Mechanism phenyl azide formation

Qf-receptor blocking agent, 1, 176 Phenylalanine hydroxylase in tyrosine synthesis from phenylalanine, 1, 261 L-Phenylalanine hydroxylase mechanism, 1, 261 Phenyl azide formation... 

Quantitative formation of 118-phenyldihydrotriazole (II) by heating Compound 118 with an excess of phenyl azide liquid, in absence of solvent, and then removing unreacted azide with a mechanical vacuum pump. (Because of the small amounts of 118 and phenyl azide reacting, the presence of conveniently measured amounts of solvent makes the rate of dihydrotriazole formation too slow to be practicable.)... 

The thermal cycloaddition of azides to acetylenes is the most versatile route to 1,2,3-triazoles, because of the wide range of substituents that can be incorporated into the acetylene and azide components. The accepted mechanism for the reaction is a concerted 1,3-dipolar cycloaddition. The rates of addition of phenyl azide to several acetylenes have been measured the rates of formation of the aromatic triazoles are not appreciably different from the rates of cycloaddition to the corresponding olefins, indicating that the transition-state energy is not lowered significantly by the incipient generation of an aromatic system. 

On the basis of their observations, the authors drew up the following reaction mechanism for the formation of phenyl azide ... 

The parent phenylnitrene has been studied in detail.406 The thermal or photochemical decomposition of phenyl azide and most of its derivatives in solution results in the formation of intractable polymeric tars. Meaningful mechanistic studies became possible only when it was found that amines intercept the intermediate formed by thermal409 or photochemical410 decomposition of phenyl azide in solution that was responsible for polymerization. The current knowledge about the mechanism of phenyl azide photochemistry is summarized in Scheme 5.7. 

Due to its short lifetime, the singlet phenylnitrene 52 has never been trapped chemically, the only exception being protonation with the formation of phenyl nitfenium ion 54.200-202 jj. predicted theoretically, that 54 has a singlet ground state which is favored by 21.2kcal/mol over the lowest triplet state.20" Protonation of 52 competes with its isomerization to 51 only at low pH < I.200 McClelland and co-workers performed product analysis and LFP stndies of the photolysis of phenyl azide in water at pH = 0-2 and proposed a mechanism of reactions of nitrenium ion 54 (Scheme 11.29).2 2o> The nitreninm ion 54 reacts with water or other nucleophiles yielding substituted anilines.2 2 Note, that lifetime of 52 in water (25-50 ps) was estimated to be much shorter than in hydrocarbons ( lns). 

Since vinyl azides like 34 are electron-rich olefins, [2 + 2] cycloaddition with electron-deficient alkenes such as diphenylketene could lead to azidocyclobutanes. " The stability of the cycloadducts 211, prepared from 34 or 52 and tetracyanoethene (TCNE), allowed characterization in solution but not isolation of these products because rapid ring-expansion regioselectively afforded the dihydropyrroles 212 already at room temperature (Scheme 5.25). "" A similar mechanism via [2 + 2] cycloaddition and quick ring-enlargement may perhaps explain the formation of 213 from 52 and 4-phenyl-l,2,4-triazole-3,5-dione (PTAD). In this " " and other " " cases, however, different interpretations were offered. The 2-azidobuta-l,3-dienes 92a,b underwent [4 + 2] cycloaddition in the... 

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