Catecholatoiron complex

Monooxygenation of aromatics, alicyclic and linear alkanes with molecular oxygen is catalyzed by nonheme iron complexes in the anhydrous organic solvents in the presence of hydroquinones as electron and proton donors. Iron complexes are prepared in situ by stirring FeCla, pyrocatechol, and pyridine (mole ratio is 1 1 2) in acetonitrile or in pyridine. Isolated catecholatoiron complex is also used. Catalytic activity is greatly dependent on the kinds of hydroquinone and increases in the order of 2,5-di-t-butyl- t-butyl->methyl->H-hydroquinone. Non-substituted hydroquinone hardly exhibits activity, and the activity is controlled by the oxidation potential and steric effect of hydroquinones. 

A different type of complex, which exhibits a characteristic band near 700 nm, is also formed when catechols are mixed with FeCl3 in various organic solvents [35]. Funabiki et al have isolated catecholatoiron complexes which are formulated either as [FeCl2(Cat)py] [H] (Cat H-Cat and 4-Me-Cat) or as FeCl(Cat)py2 (Cat 3-Me-Cat and DTBC) [32]. These complexes, which are dimeric in the solid state, become monomeric in THF and exhibit characteristic bands near 700 nm (H 630 nm, 4-Me 646 nm, 3-Me 658 nm, 3,5- Bu2 722 nm). The spectra have not, so far, been well explained. Although it is reported that various metal-semiquinonate complexes show a characteristic band in these regions [82], this is not a strong support for the formation of a stable complex of a Fe -SQ form in solution. Differences between M-DTBC and M-DTBSQ in electronic spectra have been discussed based on data for the Zn complexes [102]. 

Funabiki et al. have performed a Extended Hiickel calculation study on the relative stability of the probable catecholatoiron complexes [118]. 3,5-Dimethylcatecholato-(tetraammonium)iron(III) complexes are used for simplification. The results support the radical character of the catecholate ligand and suggests that the direct attack of free oxygen on aromatics, if it occurs, can be at the C-2 and C-6 positions from the side perpendicular to the aromatic plane. This parallels the two routes in Scheme 8 and path A in Scheme 9. In the model species of type 50, the most electrodeficient C-1 is favored over C-2 or C-6 for the attack by peroxide if importance is attached to the anionic character rather than the radical character of the peroxide. 

As described in 3.1.1, various types of catecholatoiron model complexes in the form of the chelate coordination have been isolated and found to give the oxygen insertion products by the reaction with molecular oxygen. Different from the enzyme structure, however, most of them are in the six-coordinate configuration. 

Funabiki et al. developed an monooxygenation system using hydroquinones with catecholatoiron(III) complexes in acetonitrile in the presence of pyridine."" First the system was applied to hydroxylation of aromatics."" "" Catalytic activity depends greatly on the snbstituents on hydroquinones (R-HQ) and increases with the electron-donating property R= H < Me < Bu < Bui. "" In the oxygenation of... 

The other important contribution of the model systems to the progress in enzymatic studies is to obtain information about structures and reactivities of substrate-metal intermediates. One of the characteristic examples is the isolation of mono- and bi-dentate catecholatoiron(III) complexes in relevance to catechol dioxygenases. Reactivities of these... 

Figure 13. Electronic spectra of monodentate and bidentate catecholatoiron(III) complex [88].

The extradiol cleavage of the catecholatoiron(III) complex reported by Dei et al. [46] was reexamined by Ito and Que [173]. The complex [Fe(TACN)(DTBC)Cl] formed in CH3CN in the presence of AgBF4 gave a 55% yield of extradiol cleavage products 4,5-di-t-butyl-2-pyrone (35%) and 3,5-di-t-butyl-2-pyrone (24, 20%). The higher selective extradiol cleavage (90%) was obtained in pyridine. 

---