Regioselective Synthesis of [6,6]-Phenyl-C₇₁-Butyric Acid Methyl Esters via Sulfur Ylides for Use in Bulk Heterojunction Solar Cells

Abstract

We herein report the development of a regioselective synthetic route to [70]methanofullerenes, which have potential for application as electron acceptors in solution-processed bulk heterojunction solar cells. Sterically hindered sulfur ylide, generated in situ from their corresponding sulfonium salts in the presence of 1,8-diazabicy­clo[5.4.0]undec-7-ene (DBU) as the base, smoothly reacted with [70]fullerene to afford the α-type [6,6]-phenyl-C₇₁-butyric acid methyl ester ([70]PCBM) in high purity (up to 98%). Furthermore, we successfully demonstrated the gram-scale synthesis of [70]PCBM.

Key words

bulk heterojunction (BHJ) organic solar cells - methanofullerene - [70]PCBM - regioselective synthesis - sulfur ylide

• References and Notes

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For reviews, see:
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• 15 Methyl 5-bromo-5-phenylpentanoate (1) and dimethyl (5-methoxy-5-oxo-1-phenylpentyl) sulfonium tetrafluoroborate (2a) were prepared according to reference 12a.
• 16 Representative Procedure for the Preparation of Sulfonium Salts; Dodecyl Methyl (5-Methoxy-5-oxo-1-phenylpentyl) Sulfonium Tetrafluoroborate (2b): To a solution of methyl 5-bromo-5-phenylpentanoate (1; 271 mg, 1.0 mmol) and dodecyl methyl sulfide (216 mg, 1.0 mmol) in CH₂Cl₂ (2 mL) was added silver tetrafluoroborate (195 mg, 1.0 mmol) at 0 °C. The resulting mixture was then warmed gradually to r.t. over 4 h with stirring. After filtration through Celite^® to remove the precipitate, the filtrate was concentrated under reduced pressure. The residue was then washed twice by decantation with hexane (10 mL), and the solvent was removed in vacuo to give a diastereomeric mixture (ca. 1:1) of sulfonium salt 2b as a viscous oil in 87% yield (430 mg). All sulfonium salts were stored at –15 °C prior to use to inhibit their gradual degradation. ¹H NMR (270 MHz, CDCl₃): δ = 7.49 (s, 5 H), 4.97 (t, J = 5.9 Hz, 1 H), 3.64 (s, 1.5 H), 3.63 (s, 1.5 H), 3.43 (t, J = 8.1 Hz, 1 H), 3.14–3.24 (m, 0.5 H), 3.05 (s, 1.5 H), 2.78–2.88 (m, 0.5 H), 2.57 (s, 1.5 H), 2.21–2.41 (m, 4 H), 1.26–1.96 (m, 22 H), 0.88 (t, J = 6.2 Hz, 3 H). ¹³C NMR (68 MHz, CDCl₃): δ = 173.27, 131.02, 130.72, 130.66, 130.50, 130.01, 129.63, 129.15, 60.66, 59.99, 51.61, 40.77, 40.73, 35.97, 32.54, 32.50, 31.86, 30.31, 29.98, 29.56, 29.50, 29.43, 29.34, 29.29, 29.17, 29.07, 28.92, 28.68, 28.59, 28.38, 28.04, 24.88, 24.38, 22.64, 21.78, 21.65, 21.44, 20.15, 17.31, 14.07. IR (neat): 2925, 2854, 1735, 1458, 1437, 1060, 710 cm^–1. HRMS (ESI): m/z [M – BF₄]⁺ calcd for C₂₅H₄₃O₂S⁺: 407.2978; found: 407.2955. Isopropyl Methyl (5-Methoxy-5-oxo-1-phenylpentyl) Sulfonium Tetrafluoroborate (2c): White solid, 74% (273 mg) yield, diastereomeric mixture (ca. 1:1.2); mp 95.3–96.2 °C. ¹H NMR (300 MHz, CDCl₃): δ = 7.46–7.51 (m, 5 H), 4.89 (t, J = 9.6 Hz, 0.45 H), 4.80 (t, J = 6.6 Hz, 0.55 H), 3.94 (quin, J = 6.9 Hz, 0.55 H), 3.64 (s, 1.65 H), 3.62 (s, 1.35 H), 3.39 (quin, J = 6.9 Hz, 0.45 H), 3.01 (s, 1.35 H), 2.47 (s, 1.65 H), 2.22–2.43 (m, 4 H), 1.46–1.66 (m, 5.3 H), 1.39 (d, J = 6.9 Hz, 1.35 H), 1.30 (d, J = 6.9 Hz, 1.35 H). ¹³C NMR (75 MHz, CDCl₃): δ = 173.18, 131.62, 130.66, 130.58, 130.45, 130.03, 129.96, 129.29, 128.74, 58.36, 58.19, 51.56, 51.52, 45.14, 44.98, 32.69, 32.52, 30.62, 30.36, 21.64, 21.20, 19.11, 18.68, 17.01, 16.53, 16.38, 15.90. IR (neat): 3021, 2943, 1731, 1459, 1440, 1265, 1151, 1054, 733, 711 cm^–1. HRMS (ESI): m/z [M – BF₄]⁺ calcd for C₁₆H₂₅O₂S⁺: 281.1570; found: 281.1547. Isopropyl Propyl (5-Methoxy-5-oxo-1-phenylpentyl) Sulfonium Tetrafluoroborate (2d): Viscous oil, 65% (258 mg) yield, diastereomeric mixture (ca. 1:1). ¹H NMR (300 MHz, CDCl₃): δ = 7.44–7.58 (m, 5 H), 5.03 (dd, J = 1.3, 8.4 Hz, 0.5 H), 4.88 (dd, J = 3.5, 9.6 Hz, 0.5 H), 4.08 (sept, J = 6.6 Hz, 0.5 H), 3.37–3.68 (m, 4 H), 3.28 (sept, J = 6.6 Hz, 0.5 H), 3.07–3.16 (m, 0.5 H), 2.86–2.96 (m, 0.5 H), 2.17–2.46 (m, 4 H), 1.21–2.05 (m, 12 H), 1.17 (t, J = 7.8 Hz, 1.5 H), 0.79 (t, J = 7.8 Hz, 1.5 H). ¹³C NMR (68 MHz, CDCl₃): δ = 172.79, 131.36, 130.93, 130.05, 129.97, 129.53, 129.41, 129.02, 128.38, 65.23, 57.89, 57.30, 51.06, 44.75, 44.50, 36.97, 36.42, 32.28, 32.11, 30.26, 30.08, 20.99, 20.91, 19.91, 19.26, 18.39, 18.22, 16.42, 15.82, 14.70, 12.35, 11.98. IR (neat): 2970, 2879, 1731, 1456, 1438, 1381, 1057, 709, 520 cm^–1. HRMS (ESI): m/z [M – BF₄]⁺ calcd for C₁₈H₂₉O₂S⁺: 309.1883; found: 309.1858. Diisopropyl (5-Methoxy-5-oxo-1-phenylpentyl) Sulfonium Tetrafluoroborate (2e): White solid, 82% (325 mg) yield; mp 67.6–68.5 °C. ¹NMR (300 MHz, CDCl₃): δ = 7.32–7.57 (m, 5 H), 4.94 (dd, J = 5.5, 9.9 Hz, 1 H), 4.11 (sept, J = 7.0 Hz, 1 H), 3.63 (s, 3 H), 3.47 (sept, J = 7.0 Hz, 1 H), 2.19–2.45 (m, 4 H), 1.46–1.78 (m, 8 H), 1.44 (d, J = 8.2 Hz, 3 H), 1.23 (d, J = 7.0 Hz, 3 H). ¹³C NMR (68 MHz, CDCl3): δ = 172.67, 131.52, 129.78, 129.31, 128.66, 54.56, 50.95, 44.39, 43.88, 32.06, 31.28, 20.72, 19.51, 18.88, 18.74, 18.60. IR (neat): 2950, 1731, 1456, 1439, 1271, 1083, 720, 533, 522 cm^–1. HRMS (ESI): m/z [M – BF₄]⁺ calcd for C₁₈H₂₉O₂S⁺: 309.1883; found: 309.1857. Diisobutyl (5-Methoxy-5-oxo-1-phenylpentyl) Sulfonium tetrafluoroborate (2f): Viscous oil, 61% (259 mg) yield. ¹H NMR (300 MHz, CDCl₃): δ = 7.47–7.59 (m, 5 H), 5.06 (dd, J = 8.5, 1.3 Hz, 1 H), 3.63 (s, 3 H), 3.34–3.53 (m, 2 H), 3.11–3.18 (m, 1 H), 2.82–2.89 (m, 1 H), 2.03–2.46 (m, 5 H), 1.45–1.72 (m, 2 H), 1.26–1.41 (m, 1 H), 1.18 (dd, J = 8.5, 1.8 Hz, 6 H), 0.89 (d, J = 6.6 Hz, 3 H), 0.76 (d, J = 6.6 Hz, 3 H). ¹³C NMR (68 MHz, CDCl₃): δ = 173.05, 131.00, 130.50, 129.73, 129.20, 60.76, 51.35, 49.03, 48.71, 32.38, 29.96, 26.09, 25.79, 21.61, 21.47, 21.14, 20.90. IR (neat): 3064, 2965, 2937, 2877, 1733, 1467, 1458, 1438, 1200, 1173, 1058, 735 cm^–1. HRMS (ESI): m/z [M – BF₄]⁺ calcd for C₂₀H₃₃O₂S⁺: 337.2196; found: 337.2166
• 17 Preparative Synthesis of α-[6,6]-Phenyl-C₇₁-butyric Acid Methyl Ester (α-[70]PCBM): To a stirred mixture of sulfonium salt 2e (943 mg, 2.38 mmol) and fullerene C₇₀ (1.0 g, 1.19 mmol) in ODCB (65 mL) was added a solution of DBU (545.2 mg, 3.57 mmol) in ODCB (15 mL) at 0 °C. After stirring at this temperature for 16 h, AcOH (ca. 3 equiv) was added. The resulting solution was then evaporated to half its original volume, and the resulting reaction mixture was purified by silica gel column chromatography. The desired [70]PCBM was obtained in 51% isolated yield (631 mg) in addition to 15% of the bis-[70]PCBM (215 mg). α-[70]PCBM: ¹H NMR (300 MHz, CDCl₃): δ = 7.40–7.92 (m, 5 H), 3.67 (s, 3 H), 2.42–2.53 (m, 4 H), 1.99–2.25 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 173.42, 155.97, 155.26, 152.15, 151.92, 151.48, 151.20, 151.13, 150.89, 150.83, 150.59, 150.52, 149.43, 149.39, 149.20, 149.13, 148.58, 148.55, 148.52, 148.42, 148.31, 148.02, 147.96, 147.90, 147.56, 147.52, 147.42, 147.36, 147.02, 146.86, 146.29, 146.09, 145.92, 145.82, 145.64, 144.88, 144.50, 144.09, 143.92, 143.87, 143.74, 143.63, 143.32, 143.24, 142.64, 142.50, 141.71, 141.58, 141.41, 140.95, 140.14, 139.29, 138.90, 137.91, 137.28, 134.00, 133.84, 132.84, 131.59, 130.90, 130.76, 130.65, 130.42, 128.54, 128.21, 71.87, 69.79, 51.72, 35.88, 34.09, 33.81, 21.70. IR (KBr): 2941, 2923, 1737, 1429, 795, 726, 699, 674, 643, 579, 534, 459 cm^–1. MS (MALDI): m/z [M]^– calcd for C₇₂H₁₄O₂: 1030; found: 1030. Bis-[70]PCBM:^{18 1}H NMR (300 MHz, CDCl₃): δ = 7.37–7.93 (m, 5 H), 3.65–3.67 (m, 3 H), 2.10–2.50 (m, 6 H). MS (MALDI): m/z [M]^– calcd for C₇₂H₁₄O₂: 1220; found: 1220.
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