Dihydro-1,2,4-triazines, stability

The first systematic theoretical study on dihydro-1,2,4-triazines was recently carried out (98JOC5824) the stabilities of all the possible unsubstituted dihydro-1,2,4-triazines were calculated using various theoretical methods, all reliable calculation methods consistently show that the 2,5-dihydro isomer 98 is the most stable. This is in perfect agreement with the experimental observations all the synthetic methods used for the preparation of dihydro-1,2,4-triazines result in 2,5-dihydro isomer 98, provided the structures of the reactants and the reaction mechanism allow its formation. Thus, although Metze and Scherowsky (59CB2481) claimed the formation of 1,2-dihydro-1,2,4-triazine 92 (R = = Ph) in the reduction... 

If the formation of a 2,5-dihydro isomer 98 is not allowed, other dihydro-1,2,4-triazines are formed aeeording to their order of stability ealeulated by various theoretieal methods (see Table III) (98JOC5824). 

Studies on the stability of 1,2-dihydro-2,4,6-triphenyl-1,3,5-triazine (369) included its synthesis from the aromatic 1,3,5-triazine 368. However, in the presence of aluminum chloride rearrangement occurs to give a 16% yield of 2,4,5-triphenylimidazole (370). A suggested mechanism proposed that the unstable dihydrotriazine undergoes a radical-induced ring opening and contraction to 370. ... 

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. 

Stabile Derivate des 1,2,4-Triazins (z.B. 3-Hydroxy-5,6-dipnenyl-l,2,4-triazin) konnen unter Erhaltung des Rings Wasserstoff anlagem, wobei im neutralen oder alkali-schen Medium die Dihydro-, im sauren Milieu die Tetrahydro-Derivate entstehen z.B.11 12 ... 

Stabilities of o -adducts derived from nucleophilic addition of alcohols and alkox-ide ions to azaaromatic compounds are varied to a great extent. A whole number of alkoxy adducts of 1,4-diazinium and 1,2,4-triazinium cations have been registered by NMR [114] however, attempts to isolate them failed. Contrary to that, treatment of 3-aryl-l,2,4-triazin-5-ones with primary or secondary alcohols in the presence of acetic anhydride results in the formation of rather stable 6-aIkoxy-l-acetyl-l,6-dihydro-l,2,4-triazin-5-ones (Scheme 38) [117, 140, 141],... 

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