Dioxygenase-model oxygenations

Balogh-Hergovich, E. Kaizer, J. Speier, G. Kinetics and mechanism of the Cu(I) and Cu(II) flavonolate-catalyzed oxygenation of flavonols. Functional quercetin 2,3-dioxygenase models, J. Mol. Catal. A Chem., 2000,159, 215. 

Fig. 7. Model for oxygen activation by intradiol dioxygenases. PCA, the substrate pro-tochatechuic acid.

Cobaltn-Schiff base complexes, e.g. Co(salen),567 Co(acacen)568 and cobalt(II) porphyrins,569 e.g. Co(TPP), are effective catalysts for the selective oxygenation of 3-substituted indoles to keto amides (equation 249), a reaction which can be considered as a model for the heme-containing enzyme tryptophan-2,3-dioxygenase (equation 21).66 This reaction has been shown to proceed via a ternary complex, Co-02-indole, with probable structure (175), which is converted into indolenyl hydroperoxide (176). Decomposition of (176) to the keto amide (174) readily occurs in the presence of Co(TPP), presumably via formation of a dioxetane intermediate (177).569,56 Catalytic oxygenolysis of flavonols readily occurs in the presence of Co(salen) and involves a loss of one mole of CO (equation 251).570... 

The visible spectra of the intradiol dioxygenases (Fig. 2) are characterized by a broad absorption band centered near 460 nm with molar extinction coefficients of 3000-4000 M-1 cm-1 16>34). The color disappears upon reduction of the ferric ion with dithionite and is regenerated upon exposure of the solution to oxygen. Resonance Raman studies on these enzymes15 35-38) have been reported by several laboratories (Table 4). These spectra are characterized by a set of four peaks at ca. 1605, 1505, 1270, and 1175 cm-1, which are assigned to ring vibrations of Fe(III) coordinated tyrosinate ligands. Similar spectra are obtained for the transferrins as well as for model iron-phenolate complexes (Table 4). A new class of iron proteins seems to... 

In these enzymic oxygenations no evidence exists that excited-state products are formed. In fact, 1,2-dioxetane intervention has not been soundly established, except that it constitutes a mechanistic convenience. The dioxygenase action entails cleavage of aromatic substrates [Eq. (34a)] or their cis-dihydroxylation [Eq. (34b)].91 A recent chemical model study on pyrocatechase discounts 1,2-dioxetane formation.92... 

Step behind their counterparts, the intradiol-cleaving dioxygenases. In particular, no iron(II) catecholate complexes have been reported to carry out the oxygenation chemistry of the enzymes. The development of model systems that proceed via the same pathway as the enzymes presents itself as the next challenge in the effective development of functional model systems. 

---