Extradiol oxygenation

It has been found later that iron(III) works equally well in catechol oxygenation by the above system [104], in which the first process anyway is oxidation to iron(III). Product analysis has revealed that both intradiol and extradiol oxygenation take place. 

Recently, experimental supports for the oxygen activation process (B) were reported to explain the extradiol oxygenation of the Fe" -(DTBC) complex. The process is favored by the complex having facial tridentate ligands rather than tetradentate ligands in the former 02 and substrate can occupy the opposite face and form an intermediate that leads to the extradiol products (Fig. 9). The A process was proposed for the intradiol oxygenations for the complexes with meridional... 

This result indicates that modification of ligands provides good functional models for catechol dioxygenases. Recently, both functional and structural model studies for Fe -enzymes have started. This is a challenge to the model studies for the V 102 systems which seem to have the higher barrier to be overcome than the Fe /02 system. It is expected that catalytic systems for selective extradiol oxygenation of catechols by iron complexes will be developed in future. 

Figure 3. Effect of the water concentration on the intra- and extradiol oxygenations of DTBC by FeCl2 and FeCl3 in THF/H2O [31].

Scheme 11. Intra- and extradiol oxygenations of DTBCH2 by Fe " and Fe + complexes [31],...

Compared with extensive studies on the intradiol oxygenation by model complexes, the extradiol oxygenation has been little studied. The extradiol oxygenation is performed not only by Fe -enzymes but also Fe -enzymes in the case of 3-substituted catechols [119]. Thus, the explanation for the extradiol oxygenation by an Fe complex in Scheme 8 is reasonable. However, the remarkable promotion of extradiol oxygenation by using Fe " " in place of Fe " has indicated that such oxygenation is catalyzed by Fe rather than Fe " ", as in enzymatic systems. The reaction has been explained by Scheme 11 [31]. The Fe " " species may be formed from the Fe species by the side-reaction of oxidation of catechol to quinone. 

Figure 19. Posshible p-peroxo-iron species for extradiol oxygenation.

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