Iodoform, photolysis

The photolysis of polyhalogenated compounds forms the basis for another monochrome system. Iodoform can undergo photolysis to produce hydrogen iodide, which subsequendy reacts with a di(2-furfuryl) derivative (3) and aromatic amines to produce a colored dye adduct (4) (29). The photolysis scheme and subsequent reactions can be shown by the following ... 

A quantitative determination of two-photon absorption cross-sections from direct measurements of chemical yield was performed by Speiser and Kimel i ) who studied the two-photon-induced photolysis of iodoform with a Q-switched ruby laser. 

The photolysis of iodoform presumably generates a free iodocarbene which reacts with an alkene in a stereospecific manner to form iodocyclopropanes" (see Table 3 and Houben-Weyl, Vol. E19b, pp 1623-1624). 

Singlet iodocarbene is probably generated in this reaction. To remove iodine, the process is carried out in the presence of aqueous sulfurous acid," aqueous sodium hydroxide, or sodium sulfite." The competitive reaction observed during photolysis of iodoform is the free radical cleavage of the C-I bond the free radical thus formed undergoes addition to an alkene. 

Table 3. lodocyclopropanes from Iodoform, Alkenes and Photolysis or Diethylzinc... 

An excellent alternative to the classical Hunsdiecker reaction and its variants, which totally avoids the use of heavy metal salts and potent electrophilic reagents, consists of the simple photolysis or thermolysis of Barton esters in refluxing bromotri-chloromethane for the bromides or tetrachloromethane for the chlorides [4], The analogous decarboxylative iodination can also be achieved using iodoform as the reagent in a benzene/cyclohexene solvent system (Scheme 5). For the cases of vinylic and aromatic acids, where the usual problems of chain efficiency are encountered, the addition of azobisisobutyronitrile (AIBN) is also required [10]. Nevertheless, since this method can operate on both electron-rich and electron-poor aromatic systems, and moreover does not suffer from the competitive electrophilic aromatic bromination found with electron rich aromatics under normal Hunsdiecker conditions, this route to synthetically useful aryl iodides and bromides should find widespread application. 

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