Electron deficient manganese(III) corrole catalyzed oxidation of alkanes and alkylbenzenes at room temperature

Article abstract of DOI:10.1016/j.catcom.2011.04.026

At room temperature electron deficient manganese (III) corrole complexes (1-3) were successfully employed as catalysts in the oxidation of alkanes and alkylbenzenes using m-chloroperbenzoic acid (m-CPBA) as the terminal oxidant. Adamantane has been selectively hydroxylated to adamantane 1-ol and 2-ol with higher preference for the tertiary position. Cyclohexane has also been oxidized. The present oxidizing system also oxidizes toluene, ethylbenzene and diphenylmethane. High valent oxomanganese(V) species has been proposed to be the active oxidant. The high-valent oxomanganese(V) corrole undergoes hydrogen atom transfer (HAT) reaction with 2,4,6-tri-t-butylphenol (TTBP) resulting in the formation of oxidized phenoxyl radicals. Kinetic studies have led to the determination of second-order rate constants for the hydrogen atom transfer reactions. The kinetic experiments reveal a first order reaction rate dependence on the concentration of catalyst as well as on that of the oxidant.

Full text of DOI:10.1016/j.catcom.2011.04.026

Catalysis Communications 12 (2011) 1193–1197
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Catalysis Communications
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Short Communication
Electron deficient manganese(III) corrole catalyzed oxidation of alkanes and
alkylbenzenes at room temperature
⁎
Suranjana Bose, Anand Pariyar, Achintesh Narayan Biswas, Purak Das, Pinaki Bandyopadhyay
Department of Chemistry, University of North Bengal, Raja Rammohunpur, Siliguri 734013, India
a r t i c l e i n f o
a b s t r a c t
Article history:
At room temperature electron deficient manganese (III) corrole complexes (1–3) were successfully employed
as catalysts in the oxidation of alkanes and alkylbenzenes using m-chloroperbenzoic acid (m-CPBA) as the
terminal oxidant. Adamantane has been selectively hydroxylated to adamantane 1-ol and 2-ol with higher
preference for the tertiary position. Cyclohexane has also been oxidized. The present oxidizing system also
oxidizes toluene, ethylbenzene and diphenylmethane. High valent oxomanganese(V) species has been
proposed to be the active oxidant. The high-valent oxomanganese(V) corrole undergoes hydrogen atom
transfer (HAT) reaction with 2,4,6-tri-t-butylphenol (TTBP) resulting in the formation of oxidized phenoxyl
radicals. Kinetic studies have led to the determination of second-order rate constants for the hydrogen atom
transfer reactions. The kinetic experiments reveal a first order reaction rate dependence on the concentration
of catalyst as well as on that of the oxidant.
Received 10 December 2010
Received in revised form 19 April 2011
Accepted 22 April 2011
Available online 30 April 2011
Keywords:
Manganese Corrole
Catalysis
Oxygenation
Alkane
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Gross et al. [14]. The substitution of the β-pyrrolic hydrogens of corrole
macrocycles with bromine [15] or fluorine [16] enhances the catalytic
The oxidative functionalization of saturated hydrocarbons to more
valuable products is of great interest in synthetic organic chemistry
and industrial chemistry [1,2]. The inertness of alkanes towards
chemical conversion demands a great deal of effort to develop
efficient catalysts operating at ambient conditions [3]. Considerable
attention has been focused on a variety of coordination complexes to
utilize them as efficient homogeneous catalysts for oxygenation of
hydrocarbons. In this direction transition metal complexes of porphy-
rins, phthalocyanines, chlorins, triazacylononanes and Schiff bases
have been explored as homogeneous catalysts for oxygenation of
alkanes and alkenes [4–6]. Metalloporphyrin catalyzed oxidation of
hydrocarbons has been most extensively studied [7–9]. Corrole, another
tetrapyrrolic macrocycle similar to porphyrin, is gaining importance due
to the applications of its metallo-derivatives as catalysts in diverse areas
like oxygenation of organic compounds, cyclopropanation and aziridi-
nation [10–12]. The metallocorrole catalyzed hydrocarbon oxidation is
mainly confined to iron–corrole and manganese–corrole systems [12]
and the only other instance is photocatalytic oxygenation of cumene by
difluoroantimony(V)corrole [13]. Manganese corroles have evoked
more interest than iron–corroles as catalysts in hydrocarbon oxida-
tion [12,14–16]. The synthesis of manganese(III) complex with 5,10,15-
tris(pentafluorophenyl)corrole, [Mn(III)(tpfc)], and its successful use as
catalyst in the oxidation of styrene with PhIO were first reported by
activity of the manganese(III) corroles in terms of yield and reaction
time. The catalytic activity of (tpfc)Mn(V)O in alkeneoxidation has been
found to be enhanced by the axial coordination with imidazole [17].
Despite considerable progress in manganese(III) corrole catalyzed
alkene oxidation, only one report on the catalytic oxygenation of
alkanes by manganese(III) corrole has appeared so far [18].
In continuation of our earlier work [18–21] on metallocorrole
catalyzed oxidation of hydrocarbons, here we wish to report the
manganese(III) corrole catalyzed oxygenation of adamantane, cyclo-
hexane, toluene, ethylbenzene and diphenylmethane at room temper-
ature with m-CPBA as terminal oxidant. The kinetic studies of the
present oxidizing systems with 2,4,6-tri-t-butylphenol (TTBP) have also
been undertaken. A plausible reactionmechanismhas been proposed on
the basis of experimental results.
2. Experimental
2.1. Instrument and reagents
Acetonitrile and dichloromethane were distilled under argon from
CaH2 and CaCl2 and stored over molecular sieves (4 Å). 2,4,6-tri-t-butyl
phenol (TTBP) and m-CPBA were purchased from Aldrich and purified
accordingly [22]. Other substrates, all the reaction products and
dodecane (internal standard) were purchased from Aldrich and used
as received. The exact active oxygen content of the oxidant was
determined iodometrically prior to use. UV–vis spectral measurements
⁎
Corresponding author. Tel.: +91 353 2776381; fax: +91 353 2699001.
E-mail address: pbchem@rediffmail.com (P. Bandyopadhyay).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.04.026

1194
S. Bose et al. / Catalysis Communications 12 (2011) 1193–1197
were taken with a JASCO V 530 spectrophotometer connected with a
thermostat at 25 1°C.
cuvette was degassed by blowing argon over it for 15 min. Degassed
acetonitrile (2 mL) was used to dissolve the TTBP in the cuvette. A
standard solution of [Mn(III)(tpfc)] in acetonitrile was added so that
the final concentration of the catalyst was 13 μM. The m-CPBA was
prepared in degassed acetonitrile (20 mg in 2 mL). An aliquot volume
(44 μL) of this stock solution was added to the cell to initiate the
reaction. The cell was vigorously shaken and was placed immediately
in a thermostatted cell holder in a spectrophotometer and the
absorbance data at 630 nm were collected at 5-second intervals.
The product analysis was done by Perkin Elmer Clarus-500 GC with
FID (Elite-I, Polysiloxane, 15-meter column) by injecting 1 μL aliquot
from the reaction vial taken after addition of dodecane as internal
standard. The identification and quantification of the products were
done from the response factors of standard product samples.
2.2. Synthesis of the catalysts
The syntheses of the free base corroles, 5,10,15-tris(2,6-difluor-
ophenyl)corrole (tdfc) and 5,10,15-tris(pentafluorophenyl)corrole
(tpfc) were carried out by using ‘solvent free’ condensation of pyrrole
and the respective aldehydes described by Gross et al. [23]. The
respective manganese(III) corroles were prepared by refluxing the free
base corrole with Mn(OAc)2. 4H2O in DMF [14,18]. Perbrominated
manganese(III) corrole [Br8Mn(III)(tpfc)] (3) was prepared and
characterized following the reported procedure [15].
3. Results and discussion
3.1. Catalytic activity studies
Initially the catalytic activities of the manganese(III) corroles (1–3)
(Fig. 1) were examined in oxidizing adamantane by m-CPBA in
acetonitrile medium at room temperature.
The reason behind the choice of adamantane as a substrate is based
on the fact that the oxidation of adamantane is a useful probe to test
the regioselectivity between tertiary (3°) and secondary (2°) C―H
bonds [24]. Moreover, there is a wide applicability of its hydroxylated
derivatives in pharmaceutical intermediates, polymers and electronic
industry [25]. However, the conversion of relatively inert C―H bond
of adamantane tricycles to C―OH bond is quite difficult [25]. The
present oxidizing system selectively oxidizes adamantane to the
corresponding adamantanols at room temperature with a strong
preference for the hydroxylation at the tertiary centre. The Mn(III)
corrole complexes (1–3) have emerged as almost equally efficient
catalysts, affording a total yield of 38% to 42% of adamantanols within
2 hours. The results are summarized in Table 1. The reactions showed
selectivity for oxidation at the tertiary position, with a 3°/2° ratio of
7·5–8·7 (normalized on a per-hydrogen basis).
2.3. Catalytic oxidation
Catalytic reactions were carried out in small screw capped vials
fitted with PTFE septa. In a typical reaction 25 μM of catalyst and
200 mM (100 mM in case of adamantane) of substrate were dissolved
in 2 mL of acetonitrile or 3:2 acetonitrile/dichloromethane in case of
adamantane. The oxidation reaction was initiated by adding 2 mM of
m-CPBA and the contents were magnetically stirred. After periodic
time intervals standard solution of dodecane was added to this
reaction mixture and an aliquot was injected into a capillary column
of a preheated GC.
2.4. Kinetic measurements
These results encouraged us to examine the catalytic oxygenation of
cyclohexane with manganese(III) corrole by m-CPBA. Under these
reaction conditions cyclohexane has been converted to cyclohexanone
In a typical kinetic experiment, TTBP (24 mg, final concentration
44 mM) was taken in a cuvette fitted with silicon rubber septa. The
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
N
N
N
N
N
N
N
N
III
Mn
III
Mn
F
F
F
1
2
F
F
F
F
Br
Br
F
N
N
Br
F
Br
F
F
N
F
F
III
F
Mn
F
N
F
F
F
Br
Br
Br
Br
3
Fig. 1. Manganese(III) corrole complexes employed in this study.

S. Bose et al. / Catalysis Communications 12 (2011) 1193–1197
1195
Table 1
Oxidation of alkanes and alkyl benzenes by Mn(III) corroles and m-CPBA.
Substrate
Time
(h)
Catalyst
Product profile (%)a
Adamantane-1-ol
Adamantane-2-ol
Adamantane
2
2
2
1
2
3
33
31
30
9
8
8
Cyclohexanol
Cyclohexanone
Cyclohexane
Toluene
2
2
2
1
2
3
4
10
12
2
2
3
Benzaldehyde
16
12
13
Benzyl alcohol
4
3
3
1
1
1
1
2
3
Phenyl acetaldehyde
1-phenyl ethanol
Acetophenone
Ethylbenzene
2
2
2
1
2
3
9
12
15
4
8
9
2
6
5
Diphenylmethanol
Diphenylmethane
2
2
2
1
2
3
20
26
29
a
Yields are based on concentration of oxidant.
and cyclohexanol. In this case, the perbrominated Mn(III) corrole (3)
has been found to be most effective catalyst affording cyclohexanol and
cyclohexanone with A/K ratio of 4.0 (Table 1).
established that manganese(III) corroles react with PhIO and form
oxomanganese(V) species. However, the lack of reactivity of this
oxospecies towards oxygenation reactions led to the proposal that a
high valent Mn(VI)O species, generated by disproportionation of Mn(V)O,
is the active oxidant [14,17]. Oxomanganese corroles have been shown to
be effective oxo-transfer agents towards olefinic substrates like cyclohex-
ene and styrene [14–17]. Very recently, Kumar et al. reported oxygen
atom transfer reactions from isolated oxomanganese(V) corroles to
organic sulfides [29]. However, C―H bond activation of alkanes using
high-valent oxomanganese corroles is still unknown.
Treatment of the manganese(III) corroles (1–3) with m-chloroper-
benzoic acid (m- CPBA) in acetonitrile medium at room temperature
results in the formation of the oxomanganese(V) species [14]. The UV–
vis spectral change brought about by the treatment of m-CPBA to
[Mn(III)(tpfc)] (2) has been shown in Fig. 2.
Oxidation of alkylbenzenes is important chemical transformations
in chemical synthesis, which normally undergo with a low selectivity,
with the formation of various by-products [26]. In general, oxidation
of toluene usually affords benzaldehyde, benzyl alcohol, cresols,
benzoic acid and dibenzyl and thus design of efficient process for the
selective oxidation of toluene is highly desirable. With the present
oxidizing system, toluene has been oxidized under ambient condition
to form benzaldehyde as the major product, while benzyl alcohol is
generated as the minor one. No oxidation of the aromatic ring was
observed. The results summarized in Table 1 show that overall yields
of 16% to 20% were achieved within 1 hour.
The present oxidizing system has also been found effective in oxi-
dizing ethylbenzene and diphenylmethane at room temperature. Ethyl-
benzene has been converted into phenylacetaldehyde, 1-phenylethanol
and acetophenone (Table 1). The reaction proceeded with a greater
selectivity for the formation of phenylacetaldehyde and highest conver-
sion (29%) has been achieved with the catalyst 3. The formation of
phenylacetaldehyde in the present case is surprising. This may arise from
the oxidation of ethylbenzene to styrene, followed by the oxidation of
styrene by the present oxidizing system.
Titration experiments reveal that the characteristic peaks of the
catalyst 2 gradually disappear on addition of m-CPBA with emergence
Diphenylmethane, on the other hand, is selectively oxidized to
diphenylmethanol (Table 1). Significantly, acetophenone, the over
oxidized product, was not at all detected under the reaction conditions
employed. Among the manganese(III) corroles, catalyst 3 has been
found to be most effective in bringing about oxidation of diphenyl-
methane; the relative activity of the catalysts follows the order
3→2→1.
The overall yield of the oxidized products has been found to be
moderate. However, the total amount of oxidant may be accounted for by
considering the pronounced anti-oxidant property of the manganese(III)
corroles [27,28]. It has been demonstrated that manganese(III) corroles
effectively decompose reactive oxygen species and this aspect has been
receiving continuous attention for its potential application in combating
against several neurodegenerative disorders [28].
3.2. Mechanistic considerations
High valent oxomanganese corroles have been frequently invoked as
the key oxidant in the manganese(III) corrole catalyzed oxidation
reactions with iodosylbenzene as the oxidant [14–17]. It has been
Fig. 2. UV–visible spectroscopic changes of the catalyst 2 (green) in acetonitrile at room
temperature upon addition of 1 eqv. (yellow), 2 eqv. (red), 3 eqv. (blue) and 4 eqv.
(brown) m-CPBA solution.

1196
S. Bose et al. / Catalysis Communications 12 (2011) 1193–1197
32
Table 2
[Mn(III)(tpfc)]-catalyzed oxidation of TTBP by m-CPBA in acetonitrile at 25 1 °C.
30
28
26
24
22
20
18
Entry no.
TTBP
(mM)
Catalyst
(μM)
Oxidant
(mM)
(dA/dt)0
Yield
(%)a
Catalyst
survival (%)
s−1 ×104
1
2
3
4
5
6
7
8
17.53
20.40
31.24
42.48
60.97
83.83
95.26
106.69
162.85
44.82
44.55
44.58
44.28
44.02
44.15
44.11
44.08
12.97
12.97
12.98
12.97
12.97
12.97
12.97
12.97
12.97
13.02
28.02
37.68
51.72
12.97
12.97
12.97
12.97
0.96
0.97
0.98
0.97
0.98
0.97
0.96
0.96
0.97
0.97
0.98
0.97
0.98
0.96
1.92
2.91
3.84
7.922
10.029
12.958
21.052
29.832
28.227
28.021
26.926
25.034
23.522
28.295
32.827
34.511
22.859
24.947
27.607
29.993
89.40
93.30
96.02
98.75
99.93
99.20
98.50
97.40
84.26
98.90
99.40
99.76
99.25
99.12
95.51
88.47
80.15
Bleached
Bleached
24
48
59
65
82
88
93
–
0.6
1.6
2.6
3.6
9
[m-CPBA] (mM)
10
11
12
13
14
15
16
17
–
–
Fig. 4. Plot of (dA/dt)0 vs. [m-CPBA].
–
–
–
tion of substrate (Table 2, entries 1 and 2) the yield is not quantitative;
again decrease in the overall yield is observed on increasing the
substrate concentration (entry 9). It is clear from Table 2 that
considerable catalyst bleaching takes place at lower concentration of
TTBP (entries 1–5). The highest catalytic activity of manganese(III)
corrole has been observed in the range of 30 to 100 mM substrate
concentration.
The kinetic plot of absorbance at 630 nm vs. time is shown in Fig. 3 as
a representative case. The absorbance vs. time plot (Fig. 3) shows that
the increase in absorbance does not fit with a simple first or second
order kinetic trace.
–
–
a
Yields are based on the total amount of m-CPBA used.
of new peaks at 348 nm, 410 nm and 518 nm. It has been observed
that the complete conversion of the manganese(III) corrole to the
oxomanganese(V) species requires two equivalents of m-CPBA.
3.3. Kinetics of the catalytic reactions
Therefore, the data obtained were analyzed by ‘Initial-rate’ method
[35]. All runs were carried out in at least duplicates and the values of
(dA/dt)0 given in the tables are the average of the runs. The values of
(dA/dt)0 at varying initial concentrations of the reaction components
are presented in Table 2. The plot of (dA/dt)0 vs. [m-CPBA] at constant
[Mn(III)(tpfc)] and that of (dA/dt)0 vs. [Mn(III)(tpfc)] at fixed [m-CPBA]
are shown in Figs. 4 and 5 respectively. It is clear that the data are best
fitted by a first order dependence of (dA/dt)0 on the concentration of
m-CPBA (Fig. 4) and also on the concentration of catalyst [Mn(III)(tpfc)]
(Fig. 5).
The formation of oxomanganese(V) corrole having been confirmed
in the present oxidizing system, we decided to follow the kinetics of the
reaction with 2,4,6-tri-butylphenol (TTBP) as the substrate. The
rationale behind the choice of this particular substrate was its ability
to form stable phenoxyl radical via formal H-atom abstraction, which
could easily be identified by UV–vis spectroscopy [30,31]. It is
noteworthy that MnV≡O unit is a potentially important intermediate
in Photosystem II and is responsible in hydrogen atom transfer (HAT)
processes with a nearby tyrosyl radical [32]. Moreover, HAT reactions of
TTBP with imidomanganese(V) corrole [33] and oxomanganese(V)
corrolazine [34] have been reported recently and the reaction provided
significant insight into a wide range of enzymatic and synthetic
processes involving high-valent metal-oxo intermediates.
The dependence of (dA/dt)0 on substrate concentration has been
ignored due to high excess presence of the substrate and overall we
propose the following relation (1).
The rate of the catalyst oxidation with m-CPBA was studied using
2,4,6-tri-t-butylphenol (TTBP) under argon atmosphere. The forma-
tion of oxidized product 2,4,6-tri-t-butylphenoxy radical was moni-
dA= dt∝½m−CPBAꢀ ½MnðIIIÞðtpfcÞꢀ
ð1Þ
tored by measuring its absorbance at 630 nm (∈=385 mol−1 cm−1
)
From the slope of the (dA/dt)0 vs. [m-CPBA] at constant [Mn(III)(tpfc)],
second order rate constant 1.93×104 dm3 mol−1 is obtained whereas
the (dA/dt)0 vs. [Mn(III)(tpfc)] plotatconstant [m-CPBA]gives a valueof
3.08×104 dm3 mol−1. Lower value of the second order rate constant in
the first case may be rationalized in terms of greater degree of catalyst
bleaching at higher concentration of the oxidant.
[30,31]. The yield of the radical was found to be dependent on
substrate (TTBP) concentration. It has been observed that a minimum
concentration of 40 5 mM of TTBP is required to trap all the reactive
intermediates in these reactions in acetonitrile. At lower concentra-
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
38
36
34
32
30
28
26
24
22
20
0
500
1000
1500
2000
2500
10
20
30
40
50
Time (seconds)
[Mn(III)(tpfc)] (µM)
Fig. 3. Absorbance vs. time plot of 2,4,6-tri-t-butylphenoxy radical formation in
acetonitrile at 25 1 °C. TTBP=20.40 mM; catalyst=12.97; M m-CPBA=0.97 mM.
Fig. 5. Plot of (dA/dt)0 vs. [Mn(III)(tpfc)].

S. Bose et al. / Catalysis Communications 12 (2011) 1193–1197
1197
(HAT) reaction with 2,4,6-tri-t-butylphenol. Kinetic investigations
with 2,4,6-tri-t-butylphenol reveals a first order rate dependence on
the concentration of the catalyst as well as on that of the oxidant. The
results further supports the conclusion of Goldberg et al. that
oxomanganese(V) transients are effective in HAT processes despite
having very low redox potentials [34].
Acknowledgments
The financial support (SR/S1/IC-08/2007) from DST, Government
of India, is gratefully acknowledged. We thank UGC (India) for the
award of fellowships to SB and AP.
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Scheme 1. Manganese(III) corrole catalyzed oxidation of 2,4,6-tri-t-butylphenol.
The proposed route for the HAT process has been outlined in
Scheme 1. The (oxo)manganese(V) corrole generated in situ by the
reaction of manganese(III) corrole and peracid oxidizes the phenol
(TTBP), via a formal hydrogen-atom abstraction and itself converts to
MnIV–OH species. The involvement of similar species has been proposed
in case of the hydrogen-atom abstraction of phenols by high-valent
manganese(V)-oxo corrolazine [34]. The putative MnIV–OH species is
itself highly reactive [34] and abstracts a hydrogen atom from another
molecule of TTBP regenerating the manganese(III) corrole (Scheme 1).
An alternative mechanism that can account for the phenol oxidation in
the present case involves the disproportionation of the MnIV–OH
producing the Mn(III) catalyst and the (oxo)manganese(V) corrole, that
oxidizes the second phenol substrate. However, considering much
greater reactivity of the MnIV–OH species than (oxo)manganese(V)
species [34], this mechanistic pathway appears to be less favored.
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