Azirines formation

H-azirine formation no conjugation of the imine function with the ester... 

The first example of azirine formation from a carbene and a nitrile has... 

Laser flash photolysis of a series of fluorinated aryl azides produces the transient spectra of the corresponding singlet nitrenes. ° With the exception of singlet o-fiuorophenylnitrene (39s), the rate of decay of the singlet nitrene was equal to the rate of formation of the reaction products, for example, didehydroazepines and triplet nitrenes. Values of fejsc and the Arrhenius parameters for azirine formation are summarized in Table 11.5. 

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). 

Azirine, formation, 56, 372 2W-Azirine-2-acrylates, 3-phenyl-reaction with formamidine, 57, 62, 63 guanidine, 57, 63 hydrazine, 57, 62... 

Introduction of oxygen into a Minisci-type reaction mixture leads to formation of amino ketones (Scheme 94). The mixed acetate/azide of lead(IV) with styrene in acetonitrile at -20 C yields phen-acyl azide (60%). One example of azirine formation has already been discussed (Scheme 87). Other related syntheses from vinyl azides are included in a recent review. ... 

The generality of azirine formation from vinyl azides was demonstrated by Hassner and co-workers,25 who also isolated the bicyclic azirine 7 from... 

The mechanism of azirine formation has been discussed by L abbe.20 A priori, the three paths shown in Scheme 1 can be considered. From a comparison of the activation parameters27 for vinyl azide decomposition ( a 26-30 kcal/mol, AS - 3 to + 5 eu) with those for aryl azides (Ea 35-39, AS 18) it appears that the nitrene pathway a can be excluded, but a clear choice between paths b and c cannot be made at this time.20... 

The simplest member of the class of vinyl azides, H2C=CH-N3, has been known for about 100 years. However, it was not until the late 1960s that vinyl azides became an important and synthetically useful class of organic compound. The most interesting and important reaction of vinyl azides is the formation of azirines produced upon photolysis and thermolysis of vinyl azides. Smolinsky and Pryde first observed azirine formation, together with a small amount of keteneimine, by gas-phase pyrolysis of a-aryl-substituted vinyl azides. 

Several compntational methods have been used to evaluate the mechanism of azirine formation. 

Computational Results on the Mechanism of Azirine Formation and Properties of Vinyinitrene... 

According to MNDO calculations, the concerted mechanism of azirine formation (b) was found to be the most reasonable process (see Scheme 5.0). The free nitrene route (a) for the thermal reaction was excluded on the basis of very high activation energy. A similar conclusion was made on the basis of HF, MP2, and CISD calculations with a minimal STO-3G basis set. But as noted before, the authors of both publications considered the formation and rearrangement of the closed-shell singlet nitrene ( A -VN). 

Smolinsky and Pryde first observed azirine formation, together with small amount of ketenimin, by gas-phase pyrolysis of a-aryl substituted vinyl azides. 

The decay of vibrationally cooled 67b was accompanied by the growth of isocarbazole 72b with the rate constant bs = 3.8 0.8 x 10 s (t = 260 70ps). A considerable acceleration of the singlet nitrene 67b rearrangement was observed in methanol. The rate constantof its decay was found to be 1.6 0.2 X 10 s (t=62 lOps). Nevertheless, this value is about 4 times lower than that for 67a in CH3CN (6.3 x 10 s ). The main contribution to the latter process is the reaction of the azirine formation ( 67a 69a). 

In essence, the stage of azirine formation resembles the azirine synthesis from oximes or their ethers by the Hoch-Campbell reaction [344—348]. 

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