A highly selective Pd(OAc)2/pyridine/K2CO3 system for oxidation of terpenic alcohols by dioxygen

Article abstract of DOI:10.1007/s10562-011-0754-4

Molecular sieves, complex organic bases and radical oxidants are commonly used in alcohols oxidation reactions. In this work, we have evaluated the beneficial effects of addition of K₂CO₃ to Pd(II)-catalyzed oxidation alcohols, which resulted in a remarkable increase in the oxidation reaction rates without selectivity losses. Herein, in a metallic reoxidant-free system, terpenic alcohols (β-citronellol, nerol and geraniol) were selectively converted into respective aldehydes from Pd(II)-catalyzed oxidation reactions in presence of dioxygen. High conversions and selectivities (greater than 90%) were achieved in the presence of the Pd(OAc)₂/K₂CO₃ catalyst and pyridine excess. The exogenous role of others auxiliary anionic and nitrogen compounds was appraised. Graphical Abstract: Reaction conditions: β-citronellol (2.75 mmol); Pd(OAc)₂ (0.05 mmol); pyridine (5.0 mmol); K ₂CO₃ (2.5 mmol); toluene (10 mL); MS3A (0.5 g); O2 (0.10 MPa); 60 °C.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-011-0754-4

Catal Lett (2012) 142:251–258
DOI 10.1007/s10562-011-0754-4
A Highly Selective Pd(OAc)2/Pyridine/K2CO3 System
for Oxidation of Terpenic Alcohols by Dioxygen
•
Danieli M. Carari Marcio J. da Silva
´
Received: 6 June 2011 / Accepted: 3 December 2011 / Published online: 15 December 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Molecular sieves, complex organic bases and
radical oxidants are commonly used in alcohols oxidation
reactions. In this work, we have evaluated the beneficial
effects of addition of K2CO3 to Pd(II)-catalyzed oxidation
alcohols, which resulted in a remarkable increase in the
oxidation reaction rates without selectivity losses. Herein,
in a metallic reoxidant-free system, terpenic alcohols
(b-citronellol, nerol and geraniol) were selectively con-
verted into respective aldehydes from Pd(II)-catalyzed
oxidation reactions in presence of dioxygen. High con-
versions and selectivities (greater than 90%) were achieved
in the presence of the Pd(OAc)2/K2CO3 catalyst and pyri-
dine excess. The exogenous role of others auxiliary anionic
and nitrogen compounds was appraised.
process), which is a Rh/phosphine-catalyzed multi-step
process under low temperatures and inert atmosphere,
using myrcene as a raw material [5]. Because the direct
synthesis of terpenic aldehydes from monoterpenes is dif-
ficult due the differences in reactivity of the double bonds,
which may vary significantly in electron density and steric
accessibility [6], an interesting approach may be its syn-
thesis from respective terpenic alcohols, which are also
abundant feedstocks.
The development of oxidative processes based on an
abundant non-toxic green oxidant such as dioxygen, is an
important and challenging goal in oxidation chemistry [7].
There is an urgent demand for greener catalytic processes
that employ clean oxidants such as dioxygen [8, 9]. A
typical example is the Wacker system, (PdCl2/CuCl2/O2)
which is used in olefins oxidation [10]. On the other hand,
in palladium-catalyzed alcohol oxidation, metal reoxidant-
free systems have been proposed. Larock and co-workers
described the oxidation of benzylic alcohols by dioxygen
catalyzed by Pd(OAc)2/K2CO3 in DMSO solutions [11].
However, this catalyst showed a very low activity,
requiring a reaction time as long as 24–72 h for oxidation
of others alcohols. Alternatively, several researchers have
proposed catalysts based on palladium/nitrogen com-
pounds, such as sparteine [12], bathophenantroline [13] and
triethylamine [14]. Both catalytic systems based on alka-
line or nitrogen bases allowed the direct oxidation of Pd(0)
by dioxygen, without a reoxidant. However, among the
most successful and versatile oxidative catalysts described,
Pd(OAc)2/pyridine/MS3A system (Uemura catalyst)
deserves highlights [15]. Uemura system efficiently cata-
lyze the aerobic oxidation of different types of alcohols: a
variety of aldehydes was obtained in 86–95% yield from
alicyclic [16], benzylic, saturated primary alcohols using
Pd(OAc)2/pyridine/MS3A in toluene, at 80 °C for 2 h,
Keywords Palladium Á Terpenic alcohol Á Nitrogen
ligand Á Dioxygen
1 Introduction
Monoterpenes are renewable, abundant and inexpensive
naturally occurring compounds, and for these reasons are
attractive feedstocks in several fine chemical industries [1].
Oxygenated terpenes are predominantly used as ingredients
in the flavor and fragrance industries [2, 3]. For instance,
(R)-citronellal which is known be an active insecticide, is
also an important raw material in the synthesis of phar-
macos [4]. A total of 2000 tons per year of (R)-citronellal
are produced by Takasago International Co. (Takasago
D. M. Carari (&) Á M. J. da Silva
´
Departamento de Quımica, Universidade Federal de Vic¸osa,
Vic¸osa, Minas Gerais 35690-000, Brazil
e-mail: silvamj2003@ufv.br
123

252
D. M. Carari, M. J. da Silva
under 1 oxygen atmosphere [17]. However, Uemura system
is less effective when the substrates used are unsaturated or
sterically hindered alcohols. For instance, a longer reaction
time or a higher pyridine load was required in oxidation of
b-citronellol by dioxygen using a Pd(OAc)2: pyridine
molar ratio of 1:4; 75% isolated yield was achieved after
15 h reaction. Conversely, with a Pd(OAc)2: pyridine
molar ratio equal to 1:100, it was achieved an isolated yield
of 81% after 6 h of reaction [18]. Recently, Kumpulainen
and Koskinen described the use of a modified copper-
TEMPO catalyst (Cu(OTf)3/N-methylimidazol)/bipy/1,8-
diazabicyclo[5.4.0]undec-7-ene) in aerobic oxidation of
b-citronellol, obtaining high yields and conversions [19].
In this paper, significantly enhanced activity of the
Pd(OAc)2/pyridine system was achieved with the addition of
K2CO3 to the system. In toluene solutions containing a
excess pyridine (100-fold in relation to palladium),
Pd(OAc)2/K2CO3 catalyst promoted an efficient and selec-
tive oxidation of terpenic alcohols (b-citronellol, nerol and
geraniol) by dioxygen into respective aldehydes. Noticeably,
Pd(OAc)2/pyridine/K2CO3 system was as effective as the
Pd(OAc)2/pyridine/MS3A system. This simple and envi-
ronmentally friendly catalyst proposed herein does not
requires molecular sieves or metal reoxidant, and uses
oxygen as stoichiometric oxidant. Moreover, Pd(OAc)2/
K2CO3 catalyst require smaller loads of both palladium and
pyridine than Uemura system. The role of the main reaction
variables, including temperature and nature of others anionic
and nitrogen bases, were also assessed.
base (pyridine, sparteine or triethylamine) were dissolved in
toluene solution (10.0 mL) under dioxygen pressure
(0.10 MPa). Furthermore, after addition of the terpenic
alcohol (2.75 mmol) to the reaction solution, it was heated to
60 °C and magnetically stirred for 8–12 h. When appropri-
ate, before initiating the reaction, 2.5 mmol of the auxiliary
anionic base was added. Blank reactions were carried out in
the same conditions cited in the absence of the catalyst.
2.3 Reaction Monitoring
Reactions were monitored by analyzing aliquots taken at
regular time intervals by gas chromatography (Varian 450
instrument), equipped with a flame ionization detector and
fitted with a Carbowax 20 M capillary column (30 m length,
0.25 mm i.d., 0.25 mm film thickness). Gas chromatography
conditionswereasfollows:80 °C(3 min);rateoftemperature
increase: 10 °C/min; final temperature: 260 °C; injector
temperature: 250 °C; and detector temperature: 280 °C.
Conversion was estimated from the corresponding chro-
matographic peak areas in comparison with the corresponding
calibrating curve. Dodecane was used as an internal standard.
2.4 Products Chromatographic Analysis
Compounds were identified by GC/MS analyses (Shimadzu
MS-QP 5050A mass spectrometer instrument operating at
70 eV electronic impact mode coupled with a Shimadzu
17A GC). Additionally, the aldehydes were also identified
by co-injection with the authentic samples in GC as
described in the Sect. 2.3.
2 Experimental Procedures
2.5 Synthesis of Pd(OAc)2 Catalyst Supported
on K2CO3
2.1 Materials
All chemicals were purchased from commercial sources.
Pd(OAc)2 was acquired from Sigma-Aldrich (99% w/w).
Molecular sieves (MS3A; Merck) were used after thermal
activation at 400 °C. b-citronellol (racemic mixture 90–95%
w/w, Sigma Aldrich), nerol (Sigma Aldrich, 97% w/w) and
geraniol (Sigma Aldrich, 98% w/w) were used as received.
Toluene (99%, w/w, Merck) was used without treatment.
Carbonate salts were acquired from Sigma-Aldrich (99%
w/w) and were dried at 100 °C before use. Pyridine and
sparteine (99% w/w, Sigma-Aldrich), and triethylamine
(99% w/w, Merck) were also used as received.
The Pd(OAc)2 catalyst supportedon K2CO3 was obtained via
impregnation. Palladium acetate (125.0 mg; 0.557 mmol),
was dissolved in toluene (30 mL) and magnetically stirred at
80 °C temperature. Then, potassium carbonate (3.338 g)
was added and the suspension resulting maintained under
stirring by 2 h at 80 °C temperature. After decantation, the
solid catalyst was filtered and washed with toluene and three
times with a minimal of ethyl ether. After evaporation sol-
vent under air, the catalyst solid was dried at 100 °C tem-
perature by 2 h, resulting in a brown solid.
2.2 Catalytic Tests
3 Results and Discussion
Reactions were carried out in a glass reactor (50 mL)
equipped with a magnetic stirrer, with sampling system and
connected to a gas burette to monitor dioxygen uptake. In a
typical run, the Pd(OAc)2 catalyst (0.05 mmol), nitrogen
3.1 General Aspects
The terpenic alcohols oxidation reactions are normally
complicated by the presence double bonds that may also be
123

Pd(OAc)2/Pyridine/K2CO3 System
253
oxidized or undergoes isomerization along reaction.
Although Pd(II) catalysts are well explored in oxidation
reactions, to develop oxidative processes in which the
Pd(0) is directly recovered by dioxygen is the goal of
several researchers [20]. Herein, a combination of the
catalysts proposed separately by Larock [11] and Uemura
[15–18] was considered, and after an investigation of role
of several anionic and nitrogen bases, an oxidative system
can be obtained: Pd(OAc)2/pyridine/K2CO3 system.
Table 1 Effect of Pd(OAc)2/nitrogen compound on b-citronellol
oxidation by dioxygen
Run
Nitrogen
compound
Pd(OAc)2: nitrogen
compound molar ratio
Time
(h)
Yields
(%)b
1
Pyridine
1:4
1:100
1:4
8
8
4
8
2
8
9
22
8
2
3c
Pyridine
Triethylamine
Triethylamine
Sparteine
4
1:100
1:4
36
\3
13
5c
6
Sparteine
1:100
3.2 Effects of the Nitrogen Compounds
on the Pd(OAc)2-Catalyzed Oxidation Reaction
of b-Citronellol by Dioxygen
Reaction conditions: b-citronellol (2.75 mmol); Pd(OAc)2
(0.05 mmol); nitrogen compound (0.20 or 5.0 mmol); O2 (0.10 MPa);
60 °C
b
Determined by GC
Oxidatively stable ligands such as nitrogen bases play a
crucial role in oxidation reactions by minimizing catalyst
deactivation, promoting the direct reaction between palla-
dium reduced species and dioxygen, as well as modulating
substrate reactivity [21]. Moreover, the exogenous action of
nitrogen bases is also a key-factor in both oxidation reac-
tions and oxidative kinetic resolution of alcohols [12, 22].
Firstly, we have investigated the effect of the nature of
nitrogen compounds on the Pd(OAc)2-catalyzed b-citro-
nellol oxidation reaction by dioxygen (Fig. 1). In this step,
b-citronellol was the substrate and pyridine, sparteine and
triethylamine were the bases selected. When mixtures of
Pd(OAc)2 and nitrogen compounds are used, the number
maximum of ligands (L) (L = triethylamine, sparteine or
pyridine) coordinated to palladium is two, leading to
Pd(OAc)2L2 catalyst. Herein, were used four equivalents of
L; consequently, the action of nitrogen compounds is both
as ligand and exogenous base. In Table 1 are showed
conversion results obtained in reactions of b-citronellol
oxidation by dioxygen in Pd(OAc)2/nitrogen compound
system. Reactions were performed using excess of nitrogen
compound in relation to palladium (1:4 and 1:100 molar
ratio). Under these reaction conditions the nitrogen com-
pound acts as ligand and as exogenous base.
c
Reaction was stopped due to palladium black formation
addition, these catalysts remained stables for a minimum of
4 h reaction (Table 1). Contrarily, under similar reaction
conditions, palladium/sparteine system quickly was con-
verted into Pd(0) species after time reaction lower than 2 h.
This is suggestive that if a low concentration of nitrogen
compound is used, less electron donating ligands and with
steric hindrance (e.g. sparteine) results in a catalyst palla-
dium less stable. On the other hand, a large excess of the
nitrogen compound may be unfavorable, because it
may occupy a palladium coordination site required for
b-hydride elimination, which is normally the rate-limiting
step in most of these reactions [23]. Nevertheless, pyridine
may also prevent the decomposition of the catalyst into the
inactive bulk metal (palladium black), preventing its
aggregation as well as enhancing the oxidation reaction
rate between reduced palladium and dioxygen [24, 25].
Here, when a large excess of nitrogen compound was
employed, highest yield was achieved in the reaction per-
formed in Pd(OAc)2/triethylamine system. According to
Fig. 2, the reaction yield is higher with Pd(OAc)2/trieth-
ylamine than with Pd(OAc)2/pyridine system. In contrast, it
seems that the stability of the Pd(OAc)2/pyridine system is
higher than the stability of Pd(OAc)2/triethylamine (see
results after 8 h).
GC and GC–MS analyses showed that b-citronellal was
the principal product formed in all reactions (greater than
90% selectivity). When the palladium: nitrogen compound
molar ratio used was equal to 1:4, triethylamine and pyri-
dine were the most efficient nitrogen compounds. In
Highlighted, in all catalytic runs performed with excess
of nitrogen compound a non formation of palladium-black
was observed. However, despites yields have been higher
than those obtained at palladium: nitrogen compound
molar ratio equal to 1:4 (Table 1), after an 8 h the reaction
rate remain almost constant. Thus, this suggestive that
the inhibitory effect caused by excess of nitrogen com-
pounds is more pronounced when the olefin concentration
is lower.
O
Pd(OAc)2/nitrogen compound
OH
H
toluene, 60 0C, 0.10 MPa O2
Kinetic curves (Fig. 2) reveal that although in the same
concentrations (i.e. 1:100 Pd:L molar ratio), nitrogen com-
pounds affected differently the reaction rate. Noticeably, the
Fig. 1 Pd(OAc)2/nitrogen compound-catalyzed oxidation reaction of
the b-citronellol by dioxygen
123

254
D. M. Carari, M. J. da Silva
towards the efficiency of palladium (II)-catalyzed reac-
tions, should be more subtle than normally considered.
Conversely, due to higher electrodonating character of
triethylamine when compared to others nitrogen com-
pounds studied, palladium was more efficient regenerated
by dioxygen and high yields were then reached (ca. 36%
after 6 h reaction, Fig. 2). Contrarily, in Pd(OAc)2/sparte-
ine-catalyzed reactions the maximum yield achieved was
15%, after 6 h of reaction. The cyclohexyl groups in
sparteine may hamper substrate complexation to Pd(II),
resulting in a lower catalytic activity. This result is in
agreement with the fact that the bipyridine, another bulky
ligand, significantly inhibits catalytic turnover of palla-
dium-catalyzed oxidation reactions of benzylic alcohol
[31]. Moreover, due sparteine is the weakest base, its
exogenous action was drastically compromised. Con-
versely, although steric hindrance effect is almost absent on
pyridine, its role as exogenous base is also affected by its
lower basicity, resulting in a less active catalyst than the
Pd(OAc)2/triethylamine catalyst.
40
35
30
25
20
15
10
5
triethylamine
pyridine
sparteine
0
0
2
4
6
8
time (h)
Fig. 2 Kinetic curves obtained from Pd(OAc)2-catalyzed b-citronel-
lol oxidation by dioxygen in presence of different nitrogen com-
pounds. Reaction conditions: b-citronellol (2.75 mmol); Pd(OAc)2
(0.05 mmol); nitrogen base (5.0 mmol); toluene (10 mL); O2 (0.
10 MPa); 60 °C
3.3 Effects of Molecular Sieves in the Pd(OAc)2/
Nitrogen Compound-Catalyzed Oxidation Reaction
of b-Citronellol by Dioxygen
triethylaminecausedahigherincreaseoninitialrateofreaction
thanothersnitrogencompounds;consequently,ahighyieldwas
reachedaftera2 hreaction.Anitrogencompoundmaypartici-
pate in the deprotonation step of a key-intermediate of these
reactions(i.e.palladium-alkoxide),whichnormallyisinvolved
inrate-determiningstep[25].
The effects of molecular sieves on these reactions were
investigated and the results are displayed in Fig. 3.
Remarkably, in contrast to what was observed in the
absence of MS3A (Fig. 2), the Pd(OAc)2/pyridine system
was more efficient than the Pd(OAc)2/triethylamine
system.
If this nitrogen compound is not sufficiently basic, de-
protonation of the palladium-alkoxide intermediated will
be highly disfavored and reduce the catalyst activity. This
principle has been detected in the PdCl2/sparteine-cata-
lyzed reactions where the anionic ligand, chloride, is suf-
ficiently nonbasic that an exogenous base is required to
achieve even stoichiometric alcohol oxidation [25]. Thus,
is possible that exogenous effect is directly related to
ligand basicity. Literature reports the allowable pkb values
for bases: sparteine ([11.8) [26], pyridine (8.75) and tri-
ethylamine (3.35) [27]. Reaction in the presence of an
excess stronger nitrogen base (i.e. triethylamine) was that
which greatest yield was achieved (ca. 37%). A similar
result has been observed in benzylic alcohol oxidation at
room temperature, where triethylamine was more effective
than other nitrogen bases [28]. Although PdX2 catalyst may
reacts with tertiary amines to form Pd(0) species [23], we
verified that under studied reaction conditions (i.e. excess
of triethylamine) this did not happen. According to litera-
ture, triethylamine can reduce Pd(II) into Pd(0), but their
coordination to key Pd(II) species could be also involved
[29]. Indeed, literature also report that this reduction is
absent when triethylamine is dried [30]. Thus, we can
conclude that the role of the added nitrogen compound,
pyridine
80
triethylamine
sparteine
70
60
50
40
30
20
10
0
0
2
4
6
8
time (h)
Fig. 3 Effect of MS3A on the Pd(OAc)2-catalyzed b-citronellol
oxidation by O2. Reaction conditions: b-citronellol (2.75 mmol);
Pd(OAc)2 (0.05 mmol); nitrogen compound (5.0 mmol); toluene
(10 mL); O2 (0. 10 MPa); MS3A (0.5 g) 60 °C
123

Pd(OAc)2/Pyridine/K2CO3 System
255
Although the beneficial role of MS3A in these reactions
is not yet fully understood, an important work explored the
effects of MS on the Pd(OAc)2/pyridine catalyst systems
by performing kinetic studies of benzylic alcohol oxidation
[32]. Those authors found that molecular sieves molecular
sieves enhance the rate of the Pd(OAc)2/pyridine-catalyzed
oxidation of alcohols This effect was attributed to the
ability of molecular sieves to serve as a Brønsted base to
enhance the reaction.
It is also possible that both solid potassium and cesium
carbonates act as a water scavenger. Apparently, most bulk
cations such as K? and Cs? favor its reaction. Because an
improvement in catalytic activity was obtained in presence
of carbonate ions, it can be assumed that this proves that
anionic ligands facilitate Pd(0) species oxidation. We
believe that similarly to the acetate ion coordinate to pal-
ladium, carbonate ions may participate directly on dispro-
tonation step of the b-citronellol and favor the formation of
the palladium-alkoxide intermediate [23, 35].
Thus, greater conversion of b-citronellol into citronellal
was obtained with the Pd(OAc)2/pyridine catalyst (ca.
78%). Although this result is in agreement with literature
[33], it must be mentioned that in the present study a lower
concentration of palladium was used (1.8 mol% compared
to 5 mol %) and the reaction temperature was also lower
(60 °C instead of 80 °C).
In Fig. 5, a comparison of catalytic systems based on
palladium used on b-citronellol by dioxygen is displayed.
Remarkably, the addition of K2CO3 to Pd(OAc)2/pyridine
system became this system as effective as Pd(OAc)2/pyri-
dine/MS3A.
Enhanced activity obtained by the addition of K2CO3 (or
Cs2CO3) to the Pd(OAc)2/pyridine system may be attrib-
uted to fact of the insoluble K2CO3 can act as a hetero-
geneous support for the reaction; the active catalyst appears
to be heterogeneous and was adsorbed by the insoluble
K2CO3. Moreover, besides its role as a solid support, the
carbonate anion may act as a Brønsted base in a key step of
reaction, such as deprotonation of a palladium-alkoxide
intermediate, commonly formed in alcohol oxidation
reactions [36]. This step should occur on the solid surface.
We believe that if a minimal number of carbonate ions are
in the solution they would react with HOAc generated
along reaction and consequently, the regeneration of active
Pd would not occur through this way. Is important note that
the HOAc role did not affected by the presence of pyridine
in excess.
3.4 Effects of an Anionic Base in the Pd(OAc)2-
Catalyzed Oxidation Reaction of b-Citronellol
by Dioxygen in Presence of Pyridine Excess
The kinetic curves shown in Fig. 4 reveal that only
Cs2CO3, and more remarkably K2CO3, improved perfor-
mance of the Pd(OAc)2/pyridine system.
The efficiency of Cs2CO3 and K2CO3 salts was previ-
ously reported by Stoltz and co-workers while investigating
the Pd(nbd)Cl2/sparteine-catalyzed aerobic oxidative
kinetic resolution of secondary alcohols [34]. The authors
found that Cs2CO3 and K2CO3 were that salts that dra-
matically accelerated those reactions.
For these reasons, was evaluated the catalytic activity of
Pd(OAc)2 supported on K2CO3. The results obtained with
100
K2CO3
Cs2CO3
100
Pd(OAc)2/piridine
Na2CO3
80
Pd(OAc)2/piridine/MS3A
Pd(OAc)2/piridine/K2CO3
90
80
70
60
50
40
30
20
10
0
CaCO3
BaCO3
(NH4)2CO3
60
40
20
0
0
2
4
6
8
10
12
0
2
4
6
8
10
12
time (h)
time (h)
Fig. 4 Effect of an anionic base in b-citronellol oxidation by
dioxygen performed in Pd(OAc)2/pyridine system. Reaction condi-
tions: b-citronellol (2.75 mmol); Pd(OAc)2 (0.05 mmol); pyridine
(5.0 mmol); anionic bases (2.5 mmol); toluene (10 mL); O2 (0.
10 MPa); 60 °C
Fig. 5 Comparison of systems used in Pd(OAc)2-catayzed oxidation
of b-citronellol by dioxygen. Reaction conditions: b-citronellol
(2.75 mmol); Pd(OAc)2 (0.05 mmol); pyridine (5.0 mmol); K2CO3
(2.5 mmol); toluene (10 mL); MS3A (0.5 g); O2 (0. 10 MPa); 60 °C
123

256
D. M. Carari, M. J. da Silva
80
70
60
50
40
30
20
10
0
Table 2 Oxidation of b-citronellol by dioxygen in Pd(OAc)2/pyri-
dine/K2CO3 system in homogeneous and heterogeneous conditions
K2CO3
Cs2CO3
Na2CO3
CaCO3
BaCO3
Exp. Pd(OAc)2/K2CO3 Pyridine Conversion Palladium black
system
(mmol)
(%)
formation
1
2a
Homogeneous
Heterogeneous
Homogeneous
Heterogeneous
Homogeneous
Heterogeneous
6.0
6.0
87
38
90
45
90
55
Yes
Yes
No
(NH4)2CO3
3
12.0
12.0
18.0
18.0
4a
5
6a
Yes
No
Yes
Reaction conditions: b-citronellol (2.75 mmol); Pd(OAc)2
(0.05 mmol); pyridine (6–18.0 mmol); K2CO3 (2.5 mmol); toluene
(10 mL); O2 (0. 10 MPa); 60 °C
a
The supported catalyst was used containing a palladium carbonate
molar ratio equal to homogeneous system
0
2
4
6
8
10
12
time (h)
Fig. 6 Effect of anionic bases on Pd(OAc)2-catalyzed b-citronellol
oxidation with dioxygen in presence of pyridine excess and MS3A.
Reaction conditions: b-citronellol (2.75 mmol); Pd(OAc)2
(0.05 mmol); pyridine (5.0 mmol); anionic base (2.5 mmol); toluene
(10 mL); MS3A (0.5 g); O2 (0.10 MPa); 60 °C
supported catalyst via impregnation and the palladium
catalyst homogeneous are shown in Table 2.
The supported palladium catalyst on potassium car-
bonate was less stable than homogeneous palladium cata-
lyst. In all catalytic runs, the formation of palladium black
species formation was observed at reaction initial step.
Lower conversions were obtained if compared to run per-
formed with palladium homogeneous catalyst. Moreover,
yet under a large pyridine excess, the heterogeneous cata-
lyst was deactivated and reduced palladium species were
formed.
100
80
60
40
20
0
100
80
60
40
20
0
°
conversion 80 C
3.5 Effects of the Simultaneous Addition of an Anionic
Base and MS3A in the Pd(OAc)2-Catalyzed
Oxidation Reaction of b-Citronellol by Dioxygen
in Presence of Pyridine Excess
°
conversion 60 C
°
selectivity 80 C
°
selectivity 60 C
The possibility of synergism between molecular sieves and
anionic bases was investigated and the results are displayed
in Fig. 6.
0
2
4
6
8
10
12
time (h)
Fig. 7 Effects of the temperature on oxidation reaction of b-
citronellol by dioxygen in Pd(OAc)2/pyridine/K2CO3 system Reac-
tion conditions: b-citronellol (2.75 mmol); Pd(OAc)2 (0.05 mmol);
pyridine (5.0 mmol); K2CO3 (2.5 mmol); toluene (10 mL); O2
(0.10 MPa); 60 °C
While the addition of MS3A to Pd(OAc)2/pyridine/
K2CO3 or Pd(OAc)2/pyridine/Cs2CO3 systems does not
enhance their activities, a slight increase in conversion was
observed for the other salts, especially in the case of
Na2CO3. However, the observed conversion was lower
than that achieved by Pd(OAc)2/pyridine/MS3A without
carbonate ions.
Is important to note that in reactions performed at 80 °C
temperature the system showed less stable, and conse-
quently Pd(0) species were formed. However, it did not
occur with pyridine loadings up to 12.0 mmol, as will be
described in the next section.
3.6 Effects of Temperature on Oxidation Reaction
of b-Citronellol by Dioxygen in Pd(OAc)2/
Pyridine/K2CO3 System
3.7 Effects of Pyridine Concentration
An increase in reaction temperature resulted in a conse-
quential increase in the initial rate of reaction. Indeed, high
aldehyde selectivity was obtained from the initial period of
reaction (Fig. 7).
Literature suggests that an excess of pyridine may inhibit
palladium catalytic activity. However, in all pyridine
concentration ranges studied (ca. 3.0–18.0 mmol), high
123

Pd(OAc)2/Pyridine/K2CO3 System
257
100
90
80
70
60
50
40
30
20
10
0
100
80
60
40
20
0
0.0 mmolsc
3.0 mmolsd
6.0 mmolsd
12.0 mmols
18.0 mmols
°
geraniol 60 C
°
geraniol 80 C
°
nerol 60 C
°
nerol 80 C
0
2
4
6
8
10
12
0
2
4
6
8
10
12
time (h)
time (h)
Fig. 9 Effect of temperature on oxidation reaction of geraniol and
nerol by dioxygen in Pd(OAc)2/pyridine/K2CO3 system. Reaction
conditions: substrate (2.75 mmol); Pd(OAc)2 (0.05 mmol); pyridine
(5.0 mmol); K2CO3 (2.5 mmol); toluene (10 mL); O2 (0.10 MPa)
Fig. 8 Effect of the pyridine loading on Pd(OAc)2/K2CO3-catalyzed
b-citronellol oxidation by O2a-d aReaction conditions: b-citronellol
(2.75 mmol); Pd(OAc)2 (0.050 mmol); K2CO3 (2.5 mmol) toluene
b
(10 mL); 80 °C temperature; O2 (0.10 MPa); 12 h. Conversion and
selectivity determined by GC. In all catalytic runs the aldehyde
selectivity was greater than 90% (not shown in Fig. 9 for simplifi-
cation). cFormation of Pd(0) species after 5 min of reaction. dIn
catalytic runs with low pyridine concentration (ca. 3.0 and 6.0 mmol),
the formation of Pd(0) species begins after 30 min of reaction
O
CH2OH
H
Pd(OAc)2/K2CO3/pyridine
toluene, 80 0 C, 0.10 MPa O2
values of conversion of b-citronellol were obtained
(Fig. 8).
Although with pyridine concentrations between 3.0 and
6.0 mmol, the formation of Pd(0) species begins after
30 min of reaction, it did not affected the final conversion.
Moreover, when a pyridine excess was employed no
inhibitor effect was verified and palladium catalyst remains
active.
geraniol
trans-citral
Pd(OAc)2/K2CO3/pyridine
toluene 80 0 C, 0.10 MPa O2
CH2OH
H
3.8 Oxidation of Geraniol and Nerol by Dioxygen
in Pd(OAc)2/Pyridine/K2CO3 System
O
Heartened by the observation of selective oxidation of the
b-citronellol by dioxygen using Pd(OAc)2/pyridine/K2CO3
system, we investigated the oxidation of two others terpene
alcohols (i.e. geraniol and nerol) as examples of alcohols
that could be oxidized, or react in other ways. The palla-
dium-catalyzed oxidation reactions by dioxygen of gera-
niol and nerol were performed at 60 and 80 °C temperature
(Fig. 9).
nerol
cis-citral
Fig. 10 Oxidation of geraniol and nerol by dioxygen in Pd(OAc)2/
pyridine/K2CO3 system
pyridine/K2CO3 system proved highly selective and the
aldehydes (i.e. trans-citral an cis-citral (neral)) were
obtained with selectivity upper of 90%, independently of
the reaction temperature (Fig. 10).
Expectedly, an increase in reaction temperature resulted
in a consequential increase in the conversion of the terpenic
alcohols into respective aldehydes.
However, in all catalytic runs no decreases on aldehyde
selectivity it was found verified; selectivities higher than
90% were reached in all cases, as showed GC analysis (no
displayed in Fig. 9 by simplification). The oxidation of
both allylic terpenic alcohols by dioxygen in Pd(OAc)2/
4 Conclusions
In summary, the Pd(OAc)2/K2CO3/pyridine system was
highly efficient in promoting selective oxidation by diox-
ygen of terpenic alcohols (i.e. b-citronellol, nerol and
123

258
D. M. Carari, M. J. da Silva
9. da Silva MJ, Gusevskaya EV (2001) J Mol Catal A 176:23
¨
geraniol) into aldehydes in toluene solutions. Noticeably,
the Pd(OAc)2/K2CO3/pyridine system was as effective as
Pd(OAc)2/pyridine/MS3A system (Uemura system). As
described in this work, selection of the nitrogen compound
and the auxiliary anionic base is crucial in developing
robust and active aerobic alcohol oxidation catalysts. The
results obtained with palladium in homogeneous phase
suggested that their action may be promoted by solid
K2CO3, which may act as a heterogeneous support for the
reaction and as a Brønsted base. However, Pd(OAc)2
supported on K2CO3 catalyst showed unstable even at
presence of pyridine excess. This protocol is a straight-
forward synthesis method of terpenic aldehydes in metal
reoxidant-free catalytic system for palladium and which
use molecular oxygen as stoichiometric oxidant.
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