Metal-carbene complexes Electrophilic

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. 

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... 

The metal-carbene complexes are electrophilic in character. They can, in fact, be represented as metal-stabilized carbocations. 

Thus the reactivity of transition metal-carbene complexes, that is, whether they behave as electrophiles or nucleophiles, is well explained on the basis of the frontier orbital theory. Studies of carbene complexes of ruthenium and osmium, by providing examples with the metal in either of two oxidation states [Ru(II), Os(II) Ru(0), Os(O)], help clarify this picture, and further illustrations of this will be found in the following sections. 

The interaction between catalyst and diazo compound may be initialized by electrophilic attack of the catalyst metal at the diazo carbon, with simultaneous or subsequent loss of N2, whereupon a metal-carbene complex (415) or the product of carbene insertion into a metal/ligand bond (416) or its ionic equivalent (417) are formed. This is outlined in a simplified manner in Scheme 43, which does not speculate on the kinetics of such a sequence, nor on the possible interconversion of 415 and 416/417 or the primarily formed Lewis acid — Lewis base adducts. 

Transition metal carbene complexes have broadly been classified into Fischer-type and Schrock-type carbene complexes. The former, typically low-valent, 18-electron complexes with strong 7t-acceptors at the metal, are electrophilic at the carbene carbon atom (C ). On the other hand, Schrock-type carbene complexes are usually high-valent complexes with fewer than 18 valence electrons, and without n-accepting ligands. Schrock-type carbene complexes generally behave as carbon nucleophiles (Figure 1.4). 

In addition to catalytically active transition metal complexes, several stable, electrophilic carbene complexes have been prepared, which can be used to cyclopropanate alkenes (Figure 3.32). These complexes have to be used in stoichiometric quantities to achieve complete conversion of the substrate. Not surprisingly, this type of carbene complex has not attained such broad acceptance by organic chemists as have catalytic cyclopropanations. However, for certain applications the use of stoichiometric amounts of a transition metal carbene complex offers practical advantages such as mild reaction conditions or safer handling. 

However, with substrates prone to form carbocations, complete hydride abstraction from the alkane, followed by electrophilic attack of the carbocation on the metal-bound, newly formed alkyl ligand might be a more realistic picture of this process (Figure 3.38). The regioselectivity of C-H insertion reactions of electrophilic transition metal carbene complexes also supports the idea of a carbocation-like transition state or intermediate. 

A variety of transition metal-carbene complexes have been prepared and characterized. None of these are known to efficiently effect intermolecular C-H insertion. An electrophilic iron carbcne complex can, however, participate in intramolecular C-H insertions (Section I.2.2.3.2.I.). More commonly, transition metal complexes are used to catalyze intramolecular C-H insertion starting with a diazo precursor. In these cases, the intermediate metal carbene complexes are not isolated. 

Transition-metal catalysis, especially by copper, rhodium, palladium and ruthenium compounds, is another approved method for the decomposition of diazo compounds. It is now generally accepted that short-lived metal-carbene intermediates are or may be involved in many of the associated transformations28. Nevertheless, these catalytic carbene transfer reactions will be fully covered in this chapter because of the close similarity in reaction modes of electrophilic carbenes and the presumed electrophilic metal-carbene complexes. 

Transition metal carbene complexes can be divided into two classes electrophilic carbenes (Fischer carbene [69-71], Casey carbene [72,73]) and nucleophilic carbenes (Osborn carbene [74,75], Schrock carbene [76-79]) ... 

As mentioned above, the electrophilic metal carbene complexes are stabilised by the presence of heteroatoms or phenyl rings at the divalent carbon atom, while hydrogen or alkyl groups stabilise the nucleophilic complexes. Therefore, there is a distinction between carbenoids and alkylidenes when designing carbene ligands corresponding to the former or the latter class. 

Electrophilic metal carbene complexes such as (CO)5W=C(Ph)OMe generally exhibit poor activity as catalysts for metathesis polymerisation, and higher temperatures are required to bring about the polymerisation of high-strained cycloolefins such as norbornene or cyclobutene [84,85], However, their activity can be enhanced by the addition of a Lewis acid such as TiCL into the polymerisation system [86]. Electrophilic complexes such as (CO)5W=CPh2 also generally exhibit poor activity but they are more active than those mentioned above and enable the polymerisation of various cycloolefins [87,88],... 

There are essentially three different types of transition metal carbene complexes featuring three different types of carbene ligands. They have all been named after their first discoverers Fischer carbenes [27-29], Schrock carbenes [30,31] and WanzUck-Arduengo carbenes (see Figure 1.1). The latter, also known as N-heterocycUc carbenes (NHC), should actually be named after three people Ofele [2] and Wanzlick [3], who independently synthesised their first transition metal complexes in 1968, and Arduengo [1] who reported the first free and stable NHC in 1991. Fischer carbene complexes have an electrophilic carbene carbon atom [32] that can be attacked by a Lewis base. The Schrock carbene complex has a reversed reactivity. The Schrock carbene complex is usually employed in olefin metathesis (Grubbs catalyst) or as an alternative to phosphorus ylides in the Wittig reaction [33]. 

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. ... 

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. 

Transition metal-carbene complexes possess several sites where nucleophiles, electrophiles, oxidizing agents, and protic acids might attack these are depicted... 

At the beginning of Section 10-3, we commented that metal-carbene complexes exhibit a spectrum of reactivities with nucleophiles and electrophiles, especially at Qarbene- Carbene complexes of mid-transition metals (Groups 7-9) without heteroatomic substituents at Ccarbene may show electrophilic behavior depending upon the nature of other ligands, oxidation state of the metal, and overall charge on the complex. From some observations listed below, we may be able to discern a pattern of reactivity.63... 

In certain cases, the metal-carbene complex derived from an unsaturated diazocarbonyl compound can be trapped intramolecularly in reactions other than cyclopropanation, e.g. C-H insertion and ylide formation by interaction with a heteroatom with a lone pair. Since the chemoselectivity is influenced by the electrophilicity of the metal-carbene complex, it may be controlled in favorable cases by the catalyst metal and its ligands or by the second substituent at the carbenoid carbon atom. 

Electrophilic transition-metal-carbene complexes (Fischer carbene complexes) serve as formal carbene transfer reagents in reactions with alkenes to give functionalized cyclopropanes. This reaction behavior is well documented for alkoxycarbene complexes of elements of group In contrast, aminocarbene complexes exhibit a different reactivity over a wide range of conditions and [2 + 1] cycloadditions to alkenes represent exception. 

New evidence as to the nature of the intermediates in catalytic diazoalkane decomposition comes from a comparison of olefin cyclopropanation with the electrophilic metal carbene complex (CO)jW—CHPh on one hand and Rh COAc) / NjCHCOOEt or Rh2(OAc)4 /NjCHPh on the other . For the same set of monosubstituted alkenes, a linear log-log relationship between the relative reactivities for the stoichiometric reaction with (CO)5W=CHPh and the catalytic reaction with RhjfOAc) was found (reactivity difference of 2.2 10 in the former case and 14 in the latter). No such correlation holds for di- and trisubstituted olefins, which has been attributed to steric and/or electronic differences in olefin interaction with the reactive electrophile . A linear relationship was also found between the relative reactivities of (CO)jW=CHPh and Rh2(OAc) NjCHPh. These results lead to the conclusion that the intermediates in the Rh(II)-catalyzed reaction are very similar to stable electrophilic carbenes in terms of electron demand. As far as cisjtrans stereoselectivity of cyclopropanation is concerned, no obvious relationship between Rh2(OAc) /N2CHCOOEt and Rh2(OAc),/N2CHPh was found, but the log-log plot displays an excellent linear relationship between (CO)jW=CHPh and Rh2(OAc) / N2CHPh, including mono-, 1,1-di-, 1,2-di- and trisubstituted alkenes In the phenyl-carbene transfer reactions, cis- syn-) cyclopropanes are formed preferentially, whereas trans- anti-) cyclopropanes dominate when the diazoester is involved. 

Mechanistic studies of rhodium porphyrins as cyclopropanation catalysts have resulted in the spectroscopic identification of several potential intermediates in the reaction of ethyl diazoacetate with olefins, including a diazoniumfethoxy-carbonyl)methyl-rhodium complex formed by electrophilic addition of the rhodium center to the a-C atom of ethyl diazoacetate [29]. It is not known if analogous intermediates are also formed in analogous reactions of copper catalysts. However, the initial part of the catalytic cycle leading to the metal carbene intermediate is not of primary concern for the enantioselective reactions described herein. It is the second part, the reaction of the metal-carbene complex with the substrate, that is the enantioselective step. 

The metal-carbene complexes postulated as intermediates in transition metal-catalyzed reactions of diazo compounds are electrophilic species (especially if they are derived from a-diazocarbonyl compounds). Accordingly, electron-rich olefins are the most suitable substrates for copper-catalyzed cyclopropanations, whereas electron-poor substrates such as a,P-unsaturated carbonyl compounds in general are not sufficiently reactive. 

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