Copper-mediated aerobic oxidative cleavage of α,β-unsaturated ketones to 1,2-diketones

Abstract

The copper-mediated aerobic oxidative cleavage of α,β-unsaturated ketones to synthesize 1,2-diketones by using potassium acetate as a catalyst and sodium iodide as a promoter in acetic acid is described. The protocol has the advantages of using inexpensive and non-toxic raw materials, high yield and simple work-up procedure.

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Introduction

It is known that 1,2-diketones are important synthetic building blocks¹ and have attracted the attention of many organic chemists, especially for use as precursors to some compounds such as α-hydroxy ketones² and heterocyclic compounds.³ They have also shown interesting applications as photoinitiators⁴ and corrosion inhibitors.⁵

The reported synthetic methods for 1,2-diketones (Scheme 1), for example 1,2-diaryl-1,2-diones, included: (1) the C–H oxidation of 1,2-diarylethanes using graphite oxide (GO);⁶ (2) the oxidation of stilbenes using GO;⁶ (3) the oxidation of 1,2-diarylethynes using DMSO catalyzed by Pd–Cu;⁷ (4) the oxidation of deoxybenzoins using iodine and DMSO catalyzed by CuO;⁸ (5) the aerobic oxidation of benzoins catalyzed by zinc;⁹ (6) the aerobic oxidation of 2-bromo-1,2-diarylethan-1-ones catalyzed by ruthenium complexes;¹⁰ (7) the oxidation of 1,2-diaryl-1,2-dibromoethanes using DMSO;¹¹ and (8) the oxidation of hydrobenzoins using t-BuOOH (TBHP) catalyzed by vanadium complexes, etc.¹²

Scheme 1 The general synthetic methods for 1,2-diketones.

The recent research showed that 1,2-diketones could also be synthesized by oxidative cleavage of 1,3-diketones using different oxidants (Scheme 2).¹³

Scheme 2 Synthesis of 1,2-diketones by oxidative cleavage.

It is worthy to mention that McKillop reported a method to 1,2-diketones from chalcones using thallium(III) nitrate as a oxidant in perchloric acid medium (Scheme 3).¹⁴ However, this method required highly toxic thallium salt as a strong oxidizing agent in strong acid medium, meanwhile only middle yield was afforded in refluxing condition. Therefore, it is necessary to find new methods for 1,2-diketones. Herein, we report the copper-mediated aerobic oxidative cleavage of α,β-unsaturated ketones to synthesize 1,2-diketones under mild conditions.

Scheme 3 Synthesis of 1,2-diketones from chalcones.

Results and discussion

Initially, the investigation was selected chalcone (1a) as substrate. The reactions were ever conducted under the condition of sodium iodide as a promoter and potassium acetate as a catalyst under refluxing condition in acetic acid. However, the corresponding product benzil (2a) was not observed (Table 1, entry 1). In the later research, it was found that the reaction could efficiently give 2a using some copper salts as mediators. The use of copper chloride, copper sulfate and copper acetate gave relatively low efficiency, while copper chloride was almost inactive (Table 1, entries 2–4). However, the reaction was effective under the condition of copper nitrate pentahydrate as a mediator to afford 2a in high yield (Table 1, entry 5). The possible reason is that copper nitrate pentahydrate may have dual functions in the reaction, in addition to copper as a mediator, nitrate as a oxidant as well as air. It was also noteworthy to mention that the absence of either of sodium iodide or potassium acetate in the reaction system could cause the obvious decrease of the reaction yield (Table 1, entries 6 and 7).

Table 1 The effect of mediators on the yield of 1a to 2a^a

Entry	Mediator	Promoter	Catalyst	Yield^b (%)
a Reaction condition: 1a (1 mmol), mediator (1.1 mmol), NaI (1.1 mmol), KOAc (0.2 mmol) in 10 mL of HOAc at 100 °C for 9 h.b Isolated yields.
1	None	NaI	KOAc	0
2	CuCl2	NaI	KOAc	5
3	CuSO4	NaI	KOAc	21
4	Cu(OAc)2	NaI	KOAc	34
5	Cu(NO3)2·5H2O	NaI	KOAc	83
6	Cu(NO3)2·5H2O	None	KOAc	51
7	Cu(NO3)2·5H2O	NaI	None	47

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The solvents also played an important role in the reaction of 1a to 2a. The reaction in many solvents such as water, 1,4-dioxane, toluene and methylene chloride was not occurred, while the reaction in ethanol or acetonitrile only gave unidentified by-products in low yield by comparison with corresponding product using TLC. However, acetic acid was an efficient solvent for the reaction to afford the corresponding product in 83% yield. In fact, the role of acetic acid in the reaction is both as solvent and as acidic medium.

Based on the above promising findings, a series of α,β-unsaturated ketones were attempted for aerobic oxidative cleavage to synthesize 1,2-diketones using copper nitrate pentahydrate as a mediator, sodium iodide as a promoter and potassium acetate as a catalyst in acetic acid (Scheme 4 and Table 2). It was found that α,β-unsaturated ketones bearing both electron-donating (CH₃, CH₃O) and electron-withdrawing (Cl, F, NO₂) groups on aromatic rings could participate in reactions. However, the former afforded the desired 1,2-diketones in slightly higher yield than the latter.

Scheme 4 The aerobic oxidative cleavage of α,β-unsaturated ketones to 1,2-diketones.

Table 2 The copper-mediated aerobic oxidative cleavage of α,β-unsaturated ketones to 1,2-diketones^a

Entry	R^1	R^2	Yield^b (%)	Mp (°C) (lit.)
a Reaction condition: α,β-unsaturated ketone (1 mmol), Cu(NO3)2·5H2O (1.1 mmol), NaI (1.1 mmol), KOAc (0.2 mmol) in 10 mL of HOAc for 9 h.b Isolated yields.
1	C6H5	C6H5	83	83–84 (94–95)^7
2	4-CH3C6H4	4-CH3C6H4	78	82–83
3	C6H5	4-CH3OC6H4	86	49–50
4	C6H5	4-CH3C6H4	71	82–83 (96)^7
5		C6H5	88	96–98
6		4-CH3C6H4	82	102–103
7	4-FC6H4	4-CH3OC6H4	75	62–64
8	4-FC6H4	C6H5	72	50–51
9	4-ClC6H4	4-CH3OC6H4	68	118–120
10	2-ClC6H4	4-CH3OC6H4	75	90–92
11	2-ClC6H4	4-CH3C6H4	73	66–67
12	4-ClC6H4	4-CH3C6H4	82	116–117
13	2-ClC6H4	4-ClC6H4	70	80–82
14	2,4-Cl2C6H3	4-ClC6H4	66	82–83
15	C6H5	4-ClC6H4	73	66–67
16	4-NO2C6H4	C6H5	61	131–132 (141)^7

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A plausible mechanism for the aerobic oxidative cleavage of α,β-unsaturated ketones is presented in Scheme 5. The initial step of the reaction involves the formation of epoxide A from α,β-unsaturated ketone 1 under air. Then A is attacked by I⁻ in acetic acid to get B and C. C is further oxidized to D under air. Then I⁻ of D is replaced by hydroxy from water to α-hydroxy dicarbonyl compound E, followed by a further oxidation to tricarbonyl compound F under air. The carbonyl of F is coordinated with Cu(II) to form a Cu(II) complex G, which undergoes a 1,2-Wagner-Meerwein-type rearrangement of a carbonyl group with an electron pair to the electrophilic carbon resulting in the formation of intermediate H.^13,15 The subsequent (or simultaneous) liberation of carbon monoxide and Cu(II) from intermediate H results in the final product 1,2-diketone 2.

Scheme 5 Proposed mechanism for the formation of 1,2-diketones.

Conclusions

In summary, an efficient method has developed for the aerobic oxidative cleavage of α,β-unsaturated ketones to synthesize 1,2-diketones using copper nitrate pentahydrate as a mediator, sodium iodide as a promoter and potassium acetate as a catalyst in acetic acid. The protocol has advantages of using inexpensive and non-toxic raw materials, high yield and simple work-up procedure.

Experimental

IR spectra were recorded using KBr pellets on an Alpha Centauri FTIR spectrophotometer. ¹H NMR and ¹³C NMR spectra were recorded on a Mercury-400BB instrument using CDCl₃ as solvent and Me₄Si as internal standard. Elemental analyses were performed on a Vario El Elemental Analysis instrument. Melting points were observed in an electrothermal melting point apparatus. α,β-Unsaturated ketones were prepared according to literature procedure.¹⁶

The general procedure for the preparation of 1,2-diketones

To a stirred mixture of copper nitrate pentahydrate (1.1 mmol), sodium iodide (1.1 mmol), potassium acetate (0.2 mmol) in 10 mL of acetic acid, α,β-unsaturated ketone (1 mmol) was added. Then the reaction system was stirred under air at 100 °C for 9 h. After the completion of the reaction, the mixture was cooled to room temperature, diluted with ethyl acetate and washed with saturated sodium carbonate solution. The resulting organic phase was dried over anhydrous magnesium sulfate, and concentrated under reduced pressure. The residue was isolated by column chromatography using petroleum ether and ethyl acetate (100 : 1) as eluent to give pure product. The final product can also be further recrystallized from petroleum ether to give purer product. The analytical data for products are given below.

Benzil

Yellow solid (174.3 mg, 83% yield); Mp: 83–84 °C; IR (KBr, cm⁻¹) ν 3064, 1662, 1589, 1449; ¹H NMR (400 MHz, CDCl₃): δ 7.98 (d, J = 7.6 Hz, 4H, ArH), 7.64–7.68 (m, 2H, ArH), 7.49–7.53 (m, 4H, ArH); ¹³C NMR (100 MHz, CDCl₃): δ 194.5, 134.8, 133.0, 129.9, 129.0. Anal. calcd for C₁₄H₁₀O₂: C, 79.98; H, 4.79. Found: C, 79.76; H, 4.77%.

1,2-Di(p-tolyl)ethane-1,2-dione

Yellow solid (185.6 mg, 78% yield); Mp: 89–92 °C; IR (KBr, cm⁻¹): ν 3053, 2917, 2849, 1664, 1601; ¹H NMR (400 MHz, CDCl₃): δ 7.78 (d, J = 8.0 Hz, 4H, ArH), 7.22 (d, J = 8.0 Hz, 4H, ArH), 2.35 (s, 6H, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ 194.5, 146.0, 130.7, 130.0, 129.7, 21.9. Anal. calcd for C₁₆H₁₄O₂: C, 80.65; H, 5.92. Found: C, 80.78; H, 5.94%.

1-(4-Methoxyphenyl)-2-phenylethane-1,2-dione

Yellow solid (206.4 mg, 86% yield); Mp: 49–50 °C; IR (KBr, cm⁻¹): ν 2918, 2848, 1668, 1599, 1508, 1454; ¹H NMR (400 MHz, CDCl₃) δ 7.94–7.96 (m, 4H, ArH), 7.63–7.65 (m, 1H, ArH), 7.50 (d, J = 7.6 Hz, 2H, ArH), 6.97–6.99 (m, 2H, ArH), 3.89 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ 194.8, 193.1, 165.0, 134.7, 133.2, 132.3, 129.9, 128.9, 126.1, 114.4, 55.6. Anal. calcd for C₁₅H₁₂O₃: C, 74.99; H, 5.03. Found: C, 75.11; H, 5.01%.

1-Phenyl-2-p-tolylethane-1,2-dione

Yellow solid (159.1 mg, 71% yield); Mp: 82–83 °C; IR (KBr, cm⁻¹): ν 3054, 2926, 2854, 1674, 1603, 1449; ¹H NMR (400 MHz, CDCl₃) δ 7.88 (d, J = 8.0 Hz, 2H, ArH), 7.78 (d, J = 8.4 Hz, 2H, ArH), 7.54–7.58 (m, 1H, ArH), 7.40–7.44 (m, 2H, ArH), 7.22 (d, J = 8.0 Hz, 2H, ArH), 2.35 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃): δ 194.7, 194.2, 146.1, 134.7, 133.1, 130.6, 130.0, 129.8, 129.7, 128.9, 21.9. Anal. calcd for C₁₅H₁₂O₂: C, 80.34; H, 5.39. Found: C, 80.25; H, 5.41%.

1-(Benzo[d][1,3]dioxol-5-yl)-2-phenylethane-1,2-dione

Yellow solid (223.5 mg, 88% yield); Mp: 96–98 °C; IR (KBr, cm⁻¹): ν 3082, 2918, 2849, 1659, 1595, 1488, 1444; ¹H NMR (400 MHz, CDCl₃) δ 7.96 (d, J = 8.4 Hz, 2H, ArH), 7.63–7.67 (m, 1H, ArH), 7.48–7.52 (m, 4H, ArH), 6.87 (d, J = 8.8 Hz, 1H, ArH), 6.09 (s, 2H, CH₂); ¹³C NMR (100 MHz, CDCl₃) δ 194.5, 192.7, 153.4, 148.6, 134.7, 133.1, 129.8, 128.9, 127.8, 108.3, 108.2, 106.8, 102.2. Anal. calcd for C₁₅H₁₀O₄: C, 70.86; H, 3.96. Found: C, 70.94; H, 3.97%.

1-(Benzo[d][1,3]dioxol-5-yl)-2-p-tolylethane-1,2-dione

Yellow solid (219.8 mg, 82% yield); Mp: 102–103 °C; IR (KBr, cm⁻¹): ν 3082, 1664, 1658, 1599, 1488, 1447; ¹H NMR (400 MHz, CDCl₃) δ 7.85 (d, J = 8.0 Hz, 2H, ArH), 7.46–7.49 (m, 2H, ArH), 7.30 (d, J = 8.0 Hz, 2H, ArH), 6.86 (d, J = 8.8 Hz, 1H, ArH), 6.08 (s, 2H, CH₂), 2.43 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ 194.2, 192.9, 153.3, 148.6, 146.0, 130.2, 129.9, 129.6, 129.1, 127.8, 108.3, 108.2, 102.2, 21.9. Anal. calcd for C₁₆H₁₂O₄: C, 71.64; H, 4.51. Found: C, 71.51; H, 4.49%.

1-(4-Fluorophenyl)-2-(4-methoxyphenyl)ethane-1,2-dione

Yellow solid (193.5 mg, 75% yield); Mp: 62–64 °C; IR (KBr, cm⁻¹): ν 3074, 2917, 2847, 1660, 1599, 1506; ¹H NMR (400 MHz, CDCl₃) δ 8.00–8.03 (m, 2H, ArH), 7.95 (d, J = 9.0 Hz, 2H, ArH), 7.16–7.20 (m, 2H, ArH), 6.98 (d, J = 9.0 Hz, 2H, ArH), 3.89 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ 192.9, 192.5, 167.8, 165.3, 165.0, 132.6, 132.5, 132.3, 129.6, 125.8, 116.3, 116.1, 114.3, 55.6. Anal. calcd for C₁₅H₁₁FO₃: C, 69.76; H, 4.29. Found: C, 69.83; H, 4.30%.

1-(4-Fluorophenyl)-2-phenylethane-1,2-dione

Yellow solid (164.2 mg, 72% yield); Mp: 50–51 °C; IR (KBr, cm⁻¹): ν 3071, 1665, 1593, 1503, 1448; ¹H NMR (400 MHz, CDCl₃) δ 8.01–8.04 (m, 2H, ArH), 7.97–7.98 (m, 2H, ArH), 7.66–7.69 (m, 1H, ArH), 7.50–7.54 (m, 2H, ArH), 7.17–7.22 (m, 2H, ArH). ¹³C NMR (100 MHz, CDCl₃) δ 194.0, 192.7, 168.0, 165.5, 134.9, 132.8, 132.7, 132.6, 129.9, 129.5, 129.0, 116.5, 116.2. Anal. calcd for C₁₄H₉FO₂: C, 73.68; H, 3.98. Found: C, 73.77; H, 3.99%.

1-(4-Chlorophenyl)-2-(4-methoxyphenyl)ethane-1,2-dione

Yellow solid (186.7 mg, 68% yield); Mp: 119–120 °C; IR (KBr, cm⁻¹): ν 3082, 1658, 1599, 1488, 1447; ¹H NMR (400 MHz, CDCl₃) δ 7.91–7.95 (m, 4H, ArH), 7.48 (d, J = 8.8 Hz, 2H, ArH), 6.98 (d, J = 8.8 Hz, 2H, ArH), 3.89 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ 193.3, 192.4, 165.1, 141.3, 132.4, 131.6, 131.2, 129.3, 125.9, 114.4, 55.6. Anal. calcd for C₁₅H₁₁ClO₃: C, 65.59; H, 4.04. Found: C, 65.69; H, 4.03%.

1-(2-Chlorophenyl)-2-(4-methoxyphenyl)ethane-1,2-dione

Yellow solid (205.9 mg, 75% yield); Mp: 90–92 °C; IR (KBr, cm⁻¹): ν 3068, 3007, 2935, 2843, 1666, 1595, 1570, 1508; ¹H NMR (400 MHz, CDCl₃) δ 8.02 (d, J = 8.8 Hz, 2H, ArH), 7.87–7.90 (m, 1H, ArH), 7.50–7.54 (m, 1H, ArH), 7.41–7.44 (m, 2H, ArH), 7.01 (d, J = 8.8 Hz, 2H, ArH), 3.90 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ 193.8, 190.7, 164.7, 134.3, 134.1, 133.8, 132.6, 132.1, 130.5, 127.2, 125.5, 114.2, 55.6. Anal. calcd for C₁₅H₁₁ClO₃: C, 65.59; H, 4.04. Found: C, 65.48; H, 4.05%.

1-(2-Chlorophenyl)-2-p-tolylethane-1,2-dione

Yellow solid (188.7 mg, 73% yield); Mp: 66–67 °C; IR (KBr, cm⁻¹): ν 3066, 1665, 1598, 1439; ¹H NMR (400 MHz, CDCl₃) δ 7.89–7.95 (m, 3H, ArH), 7.51–7.56 (m, 1H, ArH), 7.41–7.45 (m, 2H, ArH), 7.34 (d, J = 8.0 Hz, 2H, ArH), 2.45 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ 193.7, 191.8, 145.7, 134.4, 133.9, 132.1, 130.5, 130.3, 130.0, 129.6, 127.3, 21.9. Anal. calcd for C₁₅H₁₁ClO₂: C, 69.64; H, 4.29. Found: C, 69.53; H, 4.30%.

1-(4-Chlorophenyl)-2-p-tolylethane-1,2-dione

Yellow solid (212.0 mg, 82% yield); Mp: 116–117 °C; IR (KBr, cm⁻¹): ν 3092, 3064, 1662, 1600, 1483; ¹H NMR (400 MHz, CDCl₃) δ 7.91 (d, J = 8.4 Hz, 2H, ArH), 7.86 (d, J = 8.0 Hz, 2H, ArH), 7.48 (d, J = 8.0 Hz, 2H, ArH), 7.31 (d, J = 8.0 Hz, 2H, ArH), 2.44 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ 193.5, 193.2, 146.4, 141.4, 131.5, 131.2, 130.4, 130.0, 129.8, 129.3, 21.9. Anal. calcd for C₁₅H₁₁ClO₂: C, 69.64; H, 4.29. Found: C, 69.70; H, 4.28%.

1-(2-Chlorophenyl)-2-(4-chlorophenyl)ethane-1,2-dione

Yellow solid (195.3 mg, 70% yield); Mp: 80–82 °C; IR (KBr, cm⁻¹): ν 3083, 1672, 1585, 1439; ¹H NMR (400 MHz, CDCl₃) δ 7.99 (d, J = 8.4 Hz, 2H, ArH), 7.89 (d, J = 8.4 Hz, 1H, ArH), 7.51–7.57 (m, 3H, ArH), 7.42–7.47 (m, 2H, ArH); ¹³C NMR (100 MHz, CDCl₃) δ 193.0, 190.3, 140.9, 134.4, 133.6, 133.5, 131.7, 131.2, 130.5, 130.1, 129.0, 127.1. Anal. calcd for C₁₄H₈Cl₂O₂: C, 60.25; H, 2.89. Found: C, 60.31; H, 2.88%.

1-(4-Chlorophenyl)-2-(2,4-dichlorophenyl)ethane-1,2-dione

Yellow solid (206.9 mg, 66% yield); Mp: 82–83 °C; IR (KBr, cm⁻¹): ν 3074, 1661, 1581, 1469; ¹H NMR (400 MHz, CDCl₃) δ 7.97 (d, J = 8.4 Hz, 2H, ArH), 7.85 (d, J = 8.0 Hz, 1H, ArH), 7.52 (d, J = 8.4 Hz, 2H,ArH), 7.43–7.46 (m, 2H, ArH); ¹³C NMR (100 MHz, CDCl₃) δ 192.1, 190.2, 141.4, 140.7, 134.7, 132.9, 132.2, 131.5, 130.6, 130.4, 129.4, 128.0. Anal. calcd for C₁₄H₇Cl₃O₂: C, 53.63; H, 2.25. Found: C, 53.50; H, 2.26%.

1-(4-Chlorophenyl)-2-phenylethane-1,2-dione

Yellow solid (178.5 mg, 73% yield); Mp: 66–67 °C; IR (KBr, cm⁻¹): ν 1668, 1585, 1485, 1449; ¹H NMR (400 MHz, CDCl₃) δ 7.84–7.90 (m, 4H, ArH), 7.58–7.62 (m, 1H, ArH), 7.41–7.47 (m, 4H, ArH); ¹³C NMR (100 MHz, CDCl₃) δ 194.7, 194.2, 146.1, 134.7, 133.1, 130.6, 130.0, 129.8, 129.7, 128.9. Anal. calcd for C₁₄H₉ClO₂: C, 68.73; H, 3.71. Found: C, 68.68; H, 3.70%.

1-(4-Nitrophenyl)-2-phenylethane-1,2-dione

Yellow solid (155.6 mg, 61% yield); Mp: 131–132 °C; IR (KBr): ν 3108, 1663, 1597, 1527, 1448; ¹H NMR (400 MHz, CDCl₃) δ 8.36 (d, J = 8.8 Hz, 2H, ArH), 8.18 (d, J = 8.8 Hz, 2H, ArH), 7.99 (d, J = 7.2 Hz, 2H, ArH), 7.70–7.74 (m, 1H, ArH), 7.54–7.58 (m, 2H, ArH); ¹³C NMR (100 MHz, CDCl₃) δ 192.8, 192.0, 151.2, 137.3, 135.4, 132.4, 130.9, 130.0, 129.2, 124.1. Anal. calcd for C₁₄H₉NO₄: C, 65.88; H, 3.55, N, 5.49. Found: C, 65.94; H, 3.56, N, 5.51%.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21162024) and Key Laboratory of Eco-Environment-Related Polymer Materials for Ministry of Education for the financial support of this work.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Copies of IR, ¹H and ¹³C NMR spectra of all compounds. See DOI: 10.1039/c4ra05307a

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