Epoxidation of cyclic enones with hydrogen peroxide catalysed by alkylcarboxylate-intercalated Ni–Zn mixed basic salts

Abstract

The interlayer spacing of layered Ni–Zn mixed basic salts (NiZn) can be precisely controlled by the intercalation of various long alkyl chain carboxylate anions into the NiZn interlayer. The butyrate-exchanged NiZn (C₃H₇COO⁻/NiZn) catalyst effectively promotes the epoxidation of various cyclic enones with aqueous hydrogen peroxide in remarkably high yields. The C₃H₇COO⁻/NiZn-catalysed epoxidation of 2-cyclohexen-1-one with an equimolar amount of H₂O₂ proceeds in a highly efficient manner, with 97% efficiency of H₂O₂ utilization. This C₃H₇COO⁻/NiZn catalyst can be reused without any loss of its catalytic activity and selectivity.

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Introduction

Anion-exchange within layered inorganic materials has received much attention as a means to create various types of useful inorganic–organic nanocomposites.¹ Layered double hydroxides (LDHs),² with the formula [M²⁺_1−xM³⁺_x(OH)₂]^x+[Aⁿ⁻_x/n]^x−·mH₂O, where (0.51 < x < 1.00), are a representative group of this class of solids. Because of their low toxicity, LDHs have been used as drug delivery matrices by intercalating active pharmaceutical ingredients into the interlayer of the inorganic materials.³ Among the many types of LDHs, rhombohedral Mg–Al hydrotalcites (M²⁺: Mg²⁺, M³⁺: Al³⁺, and Aⁿ⁻: CO₃²⁻) have been widely used as heterogeneous catalysts or catalyst supports.⁴ Because of their great potential for use in the design of high-performance heterogeneous catalysts, various strategies for the preparation of such materials have been widely reported in recent years. The acid–base properties of hydrotalcites can be controlled by changing the identity of the metal species and/or the M²⁺/M³⁺ ratio within the structure.⁵ Furthermore, catalytically active species, such as metal salts, organometallic complexes, or nanoparticles, can be supported on their cationic layer sheets, surfaces, or interlayer spaces.⁶ Recently, it has been found that environmentally benign organic transformation reactions can be achieved by using the combined catalytic systems composed of both a solid acid catalyst and a solid basic hydrotalcite.⁷

Layered hydroxy double salts (HDSs), which consist of positively charged layers and exchangeable interlayer anions, have also received considerable interest as anion-exchangeable layered compounds.⁸ Compounds in the HDS family follow the general formula M_A²⁺_1−xM_B²⁺_2xAⁿ⁻_2x/n(OH)₂·mH₂O (0.15 < x < 0.25), where M_A²⁺ and M_B²⁺ are divalent metal cations such as Zn²⁺, Cu²⁺, Co²⁺, or Ni²⁺, and Aⁿ⁻ represents various interlayer anions. The anionic guests that can be used range from simple inorganic anions (NO₃⁻, ClO₄⁻, Cl⁻, MnO₄⁻, SO₄²⁻, etc.) to anionic metal complexes to organic anions including drugs and dyes.⁹ It can be considered that HDSs are also powerful candidates for advanced nanoscaled catalysts or catalyst supports that allow for control over the location of the catalytically active metal species.

The epoxidation of C=C double bonds is one of the platform synthetic processes in basic research and industrial applications, because epoxides and their intermediates are widely employed in the synthesis of high-valued chemicals, such as cosmetics, pharmaceuticals, and polymer materials.¹⁰ The epoxidation of electron-deficient olefins in particular, such as α,β-unsaturated ketones, has been an important target for the functionalisation of ketones. It is well known that this type of epoxidation generally requires a nucleophilic oxidant, such as OOH⁻ species.¹¹ Many epoxidation procedures employ H₂O₂/Bu₄NF, TBHP/DBU, mCPBA/KOH, NaOCl/cinchona alkaloid, NaBO₃/THAHS, iodosyl benzene, dioxirane, or TBA₂S₂O₈ as an oxidant,¹² however, these oxidants often yield stoichiometric amounts of their deoxygenated forms as a waste by-product. In contrast, hydrogen peroxide is an ideal oxidant because water is the only by-product. Although organometallic complexes, heteropolyacids, organic molecules, and their immobilized materials can act as effective catalysts for the epoxidation of enones under neutral conditions,¹³ utilization of homogeneous reaction systems with H₂O₂ under alkaline conditions (NaOH, KOH, LiOH, Na₂CO₃, or K₂CO₃) is the most common procedure. To replace these standard homogeneous systems with more practical heterogeneous alternatives, a number of solid bases such as Mg–Al hydrotalcite, KF/alumina, hydroxyapatite, or sodalite catalysts have been developed.¹⁴

We have previously developed a Ni–Zn mixed basic salt (NiZn) as a solid catalyst or catalyst support based on the design idea of “Intercalation Catalysts”.¹⁵ Ni_1−xZn_2x(OCOCH₃)_2x(OH)₂·nH₂O (0.15 < x < 0.25; NiZn), classified as a layered HDS, consists of positively charged layers and exchangeable interlayer anions. Our selection of NiZn as a catalyst material is motivated by its unique characteristics such as (i) simple preparation, (ii) high crystallinity, (iii) strong electrostatic interactions between guest anions and Zn²⁺ cations, and (iv) high anion exchange capacity, as well as (v) the ability to fine-tune the interlayer space by modifying the size of the guest anion. Recently, interlayer space-controlled NiZn with alkyl carboxylate anions intercalated into the interlayer has been found to be an effective heterogeneous Brønsted base catalyst for Knoevenagel condensation in water.^15c Here, we report the successful epoxidation of cyclic enones using hydrogen peroxide in the presence of NiZn as a basic catalyst.

Results and discussion

In accordance with the previously reported literature,¹⁶ the acetate-intercalated Ni–Zn mixed basic salt, CH₃COO⁻/NiZn, Ni_0.63Zn_0.37(OAc)_0.37(OH)₂·1.93H₂O (Ni/Zn = 2.20), was synthesized.¹⁷ Anion exchange in the NiZn interlayer was performed using the same procedure in water. Treatment of CH₃COO⁻/NiZn with various sodium alkyl carboxylates yielded anion-exchanged NiZn catalysts (C_n−1H_2n−1COO⁻/NiZn, n = 1–10) as green powders.

The powder X-ray diffraction (XRD) profiles of the synthesised NiZn catalysts are shown in Fig. 1. From the XRD results, the layered structure of NiZn was maintained in each case; however, the calculated clearance spaces of each material (C.S. = basal spacing (d₀₀₁) – thickness of layer (0.46 nm)) were changed after the intercalation procedure, also shown in Fig. 1. The C.S. of each was proportional to the length of the alkyl groups intercalated (slope: 0.17 nm per CH₂, Fig. 2). Taking the C.S. and the size of the pillared alkyl carboxylate anions into account, a suitable conformation of C_n−1H_2n−1COO⁻/NiZn can be a bilayer arrangement with a tilting angle of approximately 40 degrees.

Fig. 1 XRD profiles of (a) CH₃COO⁻/NiZn, (b) HCOO⁻/NiZn, (c) C₃H₇COO⁻/NiZn, (d) recovered C₃H₇COO⁻/NiZn after epoxidation of 2-cyclohexen-1-one (1), (e) C₅H₁₁COO⁻/NiZn, (f) C₇H₁₅COO⁻/NiZn, and (g) C₉H₁₉COO⁻/NiZn. The C.S. values were also given.

Fig. 2 The relation between C.S. and carbon number of alkyl groups in the carboxylate anion of C_n−1H_2n−1COO⁻/NiZn catalysts.

To explore the catalytic abilities of the anion-exchanged NiZn, the epoxidation of 2-cyclohexen-1-one (1) into 1,2-epoxycyclohexanone (2) was carried out in the presence of 4 equivalents of aqueous hydrogen peroxide (30 wt%) relative to the amount of 1. The results and the C.S. values of the C_n−1H_2n−1COO⁻/NiZn catalysts are shown in Table 1.

Table 1 Epoxidation of 2-cyclohexen-1-one (1) with H₂O₂^a

Entry	Catalyst	C.S.^b (nm)	Time (h)	Conv.^c (%)	Yield^c (%)
a 1 (0.5 mmol), NiZn catalyst (0.05 g), DMF (2 mL), 30 wt% H2O2 aq. (4 eq. relative to the substrate), 60 °C. b Calculated from XRD data. c Determined by GC using an internal standard technique. d Ni catalyst (Ni: 0.26 mmol). e Zn catalyst (Zn: 0.13 mmol). f Hydrotalcite (0.05 g), purchased from WAKO, was used as a catalyst. g C3H7COONa (1.47 mmol) was used as a catalyst.
1	HCOO^−/NiZn	0.45	1	7	6
2	CH3COO^−/NiZn	0.84	1	45	45
3	C3H7COO^−/NiZn	1.06	1	55	55
4	C5H11COO^−/NiZn	1.49	1	28	28
5	C7H15COO^−/NiZn	1.74	1	24	22
6	C9H19COO^−/NiZn	2.11	1	19	16
7^d	Ni(OCOCH3)2·4H2O	—	3	trace	trace
8^e	Zn(OCOCH3)2·2H2O	—	3	trace	trace
9^d	Ni(OH)2	—	3	trace	trace
10^e	Zn(OH)2	—	3	trace	trace
11^f	Mg6Al2(CO3)(OH)16·4H2O	0.30	3	18	18
12^g	C3H7COONa	—	3	35	35
13	Blank	—	3	no reaction

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In the case of HCOO⁻/NiZn, the catalytic activity was low (entry 1), most likely due to the mass transfer limitation within the narrow interlayer space. The parent CH₃COO⁻/NiZn catalysed this epoxidation efficiently, yielding 45% of 2 after 1 h (entry 2). It should be noted that the C₃H₇COO⁻/NiZn catalyst exhibited the highest activity, yielding only 2 in 55% yield after 1 h (entry 3).¹⁸ On the other hand, the catalysts which have more enlarged interlayer spaces such as C₅H₁₁COO⁻/NiZn, C₇H₁₅COO⁻/NiZn, and C₉H₁₉COO⁻/NiZn did not promote this epoxidation reaction effectively (entries 4–6). It is likely that the substrate could not diffuse smoothly into the interlayer space of these catalysts because these spaces are occupied by the alkyl chains of the guest anions. Ni(OCOCH₃)₂·4H₂O, Zn(OCOCH₃)₂·2H₂O, Ni(OH)₂, and Zn(OH)₂, the precursors of NiZn and some of the components of the brucite-like layer of NiZn, did not show any significant catalytic activity under the test reaction conditions (entries 7–10). It is well known that Mg–Al hydrotalcite can heterogeneously catalyse the epoxidation of electron-deficient olefins with H₂O₂.^14a–c However, the reaction rate over the Mg₆Al₂(CO₃)(OH)₁₆·4H₂O catalyst was slow under the same reaction conditions (entry 11). When C₃H₇COONa (1.47 mmol) was used as a catalyst under the same reaction conditions, only 35% yield of 2 was obtained after 3 h (entry 12). The epoxidation did not proceed at all without the catalyst (entry 13).

The screening of solvents to determine their effect on the epoxidation of 1 over the C₃H₇COO⁻/NiZn catalyst at 50 °C showed that N,N-dimethylformamide (DMF) was superior to the other solvents tested, giving only 2 as a product in 83% yield within 3 h (Table 2, entry 1), whereas N,N-dimethylacetamide (DMA), acetonitrile, methanol, n-hexane, 1,2-dichloroethane, toluene, and water were not effective (entries 2, 4–5, and 7–10). When a combined oxidant of benzonitrile, acetonitrile, or acetamide and hydrogen peroxide was used in the presence of the C₃H₇COO⁻/NiZn catalyst, the enone epoxidation did not proceed effectively (entries 3, 4 and 6). The fact that no benzamide or acetamide was formed in nitrile solvents indicates that the active peroxy intermediates from nitrile or amide, which were used as oxidants, were not involved in this catalytic system.¹⁹

Table 2 Effect of solvent on the epoxidation of 1 with H₂O₂ catalysed by C₃H₇COO⁻/NiZn^a

Entry	Solvent	Conv.^b (%)	Yield^b (%)
a 1 (0.5 mmol), C3H7COO^−/NiZn catalyst (0.05 g), solvent (4 mL), 30 wt% H2O2 aq. (2 eq. relative to the substrate), air, 50 °C, 3 h. b Determined by GC using an internal standard technique. c With acetamide (2 mmol).
1	N,N-Dimethylformamide	83	83
2	N,N-Dimethylacetamide	39	39
3	Benzonitrile	7	5
4	Acetonitrile	57	57
5	Methanol	38	38
6^c	Methanol	42	38
7	n-Hexane	39	34
8	1,2-Dichloroethane	3	1
9	Toluene	9	8
10	Water	68	68

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In order to further demonstrate the requirement of the heterogeneous C₃H₇COO⁻/NiZn catalyst, the catalyst was removed by filtration when epoxidation of 1 had reached almost 70% conversion (Fig. 3). After removal of the catalyst, the reaction was monitored for an additional hour, and no additional formation of 2 was observed. These results demonstrate that oxidation proceeds in the NiZn interlayer and that any dissolved anionic species are not involved in this reaction. After the epoxidation was completed, it was possible to easily separate the C₃H₇COO⁻/NiZn catalyst from the reaction mixture by simple centrifugation (Fig. 4). From the XRD profile of the recovered C₃H₇COO⁻/NiZn catalyst, it was found that the crystallinity of NiZn had decreased and its C.S. expanded from 1.06 nm to 1.36 nm (Fig. 1d) due to the intercalation of water molecules and/or DMF solvent during the reaction.²⁰

Fig. 3 Hot filtration experiment. Reaction conditions were as follows: 1 (0.5 mmol), C₃H₇COO⁻/NiZn catalyst (0.1 g), DMF (2 mL), 30 wt% H₂O₂ aq. (4 eq. relative to the substrate), 60 °C.

Fig. 4 The photograph of the C₃H₇COO⁻/NiZn catalyst (a) in the reaction mixture during catalytic epoxidation of 1 and (b) after phase separation by centrifugation.

The epoxidation of various cyclic enones using the C₃H₇COO⁻/NiZn catalyst was carried out in DMF at 60 °C (Table 3). 2-Cyclopenten-1-one and 1 were successfully oxidised into their corresponding epoxyketones within 3 and 6 h, respectively (entries 1 and 3). The products expected from an aldol-type condensation were scarcely observed under the reaction conditions when performed over the C₃H₇COO⁻/NiZn catalyst. The recovered C₃H₇COO⁻/NiZn catalyst was found to be reusable without any loss of its activity and selectivity at least four times (Table 1, entry 3 and Table 3, entries 4–7). After the 4th recycling experiment (Table 3, entry 7), any nickel and zinc species and butylate anion leached into the reaction mixture were in amounts small enough to be undetectable by atomic absorption spectroscopy and ion chromatography. The reactivity is affected by various factors such as the orbital energy in LUMO of substrates, contact probability, the formation rate of HOO⁻, inhibition of side reaction, and so on. Especially, the reactivity of enones can be explained by using the orbital energies in LUMO of the substrates.²¹ Because the nucleophilic attack of HOO⁻ to the substrate is known as the rate-determining step,²² LUMO energies of substrates are critical to explain the difficulty of the epoxidation of enones. In the case of the epoxidation of β-substituted cyclic enones,²³ such as 3-methyl-2-cyclopenten-1-one (LUMO energy: −0.128 eV) and 3-methyl-2-cyclohexen-1-one (−0.121 eV), the reaction rates were decreased (entries 2 and 8). The lower reactivity of these compounds compared with the non-substituted ones (1: −0.116 eV, 2-cyclopenten-1-one: −0.126 eV) might be explained by the steric hindrance about the C=C double bond of substrates. The epoxidation of α-isophorone hardly occurs (entry 11) due to the steric hindrance of the methyl group at the β-position and the gem-dimethyl group.²⁴

Table 3 Substrate scope for C₃H₇COO⁻/NiZn-catalysed epoxidation with H₂O₂^a

Entry	Substrate	Product	Time (h)	Conv.^b (%)	Yield^b (%)
a Substrate (0.5 mmol), C3H7COO^−/NiZn catalyst (0.05 g), DMF (2 mL), 30 wt% H2O2 aq. (4 eq. relative to the substrate), 60 °C, air. b Determined by GC using an internal standard technique. c Value in parenthesis is isolated yield. d 1st reuse. e 2nd reuse. f 3rd reuse. g 4th reuse. h Methanol (2 mL) was used as a solvent.
1			3	97	97
2			12	62	62
3			6	98	98 (91)^c
4^d			1	55	55
5^e			1	56	55
6^f			1	54	54
7^g			1	53	52
8			12	70	70
9^h			5	97	96
10			9	16	11
11			24	5	n.d
12			24	13	10
13			24	20	20

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The β,β-disubstituted acyclic enone, (R)-(+)-pulegone, with a high LUMO energy of 0.273 eV, was not transformed into an epoxyketone, even after 24 h. In contrast, the C₃H₇COO⁻/NiZn-catalysed epoxidation of 1,4-naphthoquinone, which has a low LUMO orbital energy (−1.529 eV), totally proceeded, and the corresponding product was formed almost quantitatively (entry 9). The high reactivity of the substrates possessing low LUMO orbital energies supports the hypothesis that the catalytic epoxidation process occurs via the nucleophilic attack of a perhydroxyl species. Conversely, 3-nonen-2-one with a high LUMO orbital energy²⁵ gave an extremely low yield of the corresponding epoxyketone (entry 10). In addition, the epoxidation of bulky substrates such as benzalacetone and chalcone resulted in only 10% and 20% yields of 2,3-epoxy-1,3-diphenylpropanone and 2,3-epoxy-1,3-diphenyl propanone, respectively (entries 12 and 13).

Moreover, the C₃H₇COO⁻/NiZn-catalyzed epoxidation of 1 with an equimolar amount of H₂O₂ proceeded in a highly effective manner,²⁶ giving 97% of 2 at 60 °C in 8 h, with 97% efficiency of H₂O₂ utilization, as highlighted in Scheme 1.²⁷

Scheme 1 C₃H₇COO⁻/NiZn-catalysed epoxidation of 1 with an equimolar amount of H₂O₂. Reaction conditions: 1 (0.5 mmol), C₃H₇COO⁻/NiZn catalyst (0.05 g), 30 wt% H₂O₂ aq. (0.5 mmol), DMF (2 mL), 60 °C, 8 h.

Based on these results, the most plausible mechanism for C₃H₇COO⁻/NiZn-catalysed epoxidation is as follows. A perhydroxyl anion, which is formed by the reaction between the base site of NiZn and H₂O₂, attacks the β-olefinic carbon atom of an α,β-unsaturated ketone. Following the ring enclosure, the corresponding epoxyketone is formed and anion exchange between OH⁻ and C₃H₇COO⁻ takes place.

Conclusions

We conclude that the C₃H₇COO⁻/NiZn catalytic system presented here allows simple and practical epoxidation of cyclic enones, forming fine chemicals and pharmaceuticals in a manner that meets the increasing demand for environmentally friendly chemical processes.

Acknowledgements

This study was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (25420824).

Notes and references

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17. The chemical formula of the synthesized CH₃COO⁻/NiZn was confirmed by elemental analysis, XRF, and TG-DTA. The anion exchange capacity was found to be 2.44 mmol g⁻¹..
18. In the case of simple alkenes such as cyclooctene, 1-decene, and styrene, the corresponding epoxides were not obtained at all..
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20. The molecular size of DMF and water are ca. 0.43 nm and 0.15 nm, respectively. The results of molecular modeling are shown in the ESI.†.
21. The electrophilicity of enones can be described with LUMO orbital energies, see R. J. Loncharich, F. K. Brown and K. N. Houk, J. Org. Chem., 1989, 54, 1129.
22. H. O. House and R. S. So, J. Am. Chem. Soc., 1958, 80, 2428.
23. Kaneda and co-workers calculated the LUMO orbital energy of enones, shown in ref. 14a..
24. (R)-(−)-Carvone also did not react under these reaction conditions..
25. In ref. 14a, the LUMO energies of 4-hexen-3-one were calculated to be −0.008 eV (cis) and −0.047 eV (trans)..
26. K. Kamata, K. Yonehara, Y. Sumida, K. Yamaguchi, S. Hikichi and N. Mizuno, Science, 2003, 300, 964.
27. The decomposition of H₂O₂ might be inhibited by the buffering effect of the solid NiZn catalyst..

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cy01063a

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