Azirines, nitrile ylide structure

It was found that the azirine-nitrile ylide isomerization was a completely reversible process. The unlabeled nitrile ylide showed a prominent band at 1926 cm that underwent a 66-cm shift with N substitution. This shift was interpreted as being consistent with an allene-like skeleton (8) rather than the alternative pwpargyl-like stmcture (9). This conclusion was supported by the spectra from the C- and H-labeled variants. Warming the nitrile ylide in a xenon matrix from 12 to 82 K provided no new absorptions suggesting that the allene-like structure may also be adopted in solution. Some absorption spectra for benzonitriho benzylide (DPNY) and some substituted benzonitrilio methylides obtained via pulsed-laser photolysis of azirines are given in Table 7.1 (5). 

Studies of the reaction of photochemically generated carbenes with nitriles to give nitrile ylides have again attracted attention. Flash photolysis of 1-naphthyldiEizomethane (51) in acetonitrile or of the azirine (52) gave the same transient species to which the nitrile ylide structure (53) was assigned. The first stable nitrile ylide was also obtained in this way irradiation of diazotetrakis(trifluoromethyl)cyclopentadiene (54) in the presence of 1-cyanoadamantane (55) gave the crystalline ylide (56).An X-ray structure determination showed that the 1,3-dipole is essentially linear. 

Figure 6.8 The 2i7-azirine nitrile ylide conversion structure at the Sj-So conical intersection as determined by semiempirical MNDOC-CI calculations (AS = 2-2 , reference configurations and...

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. 

Work on the molecular structure of benzonitrilio methylide 8/9 has been carried out via Fourier transform infrared (FTIR) smdies on it and five isotope-labeled variants. The nitrile ylides were generated in nitrogen matrices at 12 K either directly, by photolysis of the azirine 7, or indirectly from the azidostyrenes 6 (4). 

Orton, E., Collins, S. T., Pimentel, G. C., Molecular Structure of the Nitrile Ylide Derived from 3 Phenyl 2H azirine in a Nitrogen Matrix, J. Phys. Chem. 1986, 90, 6139 6143. 

The iminocarbenes 33 (Scheme 7) may also be regarded as nitrile ylides (35). Ab initio calculations45 indicate that nitrilium betaines are best described as hybrids of the bent dipolar and carbenic structures (e.g., 33 and 35). Consequently, the generation of the nitrile ylide 35 in the gas phase should give the same product as obtained from the azirine 31. This was confirmed by flash vacuum pyrolysis of the appropriate oxazol-5(2//)-one (36), which gave a quantitative yield of the azadiene 34.46... 

Starting with y= 120° and 0 = 0°, the MNDOC-CI method yields a conical intersection, the structure of which is shown in Figure 6.8. The values 0=0° for the rotational angle and 7 = 143,7° for the bond angle are as expected the bond distances are between those of azirine and nitrile ylide, especially because the distance between nitrogen and the methylene carbon r N = 1.34 A has decreased and is equal to the double-bond value r N = 1.34 A in the nitrile ylide. To completely establish the mechanism, it remains to be shown that the conical intersection is accessible without a barrier from the Franck-Condon geometry and that the nitrile ylide can be reached from the conical intersection. This is easily... 

Irradiation of phenyl-2//-azirines in the presence of carbon dioxide leads to the formation of the 3-oxazoline-5-one system121-123 and, in some cases, to the isomeric 2-oxazolin-5-one122 [Eq. (24)1. The azirines serve as incipient nitrile ylides, whose 1,3-dipolar structure permits cycloaddition to the dipolarophile C02123 [Eq. (25)1. The reverse reaction, photolytic extrusion of C02 from pseudoxazolones, is synthetically useful, since the dipolar nitrile ylide thus formed can be trapped with a variety of dipolarophiles. Thus, 2,2,4-triphenyl-3-oxazolin-5-one (48) is readily converted into the stabilized ylide (49)124 [Eq. (26)1, and the use of methyl acrylate,122 acrylonitrile,122 and dimethylacetylene dicarboxy-... 

Of all the methods available to generate this 1,3-dipole, the photochemical generation of nitrile ylides from 2tf-azirines offers the greatest opportunity for structural variation. The 2/f-azirine ring can be readily prepared in large quantities using either the vinyl azide route developed by Hassner and coworkers or the modified Neber reaction sequence. 3-Aryl-2 -azirines (15) can also be obtained in good yields by reaction of alkylidene phosphoranes... 

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