Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Complex, metal-carbene

Grubbs, contains a ruthenium atom linked by a double bond to carbon, an example of a so-caUed metal carbene complex. Metal carbenes wctc proposed (by Chauvin) and confirmed as intermediates in alkene metathesis decades ago. Subsequently,... [Pg.525]

Many other organometaUic compounds also react with carbonyl groups. Lithium alkyls and aryls add to the ester carbonyl group to give either an alcohol or an olefin. Lithium dimethyl cuprate has been used to prepare ketones from esters (41). Tebbe s reagent, Cp2TiCH2AlCl(CH2)2, where Cp = clyclopentadienyl, and other metal carbene complexes can convert the C=0 of esters to C=CR2 (42,43). [Pg.389]

MII Transition Metal Carbene Complexes (F. R. Kreissel, ed.), VCH,... [Pg.172]

The first reaction pathway for the in situ formation of a metal-carbene complex in an imidazolium ionic liquid is based on the well loiown, relatively high acidity of the H atom in the 2-position of the imidazolium ion [29]. This can be removed (by basic ligands of the metal complex, for example) to form a metal-carbene complex (see Scheme 5.2-2, route a)). Xiao and co-workers demonstrated that a Pd imida-zolylidene complex was formed when Pd(OAc)2 was heated in the presence of [BMIMjBr [30]. The isolated Pd carbene complex was found to be active and stable in Heck coupling reactions (for more details see Section 5.2.4.4). Welton et al. were later able to characterize an isolated Pd-carbene complex obtained in this way by X-ray spectroscopy [31]. The reaction pathway to the complex is displayed in Scheme 5.2-3. [Pg.223]

However, formation of the metal carbene complex was not observed in pure, halide-free [BMIM][Bp4], indicating that the formation of carbene depends on the... [Pg.223]

Another means of in situ metal-carbene complex formation in an ionic liquid is the direct oxidative addition of the imidazolium cation to a metal center in a low oxidation state (see Scheme 5.2-2, route b)). Cavell and co-workers have observed oxidative addition on heating 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPli3)4 in refluxing THF [32]. The Pt-carbene complex formed can decompose by reductive elimination. Winterton et al. have also described the formation of a Pt-car-bene complex by oxidative addition of the [EMIM] cation to PtCl2 in a basic [EMIM]C1/A1C13 system (free CP ions present) under ethylene pressure [33]. The formation of a Pt-carbene complex by oxidative addition of the imidazolium cation is displayed in Scheme 5.2-4. [Pg.224]

In the light of these results, it becomes important to question whether a particular catalytic result obtained in a transition metal-catalyzed reaction in an imidazolium ionic liquid is caused by a metal carbene complex formed in situ. The following simple experiments can help to verify this in more detail a) variation of ligands in the catalytic system, b) application of independently prepared, defined metal carbene complexes, and c) investigation of the reaction in pyridinium-based ionic liquids. If the reaction shows significant sensitivity to the use of different ligands, if the application of the independently prepared, defined metal-carbene complex... [Pg.224]

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... [Pg.6]

A decade after Fischer s synthesis of [(CO)5W=C(CH3)(OCH3)] the first example of another class of transition metal carbene complexes was introduced by Schrock, which subsequently have been named after him. His synthesis of [((CH3)3CCH2)3Ta=CHC(CH3)3] [11] was described above and unlike the Fischer-type carbenes it did not have a stabilizing substituent at the carbene ligand, which leads to a completely different behaviour of these complexes compared to the Fischer-type complexes. While the reactions of Fischer-type carbenes can be described as electrophilic, Schrock-type carbene complexes (or transition metal alkylidenes) show nucleophilicity. Also the oxidation state of the metal is generally different, as Schrock-type carbene complexes usually consist of a transition metal in a high oxidation state. [Pg.9]

The subsequent insertion of the alkyne into the metal-carbene bond affords the (r]1 r]3)-vinylcarbene complex D, which may exist either as a (Z)- or an ( )-metallatriene. This intermediate maybe considered as a branching point in the benzannulation reaction as three diverging routes starting from this point have been explored. [Pg.126]

The inherent plane of chirality in the metal carbene-modified cyclophane 45 was also tested in the benzannulation reaction as a source for stereoselectivity [48]. The racemic pentacarbonyl(4-[2.2]metacyclophanyl(methoxy)carbene)-chromium 45 reacts with 3,3-dimethyl-1-butyne to give a single diastereomer of naphthalenophane complex 46 in 50% yield the sterically less demanding 3-hexyne affords a 2 1 mixture of two diastereomers (Scheme 30). These moderate diastereomeric ratios indicate that [2.2]metacyclophanes do not serve as efficient chiral tools in the benzannulation reaction. [Pg.140]

Casey CP (1981) Metal-carbene complexes. In Jones M Jr, Moss RA (eds) Reactive intermediates. Wiley, New York, p 135... [Pg.154]

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. [Pg.157]

Keywords Metal carbenes Photochemical reactions Metal-ketene complexes... [Pg.157]

Keywords Metathesis Alkenes Catalysis Ruthenium Metal carbene complexes... [Pg.224]

Although olefin metathesis had soon after its discovery attracted considerable interest in industrial chemistry, polymer chemistry and, due to the fact that transition metal carbene species are involved, organometallic chemistry, the reaction was hardly used in organic synthesis for many years. This situation changed when the first structurally defined and stable carbene complexes with high activity in olefin metathesis reactions were described in the late 1980s and early 1990s. A selection of precatalysts discovered in this period and representative applications are summarized in Table 1. [Pg.226]

It has been demonstrated that group 6 Fischer-type metal carbene complexes can in principle undergo carbene transfer reactions in the presence of suitable transition metals [122]. It was therefore interesting to test the compatibility of ruthenium-based metathesis catalysts and electrophilic metal carbene functionalities. A series of examples of the formation of oxacyclic carbene complexes by metathesis (e.g., 128, 129, Scheme 26) was published by Dotz et al. [123]. These include substrates where double bonds conjugated to the pentacarbonyl metal moiety participate in the metathesis reaction. Evidence is... [Pg.259]

The aim of this volume is to convince the reader that metal carbene complexes have made their way from organometallic curiosities to valuable - and in part unique - reagents for application in synthesis and catalysis. But it is for sure that this development over 4 decades is not the end of the story there is both a need and considerable potential for functional organometallics such as metal carbon multiple bond species which further offer exciting perspectives in selective synthesis and catalysis as well as in reactions applied to natural products and complex molecules required for chemical architectures and material science. [Pg.369]

Free carbenes can also be avoided by using transition metal-carbene complexes L M—CRR (L = a ligand, M = a metal),which add the group CRR to double bonds.An example is ... [Pg.1086]

Carboxylic esters undergo the conversion C=0— C=CHR (R = primary or secondary alkyl) when treated with RCHBr2, Zn, and TiCl4 in the presence of A,A,A, iV -tetramethylethylenediamine. Metal carbene complexes R2C=ML (L = ligand), where M is a transition metal such as Zr, W, or Ta, have also been used to convert the C=0 of carboxylic esters and lactones to CR2. It is likely that the complex Cp2Ti=CH2 is an intermediate in the reaction with Tebbe s reagent. [Pg.1238]


See other pages where Complex, metal-carbene is mentioned: [Pg.212]    [Pg.393]    [Pg.394]    [Pg.151]    [Pg.212]    [Pg.393]    [Pg.394]    [Pg.151]    [Pg.291]    [Pg.687]    [Pg.192]    [Pg.11]    [Pg.13]    [Pg.223]    [Pg.224]    [Pg.225]    [Pg.154]    [Pg.8]    [Pg.10]    [Pg.13]    [Pg.47]    [Pg.86]    [Pg.116]    [Pg.124]    [Pg.145]    [Pg.159]    [Pg.168]    [Pg.329]    [Pg.368]    [Pg.368]    [Pg.369]   
See also in sourсe #XX -- [ Pg.1086 ]




SEARCH



Acyclic carbene-metal complexes

Alkali metal complexes with carbenes

Alkaline-earth metals, carbene complexes

Alkene metathesis metal carbene complexes

Alkenes via metal carbene complexes

Alkenyl halides via metal carbene complexes

Alkyne insertion metal carbene complexes

Amino carbene metal complexes

An Extension Metal Complexes with Unsaturated Carbenes

Carbene alkali metal complexes

Carbene complexes carbon-metal bond

Carbene complexes metal carbonyls

Carbene) Complexes of Transition Metals

Carbene-olefin metal complex

Carbenes alkali metal complexes

Carbenes alkaline earth metal complexes

Carbenes metal carbene complex

Carbenes metal carbene complex

Carbenes metal complexes

Carbenes metal complexes

Carbenes transition metal complexes

Carbenes transition metal complexes, catalytic

Carbenes, alkynyltransition metal complexes

Carbenes, alkynyltransition metal complexes 2 + 2] cycloaddition reactions

Carbenes, alkynyltransition metal complexes cycloaddition reactions with 1,3-dienes

Carbenes, alkynyltransition metal complexes ene reactions

Carbenes, complexes with transition metals

Carbenes, complexes with transition metals electronic structure

Carbenes, complexes with transition metals rearrangement

Carbenes, generation metal complexes

Carbonyl Olefination Utilizing Metal Carbene Complexes

Chemistry of Transition Metal Carbene Complexes

Classification of transition metal-carbene complexes

Complexes metal-carbene, protonated

Cyclization reactions carbene transition metal complexes

Cycloaddition reactions carbene transition metal complexes

Detection of propagating metal-carbene complexes

Early Transition and Rare Earth Metal Complexes with N-Heterocyclic Carbenes

Fischer-type carbenes transition metal complexes

From metal carbene complexes

Hydrosilylation metal-carbene complexes

Initiation efficiency metal carbene complexes

Ketene chemistry metal carbene complexes

Kinetics metal carbene complexes

Lipotoxins via metal carbene complexes

Metal carbene complex propagation mechanism

Metal carbene complex reactivity

Metal carbene complexes 18-electron

Metal carbene complexes Fischer-type

Metal carbene complexes chiral

Metal carbene complexes detection

Metal carbene complexes electron-deficient

Metal carbene complexes enantioselectivity

Metal carbene complexes in olefin metathesis

Metal carbene complexes propagating

Metal carbene complexes rotational barriers about

Metal carbene complexes structure

Metal carbenes

Metal carbenes Fischer carbene complexes

Metal-Carbene, -Methylene, -Carbyne and -Methylidyne Complexes

Metal-carbene complexes Bonding

Metal-carbene complexes Classification

Metal-carbene complexes Electrophilic

Metal-carbene complexes Fischer

Metal-carbene complexes Heterocyclics

Metal-carbene complexes NHCs)

Metal-carbene complexes Ruthenium

Metal-carbene complexes alkoxy substituted

Metal-carbene complexes amino substituted

Metal-carbene complexes anions

Metal-carbene complexes decomplexation

Metal-carbene complexes ligand substitution reactions

Metal-carbene complexes protecting group

Metal-carbene complexes reaction with alkenes

Metal-carbene complexes reaction with ylides

Metal-carbene complexes reactions with

Metal-carbene complexes spectra

Metal-carbene complexes synthesis

Metal-carbene complexes thermal decomposition

Metal-carbene-hydride complexes

Metal-carbene-olefin complexes decomposition

Metal-carbene-olefin complexes propagating, detection

Metal-ligand bonds carbene complexes

N-heterocyclic carbenes metal complexes

Nucleophilic metal-carbene complexes

Nucleophilic reactions Metal carbene complexes

Olefin metathesis using metal carbene complexes

Origins of Carbene-Metal Complexes

Reactions of Metal-Carbene Complexes

Reactivity of transition metal-carbene complexes

Schrock carbenes transition metal complex bonding

Schrock-type carbene complexes, transition metal

Silanes, alkenylsynthesis via metal carbene complexes

Sulfides, alkenyl via metal carbene complexes

Sulfur ylides, from metal carbene complexes

Synthesis of Carbene Ligands and Their Metal Complexes

Synthesis of Metal Carbene Complexes

Synthetic Reactions via Transition Metal Carbene Complexes

Transition Metal-Carbene Complexes in Olefin Metathesis and Related Reactions

Transition metal carbene complexes

Transition metal complexes carbene synthesis

Transition metal compounds chemical carbene complexes

Transition metal-carbene complexes, review

Tungsten complexes metal carbene catalysts

© 2024 chempedia.info