Radical reactions iodination

Interniolecular hydrogen abstraction, 239 Intramolecular free radical reactions, 238 Intramolecular hydrogen abstraction, 239 Iodine azide, 24... 

Radical reactions of Hoffmann-Loffler-Freytag type with participation of iodine(III) compounds leading to five-member N-heterocycles 97YGK90. 

On the basis of all these results and his own investigations on chloro- and bromo-de-diazoniations (Galli, 1981), Galli proposed in 1988 that iodo-de-diazoniation, after formation of the aryl radical in the initiation reaction (Scheme 10-22) follows three coupled iodination chain reactions based on the formation of the I2 molecule and the If anion in the step shown in Scheme 10-23, namely iodine atom (I ) addition (Scheme 10-24), and iodine abstraction from I2 and If in Schemes 10-25 and 10-26 respectively. Aryl radicals and iodine molecules are regenerated as indicated in Scheme 10-27. The addition of iodide ion to aryl radicals forming the radical anion [Arl] -, as in Scheme 10-28, is considered an unlikely pathway, as that reaction has been found to be reversible (Lawless and Hawley, 1969 Andrieux et al. 1979). 

The cleavage of phenylmercuric iodide by iodine in the presence of excess iodide ion (to suppress free-radical reactions) at 25 °C in aqueous dioxan was reported to be first-order in both aromatic and tri-iodide ion, and faster than the reaction of alkylmercuric iodides724. A further study, together with bromodemercuration, both reactions being generally represented by... 

The results were found not to be very sensitive to the presence of scavengers, such as atmospheric oxygen, iodine and FeClj, although the effect was sufficient to indicate clearly the involvement of radical reactions... 

Other than the iodine compound, the hydrogen halides can be obtained by reactions of the elements, but the reactions can be explosive. A mixture of H2 and Cl2 will explode if the reaction is initiated with a burst of light, which separates some Cl2 molecules to produce C1-. The reaction between H2 and Cl2 is a classic case of a free radical reaction. A vast number of hydrolysis reactions yield hydrogen halides. 

Marcus, R. A and Rice, O. K., The kinetics of the recombination of methyl radicals and iodine atoms.. /. Phys. Chem 55, 894 (1951). Marcus, R. A., Unimolecular dissociations and free radical recombination reactions. J. Chem. Phys. 20, 359 (1952). 

In Quest of Friendly Free Radicals Reactions of Iodine Atom Free Radicals with Some Biologically Important Compounds... 

In an extension of atom-transfer radical reactions to heterocyclic systems, Byers has introduced a novel methodology for the addition of electron-deficient radicals to unprotected pyrroles and indoles in a stannane-fi ee, non-oxidative process <99TL2677>. For exanqrle, photochemical reaction of pyrrole (33) with etl l iodoacetate (34) in presence of thiosulfiite as an iodine reductant, phase transfer catalyst and propylene oxide led to high yields of the 2-alkylated pyrrole 35 <99TL2677>. 

These reactions are related to the reaction of aryl diazonium salts with iodide yielding iodoaryls, the mechanism of which seems to be a one-electron transfer (radical) reaction and not a nucleophilic displacement. Just as iodide is easily oxi- zed to iodine by the aryl diazonium cation, 2.4.6-triphenyl-X -phosphorin is oxidized to the radical cation 58. 

A similar three-component transformation can be achieved using triethylborane-induced radical reactions (Scheme 6.34) [53]. On exposure to air, triethylborane generates the ethyl radical, which abstracts iodine from alkyl iodides to generate the t-butyl radical. Addition of the resulting t-butyl radical to methyl vinyl ketone produces a radical a to the carbonyl group, which is trapped by triethylborane to form a boron enolate with the liberation of ethyl radical, thus creating a chain. 

When molecular bromine or molecular iodine is used instead of molecular chlorine in this reaction, the chain reaction does not proceed effectively. The bond dissociation energies of Br-Br and I-I are 46 and 36 kcal/mol in the starting materials, and those of CH3-Br, CH3-I, H-Br, and H-I in the products are 70, 56, 88, and 71 kcal/mol, respectively. Thus, the difference in the bond dissociation energies between the starting materials and the products in these reactions tends to be small. Especially, iodination does not proceed at all. Therefore, the considerable difference in bond dissociation energies between the starting materials and the products is the driving force of radical reactions. 

This work was useful in establishing that the low quantum yield in the photolysis of pure methyl iodide was due to back reactions and that the initial decomposition into a methyl radical and iodine atom occurred with approximately 100% efficiency. In 1942, Blaedel et al.22 showed that the quantum yield for the decomposition of methyl iodide was, in fact, somewhat less than unity in the presence of oxygen because Bates and Spence11 had used an incorrect value for the quantum yield of the chloracetic acid actinometer. They suggested the reactions... 

Homolytic cleavage of a tin carbon bond in a tetraorgan-otin compound is less frequently encountered than is heterolytic cleavage. Such radicals as Bn, , and the suc-cinimidyl radical can take part in Sn2 displacement with tetraorganotins (Scheme 8). The reactivity of ITiSn in MezCO towards the succinimidyl radical is in the sequence of R = Me > Et > Pr > Bn this is the reverse of that for radical reactions of iodine and bromine. 

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