Oxidation of hydrocarbons with H2O2/O2 catalyzed by osmium complexes containing p-cymene ligands in acetonitrile

Article abstract of DOI:10.1039/c4cy00492b

The soluble osmium complexes containing p-cymene (π-p-cym) ligands, [(η⁶-p-cym)OsCl₂]₂ (1), [(η⁶-p-cym)Os(bipy)Cl]PF₆ (2), and [(η⁶-p-cym)₂Os₂(μ-H)₃]PF ₆ (3), are efficient catalysts for the oxidation of alkanes (cyclohexane, n-heptane, methylcyclohexane, isooctane, and cis- and trans-1,2-dimethylcyclohexane) with hydrogen peroxide in air to the corresponding alkyl hydroperoxides in acetonitrile solution if a small amount of pyridine is present in the solution. The binuclear complex 1 is the most active precatalyst in the oxidation whereas compound 2 containing the bipyridine ligand is much less efficient. The oxidation of cyclohexane at 60 °C and low concentration [1]₀ = 10^-7 M gave a turnover number (TON) of 200200 after 24 h. A study of the selectivity parameters in the oxidation of linear and branched alkanes and the kinetic peculiarities of the cyclohexane oxidation led to the conclusion that the main reaction mechanism includes the formation of hydroxyl radicals. The effective activation energy E_a for the cyclohexane oxidation catalyzed by complex 1 was 10 ± 2 kcal mol^-1. A kinetic analysis verified also that monomerization of complex 1 occurs before the oxidizing species is involved in the catalytic cycle. The 1-catalyzed reaction of cyclohexane, c-C₆H₁₂, with H₂¹⁶O₂ in an atmosphere of ¹⁸O₂ gave labeled cyclohexyl hydroperoxide, c-C ₆H₁₁-¹⁸O-¹⁸OH. In addition, a small amount of "light" cyclohexanone, c-C₆H₁₀ ¹⁶O, is produced apparently via a mechanism which includes neither hydroxyl radicals nor incorporation of molecular oxygen from the atmosphere. The oxidation of benzene with H₂¹⁶O₂ under ¹⁸O₂ gave phenol which did not contain the ¹⁸O isotope. The reactions with cyclohexane and benzene were shown to proceed also via an alternative minor mechanism with oxo derivatives of high-valent osmium "Os=O" as key oxidizing species. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cy00492b

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Oxidation of hydrocarbons with H O /O
2
2
2
catalyzed by osmium complexes containing
Cite this: Catal. Sci. Technol., 2014,
p-cymene ligands in acetonitrile†
4, 3214
ab
c
b
a
Mikhail M. Vinogradov, Yuriy N. Kozlov, Dmytro S. Nesterov, Lidia S. Shul'pina,
b
c
Armando J. L. Pombeiro and Georgiy B. Shul'pin*
6
The soluble osmium complexes containing p-cymene (π-p-cym) ligands, [(η -p-cym)OsCl
2
]
2
(1),
6
6
6 2 2 3 6
[
(η -p-cym)Os(bipy)Cl]PF (2), and [(η -p-cym) Os (μ-H) ]PF (3), are efficient catalysts for the oxidation
of alkanes (cyclohexane, n-heptane, methylcyclohexane, isooctane, and cis- and trans-1,2-
dimethylcyclohexane) with hydrogen peroxide in air to the corresponding alkyl hydroperoxides in acetoni-
trile solution if a small amount of pyridine is present in the solution. The binuclear complex 1 is the most
active precatalyst in the oxidation whereas compound 2 containing the bipyridine ligand is much less effi-
−7
cient. The oxidation of cyclohexane at 60 °C and low concentration [1]
TON) of 200 200 after 24 h. A study of the selectivity parameters in the oxidation of linear and branched
alkanes and the kinetic peculiarities of the cyclohexane oxidation led to the conclusion that the main
0
= 10 M gave a turnover number
(
a
reaction mechanism includes the formation of hydroxyl radicals. The effective activation energy E for
the cyclohexane oxidation catalyzed by complex 1 was 10 ± 2 kcal mol 1. A kinetic analysis verified also
−
that monomerization of complex 1 occurs before the oxidizing species is involved in the catalytic cycle.
1
6
18
The 1-catalyzed reaction of cyclohexane, c-C
6
H
12, with H
2
O
2
in an atmosphere of
2
O gave labeled
1
8
18
6 11
cyclohexyl hydroperoxide, c-C H – O– OH. In addition, a small amount of “light” cyclohexanone,
1
6
Received 15th April 2014,
Accepted 30th May 2014
c-C H10 O, is produced apparently via a mechanism which includes neither hydroxyl radicals nor
6
16
incorporation of molecular oxygen from the atmosphere. The oxidation of benzene with H
2 2
O under
1
8
18
2
O gave phenol which did not contain the O isotope. The reactions with cyclohexane and benzene
DOI: 10.1039/c4cy00492b
were shown to proceed also via an alternative minor mechanism with oxo derivatives of high-valent
osmium “OsO” as key oxidizing species.
www.rsc.org/catalysis
2
a
2b
organic reactions, for example, alkylation, dihydroxy-
1
. Introduction
2
c,d
2e
2f
lation,
genation, cyclization,
hydrophenylation of ethylene, carbonylation of methanol,
oxyamination, hydrogenation, alcohol dehydro-
2
g
2h,i
2j
In recent decades, many soluble derivatives of transition
metals were reported as efficient catalysts for oxidation of
organic compounds and particularly of hydrocarbons with
oxidative cleavage of olefins,
2
k
2l
2
m–o
and oxidation of C–H compounds with various oxidants,
including photochemical coupling of arenes and oxygena-
tion of alkanes catalyzed by osmium chlorides, only a few
examples of hydrocarbon oxidation with peroxides have been
1
2p
molecular oxygen and peroxides. In contrast to complexes of
2
q
iron, manganese or copper, much fewer publications have
been devoted to osmium compounds as catalysts. Although
soluble osmium complexes are known to catalyze some
2
r,s
reported.
Previously, some of us demonstrated that simple osmium
salts (OsCl , Na OsCl ) are good catalysts for the oxidation of
a
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
3
2
6
Ulitsa Vavilova, dom 28, Moscow 119991, Russia
b
alkanes and some other organic compounds with hydrogen
Centro de Química Estrutural, Complexo I, Instituto Superior Técnico,
3
a,b
peroxide.
The oxidation of cycloheptane in MeCN with
Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
c
−
3
H O in air in the presence of OsCl (1.0 × 10 M) gave cyclo-
Semenov Institute of Chemical Physics, Russian Academy of Science, Ulitsa Kosygina,
2
2
3
dom 4, 119991 Moscow, Russia. E-mail: Shulpin@chph.ras.ru, gbsh@mail.ru;
Fax: +7499 1376130, +7495 6512191; Tel: +7 495 9397317
heptanol and cycloheptanone, and the total turnover number
TON) after 3 h was 63. A comparison of the chromatograms
of the reaction samples before and after their treatment with
PPh (see below, section 2.2) demonstrated that concentra-
tions of alkyl hydroperoxides were very low. Thus, under
these conditions all of the cycloheptyl hydroperoxide
(
†
Electronic supplementary information (ESI) is available: Additional details of
results and discussion. Data on oxidations of linear, branched and cyclic
alkanes: regio-, bond-, and stereo-selectivity parameters. Details of kinetic experi-
3
1
8
ments. Details of chromato-mass experiments with
c4cy00492b
2
O . See DOI: 10.1039/
3214 | Catal. Sci. Technol., 2014, 4, 3214–3226
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decomposed in the course of the oxidation reaction. Addition
of a small amount of pyridine led to a noticeable increase in
the yield and to the predominant formation of the ketone
0.063 mmol) in 2 mL of methanol was stirred for 24 h at
ambient temperature. Then a solution of KPF6 (100 mg,
4.3 mol equiv.) in 10 mL of water was added. The reaction
mixture was extracted with CH Cl , extracts were dried with
(
after 90 min TON was 112). The OsCl salt catalyzes the oxi-
3
2
2
dation of alcohols. Thus, the oxidation of 2-cyanoethanol
with hydrogen peroxide produces the corresponding aldehyde
and acid in a yield of up to 90% and a TON of up to 1500.
anhydrous Na SO and the solvent volume was reduced to
2 4
~3 mL under reduced pressure. After addition of diethyl ether
to the CH Cl solution the product precipitated as a yellow
3
c,d
2
2
Introduction of a π-coordinated olefin to a carbonyl osmium(0)
complex led to noticeable enhancement of activity: the olefin
complex (2,3-η-1,4-diphenylbut-2-en-1,4-dione)undecacarbonyl
triangulotriosmium efficiently catalyzed the alkane oxidation
crystalline powder which was filtered off, washed with diethyl
ether and dried. Yield: 79.5 mg (96%).
2.2. Oxidation experiments
3
e,f
with hydrogen peroxide (TONs were up to 2400).
It turned
The catalysts and the co-catalyst (pyridine) were used in the
form of stock solutions in acetonitrile. (CAUTION: osmium
derivatives are toxic and all experiments should be carried
out in a hood.) Aliquots of these solutions were added to the
reaction mixtures in the oxidations of alkanes. The oxidation
reactions were typically carried out in air in thermostated
Pyrex cylindrical vessels with vigorous stirring; the total vol-
ume of the reaction solution was 5 mL. (CAUTION: the com-
out that addition of pyridine improved the oxidation reactions.
Thus, in the alkane oxidation osmium carbonyl Os (CO)
3
12
3
g,h
exhibited
a very high TON (60 000). The carbonyl hydride
Os (CO) (μ-H) with a similar structure was less efficient (TON
3
10
2
3i
was 1400). A high activity calculated per one metal ion (TON =
1 000, TOF = 6000 h ) was shown by decamethylosmocene
−
1
5
3
j
Cp *Os as the catalyst. In the present work, we studied some
2
p-cymene osmium complexes and found that they are very effi-
cient catalysts for hydrocarbon oxidation with hydrogen perox-
ide exhibiting TON values much higher than those reported so
far for complexes of other metals (recently, high TONs have
been reported for the metalloporphyrine-catalyzed cyclohexane
2 2
bination of air or molecular oxygen and H O with organic
compounds at elevated temperatures may be explosive!). The
reactions were stopped by cooling and analyzed twice, i.e.,
before and after addition of an excess solid PPh
method was developed and used previously by Shul'pin
3
. This
3
g,i,5
3k
oxidation with molecular oxygen, although this reaction proba-
bly proceeds at 155 °C via a classical radical chain mechanism).
This publication is part 5 from the series “Oxidation reactions
catalyzed by osmium compounds”; for part 4, see ref. 3h.
for the analysis of reaction mixtures obtained from various
alkane oxidations and was used in recent years by other
6
chemists. In the meantime some scientists continue to mea-
sure concentrations of alcohols and ketones by injecting the
samples directly to the chromatograph although this does
not give any valuable information about the existence or non-
existence of alkyl hydroperoxides in the reaction solution.
Applying in the present work for the oxidation of cyclohexane
the comparison of chromatograms before and after reduction
2
. Experimental section
2
.1. Synthesis of catalysts
1
The H NMR spectra were recorded using a Bruker AMX-400
spectrometer (400.13 MHz) relative to residual signals of the
with PPh we demonstrate that the reaction affords predomi-
6
3
solvent (for the spectra, see Fig. S1, S2†). Complex [(η -p-
nantly cyclohexyl hydroperoxide as a primary product. The
hydroperoxide only slowly decomposes to form cyclohexanol
and cyclohexanone. In our kinetic studies for precise determi-
nation of oxygenate concentrations only the data obtained
after reduction of the reaction sample with PPh were used.
3
A Fisons Instruments GC 8000 series gas chromatograph
with a capillary column (30 m × 0.32 mm × 25 μm, DB-WAX
4
a
2 2 3 6
cym) Os (μ-H) ]PF was prepared by a known procedure.
6
6
Compounds [(η -p-cym)Os(bipy)Cl]PF (ref. 4b) and [(η -p-
6
cym)OsCl ] (ref. 4c,d) were synthesized by substantially mod-
2
2
ified methods.
6
[
(η -p-cym)OsCl ] .
A
heavy-walled microwave vessel
2
2
containing Na OsCl (635 mg, 1.41 mmol), 3 mL of
2
6
γ-terpinene, 8 mL of EtOH (96%) and a stirring bar was filled
bubbled through the solution) with argon, sealed and placed
(
J&W)) and a Perkin-Elmer Clarus 600 gas chromatograph,
(
equipped with a Perkin-Elmer Clarus 600 C mass spectrome-
ter (electron impact), with a BPX5 capillary column (SGE)
into a microwave reactor. The mixture was allowed to react
for 4 h at 100 °C and a maximum power of 200 W. The reac-
tion mixture was cooled to room temperature and left at
(
helium was the carrier gas; the internal standard was nitro-
methane) were used. In the kinetic experiments each oxida-
tion was carried out at least twice and the average value is
shown in the corresponding figure. The experimental error
was 10%. The concentration of H O in the course of the oxi-
~
20 °C overnight. The orange precipitate was filtered off and
washed successively with EtOH, water, EtOH and diethyl
ether. After drying in air the yield was 417 mg (75%) of the
product as orange crystals. The reaction mixture after filtra-
tion and washing with EtOH (water washes were not added!)
was evaporated under reduced pressure, and the residue was
washed with diethyl ether and crystallized from EtOH/diethyl
2
2
dation reactions was determined by the Ti(IV) reagent.
1
8
2.3. Experiments with
O
2
ether. That gave an additional 104 mg (19%) of the product.
A Perkin-Elmer Clarus 600 C mass spectrometer (electron
impact) equipped with two capillary columns (SGE BPX5;
30 m × 0.32 mm × 25 μm), one with an EI-MS detector and
6
[
(η -p-cym)Os(bipy)Cl]PF . A solution of 2,2′-bipyridine
6
6
(
24 mg, 0.15 mmol) and [(η -p-cym)OsCl ] (49.5 mg,
2 2
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the other one a FID detector, was used for analyses of the
reaction mixtures. Helium was used as the carrier gas. All EI
mass spectra were taken with 70 eV energy.
3.2. Main features of alkane oxidation
We have found that the p-cymene compounds 1, 2, and 3 cat-
alyze the oxidation of cyclohexane with H O /O in acetoni-
trile at 60 °C. Addition of a small amount of pyridine
improves the reaction. As demonstrated by a comparison of
chromatograms obtained before and after the reduction of
2
2
2
1
8
Labeled dioxygen (99% of
O) was purchased from
CortecNet. Freshly prepared catalytic reaction mixtures
contained in Schlenk flasks were frozen with liquid nitrogen,
pumped and filled with N a few times in order to remove
2
3
the samples with PPh (for this method, see section 2.2 and
air. Then the cooled mixtures were pumped again, the vac-
uum pump was turned off, and the evacuated mixtures were
ref. 3g,i,5), cyclohexyl hydroperoxide is the main primary
product (the sole product at the initial period of the reaction)
which only slowly decomposes partly in the course of the oxi-
dation reaction to afford small amounts of cyclohexanol and
cyclohexanone. For precise determination of oxygenate con-
centrations, we used only the data obtained after reduction
1
8
heated up to 20 °C and immediately filled with O gas with
2
a syringe through a septum. The mixtures were then heated
up to 60 °C with a possibility of gas flow to compensate for
1
6
18
excessive pressure. The O and O compositions of the oxy-
genated products were determined by the relative abun-
dances of mass peaks at m/z = 57/59 (for cyclohexanol),
of the reaction sample with PPh
in the cyclohexane oxidation catalyzed by compounds 1, 2,
and 3 is shown in Fig. 1. The maximum initial rates W were
3
. Accumulation of products
9
8/100 (for cyclohexanone) and 94/96 (for phenol), unless
0
stated otherwise.
measured from the slopes of tangents (dotted straight lines)
to the kinetic curves of oxygenate accumulation. It can be
3
. Results and discussion
In this work, we used for the first time p-cymene complexes
of osmium, the binuclear 1 and 3 and the mononuclear com-
plex 2 (Scheme 1).
3
.1. Synthesis of catalysts
A few publications which describe the synthesis of the com-
6
2 2
pound [(η -p-cym)OsCl ] are known. All methods comprise
interaction of α-phellandrene [2-methyl-5-(l-methylethyl)cyclo-
hexa-1,3-diene] with OsCl ·3H O (ref. 7) or Na OsCl (ref. 4c)
3
2
2
6
2 6
in ethanol. Reaction of Na OsCl with α-phellandrene proceeds
slowly and requires prolong refluxing (100 h). Very recently, the
6
benzene analog [(η -benzene)OsCl ] was synthesized from
2
2
2 6
Na OsCl in excellent yield and low reaction time using micro-
4d
wave oven heating. We applied this approach in the synthesis
Fig. 1 Accumulation of the sum of oxygenates (predominantly
cyclohexyl hydroperoxide) with time in the cyclohexane oxidation with
6
of the complex [(η -p-cym)OsCl ] . The reaction of Na OsCl
2
2
2
6
with γ-terpinene [2-methyl-5-(l-methylethyl)cyclohexa-1,4-diene]
in ethanol solution and microwave heating allowed us to
obtain complex 1 in 94% yield after only 4 h. Complex 2 was
synthesized by a known method using a modified set up of
the reaction mixture. This modification provides complex 2 in
almost quantitative yield. Complex 3 was synthesized by a pro-
cedure described in ref. 4a.
H
2
O
2
in air catalyzed by complexes 1, 2 and 3 in the presence of
pyridine in acetonitrile. Curve a: in the absence of any catalyst. The
maximum initial rates W were measured from the slopes of tangents
(dotted straight lines) to the kinetic curves of oxygenate accumulation.
0
4
b
−
5
Conditions: [catalyst]
.46 M, [H = 2.2 M, [H
were measured as a sum of cyclohexanol and cyclohexanone after
reduction of the reaction sample with PPh
0
= 1 × 10 M, [py] = 0.05 M, [cyclohexane]
0
=
0
2
O
2
]
0
2
O]total = 4.15 M, 60 °C. The concentrations
3
.
Scheme 1 Complexes used as catalysts.
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clearly seen that complex 1 exhibits the highest oxidation rate
and highest yield of products. Compound 3 is insignificantly
less efficient whereas the activity of complex 2 is noticeably
lower. In almost all further experiments we used the most
active compound 1 as the catalyst.
As shown in Fig. 2 at [cyclohexane]
0 2 2 0
= 0.46 M and [H O ]
=
0.35 M the oxidation stops after 500 min. Nevertheless,
when new portions of the alkane and hydrogen peroxide are
added the reaction begins again with the same rate (curve 1).
A similar situation was noticed when only a new portion of
H O (without cyclohexane) was introduced into the reaction
2
2
solution (curve 2). This clearly testifies that an Os-containing
catalytically active species is not destroyed during the oxida-
tion reaction.
We have also studied the dependences of the initial oxida-
tion rates on initial concentrations of the reactants: catalyst 1
Fig. 3 Graph A: dependence of initial rate W
oxidation with H /O
Conditions: [py] = 0.05 M, [cyclohexane]
O]total = 4.15 M, 60 °C. For the original kinetic curves obtained at
various concentrations of 1, see Fig. S3 .† Graph B: dependence of [1] /W
on W which is a straight line in accordance with eqn (11).
0
of the cyclohexane
2
O
2
2
catalyzed by complex 1 on its concentration.
= 0.46 M, [H = 2.2 M,
0
2 2 0
O ]
[H
2
(
(
Fig. 3A), pyridine (Fig. 4A), hydrogen peroxide (Fig. 5), water
Fig. 6), and cyclohexane (Fig. 7A). Kinetic analysis of these
0
0
0
data (see below, section 3.5) led to the results shown in insets
B for the figures. We also measured the initial reaction rates
at different temperatures and evaluated the effective activa-
−
1
tion energy Ea = 10 ± 2 kcal mol (see Fig. S8; see also
Fig. S9† for original kinetic curves).
We found that benzene can also be oxidized to phenol
with H O if complex 1 and pyridine are used as components
2
2
of the catalytic system (Fig. 8, curve 3). It turned surprisingly
out that addition of benzene to the solution containing cyclo-
hexane does not affect the rate of cyclohexane oxidation
Fig. 4 Curve A: dependence of initial rate W
oxidation with H /O catalyzed by complex 1 on concentration of
added pyridine. Conditions: [1]
= 1.1 M, [H O]total = 2.1 M, 60 °C. For the original kinetic
curves obtained at various concentrations of pyridine, see Fig. S4.† Line
B: dependence of W on [py] calculated by taking into account the
inhibiting” effect of pyridine in accordance with eqn (5) and (6).
0
of the cyclohexane
2
O
2
2
−
5
0
0
= 5 × 10 M, [cyclohexane] = 0.46 M,
[
H
2
O
2
]
0
2
1
0
“
(
compare curves 1 and 2 in Fig. 8; see also Fig. S10;† a discus-
sion of this phenomenon is presented below, in section 3.5).
3.3. Turnover numbers in the oxidation catalyzed by Os
complexes
Fig. 2 Accumulation of the sum of oxygenates (predominantly
Under similar conditions (concentration of the catalyst was
cyclohexyl hydroperoxide) with time in the cyclohexane oxidation with
−
5
8
5
× 10 M) the TON values for compounds 1, 2, and 3 after
2 2
H O catalyzed by complex 1 in acetonitrile in air. Conditions:
−
5
[
=
catalyst]
0.35 M, [H
new portions of cyclohexane (0.46 M) and H
0
= 5 × 10 M, [py] = 0.1 M, [cyclohexane]
O]total = 0.67 M, 60 °C. At the time denoted by an arrow,
(0.35 M) were
0 2 2 0
= 0.46 M, [H O ]
5.5 h were 11 100, 980, and 7500, respectively. At a very
−
7
2
low concentration of the catalyst 1 (1 × 10 M; entry 1 in
Table S1;† see also Fig. 9) the TON (after subtracting the con-
centration of products formed in the reaction in the absence
of 1) was 200 200. The reaction catalyzed by 1 proceeds in
24% yield based on starting cyclohexane (see Fig. 1, curve for
2
O
2
simultaneously added to the reaction mixture (curve 1) or a new
portion of only hydrogen peroxide was added (curve 2). The
concentrations were measured as the sum of cyclohexanol and
3
cyclohexanone after reduction of the reaction sample with PPh .
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0
Fig. 5 Dependence of initial rate W of the cyclohexane oxidation
with H
Conditions: [1]
O]total = const = 5.9 M, 60 °C. For the original kinetic curves
obtained at various concentrations of pyridine, see Fig. S5.†
2 2 2 2 2
O /O catalyzed by complex 1 on initial concentration of H O .
−
5
0
0
= 5 × 10 M, [py] = 0.05 M, [cyclohexane] = 0.46 M,
[
H
2
Fig. 7 Graph A: dependence of initial rate W0 of the cyclohexane
oxidation with H O /O catalyzed by complex
2
2
2
1
on initial
5
−
concentration of cyclohexane. Conditions: [1]
0
= 5 × 10 M, [py] =
0
.05 M, [H = 1.1 M, [H O]total = 2.1 M, 60 °C. For the original kinetic
2
O
2
]
0
2
curves obtained at various concentrations of pyridine, see Fig. S7 .† Graph
B: the simulation of curve from graph A in accordance with eqn (6).
Fig. 6 Dependence of initial rate W
with H /O catalyzed by complex 1 on total concentration of water.
Conditions: [1]
cyclohexane]
0
of the cyclohexane oxidation
2
O
2
2
−
5
0
= 5 × 10 M, [py] = 0.05 M, [H
2 2 0
O ] = 1.1 M,
[
0
= 0.46 M, 60 °C. For the original kinetic curves
obtained at various concentrations of pyridine, see Fig. S6.†
complex 1). Fig. S1b† shows that the concentration of oxygen-
Fig. 8 Accumulation of the sum of oxygenates (cycohexanol +
cyclohexanone after reduction with PPh
3
) with time in the
ates after 3 h reaches 0.14 M. It means that under the condi-
−
5
cyclohexane oxidation with H /O catalyzed by complex 1 in
2
O
2
2
tions ([1]
0
= 5 × 10 M, [py] = 0.05 M, [cyclohexane]
0
= 0.46 M,
acetonitrile in the absence (curve 1) and in the presence of benzene
[H
2
O
2
]
0
= 2.2 M, [H
2
O]total = 4.15 M, 60 °C.) the yield of prod-
−5
(
1
0.02 M; curve 2). Conditions: [1]
.1 M, [H O]total = 2.1 M, [cyclohexane]
accumulation of phenol with time in benzene (0.46 M) oxidation with
catalyzed by complex 1 in acetonitrile under the experimental
conditions shown by curves 1 and 2.
0
= 5 × 10 M, [py] = 0.05 M, [H
2
O
2
]
0
=
ucts was 30%. It follows from Fig. 2 that the yield of oxygenates
after 700 min equals 23% based on hydrogen peroxide taken in
concentrations lower than the concentration of cyclohexane.
Table S1† (entries 1–3) shows that osmium complexes 1,
2
0
= 0.46 M, 60 °C. Curve 3:
2 2
H O
3
Os (CO)12 and decamethylosmocene bearing different ligands
catalyze the alkane oxygenation with comparable initial reac-
tion rates. Thus, we can conclude that the nature of ligands
surrounding the osmium ion does not dramatically affect the
catalyst's power. It follows also from Table S1† that the parame-
3
.4. Selectivity parameters of alkane oxidation
In order to get an insight into the mechanism of oxygenation
of alkane C–H bonds with H /O catalyzed by the osmium
O
2
2
2
ters of initial reaction rates W per one catalytic center (calcu-
complexes we studied the oxidation of some assay alkanes in
the presence of compound 1 and pyridine. The value of the
regioselectivity parameter for n-octane oxidation with the
0
lated as TONs per one hour) attained in our studies on
catalysis by osmium complexes are comparable with the
parameters measured for the alkane oxidations catalyzed by
1/py/H
2
O
2
system was based on the distribution of isomeric
) the
9
various enzymes.
alcohols. The oxidation gave (after reduction with PPh
3
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involvement solely of a radical-chain mechanism has been esti-
mated to be 22000 years.
The data described in section 3.4 are in good agreement
with an assumption that the main route of the alkane oxida-
tion proceeds with the formation of hydroxyl radicals. Addi-
tional support is obtained from the measurement of the
kinetic isotope effect (KIE = k /k = 1.25 for 1 at 60 °C) and
H
D
experiments with addition of CCl Br (in this case bromo-
3
cyclohexane was found after reduction with PPh
cyclohexanol and cyclohexanone).
3
instead of
This reaction pathway is also in agreement with the kinetic
data obtained in the present study. Thus, we have found that
addition of cyclohexane, CyH or RH, into the reaction solution
does not affect the hydrogen peroxide decomposition rate,
Fig. 9 Accumulation of the sum of oxygenates (predominantly
cyclohexyl hydroperoxide) with time in the cyclohexane oxidation
with H O /O catalyzed by complex 1 in acetonitrile. The maximum
2 2 2
initial rate was measured from the slopes of tangents of dotted
straight lines to the kinetic curves of oxygenate accumulation.
0
whereas the rate W = d[ROOH]/dt of cyclohexyl hydroperoxide,
ROOH, formation increases with increasing [CyH] (Fig. 7A).
0
This testifies that: (i) cyclohexane does not affect the catalyst
reactivity, because it does not, inter alia, form any adduct with
the catalytically active species; (ii) the oxidation of CyH is
induced by an intermediate species X of the oxidative nature
which is generated in the process of hydrogen peroxide decom-
position; and (iii) the interaction between CyH and X is only
one of a few pathways leading to the consumption of species X
(contribution of this canal grows with growing the initial con-
centration of hydrocarbon RH). Based on this statement we
can consider the following kinetic scheme of the competitive
interaction of RH with species X:
−
7
Conditions: [1]
= 2.2 M, [H
measured as sum of cyclohexanol and cyclohexanone after
reduction of the reaction sample with PPh
0
= 1 × 10 M, [py] = 0.05 M, [cyclohexane]
0
= 0.97 M,
[
H
2
O
2
]
0
2
O]total = 4.15 M, 60 °C. The concentrations were
a
3
.
following alcohols (M): octanol-1 (0.0005), octanol-2 (0.0014),
octanol-3 (0.0014), and octanol-4 (0.0013). This corresponds
to the selectivity parameter C(1) : C(2) : C(3) : C(4) = 1 : 2.8 : 2.8 : 2.6.
This parameter is low and comparable with the corresponding
1
0
values for some other systems which oxidize with hydroxyl
radicals: the oxidation by hydrogen peroxide in air under UV
irradiation or in the presence of an Fe(II) salt (the Fenton's
Reactants → X
(1)
(2)
−
reagent) and the oxidation by the H O /VO /PCA and the
2
2
3
−
+

+
H
2
O
2
/VO
3
/H systems. A chromatogram obtained for the prod-
.
O ,e ,/H
2
X  RH  R ROOH
ucts of the methylcyclohexane oxidation is shown in Fig. S11.†
The selectivity parameter based on concentrations of alcohols
1
° : 2° : 3° equals 1 : 5.2 : 13.8 which is typical for the oxidation
X + L
i
→ … → products
(3)
with hydroxyl radicals. The oxidation with the 1/H system
2 2
O
of cis-1,2-DMCH and trans-1,2-DMCH (where DMCH is
dimethylcyclohexane) proceeds non-stereoselectively. Parame-
ter trans/cis for the oxidation of cis-1,2-DMCH was 0.84 and
for trans-1,2-DMCH it was 0.88. Thus, we can conclude that
selectivity parameters testify that in the main pathway
alkanes are oxidized by the system under discussion with
hydroxyl radicals.
Here reaction (1) is a stage of the oxidative species X gen-
eration in the catalytic process of H O decomposition with
2
2
rate W ; (2) is the sequence of transformations of RH into
1
ROOH with the rate limiting step of interaction between X
and RH (this step is characterized by a bimolecular rate con-
stant k ); (3) is the interaction of species X with various reac-
2
tants present in the reaction mixture L . Reactants L compete
i
i
with hydrocarbon RH for the oxidizing species X, and the
limiting steps of these competitions are characterized by
bimolecular rate constants k (L ).
3
.5. Kinetic analysis of the alkane oxidation
The kinetic analysis of the liquid-phase alkane (RH) oxidation
initiated with such reactive radicals as hydroxyl or alkoxyl at low
temperatures shows that this process is possible to some extent
via a classical radical-chain mechanism only in the case of RH
having weak C–H bonds (for example, tetralin, decalin, cumene).
This mechanism involves the reaction RH + ROO˙ → R˙ + ROOH
as a key step. Indeed, if the 10% conversion time at 100 °C for
tetralin and cumene is only 0.2 h, the same parameter for
cyclohexane and methane is 6.5 and 30 days, respectively. The
time necessary for 10% methane conversion at 30 °C with
3
i
Using the method of quasi-stationary concentrations rela-
tive to the intermediate X we can deduce the following
i
expression:


W  k

RH 0


i k3 
Li


Li 
X

(4)
1

2



k3

Li


Li 
X
 is the sum of all possible reactions of
where i
the X consumption excluding the interaction of X with RH.
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In this case we come to the following equation:
and
k3

MeCN
k3 py




MeCN  0.15 M
d

RH 
dt
d

ROOH

1.25,


py
py
MeCN

0.12 M
dt
(
5)
W1k2
RH 0

RH 0

k
3


that is
 290

k2



i k3



Li

Li
 
k3
Both results are in satisfactory agreement with our
assumption that the oxidizing species is the hydroxyl radical
for which the rate constants are known from radiation-chemical,
photochemical and chemical experiments (see ref. 3g,10a,11)
This equation can be presented in the form:

RH

1 
1



0


RH0

k3

Li

Li
 


(6)
d

ROOH

W1
k2

i
3 3
where k (py)/k (MeCN) ≈ 300.
dt
Taking into account the results discussed above we can
easily find a correlation between initial rates of the ROOH
formation and initial rates of the active oxidizing species gen-
The experimental data given in Fig. 7 are in agreement
with eqn (6). Indeed, the value [RH] /(d[ROOH]/dt) linearly
0
eration W in the decomposition of H O , as it is reflected by
depends on [RH]
0
as shown in Fig. 7B. In accordance with
1
2 2
eqn (5) and (6). For the conditions under which we studied
the dependence of (d[ROOH]/dt) on initial concentration of
catalyst [1] , W = 1.5(d[ROOH]/dt) and W does not depend
eqn (6) the reciprocal tangent of the slope of the straight line
in Fig. 7B corresponds to the X generation rate W . The ratio
0
1
of a segment that is cut off on the y-axis to the slope tangent
corresponds to the value Ψ
0
1
0
1
on [1] . Thus, dependence of W on [1] is analogous to the
0
1
0
experimental dependence shown in Fig. 3, taking, however,
into account that rate values on the Y axis should be multi-
plied by 1.5. The data in Fig. 3 testify that the oxidation reac-
tion order in pre-catalyst 1 is lower than the first order. At
1


k3

Li



Li .
i
k2
Under the experimental conditions presented in Fig. 7
high [1] the reaction order is close to 0.5. Such a type of
0
we have
dependence of W = (d[ROOH]/dt) (and, consequently, also
0
0
1 0
of W ) on [1] allows us to assume that a catalytically active
−
5
−1
W = 2.1 × 10 M s
i
species is a monomeric form of the osmium compound (M)
which is produced by the dissociation of the starting dimeric
form of pre-catalyst 1:
and the parameter Ψ
1


i k3

Li


Li

 0.27 M.
1 ⇄ 2M
(7)
k2
The dissociation constant is small and at a high value of
1] the degree of its dissociation is not high. Let us assume
The latter value Ψ = 0.27 M is higher than the average
values (0.15 M) obtained previously from the data for the
alkane oxidation by other systems which are believed to oper-
[
that K is the concentration constant of this equilibrium and
k is the effective rate constant of the oxidizing species genera-
tion. The latter reaction is pseudo-first order for M. To
describe the experimental dependence shown in Fig. 4A we
have:
1
0a,11a
ate with hydroxyl radicals.
In previously studied sys-
tems the solvent acetonitrile played the role of a competitor
to RH for the hydroxyl radical. The enhanced value of param-
eter Ψ measured in the present work reflects the fact that pyr-
idine in addition to acetonitrile is an efficient rival for the
hydroxyl radical. In summary, we have:
2
W
1
= k[M]; K[1] = [M] and [1]
0
= [1] + [M]/2
(8)
k3

MeCN


MeCN

 k3

py


py  0.27 M
The following equation can be deduced from these
expressions:
k2
2
1
W
W1
and in accordance with the parameters listed in ref. 11:
k3 MeCN
MeCN  0.15 M
k2
1
 0


(
9)
2
k K 2k



Eqn (9) can be transformed into eqn (10) which is more
convenient for treating the experimental results.
It means that at [MeCN] = 18 M and [py] = 0.05 M under
the experimental conditions shown in Fig. 7 we have
k3

py


py  0.12 M, that is k3

py  2.4

1
0 
W
1

(10)
2
k2
k2
W1 k K 2k
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Taking into account that W = 1.5W we come to eqn (11):
We expected that addition of benzene as competing for
the hydroxyl radical reagent would decrease the rate of RH →
ROOH transformation. Surprisingly, the experimental data
have demonstrated that the ROOH formation rate does not
practically depend on the presence or absence of added ben-
zene (see Fig. 8, curve 2 and Fig. 9). It is possible to assume
that this unexpected result reflects the fact that radicals gen-
erated in the interaction between benzene and hydroxyl radi-
cals react efficiently further with the alkane RH to produce
1
0

1
0
2.25
0.75

W 
(11)
2
0
W0 k K
k
Treating the data shown in Fig. 3A by the least-square
method applying eqn (11) leads to the conclusion that our
experimental results are satisfactorily described by eqn (11)
which is demonstrated by Fig. 3B. In accordance with these
2
5
results we can calculate 2.25/k K to be 0.77 × 10 and 0.75/k
radicals R˙ which under an O atmosphere are transformed
2
to be 0.29 s. Consequently, for the conditions of our experi-
ments K = 4.3 × 10 M and k = 2.6 s .
into ROOH. We realize that an additional special study is
required to get an insight into this intriguing phenomenon.
−
6
−1
In the reaction under consideration pyridine plays a dou-
ble role. First, addition of pyridine substantially enhances
the reaction rate which in its absence is very low. This effect
can be due to the facilitation of the 1 monomerization that is
pyridine shifts the equilibrium (7) to the reactive monomer
M. Besides, binding the monomer with pyridine can enhance
the reactivity of the monomer. We cannot exclude the simul-
taneous effect of both factors. Second, pyridine is a competi-
tor of the alkane and acetonitrile for hydroxyl radicals
leading, by this way, to the decrease of the RH oxidation rate
1
8
3.6. The oxidation under an O atmosphere: incorporation
of
into phenol
2
1
8
2
O into produced cyclohexyl hydroperoxide and not
The interaction of alkyl radical with molecular oxygen is a
principal step in the mechanism of oxygenation of both satu-
rated and aromatic hydrocarbons with the participation of
2
hydroxyl radicals (R′R′′CH˙ + O → R′R′′CH–O–O˙). When
atmospheric air in the reaction vessel is partly (some air is
still present in the solution) replaced with a stream of argon
the rate of oxygenate formation decreases (an example is
shown in Fig. S4,† graph f, curve A), and this indicates that
molecular oxygen takes part in the oxygenation reaction. The
experiments with isotopically labeled
(
for the oxidation of acetonitrile by this system, see below,
section 3.6 and Fig. S14 and S15†). The relationship of these
factors governs the experimental dependence of W on [py]
0
(
Fig. 4A): at low [py] this linear dependence reflects mainly
18
2
O are a useful mech-
the activating effect of pyridine; whereas the decrease of the
reaction order at relatively high concentrations of pyridine is
due to the decrease of the fraction (relative to the total
amount) of hydroxyl radicals which react with RH in agree-
ment with eqn (5). If we take into account the “inhibiting”
effect of pyridine in accordance with eqn (5) and (6) it is pos-
sible to calculate the activating effect of pyridine at its differ-
ent concentrations using the data in Fig. 4. This analysis
shown in Fig. 4B demonstrates that in all of the studied
intervals of [py] values there is a linear dependence of pyri-
dine activation on the rate of hydroxyl radical generation.
The proposed mechanism of the cyclohexyl hydroperoxide
formation is a radical one but it is not a radical-chain mecha-
nism. Radical chain mechanism is typical for the autoxida-
anistic probe to test the involvement and the mode of
involvement of molecular oxygen into the radical reactions. It
has previously been shown that the cyclodecane oxidation
III
under Gif conditions (Fe /Pyridine/CH COOH/H O ) under
3
2
2
18
18
an
2
O atmosphere results in a ca. 50% degree of O incor-
poration in the cyclodecanone where the yield of ketone was
ca. 15%. However, the yield of the corresponding alcohol (as
well as some important reaction conditions) was not
12a
reported. That results clearly led to an involvement (reduc-
tion) of air/oxygen in open-air reactions and inspired us to
investigate the process of such a type in more detail using
the catalytic system based on osmium complex 1.
The accumulation of labeled oxygenated products with
−5
time was studied for conditions [1]
under the atmosphere of
0
= 5 × 10 M and 60 °C
1
1b
tion of alkanes.
18
2
O . The yields and isotopic abun-
It is well known that benzene is an efficient acceptor of
hydroxyl radicals and due to this the phenol formation in the
catalytic system under discussion is not surprising (Fig. 8,
curve 3). The rate of phenol accumulation is low in compari-
son with the rate of the ROOH formation in RH oxidation.
This can be explained if we assume that for the transforma-
tion of benzene to phenol the system requires two oxidative
equivalents produced by the system (for example two
hydroxyl radicals) whereas in the RH oxidation hydroxyl radi-
cals provide one oxidative equivalent and the subsequent
transformations of radical R˙ occur with the participation of
molecular oxygen from the atmosphere. This assumption is
supported by our experiments with labeled dioxygen: molecu-
lar oxygen from the atmosphere is incorporated into ROOH
but not into C H OH (see below, section 3.6).
dances were measured typically after reduction of the reac-
18
tion samples with PPh
3
. The highest degree (66%) of
O
incorporation into cyclohexanol was observed at the begin-
ning of reaction (Fig. 10, graph A). The percentage of labeled
alcohol decreases with reaction time reaching 53% of
18
Cy– OH after 6 h. This effect can be explained by a weak cata-
lase activity of the catalytic system which produces unlabeled
16
16
oxygen
2 2 2
O from the hydrogen peroxide H O . One may
18
expect the incorporation of the O isotope into the formed
triphenylphosphine oxide (OPPh ) via the following reac-
3
tion scheme where the alkyl hydroperoxide is reduced to alco-
hol by phosphine:
18 18
18
18
Cy– O OH + PPh
3
→ Cy– OH + OPPh
3
(12)
6
5
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Fig. 10 Incorporation of the labeled oxygen into cyclohexanol and
cyclohexanone (graph A) and triphenylphosphine oxide (graph B;
squares are experimental data, solid line is an exponential fit) in the
−
5
course of the cyclohexane oxidations. Conditions: [1]
0
= 5 × 10 M;
O]total = 2.1 M; [cyclohexane]
2
O , 1 bar. The yields and isotopic abundances were
[
py] = 0.05 M; [H
2
O
2
]
0
= 1.1 M; [H
2
0
=
1
8
0
.46 M; 60 °C;
Fig. 11 Kinetic curves of accumulation with time of cyclohexanol
3
measured after reduction of the reaction samples with PPh .
(
graph A) and cyclohexanone (graph B) containing both partially
16
18
18
labeled ( O + O) and completely O-labeled oxygenates. The yields
and isotopic abundances were measured after reduction of the
3
The analysis of spectral patterns of OPPh which repre-
3
reaction samples with PPh .
1
2b
+
+
sent overlapped sets of peaks
of [M]˙ and [M–H]˙ reveals
the highest degree of O incorporation that is equal to ca.
0% (Fig. 10, graph B). The mode of the experimental data
1
8
3
amount of cyclohexanone which is really present in the reac-
tion mixture (the amount observed after reduction of CyOOH
by PPh ) does not contain O we can conclude that this
3
resulted in the curve shown in Fig. 10, graph B, which resem-
bles the overall dependence of product accumulation
1
8
(
Fig. 11, graph A). These results can be expected taking into
portion of cyclohexanone is formed not from CyOOH. It is
reasonable to assume that this unlabeled cyclohexanone is
produced in an alternative pathway which apparently does
not involve either hydroxyl radicals or ROOH. Molecular
oxygen from the atmosphere is not incorporated into the
product in this route.
A summary of two competitive routes is depicted by eqn (13).
The second route apparently involves the direct oxygenation of
the alkane by a high-valent osmium-containing species.
Although such a mechanism leads to a sufficiently lower
amount of the product (concentration of cyclohexanone is ca. 15
times lower than the concentration of the corresponding alco-
hol) this pathway is of significant interest because it does not
involve the formation of free radicals. Possibly, our observations
are relevant to the studies made by Mayer and co-workers who
account the reaction scheme reflected by eqn (12).
Dependence of the O incorporation into cyclohexanone
drastically differs from that obtained for the alcohol (Fig. 11).
Maximum concentration of the O-labeled ketone does not
exceed 5 × 10 M (yield is 0.1% based on cyclohexane). We
can clearly see in Fig. 11B that the yield of cyclohexanone–
1
8
1
8
−
4
1
6
O is much higher than that of labeled cyclohexanone. It is
necessary to emphasize that the yield of a ketone after reduc-
tion with PPh is equal to the real yield of this compound in
the reaction. Unlabeled ketone cannot be formed from the
3
1
6
16
unlabeled peroxide Cy– O– OH. Indeed, the labeled hydro-
peroxide Cy– O– OH is present in the reaction solution and
its decomposition would lead to the formation of cyclohexa-
none– O in addition to cyclohexanol– O. Thus, as some
1
8
18
1
8
16
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reported the data on oxidations (which require the presence
of pyridine) of alkanes, silanes, alkenes and H with OsO
oxidative hydrocarbon transformations. Initially, a starting
osmium complex is transformed under the action of H O
(and also possibly pyridine) into catalytically active species.
This could be a derivative of Os(II) which in the catalytic cycle
can be oxidized to Os(III). In the Fenton-like mechanism the
Os(III)–Os(II) manifold will further operate:
2
4
2
2
2
m,13a–c
“
consistent with a concerted [3 + 2] mechanism”.
It
3
i
should also be noted that our recent kinetic study of the
alcohol oxidation with H O catalyzed by the trinuclear
2
2
carbonyl hydride cluster, Os
3
2
(CO)10(μ-H) , led to the conclu-
sion that oxidation of alcohols does not involve hydroxyl
radicals as main oxidizing species and apparently proceeds
with the participation of osmyl species, ‘OsO’. Periana and
+
Os(III) + H O → Os(II) + HOO˙ + H
2
2
1
3d
co-workers
established “the viability of using cis-dioxo
−
metal compounds, such as OsO , as reagents for the facile,
Os(II) + H O → Os(III) + HO˙ + HO
2 2
4
selective functionalization of nucleophilic metal alkyl species
via a low energy (2 + 3) transition state”.
Hydroxyl radicals thus formed are the key oxidizing spe-
cies which attack either the alkane RH
RH + HO˙ → R˙ + HOH
or benzene
(
13)
6 6 6 6
C H + HO˙ → HOC H ˙
Under the action of molecular oxygen these radical species
are transformed into oxygenates via different mechanisms:
The chromatogram of a sample taken after 1.2 h and
injected without addition of PPh revealed almost equal
3
R ˙ + O → ROO˙ → → ROOH
2
amounts of the alcohol and ketone produced via decomposi-
tion of CyOOH in the injector (see above, sections 2.2, 3.2,
and ref. 3g,i,5) heated up to 200 °C. Both products were
HOC H ˙ + O → → HOC H
5
6
6
2
6
1
8
found to be half-labeled (55 and 66% of O in cyclohexanol
and cyclohexanone, respectively) confirming that these prod-
ucts originated from the 66% labeled cycloalkyl hydroperox-
ide. A detailed description and discussion of over-oxidation
products is given in the ESI† (Appendix. Products of the
Primary products (alkyl hydroperoxide or phenol) can be
further transformed into more stable products (alcohol and
ketone or quinone, etc.).
In a minor pathway, the interaction between complex 1
1
6
18
cyclohexane over-oxidation with the H
2
O
2
/ O
2
system; see
16
and H2 O leads to the formation of an oxo–osmium deriva-
2
16
also ref. 14 in the ESI†).
tive Os O which reacts with the alkane, RH, to produce
cyclohexanol– O with further oxidation to cyclohexanone–
O. Direct formation of cyclohexanone– O from Os
and CyH is also possible via the insertion of O into the C–H
bond. Similarly, the interaction between the Os O species
and benzene can afford phenol C H – OH at least in one of
The accumulation of labeled phenol in the oxidation of
benzene by the 1/py/H system was studied using the same
16
2
O
2
16
16
16
O
reaction conditions and procedures as for cyclohexane oxida-
tion. The GC-MS spectra of the samples taken from the reac-
tion mixture clearly showed that both phenol (main product)
16
16
16
6
5
1
8
and benzenediols (byproducts) are free from O at any reac-
tion time. These results are in agreement with the earlier
studied mechanism of benzene interaction with hydroxyl
the possible pathways.
1
3e
radical which proceeds in a substantially different manner
compared to the case of saturated hydrocarbons as shown in
Scheme 2, eqn (15) (for eqn (14), see the ESI†). Alternatively,
the mechanism analogous to those assumed for the reaction
by cytochrome P450 with the participation of an osmium–oxo
species affords phenol via the intermediate epoxide (with the
possible NIH shift; see ref. 1d, page 471) as depicted by eqn
4
. Conclusions
6
Osmium complexes containing η -coordinated p-cymene
ligands are efficient robust catalysts for the oxidation of
alkanes and benzene with H O /O to afford alkyl hydroper-
2 2 2
oxide or phenol, respectively. The catalytic studies reveal that
the complex 1 is the most efficient precatalyst, showing high
(
16) in Scheme 2. Both routes lead to the formation of
5
3
−1
turnover numbers (TON) of 2 × 10 and 15.6 × 10 h . As far
as we are aware, these parameters are the highest ones
observed for catalytic oxidation of alkanes, and even compa-
rable to enzymatic oxidation of alkanes. The yield of cyclo-
hexyl hydroperoxide, based on cyclohexane, is up to 30%,
which is also among the highest reported.
unlabeled phenol.
3
.7. On the mechanisms of alkane and benzene oxygenation
Based on the results and discussions presented in the previ-
ous sections we can propose a simplified scheme of the
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Paper
Catalysis Science & Technology
Scheme 2 Proposed mechanisms for the benzene hydroxylation to phenol under the action of hydroxyl radical/molecular oxygen or the osmyl
species “OOs” or “OOsO”.
The selectivity tests, as well as the kinetic studies and the-
oretical fitting of the kinetic data, show that the main route
of the reaction involves the generation of hydroxyl radicals.
The theoretical modeling allowed us to study the formation
of the catalytically active species in detail and estimate the
respective reaction rate constants. Particularly, it is shown
that the monomeric form of the osmium complex 1 is the
catalytically active species.
2 2
industry. The system uses relatively expensive H O and also
very small amounts of Os complexes and, naturally, it is
expensive for the industrial oxidation of cyclohexane.
However it could be considered for the oxidation of valuable
CH compounds such as natural products, terpenes, steroids,
etc. Cyclohexane is an excellent model compound for these
studies.
For the most active compound 1 the kinetic tests under the
1
8
Acknowledgements
atmosphere of isotopically labeled
2
O have been performed,
in order to understand the peculiarities of the catalytic mecha-
This work was supported by the Russian Foundation for
Basic Research (grant 12-03-00084-a) and the FCT (Ciência
and PPCDT programmes, projects PTDC/QUI-QUI/102150/
1
8
nisms. While the incorporation of O into cyclohexanol,
formed from cyclohexyl hydroperoxide, was found to be at ca.
1
8
6
0% level, most of cyclohexanone was found to be O-free.
2
2
008, PTDC/QUI-QUI/121526/2010 and PEst-OE/QUI/UI0100/
013; fellowship SFRH/BPD/42000/2007) (Portugal). L.S.S. and
The formation of unlabeled cyclohexanone is fully unexpected
and contrasts with the hydroxyl radical pathway, proposed as
the main oxidation mechanism. We assume that unlabeled
cyclohexanone is formed not from CyOOH but is produced in
an alternative route which apparently does not involve hydroxyl
radicals and ROOH. These observations were confirmed by
G.B.S. express their gratitude to the FCT and Group V of
Centro de Química Estrutural for making it possible for them
to stay at the Instituto Superior Técnico, University of Lisbon,
as invited scientists and to perform a part of the present
work (all of the funding for the invited scientist fellowship
comes entirely from this group). This paper is dedicated in
memory of Aleksandr Evgenievich Shilov (1930–2014).
1
8
studying the incorporation of O into the main by-products
cyclohexanediols and hydroxycyclohexanones), where ketones
(
were suggested to not contain labeled oxygen. Possibly, the
minor amounts of the oxygenates, such as ketones, can be
assumed to be formed via the interaction of an osmyl species
References
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OOs or OOsO) with an alkane or benzene, which is in
18
agreement with the absence of O in the respective products.
The present work combines the comprehensive catalytic
studies of the osmium complexes, where a wide range of
kinetic, isotopic and theoretical methods are applied together
toward precise identification of the peculiarities of the cata-
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3224 | Catal. Sci. Technol., 2014, 4, 3214–3226
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