1.1- diiodo adduct with iodine

Iodination of terminal allenes has been reported to occur on the end carbon of the al-lenic system to give cis/trans mixtures of 1,2-diiodo adducts [64—66], Reaction of a nucleophile with these adducts proceeds solely with displacement of the allylic iodine [65], Iodi-... 

Carbon-carbon double bonds, whether attached directly to or are distant from the phosphorus, can undergo some unusual reactions, particularly when treated with electrophilic reagents such as the halogens or pseudo-halogens, a behaviour attributable to phosphoryl nucleophilic character. Ethenylphosphonic acid and its simple derivatives add chlorine or bromine with the expected overall results, but the normal addition of the latter is rapidly accelerated by UV radiation and inhibited by added iodine, suggestive of a homolytic nature Dialkyl (pent-4-enyl)phosphonates (373 R = RO, X = CH2, n = 2) equally add bromine to give the expected dibromo adducts, but their behaviour towards iodine is more complex (Scheme 49). In reactions in CHCI3 at ambient temperature, not only are the expected 4,5-diiodo adducts 374 formed, but so is an additional species, probably a quasi-... 

Because iodine is the best of the halogens at bridging, we might expect the iodonium ion to be the intermediate in the addition of iodine to alkenes. Indeed, an iodonium ion was isolated from the reaction of iodine with adamantylideneadamantane. The addition of iodine to alkenes is generally not synthetically useful, however, primarily because the formation of 1,2-diiodo compounds from I2 and alkenes is not as exothermic as are the additions of other halogens to alkenes. For example, the AH for addition of iodine to ethene is exothermic by only about 11 kcal/mol in the gas phase. Since AS is -31.5 eu, adduct stability decreases with increasing temperature. In one study it was found that a mixture of styrene and iodine produced styrene diiodide the product could be collected by filtration at 0°C, but it decomposed at room temperature to styrene and iodine. ... 

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