Aryl azides, reduction

There now are available a number of alkyl azide compounds that may be used in click chemistry reactions and the Staudinger ligation processes. It is not recommended, however, to use aryl azide compounds, as these are light sensitive and photoreactive as well as highly susceptible to reduction in the presence of thiols. Unfortunately, at the time of this writing there are fewer choices in aryl phosphine compounds to participate in this reaction, as commercial sources of labeling reagents are limited. 

Aryl azides are converted into the corresponding anilines by polymer-supported borohydride [33]. Simple aliphatic azides are not reduced under similar conditions and the reduction of benzyl azides is slow. 

By the late 1980s it was clear that a significant number of thermal and photochemical reactions of arylhydroxylamines and their derivatives, N-chloroanilines, aryl azides, anthranilium salts, and other compounds could be explained in terms of nitrenium ions or transition states that resembled nitrenium ions. Since no monoarylnitrenium ion had been directly observed, and data on the lifetimes and quantitative reactivity/selectivity of these species were not available, it was not possible to assess whether the reactions that had been observed were due to free ions, or ion pairs, or preassociation processes. In many cases Sn2 reactions could not be ruled out because appropriate kinetics experiments had not been performed. Most authors had attributed the presence of reduction products in thermal and photochemical reactions to triplet ions, but calculations suggested that the triplet species may not be accessible in thermal processes. It was clear that singlet ions could be reduced under certain conditions, so the presence of the... 

The photochemistry of aryl azides is quite complex, suggesting that the nitrene 14 may not be the only reactive intermediate and that insertion reactions may not be the only route to form photoconjugates.Although aryl nitrenes are much less susceptible to rearrangements than acyl nitrenes, they may still occur and lead to the formation of reactive intermediates such as azepines, which may go on to react with nucleophiles.[911 141 Addition of nitrenes to double bonds will generate azirines, while dimerization will produce azobenzenesJ11 Aryl azides are stable to most of the procedures used in the course of peptide synthesis except for reduction reactions. Non-photochemical reduction of aryl azides to the primary amines by thiols has been reported by Staros et al.[15]... 

It was concluded that dithiols should not be used in photoaffinity labeling experiments with aryl azides and monothiols should be used with caution. The reduction reaction can often be monitored spectrophotome-trically, e.g. 2-nitro-4-azidoanilines which are red-orange (Xmax 480 nm in H20) are converted to purple diamines (Xmax 550 nm). TLC and IR spectroscopy are also useful (Staros et al., 1978). 

Like aryl azides, diazo compounds are susceptible to reduction by thiols and a study of the diazotrifluoropropionyl group has been made by Taka-gaki et al. (1980) who showed that the corresponding hydrazone is formed (Fig. 3.6). For example, one reagent 20 pM) was converted to the extent of 24% in 3 h at room temperature by 30 mM cysteine, at pH 8.0. 

Reaction of aryl azides with hydrazine hydrate can result in reductive cleavage of the azido group. Azidobenzene gives a mixture of benzene and aniline when heated with an excess of hydrazine hydrate in ethanol. If there is an acyl or an alkoxycarbonyl group at the ortho position, cyclization products are formed instead for example, methyl 2-azidobenzoate gave indazolinone (13 90%). 

Rates of reduction for the series of aryl azides studied are summarized in Table 3.8. It is evident that, for 3- and 4-substituents, the expected pattern of reactivity was observed, with electron-withdrawing substituents (Br, Cl and NO2) resulting in a much faster rate of reduction than for unsubstituted phenyl azide, which was in turn reduced more rapidly than those aryl azides bearing an electron-donating methoxy substituent. In order to obtain some quantitative measure of the contribution of these substituents to reaction rates, the results were modelled using the Hammett relationship [114], and the linear nature of the plot of log k against Hammett constant ([Pg.157]

Interestingly, our previously described studies concerning the in vitro biotransformation of aryl azides in murine microsomes [93] generally parallel the above results observed for the chemical reduction of azides by thiols, with electron-deficient aryl azides also undergoing metabolic reduction much more rapidly than their electron-rich counterparts or alkyl azides. These similarities have led us to speculate that a possible mechanism for the enzyme-catalyzed reduction of aryl azides may entail analogous initial attack by a sulphydryl group at the active site of the appropriate enzyme, although this hypothesis remains to be confirmed. 

Samarium. 14,275 17,301 Reductions. In the re included. For example, iodii aryl azides, and sodium alk Titanium(rV) chloride an e.g., in the transformation of amines, respectively. 

Reductions. In the reduction of organic compounds by Sm some additives are often included. For example, iodine is present in the system for the reduction of nitroarenes and aryl azides, and sodium alkyl thiosulfates.- ... 

Other examples of photo-induced transformations in monolayers include reduction of p-nitrothiophenol assembled on a silver electrode to p-aminothiophenol initiated by laser radiation (monitored by SERS)350. UV irradiation of a monolayer terminated with aryl azide in the presence of amines results in incorporation of the amines in the monolayer as azepines and hydrazines. No surface attachment takes place without irradiation, and this makes it a convenient method to prepare photopattemed monolayers351. 

Reduction Aryl azides are reduced to arylamines by NaI-BF3 OEt2 in MeCN at... 

Reductions. Nitrones and N-oxides are deoxygenated by (BnNEtjljMoS. Acyl azides give amides. Alkyl azides undergo a homocoupling reaction to form imines, whereas stabilized azides, such as acyl, sulfonyl, and aryl azides, undergo reductive elimination of... 

Reductions. VCI2 (also T1CI3) reduces benzils to benzoins (THF-H2O, 80-907o yield). Quinones are also reduced to hydroquinones (90-95 yield). Aryl azides are cleaved by VCI2 to arylamines and nitrogen (70-95% yield). ... 

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