exo/endo Preferences of Double Bonds in Three‐Membered Ring Compounds − The Bias Toward Endocyclic Unsaturation in 3‐Alkyl‐ and 3‐Amino‐2H‐azirines: A Theoretical and Experimental Study

Irradiation of the amino-4H-1,2,3-triazoles 7 (λ ⩾ 280 nm) affords molecular nitrogen and 3-amino-2H-azirines 8 in high yields. As shown on the basis of spectroscopic evidence, the photoproduct of 7a exists exclusively as 3-amino-2H-azirine 8a rather than in an equilibrium with the hypothetical iminoaziridine 10. Ab initio quantum chemical calculations up to the Gaussian-3 level were performed for pairs of 3-substituted 2H-azirines (1a, 3a, 5)/aziridines (2, 4, 6) and 1-substituted cyclopropenes (11, 13, 15)/cyclopropanes with exocyclic double bonds (12, 14, 16). The G3 energies of acyclic pairs of tautomers 17−22 and other compounds were calculated for comparison. Ring strain was assessed with the help of two groups of homodesmotic reactions involving either hydrogen transfer (Equations 1−4) or exchange of carbon for nitrogen (Equations 5−7). In the cyclopropane series, relative ring strain and the stabilities of the functional groups both favour the same tautomer. The strain in aziridines − and in 2H-azirines in particular − is lower than in the corresponding cyclopropanes. This is the reason why the relative stabilities of the functional groups present in the N-heterocycles outweigh differences in strain that may be caused by the endo- or exocyclic location of the double bonds. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)