Nitrenes vinylnitrenes

Important synthetic paths to azirines and aziridines involve bond reorganization, or internal addition, of vinylnitrenes. Indeed, the vinylnitrene-azirine equilibrium has been demonstrated in the case of trans-2-methyl-3-phenyl-l-azirine, which at 110 °C racemizes 2000 times faster than it rearranges to 2-methylindole (80CC1252). Created in the Neber rearrangement or by decomposition of vinyl azides, the nitrene can cyclize to the p -carbon to give azirines (Scheme 4 Section 5.04.4.1). 

Extension of these studies to benzologues of tetrazolo[l,5- ]pyridine, that is, for tetrazolo[l,5- ]quinoline 21 and tetrazolo[5,l- ]isoquinoline 22, led to interesting results <2003JOC1470> as shown in Scheme 7. Both of these fused tetrazoles resulted in formation of a nitrene 23 and 24, respectively, which could be interconverted via formation of the fused cyclic carbodiimide derivative 25. Isoquinolylnitrene 24, furthermore, was found to undergo subsequent reactions ring opening afforded the vinylnitrene 26, which was transformed to o-cyanophe-nylacetonitrile 27 by a 1,2-H shift and to 4-cyanoindole 28 by an intramolecular cyclization in 40% and 25% yields, respectively. 

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. 

Although vinylnitrenes are not involved in the formation of 2ff-azirines from vinyl azides, such nitrenes must be invoked in order to explain the further reactions of the azirines (Scheme 2). Thermal or photochemical ring opening of the azirine to a nitrene, and subsequent 1,2-shifts explain the formation of ketenimines and benzyl cyanides. A competing electrocycliza-tion of the nitrene onto a phenyl substituent in the /(-position leads to the... 

Note that in the early literature vinylnitrene was thought to be a closed-shell dipolar species. Therefore, attempts were made to intercept this nitrene using 1,3-dipo-... 

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. ... 

This tendency is also seen in comparing the CASPT2 and multireference configuration interaction (MR-CISD -1- Q) values for the energy difference between open-shell singlet (W) vinylnitrene and 2//-azirine. The nitrene and azirine... 

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 whole range of vinylnitrene reactivity was demonstrated in the case shown in Scheme 4. The 2-(2-benzofuryl)-2//-azirine 23 was produced by photolysis of the azide 22 at 0°C. The azirine underwent thermal ring opening to the nitrene 24 at 55°C. Depending on the substituents, a condensed pyrrole, pyridine, or azepine was obtained, as illustrated in the scheme. The same... 

However, singlet vinylnitrenes have never been observed by time resolved spectroscopy and triplet vinylnitrenes have not been observed by either time resolved or matrix spectroscopy. The formation of an azirine was observed upon photolysis of a-azidostyrene in an argon matrix at cryogenic temperature (8 K), but even under these conditions nitrene... 

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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