Ring-opening of Azirines

Substituted azirines add photocliemically to olefins and ketones to yield five-membered rings of the t5q e of pyrroline ((X=CRiR2) or S.oxazoline (X=0) 200,203), yjie primary nTi excitation, like the nre excitation of ketones, leads to the cleavage of a C—C single bond to the site of 

---

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... 

SCHEME 5.33 Nitrile ylide from photolytic ring opening of azirines. 

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. 

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). 

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... 

The photolytic ring opening of 2//-azirines yields nitrile ylides which can be trapped as pyrrolines (97) (73JA1945). 

The formation of the [3 + 2] cycloadduct 89 can be explained by two different reaction pathways (Scheme 36). Irradiation at 300 nm leads to ring opening of the azirine 88 to the nitrile ylide as an intermediate which adds to C6o yielding the observed product 89. No product formation is observed at wavelengths > 400 nm. Instead, under photoinduced electron transfer conditions in the presence of... 

Ring opening of aziridines and azirines by hydrofluorination 91T5329. Synthesis of JV-arylaziridines based on l-chloroalkyl(aryl)sulfoxides 91YZ205. 

The photolysis of 4-azidopyridazines gives significant yields of the ring-expansion products, unsaturated 1,2,5-triaze-pines. Thus, 3-methoxy derivative 801 reacts, via formation of a nitrene and ring opening of an intermediate fused azirine, with methoxide or diethylamine to give 4-methoxy (or4-diethylamino)-l,2,5-triazepines 802 (X = OMe, NEt2). 

An interesting application of PET mediated bond cleavage reaction from azirine 63 has been reported by Mattay et al. [67] for synthesizing N-substituted imidazoles (65) via the (3 + 2) cycloaddition reaction of resultant 2-azaallenyl radical cations with imines 64. Synthesis of pyrrolophane 3,4-dimethyl ester (68) has been reported recently [68] by the ring opening of 66 followed by inter-molecular cycloaddition with dimethyl acetylene dicarboxylate (67) as shown in Scheme 13. 

Scheme 34. Preparation of porphyrin 34 initiated through electrocyclic ring opening of four azirine radical cations to 2-azaallenyl radical cations [373c].

---