The role of the carboxylic acid as co-catalyst in metalloporphyrin catalytic hydrogen peroxide oxidations is discussed, taking into account its dependence on the ratio relatively to the catalyst. The catalytic efficiency and stability of the catalyst in the presence of an excess of carboxylic acid suggests that a metallo-acylperoxo complex can be the effective oxidation intermediate.http://www.sciencedirect.com/science/article/B6TGM-42G6XGY-4/1/0ef5cfc94471f6985d77385ef339282

António M d’A. Rocha Gonsalves

Arménio C Serra

Banfi

Copéret

Fujii

Gross

Groves

Hamada

Kamaraj

Linker

Machii

Mee Goh

Mirla

Novak

Palucki

Quici

Rocha Gonsalves

Traylor

Yamada

d'A. Rocha Gonsalves, António M.

Serra, Arménio C.

Estudo Geral

Journal of Molecular Catalysis A: Chemical 168 (2001) 25–32
On the mechanism of carboxylic acid co-catalyst assisted
metalloporphyrin oxidations
António M. d’A. Rocha Gonsalves, Arménio C. Serra
Departamento de Quı´mica, Faculdade de Ciências e Tecnologia, Universidade de Coimbra,
Rua Larga, P-3000 Coimbra, Portugal
Received 4 August 2000; accepted 29 September 2000
Abstract
The role of the carboxylic acid as co-catalyst in metalloporphyrin catalytic hydrogen peroxide oxidations is discussed,
taking into account its dependence on the ratio relatively to the catalyst. The catalytic efficiency and stability of the catalyst
in the presence of an excess of carboxylic acid suggests that a metallo-acylperoxo complex can be the effective oxidation
intermediate. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Metalloporphyrin; Catalysis; Hydrogen peroxide; Oxidation
1. Introduction
It has been generally accepted that the mechanism
of catalytic oxidation by metalloporphyrins involves a
metallo-oxo structure as the direct substrate oxidant.
This basic structure may vary between the one hav-
ing a pure oxo character such as in manganese met-
alloporphyrins [1–3] and that with a marked radical
nature such as in iron-porphyrin complexes [4–7]. In
similar oxygenation systems based on the manganese
salen catalyst the role of a metallo-oxo complex as
intermediate was also discussed [8–10].
However, the role of metallo-oxo species as the
oxidative intermediate in catalytic metalloporphyrin
oxidations has already been questioned and other
intermediates have been suggested [11–14]. Our pre-
vious results on the study of hydrogen peroxide oxi-
dations catalysed by metalloporphyrins in a two-phase
system assisted by a carboxylic acid co-catalyst [15]
 Corresponding author. Fax: C351-239-826068.
E-mail address: arg@qui.uc.pt (A.M. d’A. Rocha Gonsalves).
suggest that the metallo-acylperoxo species involved
is not a simple precursor of the metallo-oxo direct
oxidative species (Scheme 1), as explained by the
more generally accepted mechanism.
In this paper, we report further studies of hydrogen
peroxide epoxidations in a two-phase system and man-
ganese porphyrins (1 and 2) as catalysts (Scheme 2),
which support the hypothesis of metallo-acylperoxo
complex (I) being the direct oxidative species.
2. Results and discussion
As previously observed, oxidations by 5% hydro-
gen peroxide catalysed by manganese porphyrins are
much faster if an excess of benzoic acid co-catalyst
relatively to metalloporphyrin is used [15]. If we
take the most commonly accepted mechanism for
these catalysed oxidations, the large concentration of
benzoic acid would increase the rate of the reaction
because it benefits the formation of the metallo-oxo
intermediate (II) [16,17] (Scheme 3).
1381-1169/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1381 - 116 9 (00 )00490 -8
26 A.M. d’A. Rocha Gonsalves, A.C. Serra / Journal of Molecular Catalysis A: Chemical 168 (2001) 25–32
Scheme 1.
Scheme 2.
However, the preceding mechanism does not ex-
plain the higher stability of the metalloporphyrin
observed in the presence of the excess of benzoic acid
[15].
In an attempt to clarify the problem, we decided to
study the influence of the structure of the organic acid
on rate of the epoxidation reaction. The epoxidation of
cis-cyclooctene using the manganese complex of meso
(2,6-dichlorophenyl) porphyrin (1) was performed in
the presence of excess (20:1 relatively to catalyst 1) of
different organic acids (Fig. 1). The substrate conver-
Scheme 3.
sion yields were as shown in Fig. 1 and cis-cyclooctene
epoxide was obtained with a selectivity higher than
90%, in all cases, as observed by gas cromathography.
The experimental results show that the structure of
the acid has a very significant influence on the rate
of the reaction. Acids with the same or smaller Ka
values than benzoic acid give faster reactions. On the
other hand slower reactions occur with acids having
a higher Ka than benzoic acid. Particularly slow re-
actions are observed in the cases of 2-chlorobenzoic
and 2,4-dichlorobenzoic acids. The same behaviour is
A.M. d’A. Rocha Gonsalves, A.C. Serra / Journal of Molecular Catalysis A: Chemical 168 (2001) 25–32 27
Fig. 1. Epoxidation of cis-cyclooctene catalysed by 1 in the presence of several organic acids. Reaction conditions: byphasic system;
catalyst:4-tert-butylpyridine:organic acid:alkene:H2O2 (5%) D 1:1:20:300:1300 (except acetic acid with a 1:40 ratio).
observed in the case of the sulphonyl derivative 2
which has proved to be a more active catalyst than 1
[15] (Table 1).
The preceding results cannot be explained by the
mechanism outlined in Scheme 3 [16,17]. In the
metallo-acylperoxo species (I) the more acidic the
acid the better leaving group would be the con-
jugate base. This would help the cleavage of the
oxygen–oxygen bond to originate the metallo-oxo
intermediate (II) and faster reactions were expected,
Table 1
Epoxidation of cis-cyclooctene catalysed by catalyst 2 in the pres-
ence of several organic acidsa
Organic acid Conversion (%) (min)
Benzoic 98 (5)
100 (10)
2-Chlorobenzoic 62 (10)
100 (20)
2,4-Dichlorobenzoic 50 (10)
78 (20)
100 (50)
a Reaction conditions: byphasic system; catalyst:4-tert-butyl-
pyridine:organic acid:alkene:H2O2 (5%) D 1:1:20:300:1300.
contrary to what is observed. Montanari [18] stated
once, without any further comment, to have observed
that electron-withdrawing substituents on the benzoic
acid slightly decrease the rate of epoxidation while
electron-donating groups similarly increased the rate
of the reaction.
Since it became apparent that the excess of ben-
zoic acid directly intervenes on the mechanism of the
reaction, we decided to clarify the role of the acid
co-catalyst studying the influence of the same organic
acids, previously studied, on the rate of epoxidation
of cis-cyclooctene using this time a 1:1 ratio relatively
to the catalyst 1 (Fig. 2).
The reaction rate in the presence of an equimolar
amount of the organic acid is slower than the one ob-
served when an excess is used. However, under these
conditions and as shown in Fig. 2 the organic acids
with electron-withdrawing substituents originate faster
reactions than benzoic acid. Noteworthy are the cases
of 2,4-dichlorobenzoic acid and 4-nitrophenylacetic
acid which give even much faster reactions when
the acid is present in an equimolar amount than in
the presence of an acid excess. On the contrary the
acids which gave the fastest reactions when used in
20:1 ratio, phenylacetic and nonanoic acid, gave the
28 A.M. d’A. Rocha Gonsalves, A.C. Serra / Journal of Molecular Catalysis A: Chemical 168 (2001) 25–32
Fig. 2. Epoxidation of cis-cyclooctene catalysed by (1) in the presence of several organic acids. Reaction conditions: byphasic system;
catalyst:4-tert-butylpyridine:organic acid:alkene:H2O2 (5%) D 1:1:1:300:1300.
slowest reactions when used in an equimolar amount.
The mechanism presented in Scheme 3 is, therefore,
suitable to explain the results of the reactions where
equimolar amounts of the acid co-catalyst is used.
Concerning the stability of the catalyst to reaction
conditions, it also proved to be highly dependent on
the relative amount of organic acid used as shown in
Fig. 3.
It is noteworthy that the catalyst is always more
stable in the presence of an excess of organic acid such
as previously shown for benzoic acid [15]. It should be
emphasised that in the presence of excess of benzoic,
Fig. 3. Comparative values of the stability of the catalyst (1) in the reactions of Figs. 1 and 2.
phenylacetic and nonanoic acids the residual amount
of catalyst is higher even after a larger number of
catalytic cycles.
The preceding observations lead us to suggest two
different and competitive mechanistic pathways for the
oxidations under our reaction conditions (Scheme 4).
The metallo-hydroperoxo complex (III) species
can follow two different interconnected pathways de-
pending on the amount of organic acid. If only an
equimolar amount of the acid is present, the route
leading to the cleavage of the oxygen–oxygen bond
and the formation of the metallo-oxo complex (II),
A.M. d’A. Rocha Gonsalves, A.C. Serra / Journal of Molecular Catalysis A: Chemical 168 (2001) 25–32 29
Scheme 4.
path A is dominant [19]. Stronger acids will be the
most efficient, in this case, giving faster conversion
of (III) to (II) as shown in the results of Fig. 2. In
the presence of an excess of the acid co-catalyst,
pathway B becomes the dominant one leading to the
formation of the metallo-acylperoxo intermediate (I)
by nucleophilic attack of a molecule of the acid to
the oxygen atom near the metal and loss of a water
molecule aided by a second molecule of the acid.
This kind of attack was already suggested to explain
the formation of intermediate (I) by organic acids
[19–20] and the oxidation of sulphur compounds by
co-ordinated hydroperoxo complexes [21].
Scheme 5.
The formation of the metallo-acylperoxo species
(I) through pathway B can be favoured by a spe-
cific arrangement of the catalyst and organic acid at
an interface region, as we have suggested before [22]
(Scheme 5).
The excess of an acid co-catalyst having electron-
withdrawing substituents does not benefit the forma-
tion of metallo-acylperoxo species (I) because the
attack to the metallo-hydroperoxo species (III) by the
nucleophile is less efficient. That type of acid also
contributes to diminish the amount of species (III)
which is available because it promotes the formation
of the species (IV) having a carboxylate as axial
30 A.M. d’A. Rocha Gonsalves, A.C. Serra / Journal of Molecular Catalysis A: Chemical 168 (2001) 25–32
Scheme 6.
ligand [23] favouring the shift of the equilibrium
shown in Scheme 6 to the right hand side.
The increased catalytic efficiency observed in the
presence of excess of organic acid, particularly the
increased stability of the catalyst, can be under-
stood if the metallo-acylperoxo species (I) formed
is reactive enough to directly oxidise the substrate
without requiring its conversion to the metallo-oxo
intermediate (II). By avoiding the formation of this
highly oxidative species (II) which is referred as
being directly related to the oxidative destruction of
the macrocycle or axial ligand [24], route B con-
tributes to the stability of the catalyst. In the pres-
ence of hydrogen peroxide, species (II) may favour
the formation of reactive radicals [25] which can
be directly responsible for the degradation of the
catalyst.
The metallo-acylperoxo species resembling (I)
was firstly suggested as the oxidising species by
Watanabe et al. [14], and more recently the works
of Nam [26] and Bandyopadhyay [27] showed that
an metallo-acylperoxo complex is capable of directly
oxidising substrates. Similarly metallo-acylperoxo
complexes were also suggested as intermediates for
Fig. 4. Values of absorvances of samples of catalyst (1) subject to catalytic conditions but without substrate. Reaction conditions: byphasic
system; catalyst:4-tert-butylpyridine:organic acid:H2O2 (5%) D 1:1:1:1300 or 1:1:20:1300.
epoxidations of alkenes with oxygen in a reaction
where an aldehyde is the added co-oxidant [28,29].
Although the visible spectra of species (I) and (II)
of pathways A and B must be distinct [30,31], we
could not detect any spectroscopic difference for the
solution either in the presence of equimolar or ex-
cess of benzoic acid. It is not unlikely that under
the experimental conditions which were used, the
amount of any of the species (I) or (II) may well
be so low that they are not detected. They can be
quickly reduced when formed, possibly by reaction
with the hydrogen peroxide as proposed by Traylor
[25].
We studied the stability of catalyst (1), measured
by the value of absorbance for the Soret band, in the
presence of different quantities of benzoic acid and in
absence of alkene for a period of 60 min. The results
are plotted in Fig. 4.
This experiment showed that the catalyst is stable
in the absence of substrate implying that the catalyst
inactivation is promoted by intermediates generated
in the catalytic cycle. A similar behaviour was re-
ferred to in methyltrioxorhenium catalysis [32]. In an
experiment where an excess of cis-cyclooctene (200:1
relatively to the catalyst) was added after 40 min un-
der the conditions described in Fig. 4 gave only 30%
of conversion (either for benzoic acid/catalyst ratios
of 20/1 and 1/1) after an extra 40 min. This is to be
expected if destruction of the axial ligand, required
for an efficient catalytic process, occurred in the first
40 min making the reaction very slow under such
conditions.
A.M. d’A. Rocha Gonsalves, A.C. Serra / Journal of Molecular Catalysis A: Chemical 168 (2001) 25–32 31
Table 2
Epoxidation of cis-cyclooctene catalysed by catalyst 1 in the pres-
ence of organic acids in monophasic systema
Organic acid Conversion
(%) (min)
Benzoic 0 (10)
0 (20)
Phenylacetic 4 (10)
4 (20)
4-Nitrophenylacetic 4 (10)
4 (20)
a Reaction conditions: monophasic system; catalyst:4-tert-
butylpyridine:organic acid:alkene:H2O2 (30%) D 1:1:20:300:600.
The possible existence of a relation between cat-
alytic oxidative cycles and stability of the catalyst is
consistent with our proposal that route B, avoiding the
metallo-oxo, explains the higher catalytic efficiency
and catalytic stability observed with high concentra-
tion of carboxylic acid co-catalyst.
When replacing the reaction conditions of the
oxidation system, by changing the medium from
biphasic to monophasic by addition of methanol, the
rate of epoxidation is greatly affected as registered
in Table 2. The results show a complete suppression
of the reaction which may be due to destruction of
the interface and the organised structure proposed in
Scheme 5. Another possibility is the formation of
strong hydrogen bonds between methanol and the
organic acids, like the ones which occur with benzoic
acid [33], so blocking the formation of species (I)
or (II).
3. Conclusions
The metalloporphyrin catalytic oxidations have
been the object of intensive studies leading to the
acceptance of a unique simple mechanism involving
a metallo-oxo species as the direct oxidant of the
substrates. In a two-phase system and with hydrogen
peroxide as oxidant a careful study of the dependence
of the efficiency and stability of the catalyst on the
structure and amount of the carboxylic acid used as
co-catalyst proved that the metallo-oxo oxidant inter-
mediate cannot explain the observed behaviour and
that a metallo-acylperoxo species is the more likely
candidate to be the true substrate oxidant.
4. Experimental
Dichloromethane was distilled from CaCl2 before
use. Methanol was dried and distilled. Others sol-
vents used were commercially available and used as
received. cis-Cyclooctene was obtained from Aldrich
and passed through a short column of alumina be-
fore use. Hydrogen peroxide 5% was prepared be-
fore use from a concentrated solution from Riedel
titrated by iodometry. The pH of this solution was
set to 4.5–5 with hydrogen carbonate. Catalysts 1
and 2 were prepared following a previously de-
scribed procedure [22]. Benzoic and 4-methylbenzoic
acids was purchased from Fluka, 4-nitrophenylacetic
and phenylacetic acids from Merck, acetic acid
from Riedel, 2,4-dichlorobenzoic, 2-chlorobenzoic,
2-methylbenzoic, 4-methoxybenzoic, 4-nitrobenzoic,
nonanoic and dodecanoic acids from Aldrich.
Gas chromatography analyses were performed on
a Hewlett-Packard 5890A instrument with a flame
ionisation detector and equipped with a Hp-Ultra-1
(25 m  0:25 mm, i.d.) capillary column. UV/visible
spectra were obtained on a Hitachi-220S or on a Jasco
7800.
Catalytic reactions were carried out at room temper-
ature. They were monitored by removing aliquots and
examining the products by gas chromatography. The
epoxide of cis-cyclooctene were identified by compar-
ison of the chromatographic retention time with that
of an authentic sample prepared independently and
characterised by usual methods, NMR, MS and IR.
Conversions are reported relatively to bromobenzene
as internal standard.
General procedure for oxidation reactions: a 20 ml
flask is charged with 2:5  10−3 mmol of the met-
alloporphyrin and 2 ml of a solution of organic acid
in CH2Cl2 with a concentration that gave the desired
organic acid/catalyst ratio. Then the internal standard,
the axial ligand and the alkene were added. The reac-
tion was started by addition of 2 ml of hydrogen per-
oxide solution (5%) and stirred at maximal rate. For
monophasic system the organic acid were dissolved
in 2 ml of a solution of dichloromethane/methanol
(1/1) and 30% hydrogen peroxide was used.
32 A.M. d’A. Rocha Gonsalves, A.C. Serra / Journal of Molecular Catalysis A: Chemical 168 (2001) 25–32
Acknowledgements
The authors would like to thank Chymiotech-
non for financial support and Universidade Católica
Portuguesa for leave of absence (ACS).
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On the mechanism of carboxylic acid co-catalyst assisted metalloporphyrin oxidations

Gonsalves, António M. d'A. Rocha

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On the mechanism of carboxylic acid co-catalyst assisted metalloporphyrin oxidations

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