Functional model oxygenations by nonheme iron complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 185

dibridged >> tribridged (L:
bidentate nitrogen ligand, X: potentially labile ligand). 209-214 It is suggested that the
first step of the reaction is the binding of the oxidant to the iron catalyst probably
resulting in the cleavage of the dinuclear unit into monomers as a real active species.
The preparation of highly stable, easy to handle, iron complexes with highly labile
sites is thought to bring about the advantage of the dinuclear structure over the
mononuclear one. As for the cleavage of the O-O bond within the iron-peroxo
complex, both of the homolytic (with CHP) and heterolytic (with TBHP or
cleavages are proposed.
Reactivity of dinuclear iron complexes having tripodal ligands, represented
by was investigated in the presence of TBHP under
Ar 215, 216 with varying the tripodal and the bridging ligands. It was shown that the
ratio of (alcohol + ketone)/ t -BuOO-adduct) increased as the ligands become more
electron donating. Two mechanisms for the decomposition of TBHP have been
proposed: a homolytic pathway is initiated by generation of and
radicals that form and a heterolytic pathway is initiated by the
dissociation of the bridging anion from one iron center to provide a site for the
coordinating the alkyl peroxide ion. The latter metal-based mechanism involving an
alkylperoxyiron(III) intermediate was supported by the selective alcohol formation
and the large kinetic isotope effect value
2-Methyl-1-phenyl-2-propyl hydroperoxide ( MPPH ) was used as a probe
capable of distinguishing between free alkoxyl radical chemistry and radical-free
(enzyme mimetic) chemistry. 218-220 Miyake et al. studied the reaction of
with MPPH and showed that MPPH breaks down by O-O
bond homolysis, leading to the formation of the benzyl radical and a high-valent
species (Scheme 3). 221 The latter oxidizes exogenous substrates such as
thioanisole, cyclohexanol, and cyclooctene.

217