Anodic Cyclization of 1,7-Diarylheptane-1,7-diones to the Corresponding 1,2-Diaroylcyclopentanes

Abstract

A range of 1,7-diarylheptane-1,7-diones were electro­oxidized in the presence of iodide ions and a base in a mixture of methanol and toluene as the reaction solvents to give the corresponding cyclized 1,2-diaroylcyclopentanes in moderate to good yields. The reactions were carried out under extremely mild reaction conditions for which the optimal amount of electrolytic current varied from 2.05–3.17 F·mol^–1 depending on the substrates. The reaction presumably proceeds through a two-electron oxidation process in which the iodide ion plays an important role as the electron carrier.

Key words

electron transfer - oxidation - cyclization - ring closure - green chemistry - iodine

• References


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• 5 1,7-Diarylheptane-1,7-diones were prepared by using standard Friedel–Crafts acylation conditions between pimeloyl dichloride and aromatic compounds in the presence of AlCl₃.⁶ Preparative-scale electrooxidations were carried out in a tall 50-mL beaker equipped with a cylindrical fine frit cup (height: 45 mm, diameter: 12 mm) as the cathode compartment with a nickel coil cathode (length: 280 mm, diameter: 1 mm), along with a cylindrical platinum net anode (height: 35 mm, diameter: 30 mm, 50 mesh).Typical Procedure: A solution of 1,7-diphenylheptane-1,7-dione 1a (1.40 g, 5 mmol) in a mixture of MeOH (25 mL) and toluene (15 mL) containing KI (0.83 g, 5.0 mmol) and NaOMe (2.70 g, 5.0 mmol) was electrooxidized under a constant current (0.3 A) at r.t. (ca. 15 °C) with magnetic stirring. During the course of the electrooxidation, the composition of the reaction mixture was monitored by GC and/or TLC analyses. Passage of the electric current was continued until the increase in the amount of the product was no longer detected (2.24 F·mol^–1). The reaction mixture was concentrated in vacuo at approximately 60 °C to near dryness. The resulting residue was treated with brine (ca. 30 mL), then extracted with diethyl ether or chloroform (3 × 40 mL), and dried overnight over sodium sulfate. After removal of the solvent in vacuo, the crude product was purified by silica gel column chromatography (height: 350 mm, diameter: 25 mm) using dichloromethane as the eluent, to afford the pure cyclized product 2a (0.88 g, 63%). The electrooxidation products were characterized by using IR, ¹H and ¹³C NMR analyses, and by HRMS. Cyclopentane-1,2-diylbis(phenylmethanone) (2a): Yield: 0.88 g (63%); colorless, fine cubic crystals; mp 88–89 °C (MeOH); R_f = 0.64 (CH₂Cl₂). IR (KBr): 3061 (w), 2965, 1675 (s), 1595, 1450, 1322, 1284, 1194, 987, 781, 702 (s) cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 1.75–1.92 (m, 4 H, 2 × CH₂), 2.18–2.31 (m, 2 H, CH₂), 4.38–4.46 (m, 2 H, 2 × CH), 7.45–7.57 (m, 6 H, 6 × CH), 7.97–8.03 (m, 4 H, 4 × CH). ¹³C NMR (100 MHz, CDCl₃): δ = 26.29 (CH₂), 32.06 (CH₂), 48.67 (CH), 128.58 (CH), 128.68 (CH), 133.06 (CH), 136.41 (C), 201.74 (CO). MS (EI, 70 eV): m/z (%) = 278 (6) [M⁺], 260 (7), 174 (25), 173 (98) [M⁺ – C₆H₅CO], 106 (19), 105 (100) [C₆H₅CO], 95 (10), 77 (61) [C₆H₅], 67 (7), 51 (15). HRMS: m/z [M⁺] calcd for C₁₉H₁₈O₂: 278.1307; found: 278.1310. Cyclopentane-1,2-diylbis(p-tolylmethanone) (2b): Yield: 1.06 g (69%); colorless, column crystals; mp 92–93 °C (MeOH); R_f = 0.65 (CH₂Cl₂). IR (KBr): 3066 (w), 2949, 1667 (s), 1607 (s), 1412, 1335, 1222, 1178, 1016, 843, 825 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 1.75–1.89 (m, 4 H, 2 × CH₂), 2.15–2.27 (m, 2 H, CH₂), 2.38 (s, 6 H, 2 × CH₃), 4.34–4.42 (m, 2 H, 2 × CH), 7.22 (d, J = 8 Hz, 4 H, 4 × CH), 7.90 (d, J = 8 Hz, 4 H, 4 × CH). ¹³C NMR (100 MHz, CDCl₃): δ = 21.59 (CH₃), 26.32 (CH₂), 32.14 (CH₂), 48.56 (CH), 128.81 (CH), 129.25 (CH), 133.93 (C), 143.79 (C), 201.42 (CO). MS (EI, 70 eV): m/z (%) = 306 (4) [M⁺], 289 (8), 288 (35), 188 (27), 187 (86) [M⁺ – CH₃C₆H₄CO], 120 (30), 119 (100) [CH₃C₆H₄CO], 95 (13), 91 (70), [CH₃C₆H₄], 65 (29). HRMS: m/z [M⁺] calcd for C₂₁H₂₂O₂: 306.1619; found: 306.1602. Cyclopentane-1,2-diylbis[(4-ethylphenyl)methanone] (2c): Yield: 1.10 g (66%); colorless, cubic crystals; mp 56–57 °C (MeOH); R_f = 0.72 (CH₂Cl₂). IR (KBr): 3057 (w), 2966, 2933, 1665 (s), 1606 (s), 1416, 1330, 1218, 1181, 1006, 848 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 1.23 (t, J = 8 Hz, 6 H, 2 × CH₃), 1.76–1.91 (m, 4 H, 2 × CH₂), 2.18–2.28 (m, 2 H, CH₂), 2.67 (q, J = 8 Hz, 4 H, 2 × CH₂), 4.35–4.43 (m, 2 H, 2 × CH), 7.26 (d, J = 8 Hz, 4 H, 4 × CH), 7.93 (d, J = 8 Hz, 4 H, 4 × CH). ¹³C NMR (100 MHz, CDCl₃): δ= 15.16 (CH₃), 26.33 (CH₂), 28.91 (CH₂), 32.15 (CH₂), 48.61 (CH), 128.07 (CH), 128.94 (CH), 134.16 (C), 149.96 (C), 201.47 (CO). MS (EI, 70 eV): m/z (%) = 334 (3) [M⁺], 316 (15), 202 (22), 201 (76) [M⁺ – CH₃CH₂C₆H₄CO], 134 (23), 133 (100) [CH₃CH₂C₆H₄CO], 105 (26) [CH₃CH₂C₆H₄], 103 (13), 79 (19), 77 (13). HRMS: m/z [M⁺] calcd for C₂₃H₂₆O₂: 334.1933; found: 334.1937. Cyclopentane-1,2-diylbis[(4-methoxyphenyl)methanone] (2d): Yield: 1.30 g (77%); colorless, fine cubic crystals; mp 130–131 °C (MeOH); R_f = 0.43 (CH₂Cl₂). IR (KBr): 3013 (w), 2959, 1665 (s), 1600 (s), 1575, 1331, 1261, 1215, 1166 (s), 1018, 1004, 838 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 1.76–1.91 (m, 4 H, 2 × CH₂), 2.15–2.27 (m, 2 H, CH₂), 3.85 (s, 6 H, 2 × CH₃O), 4.30–4.38 (m, 2 H, 2 × CH), 6.91 (d, J = 9 Hz, 4 H, 4 × CH), 7.98 (d, J = 9 Hz, 4 H, 4 × CH). ¹³C NMR (100 MHz, CDCl₃): δ = 26.39 (CH₂), 32.28 (CH₂), 48.46 (CH), 55.45 (OCH₃), 113.72 (CH), 129.48 (C), 130.98 (CH), 163.46 (C), 200.50 (CO). MS (EI, 70 eV): m/z (%) = 338 (2) [M⁺], 320 (7), 204 (9), 203 (66) [M⁺ – CH₃OC₆H₄CO], 136 (11), 135 (100) [CH₃OC₆H₄CO], 118 (4), 107 (10) [CH₃OC₆H₄], 92 (8), 77 (11). HRMS: m/z [M⁺] calcd for C₂₁H₂₂O₄: 338.1518; found: 338.1473.Cyclopentane-1,2-diylbis[(4-ethoxyphenyl)methanone] (2e): Yield: 1.17 g (64%); colorless, cubic crystals; mp 109–110 °C (MeOH); R_f = 0.49 (CH₂Cl₂). IR (KBr): 3075 (w), 2930, 1662 (s), 1600 (s), 1573, 1509, 1321, 1260, 1190, 1169 (s), 1038, 846 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 1.42 (t, J = 7 Hz, 6 H, 2 × CH₃), 1.76–1.90 (m, 4 H, 2 × CH₂), 2.16–2.26 (m, 2 H, CH₂), 4.07 (q, J = 7 Hz, 4 H, 2 × CH₂O), 4.30–4.38 (m, 2 H, 2 × CH), 6.88 (d, J = 9 Hz, 4 H, 4 × CH), 7.97 (d, J = 9 Hz, 4 H, 4 × CH). ¹³C NMR (100 MHz, CDCl₃): δ = 14.65 (CH₃), 26.39 (CH₂), 32.28 (CH₂), 48.44 (CH), 63.70 (OCH₂), 114.14 (CH), 129.30 (C), 130.97 (CH), 162.89 (C), 200.50 (CO). MS (EI, 70 eV): m/z (%) = 366 (2) [M⁺], 348 (15), 319 (13), 218 (15), 217 (85) [M⁺ – CH₃CH₂OC₆H₄CO], 150 (15), 149 (100) [CH₃CH₂OC₆H₄CO], 121 (55) [CH₃CH₂OC₆H₄], 93 (13), 65 (8). HRMS: m/z [M⁺] calcd for C₂₃H₂₆O₄: 366.1831; found: 366.1824. Cyclopentane-1,2-diylbis[(4-fluorophenyl)methanone] (2f): Yield: 1.05 g (67%); colorless, fine cubic crystals; mp 84–86 °C (MeOH); R_f = 0.69 (CH₂Cl₂). IR (KBr): 3060 (w), 2960, 1672 (s), 1596 (s), 1504, 1320, 1303, 1231 (s), 1205, 1150, 1007, 853 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 1.75–1.90 (m, 4 H, 2 × CH₂), 2.16–2.31 (m, 2 H, CH₂), 4.30–4.39 (m, 2 H, 2 × CH), 7.06–7.16 (m, 4 H, 4 × CH), 7.99–8.07 (m, 4 H, 4 × CH). ¹³C NMR (100 MHz, CDCl₃): δ = 26.29 (CH₂), 32.08 (CH₂), 48.70 (CH), 115.74 (d, J _C–F = 21 Hz, CH), 131.35 (d, J _C–F = 10 Hz, CH), 132.80 (d, J _C–F = 3 Hz, C), 165.79 (d, J _C–F = 254 Hz, C), 200.09 (CO). MS (EI, 70 eV): m/z (%) = 314 (3) [M⁺], 192 (21), 191 (76) [M⁺ – FC₆H₄CO], 164 (7), 124 (22), 123 (100) [FC₆H₄CO], 96 (7), 95 (54) [FC₆H₄], 75 (19), 67 (9). HRMS: m/z [M⁺] calcd for C₁₉H₁₆O₂F₂: 314.1118; found: 314.1167. Cyclopentane-1,2-diylbis[(4-chlorophenyl)methanone] (2g): Yield: 1.12 g (64%); colorless, fine needle-like crystals; mp 97–99 °C (MeOH); R_f = 0.75 (CH₂Cl₂). IR (KBr): 3062 (w), 2950, 1666 (s), 1588 (s), 1568, 1400, 1331, 1211 (s), 1089, 1005, 848, 839 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 1.76–1.88 (m, 4 H, 2 × CH₂), 2.16–2.30 (m, 2 H, CH₂), 4.30–4.38 (m, 2 H, 2 × CH), 7.42 (d, J = 9 Hz, 4 H, 4 × CH), 7.93 (d, J = 9 Hz, 4 H, 4 × CH). ¹³C NMR (100 MHz, CDCl₃): δ = 26.24 (CH₂), 31.98 (CH₂), 48.66 (CH), 128.93 (CH), 130.09 (CH), 134.63 (C), 139.60 (C), 200.33 (CO). MS (EI, 70 eV): m/z (%) = 350 (< 1) [M⁺, ³⁷Cl₂], 348 (2) [M⁺, ³⁷Cl, ³⁵Cl], 346 (3) [M⁺, ³⁵Cl₂], 209 (32) [M⁺ – ClC₆H₄CO], 208 (15), 207 (83) [M⁺ – ClC₆H₄CO], 141 (35) [³⁷ClC₆H₄CO], 140 (12), 139 (100) [³⁵ClC₆H₄CO], 113 (12) [³⁷ClC₆H₄], 111 (35) [³⁵ClC₆H₄]. HRMS: m/z [M⁺] calcd for C₁₉H₁₆O₂ ³⁷Cl₂: 350.0468; found: 350.0462. HRMS: m/z [M⁺] calcd for C₁₉H₁₆O₂ ³⁵Cl/³⁷Cl: 348.0498; found: 348.0438. HRMS: m/z [M⁺] calcd for C₁₉H₁₆O₂ ³⁵Cl₂: 346.0527; found: 346.0577. Cyclopentane-1,2-diylbis[(4-bromophenyl)methanone] (2h): Yield: 1.51 g (69%); colorless, fine needle-like crystals; mp 107–109 °C (MeOH); R_f = 0.75 (CH₂Cl₂). IR (KBr): 3069 (w), 2946, 1666 (s), 1584 (s), 1565, 1397, 1330, 1210 (s), 1068, 1004, 844, 837 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ= 1.73–1.87 (m, 4 H, 2 × CH₂), 2.16–2.30 (m, 2 H, CH₂), 4.30–4.37 (m, 2 H, 2 × CH), 7.59 (d, J = 9 Hz, 4 H, 4 × CH), 7.86 (d, J = 9 Hz, 4 H, 4 × CH). ¹³C NMR (100 MHz, CDCl₃): δ= 26.22 (CH₂), 31.94 (CH₂), 48.59 (CH), 128.36 (C), 130.18 (CH), 131.91 (CH), 134.98 (C), 200.51 (CO). MS (EI, 70 eV): m/z (%) = 438 (1) [M⁺, ⁸¹Br₂], 436 (2) [M⁺, ⁸¹Br, ⁷⁹Br], 434 (1) [M⁺, ⁷⁹Br₂], 253 (89) [M⁺ – BrC₆H₄CO], 251 (90) [M⁺ – BrC₆H₄CO], 185 (99) [⁸¹BrC₆H₄CO], 183 (100) [⁷⁹BrC₆H₄CO], 157 (32) [⁸¹BrC₆H₄], 155 (31) [⁷⁹BrC₆H₄], 76 (11). HRMS: m/z [M⁺] calcd for C₁₉H₁₆O₂ ⁸¹Br₂: 437.9476; found: 437.9411. HRMS: m/z [M⁺] calcd for C₁₉H₁₆O₂ ⁸¹Br/⁷⁹Br: 435.9497; found: 435.9491. HRMS: m/z [M⁺] calcd for C₁₉H₁₆O₂ ⁷⁹Br₂: 433.9517; found: 433.9495. Cyclopentane-1,2-diylbis[(2,5-dimethylphenyl)-methanone] (2i): Yield: 1.09 g (65%); slightly yellowish viscous oily liquid; bp 206–208 °C/1.2 mbar; R_f = 0.72 (CH₂Cl₂). IR (neat): 3020 (w), 2959 (s), 2926, 1678, 1495, 1449, 1294, 1244, 1169, 1006, 817 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 1.71–1.84 (m, 4 H, 2 × CH₂), 2.02–2.14 (m, 2 H, CH₂), 2.32 (s, 6 H, 2 × CH₃), 2.38 (s, 6 H, 2 × CH₃), 4.16–4.25 (m, 2 H, 2 × CH), 7.03–7.17 (m, 4 H, 4 × CH), 7.36–7.41 (m, 2 H, 2 × CH). ¹³C NMR (100 MHz, CDCl₃): δ = 20.45 (CH₃), 20.91 (CH₃), 26.02 (CH₂), 31.14 (CH₂), 51.62 (CH), 128.96 (CH), 131.53 (CH), 131.71 (CH), 134.63 (C), 135.19 (C), 138.08 (C), 206.06 (CO). MS (EI, 70 eV): m/z (%) = 334 (7) [M⁺], 265 (12), 201 (36) [M⁺ – (CH₃)₂C₆H₃CO], 134 (24), 133 (100) [(CH₃)₂C₆H₃CO], 105 (44) [(CH₃)₂C₆H₃], 103 (19), 95 (7), 79 (24), 77 (22). HRMS: m/z [M⁺] calcd for C₂₃H₂₆O₂: 334.1933; found: 334.1904
• 6 Fuson RC, Walker JT. Org. Synth. Coll. Vol. II . John Wiley & Sons; New York: 1943: 169