Unravelling the different pathways of cyclohexene oxidation via a peroxyl radical generated from tert-butyl hydroperoxide (TBHP) by various metal salts

Abstract

Selective oxidation of cyclohexene is rather challenging due to its two bond reactive centers: allylic C–H and C=C. It is of great significance to achieve highly selective oxidation towards the two reactive centers by regulating the catalytic system. Herein, the selective oxidation towards the allylic C–H and C=C bond reactive sites of cyclohexene was achieved by using CuCl₂ and VCl₃ as the catalyst, respectively, in the presence of tert-butyl hydroperoxide (TBHP). The CuCl₂-catalyzed oxidation of cyclohexene mainly yielded oxidation products at the allylic position, while the VCl₃-catalyzed oxidation of cyclohexene mainly yielded epoxidation products at the C=C bond. The two different reaction mechanisms are mainly due to the different roles of the t-BuOO˙ radical in the respective catalytic systems. Electron paramagnetic resonance characterizations showed that the amount of t-BuOO˙ radicals in the CuCl₂ catalytic system is much lower than that in the VCl₃ system. The two different mechanisms were proposed by the means of ¹⁸O₂ experiments, KIE kinetics and EPR. These mechanisms revealed that the CuCl₂ catalyst could rapidly generate the t-BuOO˙ radical, which undergoes bimolecular decay to produce O₂. The oxygen then combines with the allyl radical of cyclohexene to produce the oxidized product at the allylic reactive center. In contrast, the VCl₃ catalyst promoted the generation of the t-BuOO˙ radical, which reacted with the C=C bond of cyclohexene to directly yield the epoxide.

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Introduction

Selective oxidation of alkenes has always attracted the attention of fundamental and applied research due to the high added value of the oxidized products.^1–4 As a readily available industrial alkene, cyclohexene can be oxidized to oxygenate compounds that are widely used in fields such as chemistry, agriculture and medicine.^5–7 As cyclohexene has two active sites—the allyl site and the carbon–carbon double bond—it can undergo two oxidation pathways, as shown in Fig. 1. One is the allylic oxidation pathway, which produces 2-cyclohexen-1-ol and 2-cyclohexen-1-one (Path A, Scheme 1). The other is the epoxidation pathway, which produces epoxide, with excessive hydrolysis leading to 1,2-cyclohexadiol (Path B, Scheme 1).^8–10 However, the two branched pathways cannot be clearly distinguished, which usually results in reaction products from both these pathways. Therefore, it is difficult to obtain a desired single product via cyclohexene oxidation.

Fig. 1 Profiles of cyclohexene oxidation with TBHP catalyzed by CuCl₂ (a) and VCl₃ (b). Reaction conditions: cyclohexene 1.6 mmol, TBHP 5.0 equiv., 30.0 °C, 4 mol% catalyst, solvent acetonitrile 5 mL.

Scheme 1 Possible reaction routes of cyclohexene oxidation.

Many efforts have been made in recent years to improve the selectivity of some desired products from cyclohexene oxidation by developing various catalytic protocols with environmentally friendly oxidants, such as molecular oxygen, H₂O₂ or TBHP (tert-butyl hydroperoxide).^11–14 In general, cyclohexene was oxidized with molecular oxygen catalyzed by various metal catalysts, such as copper,¹⁵ cobalt,¹⁶ manganese salts,¹⁷ and the main obtained products were generated from the allylic position, such as 2-cyclohexen-1-ol and 2-cyclohexen-1-one (Path A). This could be ascribed to the cyclohexenyl radical, which could be generated smoothly using a metal catalyst or via thermal dehydrogenation. Subsequently, an allylic hydroperoxide radical (ROO˙) intermediate generated easily from the reaction between the cyclohexenyl radical and O₂.¹⁸ The oxidation of cyclohexene with O₂ is subject to radical chain reactions, with ROO˙ radicals being the main chain carriers.¹⁹

Different product distributions in cyclohexene oxidation could be obtained when H₂O₂ was used as the oxidant.²⁰ The decomposition mechanism of H₂O₂ is closely related to the type of catalyst. Efficient catalysts for the oxidation of cyclohexene using H₂O₂, such as Nb silicates,²¹ Mn/Al₂O₃,²² Ag/TiO₂/SiO₂,²³ achieved a high selectivity for the epoxide products. Such catalysts promote the heterolytic cleavage of H₂O₂ to HOO˙ radicals, which can coordinate at the catalytic metal center to selectively transfer one oxygen atom to the C=C bond of cyclohexene, forming epoxides. However, various catalysts, such as Cu-zeolite-Y,²⁴ Fe-based MOF MIL-88A (Fe),²⁵ CoW composite oxides,²⁶ promoted the homolytic cleavage of H₂O₂ to ˙OH, which resulted in products from the allylic oxidation pathway (Path A).

Compared with H₂O₂, TBHP exhibits a higher stability and is widely used in the large-scale production of propylene oxide, such as the Halcon processes, which simultaneously produce tert-butanol as a byproduct.^27–29 The type of catalyst also plays a key role in the selectivity of products for cyclohexene oxidation with TBHP as the oxidant. For example, VO₂-SiO₂³⁰ and Mo-based catalysts³¹ showed an excellent epoxidation selectivity for cyclohexene oxidation in the presence of TBHP; most other metal–base catalysts, such as Cu, Co and Mn, exhibit dominant selectivity for allylic products.^32–34 The homolytic decomposition of the O–O bond in the TBHP molecule could generate t-BuO˙ and ˙OH radicals, while ROO˙ radicals are the main radicals in the heterolytic cleavage of TBHP. The difference in product selectivity could be ascribed to the generation of different types of reactive oxygen species by various catalysts.

Thus, the oxidation of cyclohexene is highly complicated, which involves a complex reaction network of various free radicals and chain reactions. Especially for TBHP, three kinds of reactive oxygen radicals (t-BuO˙, ˙OH and t-BuOO˙) could be produced via the cleavage of the TBHP molecule, which directly affect the selectivity of the products. Therefore, achieving selective generation of reactive oxygen radicals in the cleavage of TBHP is of great significance for selective control of cyclohexene oxidation products.

With this idea in mind, the selective oxidation of cyclohexene was achieved by using CuCl₂ and VCl₃ as catalysts in the presence of TBHP. Two distinct mechanisms, namely the allylic oxidation pathway and the epoxidation pathway, were obtained, respectively. A series of characterizations, such as EPR, KIE kinetics and ¹⁸O₂ experiments, showed that the different roles of the t-BuOO˙ radical in the respective catalytic systems yielded two distinct mechanisms. This work would provide the potential application prospects for improving the product selectivity of the alkene with the allyl site and C=C bond as two active sites.

Results and discussion

Selective oxidation of cyclohexene catalyzed by various metal chloride salt catalysts with TBHP as the oxidant were explored. The main products of the allyl position (Path A) are 2-cyclohexen-1-one (1), 2-cyclohexen-1-ol (2) and 2-cyclohexen-1,4-dione (3). The main products of the C=C bond position (Path B) are cyclohexene epoxide (4) and 1,2-cyclohexane-diol (5). In addition, there are other products that contain both allyl and C=C bond positions, such as 2,3-epoxy-1-cyclohexanone and 2,3-epoxy-1-cyclohexanol (Fig. S1†).

As shown in Table S1,† only 9% cyclohexene was converted in the absence of catalyst. It is known that there are significant differences in the catalytic performance of different metal chlorides. This is mainly related to the different abilities of various metal chlorides to activate TBHP for generating reactive oxygen species. Among these, CuCl₂ and VCl₃ exhibit excellent catalytic performances, with conversion rates of 93% and 90%, respectively. The effect of reaction conditions, such as the solvent and reaction temperature, on cyclohexene oxidation was explored (Tables S2–S4†). Upon closer observation, it was found that when copper chloride was used as the catalyst, the products obtained from cyclohexene oxidation were mainly via pathway A, while using vanadium chloride mainly produced pathway B products (Fig. 1 and Fig. S2†). However, for other metal chlorides, there seems to be no significant difference in the products that are generated from the two pathways. Therefore, we attempted to explore the different mechanisms for catalytic oxidation of cyclohexene catalyzed by CuCl₂ and VCl₃.

The allyl position of cyclohexene is an active reaction site, which can be determined by kinetic isotope effect (KIE) experiments (Fig. S3†).³⁵ A KIE value of 6.73 (K_H/K_D) was obtained for the oxidation of cyclohexene-H10 and cyclohexene-D10 with TBHP catalyzed by CuCl₂, which indicates that the allyl-H activation step is the rate-determining step. However, the KIE value of 0.87 demonstrates that the oxidation mechanism of cyclohexene when using VCl₃ as a catalyst is completely different from that when using CuCl₂ as the catalyst (Fig. 2). The findings obtained from the KIE tests are consistent with the above catalytic performances.

Fig. 2 KIE experiment for the oxidation of cyclohexene-H10 and cyclohexene-D10 with TBHP catalyzed by CuCl₂ (a) and VCl₃ (b). Reaction conditions: cyclohexene 1.6 mmol, TBHP 5.0 equiv., 30.0 °C, 4 mol% catalyst, solvent acetonitrile 5 mL.

Subsequently, the effect of TBHP oxidant dosage on the reaction was further investigated by using CuCl₂ as the catalyst. As a result, it was found that with the increase of TBHP dosage, the oxidation products at the allyl position gradually increased, while the products from epoxidation gradually decreased (Fig. 3a). Meanwhile, numerous gas bubbles were observed in the reaction solution during the experiments (Fig. S4b†). After collection and analysis, the generated gas in the solution was confirmed to be molecular oxygen (Fig. S5a†). The generated dioxygen content is much higher than the oxygen content in air and a standard gas containing 21% oxygen (Fig. S5b†). Therefore, it could be deduced that the produced dioxygen originates from TBHP.³⁶

Fig. 3 Effect of TBHP amount on the selective oxidation of cyclohexene catalyzed by CuCl₂, (a) cyclohexene 1.6 mmol, 30.0 °C, 4 mol% catalyst, solvent acetonitrile 5 mL, (b) cyclohexene 1.6 mmol, 30.0 °C, 4 mol% catalyst, solvent acetonitrile 5 mL, dioxygen flow rate 5 mL min⁻¹.

Afterwards, while using different amounts of TBHP as the oxidant, a certain amount of oxygen (flow rate: 5 mL min⁻¹) was introduced into the reaction system. It was found that, regardless of the amount of TBHP used, the selectivity of Path A products remained unchanged (Fig. 3b). This indicates that the role of TBHP is to promote the generation of dioxygen, which is an important participant in this catalytic process. Cyclohexene oxidation catalyzed by CuCl₂ with TBHP was then carried out under an ¹⁸O₂ atmosphere. Apart from products containing ¹⁶O, products containing ¹⁸O at the allyl position were observed by CSI-MS (¹⁸O-labeled 2-cyclohexen-1-one, m/z = 98.0618; ¹⁸O-labeled 2-cyclohexen-1,4-dione, m/z = 114.0453) (Fig. S6†).

Similarly, the effect of TBHP oxidant dosage on the oxidation of cyclohexene was also investigated with VCl₃ as the catalyst. However, the selectivity of the product hardly changed with the amount of TBHP, with the main generated products corresponding to the C=C bond position (Path B) (Fig. 4a). Meanwhile, no gas bubbles were observed in the reaction solution. After introducing oxygen at a 5 mL min⁻¹ flow rate, the selectivity of the product changed significantly. The oxidized products were transformed to those generated at the allyl position (Path A) (Fig. 4b). This also indicates that the reaction rate between allyl radicals and oxygen is much higher than that of epoxidation.

Fig. 4 Effect of TBHP amount on the selective oxidation of cyclohexene catalyzed by VCl₃, (a) cyclohexene 1.6 mmol, 30.0 °C, 4 mol% catalyst, solvent acetonitrile 5 mL, (b) cyclohexene 1.6 mmol, 30.0 °C, 4 mol% catalyst, solvent acetonitrile 5 mL, dioxygen flow rate 5 mL min⁻¹.

Both in CuCl₂- and VCl₃-catalyzed oxidations of cyclohexene with TBHP, EPR signals corresponding to radical adducts of dimethyl pyridine N-oxide (DMPO) were observed. The adducts determined by hyperfine coupling constant (HFCC) values (α_N = 14.5 G, α_H(β) = 11 G, and g = 2.0061) can be attributed to DMPO spin adducts of the peroxyl radical (t-BuOO˙) (Fig. 5a and b).³⁷ Comparing the peak intensity of peroxide radicals in the two catalytic systems, it can be seen that the amount of peroxide radicals in the CuCl₂ catalytic system is much lower than that in the VCl₃ system (Fig. 5c). The kinetics of the consumption of the peroxide radical indicated that the consumption of t-BuOO˙ radicals in the CuCl₂ reaction system was much faster than that in the VCl₃ reaction system (Fig. S7†). Combining the TBHP consumption rate curves of the two catalytic systems, it is evident that the role of the peroxide radicals is significantly different in the two systems (Fig. S8†). To further investigate whether the epoxidation of olefins is a mechanism of peroxyacids or the peroxyl radical, the epoxidations of cis-styrene catalyzed by CuCl₂ and VCl₃ were investigated. The results showed that both in the VCl₃ and CuCl₂ systems, the peroxide radical is the main species for olefin epoxidation. However, the efficiency of the CuCl₂ catalyst for catalytic cis-styrene epoxidation was much lower that of VCl₃. In addition, peroxyacids were the main species for epoxidation in the absence of catalyst (Table S5†).³⁸

Fig. 5 X -band EPR spectra of the peroxyl radical (t-BuOO˙) captured by DMPO (dimethyl pyridine N-oxide). (a) CuCl₂ as the catalyst, experimental (black) and simulated (red); (b) VCl₃ as the catalyst, experimental (black) and simulated (red); (c) EPR spectra in the CuCl₂ and VCl₃ systems.

Three types of oxygen-containing radicals, including the tert-butylperoxyl radical (t-BuOO˙), tert-butoxyl radical (t-BuO˙) and hydroxyl radical (˙OH), could be generated from the decomposition of TBHP. The adducts determined by hyperfine coupling constant (HFCC) values (α_N = 13.19 G, α_H(β) = 8.16 G, and g = 2.0059) could be attributed to DMPO spin adducts of t-BuO˙ (Fig. S9a†).³⁹ The adducts determined by hyperfine coupling constant (HFCC) values (α_N = 14.8 G, α_H(β) = 14.8 G, and g = 2.0059) can be attributed to DMPO spin adducts of ˙OH (Fig. S9b†).⁴⁰ On the basis of the literature, TBHP is induced by Cu²⁺ to generate t-BuOO˙ and simultaneously Cu²⁺ is reduced to Cu⁺. Conversely, t-BuO˙ can be formed via the reductive decomposition of TBHP, which is accompanied by the regeneration of the Cu²⁺ catalyst from Cu⁺.⁴¹ Certainly, the bimolecular decay of t-BuOO˙ is another way to provide t-BuO˙, and more significantly, the oxygen produced in this process plays a key role in this reaction as the unique oxygen source.

In addition, the adducts of the cyclohexenyl radical and cyclohexenyl peroxyl radical by DMPO were detected at a mass to charge ratio (m/z) of 194.1535 and 226.1429, respectively (Fig. S10d and Fig. S10f†). Therefore, the t-BuOO˙ radical undergoes a self-polymerization reaction catalyzed by CuCl₂ to produce O₂. The generated dioxygen could combine with the cyclohexenyl radical to generate the corresponding oxidation products (Scheme 2).⁴²

Scheme 2 Pathway for dioxygen and 2-cyclohexen-1-one generation in the CuCl₂ catalytic oxidation of cyclohexene.

According to DFT calculations, the decomposition of O–O bond homolysis to ˙OH is favored over heterolysis to the ROO˙ radical, as it requires less energy.⁴³ The O–O bond homolysis of di-tert-butyl peroxide (DTBP) produces the tert-butoxyl radical (t-BuO˙), while the O–O bond homolysis of hydrogen peroxide (H₂O₂) produces the hydroxyl radical (˙OH). Using DTBP and H₂O₂ as oxidants, some control experiments catalyzed by CuCl₂ were carried out to determine the roles of the t-BuO˙ and ˙OH radicals.

As a substrate, cyclohexane epoxide was not converted at all, regardless of the oxidant used among TBHP, DTBP and H₂O₂. This indicates that the generation of 1,2-cyclohexane-diol (5) is not obtained via the deep oxidation or hydration of epoxide (entries 1–3 in Table S6†). In contrast, cyclohexene could be converted with various conversions when using any of the three oxidants (entries 4–6 in Table S6†). The conversion was 35% and the main products were generated through Path A when H₂O₂ was used as the oxidant (entry 6 in Table S6†). However, the conversion was only 10% and the only product was the combination of the cyclohexenyl radical and t-BuO˙ when DTBP was used as the oxidant (entry 5 in Table S6†). These results indicate that both t-BuO˙ and ˙OH radicals can abstract the hydrogen atom from the α-position of cyclohexene to generate the cyclohexene radical.

No conversion was obtained in the oxidation of both cyclohexen-1-one and 2-cyclohexen-1-ol using DTBP as the oxidant (entry 8 and 10 in Table S6†). However, when using TBHP and H₂O₂ as the oxidant, the main products were 1-one and 1,4-dione in the oxidation of cyclohexen-1-ol and 2-cyclohexen-1-one, respectively. Such observations indicate that ˙OH radicals are active species that promote further oxidation of cyclohexen-1-ol and 2-cyclohexen-1-one.

From Table S2 (entry 1†), it can be seen that the main product of cyclohexene oxidation was diol in the absence of catalyst, with a selectivity of up to 43%. When adding VCl₃ catalyst into the solution, the selectivity of the diols decreased from 43% to 7%, while the selectivity of epoxide increased from 5% to 51%. Combining the results of cis-styrene epoxidation (Table S5†), the promoted generation of t-BuOO˙ by VCl₃ could be concluded, thereby enhancing the selectivity of epoxide. In addition, cyclohexanol radicals were observed in CSI-MS spectra of DMPO spin adducts at a mass to charge ratio (m/z) of 212.1640 (Fig. S10h†), which were the products of the addition reaction between the ˙OH radical and double bond of cyclohexene. Subsequently, diol was generated from the combination of the cyclohexanol radical and ˙OH radical, not the hydration of epoxide.

Based on above observations and discussions, the roles of the three kind radicals—t-BuOO˙, t-BuO˙ and ˙OH—can be summarized as shown in Fig. 6. When the oxidation of cyclohexene was catalyzed by CuCl₂, the cyclohexenyl radical was promoted to generate through t-BuO˙ and ˙OH radicals. Therefore, the main products were generated from the allyl position of cyclohexene (Route 1 and Route 2 in Fig. 6). However, the t-BuOO˙ radical was the active species when cyclohexene oxidation was catalyzed by VCl₃, which resulted in the main products being generated from the C=C bond of cyclohexene (Route 4 in Fig. 6). In addition, the ˙OH radical was the active species for generating 1,2-cyclohexane-diol when the oxidation was conducted in the absence of catalyst (Route 3 in Fig. 6).⁴⁴

Fig. 6 Selective oxidation pathways for cyclohexene in the TBHP oxidation system.

The different pathways of cyclohexene oxidation catalyzed by CuCl₂ and VCl₃ could be well demonstrated, according to the above discussion. A tentative mechanism of cyclohexene oxidation via Path A in the CuCl₂/TBHP system was proposed, as shown in Scheme 3. Initially, TBHP is induced by Cu²⁺ to generate t-BuOO˙ and the Cu²⁺ was reduced to Cu⁺ (eqn (1) in Scheme 3). Conversely, t-BuO˙ can be formed via the reductive decomposition of TBHP, which is accompanied by the regeneration of Cu²⁺ from Cu⁺ (eqn (2) in Scheme 3). Certainly, the oxygen was produced via the bimolecular decay of t-BuOO˙, also providing t-BuO˙ in the presence of CuCl₂ (eqn (3) in Scheme 3). Next, the oxygen combines with the cyclohexenyl radical to generate the cyclohexenyl peroxyl radical, and then to provide the corresponding products at the allyl position of cyclohexene (Path A) (eqn (4) and (5) in Scheme 3).

Scheme 3 Plausible mechanism of cyclohexene oxidation catalyzed by CuCl₂ with TBHP.

As mentioned above (Table S5†), trans-stilbene oxide was the predominant product in the epoxidation of cis-styrene catalyzed by VCl₃, which suggested that the t-BuOO˙ radical was the active species. Therefore, a plausible mechanism of cyclohexene oxidation via Path B in the VCl₃/TBHP system was proposed as shown in Scheme 4. The peroxyl radical (t-BuOO˙) could be generated by both the oxidation and reduction of VCl₃ (eqn (1) and (2) in Scheme 4). In addition, t-BuOO˙ can be formed via the decomposition of TBHP by t-BuO˙, which is accompanied by the generation of t-BuOH (eqn (3) in Scheme 4). Then, the t-BuOO˙ radical reacted with cyclohexene to yield the epoxide directly (eqn (4) in Scheme 4).^45–47

Scheme 4 Plausible mechanism of cyclohexene oxidation catalyzed by VCl₃ with TBHP.

Conclusions

In summary, two different mechanisms were obtained in the selective oxidation of cyclohexene catalyzed by CuCl₂ and VCl₃, respectively, in the presence of TBHP. The CuCl₂-catalyzed oxidation of cyclohexene mainly yielded allylic products, but the VCl₃-catalyzed oxidation of cyclohexene mainly yielded epoxidation products. The two opposed mechanisms are mainly caused by the different roles of the t-BuOO˙ radical in their respective catalytic oxidations, which were demonstrated by means of ¹⁸O₂ experiments, KIE kinetics and EPR characterizations. CuCl₂ could rapidly generate the t-BuOO˙ radical, which undergoes a bimolecular decay to produce O₂. The oxygen combines with the allyl radical of cyclohexene that is generated by the attack of t-BuO˙ or ˙OH radicals on cyclohexene to produce the oxidized products at the allylic site. In contrast, the t-BuOO˙ radical could react with the C=C bond of cyclohexene to yield the epoxide directly in the VCl₃ catalytic system. This work provides potential application prospects for improving the product selectivity of alkene with AN allyl site and C=C bond as two active sites.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Data for this paper, including reactions screening and optimization experiments, detailed experimental procedures, spectral data, and computational study details, are provided in the ESI.†

Author contributions

L. Z. and X. L. developed the concept, designed these experiments, analyzed experimental data and wrote the paper. H. Z, C. X and H. Z. contributed to the catalyst synthesis, catalytic experiments and the analyzed experimental data. X. Z. directed the project. All the authors discussed the results and provided comments during the manuscript preparation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 22278451, 22378436 and 21938001), Guangdong Basic and Applied Basic Research Foundation (2022B1515120057, 2023A1515012710 and 2023A1515111144) and Fundamental Research Funds for the Central Universities, Sun Yat-sen University (24lgqb014).

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, spectral data, and computational study details. See DOI: https://doi.org/10.1039/d4qo01681e

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