Cyclotrimerization cyanamides

Cyclotrimerization of nitriles with heteroatomic substituents are also facile and important reactions. Melamine is formed on heating cyanamide above its melting point (equation 74) (59HC(l3)l, p. 309). Substituted cyanamides react to give either the expected 1,3,5-triazine or the isomer (142). The 1,3,5-triazine is the preferred product at high temperatures, under acid catalysis, and for cyanamides with bulky substituents, whilst the isomer is favoured by basic conditions and low temperatures (Scheme 80). The mechanism of formation of (142) has been proposed (Scheme 81) (78RCR975). 

The cyclotrimerization of A-substituted cyanamides has been reported repeatedly 10 however, in some cases, the products were isomelamine derivatives,224-226 whereas in others substituted melamines of the normal structure were obtained.227-229... 

Although the reaction mechanism is not yet fully understood, cyanic acid or isocyanic acid seems to be the first decomposition product of urea. Trimerization of cyanic acid leads to cyanuric acid (see section on Cyanuric Acid, p 685). In the presence of catalysts, cyanic acid disproporlionates into carbon dioxide and cyanamide, which then trimerizes to melamine. Thus, decomposition products, and nol urea itself, cyclotrimerize to provide the 1,3,5-triazine rings. These reactions form the basis for the industrial production of cyanuric acid286-301 and melamine.302-315... 

Triflic anhydride was found to be efficient for the cyclotrimerization of dialkylcyanamides 166 under mild conditions. The same reaction can be applied to aryl nitriles and thiocyanates. The reaction involves the formation of the bistrifluoroisourea intermediates 167, which react quickly with two molecules of cyanamide to afford the triazines 168 (Scheme 25) <2004S503>. 

Stannous chloride Cyclotrimerization of cyanamides 1,3,5-Triazines and hexahydro-l,3,5-triazines... 

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