380 IRON-CONTAINING PROTEINS AND ENZYMES
transformation of intermediate [(TMP + • )Fe III (O − )X)], 3 - X, to the possible
intermediate [(TMP)Fe IV (O)X)].
Other examples of oxidant – iron(III) adducts as intermediates in iron
porphyrin - catalyzed reactions have been published as listed in references 54a – k.
Competitive alkene epoxidation experiments catalyzed by iron porphyrins
with peroxy acids, RC(O)OOH, or idosylarenes as oxidants have been pro-
posed to have various intermediates such as [(porphyrin)Fe III (O – O – C(O)R]
or [(porphyrin)Fe III (O – I – Ar)]. Alkane hydroxylation experiments catalyzed
by iron porphyrins with oxidant 3 - chloroperoxybenzoic acid, m - CPBA, have
been proposed to operate through the [(porphyrin)Fe III (O – O – C(O)R] inter-
mediate. J. P. Collman and co - workers postulated multiple oxidizing species,
[(TPFPP+ • )Fe IV = O] and/or [(TPFPP)Fe III (O – I – Ar)] in alkane hydroxylations
carried out with various iodosylarenes in the presence of Fe(TPFPP)Cl, where
TPFPP is the dianion ofmeso - tetrakis(pentafl uorophenyl)porphyrin. 54g
Oxoiron(V) porphyrins — red in color — an isoelectronic form of oxoiron(IV)
porphyrin cation radicals — green — have been proposed (references 55a – c ),
although the density functional theory calculations have indicated that there
are no true oxoiron(V) porphyrins (references 55d – f ). Further spectroscopic
studies are necessary to confi rm the existence of this intermediate.
Oxoiron(IV) porphyrins, one electron above the Fe(III) porphryin resting
state, are known as compounds II in the catalytic cycles of peroxidases and
catalases. These intermediates are thought to be poor oxidants, but several
research groups have observed very different reactivity for a [(TMP)Fe IV = O]
complex when compared to that for [(TMP + • )Fe IV = O]. For instance, Groves
and co - workers found a mixture of cis - and t rans - β - methylstyrene oxide ( cis/
trans∼ 1.2) in reactions involving the [(TMP)Fe IV = O] complex, whereas
[(TMP+ • )Fe IV = O] under the same conditions yielded a cis/trans ratio of approx-
imately 11 : 1. 56c In general, researchers have found that the oxidizing power of
the oxoiron(IV) porphyrin complexes can be tuned by altering the nature of
the substituents on the porphyrin ligands. The oxidation mechanism involving
these complexes is less well studied than that for other species discussed
previously.
Conclusive evidence still does not exist for the presence of thiolate - ligated
oxoiron(IV) species in cytochrome P450 catalytic cycles — although see the
Schlichting X - ray crystallographic studies in Section 7.4.4. And chemists have
attempted, rather unsuccessfully, to synthesize oxoiron(IV) porphyrin model
complexes with a thiolate axial ligand. Since the confi ning enzyme pocket
present in the natural enzyme is missing in the model compounds, and the
sulfur ligand is as easily oxidized as the iron center itself, efforts to synthesize
such complexes have largely failed. However, Eckard M ü nck and Larry Que
have recently published a report on the synthesis and characterization of
[Fe IV (O)(TMCS)] + where TMCS is a pendadentate ligand, the monoanion
of 1 - mercaptoethyl - 4,8,11 - trimethyl - 1,4,8,11 - tetraaza cyclotetradecane.^69 The
ligand provides a square pyramidal, non - heme, (N 4 SR) apical ligand set that is
similar to that in cytochrome P450. (See Figure 7.23 .)