Monosubstituted carbodiimides

Carbodiimides are a unique class of reactive organic compounds having the hetero-cumulene structure R-N=C=N-R. They can be formally considered to be the diimides of carbon dioxide and they are closely related to the monoimides of carbon dioxide, the isocyanates. The substituent R can be alkyl, aryl, acyl, aroyl, imidoyl, or sulfonyl, but nitrogen, phosphorous and metal-substituted carbodiimides are also known. The unsubstituted carbodiimide is isomeric with cyanamide, H2NCN. Also, monosubstituted carbodiimides exist only in the isomeric cyanamide structure. 

Scheme 65) <60G1290>. Oxadiazolines (156) are produced by cycloaddition of cyanamides and nitrile oxides aryl (Scheme 66) <66JPR22l>. Thus, for un- and monosubstituted cyanamides, cycloaddition occurs to the C=N double bond of a tautomeric carbodiimide form rather than to a nitrile triple bond. On the other hand, dicyandiamide (157) and benzonitrile oxide furnish (158) (Scheme 67). 

The synthesis of monosubstituted Fc-peptide esters, Fc-GlyOEt 17, Fc-AspOMe 18, Fc-GluOEt 19 (Scheme 5.6) and their disubstituted analogs Fc-[GlyOEt]j 23, Fc-[AspOBz]j 24, and Fc-[GluOEt]j 25 (Scheme 5.7) was achieved with moderate to good yield using the carbodiimide-HOBt method. 

Although additives to induce radical chemistry have allowed ligand substitutions of 18-electron complexes to be conducted under mild conditions, photochemical reactions provide a common and practical alternative. Photochemically induced dissociation of carbonyl ligands is most common, but photochemical dissociations of other dative ligands are known. Several examples are shown in Equations 5.36-5.40. These examples illustrate the dissociation of CO from homoleptic carbonyl compounds of iron - and chromium, the dissociation of CO from piano-stool carbonyl compounds, " ttie dissociation of N, and the dissociation of a carbodiimide to generate an intermediate that coordinates and cleaves the C-H bonds of alkanes. In some cases, like the formation of the two THE complexes, the products of the photochemical process are not isolated instead, they are treated in situ with a ligand, such as a phosphine, to form monosubstitution products selectively. 

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