Co(II) catalysed oxidation of α-pinene by molecular oxygen. Part 2

Article abstract of DOI:10.1016/S0040-4020(00)00742-0

This paper reports studies of factors affecting air oxidation of (-)-α-pinene catalysed by Co(II)-pyridine complexes without a concomitant co-oxidant. Observations are presented about the nature and amount of the catalyst, reaction temperature, oxygen flow and solvent vs. non-solvent conditions. In addition, attention has been paid to the influence of added base or acid, and identification of side products. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4020(00)00742-0

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 8167±8171
Co(II) Catalysed Oxidation of a-Pinene by Molecular Oxygen.
Part 2
b
b
Marja K. Lajunen,a, Tatja Maunula and Ari M. P. Koskinen
*
aDepartment of Chemistry, University of Oulu, P.O. Box 3000, FIN-90014 University of Oulu, Linnanmaa, Finland
bDepartment of Chemical Technology, Helsinki University of Technology, Laboratory of Organic Chemistry, P.O. Box 6100,
FIN-02015 HUT, Helsinki, Finland
Received 13 April 2000; revised 4 August 2000; accepted 17 August 2000
AbstractÐThis paper reports studies of factors affecting air oxidation of (2)-a-pinene catalysed by Co(II)±pyridine complexes without a
concomitant co-oxidant. Observations are presented about the nature and amount of the catalyst, reaction temperature, oxygen ¯ow and
solvent vs. non-solvent conditions. In addition, attention has been paid to the in¯uence of added base or acid, and identi®cation of side
products. q 2000 Elsevier Science Ltd. All rights reserved.
Aerobic oxidation of organic ole®nic substrates catalysed
by various metal complexes in the presence of alcohols,
aldehydes, ketones, ketoesters or acetals has recently been
the subject of several papers.1 Less attention has been paid
to catalytic air oxidation of ole®ns without co-oxidant.2 We
have reported a facile method to prepare verbenone (1) in
high yield by air oxidation of (2)-a-pinene (2) catalysed by
Co(II) complexes under solvent free conditions.3 Liquid-
phase auto-oxidation of ole®ns is a radical chain process
best results for air oxidation of a-pinene were obtained by
using [Co(4-methylpyridine)2Br2] as the catalyst.3 However,
the unsubstituted analog [Co(pyridine)2Br2] was chosen for
comparison to gain further insight into the effect of the
catalyst. Concurrent air oxidations of a-pinene at 608C
were performed by using CoBr2, CoBr2´H2O, [Co(pyri-
dine)2Br2] and weighed amounts of components needed
for the formation of the complex, CoBr2´H2O and pyridine.
In addition, a sample without a catalyst was included in the
air oxidation, but no oxidation was detected after 30 h. Air
oxidations in which the catalyst was a ready-made complex
or where it was generated in situ, proceeded equally well,
producing 27% of verbenone in 23 h. Anhydrous CoBr2 and
CoBr2´H2O were ineffective as catalysts.
The amount of catalyst used in our air oxidations of
a-pinene varied from 0.1 to 0.5 mol%, though usually it
was 0.15 mol%. The effect of the amount on air oxidation
was studied by using 7.5, 1.5, 0.15 and 0.05 mol% of [Co(4-
methylpyridine)2Br2]. The reaction with the two lowest
amounts of the catalyst started and proceeded with equal
success. Air oxidations with the largest amounts of the
catalyst (1.5 or 7.5 mol%) were slower and produced less
verbenone than the reactions with less catalyst (Fig. 1).
and the chain propagation can occur via the usual abstrac-
tion mechanism to produce allylic oxidation products or via
the addition of alkylperoxy radical to the double bond,
followed by unimolecular decomposition to give an epox-
ide.4 Herein we report our observations concerning the
nature and the amount of catalyst, the reaction temperature
and the oxygen ¯ow rate. Additionally, attention was paid to
solvent vs. non-solvent conditions, and the effect of added
base or acid.
In conclusion, pyridine has an essential role in the active
catalyst species, which can also be generated during the air
oxidation. The catalyst amount 0.05 mol% is suf®cient to
catalyse air oxidation.
Results and Discussion
Catalyst
Reaction temperature
First we investigated the nature of the Co(II) catalyst. The
We have reported that an increase of reaction temperature
with the same catalyst accelerated air oxidation of
a-pinene.3 However, completion of a-pinene air oxidation
Keywords: air oxidation; Co(II) catalyst; a-pinene.
* Corresponding author. Tel.: 18-5531-632; fax: 18-5531-629; e-mail:
marja.iajunen@oulu.®
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00742-0

8168
M. K. Lajunen et al. / Tetrahedron 56 (2000) 8167±8171
Figure 1. Percentage yield of verbenone as a function of [Co(4-mepy)2Br2] catalyst at 1008C.
at 508C took days. Furthermore, we noted that a short heat-
ing of a sample of an incomplete air oxidation at 1158C
promoted the reaction signi®cantly. To shorten the reaction
time air oxidation was performed at 80, 100 and 1208C. At
all three temperatures air oxidation commenced quickly by
formation of a-pinene oxide (3). Next was produced trans-
verbenol (4) and somewhat more slowly verbenone (1). The
amount of a-pinene oxide reached a maximum in 3 h at
808C (11%), and in 2 h at both 1008C (8%) and 120 (5%),
after which time it began to reduce. For verbenone, the most
advantageous reaction temperature was 1008C where in
6.5 h the yield rose to 37%. a-Pinene oxide (3) was the
intermediate product, but it was absent among the ®nal
products. Epoxidation of a-pinene surprisingly produced
side products with the same GC retention times as air oxi-
dation of a-pinene. To explain this behaviour (2)-a-pinene
oxide (3) was air oxidised using [Co(4-methylpyridine)2Br2]
as a catalyst at 1008C. Rearrangement started immediately
and in 6 h 75% of epoxypinane had rearranged, producing
19% of 5, 19% of 6, 11% of 7 and 7% of 8 (Scheme 1).
Formation of a ring contracted aldehyde under the in¯uence
of a Lewis acid is a typical reaction of monoterpene oxides.5
Comparison of the GC retention times of product mixtures
from air oxidation and the rearrangement reaction, showed
that numerous minor products formed during air oxidation,
resulted from the rearrangement of (2)-a-pinene oxide. The
moment when the portion of pinene oxide started to
decrease indicates that the rate of rearrangement of epoxide
has exceeded that of epoxidation.
Oxygen ¯ow
Earlier we observed that the increase of oxygen ¯ow speeds
up air oxidation of a-pinene.3 The linearity of this effect was
studied by performing air oxidation at 1008C using the O2
¯ow of 5, 10 and 15 ml/min. The increase in oxygen ¯ow
accelerated air oxidation. With the highest ¯ow, a-pinene
converted nearly completely in 24 h, producing the highest
content of pinene oxide and verbenone (Fig. 2).
Solvent
All our a-pinene air oxidations were performed without a
solvent.3 To see if a solvent would affect the course of air
oxidation the reaction was performed in toluene, acetonitrile
and dimethylformamide at 608C. Tetrahydrofuran was
excluded because it is known to be autoxidised by cobalt
complexes.2c,6 In the above solvents at 608C, no air oxida-
tion of (2)-a-pinene using [Co(4-methylpyridine)2Br2] as
catalyst was detected in 23 h. Under the parallel non-solvent
conditions 76% of (2)-a-pinene was converted producing
35% yield of verbenone. A similar retarding in¯uence has
also been reported for benzene, chlorobenzene and dichloro-
methane.2d For comparison, air oxidation in acetonitrile was
performed at 1008C. Not until after 3 h did the colour of the
reaction mixture began to turn greenish, and the ®rst signs of
product formation were detected. Only 26% of (2)-a-
pinene converted in 24 h producing 9% of verbenone.
Bubbling air through the catalyst in acetonitrile (3 h) before
Scheme 1.

M. K. Lajunen et al. / Tetrahedron 56 (2000) 8167±8171
8169
Figure 2. Typical air oxidations of (2)-a-pinene at 1008C.
addition of the substrate did not improve the reactivity. The
same reaction by using old peroxidated a-pinene started
directly but the progress was no better (10% of verbenone
in 24 h).
numerous minor side products (,2%). Mass spectrometric
analysis indicated that the product mixture contained some
verbenyl acetate when the reaction mixture contained the
high amount of acid. Using Co(AcO)2 as a catalyst, instead
of the added glacial acetic acid did not improve air oxida-
tion. Co(AcO)2 with pyridine (corresponding to 0.13 mol%
of in situ formed catalyst) showed nearly equal catalytic
activity to CoBr2 with pyridine in air oxidation.
Effect of added base or acid
Next we were interested in the effect of added base or acid
upon the conversion of a-pinene (Table 1). Weighed
amounts of CoBr2´H2O and pyridine for the catalyst
(0.13 mol%) were added to (2)-a-pinene through which
oxygen was bubbled. Pyridine or glacial acetic acid
(10 vol%) was apportioned into the reaction mixture and
the reaction was held in a 508C bath. Glacial acetic acid
signi®cantly promoted the air oxidation but also speeded
up the rearrangement of pinene oxide. Excess pyridine
retarded the reactivity of a-pinene and directed air oxidation
more toward epoxidation than allylic oxidation. When
glacial acetic acid was replaced with tri¯uoroacetic acid,
air oxidation was clearly slower.
Side products
All intermediate products had the molecular ion m/z 152 and
they experienced the loss of M-18.3 Therefore, they have to
be isomeric compounds to verbenol. Four components of
intermediate products of a-pinene air oxidation were
separated by ¯ash chromatography and were identi®ed as
a-pinene oxide (3), trans-3-pinen-2-ol (9), trans-verbenol
(4) and verbenone (1). A minor amount (2±6%) of trans-3-
pinen-2-ol (9) formed in the early stages of air oxidation and
in pinene oxide rearrangement under catalytic air oxidation
conditions. cis-Verbenol was excluded by using GC
comparison of the product verbenol with cis-verbenol
obtained from reduction of verbenone by NaBH4.7 cis-
Verbenol is a solid compound with needle-like crystals,
mp 68±708C and trans-verbenol (4) is a liquid. Under
air oxidation conditions (2)-a-pinene oxide (3) mostly
The success of glacial acetic acid led us to study its contri-
bution. Air oxidation was carried out by using 2, 5, 10 and
20 vol% of glacial acetic acid (Table 2). A high amount of
acid seemed to favour more allylic oxidation than epoxida-
tion but it also resulted in rearrangement of pinene oxide to
Table 1. The effect of the added base or acid (10%) on air oxidation of a-pinene catalysed by in situ generated [Co(II)Br2±pyridine]-complex at 508C after 22 h
reaction time
Composition of the product mixture (%)
Base/acid
a-Pinene
a-Pinene oxide
trans-Verbenol
Verbenone
Other products
None
Pyridine
Glacial acetic acid
23
47
14
14
21
±
14
10
16
32
17
46
17
5
24
Table 2. The effect of glacial acetic acid on air oxidation of a-pinene catalysed by [Co(4-methylpyridine)2Br2] at 608C
Composition of the product mixture (%)
Acid (vol%)
h
a-Pinene
a-Pinene oxide
trans-Verbenol
Verbenone
Other products
2
5
10
20
10
10
10
10
62
51
31
33
14
16
±
10
12
14
14
11
15
31
32
3
6
24
24
±

8170
M. K. Lajunen et al. / Tetrahedron 56 (2000) 8167±8171
rearranged to a-campholene aldehyde (5). Other notable
products were trans-pinocarveol (6), trans-carveol (7) and
trans-sobrerol (8) (Scheme 1).
(2)-trans-Verbenol (4,6,6,-trimethylbicyclo[3.1.1]hept-
3-en-2-ol) (4). H NMR (500 MHz, CDCl3) d 5.32 (m,
1
1H, Jˆ3.1, 1.5 Hz, H-3), 4.24 (m, 1H, Jˆ3.1, 1.5 Hz, H-
4), 2.23 (m, 1H, Jˆ9.1, 5.5 Hz, H-7a), 2.15 (tdd, 1H, Jˆ5.7,
1.8 Hz, H-5), 2.00 (td, 1H, Jˆ5.4, 1.3 Hz, H-1) 1.70 (t, 3H,
Jˆ1.6 Hz, H-10), 1.30 (s, 3H, H-8), 1.26 (m, 1H, H-7b),
0.85 (s, 3H, H-9). Lit.10 13C NMR (100 MHz, CDCl3) d
148.7 (C-2), 118.8 (C-3), 70.4 (C-4), 48.1 (C-1), 47.1
(C-5), 46.2 (C-6), 28.6 (C-7), 26.6 (C-8), 22.5 (C-10),
20.4 (C-9). Lit.8b MS [m/z (relative intensity %)]: 152 (9,
M1.), 137 (12), 134 (8), 123 (8), 121 (9), 119 (22), 110 (13),
109 (55), 107 (13), 96 (13), 95 (42), 94 (40), 93 (23), 91
(50), 83 (95), 81 (53), 79 (48), 77 (40), 69 (66), 67 (53), 65
(20), 59 (12), 55 (100), 53 (20), 51 (18), 50 (21), 43 (18), 41
(68), 39 (91). Lit.9 HMRS: Found: 168.1509. Calcd for
C11H20O, 168.1514.
In the course of these experiments the (2)-a-pinene used
came from separate lots. During air oxidations it was
detected that fresh, unperoxidated a-pinene started to
oxidise slower than a lot which contained peroxides. This
indicates that peroxides, very often occurring in a-pinene,
also participate in air oxidation as an initiator of the radical
chain reaction.
Experimental
Catalysts were prepared as before and used without recrys-
tallisation.3
a-Campholene aldehyde [(2,2,3-trimethyl-3-cyclopen-
tenyl)-ethanal] (5). H NMR (200 MHz, CDCl3) d 9.76
1
(s, 1H, CHO), 5.19 (s, 1H, H-4), 2.57±1.75 (m, 5H), 1.57
(s, 3H, CH3), 0.96 (s, 3H, gem. CH3), 0.75 (s, 3H, gem.
CH3). Lit.11 13C NMR (50 MHz, CDCl3) d 202.8 (CHO),
147.9 (C-3), 121.5 (C-4), 46.8 (C-2), 45.0 (CH2CHO), 44.1
(C-1), 35.4 (C-5), 25.5 (gem. CH3), 19.9 (gem. CH3), 12.5
(CH3). Lit.12 MS [m/z (relative intensity %): 137 (2, M1´),
119 (4), 109 (22), 108 (100), 95 (32), 93 (67), 91 (20), 81
(17), 79 (10), 77 (13), 67 (25), 55 (16), 53 (12), 51 (6), 43
(15), 41 (32), 39 (22). Lit.9,11 MS [m/z (CI, NH3)] 153
(M111).
A typical air oxidation experiment: (2)-a-Pinene (5.0 g,
0.037 mol) (99%, Fluka) air oxidation was performed in a
thermostated glass reactor (bath 50±1208C) equipped with
sintered gas inlet in the bottom and a re¯ux condenser.
Co(II) catalyst (0.15 mol%) was added and molecular
oxygen was passed through the reactor (5 ml/min) under
atmospheric pressure.
Oxygen ¯ow was controlled by Brooks Mass Flow Meter
Model 5850TR. Progress of the reaction was monitored by
TLC (on silica gel 60 F254 plates from Merck) and GC
(Perkin±Elmer AutoSystem XLe, column OV-1701, length
25 m, i.d. 0.25 mm, phase layer 0.25 mm). The TLC
chromatograms were visualised by UV light and staining
with ethanolic anisaldehyde/glacial acetic acid/H2SO4
reagent. Reaction temperatures refer to bath temperatures.
trans-Pinocarveol (6) was identi®ed by comparison of its
GC retention time with the authentic sample.
trans-Carveol
[2-methyl-5-(1-methylethenyl)-2-cyclo-
1
hexen-1-ol] (7). H NMR (200 MHz, CDCl3) d 5.55 (d,
Jˆ5 Hz, 1H, H-3), 4.70 (m, 2H, vCH2), 3.97 (m, 1H,
H-1), 2.40±1.80 (m, 4H), 1.75 (m, 3H, CH3), 1.70 (s, 3H,
CH3), 1.54 (dt, Jˆ13 Hz, Jˆ4 Hz, 1H, H-5). Lit.13 13C NMR
(50 MHz, CDCl3) d 149.1 (vC,), 134.2 (C-2), 125.1
(C-3), 108.9 (vCH2), 68.4 (C-1), 36.7 (C-5), 35.1 (C-5),
30.9 (C-4), 20.8 (2£CH3). MS [m/z (relative intensity %)]:
152 (10, M1´), 137 (12), 124 (10), 119 (22), 109 (100), 95
(18), 93 (18), 91 (31), 84 (85), 83 (35), 81 (15), 79 (15), 77
(20), 69 (33), 67 (18), 65 (13), 57 (20), 55 (48), 53 (22), 51
(13), 43 (28), 41 (62), 39 (52). Lit.9
Air oxidation products for identi®cation were separated by
¯ash chromatography (SiO2 or neutral Al2O3, eluent
hexane±ethyl acetate, 20:1) and they were identi®ed by
the 1H and 13C NMR [Bruker AM200 and DRX500 spectro-
meter operating at 200 and 500 MHz (for 1H), respectively],
GC±MS spectra (Kratos MS 80 mass spectrometer and
Varian Saturn 2000) or by comparison with the authentic
sample.
(1S)-(2)-Verbenone (4,6,6-trimethylbicyclo[3.1.1]hept-
3-en-2-one) (1). H NMR (200 MHz, CDCl3) d 5.73 (q,
1
trans-Sobrerol [4-(1-hydroxy-1-methylethyl)-2-methyl-
2-cyclohexen-1-ol] (8). 1H NMR (200 MHz, CDCl3) d
5.51 (ma, 1H, H-2), 4.05 (ma, 1H, H-6), 3.62 (ma, 1H,
H-8), 1.77 (s, 3H, H-7), 1.20 (s, 3H, H-9 or -10), 1.17 (s,
3H, H-10 or -9). Lit.11 MS [m/z (relative intensity %)]: 152
(12), 137 (18), 119 (6), 109 (60), 95 (18), 94 (23), 93 (18),
92 (20), 84 (10), 81 (12), 79 (57), 77 (11), 71 (10), 69 (12),
67 (10), 59 (100), 55 (18), 43 (79), 41 (33), 39 (20). Lit.9,11
1H, Jˆ1.5 Hz, H-3), 2.81 (dt, 1H, Jˆ9.1, 5.5 Hz, H-7a),
2.65 (td, 1H, Jˆ5.9, 1.5 Hz, H-5), 2.42 (td, 1H, Jˆ6.5,
1.4 Hz, H-1), 2.08 (d, 1H, Jˆ9.1 Hz, H-7b), 2.02 (d, 3H,
Jˆ1.5 Hz, H-10), 1.50 (s, 3H, H-8), 1.02 (s, 3H, H-9). Lit.8
13C NMR (50 MHz, CDCl3) d 203.8 (C-4), 170.0 (C-2),
121.2 (C-3), 57.6 (C-5), 53.9 (C-6), 49.7 (C-1), 40.8
(C-7), 26.6 (C-8), 23.5 (C-10), 22.0 (C-9). Lit.8b MS [m/z
(relative intensity %)]: 150 (58, M1´), 135 (76), 122 (28),
121 (10), 119 (10), 115 (6), 109 (23), 108 (24), 107 (100),
105 (27), 95 (17), 93 (17), 91 (80), 80 (23), 79 (57), 77 (37),
67 (19), 65 (26), 55 (20), 53 (15), 51 (25), 50 (20), 41 (25),
39 (63). Lit.9 [a]D20ˆ22588 (c 1.0, CH3Cl). HMRS: Found:
150.1044. Calcd for C10H11O, 150.1045.
trans-3-Pinen-2-ol (2,6,6,trimethylbicyclo[3.1.1]hept-3-
en-2-ol) (9). 1H NMR (200 MHz, CDCl3) d 6.25 (dd,
Jˆ8.6 Hz, 1H, H-4), 5.49 (dd, Jˆ8.6 Hz, Jˆ2.2 Hz, 1 H,
H-3), 2.44±1.98 (m, 3H), 1.45 (d, Jˆ9.3 Hz, 1H, H-7b),
1.34 (s, 3H, H-8), 1.30 (s, 3H, H-10), 0.94 (s, 3H, H-9).
Lit.14 13C NMR (50 MHz, CDCl3) d 138.0 (C-4), 129.9
(C-3), 74.1 (C-2), 54.0 (C-1), 46.9 (C-6), 42.7 (C-5), 33.3
(C-7), 26.9 (C-10), 25.7 (C-8), 24.1 (C-9). MS [m/z (relative
Isolated a-pinene oxide (3) was identi®ed by comparison of
its GC retention time with the authentic sample.

M. K. Lajunen et al. / Tetrahedron 56 (2000) 8167±8171
8171
intensity %)]: 152 (4, M1´), 137 (18), 134 (7), 123 (4), 119
(22), 109 (100), 97 (26), 95 (53), 93 (43), 91 (71), 84
(21), 82 (40), 81 (37), 79 (46), 77 (44), 69 (56), 67
(91), 65 (30), 55 (27), 53 (24), 51 (22), 50 (21), 43 (89),
41 (52), 39 (77).
72, 143. (d) Rothenberg, G.; Yatziv, Y.; Sasson, Y. Tetrahedron
1998, 45, 593. (e) Romano, A. M.; Ricci, M. J. Mol. Catal. 1997,
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3. Lajunen, M.; Koskinen, A. M. P. Tetrahedron Lett. 1994, 35,
4461.
4. Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidations of
Organic Compounds, Academic: New York, 1981 (and references
therein).
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The authors thank TEKES (Technology Development
Centre, Finland) for ®nancial support.
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