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9.5 Product Class 5: 1H-Azirines

DOI: 10.1055/sos-SD-009-00047

Zeller, K.-P.Science of Synthesis, (2001967.

Depending on the position of the ring double bond, two isomeric azirine sructures are possible. To distinguish the two azirine isomers 1 and 2 (Scheme 1), The Ring Index[‌1‌] and CAS apply the "indicated hydrogen" nomenclature and designate 1 and 2 as 1H-azirine and 2H-azirine, respectively. This system is also adopted here. Parallel to this naming practice, a designation to indicate the position of the double bond is widely used. With this system the names of 1 and 2 are 2-azirine and 1-azirine, respectively. It is obvious that the two competing nomenclatures may produce confusion. A cautionary note should be added to the CAS practice of naming arene imines. Such molecules, which are fused aziridines, are named as azirine derivatives, e.g. compound 3[‌2‌] is named 5,5a-dihydro-4bH-dibenz[3,4:5,6]anthra[1,2-b]azirine.

Scheme 1 Azirine Nomenclature

As a cyclic planar 4π-electron system, 1H-azirine (1) is expected to be destabilized by electron delocalization, that is, it should, like oxirene and thiirene, be antiaromatic.[‌3‌‌7‌] Typically, these three-membered antiaromatic heterocycles are very elusive, and their attempted formation is either avoided or immediately followed by rearrangement into stable isomers. A discussion of the C2H3N potential energy hypersurface has been given by Maier et al.[‌8‌] The most stable members of the C2H3N ensemble are acetonitrile and methyl isocyanide. In addition to these well-known compounds, ketenimine (H2C=C=NH), aminoacetylene (HCCNH2), hydrogen cyanide N-methylide (HCN+CH2) and 2H-azirine (2) are characterized species (enumerated with increasing energy). On the other hand, no spectroscopic evidence has so far been obtained for the existence of 1H-azirine (1) or the electron-deficient isomers aziridin-2-ylidene (cyclo-H2CCNH), formimidoylmethylene [HCCH(=NH)] and vinylnitrene (H2C=CHN).

The energy difference between 2H-azirine (2) and 1H-azirine (1) is, according to 6-31G calculations, of the order of 33 kcal·mol1,[‌7‌,‌9‌] which may explain the failure of all attempts to detect 1. Furthermore, the calculations suggest that 1 is only slightly more stable than the isomeric sextet species formimidoylmethylene and vinylnitrene.

Although stabilization of the 1H-azirine system by the correct choice of substituents at all three ring positions can be envisioned, at the time of writing there were no reports on the isolation of a 1H-azirine under standard conditions. Therefore the role of 1H-azirines as short-lived reaction intermediates is discussed hereafter. Furthermore, attempts to isolate and characterize such species by spectroscopic techniques in low-temperature matrixes are described.

For a general review on azirines, see HoubenWeyl, Vol. E16c, pp 93144, 317369.