A new evidence of the high-valent oxo-metal radical cation intermediate and hydrogen radical abstract mechanism in hydrocarbon hydroxylation catalyzed by metalloporphyrins

Article abstract of DOI:10.1016/S1381-1169(99)00444-6

Forty metalloporphyrins with different peripheral substituents (R) on the porphyrin ring, RTPPM(III)Cl and [RTPPM(III)]₂O (M= Fe, Mn), were synthesized, and were used to catalyze cyclohexane hydroxylation with iodosobenzene under mild conditions. A Hammett relationship, log k = ρσ + c, was shown to exist between the rate constants k of the cyclohexane hydroxylation and Hammett constants σ of substituents on the porphyrin rings. The value of the reaction constant ρ changed from 0.3 to 0.5 for the cyclohexane oxidation catalyzed by metalloporphyrins, and electron- withdrawing groups increased the reaction speeds. This means that the reaction mediator was a radical cation, and that the rate-determining step of the reaction was a radical reaction. These research results provided a new evidence for the high-valent oxo-metal radical cation intermediate and hydrogen radical abstraction mechanism in hydrocarbon hydroxylation catalyzed by metalloporphyrins. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S1381-1169(99)00444-6

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Journal of Molecular Catalysis A: Chemical 157 2000 31–40
www.elsevier.comrlocatermolcata
A new evidence of the high-valent oxo–metal radical cation
intermediate and hydrogen radical abstract mechanism in
hydrocarbon hydroxylation catalyzed by metalloporphyrins
Can-Cheng Guo), Jian-Xin Song, Xin-Bin Chen, Guo-Fang Jiang
College of Chemistry and Chemical Engineering, Hunan UniÕersity, Changsha 410082, People’s Republic of China
Received 22 September 1999; received in revised form 1 November 1999; accepted 6 December 1999
Abstract
III
III
Ž .
w
x
Forty metalloporphyrins with different peripheral substituents R on the porphyrin ring, RTPPM Cl and RTPPM 2O
Ž
.
MsFe, Mn , were synthesized, and were used to catalyze cyclohexane hydroxylation with iodosobenzene under mild
conditions. A Hammett relationship, log ksrsqc, was shown to exist between the rate constants k of the cyclohexane
hydroxylation and Hammett constants s of substituents on the porphyrin rings. The value of the reaction constant r
changed from 0.3 to 0.5 for the cyclohexane oxidation catalyzed by metalloporphyrins, and electron-withdrawing groups
increased the reaction speeds. This means that the reaction mediator was a radical cation, and that the rate-determining step
of the reaction was a radical reaction. These research results provided a new evidence for the high-valent oxo–metal radical
cation intermediate and hydrogen radical abstraction mechanism in hydrocarbon hydroxylation catalyzed by metallopor-
phyrins. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Metalloporphyrins; Catalysis; Hammett relationship
1. Introduction
tochrome P-450 monooxygenase was found.
Chemists have paid much interest to try to
mimic cytochrome P-450 monooxygenase using
synthetic metalloporphyrins because they bear a
strong resemblance to heme in both structures
and catalytic property. So far, they have found
Cytochrome P-450 monooxygenase with
heme as an active center has been found to be
one of two enzymes that can catalyze the oxida-
tion of the inert carbon–hydrogen bond under
w
x
w x w x
mild conditions in organisms 1,2 . People have
been interested in the hydrocarbon hydroxyl-
ation catalyzed by cytochrome P-450 mono-
oxygenase under mild conditions and tried to
understand the reaction mechanism since cy-
that iron 3–9 , manganese 10–15 , chromium
w
x
w
x
16–18 and cobalt 19–22 porphyrin com-
plexes can be used as model compounds for
cytochrome P-450 monooxygenase to catalyze
the transfer of an oxygen atom from oxidants
w
x
w
x
such as PhIO 23–28 , NaClO 29–35 , H2O2
w
x
w
x
36–41 and dioxygen 42–47 to saturated hy-
)
Corresponding author. Tel.: q86-731-882-3420; fax: q86-
drocarbons at ambient temperature and pressure,
and put forward the high-valent oxo–metal radi-
731-882-2242.
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E-mail address: ccguo@mail.hunu.edu.cn C.-C. Guo .
1381-1169r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
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PII: S1381-1169 99 00444-6

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32
C.-C. Guo et al.rJournal of Molecular Catalysis A: Chemical 157 2000 31–40
cal cation intermediate and hydrogen radical
abstraction mechanism for the hydrocarbon hy-
droxylation catalyzed by metalloporphyrins.
These results are of importance in understand-
ing the mechanism of the heme monooxygen-
ase-catalyzed reaction in biological systems. In
order to use the Hammett relationship to prove
the high-valent oxo–metal radical cation inter-
mediate and hydrogen radical abstraction mech-
anism in hydrocarbon hydroxylation catalyzed
by metalloporphyrins, we synthesized 40 metal-
loporphyrins, shown in Fig. 1, and studied their
catalysis of cyclohexane hydroxylation with
PhIO.
model compounds of cytochrome P-450 so
w
x
far 48 , and 19 m-oxo-bismetalloporphyrin
III
w
x
Ž
.
RTPPM 2O MsFe, Mn , which were re-
cently put forward as the model compounds of
w
x
cytochrome P-450 49 . There is a different
peripheral substituent R on the porphyrin ring in
these compounds. The research results show
that all the synthetic metalloporphyrins can cat-
alyze cyclohexane oxidation with PhIO at ambi-
ent temperature and pressure. Moreover, there
are the same kinetic relationships between the
yields and the reaction time in all the cyclohex-
ane hydroxylation catalyzed by these metallo-
porphyrins. For the above reaction system, it
has been found that the structure of metallopor-
phyrins can affect their catalytic property. There
are Hammett relationships between the sub-
These synthetic porphyrins included 21
monometalloporphyrin RTPPMIIICl M s
Ž
.
Fe, Mn , which have been widely used as the
III
Fig. 1. Structure of RTPPMIIICl 1–21 and RTPPM 2O 22–40 .
Ž
.
w
x
Ž
.

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C.-C. Guo et al.rJournal of Molecular Catalysis A: Chemical 157 2000 31–40
33
stituent constants s of para- and meta-sub-
stituents on the benzene ring of porphyrins
and the rate constants k of the reactions cat-
alyzed by these metalloporphyrins, whether
responding substituted tetraphenylporphine
RTPPH2. RTPPH2 reacted with FeCl2 P4H2O
and MnCl2 P5H2O in DMF respectively to
give substituted chlorotetraphenylporphinato-
III
III
III
w
x
Ž . Ž .
RTPPM Clor RTPPM 2O. There is also a
similar result in the self-oxidation of these met-
alloporphyrins, which is a competitive reaction
in the cyclohexane hydroxylation system. This
study will further offer evidence both for deter-
mining competitive mechanisms between the
reaction catalyzed by metalloporphyrins and the
self-oxidation of the metalloporphyrins, and for
the structure of the radical cation intermediate
in the above mimicry system. The research re-
sults mean that the linear free-energy relation-
ship, which commonly exists in chemical phe-
nomena, might fit in with the enzyme-catalyzed
reaction in the human body.
iron III , RTPPFe Cl 1–10 and substituted
Ž
.
chlorotetraphenylporphinatomanganese III ,
RTPPMnIIICl 11–21 using the literature
Ž
.
w
x
Ž
method 52 . The structures of compounds 1–
21 were all characterized by elementary analy-
.
Ž
sis and ultraviolet absorption spectrometry see
.
Table 1 .
[
2.3. Synthesis of m-oxo-bis tetraphenyl-
III
(
)]
[
]
porphinatoiron III compounds RTPPFe 2O
(
)
22–30
Ž
.
Compounds 22–30 were prepared accord-
ing to the following procedure: 2 g of
TRPPFeIIICl was passed through a chromato-
graphic column filled with 500 g Al2O3 using
the wet method. Benzene containing 5% of
2. Experimental
C2 H5OH was used as drip washing liquid.
2.1. Instruments and reagents
III
w
x
TRPPFe 2O was formed in the above proce-
Gas chromatography was performed on a
Shimadzu GC-16A coupled with a Shimadzu
C-R3A reporting integrator, infrared spectra
were recorded on a Perkin-Elmer 783 spectro-
photometer, visible spectra were recorded on a
Perkin-Elmer L-17 spectrometer. A Perkin-
Elmer Model 2400 elemental analysis meter
was used.
Benzene was refluxed with metallic sodium
for 10 h and redistilled before used. Pyrrole was
also redistilled before used. There was no impu-
rity found in cyclohexane by GC analysis before
being used. Oxidant PhIO was synthesized ac-
dure. The eluate was evaporated in vacuo. The
III
w
x
purity of TRPPFe 2O was checked with
HPLC or IR. If the components with the reten-
tion time of 2.87 min characteristic of
TRPPFeIIICl in HPLC or with an absorption
band at 380 cmy nFe–Cl in IR disappear, the
reaction may be thought to be complete. Other-
wise, the abovementioned procedure was re-
1
Ž
.
peated until TPPFeIIICl was transformed com-
III
w
x
pletely into TPPFe 2O. Their elementary
analysis, IR and UV–Vis data are listed in
Table 1.
w
x
cording to a method in the literature 50 . Its
purity, measured by iodimetry, was 99%. Sub-
stituted benzaldehydes and other reagents were
all of analytical or commercial chemical grade.
[
2.4. Synthesis of m-oxo-bis tetraphenyl-
III
(
)]
[
]
RTPPMn 2 O
porphinatomanganese III
(
)
compounds 31–40
Ž
.
Compounds 31–40 were prepared accord-
ing to the following procedures: 1.0 g
RTPPMnIIICl was placed in a reactor filled with
250 ml benzene, and then 50 ml 5% aqueous
NaOH was added. After vigorously stirring the
2.2. Synthesis of metalloporphyrin compounds
(
)
1–21
Substituted benzaldehyde and pyrrole were
w
x
refluxed in propanoic acid 51 to give the cor-

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34
C.-C. Guo et al.rJournal of Molecular Catalysis A: Chemical 157 2000 31–40
Table 1
Ž
.
w
x
Ž
.
Elemental analysis, IR and UV–Vis data of compounds RTPPMCl 1–21 and RTPPM 2O 22–24
Ž
.
Ž
.
Ž
cm
.
Compounds
Elemental analysis %, calculated
lmax benzene
IR KBr
y1
Ž
nm
.
Ž
.
C
H
N
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.
1
2
3
4
5
6
7
8
9
68.24 68.09
3.05 3.12
7.14 7.22
419.5, 509.4, 575.3, 651.4
420.7, 509.0, 574.8, 652.1
418.5, 505.4, 576.0, 648.0
415.6, 570.5, 618.0
420.8, 512.2, 549.2, 586.8, 651.2
418.8, 507.2, 572.4, 652.8, 685.6
418.4, 507.6, 571.6, 653.2, 685.6
420.8, 509.2, 572.0
421.2, 509.2, 571.6
421.3, 514.6, 582.6, 653.7
489.7, 594.4, 645.3
477.6, 532.0, 585.6, 621.2
477.8, 533.1, 584.5, 620.3
478.0, 582.6, 620.0
nFe – Cl 382 m
Ž
Ž
Ž
Ž
nFe – Cl 384 m
62.74 62.78
2.80 2.87
6.72 6.66
Ž
Ž
Ž
Ž
62.85 62.78
2.74 2.87
6.83 6.66
nFe – Cl 380 m
Ž
Ž
Ž
Ž
nFe – Cl 382 m
51.72 51.83
2.53 2.37
5.67 5.49
Ž
Ž
Ž
Ž
44.35 43.76
2.34 2.00
5.12 4.64
nFe – Cl 378 m
Ž
Ž
Ž
Ž
nFe – Cl 380 m
75.10 75.06
3.85 4.01
7.90 7.92
Ž
Ž
Ž
Ž
75.92 75.84
4.67 4.77
7.83 7.37
nFe – Cl 380 m
Ž
Ž
Ž
Ž
nFe – Cl 383 m
77.32 77.10
5.98 6.01
6.14 6.42
Ž
Ž
Ž
Ž
69.53 69.96
4.03 4.40
7.33 6.80
nFe – Cl 381 m
Ž
Ž
Ž
.
Ž
nFe – Cl 380 m
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
59.85 59.76
2.80 2.71
12.53 12.67
Ž
Ž
Ž
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.
Ž .
68.15 68.18
3.05 3.12
7.14 7.23
nMn – Cl 318 m
Ž
Ž
Ž
Ž
.
.
.
.
.
.
.
.
.
.
62.81 62.85
2.72 2.88
6.63 6.66
nMn – Cl 320 m
Ž
Ž
Ž
Ž
nMn – Cl 321 m
62.53 62.85
2.82 2.88
6.84 6.66
Ž
Ž
Ž
Ž
62.95 62.85
2.50 2.88
7.03 6.66
nMn – Cl 322 m
Ž
Ž
Ž
Ž
nMn – Cl 320 m
52.03 51.88
2.25 2.38
5.80 5.50
477.5, 585.2, 621.4
479.2, 583.2, 616.4
Ž
Ž
Ž
Ž
51.53 51.88
2.54 2.38
5.65 5.50
nMn – Cl 323 m
Ž
Ž
Ž
Ž
nMn – Cl 319 m
43.52 43.79
1.95 2.01
4.87 4.64
478.3, 533.5, 589.3, 625.4
477.2, 533.6, 585.2, 620.4
477.6, 532.8, 586.0, 622.8
478.1, 534.1, 587.5, 625.0
479.2, 535.2, 589.6, 627.6
407.6, 570.0, 611.2
408.0, 570.0, 610.8
408.0, 570.8, 611.2
409.2, 572.8, 612.8
409.6, 572.4, 613.6
Ž
Ž
Ž
Ž
75.32 75.16
3.95 4.01
7.80 7.97
nMn – Cl 320 m
Ž
Ž
Ž
Ž
nMn – Cl 319 m
75.66 75.94
4.71 4.78
7.43 7.37
Ž
Ž
Ž
Ž
76.96 77.18
5.88 6.01
6.25 6.43
nMn – Cl 320 m
Ž
Ž
Ž
Ž
nMn – Cl 322 m
nFe – O – Fe 895 m , 870 m
Ž . Ž
nFe – O – Fe 895 m , 875 m
nFe – O – Fe 895 m , 870 m
Ž . Ž
nFe – O – Fe 895 m , 870 m
nFe – O – Fe 895 m , 870 m
Ž . Ž
nFe – O – Fe 905 m , 880 m
nFe – O – Fe 910 m , 880 m
70.68 70.03
4.31 4.41
6.69 6.81
Ž
Ž
Ž
Ž . Ž
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.
78.09 78.11
4.23 4.17
8.05 8.28
Ž
Ž
Ž
69.81 70.58
2.95 3.21
7.74 7.49
Ž
Ž
Ž
Ž . Ž
64.73 64.90
2.88 2.97
7.11 6.88
Ž
Ž
Ž
78.23 78.68
4.86 4.95
7.66 7.68
Ž
Ž
Ž
Ž . Ž
79.27 79.61
6.25 6.20
6.64 6.63
Ž
Ž
Ž
70.01 70.58
2.95 3.21
7.61 7.49
405.6, 568.4, 607.2
407.6, 568.8, 608.8
408.0, 569.4, 611.4
406.4, 570.4, 610.4
Ž
Ž
Ž
Ž . Ž
64.52 64.90
2.78 2.97
7.03 6.88
Ž
Ž
Ž
Ž . Ž
nFe – O – Fe 910 m , 880 m
nFe – O – Fe 910 m , 885 m
nMn – O – Mn 876 m , 849 m
52.95 53.26
2.13 2.44
6.02 5.65
Ž
Ž
Ž
Ž . Ž
78.33 78.68
4.85 4.95
7.68 7.68
Ž
Ž
Ž
Ž
Ž
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.
Ž .
Ž .
77.81 78.21
4.18 4.18
8.62 8.29
616.0, 578.4, 472.9, 418.5
618.6, 581.0, 471.6, 419.6
621.6, 582.6, 472.0, 420.8
615.6, 578.4, 470.4, 419.0
616.0, 578.4, 470.2, 420.4
627.6, 586.0, 472.6, 421.8
618.0, 572.8, 474.8, 438.4
615.6, 578.0, 470.2, 420.6
655.2, 586.8, 511.8, 471.6
474.7, 419.6
Ž
Ž
Ž
78.25 78.78
4.80 4.96
7.87 7.66
nMn – O – Mn 878 w , 848 m
Ž . Ž .
Ž
Ž
Ž
79.26 79.69
5.87 6.21
6.27 6.64
nMn – O – Mn 875 s , 850 s
Ž .
Ž
Ž
Ž
Ž .
69.80 70.69
3.27 3.24
7.58 7.50
nMn – O – Mn 886 w , 852 s
Ž . Ž .
nMn – O – Mn 883 s , 850 s
Ž
Ž
Ž
64.42 64.97
3.05 2.97
6.74 6.89
Ž
Ž
Ž
Ž
.
Ž .
71.85 72.45
4.47 4.56
6.99 7.04
nMn – O – Mn 880 m , 849 s
Ž . Ž .
nMn – O – Mn 878 s , 846 s
nMn – O – Mn 887 m , 850 m
nMn – O – Mn 881 s , 845 w
Ž . Ž
nMn – O – Mn 883 m , 847 w
Ž
Ž
Ž
.
72.85 73.59
5.63 5.70
12.85 13.21
Ž
Ž
Ž
.
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.
Ž
.
Ž
.
.
64.63 64.97
3.14 2.97
6.53 6.89
Ž
Ž
Ž
Ž . Ž .
64.35 64.97
3.27 2.97
7.03 6.89
Ž
Ž
Ž
52.90 53.32
2.32 2.44
6.14 5.65
reactants for 3 h, 300 ml water was added.
2.5. Cyclohexane monooxygenation
Benzene was evaporated under vacuum. The
III
w
x
crude product RTPPMn 2O gained by crystal-
Except where special explanation is given, all
cyclohexane hydroxylations were carried out ac-
lization was washed with water until the filter
liquor became neutral. RTPPMn 2O was dried
in air and recrystallized with absolute anhydrous
III
4
cording to the following procedure: 4.5=10y
w
x
5
mol of PhIO and 3.0=10y mol metallopor-
Ž
.
benzene and cyclohexane. IR, UV–Vis and ele-
phyrin metal atom as the standard were put
into a 25-ml double-layer reactor through which
constant-temperature water could flow through.
III
w
x
Ž
.
mental analysis data of RTPPMn 2O 31–40
are listed in Table 1.

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)
C.-C. Guo et al.rJournal of Molecular Catalysis A: Chemical 157 2000 31–40
35
After air had been removed out of the reactor
with N2, 5 ml cyclohexane and 5 ml of solvent
benzene were put into the reactor. Then an
electromagnetic stirrer was started, and the reac-
tion carried out. The products were analyzed by
GC, and quantified by internal standard method.
The standard substance was chlorobenzene.
When the quantity of products did not increase
further in the GC analysis, the ultimate yield
was determined based on the quantity of PhIO
added. Kinetic analysis was made by measuring
samples that were taken directly from the reac-
tion system with a mini-injector.
catalyzed by RTPPFeIIICl with different periph-
eral substituent R on porphyrin ring is shown in
Fig. 2.
According to kinetic analysis, linear slopes in
Fig. 2 represent the rate constants of the reac-
tions. The research shows that the reaction rates
of cyclohexane hydroxylation not only change
with the change of metal atoms in metallopor-
phyrins and structures of metalloporphyrins, but
also change with the change of peripheral sub-
stituents on the porphyrin ring in the same
pattern of metalloporphyrins. The reaction yields
Ž
.
% , the rate constant k and productive selectiv-
ity in cyclohexane hydroxylation is listed in
Table 2.
2.6. Kinetic determination for the self-oxidation
of metalloporphyrins
The relationships between the reaction rate
constant k and the substituent constant s for
cyclohexane hydroxylation catalyzed by para-
and meta-substitutuents agree with linear free-
energy relationship 54 Fig. 3 : log ksrsqc.
Their mathematical equations are listed in
Table 3.
In Hammett equations, which show the linear
free-energy relationship, all the reaction con-
stants r are positive. Their values change from
0.33 to 0.53 with the change of pattern of
metalloporphyrins, but the differences in num-
bers are not large. This indicates a similar reac-
tion mechanism and the same reaction interme-
diate in cyclohexane hydroxylation catalyzed by
PhIO was dissolved in benzene to obtain a
saturated solution; its concentration was mea-
sured by iodimetry. Metalloporphyrin was quan-
titatively weighed, then dissolved and diluted to
w
x
Ž
.
5
1
the order of 10y mol ly with benzene. Ab-
sorption spectrometry was used to measure the
change of the absorbency of the metallopor-
phyrin during the reaction of metalloporphyrins
with PhIO at 258C.
3. Results and discussion
3.1. Linear free-energy relationship in cyclo-
hexane hydroxylation catalyzed by metallopor-
phyrins
Catalyzed by compounds 1–40, PhIO oxi-
dizes cyclohexane into cyclohexanol and cyclo-
hexanone at ambient temperature and pressure.
The yield of cyclohexanol changes with the
change in reaction time, which agrees with the
kinetic relationship in the reaction catalyzed by
w
x
an enzyme 53 . The linear relationship between
Ž
.
Ž .
versus reaction time t relation-
Fig. 2. Cyclohexanol yield
%
Ž
.
the yield % of cyclohexanol and the reac-
ship in cyclohexane hydroxylation catalyzed by metallopor-
Ž .
tion time t in cyclohexane monooxygenation
phyrins.

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)
36
C.-C. Guo et al.rJournal of Molecular Catalysis A: Chemical 157 2000 31–40
Table 2
Ž
.
Reaction yields % , the rate constant k and productive selectivity of cyclohexane hydroxylation catalyzed by different metalloporphyrins
6
Ž
.
Metalloporphyrin
Substituent
constant s
Yield
k=106
log k=10
Alcohol: ketone
y1
Ž .
%
Ž
.
mol min
Pattern
Substituent R
RTPPFeIIICl
p-F
p-Cl
o-Cl
p-Br
p-I
0.06
0.23
22
34
46
33
29
21
28
35
31
42
33
34
33
34
33
39
38
29
34
35
36
39
37
36
33
30
29
28
29
26
46
48
46
47
47
46
45
12.1
15.0
53.1
14.8
14.2
11.5
9.8
1.08
1.18
1.73
1.17
1.15
1.06
0.99
0.98
0.92
1.40
1.04
1.14
1.14
1.12
1.22
1.35
1.30
1.01
0.94
1.02
0.88
1.13
1.15
1.14
1.10
1.04
1.03
1.01
0.96
0.98
1.35
1.24
1.15
1.11
0.97
1.01
0.92
0.76
1.39
1.34
9.4
7.1
14.3
7.3
9.7
6.3
7.6
8.3
9.4
5.8
3.0
2.8
2.7
2.7
2.8
3.7
3.6
2.2
2.7
2.6
2.7
5.9
5.9
5.9
6.0
5.9
5.7
5.8
5.7
6.0
3.1
2.5
2.7
2.6
2.6
2.7
0.23
0.18
0.00
y0.17
y0.15
y0.27
0.78
0.06
0.23
0.23
0.18
H
p-Me
Ž
.
p- i-Pr
9.6
8.2
p-OCH3
p-NO2
p-F
p-Cl
p-Br
p-I
m-Cl
o-Cl
o-Br
H
25.1
10.9
13.9
13.7
13.2
16.5
22.5
19.7
10.3
8.7
RTPPMnIIICl
0.37
0.00
y0.17
y0.15
y0.27
0.39
0.37
0.34
0.23
0.06
p-CH3
Ž
.
p- i-Pr
10.4
7.5
p-OCH3
m-Br
m-Cl
m-F
p-Cl
p-F
III
w
w
x
RTPPFe 2O
13.5
14.1
13.7
12.6
11.1
10.7
10.2
9.0
H
0.00
m-CH3
y0.07
y0.15
y0.17
0.37
0.23
0.06
Ž
.
p- i-Pr
p-CH3
m-Cl
p-Cl
p-F
9.5
III
x
RTPPMn 2O
22.1
17.2
14.1
12.7
9.4
10.2
8.3
5.8
H
0.00
Ž
.
p- i-Pr
y0.15
y0.17
y0.27
y0.83
p-CH3
p-OCH3
p-NMe2
o-Cl
2.7
a
8.5
55
53
24.2
21.8
5.0
5.3
o-Br
a Trace ketone.
these metalloporphyrins. The values of the reac-
tion constant k for porphyrins with the same
metal atom are nearer than those for porphyrins
with different metal atoms, which means that
there might be a tiny difference between the
reaction mechanism catalyzed by iron por-
phyrins and that catalyzed by manganese por-
phyrins.
3.2. Linear free-energy relationship in self-
oxidation of metalloporphyrins
Ž
.
in
The linear relationship between the yield %
Ž .
of cyclohexanol and the reaction time
t
cyclohexane hydroxylation catalyzed by metal-
loporphyrin 1–40 exists only during 90–120
min of the early reaction stage, then this linear

(
)
C.-C. Guo et al.rJournal of Molecular Catalysis A: Chemical 157 2000 31–40
37
Just as cyclohexane hydroxylation is cat-
alyzed by metalloporphyrins, there is also linear
free-energy relationship in the self-oxidation of
Ž
.
metalloporphyrins Table 5 .
The above linear free-energy relationships
show that self-oxidation of such monometallo-
porphyrins as RTPPFeIIICl and RTPPMnIIICl is
sensitive to substituents on porphyrin rings. All
substituents accelerate the reactions, whether
electron-releasing groups or electron-attracting
groups, suggesting a U-shaped Hammett corre-
Fig. 3. Hammett relationship between rate constant k and sub-
stituent constant s for substituent on porphyrin ring in cyclohex-
lation. However, the self-oxidation of m-oxo-bi-
III
Ž .
a
ane hydroxylation catalyzed by different metalloporphyrins.
w
x
sporphinatoiron, RTPPFe 2O, is not sensitive
to substituents on porphyrin rings. Electron-at-
tracting substituents slightly favor the reaction.
Comparing the above results with Table 2, it
has been shown that the effect of substituents on
the rates in self-oxidation of metalloporphyrins
agrees with the effect of substituents on the
yields in cyclohexane monooxygenation cat-
alyzed by metalloporphyrins.
III
for RTPPMnIIICl;
for RTPPFeIIICl;
w
x
Ž .
b
Ž .
c
For RTPPMn 2O;
III
Ž .
w
x
d for RTPPFe 2O.
relationship deviates gradually when the reac-
tion time is prolonged. This relationship be-
tween the reaction yields and reaction time in
the cyclohexane hydroxylation agrees with the
w x
result reported in the literature 4 . This phe-
nomenon shows that cyclohexane monooxy-
genation and self-oxidation of metallopor-
phyrins under the action of PhIO take place
simultaneously. The latter reaction causes met-
alloporphyrins to lose their catalytic activity.
The above relationship between the reaction
yields and the reaction time is the result of
competition between the two reactions with each
other. The kinetic property of the self-oxidation
of metalloporphyrins under the action of PhIO
is found to be related to both the structures of
the metalloporphyrin and the metal atom in
metalloporphyrin. The studies prove that this is
a second-order reaction for iron-porphyrin,
RTPPFeIIICl, and a first-order reaction for man-
3.3. Hydroxylation mechanism
So far all the abovementioned studies show
that the active species formed in hydrocarbon
hydroxylation catalyzed by monometallopor-
phyrin is a radical cation of a high-valence
V
w
xq
Ž
.
metalloporphyrin, PM –O
MsFe, Mn . In
this reaction, metalloporphyrin reacts first with
the oxidant PhIO to form the radical cation
intermediate, then this intermediate abstracts a
hydrogen atom from hydrocarbon to form a
w
x
product alcohol 55–62 . In this work, the val-
ues of the reaction constant r obtained by
ganese porphyrin, RTPPMnIIICl or m-oxo-bi-
Table 3
III
w
x
sporphinatoiron, RTPPFe 2O. The differences
among the kinetic properties in the self-oxida-
tion of the different metalloporphyrins show
that there are some differences in the reaction
mechanisms of their self-oxidation. The rate
constants in self-oxidation of some metallopor-
phyrins, measured by spectrophotometry, are
listed in Table 4.
Linear free-energy relationship in cyclohexane hydroxylation cat-
alyzed by metalloporphyrins
Metalloporphyrin
Hammett relationship
Relative
coefficient r
RTPPFeIIICl
log ks0.46s y5.03
log ks0.34s y5.00
log ks0.47s y4.98
log ks0.53s y4.90
0.9963
0.9927
0.9978
0.9972
III
w
x
RTPPFe 2O
RTPPMnIIICl
III
w
x
RTPPMn 2O

(
)
38
C.-C. Guo et al.rJournal of Molecular Catalysis A: Chemical 157 2000 31–40
Table 4
Rate constant of self oxidation of some metalloporphyrins
III
III
w
x
w x
RTPPMn 2O
Substituent
Substituent
constant s
RTPPFeIIICl
RTPPFe 2O
k2 =103
log k2
k1 =102
ylog k1
k1 =102
ylog k1
y1
y1
y1
l mol miny1
min
min
Ž
.
Ž
.
Ž
.
H
p-F
p-Cl
p-Br
p-I
p-CH3
p- i-Pr
p-OCH3
m-F
m-Cl
m-Br
m-CH3
o-Cl
0.00
0.06
0.23
0.23
0.18
0.17
0.15
0.27
0.34
0.37
0.39
y0.07
2.727
3.4354
2.745
2.841
3.159
0.5614
0.5465
0.5004
2.711
4.122
0.5668
0.3849
6.786
6.770
5.3375
5.294
5.580
7.237
3.8316
3.8306
3.7274
3.7237
3.7466
3.8596
2.378
1.816
5.335
3.426
3.551
3.551
3.713
2.642
0.6238
0.7409
0.2729
0.4652
0.4497
0.4497
0.4303
0.5781
6.035
6.400
5.335
0.2193
0.1938
0.2729
Ž
.
4.901
4.189
0.3098
0.3779
0.903
2.9557
Hammett relationship changed from 0.3 to 0.5
for the cyclohexane oxidation catalyzed by met-
alloporphyrins. This means that the reaction is
the radical cation mechanism. The fact that
electron-withdrawing groups increase the reac-
tion speeds agrees with the fact that the rate-de-
termining step is hydrogen radical abstraction.
The plots in Fig. 2 have a good linear relation-
ship up to 24% conversion of the PhIO and this
can be satisfactorily explained in terms of the
radical cation and hydrogen radical abstraction
in the reactions catalyzed by them. However,
w
x
the facts reported in the literature 64,65 and
the research results show that the catalytic pro-
cess of iron porphyrins is not the same as that of
the manganese porphyrins. Based on the facts
reported in the literature and the results in this
work, a reaction mechanism can be proposed
for the reaction systems consisting of metallo-
porphyrins, PhIO and hydrocarbons, as shown
in Fig. 4.
There are the same linear free-energy rela-
tionships between the self-oxidation of metallo-
porphyrins and hydrogen radical abstraction in
hydroxylation, which show that self-oxidation
of metalloporphyrins and hydrogen radical ab-
straction in the system of the cyclohexane hy-
droxylation are two competing reactions via the
w
x
mechanism 63 . Furthermore, there are the same
linear free-energy relationships and the ap-
proaching values of reaction constant r in reac-
tions catalyzed by monometalloporphyrins or
m-oxo-bismetalloporphyrins, which shows that
there are similar intermediates and mechanisms
Table 5
Linear free-energy relationship in self-oxidation of metalloporphyrins
Metalloporphyrin
RTPPFeIIICl
Hammett relationship
Relative coefficient r
Ž
.
log k2 s1.7106sq3.4326 electron-attracting substituent
0.9989
0.9981
0.9942
0.9987
0.9990
Ž
.
log k2 sy1.5734sq3.4396 electron-releasing substituent
log k1 s0.3973sy0.5682
III
w
x
RTPPFe 2O
RTPPMnIIICl
Ž .
log k1 s0.7025sy0.5668 electron-attracting substituent
Ž
.
log k1 sy2.4870sy0.5668 electron-releasing substituent

(
)
C.-C. Guo et al.rJournal of Molecular Catalysis A: Chemical 157 2000 31–40
39
Fig. 4. .Mechanism of hydrocarbon hydroxylation catalyzed by metalloporphyrins.
same active intermediate. The values of the
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