Hetero Azides

To satisfy the bond-stabilization concept, it appears essential to reclassify the inorganic azides. Hence, five classes are identified here and are listed in Figure 1 as normal azides, mixed azides, hetero azides, azido complexes, and metalorganic azides. 

The moisture-sensitive compounds are made from sodium azide and the respective chlorides at moderate or low temperatures and are obtained as oily liquids of extreme sensitivity. Examples are, triphosphonitrile azide (a), cyanogen azide (b), and chromyl azide (c). Saltlike compounds with composite cations, such as ammonium and hydrazine azides, are not considered hetero azides, but normal azides. 

Several mixed and hetero azides of vanadium(V) as well as azido complexes of vanadium(II) and (V) have been described. The crystalline vanadyl azide trichloride, VO(N3)Cl3, is made from VOCI3 and chlorine azide as described under the titanium azide. The moisture-sensitive compound deflagrates on thermal shock it is soluble in VOCI3. The brown, solid vanadium azide tetrachloride, V(N3)Cl4, was made accordingly from solid VCI4 and chlorine azide in CCI4 media. In the dry state at room temperature it is stable for several days, but decomposes with water and, less rapidly, with organic solvents. The compound is very sensitive to friction and impact and also explodes on thermal shock [141]. 

Chromium(III) forms the normal azide, a number of basic azides and azido complexes, and a chromium(VI) hetero azide. All these compounds reflect the properties of their respective classes (see p. 21). 

Chromyl azide, Cr02(N3)2, has the properties of a typical hetero azide. The deep violet solid is Ught and moisture sensitive and decomposes above -100°C. Above -60°C the compound tends to explode spontaneously, but solutions in inert solvents, such as carbon tetrachloride or freon, are stable up to -50°C [58,146]. It was made according to... 

Ionic character of the azide bond (%) Normal azide Mixed azide Hetero azide Azido complex Metal- organic azide... 

ON—N3 is a hetero azide in which four nitrogens are linked together with covalent bonds. The yellow liquid (MP, -66°C) decomposes thermally above -50°C. It was obtained in low yield (6%) from NOCl and sodium azide in vacuo at low temperature frequent explosions occurred during the experiments [314]. 

Sulfonium ylides may be added to C N double bonds to yield aziridines in a formal [1 -t-2]-cycloaddition. Alkyl azides are decomposed upon heating or irradiating to yield ni-trenes, which may also undergo [ 1 + 2 -cycloaddition reactions to yield highly strained hetero-cycles (A.G. Hortmann, 1972). 

Some 1,3-dipolar cycloadditions to hetero ft bond systems have been reported, including a couple of examples of additions of azides to the activated nitrile function of tnfluoroacetonitrile [30, 3I (equation 27)... 

Although beyond the scope of the present discussion, another key realization that has shaped the definition of click chemistry in recent years was that while olefins, through their selective oxidative functionalization, provide convenient access to reactive modules, the assembly of these energetic blocks into the final structures is best achieved through cydoaddition reactions involving carbon-het-eroatom bond formation, such as [l,3]-dipolar cydoadditions and hetero-Diels-Al-der reactions. The copper(i)-catalyzed cydoaddition of azides and terminal alkynes [5] is arguably the most powerful and reliable way to date to stitch a broad variety... 

The three-component coupling of arylacetaldehydes, secondary amines, and (hetero)aryl azides in refluxing benzene gave triazolines which underwent thermal reanangement to quinolines and [l,7]naphthyridines <1996J(P1)1359>. 

Tridentate salen ligands (10) derived from 1 have given excellent results in the enantiocontrol of the hetero Diels-Alder addition reaction of dienes with aldehydes (eq 7) and in the asymmetric additions of TMS-azide to mc5o-epoxide and trimethylsilyl cyanide to benzaldehyde (up to 85% ee). Phosphino-oxazolines derived from 1 have been employed for the asymmetric control of palladium-catalyzed allylic substitution reactions products of 70-90% ee were obtained. Photolysis of crystalline adducts of enantiomerically pure 1 with prochiral alcohols results in asymmetric inductions of up to 79% in a rare example of a solid-state enantioselective reaction. ... 

Related methods can be used to prepare 5-hetero-substituted compounds isonitriles with A-halo-succinimides and azide give halo derivatives, aryl isothiocyanates with azide give the 5-thiols, ° which can be converted into 5-unsubstituted tetrazoles by oxidation with hydrogen peroxide or chromium trioxide. ... 

The number of five-membered heterocycles with four hetero atoms is limited to those with nitrogen atoms. Thus, the tetrazole acyclonucleo-sides were formed from sugars N,N -diphenylformazanes by the action of AT-bromosuccinimide or lead tetraacetate [133,134]. The synthesis of the tetra-O-benzoyl-D-lyxononitrile and its conversion to (o-lyxo-tetritol-l-yl)-tetrazole derivatives upon reaction with sodium azide was reported [133, 134]. A pseudo C-nucleoside including a tetrazole ring was achieved by conversion of 3- 0-benzyl- 1,2-O-isopropylidene-D-ribo-pentodialdehydo-1,4-furanose to the respective nitrile and then reaction with sodiiun azide [86]. 

The other report regarding a base-catalyzed hetero-Michael reaction was a contribution from the group of Jorgensen regarding the conjugate addition of azide to nitroalkenes (Scheme 6.17). In this case, the authors carried out an intensive effort directed toward the identification of the best catalyst and reaction conditions which would provide high yields and enantioselectivities. However, while the reaction was found to proceed with a good efficiency level... 

Dipoles always contain a hetero atom as the central atom of the trio, either sp or sp hybridised. Amongst other examples, cycloadditions have been demonstrated with azides (N=N -N -R), nitrile oxides (R-C=N -0 ) and nitrile ylides (R-C=N -C R2) where the central atom is sp hybridised nitrogen, and with nitrones (R2C=N (R)-0 ), carbonyl ylides... 

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