BIOINORGANIC CHEMISTRY A Short Course Second Edition

378 IRON-CONTAINING PROTEINS AND ENZYMES

iron(III) porphyrin complexes were found to be better catalysts in hydrocar-
bon oxygenation reactions. For instance, [Fe IV = O(TPFPP + • )] + complex (TPFPP
equals the dianion ofmeso - tetrakis(pentafl uorophenyl)porphyrin), hydroxyl-
ates alkanes rapidly and effi ciently, whereas [Fe IV = O(TMP + • )] + , TMP = dianion
ofmeso - tetramesitylphenylporphyrin, does not hydroxylate alkanes under
identical conditions. 53c Gross et al.^68 found that oxygen atom transfer to olefi ns
(epoxidation) is affected by the axial ligand trans to the oxo ligand in [Fe IV =
O(TMP+ • ) X], where X = F − , CH 3 OH, Cl − , CH 3 COO − , CF SO 33 −, ClO 4 −. The rate
of styrene epoxidation was in the order F − > > CH 3 OH > Cl − > CH 3 COO − >
CF SO 33 −. When XClO= − 4 , epoxidation of styrene did not take place under the
same reaction conditions. 53e In a continuing study of the effect of the axial
ligand on models for cytochrome P450 intermediate (compound I) oxidation
reactions, the group showed evidence consistent with a multistep catalytic
mechanism.^68 In this proposed mechanism, electron transfer from the olefi n is
followed byσ - bond formation between oxygen and the olefi n radical, rear-
rangement of the electronic structure, and, fi nally, release of the products (see
Figure 7.22 ). The study shows that the major kinetic effect of the axial ligands
occurs during intramolecular electron transfer from Fe(III) to the porphyrin
radical.^68
Spectroscopic evidence for the [Fe IV = O(TMP + • ) X] ( 1 - X) moiety included
that from NMR, EPR, and RR. The EPR spectra of frozen solutions of 1 - X

Figure 7.22 Cytochrome P450 model compound reaction profi le consistent with a
multi - step catalytic mechanism. (Adapted with permission from Schemes 7 and 8 of
reference 68. Copyright 1997, Society of Biological Inorganic Chemistry.)

FeIII

OClO 3

O

intramolecular

electron

transfer

slow step

FeIII

OClO 3

O-

(TMP)

CHCH 2

FeIV

OClO 3

O

R

R

O

R

3 -X

(TMP)

1 -X

FeIII

OClO 3

2 -X

(TMP+.) intermolecular

electron

transfer

reacts with

(TMP+.)

styrene, RCH=CH 2

epoxide

CH CH 2

O

C CH 3

O

Reaction Profile

product formed at

0 o C and R. T.

product formed at

-78o C

and

possible intermediate