Azirines ring opening

The ring opening of 2//-azirines to yield vinylnitrenes on thermolysis, or nitrile ylides on photolysis, also leads to pyrrole formation (B-82MI30301). Some examples proceeding via nitrile ylides are shown in Scheme 92. The consequences of attempts to carry out such reactions in an intramolecular fashion depend not only upon the spatial relationship of the double bond and the nitrile ylide, but also upon the substituents of the azirine moiety since these can determine whether the resulting ylide is linear or bent. The HOMO and second LUMO of a bent nitrile ylide bear a strong resemblance to the HOMO and LUMO of a singlet carbene so that 1,1-cycloadditions occur to carbon-carbon double bonds rather than the 1,3-cycloadditions needed for pyrrole formation. The examples in Scheme 93 provide an indication of the sensitivity of these reactions to structural variations. 

Aziridines, e.g. (91), undergo thermal ring opening in a conrotatory manner to generate azomethine ylides. These azomethine ylides are 47r-components and can participate in [4 + 2] cycloadditions with 1-azirines acting as the 27r-component 73HCA1351). 

The reaction of 2-phenyl-l-azirine (201) with benzoic acid gaveN-benzoylphenacylamine (204) (67BCJ2938). The overall mechanism of the reaction involves initial protonation on nitrogen followed by addition of the nucleophile to the azirinium ion and finally ring opening. 

Diels-Alder cycloaddition of 2/f-azirines 23 with cyclopentadienones provides 3//-azepines 25 in excellent yields by electrocyclic ring opening, with concomitant loss of carbon monoxide, of the initially formed, nonisolable cycloadducts 24, followed by a [1,5]-H shift in the resulting 2//-azepines.31 108... 

The 1,3-dipolar intermediate obtained in the ring opening reaction of 3H-azirines via cleavage of the C—C bond has been trapped to give oxa-zolines 355) (3.36) and pyrrolines (3.37) 356). 

I.3.4.2.2. Nonaromatic Unsaturated Heterocycles Reactions of aromatic nitrile oxides with 1-azirines are followed by the ring opening of the latter to give 4-benzamidoisoxazoles 145 (314). The structure of 145 (R = 4-C1C6H4, Ar = Ar7 = Ph) was established by single-crystal X-ray analysis. A mechanism for the formation of 145 has been proposed, (see Scheme 1.29). 

The ring opening of azirines under conditions of photoinduced electron transfer (PET) gives, not a nitrile ylide, but the short-lived 2-azaallenyl radical cation 108... 

Heimgartner and co-workers conducted a detailed mechanistic investigation of the reaction of [ NJlabeled 2,2-dimethyl-3-(dimethylamino)-27/-azirine 332 with NH-acidic heterocycles (Scheme 6.71). Based on these studies the authors proved that ring opening of 188a with 332 afforded the imidazolone 336 in which... 

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