Hydroxyl radicals, aryl halides

Complete mineralization of alkyl [159, 160], vinyl [161], and aryl [162-164] halides, as shown in Eq. 25, for example, probably requires the generation of a photogenerated hydroxyl radical. These reactions figure prominently in current photocatalysis applications dealing with environmental purification. 

Decomposition of Aryl Halides by PhotochemicaUy Produced Hydroxyl Radicals.38-12... 

These reactions do not involve excitation of the aryl halide but are mentioned here because of interest in degrading chlorinated pollutants. Wastewater treatment involving hydroxyl radical production by photolysis of hydrogen peroxide or ozone is a well-known commercial process. Fe +, HjO, and UV have been used in combination (the photo-Fenton reaction) to enhance the degradation of Aroclor 1242, a mixture of PCBs. The conventional Fenton reaction produces hydroxyl radicals from and Fe + ... 

In addition to the carbon-centered radical initiators, arenesulfonyl halides are simple and efficient functionalized initiators by introducing a variety of functional termini conjugation between the aryl and the sulfonyl groups is absent (FI-51 to FI-58 Figure 14).175 Typically, a series of monosubstituted derivatives (FI-51) are commercially available and afford hydroxyl, carboxyl, nitro, and halo groups in living... 

If the reaction is run in aqueous solutions, OH from the water can replace the halide as the aryl substituent, thus forming phenols 129). The reaction of the 2-benzoylphenyl radical to form the corresponding phenol is slower than the reaction forming the halo-aryl. The hydroxylation rate constant equals... 

The Sandmeyer reaction is not only useful for the preparation of arylbromides. The Boger group used a Sandmeyer reaction to install both a hydroxyl and chloro substituent in their synthesis of the teicoplanin aglycon (29). Diazotization of 25 followed immediately by hydroxylation with cuprous oxide (halide-free/radical conditions) fiimished phenol 26 in excellent yield, even with several sensitive functionalities present in the molecule. Several steps were then required to convert 25 into aniline 27. At this point, a Sandmeyer chlorination proceeded in moderate yield to furnish the aryl chloride 28. This particular reaction is noteworthy, as the reaction proceeded well, in the presence of a variety of functional groups, many of which were unprotected. 

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