exo/endoPreferences 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

Abstract

Keywords: Ab initio calculations / Nitrogen heterocycles / Photolysis / Small ring systems / TautomerismIrradiation of the amino-4H-1,2,3-triazoles 7 (λ Ն 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 [ ‡] The experimental results are part of the Dissertation by T.

Synthesis and Reactions of Highly Strained 2,3‐Bridged 2H‐Azirines

“…It should be possible to transfer this concept from the hydrocarbons 1 to the heterocycles 3 2. 3 The 2,3‐bridged azirine 3 d , which is easily obtainable from azide 2 d by photolysis or thermolysis, can be distilled in vacuo,4 whereas the more unstable compound 4 could be characterized only in solution 5. Until now, the even more strained azirine 3 b and many similar bridged azirines bearing a six‐membered ring can be generated only in situ and detected by trapping reactions 4a.…”

Synthese und Reaktionen hochgespannter 2,3‐verbrückter 2H‐Azirine

“…Diese Betrachtungen sollten sich von den Kohlenwasserstoffen 1 auch auf die Heterocyclen 3 übertragen lassen 2. 3 Während das durch Photolyse oder Thermolyse aus dem Azid 2 d leicht zugängliche 2,3‐verbrückte Azirin 3 d im Vakuum destilliert werden kann,4 gelingt die Charakterisierung des instabileren Derivats 4 nur noch in Lösung 5. Das noch gespanntere Azirin 3 b und zahlreiche ähnliche, einen Sechsring aufweisende verbrückte Azirine ließen sich bisher nur in situ erzeugen und durch Abfangreaktionen nachweisen 4a.…”

9 Computational organic chemistry

“…The sophisticated G3 level of theory has been used to calculate the relative energies of a series of compounds based on 13 and 14 (X = CH 2 , NH and O). 16 It was found that structures with an exocyclic double bond are uniformly preferred in the cyclopropanes, but that the preference in the nitrogen heterocycles varies depending on X. Ring strain was evaluated with the use of homodesmotic reactions.…”