Manganese(III) porphyrins as catalysts for the oxidation of aromatic substrates: An insight into the reaction mechanism and the role of the cocatalyst

Article abstract of DOI:10.1016/j.molcata.2006.02.043

Manganese porphyrins bearing electron-withdrawing nitro substituents on the beta-positions catalyse the oxidation of activated aromatic compounds by hydrogen peroxide. The reaction yields based on the starting compounds, are largely lowered by complexation of the metal by the final or starting phenolic compounds and depend on the concentration of the added co-catalyst, e.g. imidazole. Using hindered silyl or sec-butyl ether derivatives of cresol, the reaction gives better results. In this case, it is possible to isolate the corresponding benzyl alcohol and benzoic acid derivatives in a moderate yield for such substrates. For the sec-butyl ether derivative, the corresponding quinone has also been isolated. The conversion of the benzyl alcohols into the benzoic acid derivatives under the experimental conditions was also demonstrated.

Full text of DOI:10.1016/j.molcata.2006.02.043

Journal of Molecular Catalysis A: Chemical 252 (2006) 96–102
Manganese(III) porphyrins as catalysts for the oxidation of aromatic
substrates: An insight into the reaction mechanism and
the role of the cocatalyst
Pietro Tagliatestaa,∗, Davide Giovannettia, Alessandro Leonia,
b
b,∗
´
Maria G.P.M.S. Neves , Jose A.S. Cavaleiro
a Dipartimento di Scienze e Tecnologie Chimiche, Universita` di Roma-Tor Vergata, Via della Ricerca Scientifica, 00133 Roma, Italy
b Departamento de Quimica, Universidade de Aveiro, 3810-193 Aveiro, Portugal
Received 13 December 2005; received in revised form 8 February 2006; accepted 8 February 2006
Available online 27 March 2006
Abstract
Manganese porphyrins bearing electron-withdrawing nitro substituents on the beta-positions catalyse the oxidation of activated aromatic com-
pounds by hydrogen peroxide. The reaction yields based on the starting compounds, are largely lowered by complexation of the metal by the final
or starting phenolic compounds and depend on the concentration of the added co-catalyst, e.g. imidazole.
Using hindered silyl or sec-butyl ether derivatives of cresol, the reaction gives better results. In this case, it is possible to isolate the corresponding
benzyl alcohol and benzoic acid derivatives in a moderate yield for such substrates. For the sec-butyl ether derivative, the corresponding quinone has
also been isolated. The conversion of the benzyl alcohols into the benzoic acid derivatives under the experimental conditions was also demonstrated.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Manganese(III); Metalloporphyrins; Oxidation; Hydrogen peroxide; Aromatic compounds
1. Introduction
paring with the substrates. In this case, the total yield, calculated
from the oxidant, is generally high but the conversion of the
substrates looks quite negligible. However, some of us recently
found that the oxidation of some aromatic compounds can be
performed with an excess of hydrogen peroxide, depending on
the substrates, the catalyst and the reaction conditions [22–24].
These facts prompted us to investigate in detail which param-
eters affect the reaction and look for the possibility to quan-
titatively oxidize the substrates in order to obtain valuable
derivatives which can be useful for further transformations. In
particular, we focused our attention on the complexation equi-
librium of the metalloporphyrins with the phenolic compounds
which could inhibit further oxidation of the metal center.
Complexation of metalloporphyrins by added ligands [25] is
a feature well known in literature; however, to our knowledge, a
correlation between the reactivity of an aromatic ring in oxida-
tion reactions and the presence of phenolic compounds as axial
ligands is not evident. In this paper, we will report on the results
obtained for the optimization of the oxidation conditions and on
evidences of the inhibition of the catalyst activity by the final
phenolic products.
Manganese(III) porphyrins have been studied for a long time
as catalysts able to mimic the in vivo activity of the family of
the cytochrome P450 dependent monooxygenases [1,2].
Several papers have reported on the epoxidation reactions
of alkenes and hydroxylation of alkanes in association with
a wide variety of oxygen donors such as iodosylarenes [3–5],
peroxo-compounds [6–8], hypochlorites [9–12], hydrogen per-
oxide [13–16] or monopersulfates [17]. Less information was
available for the oxidation reaction of aromatic substrates catal-
ysed by metalloporphyrins although some authors reported the
formation of phenols and quinones as the final products of the
metalloporphyrinsoxidationofelectron-richaromaticsubstrates
by hydrogen peroxide [18–21].
In such papers, the yield of the reactions has been reported
versus the moles of the oxidant, which is always in defect com-
∗
Corresponding authors. Tel.: +39 06 72594759; fax: +39 06 72594754.
E-mail address: pietro.tagliatesta@uniroma2.it (P. Tagliatesta).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.02.043

P. Tagliatesta et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 96–102
97
2. Experimental
evaporated under vacuum. The residue was purified by col-
umn chromatography on silica gel, eluting with petroleum ether.
The fractions containing the desired compound were collected
and evaporated under vacuum giving 0.984 g (4.43 mmol) of a
colourless oil. Yield 91%.
2.1. General remarks
Chromatographic purifications were performed on silica gel
(35–70 mesh, Merck) columns. Thin-layer chromatography was
carried out using Merck Kiesegel 60-F254 plates. H NMR
spectra were recorded as CDCl3 solutions on a Bruker AM
400 instrument using tetramethylsilane (TMS) as an inter-
nal standard. FAB Mass spectra were measured on a VG-
Quattro spectrometer using 3-nitrobenzyl alcohol (NBA) as
a matrix. GC–MS spectra were obtained with a VG-Quattro
spectrometer equipped with a 30 m Supelco SPB-5 capillary
column.
1H NMR (400 MHz, CDCl3) δ = 7.0 (d, 2H, J = 8.0 Hz), 6.72
(d, 2 H, J = 8.0 Hz), 2.26 (s, 3 H), 0.96 (s, 9 H), 0.14 (s, 6 H); EI
MS: m/z (%) 222 (20), 165 (100), 149 (3), 135 (3), 91 (15).
1
2.6. sec-Butyl 4-methylphenyl ether, 9
1.103 g (10.2 mmol) of 4-methylphenol were dissolved,
under argon, in 10 mL of anhydrous acetone in a 100 mL round
bottomed flask. 2-Bromobutane, 4.5 g (31.1 mmol) and anhy-
drous potassium carbonate 2.0 g (14.5 mmol) were added at
room temperature. The solution was then refluxed for 14 h and
after a TLC checking, 4.5 g of 2-bromobutane were added and
the solution was further refluxed for 8 h. The solution was then
filtered and the solvent was evaporated under vacuum. The
residue, redissolved in dichloromethane, was washed with water
and the organic solution was dried on anhydrous sodium sulfate,
filtered and evaporated under vacuum. The residue was purified
by column chromatography on silica gel, eluting with petroleum
ether. The fractions containing the desired compound were col-
lected and evaporated under vacuum giving 0.888 g (5.41 mmol)
of a colourless oil. Yield 53%.
2.2. GC separation conditions
The products yield for all the reactions was determined
by GC analyses performed on a Carlo Erba HRGC 5160
instrument equipped with a 30 m Supelco SPB-5 capillary col-
umn and a FID detector. Chemical yields were determined
by adding a suitable internal standard (dodecane, tetradecane
or acetophenone) to the reaction mixture at the end of each
experiment and were reproducible within 2% for multiple
experiments.
1H NMR (400 MHz, CDCl3) δ = 7.2 (d, 2 H, J = 9 Hz), 6.9 (d,
2 H, J = 9 Hz), 4.35 (m, 1 H), 2.4 (s, 3 H), 1.7 (m, 2 H), 1.4 (d,
3 H, J = 6 Hz), 1.1 (t, 3 H, J = 7 Hz); EI MS: m/z (%) 164 (25),
108 (100), 77 (20).
2.3. Chemicals
All the reagents and solvents (Carlo Erba, Aldrich, Fluka or
Acros) were of the highest analytical grade and used without
further purification.
2.7. Methyl 4-sec-butyloxybenzoate
2.4. Synthesis
3.7 mg of 4-sec-butyloxybenzoic acid were dissolved in 1 mL
ofBF3·CH3OH(1.3 M)andthesolutionwasmaintainedat50 ◦C
for 3 h.
The free bases H2TDCPP, 1 was synthesized by literature
methods [26], while the H2(2-NO2)TDCPP, 2 was obtained by
the standard nitration reaction of the free bases with Zn(NO3)2
and subsequent demetallation by CF3CO2H [27]. The man-
ganese derivative, of H2(2-NO2)TDCPP 3 was obtained by
literature methods [28]. Mn[(Cl8)TDCPP]Cl, 5 was obtained
from Mn(TDCPP)Cl by a method reported in the literature [29].
Sodium phenoxide standard solution was obtained by reac-
tion of an excess of sodium with phenol in anhydrous THF
and the title was obtained by titration with a standard HCl
solution.
The solution was then evaporated and the residue redissolved
in ethyl acetate. This solution was washed with water, dried on
anhydrous sodium sulfate and the solvent was evaporated under
vacuum giving the desired compound quantitatively.
1H NMR (400 MHz, CDCl3) δ = 8.07 (d, 2 H, J = 9.1 Hz),
6.96 (d, 2 H, J = 9.1 Hz), 4.47 (m, 1 H), 3.83 (s, 3 H), 1.79
(m, 1 H), 1.69 (m, 1 H), 1.39 (d, 3 H, J = 6.1 Hz), 1.02 (t, 3 H,
J = 8 Hz); EI MS: m/z (%) 152 (42), 121 (100), 79 (45).
2.8. Methyl 4-tert-butyldimethylsilyloxybenzoate
2.5. tert-Butyldimethylsilyl 4-methylphenyl ether, 6
3.9 mg of 4-tert-butyldimethylsilyloxy-benzoic acid were
dissolved in 1 mL of BF3·CH3OH (1.3 M) and the solution was
maintained at 50 ◦C for 3 h. The solution was then evaporated
and the residue redissolved in ethyl acetate. This solution was
washed with water, dried on anhydrous sodium sulfate and the
solvent was evaporated under vacuum giving the desired com-
pound quantitatively.
0.526 g (4.87 mmol) of 4-methylphenol were dissolved,
under argon, in 5 mL of anhydrous DMF in a 10 mL
round bottomed flask. tert-Butyldimethylsilyl chloride, 1.016 g
(6.75 mmol) and imidazole, 0.811 g (11.9 mmol) were added at
room temperature. The solution was stirred for 2 h, after that
the solvent was evaporated under vacuum and the residue redis-
solved in dichloromethane. The organic solution was washed
with water, dried on anhydrous sodium sulfate, filtered and
1H NMR (400 MHz, CDCl3) δ = 8.04 (d, 2 H, J = 9.1 Hz),
6.92 (d, 2 H, J = 9.1 Hz), 3.85 (s, 3 H), 1.04 (s, 9 H), 0.25 (s,
6 H); EI MS: m/z (%) 238 (72), 223 (100).

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P. Tagliatesta et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 96–102
2.9. General procedure for the oxidation reactions
2.12.4. 4-sec-Butyloxy-benzyl alcohol, 10
1H NMR (400 MHz, CDCl3) δ = 7.30 (d, 2 H, J = 11 Hz), 6.89
(d, 2 H, J = 11 Hz), 4.63 (s, 2 H), 4.31 (m, 1 H), 1.75 (m, 1 H),
1.63 (m, 1 H), 1.31 (d, 3 H, J = 6 Hz), 0.99 (t, 2 H, J = 5 Hz); EI
MS: m/z (%) 180 (15), 124 (93), 106 (87), 95 (100), 78 (48).
In a 10 mL flask were introduced 3 mL of acetonitrile,
0.37 mmol of substrate, 0.37 ␮mol of the catalyst and 3.7 mmol
of ammonium acetate. 0.4 mL of a 3.5% solution of hydrogen
peroxide, prepared from the 35% commercial solution by dilu-
tion with acetonitrile, were slowly added. The progress of the
reaction was monitored by GC and the reaction was stopped
after the disappearing of the starting compound. The solution
was diluted with water and extracted with dichloromethane. The
organic solution was then washed with water, dried on anhy-
droussodiumsulfateandevaporatedundervacuum. Thereaction
products were isolated by column chromatography on silica gel,
eluting with petroleum ether/diethyl ether mixtures.
2.12.5. 4-sec-Butyloxy-benzoic acid, 11
1H NMR (400 MHz, CDCl3) δ = 8.05 (d, 2 H, J = 9.2 Hz),
6.93 (d, 2 H, J = 9.2 Hz), 4.42 (m, 1 H), 1.78 (m, 2 H), 1.67 (m,
12 H), 1.35 (d, 3 H, J = 6 Hz), 1.0 (t, 3 H, J = 8 Hz).
3. Results and discussion
The biomimetic oxidation of organic substrates by metallo-
porphyrin catalysis has been deeply studied in the last twenty
years by several groups and the use of hydrogen peroxide as
environment friendly oxygen donor has been well established
[30]. For such a reason, we decided to use, for our studies, that
oxidant which is actually considered, together with the molec-
ular oxygen, the clean reagent for a green biomimetic approach
[31].
Considering certain literature content, at the beginning of this
work we decided that the electron-poor manganese porphyrins
were our primary targets for studying the catalytic oxygenation
of aromatics [18–24]. The catalysts used for this work have been
Mn[(2-NO2)TDCPP]Cl, 3, where(2-NO2)TDCPPisthedianion
of 2-nitro-5, 10, 15, 20-tetrakis(2ꢀ,6ꢀ-dichlorophenyl)porphyrin
and Mn[(Cl8)TDCPP]Cl, 5, where (Cl8)TDCPP is the dian-
ion of 2,3,7,8,12,13,17,18-octachloro-5,10,15,20-tetrakis(2ꢀ,6ꢀ-
dichlorophenyl)porphyrin. The synthetic approaches for obtain-
ing the manganese porphyrins follow the literature procedures
[25–28] and are reported in Scheme 1.
The first observations we made on the oxidation of anisole by
hydrogen peroxide and the manganese catalysts, followed what
was previously reported by Mansuy and co-workers, and such
observations were in perfect agreement with their data [18]. In
fact, when we tried to oxidize the substrates using both porphyrin
catalysts and imidazole as axial ligand, under Mansuy condi-
tions, thetotalyieldoftheproducts, the2-and4-methoxyphenol,
based on the substrate amounts used never exceeded 1%. Further
experiments performed with excess of the oxidant, never gave
more than 1% yield in phenolic compounds.
This fact was quite surprising and the further step was then
to investigate the coordination capability of the two methoxy-
phenol derivatives towards the manganese porphyrins. In Fig. 1,
the UV–visible spectra of the manganese porphyrin 5, before
and after the addition of 2-methoxyphenol and after the addition
of imidazole, are reported.
2.10. Epoxidation of styrene in the presence of 2- or
4-metoxyphenol
In a 10 mL flask were introduced 4 mL of a solution pre-
pared by dissolving 6.8 mg (5.5 ␮mol) of the catalyst 3, 10.7 mg
(0.157 mmol) of imidazole, and 447.6 mg (4.3 mmol) of styrene
in 10 mL of 1:1 acetonitrile/dichloromethane. Hydrogen per-
oxide, 80 mL (0.91 mmol) was added in four portions every
2 min. After 4 min from the last addition, tetradecane was
added as internal standard and the mixture was analyzed
by GC.
2.11. Kinetic measurements
In a 10 mL flask were introduced 3 mL of acetonitrile,
0.44 mmol of substrate, 0.49 ␮mol of the catalyst 5, 4.4 mmol of
ammonium acetate and 0.79 mmol of acetophenone as internal
standard. The standard is stable under the oxidation conditions
even after hours. Four millilitres of a solution of hydrogen per-
oxide, prepared from the commercial 35% solution by dilution
with acetonitrile 1/10, were slowly added in 8 portions waiting
15 min between two consecutive additions. The progress of the
reaction was monitored every 15 min by GC.
2.12. Characterization of the reaction products
2.12.1. 4-tert-Butyldimethylsilyloxy benzyl alcohol, 7
1H NMR (400 MHz, CDCl3) δ = 7.24 (d, 2 H, J = 7.8 Hz),
6.85 (d, 2 H, J = 7.8 Hz), 4.62 (m, 1 H), 2.62 (s, 2 H), 1.0 (s,
9 H), 0.21 (s, 6 H); EI MS: m/z (%) 238 (25), 181 (100), 163
(20), 151 (100).
2.12.2. 4-tert-Butyldimethylsilyloxy benzoic acid, 8
1H NMR (400 MHz, CDCl3) δ = 8.02 (d, 2 H, J = 8.1 Hz),
6.90 (d, 2 H, J = 8.1 Hz), 1.02 (s, 9 H), 0.26 (s, 6 H).
From such figure, looking at the shift of the Soret band after
the addition of the phenol, it is possible to argue that the metallo-
porphyrin undergoes complexation with a coordinating species.
Other information can be obtained from the spectra reported in
Fig. 2. In this figure, the spectra of the manganese catalyst 5 after
the simultaneous addition of 5 equivalents of imidazole and 5
equivalents of 2-methoxyphenol in multiple steps are reported.
It is evident that the band at 458 nm is generated by the
complexation of an extra ligand on the metal and not from the
2.12.3. 2-sec-Butyloxy-5-methylbenzoquinone, 12
1H NMR (400 MHz, CDCl3) δ = 6.56 (s, 1 H), 5.88 (s, 1 H),
4.22 (m, 1 H), 2.07 (s, 3 H), 1.8 (m, 1 H), 1.68 (m, 1 H), 1.36 (d,
3 H, J = 6.1 Hz), 0.97 (t, 3 H, J = 6.2 Hz); EI MS: m/z (%) 194
(5), 140 (50), 110 (20), 69 (90), 56 (100).

P. Tagliatesta et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 96–102
99
Scheme 1. Synthesis of the catalysts used in this work.
imidazole which gives a complex with a band at 492 nm. The
next question to be considered was centred on which species is
so powerful to bind the metal ion making it not available for
the hydrogen peroxide coordination. The phenolate anion was a
good candidate and to prove its coordination capability, the man-
ganese porphyrin 5 was titrated with a stock solution of sodium
4-methoxyphenolate (see Section 2). The results are reported in
Fig. 3.
In this case, we have the Soret band shifting to 453 nm which
is near the position of the band when 2-methoxyphenol is added
to the metalloporphyrin; this is the proof that the phenolate anion
binds the metal centre during the oxidation reaction, thus inhibit-
ing the coordination of hydrogen peroxide. From Figs. 1 and 2,
we can also extrapolate that imidazole plays a role during the
complexation through an acid-base equilibrium with phenols.
Fig. 1. UV–vis spectra of 5, before and after the addition of 2-methoxyphenol
and after the addition of imidazole.
Fig. 3. UV–vis spectra of 5 during the titration with a stock solution of sodium
4-methoxyphenate.
Fig. 2. UV–vis spectra of 5 after the contemporary addition of 5 equivalents of
imidazole and 5 equivalents of 2-methoxyphenol in multiple steps.

100
P. Tagliatesta et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 96–102
In fact after the addition of imidazole to the metalloporphyrin
and 2-methoxyphenol mixture, the Soret band shift to 461 nm, a
valueveryclose to 453 nm. Furthermore, theoxidation of anisole
by catalyst 5, using ammonium acetate as axial ligand under the
conditions reported by other authors [32] for the epoxidation of
olefins was unsuccessful. But an experiment to prove the inhi-
bition of the catalyst has been set up by using the catalyst 5 for
the epoxidation of styrene by hydrogen peroxide.
Two reactions were performed, one of them in the presence
of 0.1 equivalent of 4-methoxyphenol. The epoxidation, in the
presence of 4-methoxyphenol, gave only 30% yield while with-
out it the yield was 92%.
After having found which was the problem in our experi-
ments for the oxidation of the aromatics, the next question was
how to oxidize biomimetically and in high yield such aromat-
ics and prevent the complexation of the metal by the phenolic
products. A possibility could consider the introduction of bulky
groups to protect the ortho and para positions, but maintaining
the electron-donating substitution.
Fig. 4. Percentage of the starting 6 and the final 7 and 8 compounds as a function
of the quantity of added oxidant.
We decided to use a new substrate, 4-methylphenol, pro-
tecting the hydroxy group with the tert-butyldimethylsilyl or
sec-butyl groups. In this way, three effects could be obtained, the
lack of the formation of phenolic complexes with manganese,
the impossibility to oxidize the position 4 and the protection
from the oxidation of the 2 and 6 positions due to the steric
hindrance. The cited compounds were easily synthesized by
standard procedures [33] and purified by column chromatog-
raphy. The oxidation reactions were performed using catalyst 3
because, from preliminary experiments, it was found to be more
active than 5.
Hydrogen peroxide was used in a slight excess (30% versus
the substrate) and the additions were always stopped when the
quantity of remaining substrates, at the GC analysis, appeared to
be constant after two consecutive additions of the oxidant. For
both substrates, the reaction products were isolated by column
chromatography and in Scheme 2 the molecular structures of
the starting compounds (6 and 9) and final products (7, 8, 10,
11, 12) are reported.
In this scheme it can be seen that both the substrates undergo
to the oxidation of the methyl group giving the correspond-
ing benzyl alcohol and benzoic acid derivatives, while only the
Scheme 2. Oxidation reactions of the substrates 6 and 9.

P. Tagliatesta et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 96–102
101
Scheme 3. Oxidation reaction of 2-tert-butyl-5-pentadecylphenol.
Table 1
cumene, the main products were the benzoic acid and the ben-
zylic alcohol derivatives [24]. Finally, the oxidation of the sub-
strate 6 with hydrogen peroxide has also been considered with
catalyst 3 in the presence of an excess of 4-methoxyphenol giv-
ing no reaction products even after several hours of reaction and
thus proving the inhibition by the phenolic products.
Yields of the oxidation reactions on 6 and 9 catalyzed by 5a
Starting
compounds
Yield (%)
Benzyl alcoholsb
Benzoic acidsc
1,4-Quinonesb
6
9
4.1
31.0d
26.8
5.0
–
14.0
a
Reactions carried out at 20 ◦C with a molar ratio substrate/oxidant/porphyrin
4. Conclusions
100/300/1.
b
c
d
Determined by GC analysis.
Determined by GC analysis on the methyl ester derivatives.
The corresponding aldehyde was detected in trace.
In this paper, the inhibition of the manganese porphyrins
catalytic efficiency by the final products of the oxidation of aro-
matic compounds by hydrogen peroxide, has been demonstrated
by UV–vis spectroscopy and reactivity studies. The investiga-
tion also demonstrated that the steric hindrance obtained by the
derivatization of phenols with hindered groups plays a key role
for obtaining the oxidation of the substrates and giving in some
cases moderate yield of quinone or benzoic acid derivatives. It
has been also demonstrated that the benzylic alcohols obtained
in low yields are intermediates in the oxidation reaction of a
methyl group into a carboxylic function.
isobutyl derivative is able to give also the corresponding 1,4-
benzoquinone derivative. In Table 1, the yields of all the reaction
products, determined by GC analysis, are reported. The benzoic
acid derivatives were converted into the methyl ester by standard
procedure [34] for both the GC analysis and characterization.
The reaction on tert-butyldimethylsilyl-4-methylphenyl
ether, 6 was also checked as a function of the quantity of the
added oxidant and the results are reported in Fig. 4. In this figure,
the molar quantities of the starting product, the benzyl alcohol,
7 and benzoic acid, 8 derivatives are reported as a percent.
From the reported data it is possible to see the decrease of
6 and the simultaneous increase of 8. The molar quantity of
alcohol derivative 7 seems to remain constant after each addi-
tion. This fact could be due to the transformation of the benzyl
alcohol into the corresponding acid; this hypothesis has been
proved by running the oxidation reaction on the benzyl alcohol
which in fact is quantitatively converted into the carboxylic acid,
under the reaction conditions. These results are in agreement
with those reported in the papers published by our laboratories
in the past on the oxidation of aromatic compounds. In fact the
oxidation of the 2-tert-butyl-5-pentadecylphenol by hydrogen
peroxide with manganese porphyrins as catalysts, gave the cor-
responding quinone derivatives as the main products with high
conversion of the substrate (see Scheme 3) [35].
This fact in our opinion is due to the steric hindrance that the
two alkyl groups exert on the 1,4-positions of the benzene ring,
avoiding the complexation of the metal by the phenolic products.
In fact the UV–vis titration of the catalyst 3 by 2-methylphenol
does not give any spectral modification even after the addition
of 50 equivalents of the phenol.
Furthermore, the previously reported oxidation of toluene
gives the quinone derivative as the main product using
manganese 2-nitro-tetrakis(pentafluorophenyl)porphyrin and
the benzoic acid when manganese 2-nitro-tetrakis(2,6-
dichlorophenyl)porphyrin was used. For ethylbenzene and
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