Diaryliodonium salt photolysis electron-transfer

Wu et al. in 1988 [43], discussed the synthesis of triarylsulfonium salts by the photolysis of a diaryliodonium salt in the presence of diphenyl sulfide. They explained the reaction using an electron transfer mechanism (see below). 

SCHEME 12.7 Photolysis of diaryliodonium salts—electron transfer. 

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide. However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts. 

The electron transfer mechanism best explains the phenomenon of onium salt photosensitization in most cases. This conclusion is supported hy the following observations. Analysis of the products formed by the photosensitized photolysis of diaryliodonium and triarylsulfonium salts are those which are predicted on the basis of the above mechanism. Moreover, these products are identical to those formed by the electrolytic reduction of these same onium salts which give rise to the same diaryliodide and triarylsulfide radical intermediates. 

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