Metal-carbene complexes alkoxy substituted

Alkoxy-substituted carbene complexes serve as valuable precursors for a wide variety of metal-carbene complexes as the carbene carbon atom is very susceptible to nucleophilic attack and the alkoxy group is a better leaving group than the entering nitrogen, sulfur, or carbon nucleophiles. 

Metal-carbene complexes are stable in dilute aqueous acid, but strongly basic aminocarbene complexes react with HCl or HBr in ether at -40° to give products derived from protonation of the carbon-metal bond (Fischer et ai, 1973b). Alkoxy-substituted carbene complexes react with mixtures of HCl... 

The use of metal-carbene complexes as an amino-protecting group in peptide synthesis has been reported by Fischer and Weiss (1973). The free amino group of an amino acid ester readily displaces the alkoxy group of an alkoxy-substituted carbene complex. The nitrogen atom of the resulting N-substituted carbene complex is nonbasic and nonnucleophilic because it bears a partial positive charge. The low reactivity of the amino-substituted carbene complex allows a series of reactions to be carried out to construct a peptide chain. Finally, the completed peptide chain can be removed from the carbene complex by treatment with trifluoroacetic acid at 20°. Scheme 16 illustrates the variety of reactions that can be carried out in the presence of the metal-carbene functionality. 

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. 

The intermediate vinylketene complexes can undergo several other types or reaction, depending primarily on the substitution pattern, the metal and the solvent used (Figure 2.27). More than 15 different types of product have been obtained from the reaction of aryl(alkoxy)carbene chromium complexes with alkynes [333,334]. In addition to the formation of indenes [337], some arylcarbene complexes yield cyclobutenones [338], lactones, or furans [91] (e.g. Entry 4, Table 2.19) upon reaction with alkynes. Cyclobutenones can also be obtained by reaction of alkoxy(alkyl)carbene complexes with alkynes [339]. 

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