No abstract

The reaction between diguanides and carbodi-imides in dimethylformamide provides a new general route to melamines. The use of diguanide and its l-mono-and 1,2-di-substituted analogues affords satisfactory yields of monosubstituted melamines, 2,4-disubstituted melamines, and 1,2,6-trisubstituted isomelamines, respectively. I n each case, the reaction is thought to involve primary addition of the reactants, followed by the cyclisation of the resulting intermediate triguanide, with loss of amine, to the heterocyclic end-product.Guanidine similarly reacts with carbodi-imides to yield, successively, 1,2-diaryldiguanides and 1,2,6-triarylisomelamines. Some properties of the new melamines are described.A NUMBER of new routes to five-membered heterocycles, particularly 1,2,4-tria~oles,~~-~ 1,3,4-thiadiazoles,lb~C and 1,2,4-thiadiazoles Idse are now available from our studies of addition reactions between systems incorporating cumulative double bonds, such as carbodi-imides, isothiocyanate and isocyanate esters , and suitable compounds related to aminoguanidine.2 The latter classes include aminoguanidine la-c itself, thiosemicarbazide,lC semicarbazide,lc nitroguanidine,l" and diamin~guanidine.~ The behaviour of diguanide and its derivatives has now been examined; this Paper describes their interaction with carbodi-imides.In their reactions with carboxylic acid derivatives, aldehydes, ketones, and ureas, diguanides are a versatile source of amino-l,3,5-triazine~,~~5a-fas are the closely related cyanoguanidines which have been particularly widely employed in the production of melamines, ammelines, and ammelides. The extensive literature has been surveyed by Smolin and Rapoport (ref. 5, ch. 3-6).The reaction between diguanides and carbodi-imides was thus expected to furnish variously substituted melamines. Cyanamide, in its di-imino-form, may be regarded as the parent compound of carbodi-imides ; the interaction of cyanamide and diguanide would thus be the simplest example of the present group of reactions but has apparently not been studied. The patent literature 6~7 describes the manufacture of melamine by heating to 140-240" mixtures of guanidine or diguanide or their salts with cyanamide or dicyandiamide, but, since each of these compounds alone yields melamine under these conditions, the course of the reactions is not clear. The only relevant example appears to be the condensation of 9-chlorophenylcyanamide and p-chloroplienyldiguanide to 2 ,4-di-(~-chlorophenyl)melamine.8The action of carbodi-imides (I) on the parent compound, diguanide (11; R' = H), in dimethylformamide at 100" afforded monosubstituted melamines (IV ; R' = H) smoothly, in good yields; the products were isolated as the crystalline toluene-9-sulphonates, from which the bases were liberated without difficulty. The reaction probably involves initial formation of the triguanide (111; R' = H). Like comparable seven-membered linear carbon-nitrogen systems 4,5a-g, this cyclises spontaneously, with elimination of amine.

Diguanide reacts slowly with isocyanate esters in dimethylformamide a t 100" to give 1 -substituted hexahydro-4,6-di-imino-s-triazin-2-ones with small quantities of ammeline. The structure assigned to the s-triazin-6-ones follows from their mode of formation, their general chemical behaviour, and their alternative production from the 2t hi ones by oxid at ive desu I p h u ra tio n.1.2-Diphenyldiguanide and aromatic isocyanates similarly yield 1 -arylhexahydro-4-imino-6-phenyliminos-triazin-2-ones, the structure of which is established unequivocally by this synthesis.The use of 1 -phenyldiguanide in this general reaction yields four main products : 6-amino-4-anilino-s-triazin-2-01, 1 -substituted hexahydro-4,6-di-imino-s-triazin-2-ones, 1 -substituted hexahydro-6-imino-4-phenyliminos-triazin-2-ones, and 1 -substituted hexahydro-4-imino-6-phenylimino-s-triazin-2-ones. The nature of the isocyanate determines which of these triazines predominate. Certain chemical properties, and the ultraviolet spectra of the new substituted ammelines are discussed.The entire group of reactions is accounted for in terms of the addition-elimination mechanism involving intermediate adducts derived from equimolar quantities of the reactants. Since loss of water from carbamoyl compounds of this type occurs much less readily than loss of hydrogen sulphide from their thiocarbamoyl analogues, melamines are not formed in this reaction.