Vinylnitrene intermediates

The thermal chemistry of a number of aryl-substituted 2H-azirines often results in the formation of indole derivatives (68TL3499). Thus, heating a sample of 2H-azirine 69 gave 3-phenylindole (72) and dihydropyrazine 73. The formation of 72 was suggested to proceed via a vinylnitrene intermediate which cyclizes and then undergoes a 1,5-sigma tropic shift (Scheme 20) (77H1563). 

It was shown that 1-azirines can serve as precursors to vinylnitrenes. A number of 1-azirines rearrange to indoles, most likely via vinylnitrene intermediates, upon heating. Two examples are known in which the nitrene formed during the thermolysis of azirines can be intercepted by tris(dimethy-lamino)phosphine or triphenylphosphine. ... 

Another apparent example of the trapping of a vinylnitrene has been reported by Nomura and coworkers.These workers found that the thermolysis of vinylazides in the presence of electron-deficient alkenes resulted in the formation of iV-vinylaziridines 277 and 2l -pyrrolines 278. Although this reaction was interpreted in terms of the trapping of a vinylnitrene intermediate (path a), it seems that concerted 1,3-dipolar cycloaddition of the azide followed by extrusion of nitrogen from a transient cycloadduct 279 could also rationalize the results (path b). 

The condensation of amines with 2-formyl-3-phenyl-2/f-azirine (350) yields the imines 352, which undergo thermal reorganization to pyrazoles 353 via 1,5-dipolar cyclization of the resulting vinylnitrene intermediate derived from CN bond rupture. The thermal reactivity of 350 and 352 parallels completely that of the 2-vinyl-substituted 2/f-azirines. 

The two isomeric spiro azirines 1 and 2 were synthesized and separated chromatographicaUy but FVP of either compound at 400 °C produced a 1.1 mixture of the two (Scheme 1) with the interconversion postulated to take place by means of a vinylnitrene intermediate (201 ICEJl0071). 

Apart from the azirine pathway, a vinylnitrene 526 was postulated as a possible intermediate in the reaction (Scheme 14). The nitrene may be formed by a base-promoted elimination of the leaving group on the nitrogen and gives rise to the 277-azirine by electrocyclization (nitrene insertion) and, in view of current data, this pathway cannot be excluded. 

By analogy with the dehydration of aldoximes in the KOH/DMSO system, which certainly involves abstraction of the proton nearest to the oxime function, in the case ketoximes, one can expect 1,3-dehydration which leads to unstable intermediates such as the 1,3-dipole 82, azirine 83, vinylnitrene 84 or nitrile, or the stable rearrangement product of one of them (Scheme 42). 

In the absence of traps, the azirines can undergo further transformations that can be rationalized in terms of vinylnitrene or nitrile ylid intermediates.185,200202 Thus, photolysis of 3,4,5-triphenylisoxazole gives some of... 

The usual photoreaction of a vinyl azide is intramolecular cyclization to yield a 2H-azirine. Although this process is a well-established path to these compounds, the details of the mechanism by which the reaction occurs is unresolved. In some instances the formation of the azirine is thought not to involve the nitrene intermediate. However, other studies have shown that azirines and vinyl nitrenes are in thermal equilibrium and thus it seems likely that a vinylnitrene is the key intermediate in such reactions. 

The situation with vinylnitrenes is analogous to methylnitrene and it is not clear if either of these singlet nitrenes are true reactive intermediates with finite lifetimes. Quantum chemical calculations can help to explain the properties of vinylnitrenes. All calculations reported in the literature have been concerned with only the simplest vinyl azide and vinylnitrene, CH2=CH-N3 and CH2=CH-N (29a), respectively. Early theoretical studies on the vinyl azide to azirine transformation employed semi-empirical calcula-tions, or ab initio calculations performed at relatively low levels of theory, and have focused only on the closed-shell singlet excited state ( A ) of vinylnitrene. We will discuss only the latest calculations. ... 

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