Halogen azides, reaction with alkenes

The seminal work of Hassner has shown that halogen azides react with alkenes both in ionic and radical pathways leading to opposite regiochemistry (Scheme 8.13). The nature of the reagent and the polarity of the media play a fundamental role. Iodine azide reacts via an ionic pathway whereas the two mechamsms may operate for bromine and chlorine azides. Apolar solvents such as pentane and irradiation of the reaction mixture favor the radical process. 

Reaction of Conjugated Alkenes with Halogen Azides... 

The analogous reactions of alkenes with halogen azides are treated in Section 7.2.2 and those with iodine isocyanate and bromocyanamide are treated in Section 7.2.6. 

The nitrogen source for the aziridination of alkenes, a nitrene or nitrenoid, can be generated in various ways (1) oxidation of a primary amine (2) base-induced -elimination of HX from an amine or amide with an electronegative atom X (X = halogen, O) attached to the NH group or by -elimination of metal halides from metal A-arenesulfonyl-A-haloamides (3) metal-catalyzed reaction of [A-(alkane/arenesulfonyl)imino]aryliodanes (4) thermolytic or photolytic decomposition of organyl azides and (5) thermally induced cycloreversion reactions . 

Both alkyl and alkenyl amino acids can be prepared by this approach. A common method for introducing the halide into an alkene-bearing molecule is illustrated by the reaction of -pent-2-enoic acid with N-bromosuccinimide to form 1.12. Subsequent treatment with ammonia led to displacement of the bromine moiety to give 4-aminopent-2-enoic acid (J.I3). An alternative method reacted 1.12 with sodium azide and then reduced the azide with zinc and acetic acid (see section l.l.B.iv). Allylic halogenation in systems such as 1.12 are well known. 

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