Oxygen 1-azirines

Prolonged reflux of 3-methoxy-2,2-bis(trifluoromethyl)-2//-azirine (16) during very slow fractional distillation with a spinning-band column gives isocyanate 17 in 20% purified yield by an alkyl shift from oxygen to carbon.50... 

Leonard and Zwanenburg55 have treated azirine (26) with anhydrous perchloric acid and studied the reactions of the protonated azirine with acetone and acetonitrile. It is believed that protonated azirine ring 109 opens to cation 110 which then adds to the carbon-oxygen double bond or carbon-nitrogen triple bond to give the observed products (111 and 112). 

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. ... 

On the other hand, the photoisomerization of oxazoles to isoxazoles involves not only the interchange of the 2 and 3 positions, but that of the 2 and 4 positions. To explain the latter reaction, the intervention of a Dewar compound was proposed (Scheme 13).41 The walk of the oxygen atom is characteristic of this mechanism. A quantum mechanical calculation suggested that the O—C-2 bond was weak (for the azirin) and the bond order between C-2 and C-5 was high (for the Dewar intermediate) in the excited state. 

Hafner and Bauer were able to isolate azirine 254 in high yield from a low-temperature photolysis of the corresponding vinyl azide. Thermolysis of 254 gave imine 285 and fluorenone 286. The thermal reaction was proposed to proceed via the 9-fluorenylidene carbene, produced by elimination of HCN from 254 followed by a subsequent reaction with starting material or with oxygen. 

The mechanisms of the [Pd(PPh3)4]-catalyzed conversion of 2-allyl-azirines to pyridines and pyrroles, e.g., equation (30), and the Pd(0)-catalyzed 1,3-oxygen-to-carbon alkyl shifts, e.g., equation (31), have been studied in detail. 

The distal arrangement of the substituents in 24 prevents cyclization to a benzoxazole-type intermediate. While stable in alcohols, this intermediate can undergo hydrolysis (in water) to give hydroxyformanilide (25). Further analysis of the reaction revealed that 24 is a secondary photoproduct of the first-formed azirine 26, in contrast with the thermally induced azirine conversion to isonitrile that is observed for 18. Triplet quenchers such as oxygen and methyl acrylate quench the reaction efficiently. The authors propose a mechanism for conversion of 23 to 26 that involves a deprotonation-reprotonation sequence preceded by intramolecular electron transfer from the triplet-excited state of 23. 

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