Azidyl radicals

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 ... 

Hypotaurine has been oxidised to taurine (2-aminoethanesulfonic acid) by 02( Ag) which has been photochemically produced using methylene blue as sensitizer. In the presence of azide, a well known quencher of 02( Ag), rather than the expected inhibition, an activating effect is observed, and this has been attributed to the generation of the azidyl radical. It is supposed that this radical participates in a strong one-electron abstraction process with the hypotaurine resulting in its oxidation. 

Other classes of radicals have also been examined with TRIR methods. The rate constant for P-scission of alkoxy radicals (Scheme 2.6) was found to be enhanced in polar solvents by monitoring the rate of production of the product carbonyl compound [74]. The decarboxlyation of peroxyesters [75], the reactivity of azidyl radicals [76], and chlorine atom abstraction reactions [77] have also been investigated. 

An intramolecular charge transfer excited state may be involved, and the inverse relationship between fluorescence and photoisomerisation suggests a singlet state mechanism. Irradiation of (Z)-urocanic acid, 3-(lH-imidazoI-4-yl)prop-2-enoic acid, in the presence of nitro blue tetrazolium and sodium azide promotes its photoisomerisation in a process which involves reversible addition of the azidyl radical to the double bond. ... 

Two Pt azido complexes, czs -Pt (PPh3)2(N3)2 and Pt (dppp)(N3)2 (dppp = 1,3-bis(diphenyl-phosphino)propane) have been studied with a combination of flash photolysis and ps-TRIR. " UV excitation was found to cause photoinduced electron transfer and loss of an azidyl radical ( N3) to form Pt (PPh3)2(N3) and Pt (dppp)(N3), identified by their IR azido absorptions at ca. 2050 cm For the PPh3 complex, photoisomerization to tra w-Pt (PPh3)2(N3)2 was a competitive pathway. Both Pt complexes were found to decay via intramolecular electron transfer to the coordinatively unsaturated products, Pt°(PPh3)2 and Pt°(dppp). 

Azide reacts with singlet oxygen with high rate constant (k = 2.2 x 10 Wilkinson and Brummer 1981), producing the strong one-electron oxidant azidyl radical, Nj (eq. [36]). 

However, under suitable conditions, azides react with a variety of radicals and this is the basis of several useful synthetic procedures for the formation of carbon-nitrogen bonds. For instance, synthesis of azides by radical addition of an azidyl radical to alkenes (Scheme 8.3a) and by reaction of an alkyl radical with an azidating reagent (Scheme 8.3b) will be presented. The reduction of azides leading to aminyl radicals (Scheme 8.3c) and the addition of alkyl radicals to alkyl azides (Scheme 8.3d) will also be discussed. 

The use of the azidyl radical in organic synthesis offers the opportunity to functionalize olefins into the corresponding alkylazides, which are equivalent forms of the ubiquitous amino group. The reaction with olefins is especially interesting due to the electrophilic character of the azidyl radical. ... 

A reaction mechanism involving the formation of the azidyl radical from an iron(III) azide was proposed (Scheme 8.5). Only catalytic amounts of iron(II) salts are actually needed to sustain the radical chain. Moreover, other types of functionalization are possible depending on the iron(III) salt used. 

A practical procedure for the generation of the azidyl radical using cerium ammonium nitrate (CAN) was developed somewhat later." This method is taking advantage of the... 

Hypervalent iodine compounds represent a nsefiil source of azidyl radicals. For instance, a mixture of iodosyl benzene (PhIO), acetic acid, and sodium azide is employed for the diazidation of several olefins. A carbocationic mechanism has been proposed bnt azidyl radicals are likely responsible for the observed reactivity. ... 

Proof of the involvement of azidyl radicals in this process was obtained by using a silyl enol ether with a fused cyclopropyl ring (Scheme 8.11). The rearranged product isolated in high yield clearly suggests the participation of radical intermediates. 

The relatively low standard potential of the Ns /Ni redox couple in aqueous solution (+1.33 0.01 V vs. NHE) is even more reduced in organic solvents, and therefore the generation of azidyl radicals by electrochemical methods is perfectly feasible. The multigram scale dimerization of styrene represents an early synthetic application of the electrochemical process (Scheme 8.15). " However, the scope of this reaction is so far limited since other substrates give poor yields and/or significant amounts of by-products. 

Owing to the low thermal stability of the reactive azidoiodinanes generated in situ (e.g., PhI(N3)OTMS or PhI(N3)2) and relatively weak hydrogen-abstracting ability of the azidyl radical, all of the azidation reactions depicted above are restricted to application of very reactive substrates. To overcome these drawbacks, Zhdankin and co-workers prepared a series of stable azidoiodinane reagents I, II,... 

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