BIOINORGANIC CHEMISTRY A Short Course Second Edition

CYTOCHROME P450 : A MONOOXYGENASE 375

found that a [(porphyrin)Fe III (O 2 )] − complex would deformylate certain alde-
hydes while a corresponding [(porphyrin + • )Fe IV – O] + complex would not.^63
Publications concerning important early cytochrome P450 model com-
pounds and proposed catalytic cycles by the J. T. Groves group became avail-
able starting in the late 1970s. 53a,64 The fi rst article on the use of a synthetic
iron(III) porphyrin complex, [Fe(TPP)Cl], appeared in 1979 (TPP equals the
dianion ofmeso - tetraphenylporphyrin). This complex, in the presence of the
oxidant iodosylbenzene, PhIO, used as a source of oxygen atoms, was found
to catalyze alkene epoxidation and alkane hydroxylation reactions. Groves
enunciated the “ oxygen rebound ” mechanism to explain the reactions. In the
hydroxylation mechanism, the oxoiron(IV) porphyrin π - cation radical inter-
mediate, [Fe IV =O(TPP + • )] + , forms in the presence of PhIO (or other oxidants
such as the commonly used 3 - chloroperoxybenzoic acid, m - CPBA), and it is
this intermediate that abstracts a hydrogen atom from the substrate (R - H) to
form the “ caged ” carbon radical and [Fe IV =OH(TPP)]. The cage ensures that
the intermediates do not diffuse away from each other. The two species then
recombine in the rebound step to produce the hydroxylated product and the
resting state of the Fe III – porphyrin complex (see Figure 7.19 ).

Figure 7.19 The Groves oxygen rebound mechanism for alkane hydroxylation.

N

N N

N

FeIII

[Fe(TPP)]+ TPP = meso-tetraphenylporphyrin

+

IO

iodosylbenzene, PhIO

N

NN

N

FeIV

O

R

CH

+

N

NN

N

FeIV

HO

R

C

C

R

OH

cage radical