Nucleophilic metal-carbene complexes

The picture is quite different for nucleophilic metal-carbene complexes. Here, contributing structures 10 and 13 seem to make the most contribution to the overall structure. Support for this observation comes from temperature-dependent NMR measurements8 of the M-C rotational barriers of various Ta-carbene complexes. The values obtained range from 12 to 21 kcal/mol, and seem to indicate considerable double bond character (structure 10). 

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. 

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. 

The reaction of isocyanide complexes with nucleophiles gives metal-carbene complexes [49], which constitute an important branch of organometallic chemistry and are effective catalyst systems for a variety of processes [50, 51]. 

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. 

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). 

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. 

Nucleophilic metal alkylidene complexes are more useful for promoting the metathesis polymerisation of cycloolefins than electrophilic metal carbenes. For instance, Br2(Me3CCH20)2W=CHCMe3 is a moderately active catalyst [75,89] that can be further activated by the addition of Lewis acids such as GaBr3 to... 

The reaction occurs well below the temperature at which most of the parent metal carbonyls exchange with free CO and so is a direct nucleophilic attack on coordinated CO, although it may alternatively proceed via a prior electron path. The resulting acyl anions can be isolated as their [R4N] " or [ (C6H5)3P 2N] salts but are reactive and are used directly in subsequent alkylations with organic halides, acetylenes, a-/i-unsaturated carbonyls and alkyloxonium salts to form organic condensation products or metal-carbene complexes. 

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. ... 

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... 

The reaction of these transition metal carbene complexes with some nucleophiles such as isocyanante, thiophenol, hydrazine, orhydroxylamine,have been studied. For example, the carbene part of the complex is converted into vinyl ether by pyridine 100>. 

Stable metal carbene complexes, such as W[=C(OMe)Me](CO)5, were first prepared by Fischer, E.O. (1964) and many hundreds are now known. Some, like 1, react with other compoimds in such a way as to indicate that the carbene ligand is electrophilic others, such as 2, react in the opposite way, indicating that the carbene ligand is nucleophilic (Grubbs 1978 Parshall 1980). 

Perfluorooxiranes in which a different perfluoroalkyl group substitutes for the CF3 of HFPO can also function as sources of difluorocarbene. In the presence of nickel powder, for example, oxirane 32 reacts with iodine to give difluorodiiodomethane (33) in high yield, accompanied by small amounts of oligomeric diiodides." Yields are very low in the absence of nickel, and it is suggested that the reaction occurs on the surface of the metal with a nucleophilic nickel-carbene complex. 

Carbenes are defined as species containing divalent carbon [1], and they may display either electrophilic or nucleophilic reactivity depending on whether the two unshared electrons on the carbon center are unpaired (triplet carbene) or paired (singlet carbene). Metal-carbene complexes can be classified in a similar way based on their reactivity toward electrophiles and nucleophiles. The resonance forms shown in Fig. 4.1 define the limiting structures, and the formal charge on the carbene carbon indicates the preferred reactivity. Those that are nucleophilic at carbon are called Schrock-type complexes or alkylidenes, and they generally... 

Various elementary processes such as oxidative addition, reductive elimination, olefin and CO insertion into the metal-to-carbon bond have found extensive applications in organic synthesis. Other processes such as attack of nucleophiles on metal-bound CO and olefins, unique reactions of metal carbene complexes, and a-bond metatheses are among the topics of special interest to organometalhc chemists as well as to synthetic organic chemists. Our aim is to provide the reader with detailed accounts of elementary processes with the hope that the information provided here is used for further development of molecular catalysis. 

However, the formation of the metal-carbene complex was not observed in pure, halide-free [BMIM][Bp4], indicating that the formation of carbene depends on the nucleophilicity of the ionic liquid s anion. To avoid the formation of metal-carbene complexes by deprotonation of the imidazolium cation under basic conditions the use of 2-methyl-substituted imidazolium is frequently suggested. However, it should be mentioned here that strong bases can also abstract a proton to form the vinyl imidazolidene species which may also act as a strong ligand to electrophilic metal centers. 

Actually, terminal metal carbene and alkylidene complexes are ubiquitous throughout the transition elements. The nomenclatural distinction between "carbene" and "alkylidene" represents a fundamental difference in reactivity. Metal carbene complexes usually behave as electrophiles, with typical reactions including cycloadditions to un-saturabed bonds (e.g. cyclopropanation of olefins). On the other hand, metal alkylidene complexes are nucleophilic, undergoing Wittig-type alkylations and olefin metathesis. 

The bonding interactions of a carb)me ligand are essentially those of a metal carbene complex, but with an additional ir-bond (Figure 2.16). One orbital of cr-symmetry and two of iT-symmetry overlap with three metal orbitals of appropriate symmetry. When considered trianionic, all three orbitals of the ligand fragment contain two electrons. Theoretical studies of heteroatom-substituted or "Fischer-type" carbyne complexes - indicate that the HOMO predominantly consists of the metal fragment, and the LUMO consists of one of the TT -orbitals of the metal-carbon bond. This result explains the tendency of nucleophiles to attack the carbyne carbon in carbyne complexes, just as they attack the carbene carbon in Fischer carbene complexes. 

As stated in Chapter 3, carbene complexes can be divided into the five classes illustrated in Figure 13.2. One class of carbene complex encompasses the Fischer carbenes that were first prepared in the laboratory of E. O. Fischer. These complexes were the first transition metal carbene complexes prepared, and they contain a ir-donating group on the carbene carbon. Complexes of these carbenes are typically electrophilic at the carbene carbon. A second class of carbene complex was first prepared by Richard Schrock. These complexes contain alkyl groups or hydrogens on the carbene carbon and are called alkylidene complexes or often "Schrock carbenes." Complexes of these carbenes are typically electrophilic at the metal and nucleophilic at the carbene carbon. 

Similar to the preceding class are reactions during which attack on CO takes place by an uncoordinated external reagent. Attack of external nucleophile occurs on the carbon atom. Alkylation of resulting acyl complexes leads to the formation of metal carbene complexes (see Chapter 5). 

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