Nitrogen-substituted carbene complexes synthesis

Nitrogen often appears attached directly to carbon in Fischer-type carbene complexes. Equations 10.823 and 10.924 provide two examples of N-substituted carbene complex synthesis. The first procedure involves attack by the amide on one of the carbonyls (analogous to equation 10.6) followed by alkylation. 

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. 

Going one step beyond, the reaction of these n-donor-substituted Group 6 allenylidenes with bifunctional N,N- or W, 5-dinucleophiles opened up a fruitful route for the synthesis of an extensive family of N- or 5-heterocyclic carbenes. Thus, treatment of complex [Cr =C=C=C(NMe2)Ph (CO)5] with benzamidine, guanidine or thioacetamide has been reported to yield the a,(3-unsaturated carbenes 54 (Scheme 16) [62], arising from nitrogen attack at Cy, subsequent HNMe2... 

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