Nitrenes vinyl

Thermolysis of the vinyl azide 26 in xylene yields ethyl 8-(mcthoxymcthyl)-l,8-dihydro-benz[2,3]azepino[4,5- >]indole-2-carboxylate (27) by attack of the vinyl nitrene at the 2-phenyl group.83... 

Figure 6. MNDO heat of formation hypersurface for singlet vinyl nitrene rearrangements to 2H-azirine, ketene imine and the thermodynamically most favorable isomer, acetonitrile.

Inspection of the MNDO hypersurface shows the assumed vinyl nitrene to lie in a very shallow minimum with the activation barrier for ring closure to 2H-azirine (Figure 1 a ) calculated to be only a few kJ/mole. Whereas the separating hypersurface "ridge" to ketene imine of AA t 70 kj/mole seems insur-... 

This last example again demonstrates the usefulness of correlating hypersurface calculations as closely as possible to measurement data although methyl and vinyl nitrenes, which by chemical intuition seem to be reasonable intermediates, do not have to occur necessarily along the reaction pathway (36,39), the predictions stimulated an experiment, which otherwise - due to the hazardous starting material (38) - hardly would have been carried out. 

The question as to whether the thermal azirine formation proceeds through a vinyl nitrene intermediate or by a concerted mechanism is not as yet resolved. A nitrene intermediate seems most probable on the basis of its similarity with cyclopropene formation from alkenylcarbenesM>> and because either thermal or photochemical decomposition of vinyl azides yields die azirines (Table XI). 

Three plausible mechanisms20 have been postulated for the thermal decomposition of vinyl azides (41) to 1-azirines (47). The azide can either (1) lose nitrogen to give the vinyl nitrene which then cyclizes to the 1-azirine, (2) lose nitrogen with simultaneous ring closure (45), or (3) first cyclize to triazole (46) which then loses nitrogen. To date little experimental evidence exists to favor any of these alternatives. 

These results suggest the intriguing possibility that the azirine may be in thermal equilibrium with the vinyl nitrene. The vinyl nitrene can then react further to give the thermodynamically more stable nitrile or, in some cases, the indole. However, the intermediacy of the vinyl nitrene in the 1-azirine pyrolysis reactions has not been determined. 

The base-catalyzed condensation of a-azido esters and ketones with aromatic aldehydes has recently been developed as a new vinyl azide synthesis.42,43 The yields range from moderate to excellent in some cases. The thermal decomposition of ethyl a-azidocinnamate (87) in xylene gives only 2-ethoxycarbonylindole (88).44 The unstable 2-ethoxycarbonyl-3-phenyl-l-azirine could be detected if the thermolysis was carried out at a lower temperature. This fact indicates that the 1-azirine is probably an intermediate leading to the indole, although the intermediacy of the vinyl nitrene could not be established. This result is similar to that observed by Isomura et al. on the pyrolysis of terminal vinyl azides.27,28... 

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... 

Another significant preparative pathway to the 2i/-azirine system is the Neber rearrangement of oxime sulfonates. The presence of strong electron-withdrawing groups in the a-position to the oxime increases the acidity of those protons, and thus favors the cycloelimination reaction under mild conditions. The Neber reaction occurs either through an internal concerted nucleophilic displacement or via a vinyl nitrene (Scheme 213) <2001EJ02401>. 

Vinyl nitrenes usually do not produce keteneimines. However, in the case of l-azido-2,2-dicyanoethylenes (57) the major products were the dicyanoketeneimines 58. Due to their instability only their addition products [i.e. with ethanol (59)] were isolated ". 

In considering a mechanistic rationale for vinyl azide chemistry one is strongly tempted to invoke a vinyl nitrene intermediate in either its singlet, 131 or triplet, 132, state. The existence of nitrenes has been... 

From mechanistic studies on the Neber reaction, the 2//-azirine has been shown to be a distinct intermediate formed by ring closure of a vinyl nitrene. The evidence for the vinyl nitrene has resulted from the reported lack of steieospecificity in converting the ( )- and (Z)-isomers of 0-tosyloximes to the same a-amino ketone. Although a vinyl nitrene has been suggested as a possible intermediate in the thermal and photochemical preparations of the azirine ring, direct evidence for such a species is lacking. In this context, the configurational stereospecificity in a modified Neber reaction has been studied with oxime... 

The major thermal reaction of 2/f-azirines involves C(2)—N bond cleavage as shown in Equation (6) to form vinyl nitrenes (47). Cleavage of the C—C bond to produce iminocarbenes (48), diradical... 

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