Tautomerism carbodiimides

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). 

Wang recently reported [30] that thermolysis of carbodiimides 15 (Scheme 1.5) in aromatic solvents is an efficient route to indoloquinolines 18 used as precursors for synthesizing naturally occurring alkaloids [31], The cyclization is thought to occur through a two-step biradical Diels-Alder reaction that gives 17, which then tautomerizes to 18. 

Cyanamide, NsC—NH, in the tautomeric form HN==C=NH, is a carbodiimide and can be used in peptide synthesis. Diethylcyanamide, although incapable of tau-tomerism, functions as a dehydrating agent for the coupling of an N-protected amino acid with an amino acid ester. Best yields are obtained by heating the three components without solvent at 100° for 2 hrs. 

The addition of hydrogen halides to cyanamide which in its tautomeric form can be written as carbodiimide (XX) was reported by Drechsel in 1875. The author obtained both the dihydrochloride XXI (X = Cl), and the dihydrobromide XXI (X = Br) of cyanamide. 

On the basis of the isoelectronic principle interpolating between the C chemical shifts of allene (Z = 6) (6 = 212.6 ppm relative to TMS) and carbon dioxide 204 (Z = 8) (5c = 125 ppm (93)) one would expect the C chemical shift of carbodiimide (203) which cannot be isolated owing to tautomerism with cyanamide H2NC=N to be observed near 169 ppm. If one assumes that the... 

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