1/7-Diazirine, antiaromaticity

It is known that unsaturated three-membered nitrogen heterocycles display tautomerism involving nonaromatic and antiaromatic (i.e., Air systems) forms. In all cases, the nonantiaromatic tautomer is the most stable 1-azirine la and 1-diazirine 2a. Nonetheless, antiaromatic tautomers are known, for instance, triazirines 3. 

A comparison between aziridine 4, as a model of nonaromatic structure, l//-azirine lb, l//-diazirine 2b, and triazirine 3 using 6-31G /MP3 calculations leads to the following values for the N-H inversion barriers 4, 86.2 (experimental value 80 kJ mol ) lb, 190.4 2b, 160.2 and 3, 246 kJ mol [89JCC468].Tlie difference in inversion barrier values between lb and 2b was attributed to a decrease in the antiaromaticity of the latter. Tire antiaromaticity of lb was examined subsequently by the same authors [89JST(201)17]. 

FVP treatment of both isolated samples 300 and 301 resulted in a mixture containing 300 + 301 this finding revealed that these two compounds can be mutually transformed into each other under forced conditions. Upon some kinetic measurements (determination of a negative activation entropy), the authors concluded that the ring transformation probably proceeds via a concerted sigmatropic shift to an antiaromatic diazirine intermediate. 

The suggestion that three-membered heterocycles (362) are involved is illustrated by the postulated generation of derivatives of IH-diazirine (315 and 345), thiiren (328), thiazirin (337), and lif-triazirine (358). These three-membered heterocycles belong to the class of 4n-antiaromatic heterocycles their possible role as reaction intermediates is of general interest. There is, however, an interesting difference between the photochemistry of sydnones (Fig. 4) and meso-ionic l,2,4-triazol-3-ones (Fig. 6). In both cases IH- azirines (315 and... 

We have studied the intramolecular PT in 87 and compared it with the [ 1,5 ]-sigmatropic rearrangement of cyclopentadiene (83) [91], noting the profound differences between these two systems. In pyrazole, the N(sp2)-N(sp2) bond is essential while in cyclopentadiene the C(sp3)-C(sp2) bond is accessory. The TS in pyrazole corresponds to a structure with the proton equidistant from both N atoms and clearly out-of-plane (HXY = 63.4°). Intramolecular PT in antiaromatic 2-diazirine and 1,2-diazepine were studied as counterexamples. 

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