Iodine atom abstraction reactions, with

In contrast to the direct dynamics displayed in the hydrogen abstraction reactions of F with halomethanes, the iodine atom abstraction reactions... 

Pig. The femtosecond dynamics of the iodine atom abstraction reaction. The corresponding structures are shown with emphasis on three molecular photographs att0, tc and tt as the reaction proceeds to completion. The calculated well depth and barrier height are also noted. 

Triethylborane in combination with oxygen provides an efficient and useful system for iodine atom abstraction from alkyl iodide, and thus is a good initiator for iodine atom transfer reactions [13,33,34]. Indeed, the ethyl radical, issued from the reaction of triethylborane with molecular oxygen, can abstract an iodine atom from the radical precursor to produce a radical R that enters into the chain process (Scheme 13). The iodine exchange is fast and efficient when R is more stable than the ethyl radical. 

Remarkably, most reactions do not take place at the unsaturated part of the molecule, presumably because of the steric shielding of the Si=Si double bond by the bulky t-Bu3Si groups, but rather proceed through substitution of the iodine atoms. The reaction of 150 with water affords the product 160, whereas that with methanol furnishes compound 158 with retention of the tetrasilacyclobutene skeleton. On treatment of 150 with NaR the tetrahedro-tetrasilane 149 — the starting material for the preparation of 150 — is reformed. Finally, the reaction with BI3 proceeds with abstraction of an iodide ion to form the ionic compound 159140. 

Based on the results of deuterium labelling and radical trapping experiments with dicyclohexylphosphine (DCHP), it was proposed that the reaction occurs by the S l mechanism. A major difference, however, is an additional propagation step involving iodine atom abstraction from 42b by the radical intermediate 49, to give the substitution product 46 and radical intermediate 50, that continues the chain propagation cycle of the S l mechanism (equation 51). 

For the endothermic reaction of an iodine atom with methane. Fact can be no less than 33 kcal, and is probably somewhat larger. Even for this minimum value of 33 kcal, an iodine atom must collide with an enormous number of methane molecules (lO or a million million at 275°) before reaction is likely to occur. Virtually no iodine atoms last this long, but instead recombine to form iodine molecules the reaction therefore proceeds at a negligible rate. Iodine atoms are easy to form it is their inability to abstract hydrogen from methane that prevents iodination from occurring. 

Suginome and coworkers have described the photochemical (2+2)-cycload-dition of alkenes to 2-acetoxynaphtho-l,4-quinone. The resultant adducts can be converted into the corresponding cyclobutanols which react with mer-cury(II) oxide/iodine to afford a cyclobutanoxyl radical. A laser-flash study has examined the photochemical behaviour of vitamin K3. This investigation sought to provide details for the hydrogen atom abstraction reactions in this system. ... 

Triethylborane in combination with oxygen provides an efficient and useful system for iodine atom abstraction from alkyl iodides and therefore is a good initiator for iodine atom transfer reactions.6 Indeed, the ethyl radical, issuing... 

Bicyclic pentenes are used as trapping agents in addition reactions of 3-butynyl radicals81. Addition occurs exclusively trans to the annulated cyclopentyl ring, while the regioselectivity is low when polar substituents are not present. Under thermal conditions, the reaction finishes after iodine atom abstraction to yield the overall trans-addition product. With photochemical initiation and in the presence of hexabutylditin, further cyclization and iodine atom abstraction occurs to form exocyclic vinyl iodides. 

The Fan group and Nicholas group independently propose the radical mechanism in the amination reaction they developed. While the source of the iodine-centered radical differs, the mechanistic concept is the same. An N-iodo species can homolytically cleave to a nitrogen-centered radical. Hydrogen atom abstraction from the benzylic C—H bond and iodine atom abstraction from the A-iodo species form a benzylic iodide. Substitution of the iodide with the amine yields the product. 

Syntheses of 5-halogenotetrazoles from metallic derivatives have met with mixed fortunes. Lithiation of 1-methyltetrazole followed by reaction at -60°C with bromine, iodine, or cyanogen bromide gave the 5-bromo and 5-iodo compounds in 36-55% yields (71CJC2139). 1,2-Disubstituted tetrazolium tetraphenylborates were lithiated in the 5-position, but subsequent reaction with chlorine or bromine failed to trap the anion. Instead, oxidation produced a radical cation, which abstracted a hydrogen atom from the solvent [91 AG(E)1162]. 

Treatment of a-iodo lactone (45) with triethylborane under oxygen atmosphere gives the corresponding a-hydroxy lactone (46), via a-lactone radical species. This reaction comprises of SH2 reaction by Ef on the iodine atom of a-iodo lactones, reaction of the formed a-lactone radical with molecular oxygen, and subsequent hydrogen-atom abstraction from the solvent to form alkyl hydroperoxide (ROOH). Finally, by the addition of dimethyl sulfide for the reduction of the peroxide, the corresponding a-hydroxy lactone is obtained (eq. 2.24) [58]. 

This reaction comprises firstly of SH2 reaction on the iodine atom of ethyl iodoacetate by an ethyl radical, formed from triethylborane and molecular oxygen, to form a more stable Chester radical and ethyl iodide. Electrophilic addition of the a-ester radical to electron-rich aromatics (36) forms an adduct radical, and finally abstraction of a hydrogen atom from the adduct by the ethyl radical or oxidation by molecular oxygen generates ethyl arylacetate (37), as shown in eq. 5.20. Here, a nucleophilic ethyl radical does not react with electron-rich aromatics (36), while only an electrophilic a-ester radical reacts with electron-rich aromatics via SOMO-HOMO interaction. 

A regioselective iodoperfluoroalkylation of terminal alkynes (R—C = CH) has been reported, and is based on photolysis ofthe C—I bond in perfluoroalkyl iodides (Rp-I). Addition of the thus-formed RF" radical onto the alkyne afforded a vinyl radical that in turn abstracts an iodine atom from the starting Rp—I to form the end olefin R-C(I)= CH-Rf. A xenon lamp through Pyrex (hv > 300 nm) was used for the reaction, where aliphatic alkynes gave a better alkylation yield with respect to phenylacetylene [81],... 

Of the two chain-propagating steps, then, step (2) is more difficult than step (3) (see Fig. 2.8). Once formed, methyl radicals react easily with any of the halogens it is how fast methyl radicals are formed that limits the rate of overall reaction. Fluorination is fast because fluorine atoms rapidly abstract hydrogen atoms from methane E cx. is only 1 kcal. lodination does not take place because iodine atoms. And it virtually impossible to abstract hydrogen from methane tLCt is more than 33 kcal. 

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