Reactions of Metal-Carbene Complexes

Because free NHCs bind so strongly to a metal, it is not necessary to use an excess amount of these ligands to make NHC-metal complexes. This contrasts with the synthesis of metal phosphine complexes, where often an excess of the ligand is required for success. There are numerous examples of simple combination of the free carbene and a suitable metal complex. Equation 10.25 illustrates one example.43 

Because back-donation of electron density to ligands is not likely with high-valent, early transition metals, it seems reasonable that complexes between such metals and strong a donor ligands such as NHCs, which are not good n acceptors, could be synthesized. Synthesis of such complexes is indeed possible, as equation 10.26 demonstrates.44 

Transition metal-carbene complexes possess several sites where nucleophiles, electrophiles, oxidizing agents, and protic acids might attack these are depicted 

Blaser, and R. Boese, Inorg. Chim. Acta, 1995, 238, 179. 

Reactive Sites of Transition Metal-Carbene Complexes 

---

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. 

An impressive series of carbene- and carbyne-bridged complexes of platinum have been formed by reaction of metal carbene complexes with a platinum complex (equation 141). These complexes have been verified by X-ray crystallography, and the synthetic method appears to be one of some generality. Among the complexes of this type formed from carbenes are ones having metal frameworks with Pt—W,409-413 Pt—Mn414-418 and Pt—Cr413,417 bonds. 

In connection with these catalytic cyclopropanation reactions, it should be mentioned that the isolable ruthenium-carbene complex 162, which is obtained from 19, [RuCMp-cymene)]2 and 2,6-bis(4-isopropyl-l,3-oxazolin-2-yl)pyridine, reacts with styrene at elevated temperature in a carbene transfer reaction83 (equation 41). Since complex 162 is also catalytically active for (alkoxycarbonyl)carbene transfer to olefins, this reaction represents one of the few connecting links between catalytic and stoichiometric carbene transfer reactions of metal-carbene complexes. 

One of the synthetic procedures of metal complexes of type 53 is the reaction of metal-carbene complexes with selenium sources such as alkyneselenolates [109]. The stability of selenobenzaldehyde is enhanced by coordinating to metal carbonyls, and the reactivity of the complexes has been studied [110]. For example, the selenobenzaldehyde complexes reacts with methylthiohexyne even at - 30 °C to afford another type of complex where the selenium atom of the selenocarbonyl group is still coordinated to the metal (Eq. 29) [llOd]. 

As is clear from the introductory discussion, most, if not all, of the d-block transition metals are expected to participate in reactions that are related to those discussed here. In addition to the Co-based methodology mentioned earlier, some related reactions of Pd and are known. Also related are the cyclization reactions of metal-carbene complexes containing Cr, Mo, W and other transition metals with alkynes and alkenes and a recently reported Nb- or Ta-promoted diyne-alkyne cyclization reaction, which appears to be closely related to a number of previously developed alkyne cyclotrimerization reactions, such as those catalyzed by Co. Investigations of reactions involving other transition metals may prove to be important especially from the viewpoint of developing asymmetric and catalytic procedures. 

Several reactions of metal-carbene complexes with alkynes leading to five-membered ring compounds have been described. The action of acetylenes on the chromium phenyl(pyrrolidino)carbene complex 472 results in mixtures of indanones 473 and indenes 474297 Terminal alkynes (pent-l-yne or hex-l-yne) react with the molybdenum carbene complex 475 to afford, after oxidative work-up, indanones 476 in contrast, trimethylsilylacetylene gave only the naphthoquinone 477. ... 

The reactions of metal carbene complexes with olefins can be readily followed by H NMR spectroscopy. An early example was the reaction of W(=C//CMe3)(=0)(Cl)2(PEt3)2/AlCl3 (5 12.03) with but-l-ene to give... 

Although the reactions of metal-carbene complexes with various functional groups can lead to catalyst-substrate compatibility problems, these same reactivity profiles can be used in tandem reaction sequences. In the following two examples, a tungsten-carbene complex (Eq. 4.9) and a titanium complex (Eq. 4.10) are... 

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. 

The ligand substitution reactions of carbene complexes such as (CO)s-CrC(OCH3)CHj allow the synthesis of many phosphine- and phosphite-substituted carbene complexes. It is expected that these complexes will have modified reactivity and will provide a means of fine tuning reactions of carbene complexes. More importantly, the substitution reactions of carbene complexes proceed by a dissociative mechanism involving coordinatively unsaturated intermediates. Study of the ligand substitution reactions can give valuable information about these coordinatively unsaturated intermediates which are also involved in the important cyclopropanation, alkene scission, and thermolysis reactions of metal-carbene complexes. 

The reaction of metal-carbene complexes with electron-rich vinyl ethers occurs under milder conditions than the reaction with electron-poor unsaturated esters. The conditions are also milder than those required for ligand substitution of carbene complexes. The reaction products depend strongly on the external CO pressure with no added CO, alkene scission products predominate under 100 atm CO pressure, cyclopropanes are formed in 60% yield (Dotz and Fischer, 1972b). The ratio of isomeric cyclopropanes formed... 

The reactions of metal-carbene complexes with silicon hydrides result in addition of the Si-H bond to the carbene ligand (Connor and Rose, 1970 Dotz and Fischer, 1972c). Tin hydrides react similarly (Connor et al., 1973). Trihalomethylmercury compounds react with carbene complexes to give dihaloethylenes in unspecified yield (DeRenzi and Fischer, 1974). 

Complexes of nucleophilic carbenes are expected to react, like ylids, with electrophiles whereas complexes of electrophilic carbenes are expected to react, like carbocations, with nucleophiles and bases. All the complexes of terminal carbenes have in common the reactions with olefins, although their nature also varies. The principles of these reactions are detailed here, and application in catalysis and organic synthesis, are exposed in Parts IV and V respectively. Reactions of metal-carbene complexes leading to metal-carbyne complexes are mentioned in section 2. 

The reactions of metal-carbene complexes with olefins are important in catalysis and organic synthesis. The first reaction disclosed by Pettit in 1966 involved transient [FeCp(CO)2(=CH2)] that reacted with styrene to give phenylcyclopropane resulting from methylene transfer from the metallacarbocation to the olefin, a reaction that was shown later to be general with iron-methylene complexes of this series. ... 

In the same way, the reactions of metal carbene complexes with alkynes give metallacyclobutenes, which leads to applications in organic synthesis. Tebbe s methylene complex is in equilibrium with the metallacyclobutane by... 

Finally, the redox reactions of metal-carbene complexes lead to the transfer of H atom, alkyl group or carbene ligand. ... 

---