1/7-Azirine, 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]. 

Aromatic cyclic 7r-electron delocalization does indeed stabilize the planar structure with bond equalization (84ZOR897)—the problem is that, in addition to that effect, there may exist some others that may eventually overshadow it. Thus, the foregoing warrants the conclusion that the preference of a planar or nonplanar geometry of heterocycle depends on a number of factors including aromaticity (antiaromaticity), which may not even be the most important. In any case, this factor should not be disregarded if one wishes to obtain a correct overall energy balance. For example, aromaticity is reflected in the values of inversion barriers. Thus, for antiaromatic 2-azirine the nitrogen inversion barrier is, as was mentioned earlier, 37.7 kcal/mol, whereas in the case of its saturated... 

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... 

Annular prototropy is not of great importance for small heterocycles. However, it should be mentioned that 1-azirine (2) is much more stable than its antiaromatic 2-isomer (3). By analogy, antiaromaticity is certainly a key factor determing instability of lH-azepines which have never been observed. Thus, demethoxycarbonylation of methyl-3,6-di-r-butylazepine-l-carboxylate (46) by DBU gives a mixture of the corresponding 2H-, 3H-, and 4//-azepines in the approximate ratio 13 56 1 (Scheme 9) (94JCS(P1)1753). The distribution of the azepine isomers is proportional to their relative thermal stabilities as they interconvert via allowed 1,5-hydrogen shifts. 

Another example where aromaticity plays an important role is the barrier to the rotation of amides (compound 18 is represented with N in the middle to indicate any azole) [31]. In classical amides, like dimethylformamide (15), the calculated barrier is 80.1-81.0 kJ mol1 (MP2/6-311++G ), which compares well with the experimental barriers of 91.2 (solution) and 85.8 kJ mol1 (gas-phase) [32], The cases of A-formylaziridine (16) and iV-formyl-2-azirine (17) are more complex due to the pyramidalization of the nitrogen atom and the presence of rotation and inversion barriers [32], The effect of the antiaromatic character of 2-azirine (four electrons) [18] on the barrier is difficult to assess due to changes in the ring strain. 

We have devoted three papers explicitly to the relationships between aromaticity and tautomerism the first to the tautomerism of 1,2,3-triazole (30) and benzotria-zole (31) [46], If, in the first case, the relative stabilities are determined by the lone-pair/lone-pair repulsion of the adjacent lone pairs that destabilize 30a, in the second case this is partly compensated by the greater aromaticity of the benzenoid structure 31a. In the second paper, we discuss the aromaticity of formal 47i-electron antiaromatic 17/-2-azirine (32), 671-electron aromatic l,27/-3-diazetine (33), pyrrole (34), and 1,2-dihydropyridazine (35) [47], Compounds 33 and 35 are not planar and not aromatic. 

Since 1-azirines are well known, it does not seem reasonable to attribute the nonexistence of 2-azirines exclusively to strain energy. An unfavorable electronic situation is most likely responsible for their alleged instability. As mentioned previously, the 2-azirine ring system is a cyclic conjugated necessarily planar n system containing four 7r electrons and Hiickel s rule would not predict it to be stabilized by delocalization. In fact, HMO theory predicts that delocalization results in a less stable 7r system than the open chain analog.5 Such systems are predicted to be relatively unstable and have recently been designated as antiaromatic.8,9... 

We have not made a calculation of the inversion barrier in 2-azirine. Clark 30) obtained 35 kcal/mol for this vs. 15.5 kcal/mol in aziridine), again presumably because of the antiaromatic effect in the planar structure. Comparison of the two isomers shows that the energy of 1-azirine is 40.5 kcal/mol less than 2-azirine (using 6-31G ). Clark 30) obtained 27 kcal/mol for this difference. The very negative bond separation... 

Vapor phase pyrolysis of two unsymmetrically substituted 1,2,3-triazole isomers, like (75) and (76), involves the antiaromatic l//-azirine derivative (77) which rearranges to two isomeric 2//-azirines (78) and (79) (71CC1519, 73JCS(Pi)555> and other products. Iminocar-benes (80) are most likely involved as intermediates. Furthermore, nitrogen has been extruded from several IH- 1,2,3-triazoles by flash vacuum pyrolysis. 1-Alkyl-1,2,3-triazoles (81) give nitriles (82) and (83) (via Wolff rearrangement) and (hydroxy)isoquinolines (84) and (85) (by [1,4-H] transfer in the iminocarbene) (75JCS(P1)1). 

Azirine The 1-azirine has a three-membered ring comprising two carbons and doubly bonded nitrogen. The 2-azirine, in which the double bond is between the carbon atoms, does not exist, possibly because it is predicted to be antiaromatic. 

Theoretical methods (see Section 1.01.2) have been used to estimate that 1 //-azirine is 33-37 kcal mol-1 higher in energy than 2//-azirine. Many theoretical studies have been carried out on the elusive heterocycle l//-azirine, which is considered to be antiaromatic. These studies and calculations on the high barrier to pyramidal inversion of 1//-azirine were discussed in Section 1.01.2. 

The barrier to pyramidal inversion in 1 A-azirine (3) has been calculated to be 33-46 kcal mol-1, reflecting the unfavorability of the planar transition state, which is proposed to be antiaromatic (Section 1.01.2). 

Due to their strain and putative electronic destabilization, such three-membered heterocycles possessing a cyclic array of 47r-electrons offer a considerable challenge to synthesis. Such molecules are expected to be both unimolecularly and bimolecularly reactive, if they exist at all as energy minima. Since no isolable tellurirenes have been reported, only some reactions of selenirenes are described in this section. Selenirene and its kindred systems, oxirene, azirine, and thiirene, are of interest because of their theoretical significance as prototypes of antiaromatic species. 

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