Metal-carbene complexes synthesis

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. 

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. 

The importance of transition metal carbene complexes (compounds with formal M=C bonds) and of transition metal carbyne complexes (compounds with formal M=C bonds) is now well appreciated. Carbene complexes are involved in olefin metathesis (7) and have many applications in organic synthesis (2), while carbyne complexes have similar relevance to... 

In spite of the fact that silver(i) X-heterocyclic carbene complexes were widely employed as carbene-transfer reagents for the synthesis of other transition metal carbene complexes, their synthesis could also be achieved by the reaction of silver salts with relatively more labile carbene metal complexes, albeit rare. Complexes 71a-71c were reported to be synthesized from the reaction of the corresponding pentacarbonyl(carbene)chromium(i) complexes with silver(i) hexafluorophosphate in CDC13 under inert atmosphere (Scheme 17).117... 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

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. 

Carbenes and carbenoids have long been recognized as a highly reactive species and are frequently used as intermediates in organic synthesis. From a synthetic perspective, however, most of the carbenes are relatively short-lived and are too reactive to be controlled. Recently, metal-carbene complexes (or metaUocarbenes) were found to be easier to control and are nowadays widely used in organic synthesis . 

Olefin metathesis has proved to be a powerful synthetic tool in organic synthesis.5 The advent of well-defined metal carbene complexes with remarkable functional group tolerance has rendered metathesis as an efficient route to the synthesis of new C-C bonds. Examples of widely used ruthenium metathesis catalysts include [Ru-1],6 [Ru-2]7 and [Ru-3] 8 (Figure 1). 

The synthesis of metal complexes of type 213 can be performed by reacting metal-carbene complexes with selenium sources such as alkyneselenolates 203.430 Also the stability of unstable selenocarbonyl compounds such as selenoaldehydes can be enhanced by coordination to metal carbonyls and the reactivity of such complexes has been studied. Complex 216 can react with methylthiohexyne and the product is a different complex 217 with the selenium atom still coordinating to the metal carbonyl fragment (Scheme 66).431... 

H. Fischer, The Synthesis of Carbene Complexes, in Transition Metal Carbene Complexes (Eds. K. H. Dotz, H. Fischer, P. Hofmann, F. R. Kreissl, U. Schubert, and K. Weiss, Verlag Chemie, Weinheim, 1983, pp. 4-8). 

Synthesis of transition metal carbene complexes is achieved by the silver(I) oxide method and subsequent transfer to the next transition metal (in this case gold) (see Figure 3.62). Two different types of silver(l) carbene complexes are realised, the monocarbene and bis-carbene complexes. Transfer of the carbene to a gold(l) centre does not always occur with retention of structure as only the gold(l) monocarbene complexes are observed. 

Yang et al. used a similar protocol (an ether functionality supported on a primary alkyl halide carrier) to introduce an acetal on either side of the imidazole ring generating an ether functionalised ionic liquid (IL) imidazolium salt [183] (see Rguie 3.58). The anion could be varied without loss of the IL property (melting point below 1(X) °C) [184]. Synthesis of the transition metal carbene complexes (palladium) was done by carbene transfer ftom the corresponding silver(I) complexes or by reaction with the metal acetate (nickel) [162] (see Figure 3.64). 

This approach makes the synthesis of neutral zwitterionic transition metal carbene complexes possible, with valuable advantages over cationic complexes [230]. The concept was previously applied to N, P and S ligands [231,232] and has a carbene precursor in Roesler s backbone boron substituted NHC ligands [233] the latter has not been coordinated to transition metals yet. 

Moore et al. [274] introduced a sulfonato functionality into a NHC wingtip group to make the resulting transition metal carbene complexes water soluble. Introduction of the functional group was achieved in a modification of the epoxide method previously used by Arnold et al. [33] and Glas et al. [12] for the synthesis of alkoxide functionalised carbenes (see Section 4.1). Reaction of an N-substituted imidazole with... 

The chemistry of transition metal carbene complexes with NHC derived from purines or xanthines has its roots in the synthesis of xanthinium betaines [75], the xanthine analogues of imidazolium salts. The synthesis is sttaightforward and involves the methylation of the xanthine with methyl tosylate (see Figure 6.32). 

Although this reaction represents the first time that a purine base was deliberately reacted to generate a transition metal carbene complex, no application for this rhodium complex was reported. One could have envisaged catalytic trials. However, the efficiency of the silver oxide route for these purine base carbene precursors was recognised and their potential for the synthesis of late transition metal carbene complexes reported. 

More recent developments in the mechanistic aspects of the alkene metathesis reaction include the observation that the alkene coordinates to the metal carbene complex prior to the formation of the metallacyclobutane complex. Thns a 2 - - 2 addition reaction of the alkene to the carbene is very unlikely, and a vacant coordination site appears to be necessary for catalytic activity. It has also been shown that the metal carbene complex can exist in different rotameric forms (equation 11) and that the two rotamers can have different reactivities toward alkenes. " The latter observation may explain why similar ROMP catalysts can produce polymers that have very different stereochemistries. Finally, the synthesis of a well-defined Ru carbene complex (equation 12) that is a good initiator for ROMP reactions suggests that carbenes are probably the active species in catalysts derived from the later transition elements. ... 

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