Abstraction iodine

Packer et al. (1981) found that y-irradiation reduces arenediazonium tetrafluoro-borates to aryl radicals. Packer and Taylor (1985) investigated the y-irradiation of 4-chlorobenzenediazonium tetrafluoroborate by a 60Co source in the presence of 1 alone or I- +13 . The major product in the presence of iodide was 4,4 -dichloroazo-benzene. With I- + 1 ", however, it was 4-chloroiodobenzene. Two other investigations of the reactivity of aryl radicals with iodine-containing species are important for the understanding of the chain process of iodo-de-diazoniation that starts after formation of the aryl radical. Kryger et al. (1977) showed that, in the thermal decomposition of phenylazotriphenylmethane, the rate of iodine abstraction from I2 is extremely fast (see also Ando, 1978, p. 341). Furthermore, evidence for the formation of the radical anion V2 was reported by Beckwith and Meijs (1987) and by Abey-wickrema and Beckwith (1987) (see Sec. 10.11). 

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 (7.63) shows an example of C—S bond formation [73,74]. In fact, the aryl radical formed by iodine abstraction by (TMS)3Si radical rearranged by substitution to the sulfur atom, with expulsion of the acyl radical and concomitant formation of dihydrobenzothiophene (60). This procedure... 

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... 

Perfluoroalkyl radicals were generated according to a procedure developed by this research group, involving iodine abstraction from perfluoroalkyl iodides by phenyl radical (Equation 14.13) this latter derives from thermal decomposition of benzoyl peroxide (Equation 14.14). 

Alkyl iodides have been widely used for selective alkylation of heteroaromatic bases. The method is based on rapid iodine abstraction by aryl radicals (obtained from benzoyl peroxide or diazonium salts) or by a methyl radical (obtained from MeCOOH, t-BuOH, t-BuOOH, (t-BuO)2, (MeCOO)2, MeS0Me/H202, or MeC0Me/H202) [2]. An example is depicted in Eq. (14) of Table 3. 

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With iodoepoxides as starting materials, interesting ring enlargement reactions have been reported. After iodine abstraction by a tributyltin radical and epoxide... 

Various functionalized derivatives can be cyclized, including a-iodoesters, a-iodoketones, and a-iodomalonates. These substituents make the radical more electrophilic and reactive toward the alkene and also facilitate the iodine abstraction step. 

Crich and co-worker report an attractive solution to this problem by a combination of efficient iodine abstraction by tributyltin or tris(trimethylsilyl)silyl radicals from an aryl iodide to form an aryl radical and successive intramolecular homolytic attack at sulfur by the aryl radicals to generate acyl radicals (Scheme 8) [60]. 

J0C2456> Radicals are generated by the Fe(II)-H20j-DMSO system which involves iodine abstraction by methyl radicals. <89J0C5224>... 

Benzylic and allylic iodides may readily be coupled in excellent yields under mild conditions via air-induced iodine abstraction using triethylborane as reagent. Mixed coupling. Air passed 45 min. into a mixture of 1 4 benzyl and allyl iodides, and triethylborane in tetrahydrofuran at a rate of 50 ml/min so that the oxygen can be consumed completely by the triethylborane present 4-phenyl-l-butene. Y 72% by GLC. - An excess of the less expensive iodide may be used to increase the conversion to the more valuable material, since the coupling gives a nearly statistical distribution of products. F. e., also prepn. of iodides from boranes, s. A. Suzuki, H. C. Brown et al.. Am. Soc. 95, 1508 (1971). 

FIGURE 11.5 Synthesis of semifluorinated alkanes by radical addition of perfluoroalkyl iodides to olefins, derived from Reference 51 (a) initiation (b) iodine abstraction from Rj (c) addition of Rf radical to the double bond (d) chain propagation. 

If the subsequent reactions involve the regeneration of trifluoromethyl radicals by iodine abstraction, e. g- in the reactions with ethylene ... 

The partially fluorinated radicals have been prepared by photolysis of the fluoroalkyl iodides (16, 17) and by the photolysis of the appropriate fluoroalkyl ketone (e. g. CH FCOCH F) (18). Fluorom ethyl and difluorom ethyl radicals have also been formed from the fluoroalkyl iodides using di-Jt-butyl peroxide as initiator (16, 17). Problems can arise in kinetic studies with these radicals because the chain carrying step may involve hydrogen as well as iodine abstraction. In practice this only seems to be of major importance when electronically excited species are present. 

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