Dehydration of ureas

Primary cycloaUphatic amines react with phosgene to form isocyanates. Reaction of isocyanates with primary and secondary amines forms ureas. Dehydration of ureas or dehydrosulfuri2ation of thioureas results in carhodiimides. The nucleophilicity that deterrnines rapid amine reactivity with acid chlorides and isocyanates also promotes epoxide ring opening to form hydroxyalkyl- and dihydroxyalkylaniines. Michael addition to acrylonitrile yields stable cyanoethylcycloalkylarnines. 

Davis studied the dehydration of urea nitrates as a route to iV-nitroureas. The nitrate salt of iV-methylurea undergoes dehydration-rearrangement on treatment with concentrated sulfuric acid to give Af-nitro-A -methylurea in 42 % yield. In this compound the nitro and methyl groups are attached to the same nitrogen and so its hydrolysis can provide a route to methylnitramine. In contrast, the nitrate salts of ethyl, n-propyl, n-butyl and n-amyl ureas, give iV-nitro-A -ethylurea (49 %), A -nitro-A -propylurea (60 %), iV-nitro-iV -butylurea (67 %) and iV-nitro-A -amylurea (67 %), respectively, on treatment with concentrated sulfuric acid. 

Other reagents that have been used to effect the dehydration of ureas to carbodiimides are alkyl chloroformates (for thioureas) [46], phosgene [47], phosphorus oxychloride [48], and phosphorus pentachloride [38, 49]. Di-chlorocarbodiimides have been shown to be intermediates when COCl2 or PC15 reacts with ureas (Eq. 13). If R1 and R3 are H, then the intermediate... 

Figure 96.2 Dehydration of urea produces melamine, ammonia, and carbon dioxide.

Guanidines may also be formed by reaction of amines with carbodi-imides. This reaction is limited by the availability of carbodiimides, which are usually formed by several methods,24 including dehydration of ureas with the Edward Burgess reagent 11 (Scheme 9).25-27... 

Catbodiimides. liven unstable carbodiimides can be obtained by dehydration of ureas with this reagent and triethylamine in cold methylene chloride (equation I). ... 

NNH2,CH3N303 mw 105.06 N 39.98% OB to C02 —7.61% colorl platelets (from ethanol plus eth) mp 158.4—.8° (decompn). Freely sol in acet, acetic acid and ethanol sol in hot w (forms cyanic acid and nitroamide) si sol in benz, chlf and petr eth. CA Registry No [556-89-8]. Prepn is by dehydration of Urea Nitrate with coned sulfuric acid. The compd can be detonated but is not sensitive to percussion or heating (Ref 7)... 

A number of novel fused 1,2,4-triazole were reported and these include [l,2,4]-triazole[l,3]-thiazinoquinolines (118, 119) and their positional isomers [95MI297], The electrocyclization of N-aziridinylimino carbodiimides generated by dehydration of urea (120), produces 5,6-dihydro-7H-imidazol[l,2-b][l,2,4]triazoles (121) and 5,6-dihydro[l,2,4]triazolo[5,l-t/]-[l,3,5]oxadiaze-pines [95TL2815], The mechanisms of each transformation is discussed. 

Dehydration of urea to carbodiimide (HN=C=NH) or its more stable tautomer cyanoamide (H2N-C=N) cannot be achieved via the zwitterion mechanism of Figure 8.3. It would have to proceed via the zwitterion HN -C (=NH)-OH2 , in which—for electronegativity reasons—both formal charges would occupy worse positions than in the zwitterion O -C (=NH)-NH3 , which is an intermediate in the transformation of urea into biuret upon heat-... 

It is synthesized by dehydration of urea nitrate with sulfuric acid. 

Sheehan and co-workers" used tosyl chloride and triethylamine to effect dehydration of ureas to curbodiimides. 

The dehydration of ureas employing tosyl chloride provides access to substituted carbodiimides, as depicted in eq 39. The urea made from treatment of ethyl isocyanate with A(,A(-dimethyl-l,3-propanediamine was treated in situ with 1.1 equiv of tosyl chloride and a large excess of Triethylamine to afford the corresponding carbodiimide in high yield. ... 

A method has been described for the preparation of carbodiimides 1699 by dehydration of ureas 1696 with p-tosyl chloride under solid-liquid phase-transfer catalytic (PTC) conditions using solid potassium carbonate as a base and a lipophilic quaternary ammonium salt as a catalyst. The method is generally applicable for the synthesis of disubstituted carbodiimides, but is especially useful for un-symmetrically substituted carbodiimides. Yields of the resulting carbodiimides 1699 vary depending on the solvent (usually used at reflux temperature) in benzene or toluene yields of 66-98% are achieved, while in chloroform they are only 30-50% [1281]. 

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