Hydrocarbon oxygenation with Oxone catalyzed by complex [Mn 2L2O3]2+ (L=1,4,7-trimethyl-1,4,7- triazacyclononane) and oxalic acid

Article abstract of DOI:10.1016/j.tet.2012.07.098

Oxone (peroxysulphate) very efficiently oxidizes benzene to p-quinone (TON 1140) and alkanes to the corresponding alcohols and ketones (aldehydes) in aqueous acetonitrile 50 °C if catalytic amounts of complex [Mn ₂L₂O₃]²⁺ (L=1,4,7-trimethyl-1,4,7- triazacyclononane) and oxalic acid are present in the solution. In contrast to the similar reaction with H₂O₂, the alkane oxidation with Oxone does not afford the corresponding alkyl hydroperoxides. Phenol was quantitatively oxidized to a mixture of p-quinone and pyrocatechol (9:1 ratio). Cyclohexanol gave cyclohexanone (TON 400). The proposed mechanism includes the formation of an oxidizing species containing the Mn(V)O fragment. A kinetic study demonstrated that an adduct of [Mn₂L₂O ₃]²⁺ and oxalic acid is formed in the initial stage. This adduct reacts with Oxone to generate the oxidizing species.

Full text of DOI:10.1016/j.tet.2012.07.098

Tetrahedron 68 (2012) 8589e8599
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Hydrocarbon oxygenation with Oxone catalyzed by complex [Mn2L2O3]2þ
(L¼1,4,7-trimethyl-1,4,7-triazacyclononane) and oxalic acid
,
a
b
c
Georgiy B. Shul’pin a , Yuriy N. Kozlov , Lidia S. Shul’pina , Armando J.L. Pombeiro
*
a Semenov Institute of Chemical Physics, Russian Academy of Sciences, ulitsa Kosygina, dom 4, Moscow 119991, Russia
b Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ulitsa Vavilova, dom 28, Moscow 119991, Russia
c
ꢀ
Centro de Química Estrutural, Complexo I, Instituto Superior Tecnico, Technical University of Lisbon, Av. Rovisco Pais, 1049e001, Lisbon, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Oxone (peroxysulphate) very efficiently oxidizes benzene to p-quinone (TON 1140) and alkanes to the
corresponding alcohols and ketones (aldehydes) in aqueous acetonitrile 50 ꢀC if catalytic amounts of
complex [Mn2L2O3]2þ (L¼1,4,7-trimethyl-1,4,7-triazacyclononane) and oxalic acid are present in the
solution. In contrast to the similar reaction with H2O2, the alkane oxidation with Oxone does not afford
the corresponding alkyl hydroperoxides. Phenol was quantitatively oxidized to a mixture of p-quinone
and pyrocatechol (9:1 ratio). Cyclohexanol gave cyclohexanone (TON 400). The proposed mechanism
includes the formation of an oxidizing species containing the Mn(V)]O fragment. A kinetic study
demonstrated that an adduct of [Mn2L2O3]2þ and oxalic acid is formed in the initial stage. This adduct
reacts with Oxone to generate the oxidizing species.
Received 14 March 2012
Received in revised form 13 July 2012
Accepted 31 July 2012
Available online 7 August 2012
Keywords:
Alkanes
Benzene
CeH activation
Manganese complexes
Oxygenation
Peroxysulfate
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
TMTACN) has been synthesized and characterized by Wieghardt
and co-workers.12 This compound is known to catalyze some
Peroxidic compounds, especially ‘green’ hydrogen peroxide and
peroxysulfates, are very efficient oxidizing reagents widely used in
organic synthesis in the presence of metal catalysts.1 Potassium
peroxymonosulfate (KHSO5) mainly used in the form of OxoneÒ
(2KHSO5$KHSO4$K2SO4) easily oxidizes2 sp3-CeH compounds,3
aromatic hydrocarbons,4 olefins,5 alcohols,6 and phenols.7
Dimanganese fragments in combination with Oxone were used
for remote regioselective catalytic oxygenation of alkyl groups in
the Brudvig-Crabtree molecular recognition systems.8 Soluble and
solid derivatives of transition metals catalyze degradation of dyes
under the action of Oxone.9 A dinuclear Mn(III)/Mn(IV) complexes
in combination with Oxone provide functional model systems for
the oxygen-evolving complex of photosystem II.10 A dinuclear
complex of Mn(II) with 2,6-pyridinedicarboxylate acts as an oxygen
evolving complex with Oxone.11
oxidations of organic compounds, such as olefins and phenols
with hydrogen peroxide.13 Previously it has been found14 that
complex 1 catalyzes the oxidations by hydrogen peroxide in
acetonitrile solution much more efficiently if a small amount of
a carboxylic acid is added to the reaction solution. It has been
demonstrated that the ’1/carboxylic acid/H2O2’ combination very
efficiently oxidizes inert alkanes14,15 to afford mainly the corre-
sponding alkyl hydroperoxides, which are transformed further
into the more stable ketones, aldehydes, and alcohols. The system
oxidizes not only alkanes but also epoxidizes olefins,15a,f,16
transforms alcohols into ketones, (aldehydes),15a,f,17 and sulfides
into sulfoxides.15a The reaction with olefins afforded the products
of dihydroxylation16a in addition to the corresponding epoxides.
Catalyst 1 was used in combination with oxalic acid in a study
devoted to the decoloration of dye Rhodamine 6G.16c Recently
Veghini, Fischer and their co-workers18 prepared the insoluble
The dinuclear manganese(IV) complex [LMn(O)3MnL](PF6)2
(complex 1;
L
is 1,4,7-trimethyl-1,4,7-triazacyclononane,
salt of formula [LMn(m-O)3MnL]2[SiW12O40], which is active as
a catalyst in the oxidation of alcohols and olefins in the presence
of carboxylic acids. Alkanes and olefins can be oxidized by tert-
butyl hydroperoxide15a,19 using complex 1 as a catalyst. The re-
action is also significantly accelerated by the presence of a small
amount of a carboxylic acid.
* Corresponding author. Tel.: þ7 495 9397317; fax: þ7 495 6512191; e-mail
addresses: Shulpin@chph.ras.ru, gbsh@mail.ru (G.B. Shul’pin).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.07.098

8590
G.B. Shul’pin et al. / Tetrahedron 68 (2012) 8589e8599
Me
Me
if some amount of hydroquinone (6) is added to the reaction mix-
ture. Fig. 2, curve D demonstrates that 6 is rapidly oxidized to p-
quinone and over-oxidized by the same system. It is known that p-
alkoxyphenols are easily oxidized to p-quinone with Oxone in
acetonitrileewater (2:1) even in the absence of a metal catalyst.20
We have found that phenol is very rapidly oxidized by the ‘1/
oxalic acid/Oxone’ system: the reaction of phenol (0.03 M) with
catalyst 1 (3.0ꢁ10ꢂ5 M), oxalic acid (0.05 M) and Oxone (0.032 M)
in aqueous acetonitrile at 50 ꢀC gave after 5 min in quantitative
yield p-quinone (0.027 M) and pyrocatechol (0.003 M). Thus, it is
reasonable to assume that p-quinone is the final product formed via
the consecutive oxidation of phenol and hydroquinone.
We have studied the kinetics of benzene oxidation by the
‘Oxone/1/oxalic acid’ system in aqueous acetonitrile. As the process
occurs with an auto-acceleration we measured maximum initial
rates W0 (as an example, see: slope of a dotted line A0 in Fig.1) of the
reaction in the initial period. The lag periods in the kinetic curves
are much shorter if relatively high concentrations of 1 are used
(compare curves A and D in Fig. 1). Figs. 3e6 present the de-
pendences of rates W0 on initial concentrations of the reactants 1, 4,
3, and 2, respectively. In the cases of the first three reactants these
dependences at their relatively low concentrations are linear.
The dependence of the maximum reaction rate W0 on the
concentration of oxalic acid (2) (Fig. 6A), as well as dependences
shown in Figs. 3e5, allow us to assume that an adduct is formed in
the initial stage. This adduct reacts with Oxone to generate the
species X, which induces the substrate oxidation:
O
N
IV
N
IV
Me
N
Mn
Mn
N
(PF6 )2
Me
N
O
O
N
Me
Me
Catalyst 1
Continuing the studies of oxidations catalyzed by the ‘1/car-
boxylic acid’ combination we decided to explore the possibility of
replacing hydrogen peroxide with Oxone. It is interesting to com-
pare the mechanisms of oxidations of the two oxidants. Moreover,
previously we were unsuccessful in our attempts to oxidize ben-
zene to phenol and/or quinone in substantial yields using H2O2.
Due to this, it was important to attempt the usage of Oxone for this
purpose. In the present work, we have studied for the first time the
oxidation of benzene and various alkanes with Oxone catalyzed by
complex 1 in the presence of a small amount of oxalic acid. We have
found that oxalic acid is an essential component of the catalytic
system.
2. Results and discussion
2.1. Oxidation of benzene
10 þ 2%adduct ðconstant K1Þ
(1)
ð10Þ
(2)
We found that the ‘1/oxalic acid (2)/Oxone (3)’ system efficiently
oxidizes benzene (compound 4) to p-quinone (7) in aqueous ace-
tonitrile (Scheme 1). The amount of compound 7 attained 0.25 mol
when one mol of 3 was used. Turnover numbers (TONs) were up to
1140 based on compound 1 as a catalyst. Fig. 1 shows some ex-
amples of the kinetic curves of the reactions carried out at different
concentrations of 1. The conditions are given in the caption. At low
concentrations of 1 (<1.0ꢁ10ꢂ5 M) the mode of kinetic accumula-
tion curves (A and B) typical for the reaction with auto-acceleration
can be clearly seen. It can be also noticed that at relatively high
concentrations of 1 the kinetic curves (C and D) have maxima be-
cause the formed p-quinone is over-oxidized with Oxone. The in-
duction period (Fig. 2, curve A) is substantially diminished (curve C)
ꢀ
ꢁ
adduct þ 3%X constant K10
X þ 4/products ðconstant k2Þ
Here species 10 is an active form of the catalyst, which is pro-
duced during the lag period. Stage (2) is the rate limiting one in the
sequence of transformations associated to the substrate oxidation.
We can write for the initial reaction rate:
W0 ¼ ꢂd½4ꢃ=dt ¼ k2½Xꢃ½4ꢃ0
(3)
OH
OH
Phenol (5)
1/oxalic acid (
2
)/Oxone ( )
3
CH3CN/H2O (2:1)
O
Benzene (4)
OH
Hydroquinone (6)
O
Quinone (7)
Scheme 1. Oxidation of benzene.

G.B. Shul’pin et al. / Tetrahedron 68 (2012) 8589e8599
8591
Fig. 3. Dependence of the maximum initial rate W0 (an example of measuring W0 is
shown for curve A in Fig. 1: the rate was determined from the slop of a dotted straight
line A0) of the accumulation of p-quinone (7) in the oxidation of benzene (0.1 M) with
Oxone (initial concentration .032 M) catalyzed by complex 1 and oxalic acid (0.05 M)
on concentration of 1. Solvent was MeCNeD2O (2:1 v/v), total volume 5 mL, temper-
ature 50 ꢀC.
Fig. 1. Accumulation of p-quinone (7) in the oxidation of benzene (4; initial concen-
tration 0.1 M) with Oxone (3; initial concentration.0.032 M) catalyzed by complex 1
and oxalic acid (2; 0.05 M) at different concentrations of 1: 0.5ꢁ10ꢂ5 (curve A; TON
was 1140 after 10 h; the maximum initial rate W0 was determined from the slop of
a dotted straight line A0), 1.0ꢁ10ꢂ5 (curve B), 10ꢁ10ꢂ5 (curve C) and 20ꢁ10ꢂ5 M (curve
D; yield after 2.5 h was 0.25 mol of 7 per 1 mol of Oxone). Solvent was MeCNeD2O (2:1
v/v), temperature 50 ꢀC, total volume of the reaction solution was 5 mL.
Fig. 4. Dependence of the maximum initial rate W0 of the accumulation of p-quinone
(7) in the oxidation of benzene (4) with Oxone (initial concentration .032 M) catalyzed
by complex 1 (5.0ꢁ10ꢂ5 M) and oxalic acid (0.05 M) on initial concentration of ben-
zene. Solvent was MeCNeD2O (2:1 v/v), total volume 5 mL, temperature 50 ꢀC.
K0 K1½10ꢃ0½2ꢃ0½3ꢃ0
1
½Xꢃ ¼
(4)
1 þ K1½2ꢃ0
Fig. 2. Accumulation of p-quinone (7) in the oxidation of benzene (4; 0.1 M) (curves A,
B and C) with Oxone (3; initial concentration 0.032 M) catalyzed by complex 1
(5.0ꢁ10ꢂ5 M) and oxalic acid (2; 0.05 M) (curve A). Curve B: the same with addition of
hydroquinone (6; 1.0ꢁ10ꢂ3 M). Curve C: the parameters of curve B after subtraction of
1.0ꢁ10ꢂ3 M (i.e., initial concentration of added hydroquinone). Curve D: accumulation
of p-quinone (7) in the oxidation of hydroquinone (6; 0.1 M) under the same condi-
tions. Solvent was MeCNeD2O (2:1 v/v), temperature 50 ꢀC, total volume of the re-
action solution was 5 mL.
In this case we can expect the linear dependence of the re-
ciprocal of the reaction rate on the reciprocal of [2]0:
ꢂ
ꢃ
1
1
1
½2ꢃ0
¼
þ K1
(5)
k2K0 K1½10ꢃ0½3ꢃ0½4ꢃ0
W0
1
The analysis of the experimental dependence of W0 on [2]0
(Fig. 6A) using the least-squares procedure and coordinates, which
correspond to the linear dependence 1/W0 on 1/[2]0 (equation (5))
led to the following result for the experimental conditions of
Fig. 6A:
The concentration X in the quasi-equilibrium approximation
taking into account that [X]<<[adduct] and concentration [adduct]
is comparable with [1]0 can be calculated using the following
equation:

8592
G.B. Shul’pin et al. / Tetrahedron 68 (2012) 8589e8599
1
¼ 980 s and
ð60Þ
k2K0 K1½10ꢃ0½3ꢃ0½4ꢃ0
1
0 1
¼ 1:75 ꢁ 105 Mꢂ1
s
ð600Þ
k2K10 ½1 ꢃ0½3ꢃ0½4ꢃ0
It follows from equations (60) and (600) that K1¼180 Mꢂ1, that is,
for the conditions of Fig. 6A we have:
1=W0 ¼ 980=½2ꢃ0 1:75 ꢁ 105 Mꢂ1
s
(7)
A close coincidence of the experimental curve A and simulated
curve B in Fig. 6 indicates that equation (5) satisfactorily describes
the experimental data. Therefore, the consequence of stages
(1)e(2) is in good agreement with the real mechanism of the
reaction.
The oxidation of benzene in the absence of oxalic acid occurs
with a very long lag period and is very slow. Thus, under the con-
ditions of the experiment shown in Fig. 1C at [2]0¼0 M, the con-
centration of p-quinone after 2 h was 0 M and only after 6 h [7]
attained 0.002 M, whereas in the presence [2]0¼0.05 M this con-
centration was 0.007 M after 2 h. It is important to emphasize that
other carboxylic acids do not remove the lag period and do not
accelerate the benzene oxidation. We have found that under the
conditions of Fig. 1C, if acetic or trifluoroacetic acid (0.1 M) are
added, no p-quinone (7) was detected in the reaction mixture after
2 h. Only after 5 h, [7] attained 0.0035 and 0.005 M for the cases of
CH3COOH and CF3COOH, respectively.
Fig. 5. Dependence of the maximum initial rate W0 of the accumulation of p-quinone
(7) in the oxidation of benzene (initial concentration 0.1 M) with Oxone (3) catalyzed
by complex 1 (5.0ꢁ10ꢂ5 M) and oxalic acid (0.05 M) on initial concentration of Oxone.
Solvent was MeCNeD2O (2:1 v/v), total volume 5 mL, temperature 50 ꢀC.
2.2. Oxidation of cyclic, linear, and brunched alkanes
The efficiency of methylene group oxidation in arylalkanes
(Fig. 7) is comparable with the oxygenation of benzene (Fig. 1, curve
D). Methylene groups in ethylbenzene are activated by the neigh-
boring phenyl ring and due to this ethylbenzene is more reactive
than cyclohexane (Fig. 8). To check whether alkyl hydroperoxides
are formed in this reaction we compared the chromatograms of the
reaction mixtures obtained before and after the reduction of the
solution with an excess of triphenylphosphine (for this convenient
and simple method developed previously by one of us, see Refs. 21).
If an excess of solid PPh3 is added to a sample of the reaction so-
lution, ca. 10 min before GC analysis, the alkyl hydroperoxide,
ROOH, present in the reaction mixture is completely reduced to the
corresponding alcohol. As a result, the chromatogram differs from
that of a sample not subjected to the reduction (the alcohol peak
rises, while the intensity of the ketone peak decreases). Comparing
the intensities of peaks attributed to the alcohol and ketone before
and after the reduction, it is possible to estimate the real concen-
trations of the alcohol, ketone, and alkyl hydroperoxide present in
the reaction solution.21 In the present study we have found for the
oxidations of ethylbenzene and cyclohexane that there is practi-
cally no difference between chromatograms obtained for the un-
treated solution and the same sample reduced with PPh3. It means
that alkyl hydroperoxides are present in negligible amounts and
the reaction gives predominantly the alcohol and ketone. The
alcohol is easily oxidized by the catalytic system to the corre-
sponding ketone, which has been demonstrated in a special ex-
periment. Heating a biphasic solution of complex 1 (0.0005 mmol)
and oxalic acid (0.01 mmol), cyclohexanol (0.48 mmol), and Oxone
(0.49 mmol) in air in a mixture of MeCN (2 mL) and H2O (1.4 mL)
at 70 ꢀC gave after 2 h cyclohexanone (0.20 mmol; yield 42%;
TON 400).
Fig. 6. Dependence of the maximum rate W0 of the accumulation of p-quinone (7) in
the oxidation of benzene (0.1 M) (curve A) with Oxone (initial concentration.0.032 M)
catalyzed by complex 1 (5.0ꢁ10ꢂ5 M) and added oxalic acid on its concentration.
Dashed curve B: simulated curve calculated in accordance with equation (7). Curve C:
the linearization of the dependences presented by curve A in coordinates 1/W0ꢂ1/
[oxalic acid]0. Curve D: the dependences presented by line C in expanded (including
one more point) scale. Solvent was MeCNeD2O (2:1 v/v), total volume 5 mL, tem-
perature 50 ꢀC.
Tables 1 (see also Section S1 and Figs. S1eS3) and 2 demon-
strate that parameters of regio-, bond-, and stereo-selectivity for

G.B. Shul’pin et al. / Tetrahedron 68 (2012) 8589e8599
8593
or tert-butoxy radicals (Table 2, Entries 19 and 20). The high
selectivity parameters in the oxidation catalyzed by complex
[O3Cu4{N(CH2CH2O)3}4(BOH)4][BF4]2 (13) suggest there is sub-
stantial steric hindrance in the reaction center, and it is possible to
assume that the reaction proceeds in a cleft.26b Very large steric
hindrance was noticed for the oxidations with H2O2 catalyzed by
complex 8 (Table 2, Entry 4)15e and titanosilicate TS-1 where the
reaction proceeds in nano channels (Table 2, Entry 22).26f The
profile of the peaks in chromatogram of hydroxylated methyl-
cyclohexanes for the catalytic oxidation by the ‘1/oxalic acid/
Oxone’ system (Fig. S2a) is similar with the profile for the oxida-
tion with the participation of small hydroxyl radicals (Fig. S2b). It
can be clearly seen that in the oxidations catalyzed by the steri-
cally hindered complexes 13 and 14 (Tables 1 and 2, Entries 18 and
19; Fig. S2c and d) isomers P6 and P7 are formed in relatively
small amounts. Another very interesting peculiarity of the 1/oxalic
acid/Oxone system is the inversion of configuration in the oxida-
tion of trans-1,2-dimethylcyclohexane: the trans/cis ratio of iso-
mers of tert-alcohols with mutual trans- and cis-orientation of the
methyl groups is only 0.2 (Table 2, Entry 1). The same phenome-
non has been found by us previously for the oxidations of trans-
1,2-dimethylcyclohexane with TBHP catalyzed by copper com-
plexes bearing voluminous ligands (Table 2, Entries 18 and 19).
Thus, based on selectivity studies of the oxidations by the ‘1/oxalic
acid/Oxone’ system, we can conclude that the bulky methylated
azacyclononane ligands surrounding the manganese active re-
action centers in 1 sterically hinder the approach of certain frag-
ments of hydrocarbon molecules.
Fig. 7. Accumulation of acetophenone and 1-phenylethanol in the oxidation of eth-
ylbenzene (initial concentration 0.16 M) with Oxone (initial concentration 0.013 M)
catalyzed by complex 1 (5.0ꢁ10ꢂ4 M) and oxalic acid (0.05 M). Solvent: a mixture of
acetonitrile (2.9 mL) and water (2.0 mL). 50 ꢀC.
2.3. On the possible mechanism of the hydrocarbon oxygen-
ation with the ‘1/oxalic acid/Oxone’ system
Previously, on the basis of kinetic peculiarities of the oxidations
of alkanes27a and olefins16a by the ‘1/carboxylic acid/H2O2 system,
0
catalytic cycles for these processes have been proposed. The in-
teraction of complex 1 with a carboxylic acid and one molecule of
hydrogen peroxide leads to the formation of a catalytically active
species Cat (see Scheme S1). Carboxylic acid and the corresponding
carboxylate fragment play the roles of proton transfer reagents.
Hydrogen peroxide reduces one of Mn(IV) ions in the binuclear
compound 1 to Mn(III). The details of this catalytic oxidation are
given in Supplementary data, Section S2. This process is similar to
the first step of the catalytic cycle proposed for the oxidations by
Fig. 8. Accumulation of cyclohexanone and cyclohexanol in the oxidation of cyclo-
hexane (initial concentration 0.17 M) with Oxone (initial concentration 0.013 M) cat-
alyzed by complex 1 (5.0ꢁ10ꢂ4 M) and oxalic acid (0.05 M). Solvent: a mixture of
acetonitrile (2.9 mL) and water (2.0 mL). 50 ꢀC.
0
the ‘vanadium derivative/pyrazine-2-carboxylic acid (PCA)/H2O2
reagent23,27b and the ‘vanadate/acid’ system,24 where V(V) ion is
reduced by H2O2 to V(IV).
the oxidations of certain alkanes by the ‘1/oxalic acid/Oxone’
(Table 2, Entry 1) system are substantially higher than the corre-
sponding parameters measured for the systems, which oxidize
with the participation of hydroxyl radicals (Table 2, Entries 5e17)
In the present work we found that the oxidation with Oxone
exhibits some features that are similar to feature previously
found by us for the oxidation with hydrogen peroxide. However,
some other properties demonstrate that the two systems are
Table 1
Accumulation of oxygenates (see also Fig. S1) in the oxidation of methylcyclohexane (MCH) with Oxone catalyzed by 1 and oxalic acida
c
Time (min)
Products (concentration, mM)b
1ꢀ
2ꢀ
3ꢀ
P1
P2
P3
P4
P5
P6
P7
P8
P9
P10
P11
P12
60
90
120
180
0.04
0.06
0.06
0.07
0.15
0.33
0.55
0.75
0.14
0.34
0.66
1.40
0.13
0.30
0.59
1.10
0.45
1.00
1.77
3.10
0.04
0.08
0.13
0.14
0.08
0.16
0.29
0.40
0.06
0.14
0.23
0.20
0.03
0.07
0.11
0.10
0.09
0.18
0.28
0.41
0.05
0.09
0.14
0.20
0.009
0.06
0.045
0.07
1
1
1
1
12
12
11
6
150
167
161
135
a
Reaction conditions: [1]0¼5ꢁ10ꢂ4 M; [oxalic acid]0¼5ꢁ10ꢂ2 M; [Oxone]0¼0.013 M; [MCH]0¼0.13 M; MeCN (2.9 mL); H2O (2.0 mL); 50 ꢀC.
b
Concentrations of isomers (after reduction with PPh3) are given. Oxygenate products: P1: cyclohexanecarboxaldehyde, P2: 2-methylcyclohexanone, P3: 3-
methylcyclohexanone, P4: 4-methylcyclohexanone, P5: 1-methylcyclohexanol, P6: trans-2-methylcyclohexanol, P7: cis-2-methylcyclohexanol, P8: trans-3-
methylcyclohexanol, P9: trans-4-methylcyclohexanol, P10: cis-3-methylcyclohexanol, P11: cis-4-methylcyclohexanol, P12: cyclohexylmethanol. Typical chromatogram of
the reaction mixture is shown in Fig. S2a. Molecular formulas of oxidation products P1eP12 are given in Fig. S3.
c
Bond selectivity, i.e., relative normalized reactivities (using concentrations of isomeric alcohols) of hydrogen atoms at primary (1ꢀ), secondary (2ꢀ) and tertiary (3ꢀ) carbon
atoms of MCH determined after reduction with PPh3.

8594
G.B. Shul’pin et al. / Tetrahedron 68 (2012) 8589e8599
Table 2
Selectivity parameters in oxidations of alkanes by certain catalytic systemsa
Entry
Catalytic system
C(1):C(2):C(3):C(4)
1ꢀ:2ꢀ:3ꢀ
MCH
trans/cis
n-Heptane
cis-1,2-DMCH
trans-1,2-DMCH
1
2
3
4
1/oxalic acid/Oxone (this work)
1/acetic acid/H2O2 (Ref. 14a)
1/oxalic acid/TBHP (Ref. 22a)
8/oxalic acid/H2O2 (Ref. 15e)b
FeSO4/H2O2 (Ref. 22b)
1:30:28:30
1:46:35:34
1:14:13:12
1:91:99:68
1:5:5:4.5
1:12:150
1:26:200
1:0.4:32
0.5
0.34
0.2
0.31
1.3
0.2
4.1
7.3
13
5
1:3:6
1.2
6
h
n/H2O2 (Ref. 22b)
1:7:6:7
0.9
7
8
9
Fe(ClO4)3/H2O2 (Ref. 22b)
9/PCA/H2O2 (Ref. 22c)c
10/H2O2 (Ref. 22d)d
1:9:9 (with n-hexane)
1:6:6:5
1:10:10:6
1:15:14:11
1:9:7:7
1:7:7:6
1:7:7:7
1:4:4:4
1:5.5:5.0:4.6
1:12:10:3.5
1:8.4:6.7:5.3
1:34:23:21
1:10:8:11
1:14:9:13
1:9:7:6.5
1:7:43
3.5
1.6
0.9
0.75
0.86
1.0
5.1
1.2
1.3
0.8
0.9
0.9
10
11
12
13
14
15
16
17
18
19
20
21
22
11/H2O2 (Ref. 22d)e
VO3ꢂPCAeH2O2 (Ref. 23)
VO3ꢂ/H2SO4/H2O2 (Ref. 24)
Cp*2Os/py/H2O2 (Ref. 25a)
Os3(CO)12/py/H2O2 (Ref. 21c)
12/H2O2 (Ref. 25b)f
1:6:18
1:8:23
1:5:11
1:4:10
0.85
0.9
OsCl3/H2O2 (Ref. 25c)
13/CF3COOH/H2O2 (Ref. 26a)g
13/TBHP (Ref. 26b)g
1:5:14
1:16:128
1:15:170
0.8
0.4
0.6
0.8
0.1
0.1
14/TBHP (Ref. 26c)h
CuðMeCNÞ4þ/TBHP (Ref. 26d)
Ti-MMM-2/H2O2 (Ref. 26e)
TS-1/H2O2 (Ref. 26f)
1:6:113
No products
0.9
0.9
1:80:193:100
No products
No products
a
Regioselectivity parameter C(1):C(2):C(3):C(4) is the relative normalized (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 n-heptane. Bond selectivity parameter 1ꢀ:2ꢀ:3ꢀ is the relative normalized reactivities of hydrogen atoms at primary, secondary and
tertiary carbons of methylcyclohexane. Stereoselectivity parameter trans/cis is 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). 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
Compound 8 is complex [Mn2(R-LMe2R)2( -O)2]3þ where R-LMe2R is (R)-1-(2-hydroxypropyl)-4,7-dimethyl-1,4,7-triazacyclononane.
m
c
d
e
f
Compound 9 is [Fe2(hptb)(
Compound 10 is [Fe2(N3O-L)2(
Compound 11 is [Fe4(N3O2-L)4(
Compound 12 is (2,3- -1,4-diphenylbut-2-en-1,4-dione)undecacarbonyl triangulotriosmium.
m
-OH)(NO3)2]2þ where hptb is N,N,N0,N0-tetrakis(2-benzimidazolylmethyl)-2-hydroxo-1,3-diaminopropane.
-O)(
-OOCCH3)]þ where L is 1-carboxymethyl-4,7-dimethyl-1,4,7-triazacyclononane.
-O)2]4þ where L is 1-carboxymethyl-4,7-dimethyl-1,4,7-triazacyclononane.
m
m
m
h
g
h
Compound 13 is [O3Cu4{N(CH2CH2O)3}4(BOH)4][BF4]2.
Compound 14 is Cu(H3L)(NCS) where H4L is N,N,N0,N0-tetrakis(2-hydroxyethyl)ethylenediamine.
different. Thus, the oxidation with the ‘1/Oxone’ combination in
the presence of other carboxylic acids (acetic or trifluoroacetic)
proceeds with a very long lag period; the alkane oxidation with
the ‘1/oxalic acid/Oxone’ system affords alcohols and ketones
3. Conclusion
The ‘1/oxalic acid/Oxone’ system described in this paper for the
first time very efficiently oxidizes benzene, phenol and hydroqui-
none to p-quinone, alkanes to alcohols and ketones, and alcohols to
ketones. The proposed mechanism includes the formation of an
oxidizing species containing the Mn(V)]O fragment. An adduct of
[Mn2L2O3]2þ and oxalic acid is formed in the initial stage. This ad-
duct reacts with Oxone to generate the oxidizing species.
0
whereas the ‘1/oxalic acid/H2O2 system generates primarily alkyl
hydroperoxides. Oxone hydroxylates trans-1,2-dimethylcyclo
hexane with partial inversion of configuration whereas hydro-
gen peroxide hydroperoxidizes this isomer with partial retention
of configuration. The reactions with both oxidizing reagents ex-
hibit a lag period and auto-acceleration in the beginning of the
processes.
4. Experimental section
On the basis of obtained kinetic and selectivity data, as well as
on the information from the literature,28e32 we propose for the
oxidation by the ‘1/oxalic acid/Oxone’ system the mechanism
shown in Scheme S2. We assume that the reaction proceeds via the
catalytically active species Cat, which is formed during the lag
period. Species Cat is the same one as 10 in equation (1). Species Cat
reacts with Oxone to produce the dinuclear [Mn(V)¼OMn(IV)ꢂ
OH]2þ complex, which interacts further with the hydrocarbon
abstracting the hydrogen atom from the alkane to form the alkyl
radical, Alkꢄ. Alkyl radicals, Alkꢄ, can in part react with molecular
oxygen present in the solution. The partial inversion of configura-
tion is due to this reaction. This mechanism is described in more
detail in Section S2. It is necessary to emphasize that an alkyl
peroxyl radical, AlkOOꢄ, formed in this route is unable to initiate the
radical-chain oxidation33of cyclohexane or other relatively inert
CeH compounds because in our case the whole process is carried
out under very mild conditions (in solution at low temper-
ature).33c,d Our kinetic calculations presented in Appendix support
this conclusion.
The catalyst 1 and co-catalyst 2 were used in the form of a stock
solution. Aliquots of these solutions were added to the reaction
mixtures in hydrocarbon oxidations. The reactions were typically
carried out in air in thermostated Pyrex cylindrical vessels with
vigorous stirring and using aqueous MeCN as solvent (the volumes
of water and acetonitrile are given in the captions to the Figs.).
Precatalyst 1 was introduced into the reaction mixture in the form
of a stock solution in acetonitrile. The substrate (benzene or alkane)
was then added and the reaction started when solid Oxone was
introduced in one portion. (CAUTION. The combination of air or
molecular oxygen and Oxone with organic compounds at elevated
temperatures may be explosive!). After certain time intervals a so-
lution of 1,4-dinitrobenzene in acetone was added to the samples at
room temperature (all sodium salts including unreacted Oxone
were precipitated during this procedure). Concentrations of prod-
ucts were measured using a 1H NMR method (D2O was used as
a
component of the solvent; ‘Bruker AMX-400’ instrument,
400 MHz). For the determination of concentrations of substrates

G.B. Shul’pin et al. / Tetrahedron 68 (2012) 8589e8599
8595
ꢀ ꢁ
kG
ROO$ þ ROO$ / ROH þ R0COR00 þ O2
and products, the corresponding signals were integrated using
G
added 1,4-dinitrobenzene as an internal standard. The reactions of
alkanes were stopped by cooling, and analyzed twice, i.e., before
and after the addition of an excess of solid PPh3. This method was
developed and used previously by one of us21 for the analysis of
reaction mixtures obtained from various alkane oxidations. A
Fisons Instruments GC 8000 series gas chromatograph with a cap-
Radicals ROꢄ and HOꢄ can take part in the fast following steps:
ðC00Þ HO$ þ RH/H2O þ R$
ðC000Þ RO$ þ RH/ROH þ R$
Oxidations of alkanes can be initiated by free radicals, Xꢄ, which
are capable of abstracting the hydrogen atom from an alkane:
illary column 30 mꢁ0.32 mmꢁ25
mm, DB-WAX (J&W) (helium was
the carrier gas; the internal standard was nitromethane) was used.
Blank experiments showed that in the absence of precatalyst 1 only
very low amount of products was formed.
ðA0Þ X$ þ RH/XH þ R$
Acknowledgements
Very reactive hydroxyl and alkoxyl radicals are among potential
initiators of alkane oxidations. Radicals initiating the oxidations can
be produced by the decomposition of H2O2 or tert-BuOOH under
the action of light or (much more efficiently) of a metal complex
catalyst:
The authors thank the Russian Foundation for ˇ Basic Research,
~
grant 12-03-00084-a, and the Fundac¸ ao para a Ciencia e a Tecno-
logia (FCT), projects PTDC/QUI-QUI/102150/2008 and PEst-OE/QUI/
UI0100/2011, for support. G.B.S. and L.S.S. express their gratitude to
the FCT for making it possible for them to stay at the Instituto Su-
H2O2/HO$ þ .
tert-BuOOH/tert-BuO$ þ .
ꢀ
perior Tecnico, TU Lisbon, as invited scientists and to perform a part
of the present work.
Molecular oxygen from air could be the oxidant. The catalyzed
oxidation of an alkane, RH, with H2O2 or tert-BuOOH affords in
many cases (at least in the beginning of the reaction) the corre-
sponding alkyl hydroperoxide, ROOH. This can testify that chain
branching steps (D) and (D0), which can give the alcohol in the
propagation step (C000) should not be involved into the reaction
mechanism.
Let us analyse the possibility of a classical radical-chain route
with the key stage (C) for the oxygenation of different alkanes
having strong and weak CeH bonds. The simplified mechanism of
an initiated non-branched radical-chain liquid-phase oxidation of
a hydrocarbon, RH, can be described by the following equations.
Chain initiation:
Appendix. Low probability of the alkane oxygenation by
peroxides with the involvement of a classical radical-chain
mechanism (ROOꢄDRH[ROOHDRꢄ)
It is known that autoxidation33 of saturated hydrocarbons in the
liquid phase is usually a branched chain process with a so-called
‘degenerate’ chain branching. This means that branching of each
chain happens much later than its termination and is caused by the
formation of a rather stable intermediate. This intermediate is
nevertheless chemically more active than the initial hydrocarbon
and can form free radicals at a greater rate than the rate of the chain
initiation process. Autoxidation of compounds containing ‘weak’
CeH bonds is of great importance for living organisms. Hydroper-
oxides are the intermediates in liquid phase oxidation. The fol-
lowing classical scheme presents a typical mechanism of liquid-
phase hydrocarbon oxidation.
Wi
RH / R$
(A1)
Chain propagation:
R$ þ O2/ROO$
(A2)
(A3)
Chain initiation:
kp
ꢀ ꢁ
kA
ROO$ þ RH / ROOH þ R$
A
RH þ O2 / R$ þ HOO$
Chain termination:
Chain propagation:
kt
ROO$ þ ROO$ / non-radical products
(A4)
kB
ðBÞ R$ þ O2 / ROO$
The rate of alkyl hydroperoxide accumulation, WROOH, corre-
sponding to the above mechanism can be described by the fol-
lowing equation:
ꢀ ꢁ
kC
C
ROO$ þ RH / ROOH þ R$
pffiffiffiffiffiffi
kp
Chain branching:
pffiffiffiffiffiffiffi
WROOH
¼
Wi½RHꢃ
(A5)
2kt
ꢀ ꢁ
kD
D
ROOH / RO$ þ HO$ or
Here kp is the rate constant of the interaction between peroxyl
radical and RH [reaction (A3)], and kt is the rate constant of the
chain termination [reaction (A4)]. The chain length of the process
n
equals the ratio of the ROOH formation rate and the chain initiation
rate:
ꢀ
ꢁ
kD0
D0
2ROOH / RO$ þ ROO$ þ H2O
WROOH
Chain termination:
n
¼
(A6)
Wi
ðEÞ R$ þ R$/R ꢂ R
Equations (A5) and (A6) show that the rate WROOH increases and
the chain length
n decreases with increasing Wi. The condition
WROOH¼Wi evaluates the rate of RH oxidation, W0ROOH, via a radi-
ðFÞ ROO$ þ R$/ROOR
cal-chain mechanism with
n
¼1:

8596
G.B. Shul’pin et al. / Tetrahedron 68 (2012) 8589e8599
Here the parameters are expressed: kp in Me1 e1, DReH (i.e.,
s
!
2
kp
bond energy for the abstracted H atom) in kcal molꢂ1, and B¼ꢂ11.5
for hydrocarbons containing secondary CeH bonds and ꢂ12.5 for
hydrocarbons with tertiary CeH bonds. For hydrocarbons having
only primary CeH bonds, we accepted the value B¼ꢂ11.5, that is,
equal to the parameter for hydrocarbons containing secondary CeH
bonds. Using the same empirical equation (A8) we calculated the
temperature dependence of kp according to the relationships:
WR0OOH
¼
ꢁ½RHꢃ2
(A7)
pffiffiffiffiffiffiffi
2kt
At WROOH<W0
a radical-chain oxidation process is possible
ROOH
(n
>1), and at WROOH>W0ROOH only the radical oxidation of RH with
<1 is possible. Thus, it may be stated that comparison of the ef-
n
fective rate of the RH oxidation with the rate calculated using
equation (A7) allows us to make a conclusion about either a fun-
damental possibility or inability to associate the efficient process
ꢄ
ꢅ
lgA ¼ ꢂB þ 0:225DRꢂH
Mꢂ1sꢂ1
(A9)
rate with the chain mechanism of the oxidation. The value of
pffiffiffiffiffiffiffi
W0
is governed by so-called oxidizability parameter, kp= 2kt,
ROOH
and the hydrocarbon concentration. The values of oxidizability
parameters are known for certain hydrocarbons, and can be cal-
culated for other ones using values kp and kt. For methane and
ethane oxidation in the liquid phase the rate constants for chain
propagation, kp, and chain termination, kt, remain unknown.
Therefore, any methods of estimation of these parameters, even
approximate, are of considerable interest because they allow us to
get knowledge about the efficiency of the initiated radical-chain
process of liquid-phase oxidation of light hydrocarbons. Calcula-
tions have been carried out also for the hydrocarbons, for which
direct experimental data are known. Table A1, Section (I) summa-
rizes the rate constants for chain propagation, lg kp, at 30 ꢀC and
their dependences on temperature for certain hydrocarbons. To
estimate the rate constants kp we used empirical equation (A8):34a
ꢄ
ꢅ
E ¼ 4:575ð5900 ꢂ 98DRꢂHÞ ꢁ 10ꢂ3
kcal molꢂ1
(A10)
Here A is preexponential factor for the rate constant and E is
activation energy of the reaction. In our calculations we used either
the known from the literature value of bond energy in the hydro-
carbon or this parameter was calculated previously according to
equation (A8) using the kp value at the fixed temperature.
The rate constants for chain termination (lg2kt) and corre-
sponding temperature dependences are summarized in Table A1,
Section (II). In the cases when the temperature dependences are
unknown the values of activation energy were accepted in accor-
dance with the analysis of the literature data34b and for radicals
CH3CH2OOꢄ and CH3OOꢄ we used parameters determined for the
gas-phase reactions. Section (III) collects also oxidizability param-
eters at 30 ꢀC,34g the same parameters extrapolated to 30 ꢀC and
5900 ꢂ 98DRꢂH
lgkp ¼ ꢂB þ 0:225DRꢂH
þ
(A8)
T
Table A1
Parameters for the liquid-phase H atom abstraction from the hydrocarbons, RH, by their peroxyl radicals, ROOꢄ (I) and for termination reaction (II), as well as oxidizability
parameters (III)
No
Hydrocarbon,
RH
(I) Reaction RHþROOꢄ/RꢄþROOHa
(II) Reaction ROOꢄþROOꢄ/a
(III) Hydrocarbon oxidizabilityb
pffiffiffiffiffiffiffi
lg kp (30 ꢀC)
lg A
E
DReH
lg 2kt (30 ꢀC)
lg 2At
Et
lg ðkp= 2ktÞ (30 ꢀC)
Eeff
(Mꢂ1 sꢂ1
)
(Mꢂ1 sꢂ1
)
(kcal molꢂ1
)
(kcal molꢂ1
)
(Mꢂ1 sꢂ1
)
(Mꢂ1 sꢂ1
)
(kcal molꢂ1
)
(L molꢂ½ sꢂ½
)
(kcal molꢂ1
)
½
1
2
3
Tetralin
0.97
0.81
5.58
4.9c
6.4
5.7c
6.82
9.93
10.8
9.8
4.3
ꢂ2.52
3.9
0.89d
0.55
5.24d
5.6
6.05d
7.0
Cyclohexene
Ethylbenzene
6.45
7.25
6.0g
3.5h
ꢂ2.56
ꢂ2.64k
ꢂ2.60d
ꢂ3.78
ꢂ3.68k
ꢂ3.17l
ꢂ3.54d
ꢂ4.64
ꢂ5.0k
3.35
7.5
0.785
0.67d
ꢂ0.16
ꢂ0.2
4.9c
5.7c
5.25d
5.98
4.83
7.1c
6.35d
8.5
7.0
ꢂ0.16
ꢂ0.17d
ꢂ0.62
ꢂ0.17
ꢂ0.4d
ꢂ0.77
ꢂ0.745
ꢂ0.76d
10.0c
8.5d
12.2c
10.2e
11.2d
10.1
8.4c
6.8l
7.15d
10.2
6.0d
8.1c
4
5
Toluene
Cumene
83
94
8.48
4.27
9.86
8.58
2.0
6.0
7.2e
7.65d
6.51
5.3c
ꢂ4.82d
ꢂ2.89
ꢂ2.82k
ꢂ2.83l
ꢂ2.85d
ꢂ4.48
ꢂ5.13l
ꢂ4.80d
ꢂ5.75
ꢂ6.55
6.25
5.9d
9.2d
6.8l
6.52d
12.0
13.0l
12.5d
16.9
20
6
Cyclohexane
ꢂ1.23f
9.6f
15.0f
6.5
11
6.0i
7
8
Ethane
Methane
ꢂ1.7
ꢂ2.2
10.5
11.6
16.9
19.2
98
103
8.1j
8.7j
8.4
8.1
0.5j
e1j
a
From Ref. 34b.
pffiffiffiffiffiffiffi
b
c
The values of oxidizability parameter, kp= 2kt, and effective activation energy, Eeff¼Epꢂ½Et, were calculated using the data from Sections (I) and (II).
The temperature dependence of kp has been calculated using kp value at 30 ꢀC34b and equation (A6).
pffiffiffiffiffiffiffi
d
For some hydrocarbons, parameters lg kp, lg A, E, lg ðkp= 2ktÞ and Eeff were taken (or extrapolated) from different references, and in the last line arithmetical mean values
calculated from upper values of the column are given.
e
The parameters were calculated in accord with equation (A8) using values for DReH taken from Ref. 34c.
f
Calculated according to equation (A8) using lg kp¼ꢂ0.275 at 60 ꢀC34c
Parameter for solution in CH3OH (Ref. 34d).
Calculated based on data from Ref. 34b.
Parameter for solution in CH3COOH (Ref. 34b).
Data for the reaction in the gas phase (Ref. 34e,f).
From Ref. 34g.
.
g
h
i
j
k
l
From Ref. 34h.

G.B. Shul’pin et al. / Tetrahedron 68 (2012) 8589e8599
8597
their temperature dependences according to the literature data,34h
as well as these parameters calculated based on the arithmetical
mean values given in Sections (I) and (II) of Table A1. Table A1
demonstrates that in the cases of hydrocarbons, for which the ex-
perimental parameters are available, the estimated results are in
tolerable agreement with the data obtained from direct measure-
ments. Taking this into account, we believe that estimated by us
the 10% transformation of ethane and methane via mechanism
(A1)e(A4) the reactions will take 6.5 and 30 days, respectively,
which indicates that the highest possible rate of the chain process is
extremely low for these hydrocarbons. However, for hydrocarbons
with weak CeH bonds, such as tetralin and cyclohexene (allylic
methylenes), as well as cumene, corresponding transformations
will take less than 1 h and consequently are possible.
Table A2
0
Highest possible rates of the hydrocarbon radical-chain low-temperature oxidation (chain length
n
¼1 and W0
¼Wi) and the minimum possible times s0:1 for the
ROOH
transformation of these hydrocarbons to the extent of 10%a
(M sꢂ1
)
Time s0:1
0
Rate W0
ROOH
No
Hydrocarbon
30 ꢀC
100 ꢀC
30 ꢀC
100 ꢀC
1
2
3
4
5
6
7
8
Tetralin
9.1ꢁ10ꢂ6
6.3ꢁ10ꢂ6
8.3ꢁ10ꢂ8
2.3ꢁ10ꢂ10
2.0ꢁ10ꢂ6
5.5ꢁ10ꢂ11
3.2ꢁ10ꢂ12
7.9ꢁ10ꢂ14
1.0ꢁ10ꢂ4
5.1ꢁ10ꢂ5
7.3ꢁ10ꢂ6
1.0ꢁ10ꢂ7
1.2ꢁ10ꢂ4
1.0ꢁ10ꢂ7
1.0ꢁ10ꢂ7
2.1ꢁ10ꢂ8
1.7 h
2.5 h
0.2 h
0.3 h
2.1 h
6.5 days
0.2 h
6.5 days
6.5 days
30 days
Cyclohexene
Ethylbenzene
Toluene
Cumene
Cyclohexane
Ethane
7.5 days
7.5 years
7.5 h
32.5 years
550 years
22,000 years
Methane
a
The parameters have been calculated in assumption of the hydrocarbon concentration of 1.0 M. At WROOH>W0
the chain length n<1, which corresponds to a non-
ROOH
0
chain process, and at WROOH<W0
parameter
n>1 and s0.1
>
s0:1
.
ROOH
pffiffiffiffiffiffiffi
values of parameter kp= 2kt for ethane and methane can differ
from the true values only by a factor of a few times. Using the ar-
ithmetical mean values from Table A1 (Section III) we calculated
maximum possible rates or the RH oxidation, equal 2W0ROOH, for
the process, which occurring in accordance with kinetic scheme
(A1)e(A4) with the chain length¼1, as well as the minimum time of
Transition metal ions are often used as catalysts in low-
temperature alkane oxidations. The classical radical-chain mecha-
nism of the alkane oxidation (steps A2, A3 and A4) is the same.
Catalysts take part in the initiation stage inducing the initiator
decomposition generating free radicals. This case is not different
from the initiated alkane oxidation considered above, and here all
parameters estimated previously can be used. On the other hand,
complex ions of transition metals could effectively interact with
alkyl hydroperoxide formed in the oxidation process even at rela-
tively low temperature.33b,34b This interaction results in the per-
oxide decomposition affording free radicals. In this case we have to
add to the scheme (A1)e(A4) the following stages (M is an ion of
transition metal in oxidized or reduced form):
10% RH conversion (
s
0.1). The parameters W0
according to equation (A7). The conversion time was determined by
the integration of equation (A11):
were calculated
ROOH
!
2
d½RHꢃ
dt
kp
pffiffiffiffiffiffiffi
ꢂ
¼ 2
ꢁ½RHꢃ2
(A11)
2kt
ROOH þ MðoxÞ/ROOꢄ þ Hþ þ MðredÞ
(A15)
This expression has been obtained for the kinetic scheme
(A1)e(A4) under conditions (A12) and (A13).
WROOH
Wi
ROOH þ MðredÞ/ROꢄ þ HOꢂ þ MðoxÞ
ROꢄ þ RH/ROH þ Rꢄ
(A16)
(A17)
n
¼
¼ 1
(A12)
If in the case of the route (A1)e(A4) the final product of the
reaction is alkyl hydroperoxide, ROOH, the pathway (A15)e(A17),
which generates branching mechanism, gives rise to the alcohol as
a main product. The analysis of the kinetic scheme in the steady-
state approximation taking into account expressions (A15)e(A17)
gives equation (A18) for the hydrocarbon consumption in the
propagation stage.
d½ROOHꢃ
dt
d½RHꢃ
¼ ꢂ
2
(A13)
dt
Using these equations we can obtain the following expression
for the 10% conversion time:
0:055
s0:1
¼
(A14)
!
2
kp
!
2
pffiffiffiffiffiffiffi
½RHꢃ0
kp
3
2
ꢁ½RHꢃ2
(A18)
2kt
pffiffiffiffiffiffiffi
WRH
¼
2kt
It can be seen that if
n
>1 we have WROOH<W0
and the s0.1
ROOH
A comparison of expression (A18) and (A7) shows that the rate
of the ROOH decomposition with the participation of a catalyst is
only 1.5 times higher than the rate of hydrocarbon consumption in
the chain termination step. It is important that in the latter case the
composition of the products can be dramatically changed. As in
some metal-catalyzed oxidations, at least at low (<50 ꢀC) temper-
atures and at least in the beginning of the reaction cyclohexane and
normal alkanes are transformed only into alkyl hydroperoxides, we
can disregard a mechanism involving steps (A15)e(A17). Taking
value will be higher. Assuming [RH]¼1 M we will obtain the ex-
pressions for parameters W0
and 0.1. Both equations will
s
ROOH
contain only the oxidizability parameters. The results of such cal-
culations are summarized in Table A2. It is follows from the data of
this table that for a hydrocarbon, such as cyclohexane and espe-
cially for much more inert ethane and methane at 30 ꢀC any really
obtained rates will sufficiently exceed value of W0ROOH, that is, in
principle, we have no reason to discuss the possibility of chain
process in accordance with scheme (A1)e(A4). Even at 100 ꢀC for

8598
G.B. Shul’pin et al. / Tetrahedron 68 (2012) 8589e8599
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this into account, we conclude that, in accordance with data sum-
marized in Table A2, the classical radical-chain mechanism
(A1)e(A4) should be neglected for this case. However, the contri-
bution of this pathway might be expected for easily oxidizable
hydrocarbons, such as tetralin, cyclohexene, etc. It should be em-
phasized that our calculations do not exclude the possibility for the
oxidation of cyclohexane, methane and other alkanes having strong
CeH bonds via a radical non-chain mechanism where WRH<Wi
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alkane molecule to afford one alkyl radical, which is further trans-
formed into one molecule of a product (e.g., of alkyl hydroperoxide).
Three possible mechanisms of alkane oxygenation induced by
a radical or radical-like species are shown in Scheme A1.
ꢀ
dron Lett. 2006, 47, 3797e3800; (d) Cubillos, J.; Vasquez, S.; de Correa, C. M.
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RH
Rebound mechanism (X=Mn).
Formation of one R−OH
per one X−O .
X−OH
X−O
Classical radical-chain oxygenation.
R−OH
Formation of many ROOH
per one X−O . Under mild conditions
is possible only for compounds with
R
weak C−H bonds.
X
O2
ROOH
Radical-non-chain oxygenation.
Formation of one ROOH
per one X−O . Operates under
mild conditions.
ROO
e−, H+
RH
X is H, Mn etc.
ROOH
RH is substrate (alkane)
R−OH, ROOH are products
Scheme A1. Three mechanisms for oxygenation of alkanes induced by radical or radical-like species XeOꢄ.
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methylcyclohexane. S2. On the possible mechanism of oxygenation
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