Carbenoid Complexes Research Articles

Oxidized Bridged Carbenoids as Viable Intermediates in a Fe(III) Catalyzed C-H Insertion Reaction.

Fe catalyzed carbene insertion reactions present an efficient route for direct C-H functionalization. The use of Fe(III) in place of the widely used Fe(II) presents several benefits. However, the mechanistic understanding of Fe(III) severely lags behind Fe(II) complexes. One of the major unsolved issues relates to the formation of bridged versus terminal metallocarbenes. Even though the oxidized bridged carbenoid complexes have been isolated and found to be thermodynamically more stable, they are generally considered a dead end for the catalytic cycle. In the current report, the formation and the subsequent reactions of the bridged carbenoid complexes for an Fe(TPP)Cl catalyzed C(sp2 )-H insertion are investigated. Using DFT calculations, it is observed that both mono and bis oxidized bridged carbenoid complexes can participate in the catalytic cycle. Importantly, for the first time, a mechanistic pathway showing that these bridged species are not a dead end in Fe catalysis is presented. Their existence in other reactions might be more prevalent than what is currently believed. The current study will have important implications in utilizing Fe(III) complexes for other insertion reactions, especially for heme containing enzymes which necessarily need to be carried out under anaerobic/reducing conditions.

Synthesis, Structure, and Reactivity of Gold(I) α-Oxo Carbenoid Complexes

The trifluoromethanesulfonate α-oxo carbenoid complexes (IPr)AuCH(OTf)COR [R = OEt (1a), p-tolyl (1b)] were isolated from reactions of (IPr)Au(OTf) with the corresponding α-diazo carbonyl compounds...

Capture and characterization of a reactive haem–carbenoid complex in an artificial metalloenzyme

Capture and characterization of a reactive haem–carbenoid complex in an artificial metalloenzyme

ChemInform Abstract: Recent Advances in Catalytic Stereocontrolled Cycloaddition with Terminal Propargylic Compounds

AbstractReview: [30 refs.

Recent Advances in Catalytic Stereocontrolled Cycloaddition with Terminal Propargylic Compounds

Terminal propargylic compounds containing an alkyne unit and an alcohol or ester group in the propargylic position have a fairly acidic acetylenic hydrogen atom; this makes them versatile substrates for further chemical transformation. Some transition metals such as ruthenium or copper exhibit specific affinity for terminal propargylic compounds, generating dielectrophilic ruthenium– or copper–allenylidene complexes that show high potential for stereoselective cycloaddition with various bis-nucleophiles. In this review, we highlight this emerging field of catalytic stereoselective cycloaddition with terminal propargylic compounds. Examples of ruthenium-, copper-, palladium-, and gold-catalyzed cycloaddition are given in the article, along with mechanistic considerations. 1 Introduction 2 Ruthenium–Allenylidene Complexes in Enantioselective Cycloaddition 3 Copper–Allenylidene Complexes in Enantioselective Cycloaddition 4 Palladium–π-Propargyl Complexes in Stereoselective Cycloaddition 5 Gold–Carbenoid Complexes in Stereoselective Cycloaddition 6 Summary and Outlook

Synthesis, Structure, and Reactivity of a Gold Carbenoid Complex That Lacks Heteroatom Stabilization

AbstractHydride abstraction from the neutral gold cycloheptatrienyl complex [(P)Au(η1‐C7H7)] (P=P(tBu)2(o‐biphenyl)) with triphenylcarbenium tetrafluoroborate at −80 °C led to the isolation of the cationic gold cycloheptatrienylidene complex [(P)Au(η1‐C7H6)]+ BF4− in 52 % yield, which was characterized in solution and by single‐crystal X‐ray diffraction. This cycloheptatrienylidene complex represents the first example of a gold carbenoid complex that lacks conjugated heteroatom stabilization of the electron‐deficient C1 carbon atom. The cycloheptatrienylidene ligand of this complex is reactive; it can be reduced by mild hydride donors, and converted to tropone in the presence of pyridine N‐oxide.

Synthesis, Structure, and Reactivity of a Gold Carbenoid Complex That Lacks Heteroatom Stabilization

Hydride abstraction from the neutral gold cycloheptatrienyl complex [(P)Au(η(1)-C7H7)] (P=P(tBu)2(o-biphenyl)) with triphenylcarbenium tetrafluoroborate at -80 °C led to the isolation of the cationic gold cycloheptatrienylidene complex [(P)Au(η(1)-C7H6)](+) BF4(-) in 52% yield, which was characterized in solution and by single-crystal X-ray diffraction. This cycloheptatrienylidene complex represents the first example of a gold carbenoid complex that lacks conjugated heteroatom stabilization of the electron-deficient C1 carbon atom. The cycloheptatrienylidene ligand of this complex is reactive; it can be reduced by mild hydride donors, and converted to tropone in the presence of pyridine N-oxide.

Synthesis and the antimicrobial activity of metalcarbene compounds of the triazole series

Мetalcarbene compounds of a series of [1,2,4] and [1,2,3]triazoles have been synthesized to study their antimicrobial activity. Monocarbene complexes of palladium (9a) and copper(I) (9b,12,15), biscarbene complexes of palladium (4a) and copper(I) (4b,14), a carbenoid complex of cobalt (5) have been synthesized by the direct interaction of stable carbenes with transition metal salts or by analogous in situ reactions. The intermediate sterically hindered carbenes – [1,2,4]triazol-5-ylidenes 3a-c have been isolated in the individual state. Being intermediate in the synthesis of mesoionic complexes 9a,b 1-adamantyl-4-phenyl-[1,2,3]triazole has been synthesized by [3+2]-cycloaddition of 1-adamantylazide to phenylacetylene in the presence of the biscarbene complex of copper(I) iodide 14. Characteristic signals of the carbenoid carbon atoms are detected in the 13С NMR spectra of complexes in the range of 165-203 ppm. A high antimicrobial activity has been found for complexes 5.12; for compound 12 it corresponds to the minimal bacteriostatic concentration (MBsC) 15.6 mkG/mL, the minimal bactericidal concentration (MBcC) 31.2 mkG/mL (М. luteum), and MBsC 31.2 mkG/mL, MBcC 62.5 mkG/mL (S. aureus). The highest activity in the series studied is observed for the carbenoid complex of cobalt (5) on the test-culture of M. luteum (MBsC 15.6 mkG/mL and MBcC 31.2 mkG/mL), and on the test-culture of C. tenuis (the minimal fungistatic concentration is 7.8 mkG/mL and the minimal fungicidal concentration is 31.2 mkG/mL).

Gold(I)-Catalyzed Benz[c]azepin-4-ol Synthesis by Intermolecular [5 + 2] Cycloaddition

A gold(I)-catalyzed intermolecular formal [2 + 5] cycloaddition for the preparation of benzofused N-heterocyclic azepine products is presented. A number of benz[c]azepin-4-ol products were readily prepared in one step from easily accessible phenylpropargyl acetals and benzaldimine substrates in the presence of a gold(I) catalyst. A direct one-pot procedure from the propargyl and the respective aldehyde and amine substrates was successful as well. The reaction to access the benzofused azepines could be rationalized by a cascade reaction, including a nucleophilic benzaldimine N-attack at a highly reactive phenylpropargyl-gold(I) carbenoid complex, generated from propargyl acetal. A subsequent deauration step promotes ring closure by 1,7-electrocyclization through an intramolecular Pictet-Spengler-type reaction with the aldiminium moiety.

Density functional theory study of the mechanism of zinc carbenoid promoted cyclopropanation of allenamides

The mono- and bis-cyclopropanation of allenamides with the zinc carbenoid Zn(CH2Cl)2 have been studied using density functional theory calculations employing the M06 functional. The monomeric and dimeric precursor complexes were both constructed to model the reaction processes. In the monomeric reaction, the formation of the endo-monocyclopropyl species takes place via a methylene transfer pathway rather than a carbometalation pathway. The formation of the exo-monocyclopropyl species does not readily occur via a methylene transfer pathway due to a high activation barrier. The corresponding carbometalation pathway was not able to be found. Following the monocyclopropanation step, the biscyclopropanation of the endo-monocyclopropyl species is facile to form amidospiro[2.2]pentane. In the aggregation model, the allenamides and the zinc carbenoid form a dimer aggregate that is then followed by two pathways. One pathway takes place via transition states inside the aggregate structure (denoted here as a closed-mode process) while the other pathway introduces another zinc carbenoid molecule from outside the aggregation species (denoted here as an open-mode process). The aggregate mechanisms are not favored because the dimeric reactant of the open-mode process is not stable to coexist with the monomer and the activation barriers of the two aggregate pathways are higher than those of the monomeric pathways. The calculation results show that the key factors in the reaction mechanisms are the co-planarity of the allenic moiety with the oxazolidinone ring, the torsional strain in the butterfly-type transition state, the ring strain in the substrate–carbenoid complexes and the coordination between the carbenoid-Zn and O(CO) atoms and other long-distance interactions.

ChemInform Abstract: Niobocene Chemistry: Acetylene Hydrido, Acetylene Alkenyl, and Carbenoid Butadienyl Complexes.

The acetylene hydrido complexes Cp 2 NbH(C 2 R 2 ) (2a, R=Ph, known; 2b, R=SiMe 3 ; 2c, R=Et) were prepared from Cp 2 NbH 3 /RC≡CR by thermal substitution, and 2d (R=Me) was obtained from 2b/MeC≡CMe by a photochemical ligand displacement reaction. The complexes 2 insert acetylenes (as e.g. MeC≡CMe, PhC≡CH, HC≡CCO 2 Me) by a nonmigratory insertion process. Stable insertion products Cp 2 Nb(Me 3 SiC≡CSiMe 3 )(CR 2 =CHR 3 ) (4g, R 2 =CO 2 Me, R 3 =H; 4h, R 2 =COMe, R 3 =H; 4i, R 2 , R 3 =CO 2 Me, E isomer) were obtained from 2b. With less sterically crowded acetylene hydrido compounds 2, insertion is followed by rearrangement to give the novel carbenoid complexes Cp 2 Nb(CR 1 CR 1 CR 2 CHR 3 )

Density functional theory studies of the mechanistic aspects of olefin metathesis reactions

The potential energy surfaces of the reactions of Cl(4)MCH(2) (M = Cr, Mo, W, Ru, Re) with ethylene, models of potential chain-carrying catalysts and olefins respectively in the transition metal-catalyzed olefin metathesis reaction, have been explored using density functional theory at the B3LYP/LACVP* level of theory. In Cl(4)MCH(2) (M = Cr, Ru), the carbenoid complexes Cl(3)MCH(2)-Cl have been found to be more stable than the corresponding carbene Cl(4)M[double bond, length as m-dash]CH(2) complexes whereas in Cl(4)MCH(2) (M = Mo, W, Re) the carbene complexes are more stable than the carbenoid complexes. The carbenoid complexes have been found not to favor the formation of metallacyclobutanes, a key step in the olefin metathesis reaction according to the Herrison-Chauvin mechanism, indicating that the active species for the metathesis reaction is a carbene complex and not a carbenoid complex. Therefore, even though the formation of metallacyclobutanes through formal [2+2] cycloaddition has been found to be a low-barrier process with each of the metal carbene complexes investigated, metathesis is likely to occur only in Cl(4)MCH(2)(M = Mo, W, Re) but not in Cl(4)MCH(2)(M = Cr, Ru) since the reaction surface in the latter complexes is likely to be populated by the carbenoid complex rather than the carbene complex. In Cl(4)W=CH(2) the kinetic and thermodynamic preference of the productive [2+2] pathway leading to the metallacyclobutane over the [3+2] pathway is unambiguous whereas in Cl(4)MoCH(2) the [3+2] pathway is likely to be competitive with the [2+2] pathway. The metallacyclobutane formation in W and Re has been found to have lower barriers than in Mo, suggesting that the W and Re complexes may have a greater metathesis activity than the Mo complex.

Catalytic reactions involving the cleavage of carbon–cyano and carbon–carbon triple bonds

The catalytic reactions that involve the cleavage of C-CN bonds and carbon-carbon triple bonds are described in this tutorial review. Regarding the cleavage of a C-CN bond, the catalytic reaction can proceed by two different mechanisms: oxidative addition and deinsertion of silyl isocyanide. A carbon-carbon triple bond can be cleaved in the absence of an organic promoter via the formation of unique organometallic species, such as allenylidene and cyclopropyl carbenoid complexes.

Density Functional Theory Study of the Platinum-Catalyzed Cyclopropanation Reaction with Olefin

A computational study of the platinum-catalyzed cyclopropanation reaction with olefin is presented. The model system is formed by an ethylene molecule and the active catalytic species, which forms from a CH2 fragment and the Cl2Pt(PH3)2 complex. The results show that the active catalytic species is not a metal-carbene of the type (PH3)2Cl2Pt=CH2 but two carbenoid complexes which can exist in almost two degenerate forms, namely (PH3)2Pt(CH2Cl)Cl (carbenoid A) and (PH3)Pt(CH2PH3)Cl2 (carbenoid B). The reaction proceeds through three pathways: methylene transfer, carbometalation for carbenoid A, and the reaction of a monophosphinic species for carbenoids (A and B). The most favored reaction channel is methylene transfer pathway for (PH3)Pt(CH2PH3)Cl2 (carbenoid B) species with a barrier of 31.32 kcal/mol in gas phase. The effects of dichloromethane, THF, and benzene solvent are investigated with PCM method. For carbenoid A, both methylene transfer and carbometalation pathway barriers to reaction become remarkably lower with the increasing polarity of solvent (from 43.25 and 52.50 kcal/mol for no solvent to 25.36 and 38.53 kcal/mol in the presence of the dichloromethane). In contrast, the reaction barriers for carbenoid B via the methylene transfer path hoist 6.30 kcal/mol, whereas the barriers do not change significantly for the reaction path of a monophosphinic species for carbenoids (A and B).

Catalytic Decomposition of Diazo Compounds as a Method for Generating Carbonyl‐Ylide Dipoles

AbstractFor Abstract see ChemInform Abstract in Full Text.

The interaction of rhodium carbenoids with carbonyl compounds as a method for the synthesis of tetrahydrofurans

The transition metal catalyzed reaction of α-diazo carbonyl compounds has found numerous applications in organic synthesis, and its use in either heterocyclic or carbocyclic ring formation is well precedented. In contrast to other catalysts that are suitable for carbenoid reactions of diazo compounds, those constructed with the dirhodium(II) framework are most amenable to ligand modification that, in turn, can influence reaction selectivity. The reaction of rhodium carbenoids with carbonyl groups represents a very efficient method for generating carbonyl ylide dipoles. Rhodium-mediated carbenoid–carbonyl cyclization reactions have been extensively utilized as a powerful method for the construction of a variety of novel polycyclic ring systems. This article will emphasize some of the more recent synthetic applications of the tandem rhodium carbenoid cyclization/cycloaddition cascade for natural product synthesis. Discussion centers on the chemical behavior of the rhodium metal carbenoid complex that is often affected by the nature of the ligand groups attached to the metal center.

Catalytic Decomposition of Diazo Compounds as a Method for Generating Carbonyl‐Ylide Dipoles

AbstractThe transition metal catalyzed reaction of α‐diazo carbonyl compounds has found numerous applications in organic synthesis, and its use in either heterocyclic or carbocyclic ring formation is well‐precedented. In contrast to other catalysts that are suitable for carbenoid reactions of diazo compounds, those constructed with the dirhodium(II) framework are most amenable to ligand modification that, in turn, can influence reaction selectivity. The reaction of rhodium carbenoids with carbonyl groups represents a very efficient method for generating carbonyl ylide dipoles. Rhodium‐mediated carbenoid–carbonyl cyclization reactions have been extensively utilized as a powerful method for the construction of a variety of novel polycyclic ring systems. This article will emphasize some of the more recent synthetic applications of the tandem cyclization/cycloaddition cascade for natural product synthesis. Discussion centers on the chemical behavior of the rhodium metal–carbenoid complex that is often affected by the nature of the ligand groups attached to the metal center.

Protonation of CpCo(C4Me4): Ring-Opening with Formation of New σ,η4-Butadienyl and η5-Pentadienyl Cobalt Complexes

Protonation of CpCoCb* (Cb* = η 4 -C 4 Me 4 ) (1) produces the carbenoid complex [CpCo(σ,η 4 -C 4 HMe 4 )] + (2 + ). The structure of (2)BF 4 displays a σ, η 4 -butadienyl-type ligand with Co-C(carbene) 1.792(6) A and a terminal methyl group in anti position. DFT calculations on the protonation of the monomethyl complex CpCo(η 4 -C 4 H 3 Me) show that protonation preferentially takes place from the endo side and is followed by a stereospecific ring-opening reaction to give the corresponding carbenoid complex [CpCo(σ,η 4 -C 4 H 4 Me)] + with the same anti stereochemistry as found in 2 + . Above -10 °C cation 2 + rearranges to produce the half-open cobaltocenium ion [CpCo(η 5 -C 5 H 4 Me 3 )] (3 + ), which on addition of hydride from DIBALH gives CpCo(η 4 -3,4-Me 2 C 6 H 8 ) (4).

DFT Study of the Olefin Metathesis Catalyzed by Ruthenium Complexes

A theoretical investigation has been carried out at the DFT (B3LYP) level on the mechanism of the metathesis reaction catalyzed by Grubbs' complexes. Two model systems have been used: (a) The first model is formed by one ethylene molecule and the Cl2(PH3)2RuCH2 complex (Grubbs' catalyst). (b) In the second model the Cl2(PPh3)2RuCH2 species has been considered. The following results are relevant: (i) The “primary” active catalytic species is a metal-carbene (PR3)2Cl2RuCH2. The corresponding carbenoid complex (PR3)2ClRu−CH2Cl is significantly higher in energy (18.45 and 19.26 kcal mol-1 for the two model systems) and thus cannot represent the starting active species of the process. (ii) The existence of three different reaction pathways has been demonstrated. One of the two most likely reaction channels is characterized by the presence of “secondary” active species of carbenoid type. These species, after olefin coordination, become slightly more stable than the corresponding carbenic forms and play a key ...

ChemInform Abstract: Olefin Isomerization by a Ruthenium Carbenoid Complex. Cleavage of Allyl and Homoallyl Groups.

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