Iodine atoms, abstraction reactions

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. 

Cyclizations involving iodine-atom transfers have been developed. Among the most effective examples are reactions involving the cyclization of 6-iodohexene derivatives. The 6-hexenyl radical generated by iodine-atom abstraction rapidly cyclizes to a cyclo-pentylmethyl radical. The chain is propagated by iodine-atom transfer. 

In this reaction, the trialkylstannane serves to initiate the chain sequence but it is present in low concentration to minimize the rate of hydrogen atom abstraction from the stannane. Under these conditions, the chain is propagated by iodine atom abstraction. 

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. 

The fact that the cyclization is directed to an acetylenic group and leads to formation of an alkenyl radical is significant. Formation of a saturated iodide would be expected to lead to a more complex product mixture because the cyclized product could undergo iodine abstraction and proceed to add to a second unsaturated center. Vinyl iodides are much less reactive, and the reaction product is stable to iodine-atom abstraction. Because of the potential for competition from reduction by the stannane, other reaction conditions have been developed to promote cyclization. Hexabutylditin is frequently used.238... 

Since the Barton reaction and the Hofmann-Lofifler-Freytag reaction generate very reactive oxygen-centered and nitrogen-centered radicals respectively, the next 1,5- and 1,6-hydrogen atom abstraction reaction readily happens. However, 1,5-H shift does not proceed effectively by carbon-centered radicals, because there is not so much energy difference between the C-H bond before and after 1,5-H shift. So the reactions are quite limited. Eq. 6.21 shows iodine transfer from reactive 1-iodoheptyl phenyl sulfone (40) to a mixture of 5-iodoheptyl phenyl sulfone (41a) and 6-iodoheptyl phenyl sulfone (41b) initiated by benzoyl peroxide, through 1,5-H shift by an sp3 carbon-centered radical [56-58]. 

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). 

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. 

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). 

Reaction conditions have been developed in which the cyclized radical can react in some manner other than hydrogen atom abstraction. One such reaction is an iodine atom transfer. The cyclization of 2-iodo-2-methyl-6-heptyne is a structurally simple example. 

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