Non-Heteroatom-Substituted Carbene Complexes

This section deals with alkylidene complexes L M=CR2 and vinylidene complexes LnM=(C)n,=CR2 in which the metal-bound carbon atom bears only hydrogen, alkyl, or aryl groups, but neither heteroatoms (halogen, nitrogen, oxygen, or sulfur) nor electron-withdrawing groups. Dimetallacyclopropanes and ketene complexes will not be discussed. 

Because hydrogen, alkyl, or aryl groups can compensate only to a limited extent the electron deficit of the carbene carbon atom, it is mainly the metal and its ligands which provide stabilization in this type of carbene complex. For this reason the reactivity of these compounds depends mainly on the nature and oxidation state of the metal and on the electronic properties of the remaining ligands. 

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Table 1.2. Chemical shifts for carbon atoms (C ) and protons (H ) in representative non-heteroatom-substituted carbene complexes L M=C (R)H .

As will be discussed more thoroughly in Section 3.2.5, transition metal carbene complexes can mediate olefin metathesis. Because heteroatom-substituted carbene complexes are usually less reactive towards olefins than the corresponding nonheteroatom-substituted complexes, it is, e.g., possible to use enol ethers to terminate living polymerization or other types of metathesis reaction catalyzed by a non-heteroatom-substituted carbene complex. Olefin metathesis can also be used to prepare new heteroatom-substituted carbene complexes (Figure 2.15, Table 2.11). 

Treatment of Fischer-type carbene complexes with different oxidants can lead to the formation of carbonyl compounds [150,253]. Treatment with sulfur leads to the formation of complexed thiocarbonyl compounds [141]. Conversion of the carbene carbon atom into a methylene or acetal group can be achieved by treatment with reducing agents. Treatment of vinylcarbene complexes with diborane can also lead to demetallation and formation of diols [278]. The conversion of heteroatom-substituted carbene complexes to non-heteroatom-substituted carbene complexes... 

Several reaction sequences have been reported in which Fischer-type carbene complexes are converted in situ into non-heteroatom-substituted carbene complexes, which then cyclopropanate simple olefins [306,307] (Figure 2.22). This can, for instance, be achieved by treating the carbene complexes with dihydropyridines, forming (isolable) pyridinium ylides. These decompose thermally to yield pyridine and highly electrophilic, non-heteroatom-substituted carbene complexes (Figure 2.22) [46]. 

Closely related to the ring-closing metathesis of enynes (Section 3.2.5.6), catalyzed by non-heteroatom-substituted carbene complexes, is the reaction of stoichiometric amounts of Fischer-type carbene complexes with enynes [266,308 -315] (for catalytic reactions, see [316]). In this reaction [2 + 2] cycloaddition of the carbene complex and the alkyne followed by [2 -t- 2] cycloreversion leads to the intermediate formation of a non-heteroatom-substituted, electrophilic carbene complex. This intermediate, unlike the corresponding nucleophilic carbene... 

Non-heteroatom-substituted carbene complexes of almost all transition metals are known. Depending on the oxidation state of the metal, the overall charge of the complex, and the properties of the additional ligands, the reactivity of alkyl or aryl carbene complexes can vary greatly. Some examples of compounds with strikingly different chemical properties are shown in Figure 3.1. 

Fig. 3.1. Non-heteroatom-substituted carbene complexes covering a broad range of different reactivities [391-393].

The impressive number of different reactivities of non-heteroatom-substituted carbene complexes parallels the many possibilities for their preparation. The most important synthetic approaches are sketched in Figure 3.2. 

Fig. 3.2. Synthetic approaches for the generation of non-heteroatom-substituted carbene complexes.

Many non-heteroatom-substituted carbene complexes have been prepared from alkyl complexes by a-abstraction, for which two mechanistically different pathways must be considered (Figure 3.3) ... 

Non-heteroatom-substituted carbene complexes can also be generated by treatment of electrophilic transition metal complexes with ylides (e.g. diazoalkanes, phosphorus ylides, nucleophilic carbene complexes, etc. Section 3.1.3). Alkyl complexes with a leaving group in the a-position are formed as intermediates. These alkyl complexes can undergo spontaneous release of the leaving group to yield a carbene complex (Figure 3.2). 

Additional methods for preparing non-heteroatom-substituted carbene complexes include nucleophilic or electrophilic additions to carbyne complexes (Section 3.1.4), electrophilic additions to alkenyl or alkynyl complexes (Section 3.1.5), and the isomerization of alkyne or cyclopropene complexes (Section 3.1.6). 

The first non-heteroatom-substituted carbene complex was prepared by Schrock in 1974 [392] (Figure 3.4). Treatment of tris(neopentyl)tantalum dichloride with neopentyllithium led to the formation of neopentane and (2,2-dimethyl-1-propylidene)tris(neopentyl)tantalum. This carbene complex reacts violently with water or oxygen, but can be sublimed (80 °C) and stored indefinitely at room temperature under argon. 

Non-heteroatom-substituted carbene complexes are in principle accessible either by electrophilic or by nucleophilic addition to alkynyl or alkenyl complexes (Figure 3.26). 

In addition to the methods described, non-heteroatom-substituted carbene complexes have also been prepared by reactions which do not belong mechanistically to any of those described in previous sections. 

Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes... 

Non-heteroatom-substituted carbene complexes play a key role both as reagents and catalysts in organic synthesis and as intermediates in the preparation of other organometallic compounds. However, discussion of applications in inorganic synthesis would surpass the scope of this book. Here the focus will be on those reactions which lead to metal-free compounds and hence are particularly relevant to the organic chemist. 

Non-heteroatom-substituted carbene complexes cover a broad spectrum of different reactivities, largely dependent on the electronic properties of the metal. In Chapter 1 the division of carbene complexes into Fischer-type and Schrock-type carbenes was discussed. This way of grouping carbene complexes, although difficult to apply... 

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