Vinylcarbene Complexes Research Articles

C-H Insertion via Ruthenium Catalyzed gem-Hydrogenation of 1,3-Enynes.

gem-Hydrogenation of an internal alkyne with the aid of [Cp*RuCl]4 as the precatalyst is a highly unorthodox transformation, in which one C atom of the triple bond is transformed into a methylene group, whereas the second C atom gets converted into a ruthenium carbene. In the case of 1,3-enynes bearing a propargylic steering substituent as the substrates, the reaction occurs regioselectively, giving rise to vinyl carbene complexes that adopt interconverting η1/η3-binding modes in solution; a prototypical example of such a reactive intermediate was characterized in detail by spectroscopic means. Although both forms are similarly stable, only the η3-vinyl carbene proved kinetically competent to insert into primary, secondary, or tertiary C–H bonds on the steering group itself or another suitably placed ether, acetal, orthoester, or (sulfon)amide substituent. The ensuing net hydrogenative C–H insertion reaction is highly enabling in that it gives ready access to spirocyclic as well as bridged ring systems of immediate relevance as building blocks for medicinal chemistry. Moreover, the reaction scales well and lends itself to the formation of partly or fully deuterated isotopologues. Labeling experiments in combination with PHIP NMR spectroscopy (PHIP = parahydrogen induced polarization) confirmed that the reactions are indeed triggered by gem-hydrogenation, whereas kinetic data provided valuable insights into the very nature of the turnover-limiting transition state of the actual C–H insertion step.

Ionization of gold (γ-methoxy)vinyl complexes generates reactive gold vinyl carbene complexes.

Cationic gold vinyl carbene/allylic cation complexes of the form (E)-[(L)AuC(H)C(H)CAr2]+ OTf- {L = IPr, Ar = Ph [(E)-5a], L = IPr, Ar = 4-C6H4OMe [(E)-5b], L = P(t-Bu)2 o-biphenyl, Ar = 4-C6H4OMe [(E)-5c]} were generated in solution via Lewis acid-mediated ionization of the corresponding gold (γ-methoxy)vinyl complexes (E)-(L)AuC(H)C(H)C(OMe)Ar2 at or below -95 °C. Complexes (E)-5b and (E)-5c were fully characterized in solution employing multinuclear NMR spectroscopy, which established the predominant contribution of the aurated allylic cation resonance structure and the significant distribution of positive charge into the γ-anisyl rings. Complex (E)-5b reacted rapidly at -95 °C with neutral two-electron, hydride, and oxygen atom donors exclusively at the C1 position of the vinyl carbene moiety and with p-methoxystyrene to form the corresponding vinylcyclopropane. In the absence of nucleophile (E)-5a decomposed predominantly via intermolecular carbene dimerization whereas formation of 1-aryl-5-methoxy indene upon ionization of (Z)-(IPr)AuC(H)C(H)C(OMe)(4-C6H4OMe)2 [(Z)-6b] implicated an intramolecular Friedel-Crafts or electrocyclic Nazarov pathway for the decomposition of the unobserved vinyl carbene complex (Z)-[(IPr)AuC(H)C(H)C(4-C6H4OMe)2]+ OTf- [(Z)-5b].

Generation of Endocyclic Vinyl Carbene Complexes via Gold-Catalyzed Oxidative Cyclization of Terminal Diynes: Toward Naphthoquinones and Carbazolequinones

Carbene cascade reactions involving carbene/alkyne metathesis have attracted much attention over the past decades because this chemistry offers great potential to build complicated cyclic molecules. However, the formed vinyl metal carbenoids in these reactions are limited to exocyclic carbenes, and the generation of endocyclic vinyl carbene complexes remains unexplored. Here, we report an unprecedented gold-catalyzed oxidative cyclization of terminal diynes. Importantly, the generation of endocyclic vinyl carbene complexes was involved in this oxidative cyclization, which is distinctively different from previous protocols. This method allows the facile synthesis of various valuable naphthoquinones and carbazolequinones from readily available diynes under exceptionally mild reaction conditions and features a broad substrate scope and wide functional group tolerance. Moreover, theoretical calculations provide further evidence on the divergent selectivity of this cyclization reaction.

Synthesis and reactivity of platinum vinylcarbene complexes prepared from activation of propargyl alcohols

Platinum vinylcarbene complexes are potentially useful for organometallic synthesis and catalysis, but have been rarely studied. This work reports a convenient route to make platinum vinylcarbene complexes. Treatment of [PtCl2(PPh3)]2 with propargyl alcohols HC≡CC(OH)RR’ (RR’ = Ph2, cyclo-C6H10(OH), (iPr)(C≡CTMS) and (H)(Ph)) in the presence of EtOH produced the vinylcarbene complexes trans-PtCl2{ = C(OEt)-CH=CRR’}(PPh3). Under similar condition, [PtCl2(PPh3)]2 reacted with HC≡CC(OH)Me2 to give cis-PtCl2{ = C(OEt)-CH=CMe2}(PPh3). Complex trans-PtCl2{ = C(OEt)-CH=CPh2}(PPh3) readily undergo a metathesis reaction with NaI to give trans-PtI2{ = C(OEt)-CH=CPh2}(PPh3) which can isomerize to cis-PtI2{ = C(OEt)-CH=CPh2}(PPh3). Complex trans-PtCl2{ = C(OEt)-CH=CPh2}(PPh3) reacted with PPh3 to give EtCl and the acyl complex trans-PtCl{C(O)CH=CPh2}(PPh3)2, which can undergo a decarbonylation reaction to give PtCl(CH=CPh2)(PPh3)2.

Diversity oriented synthesis of conjugate dienes and alkenylcyclopropanes utilizing silyl group-substituted titanium carbene complexes as bimetallic synthetic reagents

Diversity oriented synthesis of conjugate dienes and alkenylcyclopropanes by the sequential reactions using silyl group-substituted titanium carbene complexes was studied. Dienylsilanes were obtained by the olefination of carbonyl compounds with γ-silylvinylcarbene complexes. Cyclopropanation of 1-alkenes with the vinylcarbene complexes produced (E)-(β-silylvinyl)cyclopropanes with high stereoselectivity. The reaction of β-(trialkylsilyl)carbene complexes with zinc alkoxides of homopropargyl alcohols produced 6-silyl-3,5-hexadien-1-ols with high regio- and stereoselectivity. The (E)-alkenylsilanes thus obtained were transformed into a range of unsaturated compounds by the palladium-catalyzed or copper(I)-promoted cross-coupling with organic halides with retention of configuration.

Efficient synthesis of ruthenium vinyl carbene complexes

First generation Grubbs catalyst [RuCl2(PCy3)2(CHPh)] and related complexes of a general formula [RuCl2(PCy3)2(CHR)], where R is an aromatic or methyl group, undergo efficient reaction with trimethylsilylacetylene and related silylacetylenes to form vinyl carbene complexes [RuCl2(PCy3)2(CHCHCHR)] and disiloxane. The reaction requires the presence of water in the reaction medium. It is a convenient method for the high-yield synthesis of ruthenium vinyl carbene complexes. For benzylidene complexes, the reaction gives exclusively E-geometry of vinyl carbene ligand.

Chemistry at the Alkyne–Carbene Intersection: A Metallacyclobutene−η3-Vinylcarbene Equilibration

Synthesis of a sterically congested metallacyclobutene complex has led to the first observation of metallacyclobutene-η(3)-vinylcarbene equilibration. The structure of the η(3)-vinylcarbene complex was elucidated by spectroscopy, HRMS, and ab initio computations. The vinylcarbene complex was trapped by reactions with ethyl diazoacetate and (C5H5)Co(PPh3)2 to give cobalt-diene and dicobalt complexes, respectively.

ChemInform Abstract: Molybdenum Carbene Complexes: Trapping of in situ Generated Vinylcarbene Complexes with Electron-Poor Olefins.

ChemInform Abstract: Molybdenum Carbene Complexes: Trapping of in situ Generated Vinylcarbene Complexes with Electron-Poor Olefins.

ChemInform Abstract: Double Insertion of Alkynes into Chromium-Carbon Bonds of β-Donor- Substituted Vinylcarbene Complexes of Chromium - a Simple Route to Cyclopenta(b)pyrans.

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.

ChemInform Abstract: Electronic Tuning of Fischer Carbene Complexes in the Preparation of Bicyclo[3.1.1]heptanones as Taxane A-Ring Synthons.

Abstract A synthetic route to taxol and other Taxus diterpenes is described which employs as a key step the reaction between a Fischer carbene complex and a 1,6 enyne to construct 1-substituted-7,7-dimethyl-2-methylenebicyclo[3.1.1]heptan-6-ones. It was found that the reaction between complex 2 and 7-methyl-3-methylene-6-octen-1-yne (dienyne 30) yielded a mixture of bicyclo[3.1.1]heptanone 35 and cyclobutenone 36, the latter possibly arising from the migration of the chromium fragment from an electron-rich alkene to a less electron-rich alkene in the vinyl carbene complex intermediate (i.e. 40–42). On this basis, it was expected that bicycloheptanone yields would increase with increasing electron deficiency in the intermediate 40 since this should lead to more competitive CO insertion. This was observed with a series of electronically modified carbene complexes (45 and 48a–d). The more electron deficient complexes gave good yields of bicycloheptanones, thus providing an efficient means for preparing taxane A-ring synthons.

Development of Catalytic Carbene Transfer Reactions Using Alkynes as a Source of Carbenes

The in situ generation of vinylidene and alkylidene complexes based on the activation of alkynes with transition metal compounds was investigated. The cyclic oxacarbene complexes are produced through the formation of vinylidene complexes followed by the electrocyclization with the neighboring subunit of an alkyne. During the course of the study, it was found that a transition metal–induced 5–exo–dig cyclization of alkynes having nucleophilic conjugate subunits affords new types of hetero arene–substituted carbene complexes, such as (2–furyl)–, (2–pyrroryl), and (2–thienyl)carbene complexes. The 5–exo–dig cyclization of propargyl esters followed by 1,2–migration of carboxylates leads to the formation of vinylcarbene complexes. The in situ generation of metal–carbenoid species is highly atom–efficient and widely applicable to organic syntheses. In this account, the in situ generation of carbenoid species from a range of alkynes and synthetic applications to catalytic carbene transfer reactions, such as cycloaddition, annulation, ylide–associated reactions, insertion reactions, cycloisomerization, and isomerization reactions are highlighted and reviewed.

Transition Metal-Catalyzed and -Promoted Reactions via Carbene and Vinylidene Complexes Generated from Alkynes

The transition metal-induced in situ generation of carbene complexes from alkynes having a carbonyl or imino group as a nucleophilic functionality has been investigated. These reactive carbenoid species are generated with high atom efficiency through a 6-endo-dig cyclization mode based on the electrocyclization of vinylidene complexes or a 5-exo-dig cyclization mode in π-alkyne complexes, and have been found to serve as versatile intermediates in catalytic carbene transfer reactions. Highlighted and reviewed in this account are the generation and preparation of pyranylidene, furylcarbene, pyrrolylcarbene, and vinylcarbene complexes and their application to [3,3]sigmatropic rearrangement of acylcyclopropylvinylidenes, catalytic cyclopropanation reactions, [2,3]sigmatropic rearrangement or condensation reactions via ylides, ring-opening and substitution reactions with heteroaromatic compounds, and catalytic isomerization of oligoynes.

Titanacyclobutenes of Titanium Vinyl Carbene Complexes? Reactivity of Organotitanium Species Generated by the Reaction of γ‐Chloroallyl Sulfides with a Titanocene(II) Reagent.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Transition metal-catalyzed ring-opening, substitution, and cyclopropanation reactions via vinylcarbene complexes generated from O-propargyl thiocarbamates

We have developed various transition metal-catalyzed vinylcarbene transfer reactions, such as ring-opening, substitution, and cyclopropanation reactions, using O-propargyl thiocarbamates as carbene precursors. Platinum, ruthenium, rhodium, and gold complexes are effective for vinylcarbenoid formation. The highly nucleophilic nature and resonance effect of a thiocarbamoyl moiety readily permit the rearrangement of a thiocarbamoyl moiety from a propargylic position to an adjacent alkynyl carbon to give the intermediary vinylcarbene complexes.

Titanacyclobutenes or Titanium Vinyl Carbene Complexes? Reactivity of Organotitanium Species Generated by the Reaction of γ‐Chloroallyl Sulfides with a Titanocene(II) Reagent

The reactivity of the organotitanium species generated by the reductive titanation of gamma-chloroallyl sulfides with the titanocene(II) reagent [Cp(2)Ti{P(OEt)(3)}(2)] was studied. The organotitanium species formed from alpha-monosubstituted gamma-chloroallyl sulfides reacted with 1,5-diphenylpentan-3-one and styrene to produce conjugated dienes and vinyl cyclopropanes as major products, thus suggesting the formation of vinyl carbene complexes as intermediates. On the contrary, the organotitanium species generated from acyclic beta,gamma-disubstituted gamma-chloroallyl sulfides revealed titanacyclobutene-like reactivity, and their reaction with 1,5-diphenylpentan-3-one produced homoallyl alcohols. These organotitanium species did not react with styrene, but did react with dichlorophenylphosphine to afford phosphacyclobutenes. In the case of beta-monosubstituted, gamma-monosubstituted, and alpha,gamma-disubstituted gamma-chloroallyl sulfides, the organotitanium species reacted with both 1,5-diphenylpentan-3-one and styrene. The former reaction produced homoallyl alcohols and the latter gave vinyl cyclopropanes or unconjugated dienes. These results suggest that titanacyclobutenes and/or titanium vinyl carbene complexes are produced by the reductive titanation of gamma-chloroallyl sulfides depending on their substitution patterns.

Reaction of Tungsten Vinylcarbene Complexes with Enamines

In this paper we describe the reaction of vinylcarbene complexes 4 and 7 with enamines 5 and 9 to form the η1-acyl complexes 6, 8, and 10. From a mechanistic point of view, the reaction starts with...

Formation of C–C Bonds in Diiron Complexes by Addition of Carbanions to Alkynyl(methoxy)carbene Ligands

AbstractAddition of cyanide ions to the alkynyl(methoxy)carbene complexes [Fe2{μ‐CN(Me)(R)}(μ‐CO)(CO){Cα(OMe)Cβ≡CγR'}(Cp)2]+ (R = Xyl, R' = Tol, 1a; R = Xyl, R' = Ph, 1b; R = Xyl, R' = Me3Si, 1c; R = Me, R' = Tol, 1d; R = Me, R' = Ph, 1e) occurs selectively at Cα to afford the 1,1‐disubstituted σ‐propargyl complexes [Fe2{μ‐CN(Me)(R)}(μ‐CO)(CO){C(OMe)(CN)(C≡CR')}(Cp)2] (R = Xyl, R' = Tol, 2a; R = Xyl, R' = Ph, 2b; R = Xyl, R' = Me3Si, 2c; R = Me, R' = Tol, 2d; R = Me,R' = Ph, 2e). Conversely, the stabilised carbanaions[CH(R)2]– (R = CN, CO2Me) add at the Cγ position with subsequent hydrogen migration to Cβ to give the 1‐σ‐buta‐1,3‐dienyl complexes [Fe2{μ‐CN(Me)(Xyl)}(μ‐CO)(CO){Cα(OMe)=CβHCγ(Tol)=Cδ(R)2}(Cp)2] (R = CN, 3a; CO2Me, 3b). No migration is possible in the absence of hydrogen atoms at Cδ, therefore addition of [C(Me)(CO2Me)2]– to 1a results in the formation of the σ‐allenyl complex [Fe2{μ‐CN(Me)(Xyl)}(μ‐CO)(CO){Cα(OMe)=Cβ=Cγ(Tol)Cδ(Me)(CO2Me)2}(Cp)2] (4). Protonation of the neutral complexes 3b and 4affords the vinyl(methoxy)carbene complexes [Fe2{μ‐CN(Me)(Xyl)}(μ‐CO)(CO){Cα(OMe)CβH=Cγ(Tol)Cδ(R)(CO2Me)2}]+ (R = H, 5; Me, 6), which exist in solution as mixtures of (E)‐ and (Z)‐isomers in dynamic equilibrium, as shown by VT NMR studies. The cationic complex 6 shows electrophilic behaviour. Thus, addition of CN– results in the σ‐allyl complex [Fe2{μ‐CN(Me)(Xyl)}(μ‐CO)(CO){Cα(OMe)(CN)[CβH=Cγ(Tol)Cδ(Me)(CO2Me)2]}(Cp)2] (7), whereas the reaction with Me2NH affords the vinyl acyl complex [Fe2{μ‐CN(Me)(Xyl)}(μ‐CO)(CO){Cα(O)CβH=Cγ(Tol)Cδ(Me)(CO2Me)2}(Cp)2] (8). The crystal structures of 2b and 3a·CH2Cl2 have been determined. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Mechanism of Enyne Metathesis Catalyzed by Grubbs Ruthenium−Carbene Complexes: A DFT Study

The complete catalytic cycle of the reaction of alkenes and alkynes to dienes by Grubbs ruthenium carbene complexes has been modeled at the B3LYP/LACV3P**+//B3LYP/LACVP level of theory. The core structures of the substrates and the catalyst were used as models, namely, ethene, ethyne, hept-1-en-6-yne, (Me(3)P)(2)Cl(2)Ru=CH(2), and [C(2)H(4)(NMe)(2)C](Me(3)P)Cl(2)Ru=CH(2). Insight into the electronically most preferred mechanistic pathways was gained for both intermolecular as well as for intramolecular enyne metathesis. Alkene metathesis is predicted to proceed fast and reversible, while the insertion of the alkyne substrate is slower, irreversible, and kinetically regioselectivity determining. Ruthenacyclobut-2-ene structures do not exist as local minima in the catalytic cycle. Instead, vinylcarbene complexes are formed directly. The alkyne insertion step and the cycloreversion of 2-vinyl ruthenacyclobutanes feature comparable predicted overall barriers in intermolecular enyne metathesis. For intramolecular enyne metathesis, a noncyclic alkene fragment of the enyne substrate is first incorporated into the Grubbs catalyst by an alkene metathesis reaction. The subsequent insertion of the alkyne fragment then proceeds intramolecularly. Alkene association, cycloaddition, and cycloreversion to the diene product complex close the catalytic cycle. Rate enhancement by an ethene atmosphere (Mori's conditions) originates from a constantly higher overall alkene concentration that is necessary for the rate-limiting [2 + 2] cycloreversion step to the diene product complex.

Coupling of Propargylsilanes with Fischer Carbene Chromium Complexes: A New Synthesis of Conjugated Dienes.

Abstract The reaction of propargylsilanes with Fischer carbene complexes has been examined. If the silane-containing carbon is secondary the predominant pathway involves formation of conjugated dienes through a 1,2-silicon shift process of the initially formed vinylcarbene complex intermediate. If a primary propargylsilane is employed, the silicon does not shift and normal alkyne-Fischer carbene coupling processes are observed. The process is moderately stereoselective, resulting in the E enol ether and Z alkenylsilane.

Reaction of Tungsten η1-Acetylide Complexes [(η5-C5H5)(NO)(CO)W−C⋮C−R]Li with Iminium Ions

The reaction of alkynyltungsten complexes [(CO)(NO)(Cp)W−C⋮C−R]Li (R = H, C6H5, C(CH3)3) with differently substituted iminium ions is investigated. Due to the relative high acidity of the hydrogen atoms on the β-carbon atom of the η1-vinylidene complex [(CO)(NO)(Cp)WCCH2)] (1), the parent η1-acetylide complex [(CO)(NO)(Cp)W−C⋮C−H)]- is generated in situ simultaneously with the iminium ion by the reaction of η1-vinylidene complex 1 with an enamine. The reaction of 1 with enamines 2a−e leads to vinylcarbene complexes 3a−e in good yield. The first step of this transformation is a Mannich reaction on the β-carbon atom of alkynyltungsten complexes, generating the expected β-amino-alkylated η1-vinylidene complexes 6. This intermediate reacts further to 3a−e by migration of the hydrogen atom adjacent to the nitrogen atom to the α-carbon atom of the η1-vinylidene moiety. The appearance of a η1-vinylidene complex as an intermediate is supported by NMR experiments, and the postulated retro-imino-ene reaction is con...