Selective hydroxylation of alkanes catalyzed by iron(IV)corrole

Article abstract of DOI:10.1016/j.molcata.2010.04.014

The complex meso-tris(pentafluorophenyl)corrolatoiron(IV)chloride [(F₁₅TPC)FeCl] emerged as efficient catalyst in hydroxylating alkanes at room temperature. Cyclohexane and adamantane have been oxidized to the corresponding alcohols using m-chloroperbenzoic acid (m-CPBA) as terminal oxidant. Cyclohexane has been converted to cyclohexanol in 50% yield with 100% selectivity. Adamantane has also been hydroxylated up to 75% overall yield under identical reaction condition. Significantly high regioselectivity in adamantane oxidation has been observed. The reactive intermediates have been quantitatively trapped by 2,4,6-tri-t-butylphenol (TTBP). Kinetic analysis of the (F₁₅TPC)FeCl-catalyzed oxidation of TTBP has found consistent with rapid reaction of organic substrate with an intermediate formed in the first and rate-determining step.

Full text of DOI:10.1016/j.molcata.2010.04.014

Journal of Molecular Catalysis A: Chemical 326 (2010) 94–98
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Selective hydroxylation of alkanes catalyzed by iron(IV)corrole
Achintesh Narayan Biswasa, Purak Dasa, Arunava Agarwalab,
Debkumar Bandyopadhyayb, Pinaki Bandyopadhyaya,∗
a Department of Chemistry, University of North Bengal, Raja Rammohunpur, Siliguri, West Bengal 734013, India
b Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The complex meso-tris(pentafluorophenyl)corrolatoiron(IV)chloride [(F15TPC)FeCl] emerged as efficient
catalyst in hydroxylating alkanes at room temperature. Cyclohexane and adamantane have been oxidized
to the corresponding alcohols using m-chloroperbenzoic acid (m-CPBA) as terminal oxidant. Cyclohex-
ane has been converted to cyclohexanol in 50% yield with 100% selectivity. Adamantane has also been
hydroxylated up to 75% overall yield under identical reaction condition. Significantly high regioselec-
tivity in adamantane oxidation has been observed. The reactive intermediates have been quantitatively
trapped by 2,4,6-tri-t-butylphenol (TTBP). Kinetic analysis of the (F15TPC)FeCl-catalyzed oxidation of
TTBP has found consistent with rapid reaction of organic substrate with an intermediate formed in the
first and rate-determining step.
Received 19 February 2010
Accepted 22 April 2010
Available online 18 May 2010
Keywords:
Catalysis
Iron corroles
Hydroxylation
Cyclohexane
Adamantane
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
in aerobic oxidation of alkenes to the corresponding hydroper-
oxides which produce corresponding alcohols on treatment with
The functionalization of saturated hydrocarbons under mild
conditions is an important goal in basic and industrial chemistry
[1–5]. The chemical transformation of C–H → C–OH is an energy-
intensive process due to the inertness of alkanes toward chemical
conversion. However the biological world has evolved several
enzymes to accomplish such transformation selectively and under
very mild conditions [6–9]. In particular, cytochromes P-450 cat-
alyze the most energetically difficult hydroxylation of unactivated
C–H bonds of alkanes [6]. With an aim to model the alkane hydrox-
ylation, transition metal complexes of porphyrins, phthalocyanines
and Schiff bases have been extensively studied as oxidation cata-
lysts [10–17]. During the last decade metallo derivatives of corroles
[18–21], tetrapyrrolic macrocycles with just one meso carbon short
from the porphyrin skeleton have evoked keen interest in their
catalytic properties. Various metallocorroles have been prepared
to investigate a wide variety of applications in catalysis for epox-
idation [22–28], cyclopropanation [22,29–31] and aziridination
[32]. Despite progress in metallocorrole-catalyzed epoxidation [20]
reports on hydroxylation of alkanes catalyzed by metallocorroles
are sparse [22,28]. Iron(IV)corrole-catalyzed oxygenation of ethyl
benzene to 1-phenylethanol (6.6%) and acetophenone (4.2%) is
the first example of matallocorrole-catalyzed hydroxylation [22].
Difluoroantimony(V)corrole was found to act as photocatalysts
triphenylphosphine. Although this system successfully oxidizes
cumene to the corresponding hydroperoxide but it fails to oxy-
genate alkanes like adamantane and ethyl benzene [28]. In general
low selectivity is a common problem in metallocorrole-catalyzed
oxygenation reactions [20,23]. The selective hydroxylation of unac-
tivated C–H bond of alkanes still remains elusive in metallocorrole
chemistry.
Herein we wish to report the hydroxylation of cyclohexane
and adamantane at room temperature catalyzed by iron corrole
[(F15TPC)FeIVCl] with m-CPBA as terminal oxidant. To our knowl-
edge, this is first report of selective hydroxylation of alkanes at
room temperature by iron corrole complex. Kinetic analysis of the
(F15TPC)FeCl-catalyzed oxidation has also been reported.
2. Experimental
2.1. General
Acetonitrile and dichloromethane were distilled [33] prior to
use. 2,4,6-tri-t-butylphenol (TTBP) and m-chloroperbenzoic acid
(m-CPBA) were purchased from Aldrich and purified accordingly
[34] and the active oxygen concentration of m-CPBA was deter-
mined iodometrically. (F15TPC)FeCl was prepared according to
the reported procedure [29]. UV–vis spectral measurements were
taken with a JASCO V 530 spectrophotometer connected with a
thermostat at 25 1 ◦C. EPR spectral measurements were done on
a JEOL JES-FA200 spectrometer fitted with a quartz Dewar for mea-
∗
Corresponding author. Tel.: +91 353 2776381; fax: +91 353 2699001.
E-mail address: pbchem@rediffmail.com (P. Bandyopadhyay).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.04.014

A.N. Biswas et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 94–98
95
Table 1
tert-butyl hydroperoxide in bringing about selective hydroxylation
of cyclohexane to cyclohexanol [12]. Apart from this result, selec-
tivity for the formation of cyclohexanol is low in most of the cases
[36].
Hydroxylation of alkanes by F15TPCFe(IV)Cl and m-CPBA at 25 1 ◦C.
Entry No.
1
Substrate
Products
Yields (%)a,b
Cyclohexane
Cyclohexanol
50
0
58
17
0
The oxidation of cyclohexane with m-CPBA-[Fe(IV)(tpfc)Cl] pro-
ceeded efficiently giving cyclohexanol as the sole product. The
reaction was found to be highly specific and cyclohexanol was
formed exclusively within 30 min with 50% yield. Absence of
cyclohexanone is noteworthy since most of the catalytic oxida-
tion reactions of cyclohexane invariably contain this over oxidized
product. Recently NiII(TPA) [TPA = tris(2-pyridylmethyl)amine] has
been shown to hydroxylate cyclohexane by m-CPBA as the oxidant
with an A/K of 8.5 [38]. Our results show that iron corrole-catalyzed
hydroxylation of cyclohexane is much more selective and appear
as prospective catalytic system for industrial applications.
Adamantane is an important mechanistic probe in deciding the
radical character in catalytic reactions [44–47]. If C3 were defined
as the total of the products oxidized at the tertiary positions and
C2 similarly for the secondary positions, the ratio of C3/C2 would
be 0.33 assuming all hydrogens as equally reactive. Generally, in
radical reaction the tertiary position is expected to be more reactive
as may be evidenced by the facts that alkoxide radical reactions
gives C3/C2 values of 6.67 [48], whereas Gif-type reactions [49]
seem to prefer oxidizing secondary positions of adamantane giving
C3/C2 ratios at around 0.9.
In the reaction of adamantane, oxidation with m-CPBA pro-
ceeded catalytically to give 1-adamantananol as the major product
with 2-adamantananol as the minor product. The conversion of 75%
(based on m-CPBA) has been achieved (Table 1). The reaction com-
pleted within 20 min and not even a trace amount of ketone was
detected. The reaction showed selectivity for oxidation at the ter-
tiary position, with a 3◦/2◦ ratio of 10.3–10.7. In case of adamantane
oxidation by t-BuOOH catalyzed by metalloporphyrins, a normal-
ized 3◦/2◦ ratio of 10.8 was observed and it was suggested that
t-BuOO• might be the hydrogen abstracting species [50]. Our results
are quite similar to that of Minisci et al. [50] and supports the
involvement of freely diffusing radicals in the present oxidizing
system.
Cyclohexanone
Adamantan-1-ol
Adamantan-2-ol
Adamantan-2-one
2
Adamantane
a
All the reactions were run at least triplicate, and the yields reported represent
the average of these reactions.
b
Based on the amount of m-CPBA added.
surement at 133 K. The product analysis was done by Pekin-Elmer
Clarus-500 GC with FID (Elite-I, Polysiloxane, 15-m column) by
injecting 1 ␮L aliquot from the reaction vial taken after addition
of pentafluoroiodobenzene as internal standard.
2.2. Catalytic oxidation of alkanes
Catalytic reactions were carried out in small screw capped vials
fitted with PTFE septa. In a typical reaction 25 ␮M 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
m-CPBA and the contents were magnetically stirred. After periodic
time intervals standard solution of pentafluoroiodobenzene 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:
pentafluoroiodobenzene, 2 mM).
2.3. Kinetic measurements
In a typical kinetic experiment, TTBP (28 mg, final concentration
50 mM) was taken in a cuvette fitted with silicon rubber septa. The
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 F15TPCFe(IV)Cl in acetonitrile was added
so that the final concentration of the catalyst was 11 ␮M. The m-
choloperbenzoic acid was prepared in degassed dichloromethane
(9 mg in 200 ␮L). An aliquot volume (10 ␮L) of this stock solution
was added to the cell to initiate the reaction. The cell was vigor-
ously shaken and was placed immediately in a thermostatted cell
holder in a spectrophotometer and the absorbance data at 630 nm
were collected at 5-s intervals. Absorbance vs. time plots have been
analyzed in details in the following section.
3.2. Mechanistic considerations
Evidence on the nature of intermediates in iron-corroles-
catalyzed oxygenation reactions is least available in the literature
[51,52]. This prompted us to investigate the same in the present
catalytic system. Thus we turned our attention to the oxidation of
the catalyst in the absence of organic substrate. Fig. 1 shows the
UV–vis spectral changes of the parent iron(IV)-corrole complex
upon addition of large excess of m-CPBA in acetonitrile medium.
Addition of the terminal oxidant resulted in bleaching as shown
by the disappearances of the bands at 396 and 370 nm. At the
same time weak Q-bands in the visible region at 645 nm was
observed (Inset, Fig. 1). The latter formed within 25 s of addition
of m-CPBA and then decayed with time. The spectral changes are
inadequate to draw any conclusion regarding the nature of the
reactive intermediates. Thus to gain an insight into the nature of
oxidizing intermediates, EPR spectra of the catalyst in the pres-
ence of the oxidant were recorded at 133 K. The EPR spectrum
shows a sharp signal at geff ≈ 2 and another signal at geff ≈ 4. The
sharp signal at g = 2 is indicative of the formation of a free radi-
cal. The signal does not correspond to iron-coupled corrole radical.
The signal at g = 4, on the other hand, may be due to some resid-
ual iron(III)corrole (known to be S = 32 , intermediate spin) from the
reductive decay of the oxidized material. The spectral changes of
the catalyst on addition of the oxidant and also the EPR spectral data
fail to provide adequate information on the identity of the reactive
intermediates.
3. Results and discussion
3.1. Hydroxylation of alkanes
Catalytic activity of the meso-tris(pentafluorophenyl)corrolato-
iron(IV)chloride [(F15TPC)FeCl] was examined in the oxygenation
reaction of alkanes as summarized in Table 1. Adamantane and
cyclohexane have been chosen as the substrate.
The selective transformation of unactivated C–H bond of cyclo-
hexane to C–OH bond continues to be challenge to the synthetic
chemists. The oxidation of cyclohexane is an important indus-
trial process from both economic and environmental viewpoints.
Cyclohexane’s oxidized products are raw materials in the adipic
synthesis, i.e., precursors of Nylon 6 and Nylon 66. Hydroxylation
of cyclohexane has been achieved by both homogeneous and het-
erogeneous catalytic systems [35–39]. So far attention has been
paid to heme and non-heme iron complexes due to their high
catalytic activity and biological relevance [40–43]. Electron defi-
cient iron porphyrins have emerged as efficient catalysts with

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A.N. Biswas et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 94–98
Fig. 1. Overlay spectra catalyst 10.97 ␮M and m-CPBA (1.02 mM) in acetonitrile at
25
◦C (successive spectrum taken after 25 s intervals). Inset: growth of the weak
1
Q-band at 645 nm and its subsequent decay.
Fig. 2. Absorbance vs. time plot of 2,4,6-tri-t-butylphenoxy radical formation in
acetonitrile at 25 1 ◦C. TTBP = 34 mM; m-CPBA = 1.06 mM; catalyst = 11 ␮M.
3.3. Kinetics of the catalytic reactions
The rate of the catalyst oxidation with m-CPBA was stud-
ied by monitoring the oxidation of 2,4,6-tri-t-butylphenol. This
particular substrate was chosen because it is known to be very
reactive organic reductant and it’s oxidized product namely 2,4,6-
concentration. A glance at Table 2 clearly shows that at lower [TTBP]
(entries 1–5) considerable catalyst bleaching has been taken place.
Again at very high [TTBP] (entry 10) iron(IV)corrole catalyst is found
not to be oxidatively robust. By careful variation of concentrations
of the reactants it was ultimately found that catalyst deactivation
was minimal at 100 mM substrate concentration (entries 8 and 9).
Now in order to predict the reaction pathway of this reaction,
the absorbance at 630 nm due to the formation of 2,4,6-tri-t-
butylphenoxy radical was monitored. A representative kinetic plot
of absorbance at 630 nm vs. time is shown in Fig. 2.
The absorbance vs. time plot (Fig. 2) shows that the increase in
absorbance does not fit with a simple first or second order kinetic
pattern. The complicated appearance of the absorbance vs. time
plot is a characteristic feature of this type of reactions. Intervening
factors might include catalyst decomposition and additional oxi-
dant decomposition (e.g. catalase-type dismutation). Therefore, the
method of ‘Initial-rate’ [55–57] was adopted. All runs were carried
out in at least duplicates and the values of dA/dt given in the tables
are the average of the runs with the standard deviation quoted as
the uncertainty. The values of dA/dt0, at varying initial concentra-
tions of the reaction components are presented in Table 2. The plots
of dA/dt0 vs. [m-CPBA] and dA/dt0 vs. [F15TPCFe(IV)Cl] are shown in
tri-t-butylphenoxy radical absorbs at 630 nm (∈ = 385 mol−1 cm−1
)
providing a simpler tool to monitor its generation by UV–vis spec-
troscopy [34,53].
Our first objective was to account for the total terminal oxidant
in terms of 2,4,6-tri-t-butylphenoxy radical (TTBP•) formation. It
has been observed that a minimum concentration of 30 5 mM of
TTBP is required to trap all the reactive intermediates in these reac-
tions in acetonitrile. At lower concentration 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
to 200 mM. Best results were obtained in the range of 30–100 mM
substrate concentration, which shows the highest activity of the
iron(IV)corrole catalyst.
Since metallocorroles are known to get bleached almost com-
pletely [22] at the end of catalytic reactions, the percentage survival
of the catalyst was measured using the technique described else-
where [54]. Selected results are compiled in Table 2. It has been
found that catalyst survival is largely dependent on the substrate
Table 2
F15TPCFe(IV)Cl-catalyzed oxidation of TTBP by m-CPBA in acetonitrile at 25 1 ◦C.
Entry No.
TTBP (mM)
Catalyst (␮M)
Oxidant (mM)
(dA/dt)0 (×105 M s−1
)
Yield (%)a
Catalyst survival (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
12.02
22.14
32.25
33.58
42.75
53.62
78.63
98.47
104.39
193.7
54.20
53.82
54.77
54.00
54.96
54.58
54.58
54.39
10.97
10.97
10.97
10.97
10.97
10.97
10.97
10.97
10.97
10.97
10.97
10.97
10.97
10.97
16.96
22.61
28.27
33.92
1.017
1.017
1.06
1.06
1.04
1.04
1.2
6.00
5.935
8.464
7.303
8.73
74.40
90.01
99.08
98.75
99.92
98.79
92.30
99.13
97.30
57.80
69.85
34.06
15.28
10.55
98.60
97.75
99.31
99.10
Bleached
29
54
56
62
78
89
95
92
–
–
–
–
–
9.333
8.601
11.96
12.04
22.55
19.23
30.03
53.31
61.03
16.09
20.76
26.32
32.77
1.2
1.09
1.028
2.17
3.26
4.34
5.60
1.08
1.08
1.08
1.08
–
–
–
–
a
Yields are based on the total amount of m-CPBA used.

A.N. Biswas et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 94–98
97
is obtained; while the dA/dt0 vs. [F15TPCFe(IV)Cl] plot gives a value
of 1.01 × 103 dm3 mol−1. In the latter case the lower value of the
second order rate constant suggests greater catalyst decomposition
at high catalyst concentration.
4. Conclusion
A significantly high catalytic activity of the iron(IV)corrole com-
plex in the oxidation of alkanes at room temperature with m-CPBA
has been achieved. For the first time in metallocorrole chemistry,
cyclohexane has been hydroxylated with 100% selectivity produc-
ing cyclohexanol in 50% yield. This is also the first report of the
hydroxylation of adamantane catalyzed by any metallocorrole. The
overall conversion in the hydroxylation of adamantane is achieved
up to 75% with 3◦/2◦ ratio of 10.3–10.7. To the best of our knowledge
these are the highest yield conversions of C–H bonds to C–OH bonds
in metallocorrole chemistry, where the carbon is sp3 hybridized.
The kinetic investigation reveals a first order reaction rate depen-
dence on the concentration of catalyst as well as on that of the
oxidant.
Fig. 3. Plot of (dA/dt)0 vs. [m-CPBA].
Acknowledgement
The financial support (SR/S1/IC-08/2007) from DST, Govern-
ment of India, is gratefully acknowledged.
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