One-pot synthesis at room temperature of epoxides and linalool derivative pyrans in monolacunary Na7PW11O39-catalyzed oxidation reactions by hydrogen peroxide

Article abstract of DOI:10.1039/d0ra00047g

In this work, we describe a new one-pot synthesis route of valuable linalool oxidation derivatives (i.e., 2-(5-methyl-5-vinyltetrahydrofuran-2-yl propan-2-ol) (1a)), 2,2,6-trimethyl-6-vinyltetrahydro-2H-pyran-3-ol (1b) and diepoxide (1c), using a green oxidant (i.e., hydrogen peroxide) under mild conditions (i.e., room temperature). Lacunar Keggin heteropolyacid salts were the catalysts investigated in this reaction. Among them, Na₇PW₁₁O₃₉ was the most active and selective toward oxidation products. All the catalysts were characterized by FT-IR, TG/DSC, BET, XRD analyses and potentiometric titration. The main reaction parameters were assessed. Special attention was dedicated to correlating the composition and properties of the catalysts and their activity.

Full text of DOI:10.1039/d0ra00047g

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One-pot synthesis at room temperature of
epoxides and linalool derivative pyrans in
monolacunary Na7PW11O39-catalyzed oxidation
reactions by hydrogen peroxide†
Cite this: RSC Adv., 2020, 10, 7691
a
b
b
*
Castelo B. Vilanculo,
Marcio J. Da Silva,
Milena Galdino Teixeira
´
and Jesus Avendano Villarrealc
In this work, we describe a new one-pot synthesis route of valuable linalool oxidation derivatives (i.e., 2-(5-
methyl-5-vinyltetrahydrofuran-2-yl propan-2-ol) (1a)), 2,2,6-trimethyl-6-vinyltetrahydro-2H-pyran-3-ol
(1b) and diepoxide (1c), using a green oxidant (i.e., hydrogen peroxide) under mild conditions (i.e., room
temperature). Lacunar Keggin heteropolyacid salts were the catalysts investigated in this reaction. Among
them, Na7PW11O39 was the most active and selective toward oxidation products. All the catalysts were
characterized by FT-IR, TG/DSC, BET, XRD analyses and potentiometric titration. The main reaction
parameters were assessed. Special attention was dedicated to correlating the composition and
properties of the catalysts and their activity.
Received 2nd January 2020
Accepted 25th January 2020
DOI: 10.1039/d0ra00047g
rsc.li/rsc-advances
In this regard, to develop oxidative routes of linalool based
on hydrogen peroxide, an inexpensive, atom-efficient and green
1. Introduction
oxidant that generates only water as a by-product is a challenge
to overcome.11,12 Nonetheless, hydrogen peroxide requires the
presence of a metal catalyst to be activated.13
Among the various catalysts used in oxidations with
hydrogen peroxide, polyoxometalates should be highlighted
due to their high compound versatility that has been effective in
homogeneous and heterogeneous processes.14–16 Several heter-
opolyacids have redox properties that characterize them as an
efficient catalyst in oxidation reactions with oxygen or hydrogen
peroxide.17
Keggin heteropolyacids (HPAs) are the most used poly-
oxometalates in catalysis. However, they are strong Brønsted
acids, a feature that compromises their use as a catalyst in
oxidations of monoterpenes, which are substrates susceptible
to acid-catalyzed concurrent reactions such as carbon skeletal
rearrangement and nucleophilic addition.18,19 To circumvent
this drawback, a rarely explored approach is to convert Keggin
HPAs to neutral salts, exchanging their acidic protons by metal
cations.20,21 Moreover, the removal of one MO unit of hetero-
polyanions (i.e., WO or MoO) results in a lacunar salt catalyst
that may be more active than its precursor with saturated
anion.22–24
The synthesis of linalool oxides and diepoxides, using
lacunar polyoxometalates as catalysts, has not yet been reported
in the literature. In this work, we present an innovative method
of one-pot synthesis of linalool oxides with high conversion and
selectivity, using a lacunar polyoxometalate as catalyst and
hydrogen peroxide as an oxidant.
The oxidation of terpenic alcohols, which are an abundant,
renewable and attractive raw material, provide compounds
useful for the synthesis of ne chemicals, fragrances, perfumes,
and agrochemicals.1,2 Linalool is a tertiary alcohol whose cycli-
zation derivatives and epoxides are highly valuable due to their
chiral building blocks for the synthesis of drugs.3,4 The cycli-
zation of linalool is an intramolecular process that results in the
formation of tetrahydropyran and tetrahydrofuran derivatives,
which are compounds with potential biological activity.5–7
Several strategies have been developed to cyclize the linalool.
Commonly, the rst step of linalool cyclization requires
a preliminary activation of the double bond through its trans-
formation in epoxide or halogenate derivatives, which cyclizes
to give the corresponding ether derivatives containing an
additional hydroxyl or halogen functional group.8,9 Nonethe-
less, these multistep processes have low atomic efficiency and
involve the use of environmentally hazardous stoichiometric
oxidants, resulting in residual wastes.10
aChemistry Department, Pedagogic University of Mozambique, FCNM, Campus of
Lhanguene, Av. de Moçambique, Km 1, Maputo, 4040, Mozambique. E-mail:
castelovilanculo@gmail.com; Tel: +258 825573337
bChemistry Department, Federal University of Viçosa, Minas Gerais State, 36590-000,
Brazil
cChemistry Department Federal University of Minas Gerais, Minas Gerais State, 31270-
901, Brazil
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra00047g
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We synthesized three lacunar Keggin HPAs sodium salts and obtained with a count time of 1.0 s and in the 2q range of 5–80
assessed their catalytic activity on the oxidation of linalool by degrees.
hydrogen peroxide. All the lacunar salts were characterized by
Thermogravimetric analysis (TGA) was performed using
FT-IR, BET, XRD, TG/DSC techniques and their acidity strength a PerkinElmer Simultaneous Thermal Analyzer (STA) 6000, to
were measured by n-butylamine titration. The effects of the study the thermal decomposition and verify the hydration
main reaction variables were analyzed.
degree of the fresh catalyst. The masses of the samples used
were between 10–50 mg, with a heating rate of 10 ꢁC minꢀ1
under nitrogen ow. Thermogram temperatures were recorded
2. Experimental section
ꢁ
at each 0.1 ꢁC range over a range of 30–600 C.
2.1. Chemicals
The surface area and total pore volume of the catalysts were
measured by N2 physisorption/desorption technique using
NOVA 1200e High Speed, Automated Surface Area, and Pore Size
Analyzer Quantachrome Instruments. The samples were out-
gassed by 1 h. The surface area was calculated by the Brunauer–
Emmett–Teller equation applied to the isotherms.
Linalool was purchased acquired by Sigma-Aldrich (99 wt%).
Sodium hydrogen carbonate from Vetec (99 wt%). All the HPAs
(i.e., H3PW12O40, H3PMo12O40, and H4SiW12O40; 99 wt%) were
purchased from Sigma-Aldrich. Na2WO4$2H2O from Vetec
ˆ
(99 wt%), HCl(aq) (33 wt%) from Dinamica and aqueous
Catalyst acidity was estimated by potentiometric titration, as
described by Pizzio et al.26 The electrode potential variation was
measured with a potentiometer (i.e. Bel, model W3B). Typically,
50 mg of HPA salt was dissolved in a CH3CN solution,
magnetically stirred for 3 h and then titrated with a n-butyl-
amine solution in toluene (ca. 0.05 mol Lꢀ1).
H3PO4(aq) (85 wt%) was from Sigma-Aldrich. CH3CN was
acquired from Sigma (99 wt%) Aqueous hydrogen peroxide was
from Alphatec (35 wt%). All reagents were used as received
without further purication.
2.2. Synthesis of lacunar Keggin heteropolyacid salts
(Na7PW11O39
)
2.5. Identication of main reaction products
The lacunar sodium salts preparation is very similar and they
were prepared according to the reported procedure25. The only
difference is the pH range used. Only the synthesis of lacunar
The major products were previously identied in a Shimadzu
GC-2010 gas chromatographer coupled with a MS-QP 2010 mass
phosphotungstate sodium salt was highlighted. Typically, 1 g of spectrometer (i.e., electronic impact 70 eV, scanning range of m/
hydrate H3PW12O40 was dissolved in water (30 mL) and heated
to 333 K with constant magnetic stirring. The solution's pH was
adjusted to 4.8 using a NaHCO3 solution (Scheme SM1†). This
z 50–450). Aerward, they were puried by column chroma-
tography (silica 60G) using eluent hexane, containing 20% of
1
ethyl acetate. Products (1a, 1b) were then characterized by H
results in the formation of the lacunar anion [PW11O39]
7ꢀ. The
and 13C NMR and FT-IR (Varian FT-IR 660) spectroscopy anal-
ysis (see ESI†). The NMRs spectra were taken in CDCl3 solu-
tions, using a Varian 600 spectrometer at 600 and 150 MHz,
respectively. The chemical shis were expressed as d (ppm)
solution with pH 4.8 was 3 h heated at 333 K. Finally, the
Na7PW11O39 salt was obtained by solvent evaporation and
recrystallization from water, followed by drying 5 h at 373 K.
Once the synthesis of other lacunar salts (i.e., Na7PMo11O39, relative to the tetramethylsilane (TMS) as an internal reference.
Na8SiW11O39
)
being very similar, it was omitted by
Due to low yield, it was not possible to characterize diepoxide
simplication.
1c; only their GC-MS data were obtained (see ESI†).
2.3. Synthesis of saturated sodium phosphotungstate salt
(Na3PW12O40
2.6. Catalytic runs
)
The catalytic tests were carried out in acetonitrile under air
(atmospheric pressure) in a closed three-necked glass ask (25
mL), equipped with a sampling system and a reux condenser,
and immersed in a thermostatic water bath. Typically, linalool
(2.75 mmol) was solved in a CH3CN solution (ca. 10 mL),
magnetically stirred and the reactions were carried out at room
temperature. Aer the addition of the catalyst (ca. 0.33 mol%),
the reaction was started and monitored during 4 h, periodically
collecting aliquots and analyzing them in a GC equipment
(Shimadzu 2010, FID).
For comparison, the sodium phosphotungstate catalyst was
also synthesized.20 Na2WO4$2H2O (30 mmol, 10 g) was slowly
added to 20 mL of distilled water and the mixture was
magnetically stirred and warmed to 333 K. Then, aer to add
H3PO4(aq) (15 mmol, 1 mL) and HCl(aq) (100 mmol, 8 mL) the
resulting mixture was stirred for 1 h. The white solid was
washed with water and was recrystallized twice from hot water.
2.4. Characterization of catalyst
Infrared spectra (FT-IR) were obtained on Varian 660-IR spec-
trometer at wavenumber range from 500 to 1700 cmꢀ1, the
ngerprint region of the main absorption bands of Keggin
heteropolyanions. The bulk structure of the catalysts was
3. Results and discussion
3.1. Catalysts characterization
examined by X-ray diffraction (XRD) technique on a Bruker D8- The FT-IR study provides information about the Keggin anion
Discover diffractometer using Ni ltered Cu-ka radiation (l ¼ structure (i.e., primary structure); for instance, if it was retained
˚
1.5418 A), working at 40 kV and 40 mA. The measurements were or not aer the removal of one WO unit. Fig. 1 shows infrared
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Fig. 1 FT-IR spectra of phosphotungstic acid, saturated sodium salt
and lacunar sodium salt.
Fig. 2 Powdered XRD patterns of Na7PW11O39 lacunar salt, parent
H3PW12O40 and saturated Na3PW12O40
.
spectra of Keggin phosphotungstate salts (i.e., Na7PW11O39 and
Na3PW12O40, respectively) and their parent heteropolyacid.
The main absorption bands of H3PW12O40 were noticed at
wavenumbers 1080, 980, 920, and 790 cmꢀ1, which agree with
data of literature.27 These bands were assigned to the stretching
of P–Oa, W–Od, W–Ob–W and W–Oc–W bonds. The main
absorption bands of Na3PW12O40 salt were like the pattern,
meaning that the salt was successfully synthesized. The
subscripts distinguish oxygen atoms; Ob are oxygen atoms
belonging to the WO6 octahedral units sharing corners, Oc
referrers to the oxygen atoms in edges, and Od are terminal
oxygen atoms. All of them are connected to the tungsten atoms,
while Oa are bonded to the phosphorous atom.28
The splitting of P–Oa bond absorption band at wavenumber
1080 cmꢀ1, which resulted in two new bands (ca. 1020 and
1060 cmꢀ1) is the main guarantee that the lacunar hetero-
polyanion was formed.29–31 This splitting is assigned to the
decreasing symmetry of PO4 group, resultant from the removal
of the WO unit.32 The same effect occurred when the Na7-
PMo11O39 lacunar salt was synthesized, nonetheless, the
absorption bands appeared at wavenumbers 1078 and
1035 cmꢀ1 (Fig. SM1†).33 Conversely, when the Na8SiW11O39
lacunar salt was synthesized, the stretching band of Si-Oa bond
remained untouchable (Fig. SM2†).
While infrared spectra gave information about the primary
structure of Keggin HPAs, XRD spectra provides data of the
secondary structure. Fig. 2 shows clearly that the XRD spectrum
of Na7PW11O39 lacunar salt displayed different peak patterns
than those presented by parent H3PW12O40 and its saturated
salt. It showed a lower level of crystallinity, with less intense
peaks than the precursor heteropolyacid and the saturated
phosphotungstate salt. This different crystallinity may be
assigned to the exchange of protons by sodium cations and
a changing on water hydration water molecules number (see
Fig. 3). The other two lacunar sodium salts also presented
a lower crystallinity level than acid precursors too (Fig. SM3 and
SM4†).
decomposition of P–Oa–W framework followed by the notice-
able peak in DSC curves around 793 K. The nal products are an
oxides mixture (Fig. 3).
TG curves of the lacunar salt presented a weight loss of
around 10%, likewise the precursor acid (Fig. 3a and b);
nevertheless, it has occurred more quickly for the hetero-
polyacid sample, indicating that the lacunar salt was thermally
more stable. The same behaviour was also observed on TG
curves of silicotungstic and phosphomolybdic acids and their
respective lacunar sodium salts (Fig. SM5 and SM6†).
The titration curves of the lacunar sodium salts and their
precursor Keggin HPAs were obtained (Fig. 4). This procedure
allows to classify the acidity strength of acid sites; Ei > 100 mV
(very strong sites), 0 < Ei < 100 mV (strong sites), ꢀ100 < Ei <
0 (weak sites) and Ei < ꢀ100 mV (very weak sites).34
Regardless of the Keggin anion, the titration curves pre-
sented similar pattern; a quick decrease in the electrode
potential aer the addition of base minimum volume suggests
that only a residual Brønsted acidity remained aer the
exchange of protons by the sodium cations, indicating that the
protons were virtually removed.
Aer the initial period of the titration, the potential
remained practically constant. This behaviour was completely
distinct than the precursor heteropolyacids (Fig. 4, SM7 and
SM8†).
The precursor HPA presented very strong acid sites (i.e., Ei ¼
700 mV, inset in Fig. 4); the exchange of protons by the sodium
cations reduced this acidity (Ei ¼ 375 mV), however, it was
TG/DTG curves obtained from lacunar Na7PW11O39 showed
two regions of weight loss; the rst one before 473 K assigned to
loss of all water molecules. The second one, was assigned to the
Fig. 3 TG/TDTG and DSC curves: precursor H3PW12O40 (a) and
lacunar salt Na7PW11O39 (b).
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Fig. 6 Effect of catalyst nature on the conversion (a) and selectivity (b)
of the linalool oxidation reaction with H2O2. aReaction conditions:
linalool to H2O2 (1 : 2); lacunar salt catalyst (0.33 mol%); temperature
(298 K); CH3CN (10 mL).
Fig.
4 Potentiometric titration curves with n-butylamine of
H3PW12O40, Na3PW12O40 and Na7PW11O39 catalysts.
Although not included in the Fig. 6, the H3PW11O39 -cata-
lyzed oxidation reaction of linalool achieved only a poor
conversion (ca. 9%) and no oxidation product was detected.
In addition to highest activity (conversion of ca. 100%), the
lacunar Na7PW11O39 catalyst was also the most selective toward
classied as very strong. Conversely, lacunar salt was less acid
(Ei ¼ 75 mV). Their strong acid sites may be assigned to the
residual protons (Fig. 4).
the
goal-products;
cis
and
trans
2-(5-methyl-5-
The pore sizes presented by sodium salts were very similar to
those displayed by the parent acids (ca. 1–4 m2 gꢀ1), regardless
of the Keggin anion. Aiming simplies, only the isotherms and
pores size distribution of the Na7PW11O39 salt was shown in
Fig. 5. The quick initial increase corresponds to the formation
of the rst layer; therefore, an increase in pressure forms the
second layer of adsorbed molecules, followed by another layer.
The total reversibility of the adsorption–desorption isotherm
was observed (i.e., absence of hysteresis cycle) for all catalysts,
a condition noticed in this catalyst.
vinyltetrahydrofuran-2-yl propan-2-ol) (1a), cis and trans 2,2,6-
trimethyl-6-vinyltetrahydro-2H-pyran-3-ol (1b), which were
formed at equimolar proportions. The diepoxide (1c) was also
obtained in linalool (1) oxidation reaction (Scheme 1).6
Note that the 2-(5-methyl-5-vinyltetrahydrofuran-2-yl propan-
2-ol) (1a) and 2,2,6-trimethyl-6-vinyltetrahydro-2H-pyran-3-ol
(1b) oxides were the major products, with selectivity of 66 and
27%, respectively (Scheme SM2†). On the other hand, the
diepoxide (1c) had only a selectivity of 7% (Fig. 6b). It indicates
that this catalyst system is highly selective for the synthesis of
the linalool oxides. The formation of the tetrahydrofuran
derivative (1a) with a ve-members ring was more selective in all
the reactions, excepted when the catalyst was Na7PMo11O39.
Probably, (1a) is the kinetic product and thus is formed faster
than (1b) product, which has a six-member ring.
Conversely, the diepoxide (1c) was the major product in
Na7PMo11O39-catalyzed reaction and the secondary one in
reaction with Na8SiW11O39. As we will demonstrate, the epoxide
is directly involved in the formation of linalool oxides.
3.2.2. Effect of the oxidant load on the oxidation of
linalool. We suppose that an oxidant may play a crucial role in
the catalytic oxidation of linalool. Therefore, since that the
Na7PW11O39-catalyzed reaction achieved a total conversion with
a 2 : 1 ratio of H2O2 to linalool, we investigate what was the
3.2. Catalytic tests
3.2.1. Effect of catalyst nature in linalool oxidation reac-
tions with hydrogen peroxide. Initially, we have assessed the
catalytic activity of the sodium phosphotungstate salts on the
linalool oxidation with H2O2 (Fig. 6). The reaction conditions
were chosen according to the literature.1 The Na7PW11O39
catalyst was more active than the lacunar (Na7PW11O39 and
mainly Na8SiW11O39) or saturated salts (Na3PW12O40) (Fig. 6a).
This effect demonstrates the importance of the presence of the
tungsten atoms as well as a vacancy on the heteropolyanion
catalyst structure.
Fig.
5 Isotherms of adsorption and desorption (a), volume and Scheme 1 Linalool oxidation by H2O2 in presence of Na7PW11O39
diameters porous (b) of the Na7PW11O39 catalyst.
catalyst.
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effect on conversion and selectivity using a lower load of oxidant acid catalysts, the ANDN substitution at a primary carbon atom
(ca. 1 : 1.5 and 1 : 1, Fig. SM9†).
is very fast.35,36
As shown in Fig. SM9a,† the conversion of linalool to linalool
3.2.4. Effect of catalyst load. The catalyst load is another
oxides and diepoxide increased gradually with the increase of essential feature that may affect either conversion or reaction
the amount of oxidant. The nal conversions aer 4 h of reac- selectivity. Herein, we have assessed this effect in concentration
tion were ca. 38%, 74% and 100%, when the proportions of range of 0.083 to 0.5 mol% (Fig. 7).
H2O2 to linalool were 1 : 1, 1 : 1.5 and 1 : 2 respectively. None-
When we investigated the effect of H2O2 load, we have found
theless, the nal selectivity was approximately the same; 66%, that while the reaction conversions were strongly affected, the
27% and 7%, for (1a), (1b) and (1c) products, respectively reaction selectivity remained almost unaltered. The variation of
(Fig. SM9b†). When the reactions were carried out in the the catalyst load triggered the same effect. The reaction selec-
absence of the catalyst, no oxidation product was detected, tivity was practically constant, regardless of the catalyst load;
regardless of the oxidant load.
3.2.3. Insights on the mechanism for linalool oxides 0.33 mol%, raised the conversions of 59 to 100%.
synthesis. We suppose that the excess of peroxide is needed to 3.2.5. Effects of temperature on the Na7PW11O39-catalyzed
conversely, an increase of the catalyst concentration of 0.083 to
peroxidize the lacunar sodium phosphotungstate catalyst as oxidation of linalool by H2O2. To assess this effect, it was
depicted in Scheme 2. Literature has reported the same required that the Na7PW11O39 catalyst had a concentration
phenomena for reactions of oxidation with H2O2 in the pres- lower than that employed in the majority of the other reactions
ence of tungsten catalysts (Scheme SM2†).1,2,36
(ca. < 0.33 mol%), since that at room temperature, the conver-
Thus, it is plausible that the intermediate 1 may be formed, sion achieved practically 100% aer 1 h of reaction. Therefore,
which may transfer the oxygen atom to the double bond of the we selected the lowest catalyst concentration used in Fig. 7 (ca.
linalool. This transformation primarily results in an epoxide, 0083 mol%).
which may be easily cyclized to give the corresponding ether
Although at the studied concentration the initial rates of the
derivatives (i.e., linalool oxides; 1a and 1b) bearing an addi- reactions have been close, the exothermic character of the
tional hydroxyl group. Therefore, we believe that the formation oxidation of linalool was conrmed by the results shown in
of epoxide is essential for that the reaction continuance Fig. 8a. Undoubtedly, it was veried that the linalool oxidation
(Scheme 2). In Scheme 2, we will demonstrate how the epoxide to oxides and diepoxide was more selective at room tempera-
is converted to product (1a) or (1b).
ture. Nonetheless, from Fig. 8b we can also conclude that high
The intramolecular nucleophilic attack of the hydroxyl group temperatures increased the diepoxide selectivity and reduced
belonging to linalool epoxide is the key-step that governs the the formation of the tetrahydrofuran derivative.
reaction selectivity. Aer the formation of epoxide, its oxirane
3.2.6. Effect of terpene substrate on Na7PW11O39-catalyzed
ring maybe then opened when the electron pairs of hydroxyl oxidation reaction with H2O2. We have selected different alco-
group attack the less hindered electrophilic carbon. This hols; primary (b-citronellol), allylic (geraniol), tertiary (a-
intramolecular cyclization step results in the tetrahydrofuran, terpineol) as substrates (Fig. SM10†). All these alcohols have
aer a proton transfer to the anionic intermediate (route a, a double bond which may potentially be epoxidized, resulting in
Scheme 2).
Conversely, when the pair of electrons of the hydroxyl group reactions. The kinetics curves are presented in Fig. 9.
attack the more hindered electrophilic carbon, the tetrahy- When an allylic alcohol (i.e., geraniol, Fig. SM10†) was the
epoxides that may subsequently undergo nucleophilic addition
dropyran is the product obtained (route b, Scheme 2). Note that substrate, a complete conversion within 4 hours of reaction was
with the epoxides, the regioselectivity is not as simple, even with achieved with a high selectivity for geraniol-epoxide (ca. 89%),
meaning that only the allylic double bond of alcohol was reac-
tive, while the more hindered double bond which remained
intact.
Fig. 7 Impacts of catalyst load on the conversion (a) and products
selectivity (b) of Na7PW11O39-catalayzed oxidation reaction of linalool
Scheme 2 Possible reaction pathway for the formation of (1a) and (1b) with H2O2. aReaction conditions: linalool (2.75 mmol); H2O2 (5.5
linalool oxides.
mmol); temperature (298 K); CH3CN (10 mL).
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phosphotungstate (Na7PW11O39). The maximum selectivity
combined toward the main products was close to 100%, with
a conversion virtually complete of the linalool. The process
described herein has attractive aspects; the reactions were
performed at room temperature, the catalyst was easily
synthesized and achieved excellent catalytic performance, being
also efficiently recovered and reused. Therefore, this route
demonstrated to be very suitable for the conversion of linalool
into linalool oxides, which are important intermediates in
chiral synthesis. In addition, Na7PW11O39 catalyst efficiently
converted the geraniol, b-citronellol and a-terpineol to their
oxidation products (mainly epoxides) at room temperature.
Fig. 8 Effects of temperature on the conversion (a) and selectivity (b)
of Na7PW11O39-catalyzed oxidation reactions of linalool with H2O2.
aReaction conditions: linalool (2.75 mmol); H2O2 (5.5 mmol);
Na7PW11O39 (0.083 mol%); CH3CN (10 mL).
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors are grateful for the nancial support from CNPq
and FAPEMIG (Brasil). This study was nanced in part by the
˜
´
Coordenaçao de Aperfeiçoamento de Pessoal de Nıvel Superior –
Brasil (CAPES) – Finance Code 001.
References
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9 Oxidation of different terpenic alcohols with hydrogen
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alcohol (2.75 mmol); H2O2 (5.5 mmol); Na7PW11O39 (0.33 mol%);
temperature (298 K); CH3CN (10 mL).
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The primary alcohol (i.e., b-citronellol, Fig. SM10†) was less
active than the allylic alcohol; only a conversion of 61% was
achieved, with a high epoxide selectivity. Solely traces of alde-
hydes were detected. The tertiary cyclic alcohol (a-terpineol,
Fig. SM10†) was very reactive; a conversion close of ca. 90% was
achieved with a selectivity of 39, 34 and 27% for keto-alcohol,
epoxide and glycol, respectively. We suppose that the glycol
was formed through the initial epoxidation of the double bond
and subsequent opening of the oxirane ring of epoxide, trig-
gered by nucleophilic attack water present in the oxidant.
4. Conclusions
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