A Highly Selective Na2WO4-Catalyzed Oxidation of Terpenic Alcohols by Hydrogen Peroxide

Article abstract of DOI:10.1007/s10562-017-2246-7

Sodium tungstate was found to be an active and highly selective catalyst to oxidation of various primary or secondary origin renewable alcohols by hydrogen peroxide as green oxidant. Borneol, nerol, geraniol and β-citronellol were efficiently and selectively converted to respective carbonyl derivatives by hydrogen peroxide. ATR/FT-IR measurements confirmed that Na₂W(O₂)₄ was the specie active catalytically. The role of the main reaction variables, including temperature, reactants and catalyst concentration, solvent, and nature of substrate were also assessed. In addition to use a green oxidant, this simple and environmentally friendly catalyst system did not require additive to control pH, molecular sieves or phase transfer catalyst. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2246-7

Catalysis Letters
https://doi.org/10.1007/s10562-017-2246-7
A Highly Selective Na2WO4-Catalyzed Oxidation of Terpenic Alcohols
by Hydrogen Peroxide
Luna Andrade Silva Viana1 · Giovanna Rodrigues Nobile da Silva1 · Márcio Jose da Silva1
Received: 11 August 2017 / Accepted: 6 November 2017
© Springer Science+Business Media, LLC, part of Springer Nature 2017
Abstract
Sodium tungstate was found to be an active and highly selective catalyst to oxidation of various primary or secondary origin
renewable alcohols by hydrogen peroxide as green oxidant. Borneol, nerol, geraniol and β-citronellol were efficiently and
selectively converted to respective carbonyl derivatives by hydrogen peroxide. ATR/FT-IR measurements confirmed that
Na2W(O2)4 was the specie active catalytically. The role of the main reaction variables, including temperature, reactants and
catalyst concentration, solvent, and nature of substrate were also assessed. In addition to use a green oxidant, this simple and
environmentally friendly catalyst system did not require additive to control pH, molecular sieves or phase transfer catalyst.
Graphical Abstract
-
2
-
2
-
2
-
2
4 H
2O2
4 H
2O
O O
W
O
O O
W
O O
W
H
O
O
H
-
2O2
O
O
O
O
O
O
O
O
O
O
O
O
W
O
step 2
step 1
O
H
2O
-
H
2O
O
O O
O O
O
Keywords Sodium tungstate · Catalytic oxidation · Terpenic alcohols · Hydrogen peroxide
1 Introduction
stoichiometric oxidant. For instance, camphor is used for
its scent, as a flavoring agent, an ingredient in cooking, as
an embalming fluid, and for medicinal purposes, has been
industrially produced through multistep process. In this case,
camphene is converted to isobornyl acetate and then sepa-
rated and converted by sodium hydroxide alcoholic solution
to isoborneol, which is finally purified and oxidized by stoi-
chiometric oxidants (i.e., Cr or Mn salts, nitric and sulfuric
acids) providing the camphor [8]. However, these oxidants
are toxic and unfriendly environmentally, generating resi-
dues that should be discarded.
Monoterpenes are versatile and renewable feedstocks that
may replace petroleum derivative raw materials [1]. Pres-
ently, to develop friendly environmentally processes to
convert monoterpenes into fine chemicals have been a great
challenge in the fragrance and pharmaceutical industries
[2–4]. In special, monoterpenes containing carbonyl groups
are key intermediates and building blocks for synthesis of
flavors, pharmacies and agrochemicals [5–7].
Nonetheless, industrial processes to production oxygen-
ates terpenic derivative still are based on toxic and corrosive
Hydrogen peroxide is a green oxidant that has been
used to oxidize origin natural alcohols. It is an attractive
oxidant because it is an inexpensive and environmentally
benign reactant, which produces water as the only by-
product with an atom efficiency of ca. 47% [9–11]. How-
ever, it is always requires the presence of metal in order
to activate hydrogen peroxide and or organic substrate, in
* Márcio Jose da Silva
silvamj2003@ufv.br
1
Chemistry Department, Federal University of Viçosa,
Viçosa, Minas Gerais 35690-000, Brazil
Vol.:(0123456789)
1 3

L. A. S. Viana et al.
addition to phase transfer agents and alkaline additives to
control pH.
2.2 Catalytic Tests and Reaction Monitoring
There are several metal-catalyzed processes frequently
used in oxidation reactions with hydrogen peroxide; none-
theless, those based on tungsten catalysts have received
a great deal of attentions to achieve high efficiency and
selectivity mainly in alcohols oxidation [12–14]. For
instance, deserves highlight the seminal work of Noyori
et al. where they described the first use of sodium tung-
state as homogeneous catalyst in oxidation reactions [15,
16]. Those authors developed a practical, efficient and
environmentally acceptable oxidative process by using of
hydrogen peroxide as stoichiometric oxidant [17]. None-
theless, this system needs strong acidic conditions and
requires expensive phase transfer catalysts.
Reactions were carried out in a glass reactor (50 mL)
equipped with a magnetic stirrer, a sampling system
and connected to reflux condenser. In a typical run, the
Na2WO4 catalyst (0.06 mmol; 2.5 mol %), terpenic alcohol
(2.40 mmol) were dissolved in DMA (10.0 mL) and mag-
netically stirred and heated to reaction temperature. Fur-
thermore, after addition of hydrogen peroxide, the reaction
was started and followed by GC analysis (Shimadzu 2010
instrument, FID) during 4 h.
The GC analyses conditions were as follows: 80 °C
(3 min); rate of temperature increase: 10 °C/min; final tem-
perature: 260 °C; injector temperature: 250 °C; and detector
temperature: 280 °C. Conversion and selectivity were esti-
mated from the corresponding GC peak areas of substrate
and products in comparison with the corresponding calibrat-
ing curves. Naphthalene was used as an internal standard.
Although heterogeneous oxidative processes have a
great advantage of reusability of catalyst, they require
longer reaction times and higher temperatures than
homogenous ones [18–20]. Moreover, notwithstanding
the significant advances obtained in this area, the catalyst
efficiency and selectivity especially in the case of alcohol
oxidations still remains a challenging to be overcome.
In this paper, after a careful solvent selection, we
developed an efficient and selective Na2WO4-catalyzed
process to oxidize natural origin alcohols (i.e., borneol,
β-citronellol, nerol and geraniol) by hydrogen peroxide.
This simple and environmentally friendly route, no acid
or alkaline additive, molecular sieves or phase transfer
catalyst (i.e., PTC) were required. High substrate con-
versions and selectivity toward carbonylic products were
achieved. The role of the main reaction variables, includ-
ing temperature, reactants concentration and catalyst, sol-
vent and nature of substrate were assessed.
2.3 Products Chromatographic Analysis
Compounds were identified by GC/MS analyses (Shimadzu
MS-QP 2014 mass spectrometer instrument operating at
70 eV electronic impact mode coupled with a Shimadzu
2014 GC). Additionally, the main products were also iden-
tified by co-injection with the authentic samples in GC as
described in the Sect. 2.2.
3 Results and Discussion
3.1 General Aspects
The pH control has been vital for the success of the
Na2WO4-catalyzed alcohols oxidation reactions by hydro-
gen peroxide. While assessed the oxidation of 2-octanol
by H2O2, Hida and Nogusa verified that the addition of
NaH2PO4 to the DMA solution containing oxidant and
Na2WO4 catalyst dissolved resulted in an increase on pH of
5.4–6.5, which improved the yield of 2-octanone of 74–94%
[21]. They concluded that under those pH conditions, strong
donor solvents favor the reaction because they stabilize a
tetraperoxotungstate intermediate, which was isolated by
them and characterized. Those authors assumed that this
intermediate was the active catalytically specie in those
oxidation reactions. In the system Na2WO4/H2O2/DMA/
NaH2PO4 some aliphatic saturated alcohols or aromatics
containing electron donating or withdrawing groups were
efficiently oxidized [21].
2 Experimental Procedures
2.1 Materials
All chemicals were purchased from commercial sources.
Sodium tungstate was acquired from Sigma-Aldrich
(99 wt%). Borneol (99 wt%, racemic mixture Sigma
Aldrich), β-citronellol (racemic mixture 90–95 wt%,
Sigma Aldrich), linalool (Sigma Aldrich, 99 wt%), nerol
(Sigma Aldrich, 97 wt%) and geraniol (Sigma Aldrich, 98
wt%) were used as received. Toluene, dimethylacetamide
(DMA), dimethylformamide (DMF), CH3OH and CH3CN,
all were acquired from Sigma Aldrich (ca. 99 wt%) and
used as received. The 34 wt% aqueous H2O2 (Sigma) was
the oxidant in all reactions.
Herein, our intention was to extend the reaction scope
to the natural origin unsaturated alcohols (i.e., terpenic
1 3

A Highly Selective Na2WO4-Catalyzed Oxidation of Terpenic Alcohols by Hydrogen Peroxide
alcohols), which are renewable raw material and useful for
fine chemicals industries [22]. Nonetheless, the use of addi-
tives such as sodium phosphate looks for us an inconven-
ient and may trigger undesirable reactions that will com-
promise the oxidation selectivity, such as rearrangement
of carbon skeletal, isomerization, nucleophilic addition or
oligomerization.
a
N W
H
2O2
O
/
2
4
K
333
H
O
O
So, after verify the effect of solvent under our reaction
conditions (i.e., without control of pH or PTC), we per-
formed the catalytic runs without any additive. Borneol was
the terpenic alcohol selected as model-molecule and used
on assessment of the effects of main reaction parameters.
Further, the reaction scope was successfully extended to the
other monoterpene alcohols.
Scheme 1 Na2WO4-catalyzed borneol oxidation reaction by H2O2
(step 1, Scheme 1), generating the active catalytically spe-
cies such as bridging tungstoperoxo complexes.
It is noteworthy that in tungsten-catalyzed oxidation reac-
tions carried in DMA, until four peroxide groups may be
coordinate to the metal cation generating Na2W(O2)4 inter-
mediate [21]. Herein, we confirmed this hypothesis through
FT-IR analysis, recording spectra of Na2WO4 catalyst in the
presence and absence of hydrogen peroxide (Fig. 1). The
FT-IR spectra were also obtained in DMA. However, due to
great amount of absorption bands of solvent, we will omit
these data and discuss only the infrared spectra obtained
without solvent. Nonetheless, we have found that no dif-
ference in relation to the results obtained with or without
solvent was observed.
3.2 Effects of Solvent on the Na2WO4‑Catalyzed
Oxidation Reaction of Borneol by H2O2
In Table 1 are the conversion results obtained from reactions
carried out at reflux temperatures of the different solvents.
GC and GC-MS analyses showed that camphor was the prin-
cipal carbonylic product formed in all reactions.
Although the camphor has been selectively formed
regardless the solvent used, only in the reactions in the pres-
ence of strong donor solvents (i.e., DMA and DMF), the
Na2WO4-catalyzed reactions achieved high conversions.
It is important to note that whereas the data previously
reported, no additive to control pH was used [21]. Moreover,
whereas some reports on literature, no PTC was employed
[16, 23, 24].
The ATR-FTIR spectrum of Na2WO4·2H2O showed as
main absorption bands the υsym and υassym stretching of
W=O bond at wavenumbers 931.5 and 832.4 cm−1, respec-
tively. In addition, the spectrum displayed the absorption
bands characteristic of water molecules, at 3270.7 and
1675.9 cm−1, corresponding to the stretching of O–H bond
and deformation of H–O–H, respectively (see spectrum (A),
Fig. 1) [28].
The efficiency of DMA and DMF was attributed to the
effect stabilizing provoked by the interaction of these sol-
vents with a tetraperoxotungstate intermediate [21]. On the
other hand, the lower activity of catalyst in methyl alcohol
or acetonitrile solutions was ascribed to low solubility of
sodium tungstate in these solvents.
The other typical absorption bands of the tungstate anions
were observed at low frequencies (ca. 815–539 cm−1). We
have found absorption bands that may be assigned to the
bridging and non-bridging oxygen atoms containing ions,
which belong to the double or single bonded tetrahedron
groups [29]. The bands placed between 800 and 950 cm−1
are associated to the υ3 and υ1 vibrations of the tetrahedral
WO4 [30]. All these data are agreed with literature [31].
After adding aqueous hydrogen peroxide in an enable
amount to dissolve the sodium tungstate, we recorded the
ATR/ FT-IR spectrum of solution, and verified that remark-
able changes have occurred (see spectrum (B), Fig. 1).
It was verified that in addition to the absorption bands
characteristics of deformation vibration of H2O2 and H2O
molecules (ca. 1633.4 and 1365.4 cm−1, respectively), we
have found stretching vibrations of O–O and W–O bonds
(ca. 877.5 and 829.3 cm−1 respectively). Nonetheless,
the main result was the disappearance of strong peak at
931.5 cm−1 wavenumber, which was present in the FT-IR
Some insights into reaction mechanism may be done
based on literature [21, 25–27] and experimental results.
Firstly, in tungsten-catalyzed oxidation reactions, has been
commonly accepted that H2O2 first binds to the metal center
Table 1 Effect of solvent on the Na2WO4-catalyzed borneol oxidation
by H2O2
Run
Solvent
Temperature Conversion Camphor
(K)
(%)
selectivitya
(%)
1
2
3
4
DMF
DMA
CH3CN
CH3OH
343
343
323
323
80
83
15
14
91
94
83
82
Reaction conditions borneol (2.4 mmol); Na2WO4 (2.5 mol%); H2O2
(2.4 mmol), 4 h
1 3

L. A. S. Viana et al.
Fig. 1 FT-IR spectra of Na2WO4⋅dihydrate (a) and Na2WO4 dihydrate/H2O2(aq) (b)
spectrum of Na2WO4 (see (A) spectrum, Fig. 1). It is an
evidence that W=O double bonds that are typical of tung-
state anion are absent [32]. On the other hand, we can note
a notable change on absorption bands at frequencies lower
than 597 cm−1, that should corresponding tungsten–oxygen
single bonds vibrations. These bands are close to those infra-
red data for peroxotungstate complexes [26, 32].
specie on this oxidation reaction, similarly to the described
by Hida and Nogusa (Scheme 2) [21].
As can be seen in Scheme 2, the peroxided intermedi-
ate (1) has a highly electron-deficient metal center that may
undergo nucleophilic attack of borneol hydroxyl group. This
attack then results in an opening of highly unstable three-
center ring, where a hydrogen atom will be transferred from
borneol to the oxygen atom that is bonded to the tungsten,
giving OOH group and bornyl-alkoxide. It is possible that
So, we can conclude that even without pH control, the
tetraperoxotungstate complex is the catalytically active
2-
2-
2-
2-
4 H2O2 4 H2O
O O
W
O
O O
W
O O
W
OH
O
H2O2
- H2O
O
O
O
O
O
O
O
O
O
O
O
O
W
O
step-2H2O
step 1
O
O
O O
O O
O
Scheme 2 Proposal for Na2WO4-catalyzed borneol oxidation reaction by H2O2 (adapted from Ref. [21])
1 3

A Highly Selective Na2WO4-Catalyzed Oxidation of Terpenic Alcohols by Hydrogen Peroxide
in this step both groups remains coordinated to the metal.
It is very common the formation of species like these (i.e.,
intermediates with peroxide and or alkoxide groups bonded
to tungsten cation) in tungsten-catalyzed oxidations [21,
23–25]. Nonetheless, as we haven’t any experimental evi-
dence corroborating that this step has really occurred into
tungsten coordination sphere, we do not included it in the
Scheme 2.
alkyl peroxides are present, the intensity of substrate GC
peak in the sample that was added PPh3 will be increased,
due to reduction of alkyl peroxide to alcohol.
Therefore, we analyzed two aliquots that were simulta-
neously collected from the catalytic run carried out with a
proportion of 1:4 borneol:H2O2 after 2 h reaction. Before
analysis, PPh3 was added only to one of them. The GC
analyses showed that expectedly, as described in Fig. 2, the
camphor was minority in both aliquots. Nonetheless, the
PPh3-free sample presented the GC peak of borneol with
an area lower than aliquot containing phosphine.
Indeed, only in this late aliquot the mass balance of
reaction was achieved (i.e. borneol GC peak area remain-
ing in the reaction + camphor GC peak area = borneol GC
peak initial area) (Scheme 3).
In the last step, the simultaneous elimination of water
and oxidized substrate generate the tungstate anion that after
react with one mol of H2O2 is peroxided again and then can
participate of more one catalytic cycle (Scheme 2).
3.3 Effects of H2O2: Borneol Molar Ratio
on the Na2WO4‑Catalyzed Borneol Oxidation
These evidences suggest that in the presence of PPh3
the alkyl peroxide species (i.e. ROOH, undetectable by
GC analysis) formed during the reaction were reduced by
the PPh3 to borneol, regenerating the initial area of sub-
strate peak. GC-MS analysis confirmed the presence of
triphenylphosphine oxide resulting of this reaction. The
preferential decomposition of ROOH species to alcohol is
in agreement with literature [33, 34]. So, we can conclude
that in the absence of catalyst, hydrogen peroxide convert
borneol to alkyl peroxides in solution, but only a little
amount is converted to camphor, remaining the larger part
as alkyl peroxides.
The effect of variation on proportion borneol to hydro-
gen peroxide was initially assessed on the reactions in the
absence of catalyst (Fig. 2).
Figure 2 shows that a maximum conversion of 27% was
achieved in the reactions without Na2WO4 at molar ratio of
1:4 substrate:oxidant. Nevertheless, a remarkable result of
these reactions is that although the borneol has been con-
sumed, the amount of camphor formed was not compatible
with substrate consumed, suggesting the formation of not
detectable products. It was confirmed by the mass balance of
reaction. It is possible that borneol has been converted into
alkyl peroxides that probably undergone thermal decomposi-
tion on injector of the gas chromatograph and thus were not
detectable [33].
Conversely, when the reactions were performed in the
presence of Na2WO4 catalyst, the borneol was preferen-
tially converted to camphor, reaching constantly selectivity
higher than 90% (Fig. 3b).
To prove this hypothesis, we performed a test proposed
by Shulpin et al. [33, 34]. This method comprises compar-
ing the intensities of GC peaks of the alcohol in samples
with and without the addition of triphenylphosphine. If
In these runs, mass balance of reactions was always
achieved. It reinforces the importance of tungsten catalyst,
whose presence significantly improved either the reactions
conversion as well as camphor selectivity.
The GC-MS analyses showed that a mixture of minority
products (i.e. borneol isomer alcohols, camphor-deriva-
tives) was formed; however, the selectivity was lower than
10%. An increase on oxidant load of 1:1 to 1:2 improved
the initial rate as well as the conversion (Fig. 3a) and
selectivity of the reactions (Fig. 3b). The values were close
to 100% indicating the high efficiency of catalyst.
undetected products (alkyl peroxides) selectivity
conversion
camphor selectivity
90
75
60
45
30
15
0
H
-
H
OO
PP
h3
1:1
1:2
1:3
1:4
H2O2: borneol molar ratio
H
2O
-
PP
O h3
H
OO
H
O
H
O
Fig. 2 Effect of reactants stoichiometry on the conversion and selec-
tivity of the catalyst free borneol oxidation by H2O2. Reaction condi-
tions: borneol (2.4 mmol); DMA (10 mL); 2 h; 343 K
Scheme 3 Oxidation of borneol to alkyl peroxide followed by reduc-
tion with PPh3 [33, 34]
1 3

L. A. S. Viana et al.
conversion
camphor selectivity
100
80
60
40
20
0
100
80
60
40
20
0
1:1
1:2
1:3
1:4
0
50
100
150
200
250
Time (min)
1:1
1:2
1:3
1:4
borneol: H2O2 molar ratio
(b)
(a)
Fig. 3 Effect of reactants stoichiometry on the kinetic curves (a), conversion and selectivity (b) of Na2WO4-catalyzed borneol oxidation by
H2O2. Reaction conditions: borneol (2.4 mmol); Na2WO4 (2.5 mol %); DMA (10 mL); 343 K
3.4 Effects of Catalyst Concentration
3.5 Effects of Temperature in Na2WO4‑Catalyzed
Borneol Oxidation by H2O2
in Na2WO4‑Catalyzed Borneol Oxidation by H2O2
To determine what the lowest concentration provides the
highest conversion and selectivity within time studied,
we carried out reactions with different concentrations of
catalyst.
The reaction temperature can play an important role on
oxidations with hydrogen peroxide. In special, the pos-
sible peroxided intermediates may be formed but remains
intact on the solution reaction. This fact was observed in
the borneol oxidation reaction by H2O2 performed at room
temperature in the absence of catalyst (Fig. 5).
The kinetic curves shown in Fig. 4 reveal that an increase
on catalyst concentration had a beneficial effect on conver-
sion and initial rate of reaction. In general, with concentra-
tions higher than 0.63 mol% a virtually complete conversion
of borneol was achieved (i.e., values close or higher than
90%). Moreover, regardless the catalyst concentration, the
camphor selectivity was always higher than 90%.
In addition to lowest conversion, the reaction carried
out at room temperature displayed also the lowest borneol
selectivity. In all the catalyst-free reactions the alkyl per-
oxide products were preferentially formed (Fig. 5).
conversion
0.63 mol %
1.25 mol %
2.50 mol %
5.00 mol %
7.50 mol %
105
90
75
60
45
30
15
0
camphor selectivity
100
80
60
40
20
0
0.63
1.25
2.50
5.00
7.50
0
20
40
60
80
100
120
catalyst concentration (mol %)
time (min)
(b)
(a)
Fig. 4 Effect of catalyst concentration on the kinetic curves (a), conversion and camphor selectivity (b) of Na2WO4-catalyzed borneol oxidation
reaction by H2O2. Reaction conditions: borneol (2.4 mmol); H2O2 (4.8 mmol); DMA (10 mL); 343 K
1 3

A Highly Selective Na2WO4-Catalyzed Oxidation of Terpenic Alcohols by Hydrogen Peroxide
peroxides alkyl selectivity
camphor selectivity
3.6 Na2WO4‑Catalyzed Terpenic Alcohols Oxidation
Reaction by H2O2
conversion
100
80
60
40
20
0
The presence of double bonds into structure of the terpenic
alcohols, besides possibility of skeletal rearrangement reac-
tions may complicate the selectivity of oxidation reactions. It
becomes more difficult to be controlled when the oxidation
proceeds through homolytic pathway, where radical interme-
diates are formed due to presence of peroxide oxidant [35].
As borneol is a saturated alcohol, we selected unsaturated
terpenic alcohols to test the efficiency of catalyst system. To
do it, we carried out the reactions with the optima condi-
tions established for borneol. The structural of acyclic ter-
penic alcohols, the kinetic curves of reactions and selectivity
results for carbonylic products are presented in the Fig. 7a–c,
respectively.
298
323
343
353
temperature (K)
In general, the reactivity of primary alcohols is higher
than secondary one, while tertiary alcohols are not reactive.
However, in addition to the hydroxyl groups, the acyclic
terpenic alcohols studied have also double bonds that may
also be oxidized (Fig. 7a). For this reason, the linalol that is
a tertiary alcohol was included in this study. Nonetheless, as
displayed in Fig. 7b, only it was the least reactive and only
a poor conversion (ca. < 7%).
Fig. 5 Effect of reaction temperature on the conversion and selectiv-
ity of the catalyst-free borneol oxidation by H2O2. Reaction condi-
tions: borneol (2.40 mmol); H2O2 (4.80 mmol); DMA (10 mL); 2 h;
Na2WO4 (2.5 mol %)
On the other hand, in the presence of catalyst, the
conversion of borneol into camphor was notably favored
(Fig. 6). An increase of temperature resulted in a conse-
quent increase in the conversion as well as the initial rate
of reaction (Fig. 6a).
In terms of conversion, the behavior of alcohols was as
follow; geraniol ad nerol that are allylic primary alcohols
were more reactive than borneol, a saturated secondary alco-
hol. This result is in agreement with literature [36].
Conversely, although borneol is a secondary alcohol, it
was more reactive than β-citronellol, a primary alcohol, even
when this late was oxidized to higher temperature (363 K).
It is possible that high rigidity of bornyl skeletal of borneol
favor the formation of alkoxy–tungsten intermediate, formed
On catalytic runs, when the reactions were carried
out at temperatures equal or higher than 323 K, the alkyl
peroxide intermediates were decomposed by the catalyst,
resulting in the selective formation of camphor.
conversion
105
camphor selectivity
100
80
60
40
20
0
353 K
343 K
90
75
60
323 K
298 K
45
30
15
0
0
40
80
120
160
200
240
298
323
343
353
time (min)
temperature (K)
(a)
(b)
Fig. 6 Effects of temperature on the Na2WO4-catalyzed borneol oxidation by H2O2. Reaction conditions: borneol (2.40 mmol); Na2WO4
(1.25 mol %); H2O2 (4.80 mmol)
1 3

L. A. S. Viana et al.
H
O
H
O
H
O
H
O
-citronellol
β
geraniol
nerol
linalool
(a) acyclic terpenic alcohols
epoxy-aldehydes
ketone/aldehydes
borneol
β-citronellol
β-citronellol (363 K)
geraniol
100
80
60
40
20
0
80
60
nerol
linalool
CHO
O
O
CHO
CHO
O
CHO
O
40
CHO
CHO
O
CHO
20
CHO
0
0
1
2
3
4
borneol β-citronellol β-citronellol
geraniol
nerol
time (h)
(363 K)
(b) kinetic curves
(c) products selectivity
Fig. 7 Kinetic curves (a) and products selectivity (b) of Na2WO4-catalyzed oxidation reactions of terpenic alcohols Reaction conditions: borneol
(2.40 mmol); Na2WO4 (1.25 mol %); H2O2 (4.80 mmol)
during the step of hydrogen atom removal from hydroxyl
group.
Na2WO4 catalyst exhibits high activities in all runs under
environmentally friendly oxidative conditions.
Expectedly, the double bonds into terpenic alcohols struc-
ture were another active site to oxidation. In almost of case
the unsaturated aldehydes formed undergone further epoxi-
dation. However, a high combined selectivity for carbon-
ylic products (aldehydes and epoxy-aldehydes) close to ca.
90% was achieved in the oxidation reactions of β-citronellol,
geraniol, nerol (Fig. 7c).
Acknowledgements The authors are grateful for the financial support
from CAPES, CNPq and FAPEMIG (Brazil).
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