Nucleophilic Substitution by Azide Ion

In a discussion of the formation of the carbon-azide bond by nucleophilic substitution with azide ion at an electrophilic carbon centre, the consequences of mechanistic differences betv/een reactions at saturated, unsaturated and aromatic centres must be considered. 

Nucleophilic substitution (5 ) reactions of saturated aliphatic compounds may be either associative or dissociative and the majority lie between the limits set by iSnI reactions, in which the rate-determining step is heterolysis of the bond to the leaving group, and typical 5 2 reactions with fully synchronous bond-formation and bond-rupture. Nl-like reactions represent an intermediate case and are characterized by a greater extent of bond-rupture than bond-formation. Hence, in aliphatic 5 reactions the rate-limiting process involves some degree of prior or concurrent bond-rupture. 

It is not possible in such reactions to have a transition state with a high degree of bond-formation and a low degree of bond-rupture. The total bond order of the entering and leaving groups must be less than or equal to unity, otherwise the total bond order of the carbon atom at the reaction centre will exceed 4 and it is generally accepted that this does not occur. 

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Nucleophilic substitution by azide ion on an alkyl halide (Sections 8 1 8 13) Azide ion IS a very good nucleophile and reacts with primary and secondary alkyl halides to give alkyl azides Phase transfer cata lysts accelerate the rate of reaction... 

Alkyl azides prepared by nucleophilic substitution by azide ion in primary or secondary alkyl halides are reduced to primary alkylamines by lithium aluminum hydride or by catalytic hydrogenation... 

Most values of k /kw are not very different from unity (Tables 2-5) especially in view of the approximations involved in their estimation. The self-consistency of the data suggests that it is reasonable to use them to analyse the factors which control values of k /kyy and the special case of reaction of the hydronium ion has already been noted. Aromatic nucleophilic substitution by azide ion is a conspicuous exception to these generalizations. 

Aqueous cationic micelles speed and anionic micelles inhibit bi-molecular reactions of anionic nucleophiles. Both cationic and anionic micelles speed reactions of nonionic nucleophiles. Second-order rate constants in the micelles can be calculated by estimating the concentration of each reactant in the micelles, which are treated as a distinct reaction medium, that is, as a pseudophase. These second-order rate constants are similar to those in water except for aromatic nucleophilic substitution by azide ion, which is much faster than predicted. Ionic micelles generally inhibit spontaneous hydrolyses. But a charge effect also occurs, and for hydrolyses of anhydrides, diaryl carbonates, chloroformates, and acyl and sulfonyl chlorides and SN hydrolyses, reactions are faster in cationic than in anionic micelles if bond making is dominant. This behavior is also observed in water addition to carbocations. If bond breaking is dominant, the reaction is faster in anionic micelles. Zwitterionic sulfobetaine and cationic micelles behave similarly. 

A more convenient procedure for the synthesis of (113) from (5) was later elaborated <9UOM(4l2)l>. It is based on the intramolecular version of the reaction of organoboranes with organic azides. The THF complex of 1-boraadamantane (5a) is treated with iodine in the presence of an excess of sodium azide. The iodine atom in the intermediately formed borabicycle (118) undergoes an easy nucleophilic substitution by azide ions. The subsequent anionotropic rearrangement in the borabicyclic azide (119) leads (after the oxidation of the reaction mixture) to aminoalcohol (120), which is smoothly converted to 1-azaadamantane (113). The yield of (113) in this synthesis is 40-45% based on (5a), or 20-22% based on triallylborane (Scheme 44). 

Nucleophilic substitution by azide ion on an alkyl halide (Sections 8.1,8.11)... 

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