu(+) and partake in unusual ensuing coordination of liberated group 6 metal carbonyl moieties to form dinuclear products.

Computational and experimental tools in solving some mechanistic problems in the chemistry of Fischer carbene complexes

Well-established bonding situations of organometallic complexes and extensive applications in synthesis have been achieved during the past 25 years. In contrast, very little attention has been devoted to the intimate understanding of their reaction mechanisms. In this feature article, we show how the combined use of experimental and computational tools can be used to explore some reaction mechanisms of Fischer-type carbene complexes. The results obtained clearly demonstrate the usefulness of these combined tools to unravel the intimacies of different thermal and photochemical transformations, not only to explain already known processes but to predict new reactivity involving these fascinating complexes.

Selective reactivity of diamino Fischer-type carbene complexes towards 2,6-disubstituted and 2,4,6-trisubstituted pyrylium salts and 2,4,6-trichloro-1,3,5-triazine

The synthesis of new amino Fischer-type carbene complexes 2a– d with amino-containing side chain is presented. The reaction of these complexes with 2,4,6-triarylpyrylium salts gave the corresponding pyridinium salt 4a, 4c– e as expected from the initial attack of the free amino group onto the pyrylium α-carbon. When using the 2,6-diphenylpyrylium salt, a γ-addition of the amino group occurred, leading – after an hydrogen migration and a ring opening step – to the new organometallic unsaturated aminoketone 5. Finally, the reactivity of the diaminocarbene 2b with 2,4,6-trichloro-1,3,5-triazine has been investigated. The new triazine 6 bearing a ferrocenylaminocarbene group was isolated.