Transition metal-carbene complexes, review

Phenylmethoxycarbenepentacarbonyltungsten(0) allowed to react at room temp, with methylidenetriphenylphosphorane in ether, and the intermediate methyl 1-phenylvinyl ether treated with dil. HCl acetophenone. Y 82%. F. e. and limitations s. C.P. Casey and T. J. Burkhardt, Am. Soc. 94, 6543 (1972) enolethers from diazo compds. s. Tetrah. Let. 1973, 1421 transition metal-carbene complexes, review, s. D. J. Cardin et al., Chem. Soc. Rev. 2, 99 (1973). 

I hope that I have been able to demonstrate, with this small selection of our newest research results, what a variety of reaction possibilities the chemistry of transition metal carbene complexes display. In the following I review an area whose development we have made most recently our special task, namely that of transition metal-carbyne complexes. 

Although a variety of new preparative routes has been developed in recent years (for reviews see refs 1 -10), the transformation of the metal-carbonyl carbon bond of a metal-carbonyl complex into a metal-carbene carbon bond is still the most useful and versatile method for preparing transition-metal carbene complexes. The addition of a carbanion to the carbon atom of a carbonyl ligand yields an anionic acyl complex that subsequently can be reacted with an electrophile to give a neutral carbene complex. Thus, the syntheses of anionic acyl and neutral carbene complexes are closely related, for almost all the carbene complexes considered in this section acyl complexes are precursors, although most have not been isolated and characterized. The syntheses of acyl complexes via CO insertion (for reviews see refs. 11, 12) or by reaction of metal carbonyl anions with acyl halides is outside the scope of this section. 

The chemistry of transition metal carbene complexes has been examined with an eye to applications in organic synthesis ever since their discovery by Fischer in 1964, and the growth in the number of useful applications has been exponential with tirne. " There are two types of transition metal carbene complexes those which have electrophilic carbene carbons and which are typified by the pentacarbonylchro-mium complex (1), and those which have nucleophilic carbene carbons and which are typified by the biscyclopentadienyltitanium complex (2). Complexes (1) and (2) are often referred to as carbene and alkylidene complexes, respectively. This review will be limited to the chemistry of electrophilic carbene complexes of the Fischer type. The chemistry of the nucleophilic alkylidene complexes will be covered in Chapter 9.3, this volume. ... 

Stoichiometric use of transition-metal-carbene complexes in organic synthesis has been thoroughly reviewed.Various examples using carbene complexes containing cyclopropane subunits have been reported.Here, the cyclopropyl moiety is either attached directly to the carbene carbon or is placed in a more remote position. This section only discusses isolated carbene complexes. Related transition-metal-catalyzed conversions of diazo compounds containing cyclopropane subunits, which are interpreted to proceed via carbene intermediates (e.g. ref 130), are not discussed here. 

Transition metal-carbene complexes undergo many more reactions in addition to these three and have been widely applied to the synthesis of organic molecules. Because this material is outside the scope of the chapter, we direct the interested reader to reviews on other carbene-mediated transformations [19-21]. 

Pertinent reviews published during 1980 cover cyclometallation of P-donor ligands, CO insertion into metal-carbon o-bonds, mechanistic features of catalytic CO hydrogenation reactions, and stoicheiometric reactions of transition-metal carbene complexes. Other articles of interest deal with the stability of metal-carbon bonds, " transition-state geometry for insertion of metals into C-H bonds, organic synthesis using Group VIII metal complexes, and C-H bond activation by transition metals. " Molecular orbital calculations on the interconversion of metal bis(olefin) and metallocyclo-pentane complexes have been reported. ... 

This article presents the principles known so far for the synthesis of metal complexes containing stable carbenes, including the preparation of the relevant carbene precursors. The use of some of these compounds in transition-metal-catalyzed reactions is discussed mainly for ruthenium-catalyzed olefin metathesis and palladium-Znickel-catalyzed coupling reactions of aryl halides, but other reactions will be touched upon as well. Chapters about the properties of metal- carbene complexes, their applications in materials science and medicinal chemistry, and their role in bioinorganic chemistry round the survey off. The focus of this review is on ZV-heterocyclic carbenes, in the following abbreviated as NHC and NHCs, respectively. 

Ethers, sulfides, amines, carbonyl compounds, and imines are among the frequently encountered Lewis bases in the ylide formation from such metal carbene complex. The metal carbene in the ylide formation can be divided into stable Fisher carbene complex and unstable reactive metal carbene intermediates. The reaction of the former is thus stoichiometric and the latter is usually a transition metal complex-catalyzed reaction of a-diazocarbonyl compounds. The decomposition of a-diazocarbonyl compounds with catalytic transition metal complex has been the most widely used approach to generate reactive metal carbenes. For compressive reviews, see Refs 1,1a. 

Dipolar cycloaddition reactions are most commonly applied for the synthesis of five-membered heterocyclic compounds.86 87 [3+2] cycloaddition reactions of transition-metal propargyl complexes have been reviewed.88 Addition of diazomethane to carbene complexes (CO)5Cr= C(OEt)R results in cleavage of the M = C bond with formation of enol ethers H2C = C(OEt)R,3 89 but (l-alkynyl)carbene complexes undergo 1,3-dipolar cycloaddition reactions at the M = C as well as at the C=C bond. Compound lb (M = W, R = Ph) affords a mixture of pyrazole derivatives 61 and 62 with 1 eq diazomethane,90 but compound 62 is obtained as sole... 

The activity of a given catalyst system will be determined by both the nature and the concentration of the active species. In Ch. 3 we present the evidence that the active species is a metal carbene complex, formed in some way from the transition metal compound. It is useful to keep this in mind when reviewing the different types of catalyst system. 

A review on the formation of ketene by addition of metal - carbene complexes to carbon monoxide has appeared, including a special development of transition-metal-catalysed carbonylation of metal - carbene complexes. ... 

A book surveys the catalysis of industrially significant reactions and another the chemistry of metal carbene complexes with reference to their intermediacy in catalytic reactions. General discussions of transition metal catalysed processes have appeared " , and mechanistic aspects of the literature covering the period mid-1982 to end-1983 have been reviewed. ... 

A60. J. P. Candlin, K. A. Taylor, and D. T. Thompson, "Reactions of Transition-Metal Complexes. Elsevier, Amsterdam, 1968. A review of types of reactions of metal complexes (e.g., substitution, combination, redox) reactions with various reagents (e.g., hydrocarbons, halides, carbon monoxide, and isonitrile) and preparation of new stabilised organic systems (e.g., metallocenes, carbenes). Intended for research workers, consequently written at a fairly high level, with emphasis on organometallics. A61. H. J. Keller, NMR-Untersuchungen an Komplexverbindungen. Springer, Berlin, 1970. Expansion of review article 37.1. 

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