Triazines triple bonds

One obvious synthetic route to isoxazoles and dihydroisoxazoles is by [3+2] cycloadditions of nitrile oxides with alkynes and alkenes, respectively. In the example elaborated by Giacomelli and coworkers shown in Scheme 6.206, nitroalkanes were converted in situ to nitrile oxides with 1.25 equivalents of the reagent 4-(4,6-di-methoxy[l,3,5]triazin-2-yl)-4-methylmorpholinium chloride (DMTMM) and 10 mol% of N,N-dimethylaminopyridine (DMAP) as catalyst [373], In the presence of an alkene or alkyne dipolarophile (5.0 equivalents), the generated nitrile oxide 1,3-dipoles undergo cycloaddition with the double or triple bond, respectively, thereby furnishing 4,5-dihydroisoxazoles or isoxazoles. For these reactions, open-vessel microwave conditions were chosen and full conversion with very high isolated yields of products was achieved within 3 min at 80 °C. The reactions could also be carried out utilizing a resin-bound alkyne [373]. For a related example, see [477]. 

Ethynyl groups undergo addition of water to give acyl-l,2,4-triazines ethanol and amines can also be added to the triple bond.294... 

The Diels-Alder reaction with inverse electron demand has been one of the most intensively studied reactions of 1,2,4-triazines. In this reaction 1,2,4-triazines behave as electron-deficient dienes and react with electron-rich dienophiles to give, generally, pyridines (see Houben-Weyl, Vol. E7b, p 471 ff). [4 + 2] Cycloadditions of 1,2,4-triazines have been observed with alkenes, alkynes, strained double bonds, electron-rich double and triple bonds, but in a few cases also with electron-deficient alkynes C—N double and triple bonds can also be used as dienophiles. In addition to intermolecular Diels-Alder reactions, intramolecular [4 + 2] cycloaddition reactions of 1,2,4-triazines have also been studied and used for the synthesis of condensed heterocyclic systems. A review on the intermolecular Diels-Alder reaction was published by Boger and Weinreb 14 Sauer published a review on his studies in 1992,381 and E. C. Taylor published a summary of his own work on intramolecular Diels-Alder reactions in 1988.382... 

In a few cases [4 + 2]-cycloaddition reactions of 1,2,4-triazines with C—N double or triple bonds have been reported. Thus, benzimidamide reacts with 1,2,4-triazines in refluxing toluene to yield 1,3,5-triazines 30 (see Section 2.3.1.2.3.). The isolated products are rationalized by assuming that the dienophile undergoes addition to the 2- and 5-positions of the 1,2,4-triazines, as in the reactions with ynamines, then a nitrile R CN is eliminated followed by aromatization via loss of ammonia.367,459... 

Cycloaddition of 1,2,4,5-tetrazines with C-N double or triple bonds yields dihydro-1,2,4-tri-azines 13 or 1,2,4-triazines 14 (see Section B.2.2.1.3.). Various systems with C—N double bonds have been used imines, including dihydropyrroles and dihydroisoquinolines 341 344 hydra-zones, including dihydropyrazoles,343 348 oxime ethers,343 imidates, including dihydrooxa-zoles 345 357 thioiniidates, including dihydrothiazoles 345,349,350 357 358 amidrazones 357,358 amidines 359 and isocyanates or isothiocyanates.351 Nitriles are the examples of systems with a C-N triple bond.352 -356... 

Figure 5.22 Cu UPD on a Au/mica substrate modified by a hybrid nanostructure consisting of a hydrogen-bonded network and a thiol SAM. (a) Cartoon ofthe hybrid structure and molecular structures of the components. The hydrogen-bonded network is composed of 1,3,5-triazine-2,4,6-triamine (melamine, triangles) and perylene-3,4,9,10-tetracarboxylic di-imide (PTCDI, rectangles) that interact via a triple hydrogen bond (dotted lines). Pores ofthe...

Since riboflavin (RF) is surface-active and possesses a high fluorescence quantum yield, the dye is very suitable for TIR fluorescence measurements at a water/oil interface. For TIR experiments, an aqueous RF solution was poured onto a CCI4 solution containing a diamino-j-triazine derivative (DTT), which acted as a flavin receptor via triple hydrogen bonds. On the basis of fluorescence dynamic measurements of RF adsorbed on a water/oil... 

Trivalent hydrogen-bonding systems have been used very extensively for guiding and influencing the structuring of polymers. As mentioned in Sect. 1, Fig. 5, the most important triple-hydrogen bonds derive from 2,6-diamino-pyridines, 2,6-diamino-1,3,5-triazines and their complexes with flavine- and... 

Fig. 21 Assembly of polymers via triple-hydrogen bonds, a Formula of the flavine/2,6-diamino-triazine and thymine/2,6-diaminotriazine interactions, b Formation of nanopar-ticles/polymer aggregates, c Polymersome formation by aggregation of poly(norbornenes) and poly(styrenes) bearing N-bisacyl-2,6-diamino-pyridine and thymine side-chains, respectively. Reprinted with permission from [104,105]...

Tetrazines are reactive dienes in Diels-Alder reactions with inverse electron demand and react not only with C—C multiple bonds, but also with C-N triple and C—N double bonds (see Section C.5.2.5.1.). In 1969 Roffey and Verge192 reported the reaction of 1,2,4,5-tetrazines with imidates 2 (X = O) to give 1,2,4-triazines 5 via the bicyclic intermediate 3 and the dihydro-1,2,4-triazine 4. Further studies, in particular by Boger and coworkers have shown that much better yields of 1,2,4-triazines are obtained when thioimidates 2 (X = s) are used as the dienophile.193-195... 

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