Mild oxidation of hydrocarbons catalyzed by iron corrole with tert-butylhydroperoxide

Article abstract of DOI:10.1016/j.catcom.2010.05.006

Catalytic alkane and alkene oxidation by iron complex of 5,10,15 tris-(pentafluorophenyl)corrole, using 70% tert-butyl hydroperoxide as oxidant at room temperature is reported for the very first time. Involvement of freely diffusing radicals in the oxidizing system is observed. A reaction mechanism is proposed based on the experimental results.

Full text of DOI:10.1016/j.catcom.2010.05.006

Catalysis Communications 11 (2010) 1008–1011
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Mild oxidation of hydrocarbons catalyzed by iron corrole
with tert-butylhydroperoxide
⁎
Achintesh Narayan Biswas, Anand Pariyar, Suranjana Bose, 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:
Catalytic alkane and alkene oxidation by iron complex of 5,10,15 tris-(pentafluorophenyl)corrole, using 70%
tert-butyl hydroperoxide as oxidant at room temperature is reported for the very first time. Involvement of
freely diffusing radicals in the oxidizing system is observed. A reaction mechanism is proposed based on the
experimental results.
Received 29 March 2010
Received in revised form 28 April 2010
Accepted 8 May 2010
Available online 16 May 2010
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Iron corrole
Catalysis
Oxygenation
t-butyl hydroperoxide
1. Introduction
corrole catalyzed oxidation of hydrocarbons [17–20]. Although a
catalytic system with mild oxidant operating at ambient condition is
The oxidative functionalization of hydrocarbons to more valuable
products under mild conditions is an area of current interest [1,2]. The
selective insertion of one oxygen atom from oxygen donors into
various hydrocarbon molecules under mild conditions remains a
challenge in chemical science. The biological system has evolved a
group of enzymes called monooxygenases which catalyze monoox-
ygenation reaction under very mild conditions [3]. Extensive effort has
been devoted to develop synthetic homogeneous catalysts as
functional models for monooxygenases. In this direction metallopor-
phyrins catalyzed oxidation of hydrocarbons has been extensively
studied [4–6]. Transition metal complexes of phthalocyanines,
chlorins, triazacylononanes and Schiff bases have also been used as
catalysts for hydroxylation of alkanes and epoxidation of alkenes
[7–10]. There is a growing interest in catalysis by other metal
complexes, especially of different macrocyclic ligands. Metal com-
plexes of corroles [11–14], one carbon atom contracted porphyrin
analogue, appear as prospective candidates in this regard. Gross et al.
introduced a convenient method for synthesis of corroles [15,16] and
first demonstrated the catalytic behaviour of iron corrole complex
[FeIV(tpfc)Cl] (tpfc = 5,10,15-tris-(pentafluorophenyl)corrolato
anion) in oxidation of styrene and ethylbenzene with iodosylbenzene
[17]. Subsequently manganese (III) corroles have received more
attention as catalysts in hydrocarbon oxidation [18–20]. Interestingly,
only iodosylbenzene has been used as terminal oxidant in metallo-
highly desirable, metallocorrole catalyzed hydrocarbon oxidation by
mild oxidant is not reported so far.
Here we wish to report the catalytic oxygenation of alkanes and
alkenes by [FeIV(tpfc)Cl] and t-BuOOH at room temperature. To our
knowledge, this is the first time to show that a metallocorrole
efficiently catalyzes oxidation of hydrocarbons at room temperature
with t-BuOOH as terminal oxidant. A plausible reaction mechanism is
proposed based on the experimental results.
2. Experimental
2.1. Materials
Acetonitrile and dichloromethane were distilled under argon from
CaH2 and CaCl2 respectively prior to use. Cyclohexene was distilled
under argon to remove the inhibitor and passed through a silica gel
column prior to reaction. Other substrates, all the reaction products,
pentafluoroiodobenzene (internal standard) and t-BuOOH (as ∼70%
solution in water) were purchased from Aldrich and were used as
received. The exact active oxygen content of the oxidant was
determined iodometrically prior to use. The iron(IV) corrole catalyst,
[FeIV(tpfc)Cl] was prepared according to the reported procedure [17].
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 iodopentafluorobenzene as
internal standard. The identification and quantification of the products
were done from the response factors of standard product samples.
⁎
Corresponding author. Tel.: +91 353 2776381; fax: +91 353 2699001.
E-mail address: pbchem@rediffmail.com (P. Bandyopadhyay).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.05.006

A.N. Biswas et al. / Catalysis Communications 11 (2010) 1008–1011
1009
2.2. Catalytic oxidation of hydrocarbons
Catalytic reactions were carried out in small screw capped vials
fitted with PTFE septa. In a typical reaction 25 μM of catalyst and 100–
200 mM of substrate were dissolved in 2 mL of argon saturated
acetonitrile. The oxidation reaction was initiated by adding 2 mM of t-
BuOOH and the contents were magnetically stirred. After periodic time
intervals standard solution of iodopentafluorobenzene was added to
this reaction mixture and an aliquot was injected into a capillary
column (elite 1, 15 m) of a preheated GC. The identification and the
quantization of the products were done from the response factors of
standard product samples as usual (Internal standard: iodopentafluo
robenzene, 2 mM).
3. Results and discussion
The UV–visible spectrum of a 30 μM solution of [FeIV(tpfc)Cl] in
acetonitrile is markedly different from that in dichloromethane
(Fig. 1). The electronic spectrum of the catalyst in acetonitrile is
quite similar to that of [Fe(III)(tpfc)(OEt2)2], which may arise from
auto-reduction of the catalyst to form [Fe(III)(tpfc)(NCCH3)2] in the
presence of excess acetonitrile [21]. The spectral changes observed
upon addition of t-BuOOH have been shown in Fig. 2. Regeneration of
an iron(IV) species with absorption bands at 370 and 396 nm followed
by a gradual decay of the transient species was observed. Easy
accessibility to different oxidation states of iron–corrole complex in
acetonitrile encouraged us to examine the catalytic potential of iron
corrole/t-BuOOH system to oxidize alkanes and alkenes in acetonitrile.
The catalytic reactions were performed in acetonitrile at room
temperature. The iron(IV) corrole or more precisely [Fe(III)(tpfc)
(NCCH3)2] catalyzes the oxidation of cyclohexane in the presence of
TBHP (Table 1). Under these conditions, cyclohexane has been found
to be oxidized to cyclohexanol and cyclohexanone. A large excess of
substrate was used to minimize over-oxidation of alcohols to ketones
(catalyst:substrate:oxidant=1:40:4000). The reactions were per-
formed in both air and under argon. In aerobic condition the overall
yield of the oxidized product was 27% with an A/K ratio of 0.66. Under
argon atmosphere the total yield was 20% with an A/K ratio of 0.58
(Table 1).
Fig. 2. Overlay spectra of the catalyst (30 μM) and TBHP (2 mM) in acetonitrile at 25 1 °C
(successive spectrum taken after 3 min intervals).
(16%) and 2-adamantanone (7%). Under argon atmosphere lower yields
of the oxidation products were observed (18% based on TBHP) (Table 1).
The normalized C3/C2 ratio (10.2) of the oxidation products is quite close
to that observed in the adamantane oxidation by metalloporphyrins/t-
BuOOH system (10.8) [24], but different from the ratio (2.7) obtained by
Gif reactions [23].
Results summarized in Table 1 demonstrate that iron–corrole
complex catalyzes facile oxidation of alkanes at room temperature
with TBHP in acetonitrile medium. Controlled experiments in absence
of the catalyst show little or no conversions of substrate indicating
that at least one key intermediate comprises the [(tpfc)Fe] moiety.
Several species in the reaction mixture may be responsible for the C–H
cleavage of cyclohexane (and other hydrocarbons). In analogy to the
iron–porphyrine chemistry, it can be proposed that the homolysis of
O–O bond in the [(tpfc)FeIV–O–OtBu] intermediate produces tBuO•
which in turn promotes alkane oxidation via tBuO• mediated H atom
abstraction [22,23,25]. Alternatively, the hypervalent iron oxo (FeV O)
perferryl species from heterolytic scission of the O–O bond in [(tpfc)
FeIV–O–OtBu] may be involved as the H atom abstractor [26].
However, the second possibility may be excluded on the basis of
very high reactivity of (FeV O)corrole transients, which has been
confirmed recently [27]. Here, the tertiary carbon atoms of adaman-
tane are more prone to oxidation than secondary ones (Table 1),
which suggests that radicals are directly involved in abstracting
hydrogen atoms of the C–H groups [22,23]. It can be explained in
We also investigated the oxidation of adamantane, an important
mechanistic probe to diagnose the radical character of catalytic
oxidation reactions [22,23]. The catalytic reactions were performed in
1:1 acetonitrile/dichloromethane medium due to the limited solubility
of adamantane in acetonitrile. Here, the conversion is moderate and the
major reaction products are 1-adamantanol (77%), 2-adamantanol
t
terms of the ‘radical strength’ of BuO• (97 kcal/mol) [28] which is
sufficient to abstract an H atom from the C–H bonds of higher
molecular (NC4) alkanes including cyclohexane (95 kcal/mol). On the
other hand the radical strength of tBuOO• (83 kcal/mol) is not enough
Table 1
[FeIV(tpfc)Cl] catalyzed hydroxylation of alkanes with t-BuOOH at 298 K.
Substrate
Atm.
Yield (%)a TONb Selectivity (%)
Remarks
Cyclohexane Air
27
23
17
45
Cyclohexanol (38)
Cyclohexanone (62)
Cyclohexanol (40)
Cyclohexanone (60)
1-Adamamtanol (77)
2-Adamamtanone (7.5)
2-Adamamtanol (15.5)
1-Adamamtanol (72)
2-Adamamtanone (11)
2-Adamamtanol (17)
A/K=0.58
Argon 20
A/K=0.66
Adamantane Air
53
C3/C2 =10.2
Argon 18
15
C3/C2 =7.8
a
Fig. 1. The UV–visible spectra of the catalyst in dichloromethane (pink) and in
acetonitrile (blue).
Yields are based on concentration of oxidant.
Moles of product/moles of catalyst.
b

1010
A.N. Biswas et al. / Catalysis Communications 11 (2010) 1008–1011
t
Table 2
to abstract an H atom from cyclohexane, it can provide the BuO• via
the disproportionation reaction and eventually facilitates the forma-
tion of the cyclohexyl radical in the reaction mixture. The formation of
cyclohexanol and cyclohexanone in an approximately 1:1 ratio is
symptomatic of a Russel-type termination [29] of two secondary
peroxyl radicals (Eq. 1). The formation of excess ketones over alcohol
under aerobic condition explained via Eq. 2 (cross reaction).
[FeIV(tpfc)Cl] catalyzed oxygenation of alkenes with t-BuOOH at 298 K.
Substrate
Atm.
Air
Time
5 h
Total yield (%)a
96%
Selectivity (%)
Cyclohexene
Cyclohexene epoxide (7.5)
Cyclohexene 1-ol (26)
Cyclohexene 1-one (66.5)
Cyclohexene epoxide (9.5)
Cyclohexene 1-ol (40)
Cyclohexene 1-one (50)
Benzaldehyde (80)
Argon
Air
10 h
6 h
89%
99%
70%
Styrene
Phenyl acetaldehyde (2)
Styrene oxide (17)
Argon
7 h
Benzaldehyde (41)
Phenyl acetaldehyde (10)
Styrene oxide (49)
a
Yields are based on concentration of oxidant.
The plausible reaction mechanism supporting all these results
obtained in the present case has been shown in Scheme 1. The
catalytic cycle involves the following steps: (a) formation of [(tpfc)
FeIV–O–OtBu] intermediate upon addition of TBHP in acetonitrile;
(b) the O–O bond homolysis forming tBuO• and a lesser reactive
known to yield epoxides in the presence of water [30,31]. Therefore,
the reaction was carried out with anhydrous TBHP instead of aqueous
TBHP (70%) to examine whether the presence of water has any role in
epoxide formation. Only a trace amount of epoxide (b1%) is detected
in the latter case, which indicates that the products are derived mostly
from the reactions of tBuO• radical in anhydrous media. It is
noteworthy that perferryl intermediate, generated by heterolytic
scission of the O–O bond in [(tpfc)FeIV–O–OtBu] is expected to be
efficient in epoxidizing alkenes. Therefore, the above result indicates
the absence of the hypervalent iron oxo (FeV O) perferryl species in the
present oxidizing system. Furthermore, the oxidation of cyclohexene
is almost quenched in presence of radical scavenger 2,4,6-tri tert-butyl
phenol (TTBP) producing only 2-cyclohexene-1-ol (7%) confirming
the involvement of radical, generated from the homolytic scission of
the O–O bond.
t
(particularly with regards to epoxidation) [(tpfc)FeIV–OH]; (c) BuO•
radical mediated reactions following H atom abstraction from
cyclohexane and TBHP; (d) formation of cyclohexanol and cyclohex-
anone via the Russel-type radical termination.
This catalytic system was applied to alkenes to gain further insight
into the reaction pathways. Cyclohexene and styrene were chosen as
substrates and have been found to be oxidized by iron corrole/t-BuOOH.
The results are compiled in Table 2.
Allylic oxidation has been found to be favoured over epoxidation in
the case of cyclohexene. Under aerobic condition higher amount of
ketone was obtained (A/K=0.39), whereas under argon atmosphere
alcohol and ketone were formed in roughly 1:1 ratio (A/K=0.8). Small
amount of epoxide (b10%) has been obtained in the oxidation of
cyclohexene by aqueous TBHP under both aerobic and anaerobic
conditions (Table 2). Under the proposed reaction route, the formation
In [FeIV(tpfc)Cl] catalyzed oxidation of styrene under aerobic
condition, higher amount of benzaldehyde (80%) was obtained. But
the same reaction under an argon atmosphere yielded styrene oxide
(49%) together with 41% of benzaldehyde. In both cases a small
amount of phenyl acetaldehyde was obtained.
t
of epoxide is expected to be at minimum. The BuOO• radicals are
Higher selectivity for benzaldehyde can also be explained in terms
of Scheme 1. Tert-butyl hydroperoxide is known to undergo homolytic
cleavage of the peroxide O–O bond on coordination to the central iron
resulting in formation of alkoxy radicals and poorly reactive
intermediate [(tpfc)FeIV–OH] which is responsible for the low yields
of epoxide [32]. The alkoxy radicals initiate a free-radical reaction that
is propagated by the oxygen present in the reaction medium, leading
to the formation of benzaldehyde [33]. When the reaction was carried
out under an inert atmosphere, the selectivity for styrene oxide
improved (Table 2). Thus the presence of oxygen in the reactions
directly influences the benzaldehyde/epoxide ratio confirming the
free-radical mechanism for the formation of benzaldehyde. The
formation of styrene oxide under anaerobic conditions indicates the
presence of at least one more key intermediate and suggests that
competitive pathways are operative in iron corrole catalyzed
oxidation of hydrocarbons with t-BuOOH [20].
4. Conclusions
[FeIV(tpfc)Cl] complex emerged as good catalyst in oxygenation of
both alkanes and alkenes in acetonitrile at room temperature. To the
best of our knowledge, the catalytic activity of the metallocorrole
complexes with mild t-BuOOH has been reported here for the very
first time. The results demonstrated the involvement of t-BuO• as the
hydrogen abstracting species. The present work may provide
important insights into the role of iron corroles/H2O2 system in
biomimetic sulfoxidation reactions [34] and also its role as promising
cytoprotective agent against oxidative and nitrative stress [35].
Scheme 1. Plausible reaction route for the oxidation of cyclohexane with [FeIV(tpfc)Cl]
and t-BuOOH.

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1011
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