Nitrogen-substituted carbene complexes reactivity

The links between formation of an iron-alkyl complex and irreversible destruction of the heme moiety have not been forged, but model studies with diaryl- and carbethoxy-substituted carbene complexes suggest that the halogenated carbenes may shift to form a bond with a nitrogen of the porphyrin. The resulting A -haloaIkyl adduct are likely to undergo water-dependent hydrolysis and might therefore not be detected by the methods used to isolate other A -alkyl porphyrins. However, the formation of alternative reactive species that attack the protein or the heme cannot be ruled out. 

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. 

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