376 IRON-CONTAINING PROTEINS AND ENZYMES
The oxygen rebound mechanism was supported by experimental evidence
including (1) high kinetic isotope effects, (2) partial positional or stereochemi-
cal scrambling, and (3) allylic rearrangements. For instance, in the presence
of [Fe(TPP)Cl] and PhIO, cis - stilbene was stereospecifi cally epoxidized. In
addition, it was found that cis - stilbene was 15 times more reactive than trans -
stilbene in competitive epoxidations. (see Figure 7.20 ). 64b
The authors ascribed the cis - stilbene preference to steric interference
between the phenyl groups oftrans - stilbene and the phenyl groups on the
[Fe IV – O(TPP + • )] + intermediate. The Groves research group also gathered
the fi rst spectroscopic indications of a synthetic, compound I analog. 53a The
[Fe IV = O(TMP + • )] + complex (TMP equals the dianion of meso - tetramesityl-
phenylporphyrin) was prepared by them - CPBA (3 - chloroperoxybenzoic acid)
oxidation of [Fe III – O(TMP)Cl]. (See Figure 7.21 .)
The [Fe IV = O(TMP + • )] + complex exhibited a characteristic bright green
color and corresponding visible absorbance in its UV – vis spectrum. In its
NMR spectrum, the meta - proton doublet of the porphyrin mesityl groups
were shifted more than 70 ppm downfi eld from tetramethylsilane (TMS)
because they were in the presence of the cation radical, while the methyl
protons shift between 10 and 20 ppm downfi eld. In M ö ssbauer spectroscopy,
the isomer shift, δ of 0.06 mm/s, and ΔEQ value of 1.62 mm/s were similar to
those for other known Fe(IV) complexes. Electron paramagnetic resonance
(EPR),^65 resonance Raman (RR),^66 and EXAFS^67 spectroscopies provided
additional indications of an Fe IV = O π - cation radical intermediate. For instance,
Figure 7.20 Epoxidation of cis - and trans - stilbene. (Adapted with permission from
Table 1 of reference 64b. Copyright 1983 American Chemical Society.)
cis-stilbene trans-stilbenePhIO[Fe(TPP)]Cl]PhIO[Fe(TPP)]Cl]yield: 77 % cis-stilbene epoxide and
0 % trans-stilbene epoxide
H O H+15 times more cis- than trans-
stilbene epoxide product