Generation of Non-Heteroatom-Substituted Carbene Complexes

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). 

A further way of generating carbon-metal double bonds is based on the [2 -I- 2] cycloreversion of metallacyclobutanes. This method has proven particularly useful for the generation of synthetically valuable titanium and zirconium carbene complexes (Section 3.1.7). 

---

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

In Figure 2.2 the most important synthetic approaches to alkoxy- or (acy-loxy)carbene complexes from non-carbene precursors are sketched. Some of these strategies can also be used to prepare amino- and thiocarbene complexes. These procedures will be discussed in detail in the following sections. In addition to the methods sketched in Figure 2.2, many complexes of this type have been prepared by chemical transformation of other heteroatom-substituted carbene complexes. Because of the high stability of most of these compounds, many different reactions can be used to modify the substituents at C without degrading the carbon-metal double bond. The generation of heteroatom-substituted carbene complexes from other carbene complexes will be discussed in Section 2.2. 

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). 

Conjugate hydride abstractions have also been used for the generation of carbon-metal double bonds. An interesting reaction sequence, in which a (thermally unstable) cationic, non-heteroatom-substituted tungsten carbene complex is prepared by conjugate hydride abstraction, is shown in Figure 3.9. 

---