Alkane hydroperoxidation with peroxides catalysed by copper complexes

Article abstract of DOI:10.1039/b306382h

Various copper(I) and copper(II) derivatives, both "simple" ones (copper acetate, perchlorate and a complex with CH₃CN) and compounds containing N,O-chelating ligands, catalyse very efficient (turnover numbers attain 2200) oxidation of saturated hydrocarbons with peroxyacetic acid (PAA) or tert-butyl hydroperoxide (TBHP) in acetonitrile solution at 60 °C. Alkyl hydroperoxide, alcohol and ketone are formed, the main product being an alkyl hydroperoxide in the oxidation with PAA and an alcohol for the case of TBHP. It has been proposed that the oxidation with PAA is induced via the attack of species r [HO or CH₃C(=O)O ] on the alkane, RH. A competitive attack of r on the solvent, CH₃CN, also occurs. It has been assumed that in the case of the reaction catalysed by complex Cu(CH₃CN) ₄BF₄, copper is present mainly in the form of Cu ⁺ cation, and the rate-limiting step of the oxidation process is the formation of r via reaction (1): CH₃C(=O)OOH + Cu⁺ → CH₃C(=O)O + HO + Cu²⁺ or/and CH₃C(=O)OOH + Cu⁺ → CH ₃C(=O)O + HO + Cu₂₊ with initial rate W₁ = k₁[PAA][Cu(CH₃CN) ₄BF₄] and k₁ = 1.7 mol_-1 dm ³ s^-1 at 60 °C. The activity of the Cu-catalyst is dramatically changed on a small modification of N,O-chelating ligands in the catalyst.

Full text of DOI:10.1039/b306382h

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Alkane hydroperoxidation with peroxides catalysed by copper
complexes
a
b
a
Georgiy B. Shul’pin,* Julieta Gradinaru and Yuriy N. Kozlov
a
Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina,
dom 4, Moscow 119991, Russia.
E-mail: Shulpin@chph.ras.ru; http://members.fortunecity.com/shulpin
Institute of Chemistry, Moldovian Academy of Sciences, Academiei 3, Chisinau MD 2028,
Moldova
b
Received 6th June 2003, Accepted 1st September 2003
First published as an Advance Article on the web 18th September 2003
Various copper() and copper() derivatives, both “simple” ones (copper acetate, perchlorate and a complex with
CH CN) and compounds containing N,O-chelating ligands, catalyse very efficient (turnover numbers attain 2200)
3
oxidation of saturated hydrocarbons with peroxyacetic acid (PAA) or tert-butyl hydroperoxide (TBHP) in acetonitrile
solution at 60 ЊC. Alkyl hydroperoxide, alcohol and ketone are formed, the main product being an alkyl
hydroperoxide in the oxidation with PAA and an alcohol for the case of TBHP. It has been proposed that the
ؒ
ؒ
ؒ
oxidation with PAA is induced via the attack of species r [HO or CH C(᎐O)O ] on the alkane, RH. A competitive
3
᎐
ؒ
attack of r on the solvent, CH CN, also occurs. It has been assumed that in the case of the reaction catalysed by
complex Cu(CH CN) BF , copper is present mainly in the form of Cu cation, and the rate-limiting step of the
oxidation process is the formation of r via reaction (1): CH C(᎐O)OOH ϩ Cu
3
ϩ
3
4
4
ؒ
ϩ
ؒ
Ϫ
2ϩ
CH C(᎐O)O ϩ HO ϩ Cu
3
᎐
3
᎐
ϩ
Ϫ
ؒ
2ϩ
or/and CH C(᎐O)OOH ϩ Cu
CH C(᎐O)O ϩ HO ϩ Cu with initial rate W = k [PAA][Cu(CH CN) BF ]
3
᎐
3
᎐
1
1
3
4
4
Ϫ1
3
Ϫ1
and k = 1.7 mol dm s at 60 ЊC. The activity of the Cu-catalyst is dramatically changed on a small modification
1
of N,O-chelating ligands in the catalyst.
and lipoproteins, particularly with copper ion participation,
Introduction
9
play an important role in the living cell metabolism.
Oxidative activation of various C–H compounds including very
inert alkanes is an important field of contemporary metal-
complex catalysis. Copper complexes are known to catalyse
10
11
Alkyl hydroperoxides and peroxy acids are widely used as
1
oxidants in metal-catalysed functionalization of hydrocarbons.
1
They are the most important oxidizing reagents in organic
catalytic chemistry after molecular oxygen and hydrogen
peroxide.
benzylic and allylic oxidations by alkyl hydroperoxides and
2
peresters. The allylic oxidation of hydrocarbons containing
double bonds (the Kharasch–Sosnovsky reaction) is used
for the synthesis of allylic esters. Much less is known about
3
copper-catalysed oxidations of completely saturated hydro-
1c,d,4
Results and discussion
carbons.
It should be noted that copper complexes also
5
catalyse efficient oxidations of some other organic compounds
such as alcohols and aromatics. It is important to emphasize
that certain oxidizing enzymes contain copper ions
are often surrounded by N,O-chelating ligands.
We have found that various copper complexes are very efficient
catalysts in alkane oxidations by peroxyacetic acid (PAA). The
reactions were carried out in acetonitrile solution at 60 ЊC.
Table 1 demonstrates that under these conditions certain salts
of transition metals exhibit relatively low catalytic activity.
6
7
8
a–f
which
8
g–n
Hydro-
peroxides formed by the oxidation of C–H bonds in proteins
a
Table 1 Cyclohexane oxidation with peroxyacetic acid catalysed by various metal complexes
Ϫ3
Product concentration (mol dm
)
Entry
Catalyst
Time, h
Cyclohexyl hydroperoxide
Cyclohexanol
Cyclohexanone
TON
1
2
3
4
5
6
7
8
9
0
1
2
3
None
3
2
2
2
2
2
2
3
2
2
2
2
2
0.0008
0.0006
0.0018
0.0014
0.0014
0.0007
0.0340
0.0130
0.0270
0.0160
0.0180
0.0010
0
0
0
0
0
0
0
0
0
0
0
0
—
(11)
36
Ni(ClO4)2
Co(ClO4)2
Fe(ClO4)3
Mn(ClO4)2
RhCl3
Cu(MeCN) BF (1)
Cu(MeCN) BF (1)
Cu(OAc) (2)
Cu(ClO ) (3)
Complex 4
Complex 5d
Complex 6
28
28
(14)
b
0.0050
0.0030
0.0040
0.0020
0.0030
0.0020
0.0007
0.0050
0.0030
0.0040
0.0040
0.0030
0.0022
0.0008
890
1900
708
440
480
126
30
4
4
c
4
4
2
1
1
1
1
4
2
e
a
Ϫ3
Ϫ3
Ϫ3
Conditions: Total volume of the reaction solution was 5 mL; solvent acetonitrile; 60 ЊC; cyclohexane, 0.46 mol dm ; PAA, 0.4 mol dm
;
.
Ϫ5
Ϫ3
b
Ϫ3
c
Ϫ5
catalyst 5 × 10 mol dm
. Additionally, CyOAc (0.0005 mol dm ) has been also found. Concentration of catalyst was 1 × 10 mol dm
Concentrations of the products were determined only after PPh addition. Additionally, CyOAc (0.0011 mol dm ) has been also found.
d
e
Ϫ
3
3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 3 6 1 1 – 3 6 1 7
3611

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Indeed, the oxidations catalysed by cobalt(), manganese()
and iron() perchlorates (entries 3, 4, 5) give oxygenates in
concentrations that only two times exceed the concentration
found for the blank experiment (entry 1). These concentrations
are practically equal to the blank in the case of nickel and
rhodium catalysts (entries 2 and 6). Copper derivatives are in
striking contrast with the metals described above and give very
high turnover numbers (that is numbers of product moles per
one mole of a catalyst; TONs) in such oxidations.
In our determinations of oxidation products we used a very
simple method which we invented and used in our previous
1c,e,12
works on alkane oxidations.
This method is based on com-
parison of the chromatograms of the reaction solution made
before and after the treatment of the sample with triphenyl-
phosphine. If an excess of solid triphenylphosphine is added to
a solution of cyclohexane oxidation products before the GC
analysis, the resulting chromatogram in many cases differs dras-
tically from that of a sample not subjected to the reduction with
Fig. 2 Accumulation of cyclohexyl hydroperoxide (curve 1), cyclo-
PPh . Such results are due to the fact that the mixture of prod-
3
hexanone (curve 2), cyclohexanol (curve 3) and cyclohexyl acetate
ucts contains cyclohexyl hydroperoxide as the main component.
Ϫ3
(
curve 4) in the reaction of cyclohexane (0.46 mol dm ) with PAA
Ϫ3
After treatment of the reaction solution with PPh , the GC
(0.8 mol dm ) in MeCN at 60 ЊC catalysed by complex Cu(ClO )
4 2
Ϫ5 Ϫ3
3
analysis will give the amount of cyclohexanone which corre-
sponds to the real concentration of this product in the reaction
solution. The amount of the cyclohexanol obtained after the
(5 × 10 mol dm ). Concentrations of cyclohexanone and cyclo-
hexanol were measured by GC before and after the reduction of the
reaction mixture with PPh3.
reduction with PPh will give us the sum of real concentrations
3
of the cyclohexyl hydroperoxide and the cyclohexanol. Thus, by
comparing the data of the chromatographic analysis of the
reaction solution before and after reduction with triphenyl-
phosphine, we can estimate quantitatively the amounts of the
cyclohexyl hydroperoxide, really present in the solution at a
given moment. One of the merits of this method is the possi-
bility to estimate the concentration of the alkyl hydroperoxide
formed from the alkane in the presence of an excess of an
oxidant (hydrogen peroxide, alkyl hydroperoxide, peroxy acid
or metal peroxide). It should be emphasized that the method
described above is not a simple “quenching of the reaction”
(
indeed, studies which determine alcohol and ketone concen-
trations only after addition of PPh give absolutely no inform-
3
ation about the existence or non-existence of the corresponding
alkyl hydroperoxides in the solutions).
Fig.
3
Accumulation of cyclohexyl hydroperoxide (curve 1),
In the present work, comparison of chromatograms obtained
for the samples before and after the reduction with triphenyl-
phosphine demonstrated that the main products of alkane
oxidations with PAA catalysed by copper complexes are alkyl
hydroperoxides. Figs. 1–4 show the accumulation of all oxygen-
ates with time. It can be seen that the reactions begin from the
formation of cyclohexyl hydroperoxide, CyOOH, which then
cyclohexanone (curve 2) and cyclohexanol (curve 3) in the reaction
Ϫ3
Ϫ3
of cyclohexane (0.46 mol dm ) with PAA (0.8 mol dm ) in MeCN
at 60 ЊC catalysed by complex Cu(OAc)2 (5 × 10 mol dm ).
Concentrations of cyclohexanone and cyclohexanol were measured by
GC before and after the reduction of the reaction mixture with PPh .
Ϫ5
Ϫ3
3
gradually decomposes to produce cyclohexanol and cyclo-
hexanone (the maximum rate of the stable product accumu-
lation coincides with the maximum of CyOOH concentration
which is most clearly seen in Fig. 4A). There is a difference in
shapes for the kinetic curves. Thus, it can be concluded that
when Cu(ClO ) is used as a catalyst under conditions described
4
2
in the caption to Fig. 2, one is able to obtain almost pure
cyclohexyl hydroperoxide if the reaction time is not longer than
4
0 min. The catalysis with Cu(OAc) is less selective.
2
Copper() complexes 4, 5 and 6 with chelating N,O-ligands
also containing sulfur can be considered as models of coppers
derivatives of certain amino acids. It turned out that copper()
complex 1 containing acetonitrile molecules as ligands exhibits
the highest catalytic activity (entries 7 and 8) attaining TON =
Fig.
1
Accumulation of cyclohexyl hydroperoxide (curve 1),
cyclohexanone (curve 2) and cyclohexanol (curve 3) as well as of
cyclohexyl acetate (curve 4) in the reaction of cyclohexane (0.46 mol
Ϫ3
Ϫ3
dm ) with PAA (0.8 mol dm , as 32% solution in CH COOH) in
3
Ϫ5
MeCN at 60 ЊC catalysed by complex [Cu(CH CN) ]BF (5 × 10 mol
3
4
4
Ϫ3
dm ). Concentrations of cyclohexanone and cyclohexanol were
measured by GC before and after the reduction of the reaction mixture
with PPh3.
3
612
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 3 6 1 1 – 3 6 1 7

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copper symmetrically with two nitrogens and two oxygens gives
relatively lower TON (entry 12). Surprisingly, a similar complex
(which however differs from 5 by its geometry) exhibited only
6
very moderate activity (entry 13).
It follows from comparison of graphs A and B in Fig. 4 that
the selectivity can be increased by using a higher concentration
of PAA. The catalyst is not destroyed in the course of the reac-
tion, and if a new portion of PAA is added to the reaction
solution the oxidation begins with approximately the same rate
(
Fig. 4A). This means that the TON and alkane conversion
parameters can be easily increased by adding PAA portion by
portion. It should be noted that in these oxidations of very inert
alkanes, we obtained yields of oxygenates up to 10% based on
starting cyclohexane and 14% based on initial PAA, however to
optimise the yields was not the aim of this study.
The rate dependence of the cyclohexane oxygenation
catalysed by compound [Cu(CH CN) ]BF (1) is first order for
3
4
4
the initial 1 complex (Fig. 5). First order has been also found
for cyclohexane at relatively low concentrations; however at
relatively high cyclohexane concentrations the rate dependence
is zero order (Fig. 6).
Fig.
4
Accumulation of cyclohexyl hydroperoxide (curve 1),
cyclohexanone (curve 2), cyclohexanol (curve 3) and cyclohexyl acetate
Ϫ3
(
(
curve 4) in the reaction of cyclohexane (0.46 mol dm ) with PAA
Ϫ3 Ϫ3
graph A: 0.4 mol dm ; graph B: 0.8 mol dm ) in MeCN at 60 ЊC
Ϫ5
Ϫ3
catalysed by complex 4 (5 × 10 mol dm ). Concentrations of
cyclohexanone and cyclohexanol were measured by GC before and
after the reduction of the reaction mixture with PPh . At the moment
3
Ϫ3
denoted by an arrow an additional amount of PAA (0.4 mol dm ) was
added.
1
900. Copper() acetate and perchlorate are a bit less efficient
catalysts (entries 9 and 10, respectively). It is noteworthy that
activities of complexes containing chelating ligands depend
dramatically on the ligand structure. Thus a complex contain-
ing a copper() ion coordinated to three nitrogens and only one
oxygen catalyses the oxidation (entry 11) with TON comparable
with those obtained for simple inorganic derivatives (entries 9
and 10). Compound 5 which contains a ligand surrounding
Ϫ3
Fig. 5 Plot of the initial rate of the cyclohexane (0.46 mol dm
)
Ϫ3
oxidation by PAA (0.8 mol dm
) versus concentration of
Cu(MeCN) BF used as a catalyst (in acetonitrile, 60 ЊC). The reaction
rates were calculated from the slopes of kinetic curves of product
accumulation in the initial period of the reaction and concentrations of
cyclohexanone and cyclohexanol were measured by GC only after the
reduction of the reaction mixture with PPh3.
4
4
In order to get additional mechanistic information on this
reaction, we studied the oxidation of certain higher alkanes. The
results are summarized in Table 2. Corresponding selectivity
parameters are also given for certain other systems known to
oxidize alkanes via free-radical mechanism (the O –H O –
2
2
2
1
3,14
nBu NVO –PCA reagent)
and to operate without involve-
ment of free radicals: the H O –[LMn (O) Mn L](PF ) –
4
3
IV IV
2
2
3
6 2
1
1d,15
MeCO H system
and the H O –m-CPBA–Co(ClO) com-
2
2
2
4
1
6
bination. It can be seen that selectivity parameters for the
Cu-catalysed oxidations of n-hexane and isooctane by PAA are
noticeably lower than that obtained for “non-radical” systems.
These parameters are close (though a bit higher) to the corre-
sponding values known for the O –H O –nBu NVO –PCA
2
2
2
4
3
reagent which operates via the formation of free hydroxyl
1
3,14
radicals.
The Cu-catalysed oxidation occurs without reten-
tion of configuration at tert-carbons (compare corresponding
parameters in Table 2 for oxidations of cis- and trans-DMCH)
which is in agreement with free-radical mechanism of the
reaction.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 3 6 1 1 – 3 6 1 7
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a
Table 2 Selectivity parameters in the Cu-catalysed oxidation with PAA and tert-butyl hydroperoxide
Entry
Substrate
Oxidant
Catalyst
Selectivity
b
C(1) : C(2) : C(3)
1 : 13 : 11
1 : 14 : 10
1 : 12 : 13
1 : 10 : 10
1 : 15 : 12
1 : 8 : 7
1
2
3
4
5
6
n-Hexane
PAA
Cu(MeCN) BF
4
4
Cu(ClO4)2
Cu(OAc)2
Complex 4
Complex 5
c
c
H O
2
nBu NVO –PCA
2
4 3
a
C(1) : C(2) : C(3) : C(4)
7
8
9
0
1
n-Heptane
t-BuOOH
Cu(MeCN) BF
1 : 14 : 9 : 13
4
4
Complex 4
1 : 16 : 9 : 13
1 : 6 : 6 : 5
1 : 46 : 35 : 34
H O
2
nBu NVO –PCA
2
4
3
IV
IV
d
1
1
[LMn (O) Mn L](PF ) –MeCO H
Co(ClO)4
3
6
2
2
e
m-CPBA
1 : 104 : 128 : 126
f
1
؇ : 2؇ : 3؇
g
1
1
1
1
1
1
2
3
4
5
6
7
Isooctane
PAA
Cu(MeCN) BF
1 : 1.7 : 12.5
1 : 2.0 : 22.2
1 : 1.5 : 11.5
1 : 3.9 : 13.0
1 : 4.0 : 9.0
1 : 5.0 : 50.0
4
4
Cu(ClO4)2
Complex 4
Complex 5
c
H O
2
nBu NVO –PCA
2
4
3
IV
IV
d
[LMn (O) Mn L](PF ) –MeCO H
3
6
2
2
1
؇ : 3؇h
1
1
8
9
t-BuOOH
Cu(MeCN) BF
Complex 4
1 : 41h
1 : 34
trans/cis
0.66
0.7
4
4
i
j
20
21
22
23
24
25
26
27
cis-DMCH
PAA
H O
Complex 4
c
nBu NVO –PCA
2
2
4
3
IV
IV
d
d
[LMn (O) Mn L](PF ) –MeCO H
0.34
0.0005
0.45
0.8
3
6
2
2
e
m-CPBA
PAA
Co(ClO)4
Complex 4
nBu NVO –PCA
j
trans-DMCH
c
H O
2
2
4
3
IV
IV
[LMn (O) Mn L](PF ) –MeCO H
Co(ClO)4
4.1
3
6
2
2
e
m-CPBA
61
a
The parameters were calculated based on concentrations of isomeric alcohols formed in the reaction and were measured after the reduction of the
b
reaction solution with PPh3. Parameter C(1) : C(2) : C(3) : C(4) is normalized (i.e., calculated taking into account the number of hydrogen atoms at
each position) relative reactivities of hydrogen atoms in positions 1, 2, 3, and 4 of the hydrocarbon chain, respectively. PCA is pyrazine-2-carboxylic
acid; for this system, see Refs. 1c–e,13,14. L is 1,4,7-trimethyl-1,4,7-triazacyclononane; for this system, see Refs. 11d, 15. m-CPBA is
m-chloroperoxybenzoic acid; for this system, see Ref. 16. Parameter 1Њ : 2Њ : 3Њ is normalized relative reactivities of hydrogen atoms at primary,
secondary and tertiary carbons, respectively. 2,2,4-Trimethylpentane. Secondary alcohols have not been found. This parameter is the trans/cis
ratio of isomers of tert-alcohols formed in the oxidation of cis- or trans-1,2-dimethylcyclohexane. 1,2-Dimethylcyclohexane.
c
d
e
f
g
h
i
j
known to decompose “non-productively” to give molecular
17a
17b
oxygen and decarboxylation products.
Since cyclohexane is not known to form complexes with cop-
per ions, the fact of its oxidation in the system under discussion
testifies that an intermediate species inducing the process is
generated via a catalytic decomposition of PAA. We can
assume that in the first step of the reaction copper ions induce
generation of free radicals from PAA. These may be products
of homolytic splitting the O–O bond in PAA, that is radicals
ؒ
ؒ
HO or OC(᎐O)CH .
᎐
3
(
1)
Fig. 6 Plot of the initial rate of the cyclohexane oxidation by PAA
Ϫ3
Ϫ5
Ϫ3
(
0.8 mol dm ) catalysed by Cu(MeCN) BF (5 × 10 mol dm )
4 4
versus initial concentration of cyclohexane (curve 1) and the linear
anamorphosis of this dependence (curve 2) obtained using eqn. (4).
Curve 3 shows the simulated dependence of W0 on [cyclohexane]0
calculated using eqn. (4) and parameters for this equation taken from
line 2. The reaction was carried out in acetonitrile at 60 ЊC. The reaction
rates were calculated from the slopes of kinetic curves of product
accumulation in the initial period of the reaction and concentrations of
cyclohexanone and cyclohexanol were measured by GC only after the
reduction of the reaction mixture with PPh3.
Both radicals then attack the hydrocarbon abstracting its
hydrogen atom which leads to the formation of an alkyl radical,
ؒ
R . The latter reacts rapidly with atmospheric molecular oxygen
ؒ
to produce a peroxy radical, ROO , which can further add
either a hydrogen atom or electron [from copper() derivative]
followed by the proton addition. The hydrogen atom can be
abstracted from the other alkane molecule since the radical
It should be noted that the yield of oxygenates is lower than
PAA consumption (see, for example, Fig. 1). Peroxy acids are
3
614
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a
Table 3 Cyclohexane oxidation with tert-butyl hydroperoxide catalysed by copper complexes
Ϫ3
Product concentration (mol dm
Cyclohexyl hydroperoxide
)
Entry
Catalyst
Time, h
Cyclohexanol
Cyclohexanone
TON
1
2
Cu(MeCN) BF
Complex 4
5
5
0.005
0.007
0.075
0.030
0.030
0.044
2200
1760
4
4
a
Ϫ3
Ϫ3
Conditions: Total volume of the reaction solution was 5 mL; solvent acetonitrile; 60 ЊC; cyclohexane, 0.92 mol dm ; t-BuOOH, 0.78 mol dm
;
Ϫ5
Ϫ3
catalyst, 5 × 10 mol dm
.
ؒ
ؒ
chain process at elevated temperature is possible in small extent
HO and CH C(᎐O)O radicals in the competitive pair are
3
᎐
13b
ؒ
for the case of higher alkanes (ROO ϩ RH
ROOH ϩ
close.
ؒ
R ). In both cases the alkyl hydroperoxide, ROOH, is formed.
This compound decomposes slowly (copper complexes presum-
ably catalyse this process) to the more stable corresponding
alcohol and ketone (see Fig. 1).
The rate of reaction (1) should be written as follows:
W = W = k [PAA][1]
1
(5)
i
1
ؒ
A Cu() derivative can be transformed into a Cu() complex
via the interaction with PAA according to equation:
The initial rate W of r generation for the system under investi-
gation has been determined above. In order to calculate the rate
constant of this reaction we need to know the stationary con-
centration of Cu ions. In order to estimate this constant let us
use the following data. It is known from the literature that
i
II
ϩ
CH C(᎐O)OOH ϩ Cu
3
ؒ
ϩ
I
18
CH C(᎐O)OO ϩ H ϩ Cu (1a)
3
᎐
ϩ
acetonitrile stabilizes copper in the form of Cu ions in aque-
As it has been mentioned above, the data of Fig. 6 show that
with increase of the cyclohexane concentration the order of the
oxidation reaction decreases from unity to zero. Taking into
account the above proposal about the radical nature of
ous solution, and due to this acetonitrile complex of Cu() is
oxidized much slower by dioxygen and hydrogen peroxide in
ϩ
comparison with Cu ؒaq. As a result, in the system under dis-
ϩ
cussion copper is present mainly in the form of Cu cation, and
ؒ
ؒ
ؒ
ؒ
oxidation-inducing species r (where r is HO or CH CO O)
the rate-limiting step of the oxidation process is reaction (1).
This assumption easily explains the linear dependence of the
reaction rate on the concentration of complex 1, and the rate
3
we can conclude that the data of Fig. 6 are in agreement
13
with the assumption concerning competition between cyclo-
Ϫ1
3
Ϫ1
hexane, RH, and the solvent, CH CN, in the reaction with an
constant of this reaction k at 60 ЊC is 1.7 mol dm s .
1
3
active oxidising species:
It is necessary to note that neither meta-chloroperbenzoic
acid (m-CPBA) nor hydrogen peroxide oxidize cyclohexane
efficiently when the copper complexes are used as catalysts.
ؒ
ؒ
r ϩ RH
rH ϩ R
k2
(2)
(3)
Ϫ5
Ϫ3
Thus, the catalysis with compound 4 (5 × 10 mol dm ) gave
Ϫ3
Ϫ3
ؒ
0.0010 mol dm of cyclohexanol and 0.00075 mol dm of
cyclohexanone (as well as small amounts of cyclohexyl hydro-
peroxide; total TON = 35) in the experiment with m-CPBA at
r ϩ CH CN
products
k3
3
The analysis of competitive reactions (2) and (3) leads to the
following kinetic equation for the stationary RH oxidation rate:
4
0 ЊC (24 h) and only TON = 17 in the experiment with 30%
H O (40 ЊC, 1 h). On the contrary, we have found that tert-
2
2
butyl hydroperoxide very efficiently (TONs were up to 2200)
oxidizes alkanes if copper derivatives are used as catalysts. A
remarkable feature of this reaction is the predominant form-
ation of alcohols as shown for the case of cyclohexane
oxidation (Table 3). Determination of cyclohexanol and cyclo-
(
4)
ؒ
where W is the r formation rate in the catalytic PAA decom-
hexanone by GC before and after the reduction with PPh
3
i
position. Experimental parameters of Fig. 6 (curve 1) are
adequately described by eqn. (4). Indeed, the dependence of the
reversed RH oxidation rate on the reversed RH concentration is
depicted by straight line 2. The parameters for this line calcu-
lated using the least-square method are the following: the seg-
showed that, in contrast to the oxidation with PAA, the corre-
sponding cyclohexyl hydroperoxide is formed in relatively low
concentration; its maximum content was found in the begin-
ning of the reaction (Figs. 7 and 8). The values of selectivity
parameters for the oxidation with tert-butyl hydroperoxide (see
Table 2) testify that this process occurs with participation of
4
Ϫ1
3
ment cut of the Y-axis is equal to 1.5 × 10 mol dm s, the
4
ؒ
tangent of the line slope is equal to 1.1 × 10 s. According to
free radicals, most probably t-BuO .
eqn. (4) the segment cut of the Y-axis is equal to 1/W , and the
i
tangent of the slope is equal to k [CH CN]/k . Accepting
3
3
2
Conclusions
Ϫ3
[
CH CN] = 18 mol dm we will have at 60 ЊC the following
3
Ϫ5
Ϫ3 Ϫ1
parameters: k /k = 0.04, W = 6.7 × 10 mol dm
=
s
at [PAA]0
Our experiments demonstrated that while “simple” copper
complexes can be successfully used (after modifications and
optimisation) in the preparative scale for the preparation of the
solutions of alkyl hydroperoxides directly from corresponding
alkanes, complexes containing chelating N,O-ligands can be
considered as biomimetic models of copper-based enzymes. It
is noteworthy that, like in Nature, relatively minor changes
in the ligand’s structure lead to a dramatic decrease of the
3
Ϫ3
2
i
Ϫ5
Ϫ3
0.8 mol dm and [1] = 5 × 10 mol dm . Let us compare the
obtained parameters with known corresponding constant ratio
ؒ
for the rate of interaction of HO with CH CN and cyclo-
3
hexane: 0.012 at 25 ЊC. In order to extrapolate this value to the
higher temperature let us assume that the difference in k and k
3
2
values is due only to the difference in the activation energy
values. In this case it follows from the ratio k /k = 0.012 at 25 ЊC
3
2
Ϫ1
1c,e
that E Ϫ E = 11 kJ mol and consequently k /k = 0.019 at
catalytic activity.
3
2
3
2
6
0 ЊC. This means that the k /k value for the Cu-catalysed
3 2
oxidation with PAA is only two times higher than the same
parameter for the reaction with hydroxyl radicals, and we can-
not choose definitely between routes a and b of reaction (1).
One can assume that results obtained testify that reactivities of
Experimental
The oxidation experiments were carried out in MeCN at 60 ЊC
in thermostated Pyrex cylindrical vessels with vigorous stirring.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 3 6 1 1 – 3 6 1 7
3615

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Acknowledgements
The authors thank the Russian Basic Research Foundation for
support.
3
616
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 3 6 1 1 – 3 6 1 7

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O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 3 6 1 1 – 3 6 1 7
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