Preparation of Carbene Complexes

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].

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. 

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

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. 

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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