Unsymmetrical carbodiimides

This procedure for the preparation of l-ethyl-3-(3-dimethyl-amino)propylcarbodiimide and its salts is a modification of one that has been published.4 Unsymmetrical carbodiimides have also been prepared by desulfurization of the corresponding thioureas with mercuric oxide3 or by dehydration of the corresponding ureas with -toluenesulfonyl chloride in pyridine.4 Unsymmetrical 1,3-disubstituted ureas are best prepared by the reaction... 

Although this procedure offers no advantage over that of Hiinig, Lehmann, and Grimmer, it effectively illustrates a new method for the synthesis of symmetrical and unsymmetrical carbodiimides. The generality of this procedure is limited only by the number of substituted benzohydroxamoyl chlorides that can be made without difficulty, as a variety of N-sulfinylamines is easily accessible. ... 

Cyclocarbonylation of o-iodophenols 503 with isocyanates or carbodiimides and carbon monoxide in the presence of a catalytic amount of a palladium catalyst (tris(dibenzylideneacetone)dipalladium(O) Pd2(DBA)3) and l,4-bis(di-phenylphosphino)butane (dppb) resulted in formation of l,3-benzoxazine-2,4-diones 504 or 2-imino-l,3-benzoxazin-4-ones 505 (Scheme 94). The product yields were dependent on the nature of the substrate, the catalyst, the solvent, the base, and the phosphine ligand. The reactions of o-iodophenols with unsymmetrical carbodiimides bearing an alkyl and an aryl substituent afforded 2-alkylimino-3-aryl-l,3-benzoxazin-4-ones 505 in a completely regioselective manner <1999JOC9194>. On the palladium-catalyzed cyclocarbonylation of o-iodoanilines with acyl chlorides and carbon monoxide, 2-substituted-4f/-3,l-benzoxazin-4-ones were obtained <19990L1619>. 

Symmetrical carbodiimides, i.e., molecules with the same substituent on both nitrogen atoms, are best prepared from alkyl or aryl isocyanates in the presence of a phospholene oxide catalyst, the byproduct being carbon dioxide gas. No solvent is required for this reaction. Unsymmetrical carbodiimides or alkylarylcarbodiimides are also obtained from isocyanates, either by reaction with amines and subsequent dehydration of the intermediate... 

Iminophosphoranes can also be used to synthesize symmetrical carbodiimides. In this case they are reacted with carbon dioxide, thereby mimicking the synthesis of carbodiimides from amines and carbon dioxide. Another useful synthesis of unsymmetrical carbodiimides, not requiring the use of carbonyl chloride, is the reaction of carbonimidoyl dihalides with amines. The synthesis and chemistry of carbonimidoyl halides was reviewed in 1968. ... 

Vinyloxirans react with carbodiimides in the presence of Pd(dba)3 to give iminooxazolidines in good yields. Better yields are obtained when unsymmetrical carbodiimides are used in this reaction. 

Two side reactions, the O to N acyl shift and racemization, limit the application to some extent. Lower reaction temperatures, shorter reaction times or the use of unsymmetric carbodiimides suppress these reactions. The addition of reactive nucleophiles will be discussed in a later section (active esters). 

Using this method, the complex [Fe(cp)(CO) j C(0)NHMe ] has been converted into [Fe(cp)(CO) j(CNMe)]Cl, and cis-[Pt(PPh3)j C(0)NHMe Cl] has been used to prepare cj s-[Pt(PPh3)(CNMe)CIj] [635]. In an interesting variation on this procedure, the reaction of phosgene with unsymmetrical carbodiimide adducts of iron has been used to prepare iron(II) complexes containing two different isonitrile ligands at a now chiral centre [636] ... 

Unsymmetrical carbodiimides. Isocyanides react with primary amines under catalysis of NiCh. Air is the oxidant. 

The elimination of hydrogen chloride from chloroformamidine hydrochlorides is another example illustrating the utility of imidoyl halides. Thermolysis of diarylchloroformamidine hydrochlorides PCIV), obtained from N,N -disubstituted thioureas and phosgene, is the best method of synthesis of unsymmetrical carbodiimides (XV) because the generated... 

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