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Photochemistry of 1,N-Diiodoalkanes

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This article reviews the photochemistry of 1,n-diiodoalkanes, with special emphasis on the involved intermediates. Photolysis of 1,1-diiodomethane produces homolytic cleavage of the C–I bond to give the α-iodo radical, which in solid matrix cannot dissociate but collapses to isodiiodomethane. However, in solution subsequent electron-transfer produces the α-iodo cation, which is able to achieve cyclo-propanation of olefins. In the case of 1,2-diiodides, their photolysis gives rise to β-iodoalkyl radicals which usually lose iodine very rapidly, forming a carbon-carbon double bond. Homolytic cleavage of 1,n-diiodoalkanes (n>2) affords n-iodo radicals which are stabilized by participation of the remaining iodine atom. The cyclic hypervalent iodine radicals thus generated have low reactivity towards oxygen and characteristic UV spectra whose λmax depends on the ring size. These intermediates can be photolyzed, with iodine extrusion and formation of 1,2-diphenylcycloalkanes. On the other hand, photoinduced electron-transfer between carbanionic nucleophiles and 1,n-diiodoalkanes produces n-iodo radicals, which couple with the nucleophile leading to radical anions. Intramolecular electron-transfer from the radical anion to the σ* MO of the remaining C–I bond may compete with intermolecular electron-transfer to the substrate. This type of processes have been used for the synthesis of mono- and disubstituted adamantane derivatives. Finally, photoinduced electron-transfer between aromatic amines and 1,n-diiodoperfluoroalkanes (n>2) allows the preparation of fluorine containing heteroaromatic compounds.