Oxidation of alkanes and benzene with hydrogen peroxide catalyzed by ferrocene in the presence of acids This article is dedicated to the memory of Aleksandr Evgenievich Shilov (1930-2014).

Article abstract of DOI:10.1016/j.jorganchem.2015.02.002

The efficient (turnover numbers attained 1200) oxidation of alkanes to the corresponding alkyl hydroperoxides by H₂O₂ in the presence of catalytic amounts of ferrocene proceeds in MeCN at 40-50 °C. Benzene is oxidized in the same system to phenol. An obligatory component of the catalytic system for both reactions is pyrazine-2-carboxylic acid (PCA) or trifluoroacetic acid (TFA). Kinetic study as well as selectivity parameters testified that the oxidation proceeds with the participation of hydroxyl radicals. In the case of PCA as a co-catalyst the initial rate W₀ of the reaction with both cyclohexane and benzene depends quadratically on [Cp₂Fe]₀ whereas the two reactions in the presence of TFA are of half order in ferrocene. The ferrocene-catalyzed reaction of cyclohexane with H₂O₂16 in an atmosphere of labeled ¹⁸O₂ gave after 2 h a mixture of labeled and unlabeled cyclohexyl hydroperoxide (total yield 20% based on starting cyclohexane) containing up to 69% of ¹⁸O (the analysis was after reduction of cyclohexyl hydroperoxide into cyclohexanol with PPh₃).

Full text of DOI:10.1016/j.jorganchem.2015.02.002

Accepted Manuscript
Oxidation of alkanes and benzene with hydrogen peroxide catalyzed by ferrocene in
the presence of acids
Lidia S. Shul’pina, Aleksandr R. Kudinov, Dalmo Mandelli, Wagner A. Carvalho, Yuriy
N. Kozlov, Mikhail M. Vinogradov, Nikolay S. Ikonnikov, Georgiy B. Shul’pin
PII:
S0022-328X(15)00054-6
10.1016/j.jorganchem.2015.02.002
JOM 18896
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 30 December 2014
Revised Date: 31 January 2015
Accepted Date: 2 February 2015
Please cite this article as: L.S. Shul’pina, A.R. Kudinov, D. Mandelli, W.A. Carvalho, Y.N. Kozlov, M.M.
Vinogradov, N.S. Ikonnikov, G.B. Shul’pin, Oxidation of alkanes and benzene with hydrogen peroxide
catalyzed by ferrocene in the presence of acids, Journal of Organometallic Chemistry (2015), doi:
10.1016/j.jorganchem.2015.02.002.
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ACCEPTED MANUSCRIPT
REVISION - JORGANCHEM-D-14-00831 To: Journal of Organometallic Chemistry, Special Issue
Functionalization of C-H Bonds with Applications in Catalysis" dedicated to Professor Shilov "in
"
Memoriam", Research Article
Research Article
Oxidation of alkanes and benzene with hydrogen peroxide catalyzed by ferrocene in the
presence of acids
a
a
b
b
c
Lidia S. Shul’pina , Aleksandr R. Kudinov , Dalmo Mandelli , Wagner A. Carvalho , Yuriy N. Kozlov ,
a
a
c,
Mikhail M. Vinogradov , Nikolay S. Ikonnikov , Georgiy B. Shul’pin *
a
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ulitsa Vavilova, dom 28,
Moscow 119991, Russia
b
Center of Natural and Human Sciences, Federal University of ABC (UFABC), Santa Adélia Street, 166, Bangu,
Santo André, SP, 09210-170, Brazil
c
Semenov Institute of Chemical Physics, Russian Academy of Sciences, ulitsa Kosygina, dom 4, Moscow 119991,
Russia
_____________________________________________________
ARTICLE INFO
This article is dedicated to the memory of Aleksandr Evgenievich Shilov (1930‒2014).
_____________________________________________________
Keywords:
Alkanes
Arenes
Catalysis
Kinetics
Functionalization of CH bonds
_______________________________________________________________
*
Corresponding author. Tel.: +7 495 9397317; fax: +7495 6512191.
E-mail addresses: Shulpin@chph.ras.ru and gbsh@mail.ru (G.B. Shul’pin)
__________________________________________________________________
Highlights
-
-
-
-
-
_
Ferrocene catalyzes oxidation of alkanes or benzene with H O
2 2
The oxidation requires the presence of pyrazine-2-carboxylic acid or trifluoroacetic acid
The oxidation of cyclohexane or benzene co-catalyzed by PCA is the second order in ferrocene
The oxidation of cyclohexane or benzene co-catalyzed by TFA is the half order in ferrocene
Hydroxyl radicals take part in the reaction, O from atmosphere is inserted into C-H bonds
2
_______________________________________________________
1

ACCEPTED MANUSCRIPT
ABSTRACT
The efficient (turnover numbers attained 1200) oxidation of alkanes to the corresponding alkyl
hydroperoxides by H O in the presence of catalytic amounts of ferrocene proceeds in MeCN at 40‒50
2
2
ºC. Benzene is oxidized in the same system to phenol. An obligatory component of the catalytic system
for both reactions is pyrazine-2-carboxylic acid (PCA) or trifluoroacetic acid (TFA). Kinetic study as well
as selectivity parameters testified that the oxidation proceeds with the participation of hydroxyl radicals.
In the case of PCA as a co-catalyst the initial rate W of the reaction with both cyclohexane and benzene
0
depends quadratically on [Cp Fe] whereas the two reactions in the presence of TFA are of half order in
2
0
16
18
ferrocene. The ferrocene-catalyzed reaction of cyclohexane with H O in an atmosphere of labeled O
2
2
2
gave after 2 h a mixture of labeled and unlabeled cyclohexyl hydroperoxide (total yield 20% based on
1
8
starting cyclohexane) containing up to 69% of O (the analysis was after reduction of cyclohexyl
hydroperoxide into cyclohexanol with PPh3).
______________________________________________________________________
Key words: alkanes; alkyl hydroperoxides; benzene; ferrocene; hydrogen peroxide; hydroxyl radicals
____________________________________________________________________________
_
Graphical Abstract:
1. Introduction
Transition metal complexes [1], particularly, iron derivatives [2], often play roles of good catalysts
in the oxidation of saturated and aromatic hydrocarbons with peroxides, especially with green oxidants
such as hydrogen peroxide. In contrast, organometallic compounds, particularly iron complexes, have so
far been rarely used as catalysts in oxidation processes [3].
The most well-known organometallic derivative of iron, ferrocene, was used in the synthesis of
pristine and doped carbon nanotubes, the transition metal catalyzed coupling and carbonylation reactions
of ferrocene derivatives, in the exploitation for the development of mechanically interlocked and non-
interlocked synthetic molecular machines [4a,b], chiral ferrocenees are used in asymmetric catalysis [4c],
burning rate catalysts [4d], preparation of biosensors [4e], anticancer agents [4f] etc.
2

ACCEPTED MANUSCRIPT
In the present work we report that ferrocene (compound 1) used in catalytic amounts and in the
presence of certain acids in low concentrations very efficiently induces oxidative transformations of
alkanes and benzene with hydrogen peroxide. Some parts of this study have been previously published as
preliminary communications.
2. Results and Discussion
In the present article we report that cyclohexane and other alkanes as well as benzene can be
oxidized to the corresponding alkyl hydroperoxides and phenol, respectively, by hydrogen peroxide in air
in the presence of catalytic amounts of ferrocene and pyrazine-2-carboxylic acid (PCA; for co-catalytic
role of this compound in various reactions, see a review [4g]) or trifluoroacetic acid, as shown in eqs. (1)
and (2). The reaction proceeds in acetonitrile solution under mild conditions (typically at 50 °C). Alkyl
hydroperoxide is relatively stable in the solution, and can be easily reduced by PPh to the corresponding
3
alcohol.
OOH
OH
PPh3
(1)
oxidant: H O and O
2
2
2
pre-catalyst: ferrocene (compound 1)
co-catalyst: pyrazine-2-carboxylic acid (PCA)
or trifluoroacetic acid (TFA)
OH
(2)
2.1. Methods for the quantification of products
We carried out a kinetic study of the cyclohexane oxidation. Products of the hydrocarbon
oxidation were quantified by the GC method. If the direct injection of a reaction sample (of the alkane
oxidation with O or/and H O ) into the chromatograph gave comparable amounts of cyclohexanol and
2
2
2
cyclohexanone, the reduction of the sample with PPh prior to the GC analysis (for this method developed
3
by Shul’pin, see [5a-g]) led to the noticeable predominance of the alcohol. This comparison clearly
indicates that cyclohexyl hydroperoxide is formed as the main primary product [5a-g]. The formation of
cyclohexyl hydroperoxide and its reduction to cyclohexanol with PPh is depicted in eq (1). Our method
3
allowed some chemists [5h-s] to obtain a very valuable information on the existence of non-existence of
alkyl peroxides in the reaction mixture whereas other investigators who did not employed this method
were unable [5t-z] to discuss a possible mechanism of the oxidation reaction.
3

ACCEPTED MANUSCRIPT
2
.2. Hydroperoxidation of alkanes with H O catalyzed by the combination ferrocene/pyrazine-2-
2
2
carboxylic acid (PCA)
The oxidation of alkanes to the corresponding alkyl hydroperoxides by H O in the presence of
2
2
catalytic amounts of ferrocene proceeds in MeCN at 50 ºC. An obligatory component of the catalytic
system is pyrazine-2-carboxylic acid (PCA). In the cyclohexane oxidation the turnover number (TON)
and yield after 1.5 h attained 1200 and 32%, respectively.
In order to get insight into the nature of the oxidizing species and evaluate the mechanism of the
process we determined various selectivity parameters for the reaction of the H O /ferrocene/PCA system
2
2
with linear and branched alkanes. All parameters (summarized in Table 1, entry 1) were measured after
reduction of the reaction mixtures with triphenylphosphine before GC analysis. A comparison with the
selectivity parameters measured previously for oxygenations by some other systems (see Table 1 [6])
indicates that ferrocene-catalyzed reactions occur with the participation of hydroxyl radicals. Indeed,
parameter C(1):C(2):C(3):C(4) = 1:7:7:6 and parameter 1°:2°:3° = 1:10:33 measured for the oxidation of
n-heptane and methylcyclohexane, respectively, are typical values for the systems oxidizing via the
mechanism which involves hydroxyl radicals. Parameters trans/cis which are the ratio of isomers of tert-
alcohols with mutual trans- and cis-orientation of the methyl groups formed in the oxidation of cis- and
trans-1,2-dimethylcyclohexane (DMCH) were found to equal 0.8 for both cis-DMCH and trans-DMCH.
Oxidations of isomeric DMCH with hydroxyl radicals typically proceed non-stereoselectively. The
oxidations of n-heptane and methylcyclohexane (MCH) are reflected in Table 2 and 3, respectively.
Table 1
a
Selectivity parameters in the oxidation of alkanes by various systems in acetonitrile
4

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_
____________________________________________________________________________________
Entry
System
n-Heptane
MCH
c-DMCH
t-DMCH
_____________________________________________________________________________________
b
b
1
2
3
4
5
6
6
7
8
9
1–PCA–H O2
1:7:7:6
1:6:6:6
1:10:33
0.8
0.9
0.8
0.9
2
c
1–TFA–H O
1:4:15
1:7:18
1:6:17
1:6:18
2
2
d
e
f
g
1:6:20
FeSO –H O (see [6a])
1:5:5:4.5
1:7:7:7
1:6:7:5
1:7:7:6
1.3
1.0
0.75
0.85
1.2
0.9
0.8
0.9
4
2
2
Cp*Os–py–H O [6b]
1:8:23
1:9:37
1:7:26
2
2
–
VO –PCA–H O (see [6c])
3
2
2
–
VO –H SO –H O (see [6d])
3
2
4
2
2
a
Abbreviations for substrates: methylcyclohexane (MCH), cis-1,2-dimethylcyclohexane (c-
DMCH), trans-1,2-dimethylcyclohexane (t-DMCH). Parameters C(1):C(2):C(3):C(4) given for n-heptane
are relative normalized (i.e., calculated taking into account the number of hydrogen atoms at each carbon)
reactivities of hydrogen atoms at carbons 1, 2, 3 and 4, of the chain of the alkane. Parameters 1º:2º:3º
given for the oxidation of MCH are relative normalized reactivities of hydrogen atoms at primary,
secondary and tertiary carbons of this alkane. In the cases of oxidation of dimethylcyclohexanes,
parameter trans/cis is given which is the ratio of trans- and cis-isomers of tert-alcohols. All parameters
were measured after reduction of the reaction mixtures with triphenylphosphine before GC analysis and
calculated based on the ratios of isomeric alcohols.
b
This work.
Reaction time 15 min.
Reaction time 30 min.
Reaction time 2 h.
Reaction time 3 h. Total yield of products was 0.123 M (23% based on MCH).
Reaction time 4 h. Total yield of products was 21%
c
d
e
f
g
_____________________________________________________________________
Table 2
Oxidation of n-heptane
a
_____________________________________________________________________________________
Time
Products (isomeric alcohols, concentrations, M)
(min)
______________________________________________________________________________
Heptanol-4
Heptanol-3 Heptanol-2 Heptanol-1
Selectivity C(1):C(2):C(3):C(4)
_
1
____________________________________________________________________________________
5
0
0
0.005
0.009
0.010
0.022
0.012
0.020
0.021
0.046
0.013
0.020
0.021
0.047
0.003
0.005
0.007
0.011
1:6:6:5
1:6:6:6
1:4.5:4.5:4
1:6:6:5.5
2
6
20
5

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‒
3
Conditions. Ferrocene, 1 × 10 M; n-heptane, 0.5 M; H O (50%), 1.5 M; TFA, 0.05 M, 40 °C.
2
2
Concentrations we measured after addition of PPh . The total yield was 25% based on n-heptane, TON =
3
1
_
30.
____________________________________________________________________________________
Table 3
a
Oxidation of methylcyclohexane
_____________________________________________________________________________________
Time
Products (isomeric alcohols, concentrations, M)
(h)
______________________________________________________________________________
A
B
C
D
E
F
G
H
_____________________________________________________________________________________
0
0
1
2
3
4
.25
.5
0.004
0.002
0.003
0.002
0.001
0.003
0.0015
0.0020
0.0040
0.0070
0.0090
0.0070
0.0008
0.001
0.002
0.003
0.004
0.003
0.006
0.010
0.018
0.023
0.020
0.003
0.005
0.010
0.012
0.009
0.006
0.010
0.015
0.020
0.016
0.004
0.007
0.011
0.015
0.012
0.002
0.003
0.005
0.008
0.006
0.005
0.009
0.014
0.019
0.015
a
Products. A: 1-methylcyclohexanol; B: trans-2-methylcyclohexanol; C: cis-2-methylcyclohexanol; D:
trans-3-methylcyclohexanol; E: trans-4-methylcyclohexanol; F: cis-3-methylcyclohexanol; G: cis-4-
‒
3
methylcyclohexanol; H: cyclohexylmethanol. Conditions. Ferrocene, 1 × 10 M; MCH, 0.5 M; H O
2
2
(
50%), 1.5 M; TFA, 0.05 M, 40 °C. Concentrations we measured after addition of PPh . The total yield
3
was 23% based on MCH, TON = 115.
____________________________________________________________________________________
We have found that the rate dependence of cyclohexane oxygenation with H O in the presence of
2
2
–
3
the ferrocene/PCA system is second-order in PCA at its low (< 2 × 10 M) concentrations. This
dependence has a maximum (Fig. 1), thus suggesting the possible formation of adducts with PCA. The
dependence of the initial oxidation rate W is first-order in hydrogen peroxide (Fig. 2) and second-order in
0
the initial ferrocene (Fig. 3). It follows from data presented in Fig. 4 (W is approaching to a plateau at
0
high concentrations of cyclohexane) that the ratio of constants kMeCN and kCyH for the interaction of
oxidizing species with MeCN and CyH, respectively, is equal to kMeCN/kCyH = 0.011 M which is the
typical for hydroxyl radicals value [6a,7a] indicates that hydroxyl radicals as oxidizing species
competitively interact both with cyclohexane (CyH) and acetonitrile [6a,7a]. Addition of water to the
reaction solution leads to a slight decrease of the initial oxidation rate. If a new portion of hydrogen
peroxide is added when the oxidation reaction deceases, the oxidation continues although with lower rate.
6

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8
A
6
4
2
0
0
2
4
6
8
10
12
3
1
0 × [PCA] (M)
0
8
6
4
B
2
0
0
0.5
1.0
1.5
2.0
3
1
0 × [PCA] (M)
0
Fig. 1. The cyclohexane oxidation with H O catalyzed by ferrocene and PCA in MeCN. Dependence of
2
2
–
3
the initial rate W of oxygenate accumulation on the initial concentration of PCA. Graph A: (0–10) × 10
0
–
3
−3
M and Graph B: (0–2) × 10 M. Conditions: [FeCp ] = 0.5 × 10 M, [H O ] = 0.32 M, [cyclohexane]
0
2
0
2
2 0
=
0.37 M, 50 °C. Dotted curves present the simulated dependence.
3
0
0
2
10
0
0
1
2
[
H O ] (M)
2
2 0
Fig. 2. Dependence of the initial rate W of oxygenate accumulation in the cyclohexane oxidation with
0
H O catalyzed by ferrocene in acetonitrile on the initial concentrations of H O . Conditions: [FeCp ] =
2
2
2
2
2 0
−
4
−3
5
.0 × 10 M, [PCA] = 3.0 × 10 M, [cyclohexane] = 0.1 M, [H O] = const = 8.0 M, 50 °C.
0
0
2
total
7

ACCEPTED MANUSCRIPT
B
12
8
4
4
1
0 × [FeCp ] (M)
2
0
0
20
16
12
8
4
0
0
2
4
6
A
0
2
4
6
8
4
1
0 × [FeCp ] (M)
2 0
Fig. 3. Dependence of the initial rate W of oxygenate accumulation in the cyclohexane oxidation with
0
H O catalyzed by ferrocene in MeCN on the initial concentration of ferrocene (Graph A). Conditions:
2
2
−
3
[
PCA] = 3 × 10 M, [H O ] = 0.32 M, [cyclohexane] = 0.37 M, 50 °C. Graph B: linearization of the
0 2 2 0 0
½
curve shown in Graph A in coordinates [FeCp ] –W .
2
0
0
cyclohexane] − (M )
1
−1
[
0
0
2
4
6
8
10
0
.25
.20
0
2
0.15
1
2
9
6
0.10
1
3
0
0.0
0.2
0.4
0.6
0.8
1.0
[
cyclohexane] (M)
0
8

ACCEPTED MANUSCRIPT
Fig. 4. Dependence of the initial rate W of oxygenate accumulation in the cyclohexane oxidation with
0
H O catalyzed by ferrocene in MeCN on the initial concentration of cyclohexane (curve 1). Conditions:
2
2
−
4
−3
[
FeCp ] = 5.0 × 10 M, [PCA] = 3.0 × 10 M, [H O ] = 0.32 M, 50 °C. Straight line 2 is linearization
2 0 0 2 2 0
of the dependence presented by curve 1 in coordinates (1/W )–(1/[cyclohexane] ).
0
0
Based on the data presented in Figs. 1‒4 we proposed the following kinetic scheme of the alkane
+
oxygenation. The first stage is oxidation of ferrocene FeCp to ferricenium cation (FeCp ) which is in
2
2
turn transformed into species Φ, where Φ is a fragment containing one iron ion. The formation of
+
(
FeCp ) is supported by the visible spectra obtained for the reaction mixture in the initial period of the
2
reaction: a growing with time peak at λ = 625 nm corresponds to the absorption of ferricenium cation.
H+
+
FeCp + H O
2
(FeCp2)
(i.1)
(i.2)
2
2
+
(
FeCp ) → Φ
2
Reactions (i.1) and (i.2) are the fast stages of the generation from FeCp and H O the main species
2
2
2
Φ active in the catalytic process. A fragment produced in these two consecutive reactions interacts with a
PCA molecule to form complex Φ(pca):
Φ + PCA
Φ(pca)
(equilibrium constant K )
(2.1)
2
.1
Here pca is a fragment of PCA molecule (possibly pyrazinecarboxylate anion). The formed complex can
react with the second PCA molecule giving in this case an adduct containing two PCA fragments per one
iron ion:
Φ(pca) + PCA
Φ(pca)2
(equilibrium constant K2.2)
(2.2)
Two adducts Φ(pca) can dimerize to afford the dinuclear complex Φ (pca) :
2
2
Φ(pca) + Φ(pca)
Φ (pca)2
2
(equilibrium constant K2.3)
(2.3)
It is reasonable to assume that complex Φ (pca) is a catalytically active species which produces hydroxyl
2
2
radicals from H O as shown by eq. (2.4):
2
2
9

ACCEPTED MANUSCRIPT
•
Φ (pca) + H O → HO
(rate constant k )
(2.4)
2
2
2
2
2.4
Hydroxyl radicals react in parallel routes with solvent (acetonitrile) and substrate (cyclohexane, RH):
•
HO + MeCN → products
(rate constant k )
(2.5)
(2.6)
2
.5
•
HO + RH → ... → ROOH
(rate constant k )
2.6
Reaction (2.6) is the rate limiting step in the sequence of alkane (RH) transformations into cyclohexyl
•
hydroperoxide. Assuming that concentration of HO is steady-state one and concentrations of all iron
complexes are in quasi-equilibrium and taking into account the conditions [Φ(pca)] << [FeCp ] and
2
0
[
Φ (pca) ] << [FeCp ] (which mean that K and K are low values) we come to the equation (2.7) for
2 2 2 0 2.1 2.3
the initial reaction rate:
k [Φ (pca) ][H O ]
2 0
2
.4
2
2
2
d[ROOH]
W =
=
0
dt
1 + k [MeCN]/k [RH]
2.5 2.6 0
(2.7)
2
where [Φ (pca) ] = K [Φ(pca)]
2
2
2.3
K [FeCp ] [PCA]
0
2
.1
2 0
and [Φ(pca)] =
2
1
+ K K [PCA]
2.1 2.2 0
We can rewrite the equation (2.7) for the initial reaction rate in the following form:
α[PCA] 2
0
W =
0
2
2
(
1 + β[PCA] )
0
(2.8)
2
K k [H O ] (K [FeCp ] )
2
.3 2.4
2
2 0 2.1
2 0
where α =
1
+ k [MeCN]/k [RH]
2.5 2.6 0
and β = K K2
1
1
0

ACCEPTED MANUSCRIPT
–
1 –1
It is easy to calculate these values for the conditions of our experiments α = 30 M s and β = 1.1
5
–2
×
10 M . If we use these values of parameters α and β the initial reaction rates (presented by a dotted
curve in Fig. 1) can been calculated at different concentrations of PCA under conditions described in the
caption to Fig. 1. It can be seen that the simulated curve coincides closely with the experimental curve.
This is the evidence that our kinetic scheme adequately reflects the real reaction. We measured initial
oxidation rates at different temperatures. Activation energy calculated for the cyclohexane oxidation is
˗
1
equal to 15 ± 2 kcal mol .
2
.3. Hydroxylation of benzene to phenol with H O catalyzed by the combination ferrocene/PCA
2 2
Phenol is the primary product of the benzene oxidation. The kinetic features of the benzene
oxidation in the initial stage of the process also were studied. In the course of the process, we also
registered a change in the hydrogen peroxide concentration in special experiments. The H O
2
2
concentration in the samples with the reaction mixture was determined [7b] based on the optical
absorption of the titanium peroxide complex formed in the reaction between an acidified solution of
Ti(SO ) and H O in the visible spectral region. The evolution of oxygen from the reaction mixture was
4
2
2
2
also observed during the process, which testifies the presence of a catalase component in hydrogen
peroxide decomposition. The observed rate of the oxidation of benzene to phenol was notably lower than
the registered rate of decrease in the hydrogen peroxide concentration.
Fig. 5 demonstrates typical kinetic curves of the phenol accumulation. It can be seen that the
benzene oxidation process under these experimental conditions deceases mainly in 30 min regardless of
the initial concentration of ferrocene. The degree of hydrogen peroxide conversion, α = ([H O ] –
t
2
2 0
[
H O ] )/[H O ] in this time interval does not exceed 30%, and the addition of a new portion of fresh
2 2 t 2 2 0
catalyst gives rise to the decomposition of additional amounts of H O and the formation of additional
2
2
amount of phenol. Thus the termination of the reaction is due to the catalyst inactivation. The final stage
of its inactivation is the sedimentation of iron oxides that precipitate from a solution. In this work, we
measured the initial rates of the phenol formation under conditions in which precipitation is not observed;
i.e., the degree of catalyst inactivation was low.
1
1

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0.04
3
0
.03
.02
0
2
1
0
.01
.00
0
0
10
20
30
40
50
60
70
Time (min)
Fig. 5. Oxidation of benzene (0.56 M) to phenol with H O (1.28 M; 50% aqueous) catalyzed by
2
2
–
4
–4
–
ferrocene and PCA (25 × 10 M) in MeCN at 50 ºC at ferrocene concentration 1 × 10 (curve 1), 3 × 10
4
–4
(
2), 5 × 10 M (3). Concentrations of phenol at various time intervals are given in the Graph. The
maximum initial reaction rate, W , is determined by the slope of the tangent to a particular kinetic curve at
0
the maximum rate point.
The kinetic features studied for benzene oxidation, i.e., the dependences of the initial rate of
phenol formation on the initial concentrations of ferrocene, PCA, hydrogen peroxide, and benzene in
acetonitrile at 50 °C, are given in Figs. 6–9, respectively. It is evident that the initial reaction rate W0
depends quadratically on [Cp Fe] and is directly proportional to [H O ] . The pattern of the W
2
0
2
2 0
0
dependence on [C H ] corresponds to a competitive reaction of benzene and acetonitrile with the active
6
6 0
oxidizing species generated during hydrogen peroxide decomposition, as it was observed in the previous
Section in the oxidation of cyclohexane.
2
1
1
0
6
2
8
4
0
1
2
1
8
4
2
0
0
2
4
6
8
10
12
4
1
0 × [FeCp ] (M)
2
0
Fig. 6. Dependence of the initial rate W of phenol accumulation in the benzene oxidation with H O
2
0
2
catalyzed by ferrocene in MeCN on the initial concentration of ferrocene (curve 1). Conditions: [PCA] =
0
1
2

ACCEPTED MANUSCRIPT
−
3
2
.5 × 10 M, [H O ] = 1.28 M, [benzene] = 0.56 M, 50 °C. Curve 2: linearization of curve 1 in
2 2 0 0
½
coordinates [FeCp ] –W .
2
0
0
3
1
0 × [PCA] (M)
0
0
0.5
1.0
1.5
2.0
1
0
8
4
3
2
1
0
6
4
2
0
0
5
10
0 × [PCA] (M)
15
3
1
0
Fig. 7. Dependence of the initial rate W of phenol accumulation in the benzene oxidation with H O
0
2
2
–
3
catalyzed by ferrocene in MeCN on the initial concentration of PCA in the intervals (0–10) × 10 M and
–
3
−4
(
0–2) × 10 M. Conditions: [FeCp ] = 5.0 × 10 M, [H O ] = 1.28 M, [benzene] = 0.56 M, 50 °C.
2 0 2 2 0 0
Dotted curves present the simulated dependence.
8
6
4
2
0
0
0.2
0.4
0.6
0.8 1.2
1.4
[
H O ] (M)
2
2 0
Fig. 8. Dependence of the initial rate W of phenol accumulation in benzene oxidation with H O
2
0
2
catalyzed by ferrocene in acetonitrile on the initial concentrations of H O . Conditions: [FeCp ] = 5.0 ×
2
2
2 0
−
4
−3
1
0 M, [PCA] = 2.5 × 10 M, [benzene] = 0.56 M, 50 °C.
0 0
1
3

−1ACCEPTED MANUSCRIPT
benzene]0− (M )
1
[
0
4
8
12
16 20
1.0
2
5
4
3
0.8
0.6
0.4
0.2
0
2
1
1
0
0.0
0.2
0.4
0.6
[
benzene] (M)
0
Fig. 9. Curve 1: dependence of the initial rate W of phenol accumulation in the benzene oxidation with
0
H O catalyzed by ferrocene in MeCN on the initial concentration of benzene. Conditions: [FeCp ] = 5.0
2
2
2 0
−
4
−3
×
10 M, [PCA] = 2.5 × 10 M, [H O ] = 1.28 M, 50 °C. Straight line 2: linearization of the
0
2
2 0
dependence presented by curve 1 in coordinates (1/W )–(1/[benzene] ).
0
0
The observed features of benzene oxidation are similar to those ones that are described in the
previous Section for the cyclohexane oxidation. Comparative analysis of the data obtained for
cyclohexane and benzene reveals two substantial differences. The first difference is that the oxidation rate
of benzene is lower than cyclohexane oxidation rate at close concentrations of both substrates and the
same concentrations of the other reactants. Such differences are also observed in the hydrogen peroxide
decomposition rates; specifically, the rate of H O decomposition in the presence of benzene is lower than
2
2
the rate of its decomposition in the presence of cyclohexane, which is close to the rate of decomposition
in the absence of a substrate. The data in the Table 4 illustrate the above observations. The degrees of
hydrogen peroxide conversion after 5 min are nearly the same in the presence and absence of
cyclohexane. The α value is 0.2 at t = 20 min in the presence of benzene that is α ≈ 0.05 at t = 5 min in the
presence of benzene. Consequently, the hydrogen peroxide decay rate is at least 4 times lower. The
second difference is the following. The observed dependence of the initial cyclohexane oxidation rate on
its initial concentration (Fig. 4) is in agreement with the assumption that the species that induces the
oxidation of the alkane is a hydroxyl radical involved in competitive oxidation reactions of both the
substrate (cyclohexane) and solvent (acetonitrile). The formation of phenol from benzene does not
contradict the possibility that the oxidizing species is the hydroxyl radical. However, it follows from the
data presented in Fig. 9 that the ratio between the rate constants for the interactions of oxidizing species
1
4

ACCEPTED MANUSCRIPT
with benzene and acetonitrile is considerably lower than the expected value. According to the data given
•
in Ref. [7a], the rate constant ratio for the reactions of the HO radical with cyclohexane and CH CN is
3
3
•
1
.6 × 10 . As the reaction rate constant of the HO radical with benzene is 4 to 5 times higher than the rate
•
•
•
constant of the reaction between HO and cyclohexane [7c], the k(HO + benzene)/k(HO + CH CN) ratio
3
•
•
also should be significantly higher than the k(HO +cyclohexane)/k(HO + CH CN) value. It follows,
3
•
•
however, from the data of Fig. 9 that the k(HO + benzene)/k(HO + CH CN) ratio is 100. This ratio is
3
substantially lower than the expected value if we assume that the oxidizing species is hydroxyl radical
and that constants of the interaction between hydroxyl radical with acetonitrile and benzene in water and
acetonitrile are equal.
Table 4
Dependence of initial reaction rate on reactant concentration as well as degree of H O consumption
2
2
_____________________________________________________________________________________
5
5
Entry Concentration, M
αt
10 W(CyOOH) 10 W(PhOH)
‒
1
‒1
_
__________________________________________
M s
M s
4
3
1
0 [Cp Fe] 10 [PCA] [cyclohexane] [benzene]
0
2
0
0
0
_
1
2
3
4
5
6
____________________________________________________________________________________
5
2.5
3.0
2.5
2.5
2.5
3.0
0
0
0.20
0
0
5
0.46
0
0
0.25 29
0
5
0.56
0.2
0.56
0
0.20
0.18
0.06
0.05
0
0
0
8
4
5
0
2.5
1
2.5
2.5
0
0.46
0
Conditions. Acetonitrile, 50 °C, [H O ] = 1 M. Parameters α = ([H O ] – [H O ] )/[H O ] , in entries 1
2
2 0
t
2
2 0
2
2 t
2
2 0
and 2 were determined at t = 5 min, and for other runs at t = 20 min. The rate of cyclohexyl
hydroperoxide formation, W(CyOOH), was calculated for the conditions given in the table from the data
of [7a]; the rate of phenol formation W(PhOH), from the data of Figs. 6,8,9.
_____________________________________________________________________________________
A contradiction found here leads to the conclusion that the oxidizing species in the case of
benzene differs slightly in reactivity from the free hydroxyl radical assuming that rate constants of
interaction between hydroxyl radicals and acetonitrile and benzene in water and acetonitrie are equal. In
the presence of benzene, the rate of the formation of species which induce its oxidation is lower than this
parameter for the active species which induce the cyclohexane oxidation (namely, free hydroxyl radicals).
The total rate of hydrogen peroxide decomposition (which includes both catalase and oxidase routes) in
the presence of benzene is lower than in the presence of cyclohexane. These data suggest that a
1
5

ACCEPTED MANUSCRIPT
catalytically active species generated in the presence of benzene is different from the one formed in the
absence of benzene. If we use the kinetic scheme (i1)‒(2.4) we can say that in the presence of benzene a
dimeric species containing two pca fragments catalyzes hydrogen peroxide decomposition (2.4) less
efficiently. As the rates of hydrogen peroxide decomposition and benzene oxidation are symbatic (e.g.,
their dependences on [Cp Fe] are similar), the failure of the rate of hydrogen peroxide decomposition to
2
0
depend on the initial concentration of benzene within the range of 0.2 to 0.56 M leads to the conclusion
that the rate of the formation of oxidizing particles does not also depend on [benzene] within the
0
indicated interval. The dependence shown in Fig. 9, thus reflects the competitive reaction of benzene with
an oxidizing species, as in the case of the competitive reaction indicated by the dependence of the rate of
cyclohexane oxidation (Fig. 4) in which a free hydroxyl radical plays the role of an oxidizing species.
This validates our estimate of the values of k(X + benzene)/k(X +CH CN), where X is an oxidizing
3
species, because it is based on the assumption that the rate of the formation of active species does not
depend on variations in the concentration of an oxidized substrate.
On the one hand, benzene changes the catalytic activity of a catalyst, but on the other hand, the
concentration of a catalytically active species remains unchanged when varied within the range
[
benzene] of 0.2 to 0.56 M. This allows us to estimate roughly the value of equilibrium constant K for
0
the formation of the adduct A formed between benzene and a catalytically active species:
Φ (pca) + benzene (2.9)
A
2
2
If we assume that, in line with the data in Fig. 6, the rate of oxidation within the accuracy of our
experiments (~20%) is proportional to the squared concentration of Cp Fe and is not changed within the
2
–
1
benzene concentration range of 0.2 to 0.56 M, we can calculate K ≥ 70 M . This means that the A adduct
–
3
at low concentrations (e.g., 10 M) is almost completely disintegrated to produce a binuclear species and
benzene molecule, and the behavior of the system is the same as when only ferrocene is added to the
reaction mixture. Only at high concentrations of benzene the reactivity of the system differs appreciably
from that of an initial system with no aromatic component capable of π-coordination with metal ions.
2
.4. Hydroperoxidation of alkanes to alkyl hydroperoxides with H O catalyzed by the combination
2 2
ferrocene/trifluoroacetic acid (TFA)
In the alkane oxidation PCA as a co-catalyst can be replaced by trifluoroacetic acid (TFA).
Experimentally obtained kinetic data which show the main peculiarities of this reaction are presented by
Figs. 10-16.
1
6

ACCEPTED MANUSCRIPT
0.06
0.05
0.04
-ol after PPh3
0.03
0.02
0.01
-
ol before PPh3
-
one before PPh3
-
one after PPh3
0
0
10
20
30
40
50
60
Time (min)
Fig. 10. Accumulation of cyclohexanol (-ol) and cyclohexanone (-one) in the oxidation of cyclohexane
−
3
(
0.46 M) with H O (0.5 M containing 0.94 M H O) catalyzed by 1 (1.0 × 10 ) in the presence of TFA
2 2 2
(0.05 M) at 50 °C. Concentrations of -ol and -one were measured twice: before and after reduction of the
reaction sample with solid PPh3.
5
4
2
3
2
1
0
0
2
4
6
2
½
½
5
10 × [FeCp ]
(M )
2
0
4
3
2
1
1
0
0
2
4
6
3
10 × [FeCp ] (M)
2 0
Fig. 11. Dependence of the initial rate W of oxygenate accumulation in the cyclohexane oxidation with
0
H O catalyzed by ferrocene and TFA in MeCN on the initial concentration of ferrocene (curve 1).
2
2
1
7

ACCEPTED MANUSCRIPT
Conditions: [TFA] = 0.05 M, [H O ] = 0.50 M, [H O] = 0.94 M, [cyclohexane] = 0.46 M, 50 °C.
0
2
2 0
2
total
0
½
Curve 2: linearization of curve 1 in coordinates [FeCp ] –W .
2
0
0
3
2
1
1
0
0
0.05
0.10
0.15
0.20
[
TFA] (M)
0
Fig. 12. Dependence of the initial rate W of oxygenate accumulation in the cyclohexane oxidation with
0
H O catalyzed by ferrocene and TFA in MeCN on the initial concentration of TFA (curve 1). Conditions:
2
2
[
1] = 0.001 M, [TFA] = 0.05 M, [H O ] = 0.50 M, [H O] = 0.94 M, [cyclohexane] = 0.46 M, 50 °C.
0
0
2
2 0
2
total
0
1
1
6
2
8
4
0
0
0.5
1.0
1.5
[
H O ] (M)
2
2 0
Fig. 13. Dependence of the initial rate W of oxygenate accumulation in the cyclohexane oxidation with
0
H O catalyzed by ferrocene and TFA in MeCN on the initial concentration of H O . Conditions: [1] =
2
2
2
2
0
0
.001 M; [TFA] = 0.05 M, [H O] = const = 2.83 M, [cyclohexane] = 0.46 M, 50 °C.
0 2 total 0
1
8

ACCEPTED MANUSCRIPT
8
6
4
2
0
0
1
2
3
4
5
6
[
H O]
2
(M)
total
Fig. 14. Dependence of the initial rate W of oxygenate accumulation in the cyclohexane oxidation with
0
H O catalyzed by ferrocene and TFA in MeCN on the total concentration of present water. Conditions:
2
2
[
1] = 0.001 M; [TFA] = 0.05 M, [H O ] = 0.50 M, [cyclohexane] = 0.46 M, 50 °C.
0 0 2 2 0 0
3
2
2
1
0
0
0.4
0.8
1.2
[cyclohexane]0 (M)
2
.5
2
1
.0
.5
1
1
0
.0
.5
0
0.0 0.2 0.4
0.6 0.8 1.0
[
cyclohexane] (M)
0
1
9

ACCEPTED MANUSCRIPT
Fig. 15. Dependence of the initial rate W of oxygenate accumulation in the cyclohexane oxidation with
0
H O catalyzed by ferrocene and TFA in MeCN on the initial concentration of cyclohexane. Conditions:
2
2
[
1] = 0.001 M; [TFA] = 0.05 M, [H O ] = 0.50 M, [H O] = 0.94 M, 50 °C.
0 0 2 2 0 2 total
−9.9
−
1
E = 15.2 ± 2 kcal mol
a
1
−10.9
−11.9
2
E = 13.0 ± 2 kcal mol−
1
a
0.0030
0.0031
0.0032 0.0033
−
1
1
/T (K )
Fig. 16. Dependence of initial reaction rate on temperature in the cyclohexane hydroperoxidation (line 1)
and benzene oxidation to phenol (line 2) with H O catalyzed by ferrocene and TFA. Conditions: [1] =
2
2
0
0
.001 M, [TFA] = 0.05 M, [H O ] = 0.5 M, [H O] = 0.94 M, [cyclohexane] = 0.46 M. The Arrhenius
2 2 0 2 total 0
˗1
˗1
plots give the effective activation energy E = 15.2 ± 2 kcal mol and 13.0 ± 2 kcal mol for the
a
oxidations of cyclohexane and benzene, respectively.
2
.5. The ESI-MS study of the oxidation with H O catalyzed by ferrocene and TFA
2 2
To get insight into the nature of the catalytically active species we studied the interaction of
ferrocene, TFA and H O in acetonitrile by the ESI-MS method. The ESI-MS spectrum of the solution
2
2
(Fig. 17) exhibits two main peak clusters. The first peak cluster with m/z = 294 corresponds to binuclear
iron complex bearing two bridged hydroxo ligands. Another complex (m/z = 312) is a product of H O
2
addition to the iron dimer.
2
0

ACCEPTED MANUSCRIPT
Besides, we monitored by ESI-MS the catalytic hydrogen peroxide oxidation of cyclohexane in
the ferrocene/TFA system (Fig. 18). The ESI-MS spectra are complicated and exhibit few peak clusters
with m/z values up to 1265. We assume the formation in this reaction of hydroxo- and oxo- bridged
oligomers of iron with CpFe moieties similar to the species described in the literature [8]. This
assumption is in agreement with the fact of acceleration of cyclohexane oxidation in ferrocene-TFA
system by water (see Fig. 14), because water is required to formation of polynuclear iron complexes. It is
noteworthy that the ESI-MS spectra of the benzene oxidation (Fig. 19) differ significantly from that
obtained for the reaction with cyclohexane, which is probably due to the formation of ꢀ-phenoxo-bridged
complexes in the case of benzene.
Fig. 17. The ESI-MS spectrum of the mixture ferrocene/H O /TFA in acetonitrile. The spectrum was
2
2
‒
3
taken immediately after adding H O (1.5 M) to the solution of ferrocene (1 × 10 M) and TFA (0.05 M)
2
2
in acetonitrile at room temperature.
2
1

ACCEPTED MANUSCRIPT
Fig. 18. ESI-MS monitoring of the cyclohexane (0.45 M) oxidation with H O catalyzed by
2
2
ferrocene/TFA in acetonitrile at 40 °C (after 20 min). For concentrations of the reactants, see caption to
Fig. 17.
2
2

ACCEPTED MANUSCRIPT
Fig. 19. ESI-MS monitoring of the benzene (0.45 M) oxidation with H O catalyzed by ferrocene/TFA in
2
2
acetonitrile. For other conditions, see caption to Fig. 17.
2
.6. Hydroxylation of benzene to phenol with H O catalyzed by the combination ferrocene/TFA
2 2
We studied the kinetic features of the benzene oxidation catalyzed by ferrocene in the presence of
TFA. The square root dependence of the initial oxidation rate on initial ferrocene concentration (Fig. 20)
testifies that the reaction is the half order in ferrocene when TFA is used as a co-catalyst. The situation is
opposite to that found for the case of PCA (see Fig. 6) where this dependence is quadratic one. It is
noteworthy that in the cyclohexene oxidation co-catalyzed by TFA the reaction is also the half order in
ferrocene (Fig. 11).
2
2
1
0
0
2
4
6
2
½
½
1
0 × [FeCp ]
(M )
2
0
2
1
1
0
0
2
4
6
3
1
0 × [FeCp ] (M)
2 0
Fig. 20. Dependence of the initial rate W of phenol accumulation in the benzene oxidation with H O
2
0
2
catalyzed by ferrocene and TFA in MeCN on the initial concentration of ferrocene (curve 1). Conditions:
TFA] = 0.05 M, [H O ] = 0.50 M, [H O] = 0.94 M, [benzene] = 0.46 M, 50 °C. Curve 2:
[
0
2
2 0
2
total
0
½
linearization of curve 1 in coordinates [FeCp ] –W .
2
0
0
2
3

ACCEPTED MANUSCRIPT
The dependence of the initial oxidation rate W on concentration of added TFA has a maximum
0
(
Fig. 21). The curve corresponding to the dependence of W on the initial concentration of benzene
0
approaches a plateau (Fig. 22).
1
.5
1.0
1
0
.5
0
0
0.1
0.2
0.3
[
TFA] (M)
0
Fig. 21. Dependence of the initial rate W of phenol accumulation in the benzene oxidation with H O
2
0
2
catalyzed by ferrocene and TFA in MeCN on the initial concentration of TFA (curve 1). Conditions: [1]0
0.001 M, [H O ] = 0.50 M, [H O] = 0.94 M, [benzene] = 0.46 M, 50 °C.
=
2
2 0
2
total
0
2
4

ACCEPTED MANUSCRIPT
6
4
2
0
2
0
0.4
0.8
1.2
[benzene]0 (M)
1.5
1.0
1
0.5
0
0.0 0.2 0.4
0.6 0.8 1.0
[
benzene] (M)
0
Fig. 22. Dependence of the initial rate W of phenol accumulation in the benzene oxidation with H O
2
0
2
catalyzed by ferrocene and TFA in MeCN on the initial concentration of benzene. Conditions: [1] =
0
0
.001 M; [TFA] = 0.05 M, [H O ] = 0.50 M, [H O] = 0.94 M, 50 °C.
0 2 2 0 2 total
2
.7. Kinetic analysis of cyclohexane hydroperoxidation or benzene hydroxylation with H O catalyzed by
2 2
the ferrocene/TFA combination
Dependencies of rates of substrate (cyclohexane or benzene) oxidation on concentration of these
substrates (see Figs. 15 and 22) correspond to our assumption that the oxidation is induced by an
oxidizing species X which is generated in the system. Species X is consumed in two parallel routes; one
of them is the interaction of X with substrate S. We can propose the following kinetic scheme:
H O → X
(rate Wi)
(i)
2
2
X + S → → products
(2.10)
(2.11)
X + CH CN → → products
3
where (i) is the stage of X generation with rate W , (2.10) is the sequence of transformations of S into
i
products (cyclohexane into cyclohexyl hydroperoxide, benzene into phenol) with the rate constant of rate
limiting step k2.10, (2.11) is the sequence of CH CN transformations into products under the action of
3
species X with constant of this limiting step k2.11
.
2
5

ACCEPTED MANUSCRIPT
The analysis of this scheme in quasi-stationary approximation relative species X leads to the
expression (2.12):
k2.10[S]Wi
d[S]
dt
=
k2.10[S] + k2.11[CH CN]
3
(2.12)
Let us transform this equation to the form which is convenient for the analysis of experimental data:
[
S]
1
k2.11[CH CN]
3
=
[S] +
d[S] Wi
dt
k2.10
(2.13)
The experimental data presented in Figs. 15 and 22 are in accordance with eq. (2.13): a linear dependence
of [S]/( d[S]dt) on [S] exists (see Graphs B in these Figures). It follows from these data that values
k2.11[CH CN]/k in the oxidation reactions of cyclohexane CyH and benzene PhH are equal to
‒
3
2.10
k2.11[CH CN]/k = 0.14 M and k2.11[CH CN]/k = 0.68 M. At the same time, it follows from the
3
CyH
3
PhH
tangent of straight line slope that the rates of generation of active species in both cases are close: W = 2.6
i
‒
4
‒1
‒4
‒1
×
10 M s for CyH and W = 3.0 × 10 Ms for PhH. This can be easily understood because in both
i
oxidations oxidizing species are generated by the same system.
The selectivity parameters in the oxidation of branched alkanes indicate that the oxidizing species
generated by the system is hydroxyl radical. The value k2.11[CH CN]/k = 0.14 M found in our work for
3
CyH
the system under discussion is in good agreement with this statement. It is, however, noteworthy that in
the reaction of benzene oxidation the analogous parameter k2.11[CH CN]/k = 0.68 M distinguishes
3
PhH
noticeably from the expected value. It is known that in aqueous solution constant of the interaction
between hydroxyl radical and cyclohexane is 5 6 times lower than the corresponding constant for the
reaction with the participation of benzene. Thus, the expected value k2.11[CH CN]/k should be lower.
‒
3
PhH
As the data on selectivity of branched alkane oxidation relatively reliably indicate that the oxidizing
species is hydroxyl radical we have to assume that the reactivity of hydroxyl radical in acetonitrile is
sufficiently lower in comparison with reactions in aqueous solution. To our knowledge, the corresponding
data are absent in the literature. We can assume that the decrease of benzene reactivity relative hydroxyl
radical in acetonitrile solution is due to the solvation of benzene by acetonitrile molecules. Benzene is
shielded by acetonitrile molecules and is less available for hydroxyl radicals.
Method ESI-MS demonstrated that in the acetonitrile solution in the presence of TFA ferrocene
under the action of hydrogen peroxide is transformed into dimeric species whereas concentration of
monomeric particles is negligible. This indicates the existence of the equilibrium
2
FeLn
{Fe L }
(2.14)
2
2n
Here which L are certain oxygen-containing ligands coordinated to one (FeL ) or two (Fe L ) iron ions.
n
2 2n
Under conditions of our experiments the equilibrium (2.14) shifted to the right, that is [FeL ]/[{Fe L }]
n
2 2n
<< 1. In this case in the eq. (2.15)
2
6

ACCEPTED MANUSCRIPT
2
K2.14[{Fe L }] = [FeL ]
(2.15)
2
2n
n
we can accept that [{Fe L }]
≈
[FeL ] /2, and consequently
2
2n
n 0
[
FeL ]
n 0
[
FeL ] =
n
2
K2.14
(2.16)
We can see that observed oxidation rate for both cyclohexane and benzene is proportional to
½
[
FeL ] which is reflected by straight lines in Figs. 11B and 20B. This leads to the conclusion that the
n 0
generation of oxidizing species occurs with the participation of monomeric forms of iron complex.
2
.8. Competitive alkane hydroperoxidation and benzene hydroxylation with H O catalyzed by the
2 2
ferrocene/TFA combination
Figs. 23 and 24 present the data describing effect of benzene additives on the initial reaction rate
of cyclohexane oxidation (the rate of CyOOH formation) and yield of phenol (PhOH). Fig. 25 shows the
effect of cyclohexane additives on the rate of phenol formation and yield of ROOH. In both cases the
substrates are oxidized with H O catalyzed by the ferrocene/TFA combination. Figs. 26 and 27
2
2
demonstrate analogous experiments when the catalytic system (n-Bu N)VO /PCA was used. It can be
4
3
seen that in all experiments in the presence of the two substrates (cyclohexane and benzene) the growth of
concentration of one substrate leads to increase of its oxidation rate and to simultaneous decrease of the
oxidation rate of the second hydrocarbon. This testifies that a concurrent oxidation of benzene or
cyclohexane is induced by the same oxidizing species generated in the course of the hydrogen peroxide
decomposition. At the same time these data indicate that cyclohexane more efficiently interacts with the
oxidizing species in comparison with benzene. For example, benzene additives (Figs. 24 and 26) only
relatively weakly decrease the initial rate of cyclohexane oxidation at [cyclohexane] = 0.1 M. On the
0
other hand, cyclohexane additives sufficiently depress the phenol yield at [benzene] = 0.46 M (Figs. 25
0
and 27). This conclusion is supported by kinetic data presented in Fig. 23 which shows that the yield of
cyclohexane oxygenates only weakly depends on concentration of benzene additives.
2
7

ACCEPTED MANUSCRIPT
0.05
0.04
0.03
0.02
0.01
2
1
3
3a
2
a
0
0.0 10
20
30 40
50
60
Time (min)
Fig. 23. Competitive oxidation of cyclohexane and benzene with H O catalyzed by the ferrocene/TFA
2
2
system. The oxidation of cyclohexane (0.46 M) to a sum of cyclohexanol and cyclohexanone in the
absence (curve 1) and in the presence of benzene (0.2 M; curve 2 and 0.6 M; curve 3) is shown.
Accumulation of phenol in two last experiments is shown by curves 2a and 3a, respectively. Conditions:
−
3
H O (0.5 M containing 0.94 M H O), catalyst
1
(1.0 × 10 M), TFA (0.05 M) at 50 °C. Concentrations
2
2
2
of products were measured after reduction of the reaction samples with solid PPh3.
1
1
.5
.0
1
PhOH
CyOH
2
0
.5
0
0.0 0.2 0.4
0.6 0.8 1.0
[
benzene] (M)
0
Fig. 24. Dependence of the rate W of the oxidation of cyclohexane (0.1 M) with H O catalyzed by the
0
2
2
ferrocene/TFA system to (mainly) cyclohexanol (curve 1) on concentration of added benzene. The rate of
phenol accumulation is shown by curve 2. Conditions: H O (0.5 M containing 0.94 M H O), catalyst
2
2
2
−
3
ferrocene (1.0 × 10 M), TFA (0.05 M) at 50 °C. Concentrations of products were measured after
reduction of the reaction samples with solid PPh3.
2
8

ACCEPTED MANUSCRIPT
CyOH
2
1
1
0
.0
.5
.0
.5
1
PhOH
2
0
0.0 0.1 0.2
0.3 0.4 0.5
[
cyclohexane] (M)
0
Fig. 25. Dependence of the rate W of the oxidation of benzene (0.46 M) with H O catalyzed by the
0
2
2
ferrocene/TFA system to phenol (curve 2) on concentration of added cyclohexane. The rate of
cyclohexanol accumulation is shown by curve 1. Conditions: H O (0.5 M containing 0.94 M H O),
2
2
2
−
3
catalyst ferrocene (1.0 × 10 M), TFA (0.05 M) at 50 °C. Concentrations of products were measured
after reduction of the reaction samples with solid PPh3.
2
.0
.5
1
CyOH
1
1.0
2
0
.5
0
PhOH
0
.0 0.2 0.4
0.6 0.8 1.0
[
benzene] (M)
0
Fig. 26. Dependence of the rate W of the oxidation of cyclohexane (0.1 M) with H O catalyzed by the
0
2
2
(
n-Bu N)VO /PCA system to (mainly) cyclohexanol (curve 1) on concentration of added benzene. The
4 3
rate of phenol accumulation is shown by curve 2. Conditions: H O (0.5 M containing 0.94 M H O),
2
2
2
−
4
−4
catalyst (n-Bu N)VO (1.0 × 10 M), PCA (4.0 × 10 M) at 50 °C. Concentrations of products were
4
3
measured after reduction of the reaction samples with solid PPh3.
2
9