Excess Azides

A potential advantage of the Schmidt reaction is illustrated by the conversion of progesterone to the 17j5-acetylamino derivative, without affecting the A-ring. A 35 % yield is obtained when 1 mole of sodium azide in polyphos-phoric acid is used. With excess azide the A-ring is transformed into an a,jS-unsaturated lactam ... 

The later publication [1] reveals that the title compound is in fact a relatively stable compound. The previously attempted preparation of the then unknown compound from trichloroacetonitrile, sodium azide and ammonium chloride (0.14 0.42 0.2 mol) by an analogous established method [2], but at lower initial temperature because of the exothermic reaction, gave, after vacuum evaporation of solvent, an oily product. When sampled with a pipette, this evolved gas and then exploded violently. It was thought that an azidomethyltetrazole may have been formed by displacement of chloro-substituent(s) by the excess azide employed [3], An alternative hypothesis which involved isomerisation of the title compound to the open chain azidoazomethine [4] was discounted, because no trace of this could be detected [1]. 

The activating effect of the azide makes the fluorine labile, so that there is a risk of excess azide incorporation when attempting preparation by nucleophilic substitution of bromofluorocarboxylates, giving more explosive products than anticipated. 

In general most substituents show normal behaviour but an interesting reaction has been reported concerning the reactivity of the bromo substituent of (126). Thus, (126) reacts with excess azide ion in HMPA to give the isomer (127), by an unknown mechanism. Furthermore, the bromo isomer (127) is slowly converted into (117) when treated with sodium azide in HMPA D20 (30 1), apparently via nucleophilic attack at bromine with generation of a carbanion at the ring carbon (82TL299). 

Exo addition is not always the rule with cyclic dienes, and endo adducts have been identified in several instances. Although in earlier work phenyl azide was thought to yield with excess norbornadiene only a single exo adduct,25 later work indicates that the endo isomer is also formed in an exo-endo ratio of 11/1 (Scheme 12).97 In excess azide, four isomeric bistriazolines have been... 

Rate constants (k X 10 9 M-1sec-1) were determined to be 7.0, 3.5, and 1.0 for the enumerated substrates, respectively. The change in kinetics for the three cation radicals with increasing steric hindrance at the (3-carbon is in accordance with the depicted addition reaction. In contrast with that, a reaction of the azide ion with these three cation radicals in acetonitrile proceeds with rate constants that are the same in all three cases ( 3 X 109 M-1sec-1). In acetonitrile, the reaction consists of one-electron transfer from the azide ion to a cation radical. As a result, a neutral styrene and the azidyl radical are formed. The azidyl radical reacts with the excess azide ion, and the addition reaction does not take place ... 

If the solution is allowed to stand for a longer period before the nitrogen complex is precipitated, the complex may react with the excess azide, reducing the yield. 

The reaction of the azide radical Ns- with excess azide ions in solution forms the hexanitrogen radical anion = 18kJmol- ). ... 

Four years after the isolation of cephalostatin A, the first synthetic studies in this area were reported. Fuchs etal. (38) reported the conversion of steroidal a-azidoketones 48 to the C-2 symmetrical nonacyclic and trisdecacyclic pyrazines 49 through catalytic reduction (Scheme 1). In the process they also observed an unusual azide-mediated formation of an unsymmetrical heterobenzyl azide (Scheme 2). Use of excess azide served as a base to generate the a-aminoenone 50. Dimerization of 50, followed by SN2 reaction with the hydrazoic acid coproduct yielded the unsymmetrical azidopyra-zine 51. 

Compound 30 was carried forward to a-bromoketone 31 which was displaced by azide to yield a-amino enone 32 directly in good yield (Scheme 14). The formation of 32 is probably due to the use of excess azide (as discussed in Scheme 6, vide supra). 

The uranyl ion, (U02), forms yellow azido complexes that are stable in dilute (0.01 M) aqueous solutions in the presence of excess azide a cation structure [(U02)(N3)] has been established and was used in the analysis of uranium [137,138]. The anion tetraazido complex has been isolated as the tetraphenylarsonium salt, [As(Ph)4]2 [(U02)(N3)4], by admixing a solution of 1 g uranyl nitrate in 2 N nitric acid in nitrogen environment to a solution of 7 g sodium azide. At 80°C the red solution was precipitated with tetraphenylarsonium chloride to obtain yellow crystallites that decomposed at 171°C [139]. 

Complex palladium azides are obtained by dissolving palladium salts in excess azide, e.g., palladium nitrate and sodium azide (1 4) yield a triazido anion, [Pd (N3)3]" [167]. Similarly, a tetraazido anion, [Pd (N3)4] , is obtained from K2 [PdCU] by ligand exchange [162]. Another all-azido anion is the azide-bridged, binuclear complex [Pd2(N3)6] [168]. All of these have been isolated as salts of large organic cations. 

The normal cadmium azide, Cd(N3)2, is a white, crystalline solid which is hygroscopic and tends to hydrolyze. Thus, the salt turns yellow when exposed to atmospheric moisture. The azide dissolves in water, probably as an aquo complex. Upon standing, the solution turns yellow and slowly precipitates basic products [62,215,216]. Excess azide ion in the solution leads to the formation of various azido complexes with a maximal ligand number of 5 [217,218] excess pyridine forms a diazidodipyridine complex [62,188,219,220]. 

Copper(II) azide, an extremely sensitive explosive material, can be crystallized in the orthorhombic form by diffusing together solutions of CUSO4 and NaN3(5) in a solution of 1% hydrogen acetate in water [30]. Only small needle-shaped crystals have been grown by this technique. It should be mentioned that excess azide concentration in this solution must be avoided as the azide will form a soluble complex with copper ... 

In the kinetic resolution of 1-indanol with diphenylphosphoryl azide in dichloromethane (DCM) using modified guanidine [68], (7 )-excess azide compound was produced in 58% yield with 30% ee after six hours when a C2-symmetrical bicyclic guanidine 13 [37] was used as a chiral auxiliary (Scheme 4.26). 

Lewis acids can accelerate the Curtius rearrangement.The reactions studied were first order in Lewis acid when excess azide was used and first order in azide with excess Lewis acid. The Lewis acid was not consumed in the reaction. Various Lewis acids, including GaCla and AICI3 were effective. The proposed mode of catalysis was activation of the acyl azide by rapid and reversible pre-complexation with the Lewis acid, followed by rearrangement to the isocyanate with release of the catalyst (Figure 3). 

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