Azirines thermal chemistry

The thermal chemistry of a number of aryl-substituted azirines often results in the formation of indole derivatives (68TL3499). Thus, heating a sample of azirine (152) gave 3-phenylindole (155) and dihydropyrazine (156). The formation of (155) was suggested to... 

It is noteworthy that a return of 2-pyridylcarbene (185) to 3- or 4-pyridyl-carbene has not been observed.228,229 This parallels the observations from the thermal chemistry that 2-pyridylcarbene expands exclusively by insertion into the 2,3-bond, not into the 1,2-bond of pyridine.199,204,232 The nonoccurrence of this latter reaction (185 - 189) is due to the unfavorable nonbonded interaction between the nitrogen lone pair and the filled carbene 7t-orbital during such a rearrangement.202,232 In the extreme case, a bicyclic intermediate in this reaction would be the antiaromatic 1H-azirine 191 (Eq. 46). On the other hand, 3-pyridylcarbene can hardly avoid... 

The thermal chemistry of a number of related aryl-substituted 2/f-azirines were also studied. The 2-naphthyl-substituted azirines 299 were found to rearrange to benz[g]indole 300. The selectivity encountered here was attributed to the higher energy content of the intermediate leading to the alternative benz[/]indole system 302. Heteroaromatic systems were also found to undergo thermal rearrangement via highly selective reaction paths ... 

The synthesis of metal-coordinated 1-azirines and the reactions of azirines induced by metals have opened a new area in the chemistry of this small ring heterocycle. Many of the reactions encountered bear resemblance to previously discussed thermally and photo-chemically induced reactions of 1-azirines. The reaction of a series of diiron enneacarbonyls in benzene results in coupling and insertion to give diimine complexes and ureadiiron complexes as well as pyrroles and ketones (76CC191). A mechanism for the formation of these products which involves initial 1,3-bond cleavage and generation of a nitrene-iron carbonyl complex as an intermediate was proposed. 

A selective sampling of the photochemical cycloaddition and cyclization chemistry of 2H-azirines has been outlined in this chapter. Some photochemical sequences increase molecular complexity more than others, but each seems to provide complex heterocyclic structures in a very efficient manner. Indeed, many of these photoreactions rapidly construct hetero-polycyclic systems that are difficult to produce in other ways. In contrast to their photochemical behavior, the major thermal reaction of 2H-azirines generally involves C(2)-N bond cleavage to form vinyl nitrenes which further react by either insertion into an adjacent C-H bond or else undergo addition across a neighboring rc-bond. The 27i-electrons of the carbon-nitrogen double bond of 2H-azirines can also participate in thermal symmetry-allowed [4- -2]-cycloadditions with a variety of substrates. It is clear from the above discussion that the chemistry of 2H-azirines is both mechanistically complex and... 

Although acid- and thermally induced rearrangements of azido-quinones have been known for several decades, only in the last few years have efforts been made to elucidate the mechanism of these reactions. Unlike most other vinyl azides discussed in this chapter, azidoquinone chemistry generally seems not to involve azirines. Instead tiae products result from expulsion of molecular nitrogen usually followed by rupture of a carbon-carbon bond. 

In 1961, Smolinsky reported on vapor phase pyrolysis of a-azidostyrene (52, Scheme 5.8), which furnished 3-phenyl-2//-azirine as the main product and provided the first example of the synthesis of such strained heterocycles from vinyl azides. By analyzing the IR data of the pyrolysates produced from 52, it was shown one year later that N-phenylketenimine is formed as a side product. By utilizing IR and NMR spectroscopy at low temperature, Wentrup and coworkers have smdied recently the detailed structures of another azirine-ketenimine pair, generated by thermal or photochanical decomposition of an enazide. The transformation of vinyl azides into 2//-azirines is currently the most frequently used access to these heterocycles. The manifold chemistry of azirines was reviewed several times," " and most of the aspects of their synthesis from alkenyl azides are summarized in Chapter 6 (Gilchrist Alwes). Therefore, only some additional certain points are included here. 

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