Lewis acid activated oxidation of alkanes by barium ferrate

Article abstract of DOI:10.1039/b002907f

In the presence of a few equivalents of a metal chloride, barium ferrate (BaFeO₄) has been found to oxidize cyclohexane at room temperature in acetic acid-dichloromethane to give a mixture of chlorocyclohexane, cyclohexanol and cyclohexanone. The rates of oxidation for the various metal chlorides follow the order AlCl₃ > FeCl₃ > MgCl₂ > LiCl > ZnCl₂. The best yield was obtained with MgCl₂, which represents a balance between reactivity and stability. Oxidation of other organic substrates has also been carried out using the BaFeO₄-LiCl system. Notably the system is able to oxidize propane and ethane to give a mixture of chloroalkanes and carbonyl products. The deuterium isotope effect for the oxidation of cyclohexane was found to be 2.1, 1.8, and 3.0 for chlorocyclohexane, cyclohexanol and cyclohexanone, respectively. The active intermediate is proposed to be a Lewis acid-ferrate adduct formed by coordination of an oxo ligand of the ferrate to the metal ion. It is suggested that the reactivity of this adduct is reminiscent of a radical species that oxidizes alkanes via a hydrogen atom abstraction pathway.

Full text of DOI:10.1039/b002907f

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Lewis acid activated oxidation of alkanes by barium ferrate
Chi-Ming Ho and Tai-Chu Lau*
Department of Biology and Chemistry, City University of Hong Kong, T at Chee Avenue,
Hong Kong, P. R. China. E-mail: bhtclau=cityu.edu.hk; Fax: ]852 2788 7406
Received (in Montpellier, France) 10th April 2000, Accepted 30th May 2000
Published on the Web 6th July 2000
In the presence of a few equivalents of a metal chloride, barium ferrate (BaFeO ) has been found to oxidize
4
cyclohexane at room temperature in acetic acidÈdichloromethane to give a mixture of chlorocyclohexane,
cyclohexanol and cyclohexanone. The rates of oxidation for the various metal chlorides follow the order
AlCl [ FeCl [ MgCl [ LiCl [ ZnCl . The best yield was obtained with MgCl , which represents a balance
3
3
2
2
2
between reactivity and stability. Oxidation of other organic substrates has also been carried out using the
BaFeO ÈLiCl system. Notably the system is able to oxidize propane and ethane to give a mixture of chloroalkanes
4
and carbonyl products. The deuterium isotope e†ect for the oxidation of cyclohexane was found to be 2.1, 1.8, and
3.0 for chlorocyclohexane, cyclohexanol and cyclohexanone, respectively. The active intermediate is proposed to be
a Lewis acidÈferrate adduct formed by coordination of an oxo ligand of the ferrate to the metal ion. It is suggested
that the reactivity of this adduct is reminiscent of a radical species that oxidizes alkanes via a hydrogen atom
abstraction pathway.
Potassium ferrate and other group 1 and group 2 metal fer-
rates have been known for a long time and are the most stable
and well-deÐned terminal oxo species of iron. A detailed
account of their preparations and properties has appeared
recently.1 The ferrate ion is thermodynamically a stronger
oxidant than MnO ~ and Cr O 2~; the E0 for the Fe(VI)/
Li(OAc), MgCl , ZnCl , FeCl and AlCl were purchased
2
2
3
3
from Aldrich and were used as received.
Gas chromatographic analyses were performed on
a
HewlettÈPackard 5890 series II gas chromatograph with an
Ultra 2 (25 m ] 0.2 mm i.d.) or FFAP (25 m ] 0.2 mm i.d.)
column. GC-MS measurements were carried out on an HP
5890 gas chromatograph interfaced to an HP 5970 mass selec-
tive detector.
4
2 7
Fe(III) couple have been estimated to be 2.20 and 0.72 V vs.
NHE in acidic and basic solutions, eqns. (1) and (2)
respectively2
Oxidation of alkanes by barium ferrate
FeO 2~ ] 8 H` ] 3 e ] Fe3` ] 4 H O
(1)
(2)
4
2
In a typical experiment, 0.4 mmol of the metal chloride was
added to a mixture containing 1.5 ml of dichloromethane, 0.5
ml of acetic acid, 0.5 ml of cyclohexane and 5 ll of chloroben-
zene (internal standard). The mixture was stirred until all the
FeO 2~ ] 4 H O ] 3 e ] Fe(OH) ] 5 OH~
4
2
3
It should thus be feasible to use metal ferrates to oxidize
alkanes under mild conditions. The selective and efficient oxi-
dation of alkanes continues to be a challenge to chemists.3,4
Our interest in the ferrate ion also arises from its relevance to
the enzymes cytochrome P-450 and methane monooxygenase,
where it is generally believed that an iron-oxo species is the
active intermediate.5h7 Although there are a number of
reports on the use of the ferrate ion for the oxidation of alco-
hols and amines,8h11 there is only one study on the oxidation
of alkanes, where it was found that cycloalkanes could be oxi-
dized to the corresponding alcohols and ketones in 30È40%
yield at 75 ¡C by K FeO adsorbed on K10 montmorillonite
Lewis acid dissolved, and 20 mg (0.078 mmol) of BaFeO was
4
than added. Aliquots were drawn at various time intervals and
analyzed by GC and GC-MS. Elemental analyses were done
on a Elementar Vario EL Analyser.
Results
BaFeO is a stable, non-hygroscopic solid that can be readily
4
prepared by the reaction of hypochlorite with iron(III) nitrate.
It is insoluble in common organic solvents including CH Cl
2
2
4
or CH CN. Initial experiments indicated that when BaFeO
2
4
3
clay.1 Our strategy to activate the ferrate ion is to make use of
BrÔnsted and Lewis acids, since recently we found that the
reactivity of metalÈoxo species such as RuO 2~, MnO ~ and
was added to a solution of cyclohexane in these solvents at
room temperature, the purple solid remained undissolved and
unchanged even after 24 h, and no products could be detected.
4
4
Cr O 2~ towards the oxidation of alkanes can be greatly
It was then found that BaFeO is slightly soluble in acetic
2
7
4
enhanced by the addition of just a few equivalents of BrÔnsted
or Lewis acids.12h14
acid to give a red solution, which appears to be stable for at
least a few hours at room temperature since no color change
was observed and no precipitate of Fe(OH) occurred.
3
However, when cyclohexane was added to this red solution no
Experimental
reaction occurred after more than 5 h at room temperature.
As indicated in eqn. (1), the oxidizing power of ferrate, like
most metal-oxo species, is enhanced by protons. Experiments
BaFeO was prepared by a literature procedure.9 The purity
4
of the complex was determined to be 98 ^ 1% using a titra-
tion method.15 The sample was also found to be relatively free
of carbonate and water from C,H analysis (C, 0.16%; H,
0.26%). Solvents were of reagent grade and further puriÐed
according to standard methods.16 Anhydrous LiCl, Li(OTf),
were thus carried out by using a stronger acid, CF COOH.
3
Upon addition of CF COOH (0.5 ml) to a suspension of
3
BaFeO in cyclohexane (0.5 ml) and CH Cl (1.5 ml), the
4
2 2
purple solid dissolved to give a brown solution within a few
DOI: 10.1039/b002907f
New J. Chem., 2000, 24, 587È590
587
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2000

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minutes, with the concomitant production of chlorocyclohex-
ane (6%) and cyclohexanone (3%). In an attempt to improve
the yield we turned to Lewis acids, since we have found earlier
that Lewis acids such as metal ions and boron halides work
just as well as protons in activating anionic metal-oxo species
of ruthenium, chromium and manganese. Hence experiments
LiCl [ ZnCl . However, the best yield (54%) was obtained
2
with MgCl . Surprisingly BF É Et O gave very low yields
2
3
2
although it is a rather strong Lewis acid. Moreover, the metal
acetates and triÑates were much less e†ective than the chlo-
rides. However, PPh Cl is also ine†ective, indicating that the
4
activating e†ect is not due to Cl~ alone.
using metal salts to replace CF COOH were carried out.
The Lewis acidÈBaFeO system was also able to oxidize
3
4
Indeed addition of 5 equiv. of LiCl to BaFeO and cyclo-
other alkanes; again chloroalkanes, alcohols and ketones were
4
hexane in CH Cl ÈCH COOH produced chlorocyclohexane,
cyclohexanol and cyclohexanone within 2 h at room tem-
perature with an overall yield of 42% (Table 1). A brown pre-
produced and the results are tabulated in Table 2. LiCl was
2
2
3
used rather than MgCl because less chlorine-containing pro-
2
ducts were found. Notably the system was able to oxidize
cipitate was also formed, which presumably was Fe(OH) .
ethane and propane at room temperature to give pre-
dominantly chloroalkanes. Oxidation of propane occurred at
both primary and secondary CÈH bonds to produce 1-
chloropropane, 2-chloropropane and propanone. In the oxi-
dation of ethane, chloroethane and ethanal were detected;
acetic acid was probably also produced, but we were not able
to conÐrm this since the solvent used contained acetic acid.
Although both alcohol and ketone products were produced in
the oxidation of cyclohexanol, no alcohols were detected for
3
The formation of Fe(OH) from the reduction of ferrate is well
3
documented in the literature.1,11 The acetic acid functions as a
solvent for the metal chloride and the barium ferrate. In the
absence of CH COOH, both the Lewis acid and the BaFeO
3
4
remained undissolved in CH Cl and no products were found.
2
2
Besides LiCl, activating e†ects were also observed with other
metal chlorides. The rates of formation of products for the
metal chlorides follow the order: AlCl [ FeCl [ MgCl [
3
3
2
Table 1 E†ect of Lewis acids on cyclohexane (CyH) oxidation by barium ferrate in CH COOHÈCH Cl a
3
2 2
Products (% yield)b
Lewis acid
Time
Chlorocyclohexane
Cyclohexanol
Cyclohexanone
TFAc
20 min
5 min
10 min
5 min
1.5 h
6 h
2 h
2 h
2 h
d
6
d
3
5
2
1
1
5
13
4
13
d
AlCl
Al(OTf)
24
d
14
d
3
5
2
3
5
10
3
8
d
3
FeCl
3
BF É Et O
3
2
ZnCl
5
2
MgCl e
31
d
21
d
2
Mg(OTf)
LiCl
Li(OTf)
Li(OAc)
2
24 h
24 h
24 h
d
d
PPh Cl
\1
3
1
4
a Reaction conditions: temperature, 23 ¡C; cyclohexane, 0.5 ml; CH Cl , 1.5 ml; CH COOH, 0.5 ml; Lewis acid, 0.4 mmol; barium ferrate, 0.078
2
2
3
mmol. Mole ratio of Lewis acid to barium ferrate is approximately 5 : 1. b Mol of product/mol of barium ferrate ] 100. c CF COOH (0.5 ml) was
used instead of CH COOH. d Not detected. e CH Cl , 1 ml; acetic acid, 1 ml.
3
3
2 2
Table 2 Oxidation of organic substrates by BaFeO ÈLiCl in CH Cl ÈCH COOHa
4
2
2
3
Substrate (amount)
Cyclohexane (0.5 ml)
Cyclohexane (0.5 ml)c
Cyclohexane (0.5 ml)d
n-Hexane (0.5 ml)
t/min
120
120
120
120
120
Product (% yield)b
Chlorocyclohexane (21), Cyclohexanol (8),
Cyclohexanone (13)
Chlorocyclohexane (29), Cyclohexanol (3),
Cyclohexanone (5)
Chlorocyclohexane (51), Cyclohexanol (7),
Cyclohexanone (14)
1-Chlorohexane (2), 2-Chlorohexane (7),
3-Chlorohexane (7), Hexan-2-one (6), Hexan-3-one (6)
1-Chloropropane (10), 2-Chloropropane (19),
Propanone (6)
Propane (2 atm)e
Ethane (2 atm)e
Methylbenzene (0.5 ml)
Ethylbenzene (0.5 ml)
120
90
90
Chloroethane (28), Ethanal (4)
Benzyl chloride (2), Benzaldehyde (6)
1-Chloro-1-phenylethane (6), Acetophenone (13),
1-Phenylethanol (7)
Adamantane (50 mg)
120
1-Chloroadamantane (22), 2-Chloroadamantane (9),
1-Adamantanol (17), 2-Adamantanol (2),
2-Adamantanone (5)
Cyclohexene (0.5 ml)
90
trans-1,2-Dichlorocyclohexane (7),
3-Chlorocyclohexene (3), Cyclohexen-3-ol (10)
Benzaldehyde (12)
Styrene (0.5 ml)
trans-Stilbene (100 mg)
Cyclohexanol (40 ll)
Triphenylphosphine (80 mg)
Benzaldehyde (17)
Cyclohexanone (36)
Triphenylphosphine oxide (100)
90
20
a Reaction conditions: temperature, 23 ¡C; BaFeO , 0.078 mmol; CH Cl , 1.5 ml; CH COOH, 0.5 ml; LiCl, 0.4 mmol. Reaction were done in
4
2
2
3
air, unless otherwise speciÐed. b Based on amount of barium ferrate used. c Reaction under argon. d In the presence of 0.03 mmol Cl . e CH Cl
(2 ml).
2
2 2
588
New J. Chem., 2000, 24, 587È590

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the less easily oxidized substrates n-hexane, propane and
ethane. It was found that cyclohexanol was readily oxidized to
cyclohexanone, with a rate that is at least a few times faster
than the oxidation of cyclohexane. Thus, for the less easily
oxidized substrates, the initial production of alcohol should be
much slower than its further oxidation to carbonyl com-
pounds. No oxidation product arising from attack of the
benzene ring was found when aromatic hydrocarbons were
used as substrates, suggesting that involvement of hydroxyl
radicals is unlikely. The oxidation of PPh proceeded with the
3
formation of PPh O in 100% yield, indicating that the system
3
is capable of transferring one oxygen atom per mole of
BaFeO . Oxidation of alkenes by the BaFeO ÈLiCl system
4
4
gave allylic oxidation products and C2C bond cleavage pro-
ducts rather than epoxides. In the case of cyclohexene, trans-
1,2-dichlorocyclohexane was also produced.
Discussion
Scheme 1
Mechanism
hexane, cyclohexanol and cyclohexanone are 2.1, 1.8 and 3.0,
respectively. For the BaFeO ÈTFA system, the KIE values for
cyclohexanol and cyclohexanone are 2.6 and 3.6, respectively.
The activation of a metal-oxo species through protonation of
an oxo ligand is well established. Thus the e†ect of
CF COOH on BaFeO can be represented by eqn. (3):
4
Similar KIE values were also obtained for cytochrome P-450
catalyzed hydrocarbon hydroxylation when the rates of
product formation from deuterated and undeuterated sub-
strates were compared, although the intrinsic isotopic e†ects
are actually much higher.5 Small KIE values were also
observed for the oxidation of cyclohexane by strongly oxidi-
zing metal-oxo species; for example, the KIE for the forma-
3
4
[FeO ]2~ ] CF CO H ] [(O) Fe(OH)]~ ] CF CO ~ (3)
4
3
2
3
3
2
The protonated ferrate species is thermodynamically a much
stronger oxidant and is capable of oxidizing alkanes at room
temperature. The low yields are probably due to the insta-
bility of [(O) Fe(OH)]~. The activating e†ects of metal chlo-
3
tion
of
cyclohexanone
by
(Bu N) (Cr O )ÈBF ,13
rides are most likely similar to that of protons, that is, an oxo
4
2
2
7
3
KMnO ÈBF 13 and CrO Cl 18 are 3.9, 4.1 and 2.5, respec-
tively. The rather small values of deuterium isotope e†ects for
ligand of the ferrate coordinates to the metal ion (which prob-
ably also has one or more chloro ligands):
4
3
2 2
the BaFeO ÈLiCl system are also in the range expected for
4
`
FeO 2~ ] Mn` ] [(O) Fe2O]M](n~2)
(4)
abstraction of hydrogen by radical reagents.19 This suggests
4
3
that the reactivity of the Lewis acidÈferrate adduct is remi-
niscent of a radical species that oxidizes alkanes via a hydro-
gen atom abstraction pathway (Scheme 1).
The withdrawal of electron density from the oxo ligand by the
metal ion acting as a Lewis acid makes the oxo-iron species
more oxidizing. The rates of formation of products for the
Oxidation of adamantane occurs predominantly at the ter-
tiary positions, the C2/C3 product ratio is 0.41, which is con-
sistent with a radical intermediate. The presence of an alkyl
radical intermediate is also supported by the use of a radical
hard Lewis acids increase in the order LiCl \ MgCl \
2
FeCl \ AlCl , in accordance with the increase in acidity
3
3
from Li` to Al3`. However, for FeCl and AlCl , although
the rates are fast, the yields are relatively low; presumably the
ferrate adducts of these Lewis acids were very unstable and
readily decomposed to iron(III) oxide. The best yields were
3
3
trap. Oxidation of cyclohexane by the BaFeO ÈLiCl system
4
(under the conditions shown in Table 2) in the presence of 0.1
ml BrCCl produced
a substantial amount of bromo-
obtained from MgCl , presumably because this represents a
3
2
cyclohexane in the dark [eqn. (5)]. The large yield of bromo-
cyclohexane suggests that a radical chain reaction involving
trichloromethyl radicals occurs.
balance between reactivity and stability. The rates and yields
with ZnCl were low, probably because of the low affinity of
2
the soft Zn2` for the hard oxo ligand. The acetate and triÑate
salts were less e†ective than the chloride salts; this is not
unexpected since the oxygen atoms on these anions should
compete e†ectively with the oxo ligand of the ferrate for the
metal ions. Attempts to isolate or detect the proposed Lewis
acidÈferrate adducts have so far been unsuccessful due to their
instability. However, adducts of less-oxidizing metal-oxo
species with Lewis acids are known in the literature. For
example, the complexes [(ReMe O) Mg(THF) ] and
(5)
4
2
4
[MRe(CH SiMe ) ON Mg(THF) ] have been prepared and
Formation of chlorinated products
2
3 4
2
2
their X-ray crystal structures showed the bonding of oxo
ligands to Mg2`.17
It was found that the yield of chlorocyclohexane as well as
cyclohexanol and cyclohexanone remained roughly the same
when 2 mol equiv. of PPh Cl was added as an external chlo-
ride source to the BaFeO ÈLiClÈcyclohexane reaction. This
suggests that the cyclohexyl radical was not further oxidized
A proposed mechanism for the oxidation of alkanes by the
Lewis acidÈferrate adduct is shown in Scheme 1. The Ðrst step
of the mechanism is the formation of alkyl radicals, this is
followed by their conversion to chlorinated and oxygenated
products.
4
4
to a carbocation, which would react rapidly with Cl~ to
produce chloroalkane. It is also known that alkyl radicals
react rapidly with oxidizing metal halides such as CuCl and
Formation of alkyl radicals
2
FeCl to produce cyclohexyl halide with the concomitant
3
Kinetic isotope e†ects (KIE \ k /k ) were determined from
the competitive oxidation of cyclohexane and d -cyclohexane
reduction of the metal center.20 In the present system, this
pathway can also be excluded since the metal chlorides used
H
D
12
by the BaFeO ÈLiCl system. The KIE values for chlorocyclo-
are not redox active. Since FeO 2~ is a very strong oxidant, it
4
4
New J. Chem., 2000, 24, 587È590
589

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is reasonable to expect that it will oxidize chloride to chlorine,
which would then react rapidly with cyclohexyl radicals to
produce chlorocyclohexane. The presence of chlorine in the
system was tested by addition of 0.1 mmol of cyclohexene to
of chlorocyclohexane under argon is a result of the removal of
dioxygen, which is a competitor of chlorine for cyclohexyl rad-
icals in the reaction.
In conclusion, metal chlorides readily activate barium
ferrate towards the oxidation of alkanes at room temperature.
Similar activating e†ects have been observed with anionic oxo
species of ruthenium, chromium and manganese. It is believed
that the metal ions function as Lewis acids, and it is remark-
able that in most cases just a few equivalents of rather mild
Lewis acids is enough to produce a large activating e†ect.
the BaFeO ÈLiClÈcyclohexane reaction. The products con-
4
sisted of 3% 3-chlorocyclohexene, 9% trans-2-chlorocyclo-
hexanol, 6% trans-1,2-dichlorocyclohexane and 11%
chlorocyclohexane ] cyclohexanol ] cyclohexanone.
The
presence of chlorine in the reaction was indicated by the for-
mation of trans-1,2-dichlorocyclohexane through the addition
reaction of cyclohexene with chlorine. If chlorocyclohexane
was produced form chlorine, then addition of external chlo-
rine to the system should increase its yield. Indeed, when chlo-
rine was added, the yield of chlorocyclohexane was greatly
increased, but the yields and distribution of the oxygenated
products were not a†ected (Table 2). This suggests that the
increase in yield of chlorocyclohexane is mainly due to a chain
reaction involving the chlorine radical, which results in the
production of more cyclohexyl radicals:
Acknowledgements
We thank the Hong Kong Research Grants Council (CERG
9040197) and the City University of Hong Kong (DAG
7100042) for Ðnancial support.
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1
2
3
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(7)
2
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4
5
6
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7
8
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9
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(9)
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This suggests that both species 2 and O are responsible for
2
producing oxygenated products from R É. The increase in yield
590
New J. Chem., 2000, 24, 587È590