Homogeneous and polymer-supported catalysts in the oxidation of α-pinene with oxygen

Article abstract of DOI:10.1023/B:RUJO.0000044540.17924.65

The catalytic activities of nitrogen-containing cobalt complexes, polymer (AN-251 anion exchanger)-supported cobalt complex, and a polymer-salt composition containing molybdenum salt and nonionogenic water-soluble polymers were compared in the oxidation o

Full text of DOI:10.1023/B:RUJO.0000044540.17924.65

Russian Journal of Organic Chemistry, Vol. 40, No. 6, 2004, pp. 790–794. Translated from Zhurnal Organicheskoi Khimii, Vol. 40, No. 6, 2004,
pp. 830–833.
Original Russian Text Copyright © 2004 by Men’shikov, Sennikov, Romanova, Sycheva, Ostroushko.
Dedicated to Full Member of the Russian Academy of Sciences
O.N. Chupakhin on his 70th Anniversary
Homogeneous and Polymer-Supported Catalysts
in the Oxidation of α-Pinene with Oxygen
1
2
1
S. Yu. Men’shikov , M. Yu. Sennikov , Yu. V. Romanova ,
2
2
N. S. Sycheva , and A. A. Ostroushko
1
Institute of Organic Synthesis, Ural Division, Russian Academy of Sciences,
ul. S. Kovalevskoi 20, Yekaterinburg, 620219 Russia
e-mail: kox@ios.uran.ru
2
Ural State University, Yekaterinburg, Russia
Received January 10, 2004
Abstract—The catalytic activities of nitrogen-containing cobalt complexes, polymer (AN-251 anion
exchanger)-supported cobalt complex, and a polymer–salt composition containing molybdenum salt and non-
ionogenic water-soluble polymers were compared in the oxidation of α-pinene.
Oxidation of α-pinene with oxygen attracts
researchers’ attention, for it utilizes an accessible and
ecologically clean oxidant [1]. This reaction leads to
practically important oxygen-containing compounds
which can be obtained with a fairly high selectivity,
especially in the presence of transition metal salts and
complexes [2]. However, analysis of published data
shows that the results of using transition metal com-
pounds as catalysts should be considered with some
criticism [3]. In particular, insufficient attention was
given to possible isomerization of the substrate and
formation of active radical intermediates. For example,
in the presence of dibromo(4-methylpyridine)cobalt(II)
portions of the catalyst was necessary to keep the
process running. Here, the yield of verbenol was
26.5%, and the yield of verbenone was 11%.
Kuznetsova et al. [5] reported on the oxidation of
α-pinene with a 1:2 oxygen–hydrogen gas mixture in
the presence of a two-component heterogeneous
catalytic system based on platinum metals and hetero-
polycompounds. The authors showed that the reaction
involves several types of active intermediates having
a radical or electrophilic nature. For example, such
heteropolycompounds as PW11 and especially PW Ti
11
and PW Zr are capable of producing peroxy inter-
11
mediates which are responsible for the transformation
of alkenes into epoxy derivatives; heteropolytungstates
containing Fe, Mn, Cr, and Co, as well as compounds
[
(4-MeC H N) CoBr ] at 72°C, the yield of verbenone
5 4 2 2
in the crude product attained 76% (the conversion of
α-pinene was complete); with (4-MeC H N) CoCl as
5
4
2
2
like PMo and PMo
V , give rise to radical oxygen
1
2
12 – n
n
catalyst (65°C), the conversion of α-pinene was 53%,
the yield of verbenone was as low as 12%, and 10%
of verbenol and 26.5% of 2,3-epoxypinane were
obtained. Presumably, the formation of the latter is
responsible for the poor yield of verbenone. In the
above cases, the oxidation of α-pinene was carried out
by bubbling air or oxygen at a flow rate of about 40–
species which promote allylic oxidation.
The activity of a series of homogeneous catalysts
and immobilized molybdenum and cobalt salts and
complexes in the oxidation of α-pinene can be es-
timated by the rate of oxygen absorption under static
conditions [6]. We previously reported on successful
peroxide oxidation of anthracene with the use of
acetylacetonato VO(II) and Co(II) complexes im-
mobilized on chloromethylated styrene–divinylben-
zene copolymer [7, 8] and of metallic molybdenum
with polyvinyl alcohol [9]. It seemed reasonable to
6
0 ml/min. On the other hand, oxidation of α-pinene in
the presence of cobalt(II) abietate in a static system at
room temperature [4] was accompanied by gradual
decrease in the rate of oxygen absorption (which was
controlled using a gas burette), and addition of 10 fresh
1
070-4280/04/4006-0790 © 2004 MAIK “Nauka/Interperiodica”

HOMOGENEOUS AND POLYMER-SUPPORTED CATALYSTS
791
a
Catalytic activity of homogeneous and polymer-supported heterogeneous catalytic systems (50°C)
Run
no.
b
Molar ratio
α-pinene–solvent–metal
–1
–1
6
Catalyst
–v
0
, mol l min
–vsp
r
–W ×10
1
CoSalen
Co(acac)
1:3.03:0.0095
0.001490
4.975
13.100
2
3
4
5
6
7
8
9
2
1:3.03:0.0095
1:3.03:0.0096
1:3.03:0.0096
1:0:0.0031
0.001260
0.000612
0.001820
0.000063
0.000130
0.000109
0.000130
0.000210
4.200
2.050
6.300
0.670
1.300
0.880
0.990
0.337
10.600
05.300
15.300
00.075
00.130
00.080
00.110
(2-MeC
5
H
H
4
4
N)
N)
2
2
CuCl
2
2
(2-MeC
5
CoCl
AHM (4.5%) + PVA
AHM (4.5%) + PVA + UV
AHM (9%) + PVA
1:0:0.0031
1:0:0.0063
c
AHM (9%) + PVA
1:0:0.0042
AN-251 + CoCl
2
1:3.03:0.021
00.375
a
–1
–1
–1
3
v
0
–
is the initial oxidation rate, vsp is the specific oxidation rate, mol l min [M] ; and W
r
is the reduced oxidation rate, m (O
2
)×
1
–1
–1
g (M) mol (α-pinene) s .
b
c
AHM stands for ammonium heptamolybdate, and PVA, for polyvinyl alcohol.
Reused catalyst.
examine the activity of polymer-supported hetero-
geneous catalysts with a view to elucidate the pos-
sibility for their repeated use. As such polymeric
catalysts we used cross-linked wide-pore styrene–
below) can be applied to estimate (at least roughly) the
kinetics of α-pinene oxidation with molecular oxygen.
The data of volumetric and titrimetric (iodometry)
analyses showed a considerable discrepancy which
attained ~40%. Presumably, this is the result of partial
decomposition of hydroperoxides in the presence of
transition metal ions.
2
-methyl-5-vinylpyridine copolymer (which is a large-
scale product) containing adsorbed Co(II) and Cu(II)
ions and a heterophase polymer–salt composition con-
sisting of polyvinyl alcohol and ammonium hepta-
molybdate (NH ) Mo O ·4H O (an aqueous solution).
A common criterion of the efficiency of homo-
geneous and polymer-supported heterogeneous catal-
ysts under the given oxidation conditions is the
specific rate of oxidation, which is calculated by
dividing the initial reaction rate v0 by the molar
concentration of metal ions (M) in solution. Some
difference between the reaction volumes with the use
of a fairly large amount of the catalyst based on
polyvinyl alcohol (without solvent addition, for in
the presence of acetonitrile as solvent coagulation of
polyvinyl alcohol occurs) and those of homogeneous
systems formally leads to higher initial concentration
of oxygen in the reaction solution and hence to lower
values of the specific initial oxidation rate.
4
6
7
24
2
Both these systems can readily be separated from the
oxidation products.
A possible catalytic activity in redox reactions of
compositions including water-soluble non-ionogenic
polymers and oxygen-containing molybdenum, tung-
sten, and vanadium salts was noted in several publica-
tions [10, 11]. An important specific feature of these
systems (which are used as solutions, gels, films, or
grains) is the occurrence therein of reversible photo-
chemical reactions leading to partial reduction of
d-metals under UV irradiation, which makes it possible
to control the catalytic process with participation of
molecular oxygen.
Although the amount of the substrate was the same
in all experiments, the substrate-to-metal ion molar
ratio was varied over a wide range. Therefore, it was
more reasonable to compare the specific initial rates of
oxidation of different samples in the presence of
polyvinyl alcohol-based catalysts only with each other.
For example, twofold increase of the concentration of
ammonium heptamolybdate in polyvinyl alcohol (from
4.5 to 9 wt %) leads to only ~20% increase in the
specific initial oxidation rate. On the other hand, UV
The results of our kinetic studies showed that,
among the examined homogeneous systems, the
catalytic properties of 2-methylpyridine Co(II) com-
plexes ensure the highest initial rate v of α-pinene
0
oxidation (see table). Their activity changes in the
following series: (2-MeC H N) CuCl < Co(acac) <
5
4
2
2
2
CoSalen < (2-MeC H N) CoCl . The initial parts of
5
4
2
2
the log c = ƒ(τ) plots (Fig. 1) are close to linear,
indicating that pseudofirst-order equation (1) (see
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 6 2004

7
92
MEN’SHIKOV et al.
logc
D
0
.495
2
1
1
0
.0
.5
.0
.5
3
3
0
0.49
.
4
9
0
0
0
.485
2
2
0
0
.
.
4
4
8
8
11
.475
0
0.47
.
4
7
.465
0.0
0
1
2
3
4
5
5
0
100
200
300
400
500
600
0
1
2
3
4
Time, h
Time, min
Fig. 1. Variation of the concentration of oxygen (logc) with
time in the presence of (1) cobalt(II) complex with 2-methyl-
pyridine, (2) cobalt(II) complex with 1,2-bis(salicylidene-
Fig. 2. Variation of the optical density D (Specol-11) of
solutions containing 2 wt % of polyvinyl alcohol and 1 wt %
of ammonium heptamolybdate at 0°C. Down and up arrows
correspond to the moments of turning UV lamp off and on,
respectively.
amino)ethane, and (3) polymer (AN-251)-supported CoCl
2
complex.
irradiation of the reaction mixture containing the same
sample over a period of 2 h results in more than 90%
increase of the specific initial rate. An analogous
situation was observed when so-called reduced oxida-
Thus, the oxidation of α-pinene under the given
mild conditions is characterized by low conversion of
the substrate and good selectivity for verbenol (accord-
ing to the GLC data); verbenone is formed in trace
amounts. Polymer-supported metal-complex catalyst
can be used repeatedly. The results confirmed the
previously presumed catalytic activity of a composi-
tion based on polyvinyl alcohol and ammonium
heptamolybdate and the possibility for controlling
the catalytic process by UV irradiation. Prolonged
periodical UV irradiation and barbotage technique are
necessary to raise the substrate conversion. The oxida-
tion catalyzed by metal complexes immobilized on
a poly(vinylpyridine) matrix should be performed
under pressure, for barbotage technique could lead to
mechanical degradation of samples.
3
tion rate W (m /s), which is related to 1 g of the metal
and 1 mol of the substrate, was used as parameter
characterizing the catalytic activity. Here, the rate of
oxidation in homogeneous catalytic systems was
considerably higher, while the reduced oxidation rates
for heterogeneous polymer-supported catalysts were
fairly similar. It should be noted that the catalyst
containing 9% of ammonium heptamolybdate can be
reused without loss of catalytic activity (see table).
While performing the oxidation under UV irradia-
tion, we found that the consumption of oxygen
increased after the UV lamp was turned off (with
account taken of the temperature conditions). Simul-
taneously, the optical density D (λ 740 nm) of the
catalytic solution increased (Fig. 2). The optical dens-
ity depends on the concentration of polymer–salt com-
plexes containing partially reduced Mo [10, 11]. Their
concentration can increase as a result of reduction of
Mo ions in the dark due to greater stability of radical
ions whose formation and disappearance are controlled
by dynamic equilibrium under UV irradiation. The
character of variation of the optical density indicates
the occurrence of a “pulsating” reaction. Probably, the
catalytic activity of such compositions depends on the
concentration ratio of the initial metal ions [M(VI)]
and the reduced species.
EXPERIMENTAL
The optical densities of solutions were measured on
a Specol-11 spectrophotometer. Ultraviolet irradiation
was generated using an HGOK-125 lamp. Samples
were analyzed by GLC on an LKhM-8MD gas
chromatograph equipped with a thermal conductivity
detector; a 3000×1.5-mm steel column was packed
with Chromaton N–AW (0.16–0.20 mm) which was
preliminarily washed with an acid and impregnated
with 15% of Apiezon L as stationary liquid phase.
The catalytic activity in the oxidation of α-pinene
with molecular oxygen was determined using a glass
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 6 2004

HOMOGENEOUS AND POLYMER-SUPPORTED CATALYSTS
793
reactor consisting of two parts where particular com-
ponents of the reaction mixture (substrate and catalyst)
could be stored without mixing until preliminary
operations were complete. The reactor was hermetic-
ally sealed and purged with oxygen (5 times), the
components were mixed, and the reaction was carried
out by continuous shaking at 50°C, measuring the
amount of absorbed oxygen. The resulting mixture was
then analyzed by GLC. All original kinetic data for
a nonzero-order reaction were treated in a simplified
version using pseudofirst-order equation (1):
Cobalt(II) complexes with a commercial sample of
styrene–2-methyl-5-vinylpyridine copolymer (AN-251)
were prepared by heating (50°C) a required amount
of the anion exchanger with 3 equiv of CoCl over
2
a period of 24 h. The product was washed in a Soxhlet
apparatus and dried. Cobalt(II) complexes with 1,2-bis-
(salicylideneamino)ethane (CoSalen) were prepared by
the procedure reported in [14]. Polymer–salt composi-
tions containing polyvinyl alcohol and ammonium
heptamolybdate were prepared as described in [10, 11].
The authors are grateful to Corresponding Member
of the Russian Academy of Sciences, Prof. A.V. Ku-
chin for providing samples of α-pinene and its
oxidation products used as reference compounds for
GLC analysis, as well as for discussing the results.
This study was performed under financial support by
the Russian Foundation for Basic Research (project
no. № 02-03-32777) and by the Ministry of Education
of the Russian Federation (program for support of
research by post-graduate students at higher school,
project no. A 03-2.11-853).
c
τ
= c
0
exp(–kτ),
(1)
where c and c are, respectively, the initial and current
0
τ
concentrations of oxygen, and k is the rate constant.
The first derivative
v
τ
= ∂c
τ
/∂τ = –c
τ
k exp(–kτ)
(2)
was extrapolated to τ = 0 to obtain the initial reaction
rate v . The current oxygen absorption measured by
0
a gas burette was converted into the consumed oxygen
concentration in an oxygen-saturated solution:
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