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

Abstract

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.

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1. Introduction

The oxidation of terpenic alcohols, which are an abundant, renewable and attractive raw material, provide compounds useful for the synthesis of fine chemicals, fragrances, perfumes, and agrochemicals.^1,2 Linalool is a tertiary alcohol whose cyclization derivatives and epoxides are highly valuable due to their chiral building blocks for the synthesis of drugs.^3,4 The cyclization 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 first step of linalool cyclization requires a preliminary activation of the double bond through its transformation in epoxide or halogenate derivatives, which cyclizes to give the corresponding ether derivatives containing an additional hydroxyl or halogen functional group.^8,9 Nonetheless, these multistep processes have low atomic efficiency and involve the use of environmentally hazardous stoichiometric oxidants, resulting in residual wastes.¹⁰

In this regard, to develop oxidative routes of linalool based on hydrogen peroxide, an inexpensive, atom-efficient and green 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 heteropolyacids 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 polyoxometalates 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 heteropolyanions (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.

We synthesized three lacunar Keggin HPAs sodium salts and assessed their catalytic activity on the oxidation of linalool by hydrogen peroxide. All the lacunar salts were characterized by FT-IR, BET, XRD, TG/DSC techniques and their acidity strength were measured by n-butylamine titration. The effects of the main reaction variables were analyzed.

2. Experimental section

2.1. Chemicals

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 (99 wt%), HCl_(aq) (33 wt%) from Dinâmica and aqueous H₃PO_4(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 purification.

2.2. Synthesis of lacunar Keggin heteropolyacid salts (Na₇PW₁₁O₃₉)

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 phosphotungstate sodium salt was highlighted. Typically, 1 g of 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 results in the formation of the lacunar anion [PW₁₁O₃₉]⁷⁻. The 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, Na₈SiW₁₁O₃₉) being very similar, it was omitted by simplification.

2.3. Synthesis of saturated sodium phosphotungstate salt (Na₃PW₁₂O₄₀)

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, after to add H₃PO_4(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 spectrometer at wavenumber range from 500 to 1700 cm⁻¹, the fingerprint region of the main absorption bands of Keggin heteropolyanions. The bulk structure of the catalysts was examined by X-ray diffraction (XRD) technique on a Bruker D8-Discover diffractometer using Ni filtered Cu-kα radiation (λ = 1.5418 Å), working at 40 kV and 40 mA. The measurements were obtained with a count time of 1.0 s and in the 2θ range of 5–80 degrees.

Thermogravimetric analysis (TGA) was performed using a PerkinElmer Simultaneous Thermal Analyzer (STA) 6000, to study the thermal decomposition and verify the hydration degree of the fresh catalyst. The masses of the samples used were between 10–50 mg, with a heating rate of 10 °C min⁻¹ under nitrogen flow. Thermogram temperatures were recorded at each 0.1 °C range over a range of 30–600 °C.

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 outgassed by 1 h. The surface area was calculated by the Brunauer–Emmett–Teller equation applied to the isotherms.

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-butylamine solution in toluene (ca. 0.05 mol L⁻¹).

2.5. Identification of main reaction products

The major products were previously identified in a Shimadzu GC-2010 gas chromatographer coupled with a MS-QP 2010 mass spectrometer (i.e., electronic impact 70 eV, scanning range of m/z 50–450). Afterward, they were purified by column chromatography (silica 60G) using eluent hexane, containing 20% of ethyl acetate. Products (1a, 1b) were then characterized by ¹H and ¹³C NMR and FT-IR (Varian FT-IR 660) spectroscopy analysis (see ESI†). The NMRs spectra were taken in CDCl₃ solutions, using a Varian 600 spectrometer at 600 and 150 MHz, respectively. The chemical shifts were expressed as δ (ppm) relative to the tetramethylsilane (TMS) as an internal reference. Due to low yield, it was not possible to characterize diepoxide 1c; only their GC-MS data were obtained (see ESI†).

2.6. Catalytic runs

The catalytic tests were carried out in acetonitrile under air (atmospheric pressure) in a closed three-necked glass flask (25 mL), equipped with a sampling system and a reflux 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. After 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).

3. Results and discussion

3.1. Catalysts characterization

The FT-IR study provides information about the Keggin anion structure (i.e., primary structure); for instance, if it was retained or not after the removal of one WO unit. Fig. 1 shows infrared spectra of Keggin phosphotungstate salts (i.e., Na₇PW₁₁O₃₉ and Na₃PW₁₂O₄₀, respectively) and their parent heteropolyacid.

Fig. 1 FT-IR spectra of phosphotungstic acid, saturated sodium salt and lacunar sodium salt.

The main absorption bands of H₃PW₁₂O₄₀ were noticed at wavenumbers 1080, 980, 920, and 790 cm⁻¹, which agree with data of literature.²⁷ These bands were assigned to the stretching of P–O_a, W–O_d, W–O_b–W and W–O_c–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; O_b are oxygen atoms belonging to the WO₆ octahedral units sharing corners, O_c referrers to the oxygen atoms in edges, and O_d are terminal oxygen atoms. All of them are connected to the tungsten atoms, while O_a are bonded to the phosphorous atom.²⁸

The splitting of P–O_a bond absorption band at wavenumber 1080 cm⁻¹, which resulted in two new bands (ca. 1020 and 1060 cm⁻¹) is the main guarantee that the lacunar heteropolyanion 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 Na₇PMo₁₁O₃₉ lacunar salt was synthesized, nonetheless, the absorption bands appeared at wavenumbers 1078 and 1035 cm⁻¹ (Fig. SM1†).³³ Conversely, when the Na₈SiW₁₁O₃₉ lacunar salt was synthesized, the stretching band of Si-O_a 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†).

Fig. 2 Powdered XRD patterns of Na₇PW₁₁O₃₉ lacunar salt, parent H₃PW₁₂O₄₀ and saturated Na₃PW₁₂O₄₀.

Fig. 3 TG/TDTG and DSC curves: precursor H₃PW₁₂O₄₀ (a) and lacunar salt Na₇PW₁₁O₃₉ (b).

TG/DTG curves obtained from lacunar Na₇PW₁₁O₃₉ showed two regions of weight loss; the first one before 473 K assigned to loss of all water molecules. The second one, was assigned to the decomposition of P–O_a–W framework followed by the noticeable peak in DSC curves around 793 K. The final 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 heteropolyacid 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; E_i > 100 mV (very strong sites), 0 < E_i < 100 mV (strong sites), −100 < E_i < 0 (weak sites) and E_i < −100 mV (very weak sites).³⁴

Fig. 4 Potentiometric titration curves with n-butylamine of H₃PW₁₂O₄₀, Na₃PW₁₂O₄₀ and Na₇PW₁₁O₃₉ catalysts.

Regardless of the Keggin anion, the titration curves presented similar pattern; a quick decrease in the electrode potential after the addition of base minimum volume suggests that only a residual Brønsted acidity remained after the exchange of protons by the sodium cations, indicating that the protons were virtually removed.

After 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., E_i = 700 mV, inset in Fig. 4); the exchange of protons by the sodium cations reduced this acidity (E_i = 375 mV), however, it was classified as very strong. Conversely, lacunar salt was less acid (E_i = 75 mV). Their strong acid sites may be assigned to the residual protons (Fig. 4).

The pore sizes presented by sodium salts were very similar to those displayed by the parent acids (ca. 1–4 m² g⁻¹), regardless of the Keggin anion. Aiming simplifies, 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 first 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.

Fig. 5 Isotherms of adsorption and desorption (a), volume and diameters porous (b) of the Na₇PW₁₁O₃₉ catalyst.

3.2. Catalytic tests

3.2.1. Effect of catalyst nature in linalool oxidation reactions 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. 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).

Although not included in the Fig. 6, the H₃PW₁₁O₃₉ -catalyzed 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 the goal-products; cis and trans 2-(5-methyl-5-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

Scheme 1 Linalool oxidation by H₂O₂ in presence of Na₇PW₁₁O₃₉ catalyst.

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 five-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 effect on conversion and selectivity using a lower load of oxidant (ca. 1 : 1.5 and 1 : 1, Fig. SM9†).

As shown in Fig. SM9a,† the conversion of linalool to linalool oxides and diepoxide increased gradually with the increase of the amount of oxidant. The final conversions after 4 h of reaction were ca. 38%, 74% and 100%, when the proportions of H₂O₂ to linalool were 1 : 1, 1 : 1.5 and 1 : 2 respectively. Nonetheless, the final selectivity was approximately the same; 66%, 27% and 7%, for (1a), (1b) and (1c) products, respectively (Fig. SM9b†). When the reactions were carried out in the absence of the catalyst, no oxidation product was detected, regardless of the oxidant load.

3.2.3. Insights on the mechanism for linalool oxides synthesis. We suppose that the excess of peroxide is needed to peroxidize the lacunar sodium phosphotungstate catalyst as depicted in Scheme 2. Literature has reported the same phenomena for reactions of oxidation with H₂O₂ in the presence of tungsten catalysts (Scheme SM2†).^1,2,36

Scheme 2 Possible reaction pathway for the formation of (1a) and (1b) linalool oxides.

Thus, it is plausible that the intermediate 1 may be formed, which may transfer the oxygen atom to the double bond of the linalool. This transformation primarily results in an epoxide, which may be easily cyclized to give the corresponding ether derivatives (i.e., linalool oxides; 1a and 1b) bearing an additional hydroxyl group. Therefore, we believe that the formation of epoxide is essential for that the reaction continuance (Scheme 2). In Scheme 2, we will demonstrate how the epoxide is converted to product (1a) or (1b).

The intramolecular nucleophilic attack of the hydroxyl group belonging to linalool epoxide is the key-step that governs the reaction selectivity. After the formation of epoxide, its oxirane ring maybe then opened when the electron pairs of hydroxyl group attack the less hindered electrophilic carbon. This intramolecular cyclization step results in the tetrahydrofuran, after a proton transfer to the anionic intermediate (route a, Scheme 2).

Conversely, when the pair of electrons of the hydroxyl group attack the more hindered electrophilic carbon, the tetrahydropyran is the product obtained (route b, Scheme 2). Note that with the epoxides, the regioselectivity is not as simple, even with acid catalysts, the A_ND_N substitution at a primary carbon atom is very fast.^35,36

3.2.4. Effect of catalyst load. The catalyst load is another essential feature that may affect either conversion or reaction selectivity. Herein, we have assessed this effect in concentration range of 0.083 to 0.5 mol% (Fig. 7).

Fig. 7 Impacts of catalyst load on the conversion (a) and products selectivity (b) of Na₇PW₁₁O₃₉-catalayzed oxidation reaction of linalool with H₂O₂. ^aReaction conditions: linalool (2.75 mmol); H₂O₂ (5.5 mmol); temperature (298 K); CH₃CN (10 mL).

When we investigated the effect of H₂O₂ load, we have found that while the reaction conversions were strongly affected, the reaction selectivity remained almost unaltered. The variation of the catalyst load triggered the same effect. The reaction selectivity was practically constant, regardless of the catalyst load; conversely, an increase of the catalyst concentration of 0.083 to 0.33 mol%, raised the conversions of 59 to 100%.

3.2.5. Effects of temperature on the Na₇PW₁₁O₃₉-catalyzed oxidation of linalool by H₂O₂. To assess this effect, it was required that the Na₇PW₁₁O₃₉ catalyst had a concentration lower than that employed in the majority of the other reactions (ca. < 0.33 mol%), since that at room temperature, the conversion achieved practically 100% after 1 h of reaction. Therefore, we selected the lowest catalyst concentration used in Fig. 7 (ca. 0083 mol%).

Although at the studied concentration the initial rates of the reactions have been close, the exothermic character of the oxidation of linalool was confirmed by the results shown in Fig. 8a. Undoubtedly, it was verified that the linalool oxidation to oxides and diepoxide was more selective at room temperature. Nonetheless, from Fig. 8b we can also conclude that high temperatures increased the diepoxide selectivity and reduced the formation of the tetrahydrofuran derivative.

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).

3.2.6. Effect of terpene substrate on Na₇PW₁₁O₃₉-catalyzed oxidation reaction with H₂O₂. We have selected different alcohols; primary (β-citronellol), allylic (geraniol), tertiary (α-terpineol) as substrates (Fig. SM10†). All these alcohols have a double bond which may potentially be epoxidized, resulting in epoxides that may subsequently undergo nucleophilic addition reactions. The kinetics curves are presented in Fig. 9.

Fig. 9 Oxidation of different terpenic alcohols with hydrogen peroxide in Na₇PW₁₁O₃₉-catalyzed reactions. ^aReaction conditions: alcohol (2.75 mmol); H₂O₂ (5.5 mmol); Na₇PW₁₁O₃₉ (0.33 mol%); temperature (298 K); CH₃CN (10 mL).

When an allylic alcohol (i.e., geraniol, Fig. SM10†) was the substrate, a complete conversion within 4 hours of reaction was achieved with a high selectivity for geraniol-epoxide (ca. 89%), meaning that only the allylic double bond of alcohol was reactive, while the more hindered double bond which remained intact.

The primary alcohol (i.e., β-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 aldehydes were detected. The tertiary cyclic alcohol (α-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, triggered by nucleophilic attack water present in the oxidant.

4. Conclusions

In this work, we developed an efficient synthesis route of the linalool oxides using an environmentally benign oxidant (i.e., H₂O₂) in lacunar Na₇PW₁₁O₃₉-catalyzed reactions at room temperature. Three valuable compounds were selectively obtained after an adequate adjust of reaction parameters; the 2-(5-methyl-5-vinyltetrahydrofuran-2-yl propan-2-ol) (cis/trans 1 : 1 proportion, 1a) and 2,2,6-trimethyl-6-vinyltetrahydro-2H-pyran-3-ol (cis/trans 1 : 1 proportion, 1b) products were the oxides obtained. A diepoxide product (1c) was also formed. The excellent performance of the catalyst was assigned to the removal of one WO unit on the saturated anion (i.e., PW₁₂O₄₀⁷⁻), to form the catalytic active lacunar sodium Keggin 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, β-citronellol and α-terpineol to their oxidation products (mainly epoxides) at room temperature.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for the financial support from CNPq and FAPEMIG (Brasil). This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra00047g

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