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Article

Practical Epoxidation of Olefins Using Air and Ubiquitous Iron-Based Fluorous Salen Complex

by
Yamato Kato
,
Miho Kanoh
,
Hina Kobayashi
,
Takayuki Shioiri
and
Masato Matsugi
*
Faculty of Agriculture, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan
*
Author to whom correspondence should be addressed.
Molecules 2024, 29(5), 966; https://doi.org/10.3390/molecules29050966
Submission received: 29 January 2024 / Revised: 15 February 2024 / Accepted: 20 February 2024 / Published: 22 February 2024
(This article belongs to the Special Issue Contemporary Research Progress in Organofluorine Chemistry)
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Abstract

:
The epoxidation of olefins by substituting “air” for potentially harmful oxidants was achieved using an oxidation method that integrated a fluorous iron(III) salen catalyst derived from common metals and pivalaldehyde. Several aromatic disubstituted olefins were converted into their corresponding epoxides with high efficiency and quantitative yields. This reaction represents an environmentally friendly oxidation process that utilizes an abundant source of air and employs a readily available metal, iron, in the form of salen complexes, making it an environmentally conscious oxidation reaction.

1. Introduction

Epoxide compounds are widely used as intermediates in pharmaceutical synthesis and functional materials [1,2]. Oxidants are required to oxidize olefins to epoxides, and air is the safest and cheapest oxidant for this purpose. To date, oxidation reactions employing molecular oxygen as an oxidant for the epoxidation of olefins, in conjunction with aldehyde reagents, have been reported [3,4,5,6,7,8,9,10,11,12,13,14,15,16].
In 1992, Kaneda et al. successfully conducted the oxygen epoxidation of olefins under oxygen-bubbling conditions using pivalaldehyde as a co-oxidant [3]. This study is significant because of its adoption of an environmentally friendly oxidation method featuring reagents with low explosiveness and no generation of byproducts from the oxidants. However, the continuous bubbling of oxygen, which is the source of oxygen, is required, making the operation cumbersome. Moreover, the use of high-concentration oxygen poses a potential risk of dust explosions. Therefore, the practical application of this method to plant-scale operations in an oxygen atmosphere is challenging. To enhance its applicability to industrial processes as an oxidant, there is a preference for utilizing the more readily available and safer “air directly” in large-scale synthesis.
Furthermore, Mukaiyama et al. presented a method for the asymmetric epoxidation of olefins using oxygen (not air) as the oxidant, combining a manganese salen complex [17,18] with pivalaldehyde [12]. However, this oxygen oxidation reaction requires the use of the rare metal manganese. Although there have been a few reports on the epoxidation of olefins using “air” as an oxidant, an efficient epoxidation reaction without using rare metals has rarely been reported.
In our previous study, we developed an asymmetric oxidation catalyst by introducing a fluorous tag at the 3,3′-position of the salen ligand. Using this complex, we achieved the asymmetric epoxidation of olefins under hypervalent iodine conditions for the first time using “iron” [19]. However, in this method, the use of oxidants, such as iodosobenzene was essential. If a reaction system using air and a common metal–iron complex can be realized, it has the potential to be applied to industrial processes as a safe and environmentally friendly synthetic method. Fluorous iron salen complexes have been suggested to have a different steric environment than normal salen complexes [19]; therefore, different and unique reactivities are expected. In this paper, we report the air epoxidation of olefins using a novel fluorous iron(III) salen complex with isoaldehyde as a co-oxidant. The reaction system using air directly as the oxidant is fundamentally different from using oxygen molecules, as it eliminates the need for the pre-preparation of oxidants. This enables the construction of an environmentally friendly and energy-efficient reaction system.

2. Results and Discussions

Two types of iron salen complexes with perfluoroalkyl groups of different lengths were synthesized based on the impact of variations in full-length fluorous-tagged iron salen complexes on catalytic activity. Fluorous iron(III) salen complex 1a with a C4F9 tag at the 3,3′-position of the ligand was prepared using our previously reported procedure [20]. Complex 1b with a C12F25 tag at the same position was also synthesized. In other words, perfluoroalkylation reactions were initiated from 5-tert-butylsalicylaldehyde 2 using V-70L [2,2′-azobis(2,4-dimethyl-4-methoxyvaleronitrile)] to obtain the perfluoroalkyl precursors 3a and 3b. Subsequently, each perfluoroalkyl precursor underwent condensation reactions with (1R,2R)-(+)-1,2-diphenylethylenediamine to form a salen ligand. The final step involved coordination exchange with iron(III) chloride to synthesize the desired fluorous iron(III) salen complexes, 1a and 1b (Scheme 1). Non-fluorous catalyst 1c was synthesized using a known synthetic method in which the central metal of the Jacobsen-type Mn salen complex [17], with the R group being the tBu group, was replaced with iron.
The epoxidation of olefins using air as the oxidant was investigated using the synthesized complexes 1a and 1b. The reaction was performed for 5 h at room temperature in acetonitrile using triphenylethylene 4a as a substrate in the presence of an iron(III) salen complex and pivalaldehyde (Table 1) [12]. The reaction with complex 1a yielded the desired epoxide 5a; however, the conversion was only 73%. Notably, the reaction with complex 1b yielded 5a quantitatively with significantly better reactivity than that with 1a. Even when the amount of 1b was reduced from 1 to 0.5 mol%, no decrease in the reactivity was observed, and the target product was successfully obtained quantitatively. However, as in a previous report [19], chirality was not induced in this reaction, and the resulting product was racemic. Even when using non-fluorous catalyst 1c under the same conditions, the reaction proceeded; however, its catalytic activity was low. These results showed that 1b is the optimal catalyst for the air oxidation reaction among these catalysts.
Next, the aldehyde structure was added as a co-oxidant in the reaction, and complex 1b was optimized (Table 2). Relatively good results were obtained when isovaleraldehyde was used; however, the yield was slightly lower than that when pivalaldehyde was used under the same conditions (Table 2, entries 1 vs. 2). When other aliphatic and alicyclic aldehydes were added, little progress was observed (Table 2, entries 3–5). Similarly, when aromatic aldehydes with nitro, methoxy, or methyl groups at the para-position, 2-naphthalaldehyde or heteroaromatic aldehydes, were used, the reaction did not proceed (Table 2, entries 6–12). These results showed that pivalaldehyde, which is used in combination with salen–manganese complexes, has been reported in the literature [12] and is the most suitable aldehyde as a co-oxidant in this reaction.
We then investigated the solvent effect on this reaction (Table 3). In addition to the successful progression observed in dry MeCN, the reaction proceeded smoothly when the protonic polar solvent, tBuOH, was used. However, at the 5-h mark, a thin-layer chromatography (TLC) test showed that the starting material had not disappeared; therefore, the starting material had to be fully consumed for 8 h (Table 3, entry 2). When dry MeOH was used, complex 1b and the substrate did not dissolve in the solvent, leading to a significant reduction in the reaction yield (Table 3, entry 3). For non-protonic polar solvents, dry DMF and dry THF were considered, but neither showed significant progress in the reaction (Table 3, entries 4 and 5). When halogenated solvents, such as dry DCM and 2,3,4,5,6-pentafluorotoluene, were used, longer reaction times were required, but high conversion rates were achieved (Table 3, entries 6 and 7). These results showed that dry MeCN was the optimal solvent for this reaction.
Next, the substrate scope of this epoxidation reaction was investigated (Table 4). Quantitatively corresponding epoxides were obtained by extending the reaction time to 6 or 7 h using disubstituted internal olefins 4b and 4c, as well as terminal olefin 4d as substrates (Table 4, entries 2–4). Interestingly, only an isomerized trans-type epoxide was produced in the reaction using cis-β-methylstyrene 4c as the substrate, and no cis-type epoxide was observed. When substrates 4e–g with electron-donating or fluorine substituents were used, the reactions proceeded smoothly and quantitatively to produce the corresponding epoxides (Table 4, entries 5–7). However, when (E)-2-styrylnaphthalene 4h was used as the substrate, a decrease in reactivity was observed, and the conversion rate remained at 52% even when the reaction time was extended to 24 h (Table 4, entry 8). When substrate 4i containing both trisubstituted and monosubstituted olefinic moieties was used, only the trisubstituted olefinic site underwent selective epoxidation with perfect regioselectivity, leading to the quantitative formation of monoxide 5i (Table 4, entry 9).
In 2008, Köckritz et al. reported the epoxidation of olefins using aldehydes as co-oxidants and highlighted the involvement of radical intermediates in the reaction [5]. Notably, in the reaction using salen iron complex 1b, isomerized trans epoxide was observed when cis-β-methylstyrene 4c was used as the substrate (Table 4, entry 3). This observation indicated that the reaction also progressed through the radical species. Therefore, we conducted a control experiment by adding 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as the radical scavenger (Scheme 2). Thus, the reaction did not proceed at all, suggesting that it also proceeds via radical species.
It is known that pivalaldehyde forms a peroxyl radical species upon interaction with oxygen molecules [4,5,6]. The corresponding trans isomer was also generated in our reaction when cis-olefin was used as the substrate. These observations show that the peroxyl radical species interacts with iron, and the reaction may proceed through an iron(III) complex of radical species, as shown in Scheme 3.

3. Conclusions

Fluorous salen complex 1b, which features iron as the central metal, was used as a catalyst to achieve the epoxidation reaction of olefins using “air” as the oxidizing agent. This reaction can be applied to various polysubstituted olefins to obtain the corresponding epoxides in high yields. This is an extremely safe reaction system that uses inexhaustible “air” as the oxidant. It is also an environmentally friendly oxidation reaction that uses iron, which is a common metal. In future studies, we aim to expand the catalytic reaction system to recycle and reuse fluorous salen complexes.

4. Experimental Section

4.1. Materials and Reagents

All the laboratory chemicals were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan), FUJIFILM Wako Pure Chemical Corporation (Richmond, VA, USA), Sigma-Aldrich Co., LLC (St. Louis, MO, USA), and Kanto Chemical Co., Inc. (Tokyo, Japan) and used without further purification unless otherwise stated. Solvents were removed using rotary evaporation under reduced pressure using a water bath at 50 °C. Nonvolatile compounds were dried in vacuo at 0.01 mbar. All reactions were stirred magnetically and monitored using thin-layer chromatography using silica gel plates. Purification by chromatography was performed on silica gel 60 N (spherical, neutral, 63–210 µm, Kanto Chemical Co., Inc.). Fluorous solid phase extraction (FSPE) was performed on FluoroFlash® SILICA (40 µm, 60 A, Sigma-Aldrich, USA).

4.2. Analytical Instruments

Melting points were determined in ATM-01 of AS ONE Corporation and are uncorrected. Nuclear magnetic resonance (NMR) spectra were recorded using JNM-ECZ400S (1H: 400 MHz, 13C: 101 MHz, 19F: 376 MHz) spectrometers. 1H NMR spectral data are reported as follows: chemical shift in ppm referenced to TMS (δ 0.00 ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet or unresolved, dd = double doublet, br s = broad signal), coupling constant (J: units in Hz), and integration. 13C NMR spectral data are reported as chemical shifts in ppm referenced to residual solvent (CDCl3 δ 77.16 ppm). 19F NMR spectral data are reported as follows: chemical shift (uncorrected), integral. High-performance liquid chromatography (HPLC) analyses were performed using DAICEL CHIRALCEL OD-3 column with UV Detector SPD-20A. High-resolution mass spectra were obtained using ExactiveTM Plus Orbitrap (Thermo Fisher Scientific, Waltham, MA, USA). The spectra were calibrated with PierceTM LTQ Velos ESI Positive Ion Calibration Solution before data acquisition.

4.3. General Procedure for the Epoxidation of Olefins Using Air

Under air atmosphere (vide Appendix A), pivalaldehyde (109 µL, 1.0 mmol, 5.0 eq.) was added to a suspension of olefin (0.2 mmol, 1.0 eq.) and Fe(III) salen complex (1.0 μmol, 0.5 mol%) in dry-MeCN (2 mL). The mixture was stirred at room temperature. After completion, the reaction mixture was diluted with ethyl acetate, and washed with saturated sodium bicarbonate aq. and brine. The organic layer was dried over Na2SO4, filtered, and concentrated under reduced pressure. The conversion was determined by 1H NMR spectra of the crude product. The ee of the product was determined by HPLC analysis of the epoxides.

4.4. Synthesis

5-(tert-Butyl)-2-hydroxybenzaldehyde (2) [21]. Paraformaldehyde (514.8 mg, 17.2 mmol, 3.0 eq.) was added to a suspension of 4-(tert-butyl)phenol (858.9 mg, 5.72 mmol, 1.0 eq.), Et3N (4.0 mL, 28.6 mmol, 5.0 eq.) and MgCl2 (1635.0 mg, 17.2 mmol, 3.0 eq.) in MeCN (8.5 mL) at 65 °C. The mixture was stirred for 1.5 h. The resulting reaction mixture was cooled to room temperature and quenched with 1 M HCl aq. After the addition of ethyl acetate, the organic layer was separated and the aqueous layer was further extracted with ethyl acetate. The combined organics were washed with brine (×3), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/EtOAc = 15:1) to give the desired product 2 (609.4 mg, 60%). Pale yellow oil; Rf 0.41 (hexane/EtOAc = 15:1); 1H NMR (400 MHz, CDCl3): δ 10.86 (s, 1H), 9.89 (s, 1H), 7.59 (dd, J = 8.8, 2.0 Hz, 1H), 7.51 (d, J = 2.8 Hz, 1H), 6.94 (d, J = 8.8 Hz, 1H), 1.33 (s, 9H).
5-(tert-Butyl)-2-hydroxy-3-(perfluorobutyl)benzaldehyde (3a) [19]. Perfluorobutyl iodide (861 µL, 5.0 mmol, 5.0 eq.) and V-70L (17.8 mg, 0.058 mmol, 6 mol%) were added to a suspension of salicylaldehyde 2 (178.2 mg, 1.0 mmol, 1.0 eq.) and cesium carbonate (1310.5 mg, 4.0 mmol, 4.0 eq.) in DMF (1 mL). The mixture was stirred for 30 min at 80 °C in oil bath. The mixture was quenched with 1 M HCl aq. After the addition of Et2O, the organic phase was separated and the aqueous phase was extracted with Et2O (×2). The combined organic phase was washed with brine (×3), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/EtOAc = 10:1, 2 times) to give the desired product 3a (126.8 mg, 32%). Yellow solid; mp 38 °C; Rf 0.48 (hexane/EtOAc = 10:1); 1H NMR (400 MHz, CDCl3): δ 11.64 (s, 1H), 9.95 (s, 1H), 7.75 (dd, J = 6.0, 2.4 Hz, 2H), 1.36 (s, 9H); 19F NMR (376 MHz, CDCl3): δ −80.8 (3F), −108.9 (2F), −122.3 (2F), −125.8 (2F).
5-(tert-Butyl)-2-hydroxy-3-(perfluorododecyl)benzaldehyde (3b). V-70L (21.6 mg, 0.07 mmol, 15 mol%) was added to a suspension of salicylaldehyde 2 (83.1 mg, 0.5 mmol, 1.0 eq.), perfluorododecyl iodide (1042.0 mg, 1.4 mmol, 3.0 eq.), and cesium carbonate (608.0 mg, 1.9 mmol, 4.0 eq.) in DMF (3.8 mL). The mixture was stirred for 2.5 h at 80 °C in oil bath. The mixture was quenched with 1 M HCl aq. After the addition of Et2O, the organic phase was separated and the aqueous phase was extracted with Et2O (×2). The combined organic phase was washed with brine (×3), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was filtered over a short pad of silica gel (hexane only to hexane/EtOAc = 10:1) and purified by fluorous solid phase extraction (FSPE) using 40% aq. THF (THF/H2O = 60:40) as the fluorophobic solvent and THF as the fluorophilic solvent. Concentration of the THF fraction under reduced pressure gave a residue that was contaminated with BHT, which was removed by silica gel column chromatography (hexane/EtOAc = 30:1) to give the desired product 3b (186.3 mg, 50%). White solid; mp 101–102 °C; Rf 0.47 (hexane/EtOAc = 10:1); 1H NMR (400 MHz, CDCl3): δ 11.65 (s, 1H), 9.95 (s, 1H), 7.76 (dd, J = 6.4, 2.0 Hz, 2H), 1.36 (s, 9H); 13C NMR (101 MHz, CDCl3): δ 196.7, 158.7, 142.7, 134.7, 133.92, 133.85, 133.77, 121.1, 118.8–107.5 (m, C12F25 tag), 34.4, 31.2; 19F NMR (376 MHz, CDCl3): δ −80.6 (3F), −108.7 (2F), −121.3 to −121.7 (16F), −122.5 (2F), −125.9 (2F); HRMS-DART (m/z): [M + H]+ calcd for C23H14F25O2+, 797.0589; found, 797.0593.
Fluorous iron(III) salen complex with C4F9 tag (1a) [19]. Under N2 atmosphere, (1R,2R)-(+)-1,2-diphenylethylenediamine (53.5 mg, 0.25 mmol, 0.5 eq.) was added to a suspension of 3a (197.7 mg, 0.50 mmol, 1.0 eq.) and 3 A MS in dry-EtOH (4 mL). The mixture was stirred at reflux in oil bath for 22 h; 3 A MS was filtered over a pad of Celite®. The filtrate was concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/EtOAc = 10:1) to give the ligand as a yellow solid. The ligand was immediately used for the next ligand exchange. Under N2 atmosphere, FeCl3 (85.1 mg, 0.52 mmol, 1.1 eq.) was added to a solution of ligand in dry-EtOH (4 mL). The resulting mixture was stirred at reflux in oil bath for 1 h. After the addition of EtOAc, the organic layer was washed with brine (×3), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/EtOAc = 10:1 to 1:1) to give the fluorous iron(III) salen complex 1a (138.0 mg, 52%). Reddish brown solid; mp 130 °C; Rf 0.40 (CHCl3/MeOH = 20:1); HRMS-DART (m/z): [M + H]+ calcd for C44H37ClF18FeN2O2+, 1058.1600; found, 1058.1592.
Fluorous iron(III) salen complex with C12F25 tag (1b). Under N2 atmosphere, (1R,2R)-(+)-1,2-diphenylethylenediamine (29.5 mg, 0.14 mmol, 0.6 eq.) was added to a suspension of 3b (183.6 mg, 0.23 mmol, 1.0 eq.) and 3 A MS in dry-EtOH (4.6 mL). The mixture was stirred at reflux in an oil bath for 30 min; 3 A MS was filtered over a pad of Celite®. The filtrate was concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/EtOAc = 10:1) to give the ligand as a yellow solid. Under N2 atmosphere, FeCl3 (37.1 mg, 0.23 mmol, 1.0 eq.) was added to a solution of ligand in dry-EtOH (4.6 mL). The resulting mixture was stirred at reflux in oil bath for 30 min. The resulting mixture was concentrated under reduced pressure. After the addition of EtOAc, the organic layer was washed with brine (×3), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/EtOAc = 10:1 to 1:1) to give the fluorous iron(III) salen complex 1b (97.6 mg, 46%). Reddish brown solid; mp 196 °C; Rf 0.24 (CHCl3/MeOH = 20:1); HRMS-DART (m/z): [M + H]+ calcd for C60H37ClF50FeN2O2+, 1858.1089; found, 1858.1087.
Ligand of 1b. Yellow solid; mp 70–72 °C; Rf 0.38 (hexane/EtOAc = 10:1); 1H NMR (400 MHz, CDCl3): δ 8.42 (s, 2H), 7.46 (d, J = 2.4 Hz, 2H), 7.32 (d, J = 2.4 Hz, 2H), 7.24–7.13 (m, 10H), 4.73 (s, 2H), 1.22 (s, 18H); 13C NMR (101 MHz, CDCl3): δ 166.5, 158.2, 141.2, 138.6, 132.9, 128.7, 128.0, 119.1, 80.3, 34.1, 31.1; 19F NMR (376 MHz, CDCl3): δ −80.7 (6F), −108.5 (4F), −121.3 to −121.8 (32F), −122.6 (4F), −126.0 (4F); HRMS-DART (m/z): [M + H]+ calcd for C60H39F50N2O2+, 1769.2208; found, 1769.2208.
Jacobsen-type iron(III) salen complex (1c) [19]. Under N2 atmosphere, (1R,2R)-(+)-1,2-diphenylethylenediamine (127.2 mg, 0.6 mmol, 0.6 eq.) was added to a suspension of 3,5-di-tert-butyl-2-hydroxybenzaldehyde (234.5 mg, 1.0 mmol, 1.0 eq.) and 3 A MS in dry-EtOH (20 mL). The mixture was stirred at reflux in an oil bath for 1 h; 3 A MS was filtered over a pad of Celite®. The filtrate was concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/EtOAc = 10:1) to give the ligand as a yellow solid (286.6 mg). Under N2 atmosphere, FeCl3 (162.6 mg, 1.0 mmol, 1.0 eq.) was added to a solution of ligand in dry-EtOH (20 mL). The resulting mixture was stirred at reflux in oil bath for 1 h. The resulting mixture was concentrated under reduced pressure. After the addition of EtOAc, the organic layer was washed with brine (×3), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/EtOAc = 10:1 to 1:1) to give the Jacobsen-type iron(III) salen complex 1c (242.8 mg, 33%). Reddish brown solid; mp 183–184 °C; Rf 0.39 (CHCl3/MeOH = 20:1); HRMS-DART (m/z): [M + H]+ calcd for C44H55ClFeN2O2+, 734.3296; found, 734.3296.
Ligand of 1c [22]. Yellow solid; mp 199–200 °C; Rf 0.50 (hexane/EtOAc = 10:1); 1H NMR (400 MHz, CDCl3): δ 8.39 (s, 2H), 7.31–7.30 (m, 2H), 7.22–7.14 (m, 10H), 6.98–6.97 (m, 2H), 4.72 (s, 2H), 1.423–1.418 (m, 18H), 1.224–1.218 (m, 18H).
2,2,3-Triphenyloxirane (5a) [23]. Following the general procedure, olefin 4a was converted to epoxide. The conversion was determined by integrating the signals of the olefin (6.97 ppm) and the epoxide (4.33 ppm). The crude product was subsequently purified through silica gel column chromatography (hexane/EtOAc = 20:1), resulting in the isolation of epoxide 5a (95% yield). White solid; mp 70–72 °C; Rf 0.43 (hexane/EtOAc = 20:1); 1H NMR (400 MHz, CDCl3): δ 7.39–7.29 (m, 5H), 7.20 (s, 5H), 7.15–7.13 (m, 3H), 7.05–7.02 (m, 2H), 4.33 (s, 1H).
(E)-1-Methyl-4-styrylbenzene (4e) [24]. Under N2 atmosphere, diethyl benzylphosphonate (1.6 mL, 7.5 mmol, 5.0 eq.) in dry-Et2O (3.0 mL) was added to a suspension of sodium tert-butoxide (721.5 mg, 7.5 mmol, 5.0 eq.) in dry-Et2O (5.0 mL). The reaction mixture was stirred at room temperature for 15 min. 4-Methylbenzaldehyde (177 µL, 1.5 mmol, 1.0 eq.) was added to the reaction mixture, and stirred for 30 min. The mixture was quenched with 1 M HCl aq. The reaction mixture was diluted with EtOAc, the organic phase was separated and the aqueous phase was extracted with EtOAc. The combined organic phase was washed with brine (×3), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/EtOAc = 30:1) to give the desired product 4e (231.1 mg, 79%). White solid; mp 114 °C; Rf 0.59 (hexane/EtOAc = 10:1); 1H NMR (400 MHz, CDCl3): δ 7.52–7.49 (m, 2H), 7.42 (d, J = 8.4 Hz, 2H), 7.35 (t, J = 7.2 Hz, 2H), 7.26–7.22 (m, 1H), 7.17 (d, J = 7.6 Hz, 2H), 7.08 (d, J = 2.4 Hz, 2H), 2.36 (s, 3H).
(E)-1-Methoxy-4-styrylbenzene (4f) [24]. Diethyl benzylphosphonate (1.1 mL, 5.5 mmol, 5.0 eq.) in dry-Et2O (3.0 mL) was added to a suspension of sodium tert-butoxide (528.0 mg, 5.5 mmol, 5.0 eq.) in dry-Et2O (3.0 mL). The reaction mixture was stirred at 50 °C for 10 min. 4-Methoxybenzaldehyde (133 µL, 1.1 mmol, 1.0 eq.) was added to the reaction mixture, and stirred for 30 min. The mixture was quenched with 1 M HCl aq. The reaction mixture was diluted with Et2O, the organic phase was separated and the aqueous phase was extracted with Et2O. The combined organic phase was washed with brine (×3), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane only) to give the desired product 4f (178.1 mg, 77%). White solid; mp 130 °C; Rf 0.31 (hexane/EtOAc = 20:1); 1H NMR (400 MHz, CDCl3): δ 7.50–7.44 (m, 4H), 7.34 (t, J = 7.2 Hz, 2H), 7.26–7.21 (m, 2H), 7.07 (d, J = 16.4 Hz, 1H), 6.97 (d, J = 16.4 Hz, 1H), 6.90 (dt, J = 8.8, 3.2 Hz, 2H), 3.83 (s, 3H).
(E)-1-Fluoro-4-styrylbenzene (4g) [25]. Diethyl benzylphosphonate (1.6 mL, 7.5 mmol, 5.0 eq.) in dry-Et2O (3.0 mL) was added to a suspension of sodium tert-butoxide (723.0 mg, 7.5 mmol, 5.0 eq.) in dry-Et2O (5.0 mL). The reaction mixture was stirred at room temperature for 30 min. 4-Fluorobenzaldehyde (158 µL, 1.5 mmol, 1.0 eq.) was added to the reaction mixture, and stirred for 30 min. The mixture was quenched with 1 M HCl aq. The reaction mixture was diluted with EtOAc, the organic phase was separated and the aqueous phase was extracted with EtOAc. The combined organic phase was washed with brine (×3), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/EtOAc = 10:1) to give the desired product 4g (239.8 mg, 81%). White solid; mp 94–95 °C; Rf 0.78 (hexane/EtOAc = 10:1); 1H NMR (400 MHz, CDCl3): δ 7.50–7.46 (m, 4H), 7.36 (t, J = 7.6 Hz, 2H), 7.28–7.24 (m, 1H), 7.12–7.00 (m, 4H); 19F NMR (376 MHz, CDCl3): δ −114.2 (1F).
(E)-2-Styrylnaphthalene (4h) [26]. Diethyl benzylphosphonate (1.6 mL, 7.5 mmol, 5.0 eq.) in dry-Et2O (3.0 mL) was added to a suspension of sodium tert-butoxide (724.7 mg, 7.5 mmol, 5.0 eq.) in dry-Et2O (5.0 mL). The reaction mixture was stirred at room temperature for 15 min. 2-Naphthaldehyde (234.6 mg, 1.5 mmol, 1.0 eq.) was added to the reaction mixture, and stirred for 30 min. The mixture was quenched with 1 M HCl aq. The reaction mixture was diluted with EtOAc, the organic phase was separated and the aqueous phase was extracted with EtOAc. The combined organic phase was washed with brine (×3), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane EtOAc = 30:1) to give the desired product 4h (228.0 mg, 66%). White solid; mp 147 °C; Rf 0.66 (hexane/EtOAc = 30:1); 1H NMR (400 MHz, CDCl3): δ 7.86–7.81 (m, 4H), 7.75 (d, J = 8.8 Hz, 1H), 7.57 (d, J = 8.4 Hz, 2H), 7.50–7.43 (m, 2H), 7.39 (t, J = 8.0 Hz, 2H), 7.31–7.22 (m, 3H).
3-(Methoxymethoxy)-3,7-dimethylocta-1,6-diene (4i). Under N2 atmosphere, MOMCl (456 µL, 6.0 mmol, 2.0 eq.) was added to a solution of linalool (467.2 mg, 3.0 mmol, 1.0 eq.), DIPEA (1.55 mL, 9.0 mmol, 3.0 eq.), and DMAP (10.3 mg, 0.084 mmol, 2.7 mol%) in dry-DCM (10 mL). The mixture was stirred at room temperature. After completion, the reaction mixture was quenched with 10% citric acid aq. After the addition of DCM, the organic layer was separated, and the aqueous layer was further extracted with DCM. The combined organics were dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/EtOAc = 2:1) to give the desired product 4i (540.9 mg, 91%). Colorless oil; Rf 0.73 (hexane/EtOAc = 5:1); 1H NMR (400 MHz, CDCl3): δ 5.87–5.80 (m, 1H), 5.181–5.178 (m, 1H), 5.15–5.14 (m, 1H), 5.12–5.08 (m, 1H), 4.69 (d, J = 6.8 Hz, 1H), 4.63 (d, J = 7.2 Hz, 1H), 3.37 (s, 3H), 2.04–1.98 (m, 2H), 1.68 (br s, 3H), 1.60 (br s, 3H), 1.59–1.55 (m, 2H), 1.32 (s, 3H); 13C NMR (101 MHz, CDCl3): δ 143.0, 131.6, 124.6, 114.6, 91.8, 78.7, 55.4, 41.1, 25.8, 23.1, 22.6, 17.8; HRMS-DART (m/z): [M + H]+ calcd for C12H23O2+, 199.1693; found, 199.1692.
trans-Stilbene epoxide (5b) [27]. Following the general procedure, olefin 4b was converted to epoxide. The conversion was calculated using the integral of the olefin signal (7.11 ppm) and the epoxide signal (3.86 ppm); conv. 100%. The obtained crude product was purified by silica gel column chromatography (hexane/EtOAc = 20:1) to give the epoxide 5b. White solid; mp 66 °C; Rf 0.51 (hexane/EtOAc = 20:1); 1H NMR (400 MHz, CDCl3): δ 7.40–7.30 (m, 10H), 3.86 (s, 2H).
trans-β-Methylstyrene epoxide (5c) [28]. Following the general procedure, cis-type olefin 4c was converted to epoxide. The conversion was calculated using the integral of the olefin signal (5.79 ppm) and the epoxide signal (3.04 ppm); trans-type epoxide only, conv. 100%. The obtained crude product was purified by silica gel column chromatography (hexane/EtOAc = 20:1) to give the epoxide 5c. Colorless oil; Rf 0.33 (hexane/EtOAc = 20:1); 1H NMR (400 MHz, CDCl3): δ 7.36–7.25 (m, 5H), 3.57 (d, J = 2.0 Hz, 1H), 3.04 (dq, J = 5.2, 2.4 Hz, 1H), 1.45 (d, J = 5.2 Hz, 3H).
α-Methylstyrene epoxide (5d) [27]. Following the general procedure, olefin 4d was converted to epoxide. The conversion was calculated using the integral of the olefin signal (5.28 ppm) and the epoxide signal (2.80 ppm); conv. 100%. The obtained crude product was purified by silica gel column chromatography (hexane/EtOAc = 20:1) to give the epoxide 5d. Colorless oil; Rf 0.26 (hexane/EtOAc = 20:1); 1H NMR (400 MHz, CDCl3): δ 7.39–7.25 (m, 5H), 2.98 (d, J = 5.2 Hz, 1H), 2.80 (dd, J = 5.6, 0.8 Hz, 1H), 1.72 (s, 3H).
2-Phenyl-3-(p-tolyl)oxirane (5e) [28]. Following the general procedure, olefin 4e was converted to epoxide. The conversion was calculated using the integral of the olefin signal (7.07 ppm) and the epoxide signal (3.83 ppm); conv. 100%. The obtained crude product was purified by silica gel column chromatography (hexane/EtOAc = 20:1) to give the epoxide 5e. White solid; mp 56 °C; Rf 0.35 (hexane/EtOAc = 20:1); 1H NMR (400 MHz, CDCl3): δ 7.40–7.31 (m, 5H), 7.26–7.23 (m, 2H), 7.19 (d, J = 8.0 Hz, 2H), 3.86 (d, J = 2.0 Hz, 1H), 3.83 (d, J = 1.6 Hz, 1H), 2.37 (s, 3H).
2-(4-Methoxyphenyl)-3-phenyloxirane (5f) [27]. Following the general procedure, olefin 4f was converted to epoxide. The conversion was calculated using the integral of the olefin signal (7.07 ppm) and the epoxide signal (3.85 ppm); conv. 100%. The obtained crude product was purified by silica gel column chromatography (hexane/EtOAc = 10:1) to give the epoxide 5f. White solid; mp 77–78 °C; Rf 0.37 (hexane/EtOAc = 10:1); 1H NMR (400 MHz, CDCl3): δ 7.39–7.24 (m, 7H), 6.91 (dt, J = 2.0, 8.8 Hz, 2H), 3.85 (d, J = 2.0 Hz, 1H), 3.81 (m, 4H).
2-(4-Fluorophenyl)-3-phenyloxirane (5g) [28]. Following the general procedure, olefin 4g was converted to epoxide. The conversion was evaluated using the substrate-derived signal (7.51–7.46 ppm) and the epoxide signal (3.83 ppm); conv. 100%. The obtained crude product was purified by silica gel column chromatography (hexane/EtOAc = 30:1) to give the epoxide 5g. White solid; mp 73 °C; Rf 0.38 (hexane/EtOAc = 20:1); 1H NMR (400 MHz, CDCl3): δ 7.41–7.29 (m, 7H), 7.10–7.04 (m, 2H), 3.85 (d, J = 1.6 Hz, 1H), 3.83 (d, J = 1.6 Hz, 1H); 19F NMR (376 MHz, CDCl3): δ −113.5 (1F).
2-(Naphthalen-2-yl)-3-phenyloxirane (5h) [29]. Following the general procedure, olefin 4h was converted to epoxide. The conversion was calculated using the substrate-derived signal (7.57 ppm) and the epoxide signal (3.97 ppm); conv. 52%. The obtained crude product was purified by silica gel column chromatography (hexane/EtOAc = 30:1) to give the epoxide 5h. White solid; mp 118–119 °C; Rf 0.34 (hexane/EtOAc = 20:1); 1H NMR (400 MHz, CDCl3): δ 7.88–7.83 (m, 4H), 7.53–7.46 (m, 2H), 7.44–7.33 (m, 6H), 4.04 (d, J = 1.6 Hz, 1H), 3.97 (d, J = 1.6 Hz, 1H).
3-(3-(Methoxymethoxy)-3-methylpent-4-en-1-yl)-2,2-dimethyloxirane (5i). Following the general procedure, olefin 4i was converted to epoxide. The conversion was calculated using the integral of the olefin signal (5.12–5.08 ppm) and the epoxide signal (2.72 ppm); monoxide only, conv. 100%. The obtained crude product was purified by silica gel column chromatography (hexane/EtOAc = 5:1) to give the epoxide 5i. Colorless oil; Rf 0.38 (hexane/EtOAc = 5:1); 1H NMR (400 MHz, CDCl3): δ 5.86–5.78 (m, 1H), 5.20–5.15 (m, 2H), 4.71–4.68 (m, 1H), 4.62 (d, J = 6.8 Hz, 1H), 3.37 (s, 3H), 2.72 (t, J = 5.6 Hz, 1H), 1.80–1.53 (m, 5H), 1.32 (d, J = 2.8 Hz, 3H), 1.31 (s, 3H), 1.27 (d, J = 2.8 Hz, 3H); 13C NMR (101 MHz, CDCl3): δ 142.7, 142.5, 115.1, 115.0, 91.8, 78.3, 64.6, 58.5, 55.4, 37.72, 37.69, 25.0, 23.67, 23.64, 23.1, 23.0, 18.78, 18.76; HRMS-DART (m/z): [M + H]+ calcd for C12H23O3+, 215.1642; found, 215.1642.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/1420-3049/29/5/966/s1, Copies of 1H NMR, 13C NMR, 19F NMR spectra of compounds (ligand of 1a–c, 3a, 3b, 5a–i) can be found in the Supplementary Materials.

Author Contributions

Conceptualization, Y.K., T.S. and M.M.; investigation, Y.K., M.K. and H.K.; Methodology, Y.K. and T.S.; project administration, Y.K.; supervision, M.M.; validation, Y.K.; visualization, Y.K.; writing—original draft, Y.K. and M.M.; writing—review and editing, Y.K., T.S. and M.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by JSPS KAKENHI 22K05469.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data from this study are reported in the text or in the Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

The pressure inside the balloon was crucial for the successful progression of the reaction. A double-layered rubber balloon (9 inches, Sansyo Co., Ltd., Tokyo, Japan) was filled with air, and connected to a 10 mL eggplant flask using a three-way valve. The dimensions of the inflated balloons are as follows: width = 7.5 inches, height = 8.3 inches.

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Molecules 29 00966 sch001
Scheme 1. Synthesis of fluorous and non-fluorous iron(III) salen complexes 1a, 1b, and 1c.
Scheme 1. Synthesis of fluorous and non-fluorous iron(III) salen complexes 1a, 1b, and 1c.
Molecules 29 00966 sch001
Molecules 29 00966 sch002
Scheme 2. Control experiment using a radical scavenger.
Scheme 2. Control experiment using a radical scavenger.
Molecules 29 00966 sch002
Molecules 29 00966 sch003
Scheme 3. Plausible reaction mechanism.
Scheme 3. Plausible reaction mechanism.
Molecules 29 00966 sch003
Table 1. Air-epoxidation reaction with fluorous iron(III) salen complexes 1a, 1b, and 1c.
Table 1. Air-epoxidation reaction with fluorous iron(III) salen complexes 1a, 1b, and 1c.
Molecules 29 00966 i001
EntryCatalyst (mol%)Conv. (%) a
11a (1)73
21b (1)100 (98) b
31b (0.5)100 (95) b
41b (0.1)6
51c (0.5)53
6-5
a Determined by 1H NMR. b Isolated yield.
Table 2. Optimization of co-oxidant aldehydes.
Table 2. Optimization of co-oxidant aldehydes.
Molecules 29 00966 i002
EntryAldehydeTime (h)Conv. (%) aEntryAldehydeTime (h)Conv. (%) a
1Molecules 29 00966 i00351007Molecules 29 00966 i00424n.r. b
2Molecules 29 00966 i00524828Molecules 29 00966 i00624trace
3Molecules 29 00966 i00724109Molecules 29 00966 i00824n.r. b
4Molecules 29 00966 i00924trace10Molecules 29 00966 i01024n.r. b
5Molecules 29 00966 i01124trace11Molecules 29 00966 i01224trace
6Molecules 29 00966 i01324n.r. b12Molecules 29 00966 i01424trace
a Determined by 1H NMR. b “n.r.” means “no reaction”.
Table 3. Solvent effects in air-epoxidation reactions.
Table 3. Solvent effects in air-epoxidation reactions.
Molecules 29 00966 i015
EntrySolventTime (h)Conv. (%) a
1dry-MeCN5100
2tBuOH8100
3dry-MeOH24trace
4dry-DMF24trace
5dry-THF24trace
6dry-DCM2496
72,3,4,5,6-pentafluorotoluene2498
a Determined by 1H NMR.
Table 4. Air-epoxidation reactions using various substrates.
Table 4. Air-epoxidation reactions using various substrates.
Molecules 29 00966 i016
EntryOlefinEpoxideTime (h)Conv. (%) aEntryOlefinEpoxideTime (h)Conv. (%) a
1Molecules 29 00966 i017Molecules 29 00966 i01851006Molecules 29 00966 i019Molecules 29 00966 i0205100
2Molecules 29 00966 i021Molecules 29 00966 i02271007Molecules 29 00966 i023Molecules 29 00966 i0245100
3Molecules 29 00966 i025Molecules 29 00966 i02661008Molecules 29 00966 i027Molecules 29 00966 i0282452
4Molecules 29 00966 i029Molecules 29 00966 i03071009 bMolecules 29 00966 i031Molecules 29 00966 i0325100
(5i:5i’ = 100:0)
5Molecules 29 00966 i033Molecules 29 00966 i0346100Molecules 29 00966 i035
a Determined by 1H NMR. b “MOM” represents the “methoxymethyl group”.
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Kato, Y.; Kanoh, M.; Kobayashi, H.; Shioiri, T.; Matsugi, M. Practical Epoxidation of Olefins Using Air and Ubiquitous Iron-Based Fluorous Salen Complex. Molecules 2024, 29, 966. https://doi.org/10.3390/molecules29050966

AMA Style

Kato Y, Kanoh M, Kobayashi H, Shioiri T, Matsugi M. Practical Epoxidation of Olefins Using Air and Ubiquitous Iron-Based Fluorous Salen Complex. Molecules. 2024; 29(5):966. https://doi.org/10.3390/molecules29050966

Chicago/Turabian Style

Kato, Yamato, Miho Kanoh, Hina Kobayashi, Takayuki Shioiri, and Masato Matsugi. 2024. "Practical Epoxidation of Olefins Using Air and Ubiquitous Iron-Based Fluorous Salen Complex" Molecules 29, no. 5: 966. https://doi.org/10.3390/molecules29050966

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