Carbene complexes ylide formation

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

The q1-coordinated carbene complexes 421 (R = Ph)411 and 422412) are rather stable thermally. As metal-free product of thermal decomposition [421 (R = Ph) 110 °C, 422 PPh3, 105 °C], one finds the formal carbene dimer, tetraphenylethylene, in both cases. Carbene transfer from 422 onto 1,1-diphenylethylene does not occur, however. Among all isolated carbene complexes, 422 may be considered the only connecting link between stoichiometric diazoalkane reactions and catalytic decomposition [except for the somewhat different results with rhodium(III) porphyrins, see above] 422 is obtained from diazodiphenylmethane and [Rh(CO)2Cl]2, which is also known to be an efficient catalyst for cyclopropanation and S-ylide formation with diazoesters 66). 

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

Electrophilic transition metal complexes can react with organic ylides to yield alkylidene complexes. A possible mechanism would be the initial formation of alkyl complexes, which are converted into the final carbene complexes by electrophilic a-abstraction (Figure 3.18). This process is particularly important for the generation of acceptor-substituted carbene complexes (Section 4.1). 

The intramolecular addition of acylcarbene complexes to alkynes is a general method for the generation of electrophilic vinylcarbene complexes. These reactive intermediates can undergo inter- or intramolecular cyclopropanation reactions [1066 -1068], C-H bond insertions [1061,1068-1070], sulfonium and oxonium ylide formation [1071], carbonyl ylide formation [1067,1069,1071], carbene dimerization [1066], and other reactions characteristic of electrophilic carbene complexes. 

Acceptor-substituted carbene complexes are highly reactive intermediates, capable of transforming organic compounds in many different ways. Typical reactions include insertion into o-bonds, cyclopropanation, and ylide formation. Generally, acceptor-substituted carbene complexes are not isolated and used in stoichiometric amounts, but generated in situ from a carbene precursor and transition metal derivative. Usually only catalytic quantities of a transition metal complex are required for complete conversion of a carbene precursor via an intermediate carbene complex into the final product. 

Several examples have been reported for furanone formation by intramolecular C-H insertion of electrophilic carbene complexes [1006,1148] (Table 4.7). Yields can, however, be low with some substrates, possibly as a result of several potential side-reactions. Oxonium ylide formation and hydride abstraction, in particular, [1090,1149-1152] (see Section 4.2.9) seem to compete efficiently with the formation of some types of furanones. 

Electrophilic carbene complexes can react with amines, alcohols or thiols to yield the products of a formal X-H bond insertion (X N, O, S). Unlike the insertion of carbene complexes into aliphatic C-H bonds, insertion into X-H bonds can proceed via intermediate formation of ylides (Figure 4.7). 

Ylide formation, and hence X-H bond insertion, generally proceeds faster than C-H bond insertion or cyclopropanation [1176], 1,2-C-H insertion can, however, compete efficiently with X-H bond insertion [1177]. One problem occasionally encountered in transition metal-catalyzed X-H bond insertion is the deactivation of the (electrophilic) catalyst L M by the substrate RXH. The formation of the intermediate carbene complex requires nucleophilic addition of a carbene precursor (e.g. a diazocarbonyl compound) to the complex Lj,M. Other nucleophiles present in the reaction mixture can compete efficiently with the carbene precursor, or even lead to stable, catalytically inactive adducts L M-XR. For this reason carbene X-H bond insertion with substrates which might form a stable complex with the catalyst (e.g. amines, imidazole derivatives, thiols) often require larger amounts of catalyst and high reaction temperatures. 

Fig. 4.8. Formation and Stevens rearrangement of ammonium ylides from acceptor-substituted carbene complexes.

The intermolecular reaction of imines with acceptor-substituted carbene complexes generally leads to the formation of azomethine ylides. These can undergo several types of transformation, such as ring closure to aziridines [1242-1245], 1,3-dipolar cycloadditions [1133,1243,1246-1248], or different types of rearrangement (Figure 4.9). 

Fig. 4.9. Formation and transformations of azomethine ylides from imines and electrophilic carbene complexes.

The generation of electrophilic carbene complexes in the presence of nitriles or other cyano-group-containing compounds can lead to the formation of nitrile ylides. With acylcarbene complexes the final products are often 1,3-oxazoles [1194], presumably formed by the mechanism sketched in Figure 4.10. 

If chiral catalysts are used to generate the intermediate oxonium ylides, non-racemic C-O bond insertion products can be obtained [1265,1266]. Reactions of electrophilic carbene complexes with ethers can also lead to the formation of radical-derived products [1135,1259], an observation consistent with a homolysis-recombination mechanism for 1,2-alkyl shifts. Carbene C-H insertion and hydride abstraction can efficiently compete with oxonium ylide formation. Unlike free car-benes [1267,1268] acceptor-substituted carbene complexes react intermolecularly with aliphatic ethers, mainly yielding products resulting from C-H insertion into the oxygen-bound methylene groups [1071,1093]. 

Fig. 4.13. Formation and reactions of carbonyl ylides from carbonyl compounds and electrophilic carbene complexes.

If acceptor-substituted carbene complexes are generated in the presence of thioethers, ylide formation is generally the mostly favored process. The resulting sulfonium ylides are often sufficiently stable to be isolated [975,1307-1309]. Typical reactions of sulfonium ylides include 1,2-alkyl migration, leading to products of... 

When thiocarbonyl derivatives are treated with an excess of electrophilic carbene complex, alkenes are usually obtained [1333-1336], The reaction is believed to proceed by the mechanism sketched in Figure 4.18, closely related to the thiocarbonyl olefination reaction developed by Eschenmoser [1337], Few examples have been reported in which stable thiiranes could be isolated [1338], The intermediate thiocarbonyl ylides can also undergo reactions similar to those of carhonyl ylides, e.g. 1,3-dipolar cycloadditions or 1,3-oxathiole formation [1338], Illustrative examples of these reactions are given in Table 4.22. 

Intramolecular C-H bond insertion and ylide formation can compete with cyclopropanation. As shown in Figure 4.21, however, the chemoselectivity of the intermediate carbene complex can sometimes be controlled by the remaining metal-bound ligands [21,990,1075,1081,1223]. 

Previous studies of the photochemistry of alkylchlorodiazirines have shown that the yield of trappable carbene is sensitive to the alkylcarbene structure. A laser flash photolysis study of phenanthridenes (91), precursors of alkylchlorocarbenes, in the presence of pyridine, has ruled out the intermediacy of a carbene-pyridine complex which partitions between pyridine-ylide formation and [1,2]-H shift. ... 

The mechanism by which this intermediate rhodium carbene complex 18 reacts can be more easily understood if it is written as the inverted ylide 19, as this species would clearly be electrophilic at carbon. We hypothesized that for bond formation to proceed, a transition state 20 in which the C-Rh bond is aligned with the target C-H bond... 

Formation of Oxygen Ylide from Metal Carbene Complexes and Subsequent Reactions 152... 

Formation of Nitrogen Ylide from Metal Carbene Complex and Subsequent Reactions 168... 

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. 

The oxygen as heteroatom in ethers or carbonyl compounds is weak to moderate Lewis base. Nevertheless, a highly reactive metal carbene complex can interact with the oxygen to generate oxygen ylide. The interaction between ether and metal carbene functional groups is believed to be rather weak as demonstrated by the facts that other metal carbene reactions, such as G-H insertion and cyclopropanation, can proceed in ethereal solvents." These experiments demonstrate that the formation of the metal ylide is much less favored in the equilibrium shown in Equation (1). ... 

The major reaction pathways for sulfonium ylide formation generated from a metal carbene complex and sulfide are [2,3]-sigmatropic rearrangement and [l,2]-shift, similar to those of the oxonium ylide formation. 

These carbene (or alkylidene) complexes are used as either stoichiometric reagents or catalysts for various transformations which are different from those of free carbenes. Reactions involving the carbene complexes of W, Mo, Cr, Re, Ru, Rh, Pd, Ti and Zr are known. Carbene complexes undergo the following transformations (i) alkene metathesis (ii) alkene cyclopropanation (iii) carbonyl alkenation (iv) insertion to C—H, N—H and O—H bonds (v) ylide formation and (vi) dimerization. Their chemoselectivity depends mainly on the metal species and ligands, as discussed in the following sections. 

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