Electrophiles with carbene ligands

All around this chapter, we have seen that a,/J-unsaturated Fischer carbene complexes may act as efficient C3-synthons. As has been previously mentioned, these complexes contain two electrophilic positions, the carbene carbon and the /J-carbon (Fig. 3), so they can react via these two positions with molecules which include two nucleophilic positions in their structure. On the other hand, alkenyl- and alkynylcarbene complexes are capable of undergoing [1,2]-migration of the metalpentacarbonyl allowing an electrophilic-to-nucleophilic polarity change of the carbene ligand /J-carbon (Fig. 3). These two modes of reaction along with other processes initiated by [2+2] cycloaddition reactions have been applied to [3+3] cyclisation processes and will be briefly discussed in the next few sections. 

Methylation of the dihaptothioacyl complex 22 affords compound 23 containing a bidentate carbene ligand, which on reaction with chloride ion leads to the neutral monodentate carbene complex 24 (50,51). The chelate carbene complex 26 is generated in a novel interligand reaction from the thiocarboxamidothiocarbonyl cation 25. The thiocarbonyl carbon acts as the electrophilic component in this reaction, and 26 is further alkylated to a bidentate dicarbene species (52). 

At the present, the most straightforward mechanism for the formation of J5 from 1 is via insertion of CO into the Th-C(acyl) bond to form a ketene (H, (eq. (4)) which subsequently dimerizes. Presumably, initial CO interaction could involve coordination either to the metal ion as shown or to the electrophilic vacant "carbene p atomic orbital. Considering the affinity of the Th(IV) ion for oxygenated ligands, interaction of the ketene oxygen atom with the metal ion seems reason-... 

Carbene complexes of transition metals [2,21,225-236] are typical representatives of compounds with a double metal-carbon bond. They are seen as derivatives of a two-covalent carbon in their singlet state [226,232,236]. As a rule, the carbene ligand is an effective a-donor and a comparatively weak n-acceptor. Formation of a cr-bond M — C takes place via transference of a nonbonding electronic pair with a nucleophilic a-orbital of the carbenic carbon to the metal atom. Simultaneously, it is also possible to form a 7t-bond as a result of the interaction of symmetrically appropriate metallic d-AO with a vacant electrophilic /7-orbital of the carbene [236,237], This situation is a key factor that determines the polarization of most of the carbene complexes according to type 145 (Fig. 2.6). 

We have already established that the carbene carbon is an electrophilic center and, hence, it should be very easily attacked by nucleophiles. In most reactions we believe that the first reaction step probably involves attachment of a nucleophile to the carbene carbon. In some cases, for instance with several phosphines (49) and tertiary amines (50), such addition products are isolable analytically pure under certain conditions (1 in Fig. 3). For the second step there exists the possibility that the nucleophilic agent may substitute a carbon monoxide in the complex with preservation of the carbene ligand (2 in Fig. 3). One can also very formally think of the carbene complex as an ester type of system [X=C(R )OR with X = M(CO)j instead of X = 0], because the oxygen atom as well as the metal atom in the M (CO) 6 residue are each missing 2 electrons for attainment of an inert gas configuration. So, it is not surprising that the... 

As mentioned earlier, the carbene ligand in our complexes shows nucleophilic character with respect to the metal fragment. Therefore, we decided to combine it with an electrophilic carbene. For this purpose we treated pentacarbonyl[methoxy(phenyl)carbene]chromium(0) with phenyl(tri-chloromethyl) mercury (85). Compounds of this kind have been studied intensively by Seyferth et al. (86) and are known as a source of dihalogeno-carbenes. The carbene complex reacted with the carbenoid compound at... 

Deviating from the route via nucleophilic attack of the carbanion at the carbon atom of a CO ligand and then reaction of the acylmetallate with an electrophile are those methods which involve (a) addition of the carbanion to the carbon atom of a carbyne ligand, (b) displacement of halides from transition-metal carbonyl halides by cyclohepta-trienyllithium, or derivatives thereof, followed by hydride abstraction or (c) substitution of a coordinated solvent from a metal-carbonyl complex (see also reaction of LiR with carbene complexes). 

A stepwise twofold electrophilic attack occurs on the carbyne complex 229 to first give the cationic t/ -carbene complex 230 and then the dica-tionic dithiatungstabicyclo[ 1.1,0]butene complex 231 156). A similar tf-carbene ligand forms in the reaction of the thiocarbyne complex [W(CO)2(CSMe) HB(pz)3)] (232) with Me2(MeS)S+(757). 

Reactions of electrophilic metal carbenes and carbynes with nucleophilic metal centers are used to prepare a large number of clusters, most containing two or more different metals. The metal-metal bond formed is usually bridged by the carbene or carbyne ligand, but ligand transfer can lead to homometallic products. ... 

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). 

Remarkably, mononitrosyl iron(—II) complexes displayed great potential in the activation of diazo compormds and carbene-transfer reactions [102]. Generally, the activation of diazo compound can be realized by electrophilic transition metal complexes. However, according to the concept of Umpoirmg [103], the electron-rich, nucleophilic iron(—II) compound 31 is expected to react with diazo compounds of electron-poor carbenes, such as ethyl diazoacetate (Scheme 42). At first the iron center would add the C=N bond of the diazo compound followed by release of N2 and formation of the electrophilic iron carbene moiety. The nitrosyl group in such transformations is assumed to support as an ancillary ligand the N2 release by pulling electron density to the iron center. 

Alternatively, the alkoxy groups of Fischer carbenes can be abstracted with a Lewis acid, as shown in Scheme 13.8. Abstraction of an alkoxy group from the Fischer carbene ligand of a neutral complex generates a cationic carbyne complex. The carbyne complex is more electrophilic than the starting carbene complex, and reaction of this carbyne complex with an alcohol regenerates an alkoxy-substituted carbene. 

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