Isomerizations of allylic azides

Isomerizations of allylic azides also appear to involve concerted, non-ionic rearrangements. Butenyl and pentenyl azides isomerize slowly at room temperature in a variety of solvents, viz. 

Allylic azide isomerizations resemble other concerted thermal isomeriza-tions of allylic compounds in their low sensitivity to solvent ionizing power and allylic substituents and also in having negative entropies and volumes of activation. Entropies of activation for isomerization of pentenyl azides average — 10 to — lieu, in pentane, ether, and acetone solutions In methylene chloride at 20°C, the volumes of activation for isomerization of a-methylallyl azide and y-methylallyl azide are —9.5 cm. mole and —7.9 cm. mole respectively.  

The insensitivity of allylic azide isomerization rates to substituent effects and solvent ionizing power, and the relatively small value of the negative volume of activation make it very unlikely that ion pair intermediates are involved in these reactions. The negative entropy and volume of activation are compatible with a concerted process involving a cyclic transition state resembling a 1,2,3-triazine. However, the known ability of alkyl and aryl azides to add to olefins - suggests the possibility of an intramolecular addition-elimination mechanism,viz. 

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Table 5.1 Synthesis of vinyl azides by DABCO-catalyzed isomerization of allyl azides ...

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