Oxidation of cyclohexene and α-pinene with O2-H 2 mixture in the presence of supported platinum or palladium catalysts

Article abstract of DOI:10.1023/A:1025688908667

Oxidation of cyclohexene and α-pinene with an O₂-H ₂ mixture in the catalytic systems containing Pt or Pd and heteropoly compounds (HPC) was studied. The main oxidation products are epoxides, allyl alcohols, and ketones. The highest yield of the oxidation products was obtained in the presence of the platinum catalyst in combination with HPC PW₁₁ or PW₁₁Fe. The reaction mechanism was proposed. A relationship between the HPC composition and the nature of intermediates involved in oxidation was examined.

Full text of DOI:10.1023/A:1025688908667

1544
Russian Chemical Bulletin, International Edition, Vol. 52, No. 7, pp. 1544—1551, July, 2003
Oxidation of cyclohexene and αꢀpinene with О2—H2 mixture
in the presence of supported platinum or palladium catalysts
N. I. Kuznetsova,aꢀ L. I. Kuznetsova,a N. V. Kirillova,a L. M. Pokrovskii,b
L. G. Detusheva,a J.ꢀE. Ancel,c and V. A. Likholobova
aG. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences,
5 prosp. Akad. Lavrent´eva, 630090 Novosibirsk, Russian Federation.
Fax: +7 (383 2) 34 3056. Eꢀmail: kuznina@catalysis.nsk.su
bN. N. Vorozhtsov Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences,
9 prosp. Akad. Lavrent´eva, 630090 Novosibirsk, Russian Federation.
Fax: +7 (383 2) 34 4752. Eꢀmail: pokrovsk@nioch.nsc.ru
cARCHEMIS, 24 Jean Jaurès av., 69153 Decines Charpieu, Cédex, France.*
Eꢀmail: JeanꢀErick Ancel@archemisꢀeu.com
Oxidation of cyclohexene and αꢀpinene with an O2—H2 mixture in the catalytic systems
containing Pt or Pd and heteropoly compounds (HPC) was studied. The main oxidation
products are epoxides, allyl alcohols, and ketones. The highest yield of the oxidation products
was obtained in the presence of the platinum catalyst in combination with HPC PW11 or
PW11Fe. The reaction mechanism was proposed. A relationship between the HPC composition
and the nature of intermediates involved in oxidation was examined.
Key words: cyclohexene, αꢀpinene, platinum, palladium, heteropoly compounds, oxygen,
hydrogen, oxidation, catalysis.
Oxidation of hydrocarbons with an O2—H2 mixture
of the Pt (or Pd)—HPC liquidꢀphase catalytic systems.
The data on the nature of active forms of oxygen are based
on changes in the activity of the systems and the composiꢀ
tion of the oxidation products with variation of the HPC
composition.
attracts interest due to ecological safety of the process,
mild reaction conditions, and selective effect of the oxiꢀ
dant.1 This oxidant is used for the selective conversion of
benzene to phenol2—4 and oxidation of alkanes5 and alkꢀ
enes.6,7 Catalysts for this process contain platinum metals
and such additives as copper chloride, halides or oxides of
other transition metals, and Ti silicates.
We have previously found8—11 that combinations of Pt
or Pd with heteropoly compounds with Keggin´s strucꢀ
ture exhibit catalytic activity in the selective oxidation of
benzene to phenol, cyclohexane to cyclohexanol and cyꢀ
clohexanone, and saturated hydrocarbons to the correꢀ
sponding alcohols and ketones with an O2—H2 mixture.
These systems operate due to the interaction of heteropoly
compounds (HPC) with metal ions (under homogeneous
conditions) or with metallic particles (in the case of solid
catalysts).
Cyclic olefins, which are autooxidized at sufficiently
high temperatures, were chosen as substrates. In the presꢀ
ence of platinum metal complexes, cyclohexene is oxiꢀ
dized with oxygen at 65 °С to form cyclohexenol and
cyclohexenone, and the fraction of epoxide is at most 3%
of the total amount of the products determined by GLC.12
The autooxidation of αꢀpinene in the presence of transiꢀ
tion metal compounds affords αꢀpinene oxide, which is
the product of the attack of the hydroperoxide radicals at
the αꢀpinene double bond,13—16 along with the products
of allylic oxidation, among which verbenol and verbenone
predominate. In biomimetic systems containing the MnIII
porphyrin complexes and an additional reducing agent,
cyclohexene is oxidized at 20 °С, yielding the products
The key stage of the liquidꢀphase reactions is believed2
to be the formation of H2O2 due to the incomplete O2
reduction with hydrogen in the presence of a Platinum
Group metal.2 The more specific details of the mechaꢀ
nism, including effect of the second component, are not
usually considered. In this work, we studied the behavior
17
with different composition.13 When H2 or ascorbate
ions18 are used in combination with colloidal platinum,
epoxide is mainly formed, which is the product of the
electrophilic attack of oxygen at the olefin double bond.19
In this work, it seemed of interest to compare the transꢀ
formations of cyclohexene and αꢀpinene to draw concluꢀ
sions on the mechanism of oxidation of hydrocarbons in
the catalytic systems under study.
* ARCHEMIS, 24 Avenue Jean Jaurès, 69153 Decines Charpieu
Cédex, France.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1462—1468, July, 2003.
1066ꢀ5285/03/5207ꢀ1544 $25.00 © 2003 Plenum Publishing Corporation

Oxidation of cyclohexene and αꢀpinene
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 7, July, 2003
1545
content of the main oxidation products (cisꢀ and transꢀverbenols,
verbenone, αꢀpinene oxide, and cisꢀpinanꢀ2ꢀol) was determined
from the surface area of the chromatographic peaks using caliꢀ
bration plots. The other products were determined from the
ratio of surface areas of the chromatographic peaks using no
calibration coefficients.
Reaction products were identified by chromatoꢀmass specꢀ
trometry (GLCꢀMS) (HewlettꢀPackard 5890/II chromatograph
and a HewlettꢀPackard MSD 5971 quadrupole mass spectromꢀ
eter). An НРꢀ5 quartz column 30 m × 0.25 mm with the film
thickness of the stationary phase 0.25 µm was used. Qualitative
analysis was carried out by comparison of retention times of
compounds in the mixture and their total mass spectra with the
corresponding data for pure components or with the spectra from
the Wiley275 Library of Mass Spectrometric Data (275000 mass
spectra).
Experimental
Preparation
of
catalysts.
Commercial
reagents
H3PW12O40•11H2O (PW12) and H3PMo12O40•14H2O (PMo12
)
(reagent grade) were used, the latter being used after purifiꢀ
cation by extraction with diethyl ether. Hereinafter in parenꢀ
theses the conventional designations of HPC are presented.
Heteropoly compounds H4PMo11VO40•13H2O (PMo11V),
H5PMo10V2O40•9H2O (PMo10V2), and H6PMo9V3O40•12H2O
(PMo9V3) were synthesized using a known procedure.20
The syntheses of H4PW11VO40•10H2O (PW11V) and
H6PW9V3O40•10H2O (PW9V3) have been described.21 The comꢀ
pound H3PMo6W6O40•13H2O (PMo6W6) was synthesized by
refluxing a solution of equimolar amounts of H3PMo12O40 and
H3PW12O40 for 1 h. Water was evaporated in air to obtain crysꢀ
tals. The compound H5PW11TiO40•6H2O (PW11Ti) was syntheꢀ
sized by extraction with diethyl ether from an aqueous solution
of the sodium salt in 5 N H2SO4,22 and H5PW11ZrO40•7H2O
(PW11Zr) was synthesized using a known procedure.23
Tetrabutylammonium salts
Results and Discussion
(Bu4N)3PW12O40 (Bu4NꢀPW12),
(Bu4N)5H2PW11O39 (Bu4NꢀPW11),
(Bu4N)4HPW11Fe(OH)O39 (Bu4NꢀPW11Fe),
(Bu4N)4HPW11Mn(H2O)O39 (Bu4NꢀPW11Mn),
(Bu4N)4PW11Cr(H2O)O39 (Bu4NꢀPW11Cr),
(Bu4N)4HPW11Co(H2O)O39 (Bu4NꢀPW11Co),
and (Bu4N)4HPW11Cu(H2O)O39 (Bu4NꢀPW11Cu)
Cyclohexene and αꢀpinene are slowly oxidized with
molecular oxygen at room temperature. The application
of the platinum catalysts both without and in combinaꢀ
tion with HPC produced no noticeable amount of the
reaction products. Under the same conditions, the cataꢀ
lytic oxidation of cyclic olefins with an O2—H2 mixture
occurs much more rapidly. A Platinum Group metal (usuꢀ
ally platinum or, in some experiments, palladium) supꢀ
ported on an inert material (carbon or silica gel) comꢀ
bined with HPC was used as catalyst. In some experiꢀ
ments, Hex4NCl or LiCl were used instead of HPC for
comparison.
were synthesized according to
a
described proceꢀ
dure.24 The salts (Bu4N)3PMo12O40 (Bu4NꢀPMo12),
(Bu4N)4PMo11VO40 (Bu4NꢀPMo11V), and (Bu4N)5PMo10V2O40
(Bu4NꢀPMo10V2) were synthesized similarly.
The Pt(5%)/C catalyst was prepared by the alkaline hyꢀ
drolysis of H2PtCl6 over the Sibunit pyrocarbon powder (Ssp
300 m2 g–1) followed by reduction with sodium formate.
=
Oxidation of cyclohexene. The oxidation of cyclohexene
with an О2—H2 gas mixture in the presence of the Pt/С
catalyst and HPC or Hex4NCl dissolved in acetonitrile
affords oxygenꢀcontaining compounds. The oxidation
products consist of cyclohexenol (1), cyclohexenone (2),
epoxide (3), cyclohexanol (4), cyclohexanone (5), and
products with two oxygenꢀcontaining functional
groups (6) and (7) (according to the GLCꢀMS data)
(Scheme 1). In addition, ∼1% cyclohexene are transꢀ
formed into cyclohexane. Cyclohexene is hydrogenated
with almost the same rate in the presence of Hex4NCl and
various HPC.
The Pt(1%)/SiO2 catalyst was prepared by the adsorption
of Pt(NH3)4Cl2 from an aqueous solution on SiO2 (Ssp
=
200 m2 g–1, granule size 0.2—0.5 mm). The sample was dried in
air first at ∼20 °C and then at 100 °С for 2 h, calcined at 300 °С
for 2 h, and reduced in an Н2 flow at 300 °С.
The Pd(1%)/SiO2 catalyst was prepared by the impregnaꢀ
tion of SiO2 with a solution of H2PdCl4. The sample was dried,
calcined, and reduced as the previous sample.
The Naꢀ(PW9O34)2Pd2Fe/SiO2 catalyst containing
1 wt.% Pd was prepared by the impregnation of SiO2 with an
aqueous solution of the sodium salt of the PdIIꢀ and FeIIIꢀconꢀ
taining HPC.9 The sample was dried in air at ∼20 °C and then at
100 °C for 2 h and reduced in an Н2 flow at 300 °С.
Cyclohexanol and cyclohexanone can be formed via
several routes: hydrogenation of the corresponding unsatꢀ
urated products or epoxide, oxidation of cyclohexane
formed in the reaction, and hydration of cyclohexene. We
studied the transformations of cyclohexenol, cycloꢀ
hexenone, and cyclohexene oxide, which were specially
introduced into the reaction medium. These compounds
turned out to be rather stable under the reactions condiꢀ
tions: they were not remarkably hydrogenated and oxiꢀ
dized within the standard reaction time. Cyclohexane
(which is present in the mixture) cannot yield the amount
of oxidation products comparable with that of cycloꢀ
Catalytic experiments. Alkenes were oxidized at an atmoꢀ
spheric pressure. The solid catalyst was loaded in a reactor in
which a constant temperature was maintained, MeCN (1.0 mL)
and then soluble HPC (or tetrahexylammonium chloride
(Hex4NCl) or LiCl) were added, and the mixture was thorꢀ
oughly stirred. An О2—Н2 (1 : 2) mixture was passed through
the reactor, which then was connected to a gas burette filled
with the same gas mixture, αꢀpinene or cyclohexene was inꢀ
jected with a syringe, and the mixture was stirred for 1 h, mainꢀ
taining an atmospheric pressure in the reactor. Then the liquid
was separated from the solid catalyst by centrifuging and anaꢀ
lyzed by GLC (Kristall 2000 chromatograph, flameꢀionization
detector, DBꢀ1701 capillary column 30 m × 0.53 mm). The

1546
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 7, July, 2003
Kuznetsova et al.
Scheme 1
hexanol was formed in comparable amounts in the presꢀ
ence of acids and tetrabutylammonium salts. It is most
likely that cyclohexanol and cyclohexanone are formed
due to oxidation at the allyl position followed by fast
hydrogenation of the double bond. Hydrogenation can be
facilitated when the allyl group is bound to the surface of
the platinum catalyst.
The mechanism of formation of compounds 6 and 7
with two oxygenꢀcontaining functional groups remains
yet unclear.
The ratio between the yields of the oxygenꢀcontaining
products depends on the composition of the second comꢀ
ponent of the catalytic system (Table 1). In the catalytic
system involving Hex4NCl, the yield of epoxide 3 is only
1.5% of the overall amount of the primary oxidation prodꢀ
ucts with one oxygenꢀcontaining group (1—5) or ∼1% of
the sum of all the products found (see Table 1, entry 1).
When Hex4NCl is replaced by Bu4NꢀPW11, the activity of
the system increases considerably, and the fraction of
epoxide 3 is ∼5% (see Table 1, entry 2). In the presence of
the coordinatively saturated HPC Bu4NꢀPW12, the sysꢀ
tem is inactive (see Table 1, entry 3).
The introduction of transition metal ions into the HPC
composition changes substantially their behavior in the
catalytic systems under study. Heteropoly acids PW11Zr
and PW11Ti do not surpass Bu4NꢀPW11 in activity. Howꢀ
ever, the fraction of epoxide in the sum of the primary and
all observed oxidation products in the presence of HPC
containing TiIV and ZrIV increases to ∼20—30% (see
Table 1, entries 4 and 5). The catalytic activity of the
system increases when the FeIII ions are introduced into
a. Homolytic mechanism; b. electrophilic oxidation; c. secondꢀ
ary reactions; d. oxidation, hydrogenation; e. oxidation.
hexene, because the cyclohexane concentration even to
the end of the reaction is 100ꢀfold lower and the reactivity
is not higher than that of cyclohexene.11 Finally, if a
saturated alcohol was formed by the hydration of cycloꢀ
hexene, it would be reasonable to expect its higher yield
in the systems with HPC. In our experiments, cycloꢀ
Table 1. Yields of the main products of cyclohexene oxidation* and their fraction in the total amount of the products (in parentheses)
Entry
Additive
Yield/µmol (mol.%)
1
2
3
4
5
6 + 7
1
2
3
4
5
6
7
8
Hex4NCl
16.0 (35.0)
46.0 (44.0)
1.3
12.0 (21.0)
30.0 (30.5)
80.0 (47.0)
23.0 (41.0)
12.0 (36.0)
1.4 (4.0)
9.7 (48.0)
25.0 (51.0)
10.0 (49.0)
8.0 (62.0)
8.6 (49.0)
29.0 (55.0)
7.8 (51.0)
8.7 (19.0)
22.0 (21.0)
0.5
0.5 (1.1)
5.6 (5.4)
—
6.5 (14.0)
10.0 (9.6)
1.0
2.5 (5.5)
3.5 (3.4)
11.0 (25.0)
17.0 (17.0)
2.0
4.2 (7.5)
3.4 (3.4)
22.0 (13.0)
12.0 (21.0)
5.0 (15.0)
3.0 (8.0)
6.0 (29.0)
12.0 (25.0)
7.2 (35.0)
2.5 (18.0)
4.2 (24.0)
12.0 (16.0)
3.3 (22.0)
Bu4NꢀPW11
Bu4NꢀPW12
PW11Ti
—
5.1 (9.0)
12.0 (21.0)
28.0 (28.0)
13.0 (7.7)
1.9 (3.4)
1.5 (4.5)
7.5 (21.0)
0.1 (0.5)
0.1 (0.2)
—
13.0 (23.0)
6.0 (6.1)
14.0 (8.3)
7.9 (14.0)
8.0 (24.0)
3.0 (8.6)
1.0 (4.9)
5.6 (11.0)
1.6 (7.8)
0.5 (3.8)
2.2 (13.0)
2.6 (5.0)
1.7 (11.0)
10.0 (18.0)
3.0 (3.0)
10.0 (5.9)
2.0 (3.5)
PW11Zr
28.0 (28.0)
30.0 (18.0)
9.5 (17.0)
7.2 (21.0)
20.0 (57.0)
3.6 (18.0)
5.0 (10.0)
1.7 (8.3)
2.0 (16.0)
2.1 (12.0)
8.4 (16.0)
2.0 (13.0)
Bu4NꢀPW11Fe
Bu4NꢀPW11Mn
Bu4NꢀPW11Cr
Bu4NꢀPW11Co
PMo12
PMo11V
PMo10V2
PMo9V3
Bu4NꢀPMo12
—
—
—
9
10
11
12
13
14
15
16
1.0 (2.1)
—
—
—
—
—
—
0.4 (2.3)
0.4 (0.8)
0.4 (2.6)
Bu4NꢀPMo11
Bu4NꢀPMo10V2
V
* Conditions: 10 mg of Pt(5%)/C, 4 mg of HPC or Hex4Cl, 1 mL of MeCN, 0.97 mmol of С6Н10, O2 : H2 = 1 : 2; 25 °C, reaction
duration 1 h.

Oxidation of cyclohexene and αꢀpinene
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 7, July, 2003
1547
the HPC composition (see Table 1, entry 6). Other reꢀ
doxꢀactive ions (MnII, CrIII, and CoII) decrease the yield
of the products (see Table 1, entries 6—9). The systems
involving HPC PꢀMo and PꢀMoꢀV afford less oxidation
products, regardless of using HPC as acids or Bu4N salts.
The yields of the oxygenation products increases on going
from PMo12 to PMo11V (see Table 1, entries 10, 11
and 14, 15). The introduction of the next V atom into
HPC has no positive effect (see Table 1, entries 12 and 16),
and the third vanadium atom worsens the catalytic propꢀ
erties of HPC (entry 13). The fraction of epoxide in the
systems with Bu4NꢀPMo12 and Bu4NꢀPMo12–nVn is lower
than 3%.
Oxidation of αꢀpinene. Under the catalytic reaction
conditions, αꢀpinene oxidation is accompanied by hydroꢀ
genation and isomerization. The complete analysis of the
products of αꢀpinene conversion was performed for three
catalytic systems: I — Pd(1%)/SiO2 + solution of LiCl in
MeCN, II — Naꢀ(PW9O34)2Pd2Fe/SiO2, and III —
Pt(1%)/SiO2 + solution of H3PMo12O40 in MeCN. The
results obtained are presented in Table 2. In addition, the
catalyzate contains 0.5—10% of nonidentified products.
The products of αꢀpinene (8) conversion are shown in
Scheme 2.
present. Myrtenal (15), the isomer of verbenone, is formed
in a lower amount than verbenone. The other oxidation
products are pinocarveol (16), pinocarvone (17), and
verbenene (18) (viz., the product of oxidation and isomerꢀ
ization of the double bond). In addition to compounds
with one oxygenꢀcontaining group, an unknown product
with the C10H16O2 composition (19) is present in considꢀ
erable amounts. Judging from the molecular weight 168,
this product contains two oxygenꢀcontaining functional
groups.
In Pdꢀcontaining systems I and II, αꢀpinene is hydroꢀ
genated to pinane (20), which is probably oxidized furꢀ
ther to transꢀpinanꢀ2ꢀol (21). The total yield of these two
products was ∼15%. In the presence of the Pt catalyst,
αꢀpinene is not hydrogenated.
αꢀPinene is isomerized more efficiently in the Pdꢀconꢀ
taining systems I and II, producing βꢀpinene (22), camꢀ
phene (23), and αꢀfenchene (24) in a 10—20% yield of
the total amount of the products formed.
The acidꢀcatalyzed25 cleavage of the С4 cycle in the
αꢀpinene molecule to form limonene (25) and terpinolene
(26) occurs only in system III at 35 °С. The oxidative
dehydrogenation of these products affords pꢀcymene (27)
(see Table 2).
At 25 °С the products of allylic oxidation are formed
in systems I and III: verbenol (9) (with predomination of
the transꢀisomer) and verbenone (10). The highest yield
of these products (30%) is achieved in system III. The
temperature increase to 35 °С decreases the yield of these
products (see Table 2). αꢀPinene oxide (11) was not
found but the products of its acidꢀcatalyzed hydration
and isomerization, viz., (1S,2S,3R,5S)ꢀpinanediol (12),
αꢀcampholenaldehyde (13), and transꢀsobrerol (14), are
The study of the products of αꢀpinene conversion in
the catalytic systems with different compositions makes it
possible to determine all compounds that formed. The
main transformations of αꢀpinene are not related to its
oxidation, and the primary oxidation products undergo
further transformations. We checked the stability of
verbenol, verbenone, and αꢀpinene oxide in the presence
of the catalyst in an inert atmosphere and on contact with
an О2—Н2 mixture after these compounds (6—32 µmol)
Scheme 2

1548
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 7, July, 2003
Kuznetsova et al.
Table 2. Influence of the catalytic system (I—III),* temperaꢀ
ture, and reaction duration on the yields of the main products of
αꢀpinene conversion with an О2—Н2 mixture (mol.% of the
total amount of the GC detected compounds) (0.1 mmol of
αꢀpinene, 1 mL of MeCN, O2 : H2 = 1 : 2)
yield of epoxide. In the oxidation of αꢀpinene in the presꢀ
ence of Bu4NꢀPW11, epoxide 11 constitutes ∼50% of the
total amount of the oxidation products 9 + 10 + 11 (see
Table 3, entry 2). The addition of FeIII decreases the
fraction of epoxide (entry 3), whereas CoII and MnII noꢀ
ticeably decrease the total amount of the oxidation prodꢀ
ucts (entries 4 and 5).
Compound
Yield (mol.%)
I
II
III (3 h)
In the systems with HPC PꢀMo and PꢀMoꢀV, less
oxidation products are formed. When the acidic form of
HPC is used, epoxide is completely converted (see Table 3,
entries 6 and 8), unlike the runs with the corresponding
Bu4N salts (entries 7 and 9). The overall amount of the
oxidation products is somewhat higher in the case of
PMo10V2 than that for PMo12. The total amount of the
products determined by GLC is smaller in the systems
with HPC PꢀMo and PꢀMoꢀV, which is caused, at least
partially, by secondary transformations of these substances
with formation of insoluble tarry compounds.
(1 h,
25 °С) 35 °С)
(1 h,
25 °С
35 °С
αꢀPinene (8)
Verbenol (9)
89.00
1.30
(trans)
62.00
—
77.00
4.90
(trans)
0.26
(cis)
1.70
—
75.00
—
Verbenone (10)
αꢀPinene oxide (11)
(1S,2S,3R,5S)ꢀPinaneꢀ
diol (12)
0.37
—
—
—
—
—
1.30
—
1.70
1.30
In the case of the Pt(5%)/C catalyst and H3PMo12O40,
we also studied the temperature effect on the composition
of the products of αꢀpinene oxidation in the interval from
–5 to 25 °С (Table 4). The total yield of verbenol 9 and
verbenone 10 depends slightly on the temperature, alꢀ
though the temperature decrease increases the fraction of
the first product. αꢀPinene oxide was not found in the
whole temperature interval studied.
Mechanism of oxidation of cyclohexene and αꢀpinene.
When the twoꢀcomponent catalytic systems are used for
the synthesis of Н2О2 from O2—H2 and for the oxidation
of organic compounds, the key point is the formation of
peroxide species coordinated with platinum1
αꢀCampholenꢀ
aldehyde (13)
transꢀSobrerol (14)
Myrtenal (15)
Pinocarveol (16)
Pinocarvone (17)
Verbenene (18)
Nonidentified
0.79
2.40
0.86
0.84
—
—
—
1.20
—
0.76
15.00
0.54
0.87
1.30
0.49
0.64
4.00
—
0.31
0.10
—
0.06
2.40
1.20
1.20
0.65
1.40
7.40
product (19)
cisꢀPinane (20)
transꢀPinanꢀ2ꢀol (21)
βꢀPinene (22)
Camphene (23)
αꢀFenchene (24)
Limonene (25)
Terpinolene (26)
pꢀCymene (27)
0.39
1.60
1.90
0.75
0.23
—
1.30
3.40
1.60
1.20
0.20
—
—
—
0.18
0.90
0.28
—
—
—
0.11
0.82
0.30
0.95
0.16
1.30
[Pt0] + O2 → [Pt2+(O2)2–].
—
—
—
—
—
0.35
In our experiments, the Pt or Pd catalysts exhibited
activity in combination with the second component, viz.,
Hex4NCl, LiCl, or HPC. The function of the second
component is first of all, probably, stabilization of the
oxidized platinum or palladium species.8 This facilitates
the twoꢀelectron transfer from the metal to oxygen to
form a peroxo complex (Scheme 3) and a peroxide comꢀ
pound of PtII. The coordination of the peroxide species to
the PtII atom impedes the further reduction of oxygen to
water and allows the peroxide O atom to be involved in
the alkene oxidation. The reactions suggested are shown
in Scheme 3. Epoxide is formed due to the electrophilic
attack of the peroxide oxygen atom at the double bond,
while the formation of the allylic oxidation products
(cyclohexenol and then cyclohexenone) occurs via oneꢀ
electron reduction of the coordinated peroxide and genꢀ
eration of the ОН• radicals.
* I — Pd(1%)/SiO2 (50 mg) + LiCl (atomic ratio Pd : Cl =
1 : 1), II — Naꢀ(PW9O34)2Pd2Fe/SiO2, Pd content 1% (50 mg),
III — Pt(1%)/SiO2 (50 mg) + H3PMo12O40 (7 mg).
were added to the components of the catalytic system.
The amount of the introduced compounds remained unꢀ
changed for 1 h (standard duration of the catalytic reacꢀ
tion) in the presence of the Pt/C catalyst both under
N2 and an О2—Н2 gas mixture. In the presence of
H3PMo12O40, verbenol is gradually consumed to be transꢀ
formed into verbenone and insoluble tarry compounds,
which were not identified by GLC. A similar pattern is
observed for αꢀpinene oxide, which is partially transꢀ
formed into campholenaldehyde (13) in the presence of
H3PMo12O40. Under the reaction conditions, verbenone
is retained in the solution.
The composition of the products of αꢀpinene converꢀ
sion is presented in Table 3 for the systems containing the
Pt(5%)/C catalyst, Hex4NCl, and HPC. The main disꢀ
tinction from the reaction of cyclohexene is the higher
In the presence of chlorides, the yield of epoxide is
low compared to the allylic oxidation products, indicatꢀ
ing of the predomination of the radical route through the
НО• radical. However, the chemical properties of the

Oxidation of cyclohexene and αꢀpinene
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 7, July, 2003
1549
Table 3. Influence of the Hex4NCl or HPC additive on the yields of verbenol (9), verbenone (10), and
αꢀpinene oxide (11) in the oxidationa of αꢀpinene
Entry
Additive
Yield/µmol (mol.%)b
9
10
11
ΣРiс/µmol
1
2
3
4
5
6
7
8
9
Hex4NCl
5.4 (60)
9.2 (37)
8.1 (48)
1.9 (100)
4.2 (42)
9.2 (70)
5.3 (64)
10.0 (74)
6.2 (58)
1.8 (20)
3.8 (16)
3.4 (20)
—
1.6 (16)
3.9 (30)
1.7 (20)
3.5 (26)
2.0 (19)
1.8 (20)
11.0 (47)
5.3 (32)
—
4.2 (42)
—
1.3 (16)
—
2.5 (23)
60
120
130
140
180
43
19
60
51
Bu4NꢀPW11
Bu4NꢀPW11Fe
Bu4NꢀPW11Mn
Bu4NꢀPW11Co
PMo12
Bu4NꢀPMo12
PMo10V2
Bu4NꢀPMo10V2
a Conditions: 0.6 mmol of αꢀpinene, 0.9 mL of MeCN, 15 °C, O2 : H2 = 1 : 2; Pt(5%)/C catalyst
(50 mg) and HPC or Hex4NCl (7 mg).
b Of the total amount of compounds 9 + 10 + 11.
c Total amount of the conversion products.
Table 4. Temperature effect on the yields of verbenol (9),
verbenone (10), and αꢀpinene oxide (11) in αꢀpinene oxidation
in the presence of the Pt(5%)/C catalyst (50 mg) + H3PMo12O40
(7 mg) (0.6 mmol of αꢀpinene, 1 mL of MeCN, O2 : H2 = 1 : 2)
A similar effect is manifested in the case of the HPC
PW11Ti and PW11Zr, which are known as catalysts of
epoxidation of alkenes with peroxide compounds.26 In
the presence of these HPC, cyclohexene epoxidation is
much more selective compared to that in the case of
Hex4NCl or other HPC. We assume that the formation of
epoxide involves the TiIV or ZrIV peroxide complexes
(Scheme 4).
The iron ions in the HPC composition direct the reacꢀ
tion predominantly toward allylic oxidation. It is most
likely that the presence of FeIII favors the formation of the
HO• radicals from peroxide. The replacement of FeIII by
other transition metals decreases the yield of the oxidaꢀ
tion products, although H2O2 decomposition in the presꢀ
ence of HPC PW11Mn, PW11Cr, and PW11Co, according
to the previously obtained data,24 is more efficient comꢀ
pared to that in the presence of PW11Fe. It is most likely
that oxidation in our systems requires the optimum activꢀ
ity of HPC in peroxide decomposition. The radical route
of alkene oxidation also predominates in the presence of
HPC PꢀMo and PꢀMoꢀV or their Bu4N salts, as shown by
the ratio between epoxide and the products of allylic oxiꢀ
Comꢀ
pound
Yield/µmol
–5 °С
5 °С
15 °С
25 °С
9
10
11
5.7
1.7
—
9.3
4.4
—
9.2
3.9
—
9.6
6.5
—
second component, for example, some HPC, can exert a
substantial effect on the behavior of the catalytic system.
It is known24 that the PW11– anions react with peroxꢀ
ides to produce peroxo complexes and catalyze the selecꢀ
tive epoxidation of alkenes. The presence of Bu4NꢀPW11
in the system under study increases the fraction of epꢀ
oxide in the oxidation products compared to Hex4NCl.
The yield of epoxide is low in the case of cyclohexene but
becomes more substantial for αꢀpinene in which the
double bond is easily subjected to the electrophilic attack.
Scheme 3

1550
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 7, July, 2003
Kuznetsova et al.
Scheme 4
ucts of its transformation (see Scheme 2), agrees with the
radical mechanism of the reaction. In the presence of H2,
αꢀpinene is oxidized in MeCN at much lower temperaꢀ
tures (≤25 °С) than the temperatures of autooxidation due
to the initiation of the chain radical process by the НО•
radicals formed on the platinum surface.
The results obtained show that the catalytic effect of
the Platinum Group metals in alkene oxidation with an
О2—H2 gas mixture is determined, to a great extent, by
the nature of the second component of the system. The
reaction involves several types of active intermediates,
which manifest the radical or electrophilic character in
the reactions with alkenes. For example, PW11 and, to a
greater extent, PW11Ti and PW11Zr can form intermediꢀ
ates of the peroxide type responsible for the transformaꢀ
tion of alkenes into epoxides, whereas Feꢀ, Mnꢀ, Crꢀ,
and Coꢀcontaining heteropolytungstates, as well as PMo12
and PMo12–nVn, generate oxygen species of the radical
character leading to oxidation at the allyl position.
dation in the reactions with cyclohexene and αꢀpinene.
The radical character of the reaction is caused by the
redox activity of these HPC, which increases in the presꢀ
ence of the VV ions in HPC.27
A specific feature of αꢀpinene oxidation should be
noted. In all systems where epoxide did not undergo inꢀ
tense secondary transformations, its yield relative to the
sum of the primary oxidation products was higher than
that in the case of cyclohexene. This was manifested even
in the systems in which H2O2 was intensely decomposed,
in particular, in the presence of the FeIIIꢀ and CoIIꢀconꢀ
taining HPC. This fact can be explained by the higher
reactivity of the double bond of αꢀpinene with respect to
the electrophilic attack of the peroxide oxygen atom. Unꢀ
like cyclohexene, the radical oxidation of αꢀpinene with
oxygen at ∼90 °C in the presence of the Co, Cr, and Cu
complexes15 and noncatalyzed autooxidation14 exhibit the
same high yield of αꢀpinene oxide: to 50% of the total
amount of verbenol, verbenone, and αꢀpinene oxide.
These reactions also afford myrtenal, pinocarveol, and
pinocarvone, viz., the products of radical transformations
of αꢀpinene. Taking into account these facts, an alternaꢀ
tive mechanism of the reaction can be proposed. Earꢀ
lier8—11 we believed that active intermediates are formed
on the metallic catalyst surface modified by the second
component, in particular, HPC. However, the escape of
some intermediates into the liquid phase bulk cannot be
excluded. In this case, oxidation proceeds via Scheme 5.
The authors thank A. S. Lisitsyn (Institute of Catalyꢀ
sis, Siberian Branch of the Russian Academy of Sciences)
for the kindly presented Pt(5%)/C catalyst.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 00ꢀ15ꢀ
97447).
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Received November 25, 2002;
in revised form March 11, 2003