Preparation of Carbenes

Scheme 2 shows the preparation of carbene complexes starting from [L M-ER3]" = [MeCp(CO)2Mn-SiMePlt2] and [(7r-arene)(CO)2Cr-SnPh3] [12,13]. By a related approach, a cationic cyclopropenylidene complex was prepared by reaction of Cp(CO)2FeSiMe3 with l-chloro-2,3-diphenylcyclopropenylium tetrafluoroborate [14],... 

The most practical approach is the direct treatment of azolium salts with metal complexes under neutral or basic conditions [39,154-159]. Alternatively, the free carbenes can be generated in the presence of a suitable metal complex by reduction of a carbene precursor, e.g. a thiourea [160]. Stable, uncomplexed imidazoline-2-ylidenes, isolated for the first time in 1991 by Arduengo [161] (for further examples see [162-166]), are also convenient starting materials for the preparation of carbene complexes [167,168]. The corresponding diaminocarbene complexes can be obtained by treatment of the stable diaminocarbenes with transition metal complexes. Finally, at high temperatures many transition metal complexes catalyze the carbon-carbon bond scission of tetraaminoethylenes, forming carbene complexes [169-171]. Examples of such preparations are given in Table 2.8. 

Donor-substituted alkynes can insert into the C-M double bond of alkoxycarbene complexes, yielding donor-substituted vinylcarbene complexes [191,192]. In addition to this, photolysis or thermolysis of a-alkoxycyclopropyl carbonyl complexes or a-alkoxycyclobutanoyl complexes can lead to rearrangement to metallacyclic carbene complexes (Table 2.11). This methodology has not been used as extensively for the preparation of carbene complexes as the other methods described above. 

Fig. 3.27. Preparation of carbene complexes by addition of electrophiles to alkynyl and alkenyl complexes [89,391,552-555],...

Fig. 3.30. Preparation of carbene complexes by sequential [2 + 2] cycloadditions and [2 + 2] cycloreversions of carbene and carbyne complexes to alkenes and alkynes [596-598].

Gold(I) isonitrile complexes were first reported (131) in 1956, when it was found that they were generally less stable than the isonitrile complexes of other transition metals. This was attributed to the low tendency of gold to form bonds with double bond character. A number of isonitrile complexes have been made, however, and they have been used extensively in the preparation of carbene and imino complexes of gold(I). 

A common method for the preparation of carbene complexes without heteroatom substituents is the reaction of a coordinatively unsaturated metal complex with a diazoalkane. This method is effective for the preparation of CpOsCl(P Pr3)(=CHPh) from CpOsCl(PTr3)2. The carbene in this complex is electrophilic and reacts with aUcyl and aryl lithium compounds and with Grignard reagents see Grignard Reagents) (Scheme 14). ... 

Another extension of the synthetic route for the preparation of carbene complexes via attack of a nucleophile at the carbon atom of a coordinated CO ligand is the replacement of the carbanion by other nucleophiles such as R NLi , PhjC=NLi, MejPLi , RjSiLi RMgCl or KOEt. ... 

Numerous functionalized aryl- ", alkyl- , and vinyllithium " compounds have been used as acylating reagents for the preparation of carbene complexes mainly of Cr and W. 

There are many methods for the preparation of carbenes. A simple approach is to photolyze ketenes (290) at 270-370 nm (100.5-76.5 kcal moT, 438.8-320.2 kJ mol l) in the presence of oxygen.226 irradiation of 290 at 270 nm gave carbon monoxide (CO, with a quantum yield = 2) and the yield of ethene was independent of the pressure of oxygen. At 370 nm, the quantum yield of CO varies with the pressure of applied oxygen and also with the temperature. The presence of additional oxygen diminishes the yield of ethene by = 3.226 xhe reaction proceeds via formation of excited ketene (291), which fragments to methylene... 

B.i. Diazomethane. Important precursors for the preparation of carbenes and related molecules are diazoalkanes. Although diazomethane is not used in carbene preparations, it has other synthetic applications There are four convenient preparations of diazomethane, which is a highly reactive gas. 

The nonspecialist reader of this section may come to the conclusion that the area of transition-metal complexes with alkyldiazenido ligands is not very large. This impression is reinforced by the fact that these complexes lose the diazo group relatively easily, as mentioned in the beginning of the section. It is wrong, however, if one considers the fact that diazoalkanes have been used, and still are, extensively in the preparation of carbenes via metastable diazenido intermediates of metal-complex catalysts in laboratory scale syntheses, in the Fischer-Tropsch reaction, and in olefin metathesis (see, e.g., Herrmann, 1978 Doyle 1986a, 1986b and Sects. 8.7 and 8.8 of this book). 

METHODS OF PREPARATION OF CARBENE COMPLEXES - a. From metal carbonyls... 

This method is often utilized for preparation of carbene compounds which do not contain heteroatoms. Trityl salts and Me3P = CH2 are utilized as hydrogen atom trapping agents ... 

Like the organic dihalides, dialkyl(chloromethylene)ammonium chlorides react with many transition metal complexes. This method allows preparation of carbene complexes in which central atoms possess high oxidation states. 

In 2007, Nolan and co-workers presented a reaction in which a Pt-coordi-nated olefin was attacked by exogenous free carbene. Considering that binding of an olefin to metals such as Pt dramatically increased their electro-philicity, it is surprising this type of reaction has not been more commonly observed in the attempted preparation of carbene complexes from this class of precursors. 

Before the first free stable carbene could be isolated, the in situ preparation of carbenes was implicitly required for synthesis of transition NHC-metal... 

In 1998, Liu et al. reported the preparation of carbene carbonyl- or biscar-bene-Pt complexes using a transfer reaction between W-complex and [(PhCN)2PtCl2]. On the other hand, the Cavell group proposed an oxidative addition of imidazolium salt to Pt complex to yield Pt -hydrido/NHC... 

Air and moisture stable imidazolium-2-carboxylates can act as NHC precursors for the preparation of carbene complexes of metals such as Rh, Ru, Ir and Pd. However, access to the carboxylate derivatives is limited. Therefore, readily available N,N -dimesitylimidazolium-2-isobutylester was used as an alternative carbene source. Exclusion of air and moisture is not necessary during the preparation of complexes from these NHC precursors, making free NHCs unlikely intermediates in the reaction. In general, the use of azolium-2-carboxylates constitutes an interesting high yield procedure for the preparation of NHC complexes under mild reaction conditions. 

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