1.2.4- Triazolin-3-ones, decomposition

When, phenylazide and 3-methyl-1-butene are mixed in benzene and kept standing for a couple of days, triazoline is formed and it gives aziridine by thermal or photodecomposition. However, when the mixture is irradiated as soon as they are mixed, the aziridine which is found in the solution is the one formed, not by the decomposition of the triazoline but, by direct photochemical reaction of phenylnitrene with 3-methy1-1-butene. 

In the case of endocyclic enamines, the imines from the triazoline decomposition are one-carbon, ring-contracted products (Scheme 179).30,39,225,510,511 Application of the reaction to cholestanonepyrrolidine... 

Triazolines substituted with two electron-withdrawing groups in the 4-position (Scheme 83) also yield aziridines as the sole products of thermal decomposition when the 1-substituent is aryl319,321,322 when one of the electron-withdrawing substituents is an acyl group, the aziridine isomer-izes spontaneously to the oxazoline via the azomethine ylide (Scheme... 

Aminoquinolines 62 have been prepared in a two-step, one-pot, three-component reaction of 2-azidobenzophenones, secondary amines and arylac-etaldehydes [110]. The microwave-assisted reaction proceeded via the initial formation of enamines 59. Subsequent addition of 2-azidobenzophenones 60 afforded the triazoline intermediates 61, which underwent thermal rearrangement and cyclocondensation to furnish 2-aminoquinolines 62 (Scheme 41). Direct comparison with conventional thermal conditions demonstrated the superiority of microwave dielectric heating in terms of yields (73% vs. 31% of heterocycle 63 after 10 min at 180 °C). Furthermore, the formation of by-products due to decomposition of azide 60 was diminished in the microwave-assisted synthesis. Purification of the products was achieved using solid-phase extraction techniques. 

The course of the decomposition reaction is much dependent on the relative stereochemistry of the substituents on the C—C bond of the triazoline At intermediate temperatures (40-90°) the stereochemistry is conserved and usually only one product is detected. Cyclopentene, cycloheptene and cu-cyclooctene reacted with phenyl azide to give the corresponding iV-phenylimino derivatives in excellent yields, while cyclohexene and (mnj-cyclooctene did not yield imines at all, iziridines being the sole products . When higher temperatures are used, the stereochemical selectivity is apparently lost and a mixture of products is generally observed. In the reaction of norbomene... 

Scheme 6.23) (see also Chapter 7). Under these conditions the intermediate triazoline is not isolable the Lewis acid appears to have the dual role of activating the enone and facilitating the decomposition of the triazoline. Triflic acid is a very efficient catalyst for promoting the formation of aziridines from azides and electron deficient alkenes such as but-3-en-2-one (Scheme 6.23). ° The acid appears to activate the alkene to nucleophilic attack by the azide it is not necessary to invoke triazolines as intermediates although they may be involved. 

Exposure of the ketene acetal (300) to ethyl azidoformate for one month at —15 °C gave the J2-l,2,3-triazoline (301) which readily decomposed on warming (5 days 35 °C) to the J -oxazoline (302) when the triazoline had a 4-substituent, the decomposition took a different course e.g. (303) gave (304) after one day at room temperature. ... 

On the other hand, triazolines prepared by the high-pressure dipolar cycloadditions of azides to norbornene derivatives were thermally more stable. For instance, cycloaddition reaction of azides to 2-azabicyclo[2.2.1]hept-5-en-3-one was accelerated by high pressure, leading to a mixture of regioisomeric triazolines in good yields [77]. Furthermore, triazolines formed by high-pressure cycloadditions (14 kbar) of azides to norbornene cyclobutene diesters could be handled without decomposition [78]. 

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