Functional model oxygenations by nonheme iron complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 173

Extradiol cleavage is performed by manganese-containing dioxygenases.
Canseshi and Dei synthesized a manganese complex, [Mn(CTH)(DTBC)]Y
which structure was analyzed by X-ray diffraction (CTH = (±) l -
5,7,7,12,14,14-hemxamethyl-1,4,8,11-tetraazacycloetradecane). 124 It was found that
the two complexes have different physical properties in the solid state, but the
spectroscopic properties of their solutions are identical. In highly polar solvents
such as DMSO, the spectra of the yellow solutions are consistent with the presence
of an chromophore, whereas in weakly polar solvents such as
toluene or acetone the spectra of the blue-green solutions are consistent with the
presence of a chromophore. No detail has been reported,
but interestingly the complex in polar solvents are stable under oxygen, while the
complex in weakly polar solvents is quickly oxidized to give DTBQ (60%) and
extradiol cleavage products, 3,5- or 4,6-bis(1,2-dimethylethyl)-2 H -pyranones (36%
+ 4%). Funabiki et al. have synthesized a complex
but it was converted selectively with molecular oxygen to an
intradiol cleavage product. 125, 126
3.1.3 Mechanisms of Oxygenations
In the mechanism of the intradiol oxygenation, the first step is the activation
of catechol by coordination to a Fe(III) center. The chelate coordination of
catechols in the enzymatic systems (protocatechuate 3,4-dioxygenase, 3,4-PCD) has
been clarified by the X-ray analysis. 56, 57, l21, 127-132 In addition, the analysis indicated
the binding of catechols to 3,4-PCD forms a five-coordinate complex with the axial
Tyr dissociation from two Tyr and two His ligands (Fig. 7).
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.