Tautomerism in Dihydro-1,3,5-triazines

Dihydro-1,3,5-triazines are of fundamental interest because of their ability to undergo amidinic tautomerism. Furthermore, as these are nitrogen-containing analogues of dihydropyrimidines (methylene at position 5 replaced by N) it would certainly be interesting to compare the effects of nitrogen substitution on the structual stability and tautomeric behavior of these compounds. 

Kobayashi et al.211 examined chemical shifts in the 19F-NMR spectra of [polyfluoro(chloro)methyl]-dihydro-1,3,5-triazines (168-171) in deuterio-chloroform and in methanol, concluding that, in these solvents, these compounds exist as an equilibrium between 1,4-dihydro (168a) and 1,2-dihydro tautomers (168b, b ). 

During our investigation on tautomerism in dihydropyrimidines, we measured the IR and H- and 13C-NMR spectra of several known dihydro-1,3,5-triazines,147 which exist in the solid state as the 1,2-dihydro isomers. The NMR measurements were carried out in two solvents, CDC13 and DMSO, and signals of the two tautomers A and B were always observed. The rate of tautomeric exchange was rather slow, very similar to that in 121, obviously owing to the presence of two phenyls at positions 2 and 4 (see Section V,C,l,d). 

All the factors affecting amidinic tautomerism in dihydropyrimidines (see Section V,C,l,b) apply equally to dihydro-1,3,5-triazine derivatives. In CDC13 the rate of tautomerism was clearly affected by the bulkiness of the 

Kobayashi, A. Ohsawa, and M. Honda, Chem. Pham. Bull. 21,1586 (1973). 

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