Addition of azide ion

Among the experiments that have been cited for the viewpoint that borderline behavior results from simultaneous SnI and Sn2 mechanisms is the behavior of 4-methoxybenzyl chloride in 70% aqueous acetone. In this solvent, hydrolysis (i.e., conversion to 4-methoxybenzyl alcohol) occurs by an SnI mechanism. When azide ions are added, the alcohol is still a product, but now 4-methoxybenzyl azide is another product. Addition of azide ions increases the rate of ionization (by the salt effect) but decreases the rate of hydrolysis. If more carbocations are produced but fewer go to the alcohol, then some azide must he formed by reaction with carbocations—an SnI process. However, the rate of ionization is always less than the total rate of reaction, so some azide must also form by an Sn2 mechanism. Thus, the conclusion is that SnI and Sn2 mechanisms operate simultaneously. ... 

Conjugate addition of azide ion to dihydropyran-2,5-diones affords the 3-amino derivative <96SL341>, whilst reaction with bisnucleophiles provides a route to piperazines, thiazines and diazepines <96JHC703>. 

Addition of azide ion to conjugated systems can be carried out by using NaN3 and acetic acid in water (Eq. 10.18).38 Some reactions were very rapid while others took 1 to 3 days to complete. Lewis base was found to catalyze such conjugate additions of azide ion to cyclic enones in water.39... 

Fig. 1 for stepwise solvolysis of R-X is due to the increase in ks (s ), with decreasing stability of the carbocation intermediate, relative to the constant value of az (M s ) for the diffusion-limited addition of azide anion. The lifetime for the carbocation intermediate eventually becomes so short that essentially no azide ion adduct forms by diffusion-controlled trapping, because addition of solvent to R occurs faster than escape of the carbocation from the solvent cage followed by addition of azide ion (k Now, the nucleophile adduct must form through a... 

An azido group could also be introduced by 1,4-addition of azide ion onto 1-Me and 2d-Me in aqueous acetic acid. The corresponding azides were isolated in 56 and 62% yield, respectively [15b]. 

Addition of azide ion and metal azides to nitriles and nitrilium ions... 

The addition of azide ion to nitriles remains the main synthetic route to 5-substituted tetrazoles. The distinction between this approach and the imidoyl azide route (Section 4.17.9.1.1) is blurred... 

The most likely explanation is as outlined in Scheme 3. Following a well-known stereoelectronic effect , wherein addition of nucleophiles to triple bonds places the developing new lone pair trans to the incoming nucleophile, a fast addition of azide ion to the diazonium ion would give the Z-diazoazide (21) which cannot form a pentazole. This is analogous to the stable imidoyl azide of tetrazole chemistry. Rapid loss of N2 from this produces the first N2 evolution. 

Harvey and Ratts [51] also reported that vinyl azides can be prepared by the addition of azide ion to conjugated allenic esters, as in Eq. (11). A terminally... 

By addition of azide ion to the reaction, the intermediate can be competitively trapped ... 

J. Andres, S. Bohm, V. Moliner, E. Silla, and I. Tunon, A theoretical study of stationary structures for the addition of azide ion to tetrofuranosides modeling the kinetic and thermodynamic controls by solvent effects, J. Phys. Chem., 98 (1994) 6955-6960. 

Values of k.v, = 5 X 109 M-1 s 1 have been determined for diffusion-controlled addition of azide ion to a variety of ring-substituted benzhydryl24 and a-substituted 4-methoxybenzyl25 carbocations. This rate constant have seen extensive use as a clock to determine absolute rate constants for addition of a variety of nucleophiles to benzylic carbocations.5 Relative rate constants for addition of azide ion... 

Fig. 1 A hypothetical plot of azide ion selectivity fcaz/fcs (M-1) against the reactivity of the carbocation intermediate of solvolysis of R-X in aqueous solution (Scheme 4). The descending limb on the left hand side of this plot is for reactions where the value of ks (s-1) is increasing relative to the constant value of k (M-1 s-1) for diffusion-limited addition of azide ion to the carbocation. The constant nucleophile selectivity is for reaction of R-X by a preassociation mechanism.

Addition of azide ion to nitriles, nitrilium ions and isonitriles The addition of the azide anion to nitriles is the most widely used route to 5-substituted tetrazoles (203). This reaction can prove highly sensitive to the nature of the cation and the type of solvent used. It is also sensitive to the nature of the substituent R of the nitrile and it works best when R contains electron-withdrawing groups. In hydroxylic or alcoholic solvents the reaction usually requires a few days at reflux temperatures and the yields may be medium or poor. The best conditions tend to be in dipolar aprotic solvents such as DMF, using ammonium azide as the source of azide ion. The work-up procedures... 

Step is the addition of azide ion. While such a reaction may be possible in some cases, this mechanism also requires that cyclization proceeds in preference to protonation of the intermediate carbanion and this seems unlikely. A more probable mechanism is a synchronous 1,3-dipolar cycloaddition, and such reactions of covalent azides are well documented... 

A useful, although somewhat limited, method of preparing vinyl azides involves the addition of azide ion to vinyl halides with subsequent elimination of halide ion. This reaction is only possible when the halogen is located to a group which can effectively stabilize the intermediate carbanion. A number of alkyl and aryl substituted )3-chlorovinyl ketones (23) have been found to react in this way ". In many cases, however, the resulting /3-azidovinyl ketones (26) cannot... 

The isolation of the furoxan 107 following reaction of 1,2-dinitro-olefins 105 and sodium azide was explained by assuming addition of azide ion to the double bond, followed by elimination of nitrite ion to form l-azido-2-nitro6lefins 106. These vinyl azides could undergo loss of nitrogen concerted with cyclization to give 107. 

The spectral changes observed on addition of azide ions to trivalent actinide or lanthanide perchlorate solutions are shown in figure 1. 

That the reaction occurs as expected between the HOMO of azide (which is, after all, an anion) and the LUMO of the electrophilic nitrile is confirmed by reactions with aryl nitriles ArCN with sodium azide. The yields are all close to quantitative but the reaction is faster when R is an electron-withdrawing group. Not everyone agrees that the reaction is concerted as the approach of the two linear molecules looks very hindered. However, a stepwise addition of azide ion 92 followed by cyclisation 93 looks, if anything, worse as two negatively charged atoms must attack each other in the cyclisation 93. However, this too is pericyclic (electrocyclic) and not ionic. 

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