Functional model oxygenations by nonheme iron complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 176

O1 and O2, and the carbons have very low spin density. Recently, Wheeler et al.
have studied on electronic structure of 3,5-di- tert -butylbenzosemiquinone (DTBSQ)
by density functional theory analysis. 150 Normal population analysis (NPA) atomic
charge densities and spin densities in parenthesis (Mulliken net, NPA atomic) are
followings. O1: -0.626 (+0.271, +0.272); O2: -0.662 (+0.245, +0.248); C1: +0.368
(+0.031, +0.038); C2: +0.362 (+0.075, +0.076); C3: -0.113 (+0.095, +0.087); C4: -
0.257 (+0.116, +0.104); C5: -0.066 (+0.072, +0.075); C6: -0.306 (+0.111, +0.098).
Most of the negative charge resides also on O1 and O2 and most of the positive
charge resides on C1 and C2. Most of the spin density is also located on O1 and O2.
The carbons attached to the tert -butyl groups (C3 and C5) have less spin density
than those attached to hydrogens (C4 and C6). In both cases, the spin density on
carbons, e.g. C2, is very low and does not link up with the direct attack of the
molecular oxygen to C2 in the step of intradiol cleavage. By calculating the total
energy change caused by approach to Fe or carbon, Funabiki et al. have proposed
the probability of the B process (oxygen activation process) rather than the A
process even in the intradiol cleavage. 149 In this calculation, the configurational
change of the catecholate ligand from chelate to monodentate form is assumed to
open a coordination site for
As for the reactivity or the semiquinonate ligand with Mialane et al.
studied the structure and electronic properties of complex (L = N,N'-
bis(4-methyl-6- tert -butyl-2-methyl-phenolato)- N,N' -bismethyl-1,2-diaminoethane).
151 In spite of the coordination of the DTBSQ monoanion, the complex was found to
be stable under supporting that the semiquinonate carbons do not bind directly
with The complex is reduced not to but to
that is also stable under Koch et al. also synthesized a complex,
This complex was also found to be converted to
with superoxide, but stable towards molecular oxygen. Since
DTBSQ is proposed to react with as an intermediate in the oxygenative cleavage
of by the results indicate that the binding of DTBSQ to
stabilizes the DTBSQ moiety against These results support the importance of the
form for the C-C bond cleavage, but do not give any
positive support for the direct attack of oxygen to the semiquinonate carbon atom.
Recently, experimental supports for the oxygen activation process (B) were
reported to explain the extradiol oxygenation of the complex. 123 The
process is favored by the complex having facial tridentate ligands rather than
tetradentate ligands; in the former 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

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