Hypohalous acid , enzymatic halogenation

A striking array of halogenated products produced by natural sources is found in the biosphere. The majority are probably formed by the enzymatic activity of haloperoxidases. The detailed mechanism by which haloperoxidases catalyze the insertions of a halogen into organic molecules is still a point of debate. However, for most of these enzymes a simple reaction mechanism has been observed that consists of a two-electron oxidation of the electron donor (Cl", Br", I") by H2O2 to hypohalous acids (equation 1) ... 

Enzymatic halogenation catalyzed by haloperoxidases and perhydrolases involves the oxidation of halide ions to a halonium ion species which leads to the formation of hypohalous acids (Fig. 16.9-1). The products obtained by enzymatic halogenation with these enzymes are the same as the products obtained by chemical electrophilic halogenation with hypohalous acids. The differences in the para ortho ratios in the halogenation of some aromatic compounds could be due to a mixture of halogenation at or near the active site and in solution. 

Haloperoxidases have been shown to transform alkenes by a formal addition of hypohalous acid to produce halohydrins. The reaction mechanism of enzymatic halogenatitHi has been debated for some time and it is now accepted that it proceeds via a halonium intermediate [1770, 1771], similar to the chemical formation of halohydrins (Scheme 2.228). The former species is derived from hypohalous acid or molecular halogen, which is in turn produced by the enzyme via oxidation of halide [1772]. In support of this, a HOCl-adduct of Fe -protoporphyrin IX was identified as a direct enzyme-halogen intermediate involved in chloroperoxidase-catalyzed halogenaticHi [1773]. 

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