[2+2] Photocycloaddition of 3-Alkoxycoumarins with C₆₀

Abstract

The [2+2] cycloaddition of 3-alkoxycoumarins with C₆₀ proceeded stereoselectively under photoirradiation conditions to produce a new class of fullerene fused oligocyclic cyclobutane products. This reaction seems to progress via radical ion pair intermediates that arise from the SET processing of 3-alkoxycoumarins with the excited form of C₆₀.

Key words

[2+2] cycloaddition - 3-alkoxycoumarin - fullerene - photoreactions - oligocyclic cyclobutane

• References and Notes


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• 9 A similar reaction with use of 425 nm LED (6 W) also proceeded but the result was inferior (25% yield) and no reaction took place under heating (80 °C).
• 10 Further reducing the amount of 1a (5 equiv) resulted in low yield (31%).
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• 16 The reactions of photoexcited C₆₀ thorough single-electron transfer process are well known. For recent reviews, see ref. 4.
• 17 Schuster and Wilson proposed the addition of triplet excited states of enones to fullerene, via triplet 1,4-biradical intermediate of enones in Ref. 8b. However, if our reaction proceeds to give the [2+2] cycloaddition products via 1,4-biradical intermediates of 3-alkoxycoumarins, the products should be given in a 1:1 diastereomixture.
• 18 Stereoselective [2+2] Photocycloaddition of 3-Alkoxycoumarins with C₆₀; Typical Procedure A 50 mL Pyrex screw-capped test tube was charged with C₆₀ (0.05 mmol), 3-alkoxycoumarin 1 (0.50 mmol), and ODCB (5 mL). The reaction mixture was bubbled through with argon gas for 10 min. Then, this test tube was purged with argon and sealed. The reaction mixture was irradiated by using a UV-LED (365 nm, 50 W). The reaction was monitored by analytical HPLC. After 16 h, the solvent was evaporated under reduced pressure, and then MeOH was added. The sediment was filtered and washed with MeOH. The residue was purified by silica-gel column chromatography (hexane/toluene, 2:1) to give 2 as a solid. Compound 2a: 14.3 mg, 31% yield as a brown solid. ¹H NMR (500 MHz, CDCl₃): δ = 0.89 (t, J = 7.6 Hz, 3 H), 1.60–1.69 (m, 2 H), 4.12–4.24 (m, 2 H), 5.46 (br, 1 H), 7.28–7.33 (m, 1 H), 7.46 (d, J = 4 Hz, 2 H), 7.70 (d, J = 6.8 Hz, 1 H). ¹³C NMR (125 MHz, CDCl₃/CS₂ = 5:1): δ = 10.47, 22.22, 55.87, 66.61 (two peaks overlap), 67.58, 96.20, 120.50, 125.05, 126.63, 128.43, 128.99, 129.15, 130.35, 138.58, 139.87, 141.42, 141.65, 142.03, 142.26, 142.40, 142.57, 142.80, 143.19, 143.55, 144.20, 144.86, 145.44, 145.56, 145.71, 145.78, 146.22, 146.34, 146.65, 146.83, 147.93, 148.55, 156.24, 169.81. HRMS (ESI): m/z calcd for C₇₂H₁₂O₃ [M – CO + K]: 935.0474; found: 935.0670. Compound 2b: 16.5 mg, 16% yield as a brown solid. ¹H NMR (500 MHz, CDCl₃): δ = 2.03–2.05 (m, 2 H), 2.61–2.71 (m, 2 H), 4.31 (dt, J = 8.4, 4.8 Hz, 1 H), 4.41 (dt, J = 8.8, 5.2 Hz, 1 H), 5.48 (s, 1 H), 7.10 (d, J = 5.6 Hz, 2 H), 7.19 (t, J = 5.6 Hz, 1 H), 7.25–7.28 (m, 2 H), 7.45 (t, J = 6.0 Hz, 1 H), 7.56 (d, J = 6.0 Hz, 1 H), 7.62 (dt, J = 6.4, 1.2 Hz, 1 H), 7.81 (d, J = 6.0 Hz, 1 H). ¹³C NMR (125 MHz, CDCl₃): δ = 30.38, 32.09, 55.57, 65.21, 66.56 (two peaks overlap), 96.20, 120.62, 125.18, 126.13, 128.47, 129.40, 130.53, 136.39, 136.90, 138.37, 139.34, 139.78, 139.86, 139.92, 139.98, 141.12, 141.47, 141.52, 141.69, 142.06, 142.17, 142.30, 142.44, 142.60, 142.74, 142.84, 142.90, 142.96, 143.09, 144.57, 144.71, 144.86, 144.91, 144.98, 145.23, 145.47, 145.51, 145.55, 145.60, 145.68, 145.82, 146.16, 146.23, 146.32, 146.36, 146.39, 146.54, 146.64, 146.69, 146.82, 146.88, 147.99, 148.26, 148.62, 152.34, 156.33, 169.92. HRMS (ESI): m/z calcd for C₇₈H₁₆O₃ [M – CO + K]: 1011.0787; found: 1011.0247. Compound 2c: 11.8 mg, 24% yield as a brown solid. ¹H NMR (500 MHz, CDCl₃): δ = 2.04–2.09 (m, 2 H), 2.36–2.43 (m, 2 H), 3.67 (s, 3 H), 4.37 (dt, J = 8.8, 4.8 Hz, 1 H), 4.43 (dt, J = 8.4, 5.2 Hz, 1 H), 5.47 (s, 1 H), 7.43–7.45 (m, 1 H), 7.55 (d, J = 5.6 Hz, 1 H), 7.60–7.62 (m, 1 H), 7.79 (d, J = 6.0 Hz, 1 H). ¹³C NMR (125 MHz, CDCl₃): δ = 23.97, 29.68, 51.73, 55.31, 64.74, 66.56 (two peaks overlap), 96.00, 120.43, 125.01, 127.69, 128.10, 129.16, 130.36, 130.50, 136.19, 136.74, 139.59, 139.74, 139.80, 141.34, 141.51, 141.90, 142.00, 142.12, 142.26, 142.42, 142.58, 142.67, 142.78, 142.91, 143.05, 144.33, 144.53, 144,68, 144.74, 144.79, 145.02, 145.30, 145.39, 145.45, 145.51, 145.62, 146.06, 146.22, 146.36, 146.48, 146.65, 146.70, 147.81, 148.06, 148.44, 152.05, 156.09, 169.65, 173.13. HRMS (ESI): m/z calcd for C₇₄H₁₄O₅ [M – CO + K]: 993.0529; found: 993.0797. Compound 2d: 6.5 mg, 13% yield as a brown solid. ¹H NMR (500 MHz, CDCl₃): δ = 2.20–2.31 (m, 2 H), 3.40–3.48 (m, 2 H), 4.42–4.48 (m, 1 H), 4.53–4.60 (m, 1 H), 5.49 (s, 1 H), 7.44 (t, J = 7.0 Hz, 1 H), 7.56 (d, J = 8.5 Hz, 1 H), 7.62 (t, J = 7.5 Hz, 1 H), 7.79 (d, J = 7.0 Hz, 1 H). ¹³C NMR (125 MHz, CDCl₃): not detected because of the low solubility of 2d in organic solvents. LRMS (APCI): m/z calcd for C₇₂H₁₁BrO₃ [M – C]: 989; found: 989. Compound 2e: 2.9 mg, 16% yield as a brown solid. ¹H NMR (500 MHz, CDCl₃): δ = 1.42–1.45 (m, 2 H), 1.70–1.74 (m, 2 H), 2.02–2.07 (m, 2 H), 4.29–4.34 (m, 1 H), 4.39–4.43 (m, 1 H), 4.96 (d, J = 10.0 Hz, 1 H), 5.00 (d, J = 16.5 Hz, 1 H), 5.48 (s, 1 H), 5.71–5.78 (m, 1 H), 7.44–7.46 (m, 1 H), 7.55 (d, J = 7.5 Hz, 1 H), 7.62 (t, J = 7.0 Hz, 1 H), 7.79 (d, J = 7.0 Hz, 1 H). ¹³C NMR (125 MHz, CDCl₃): δ = 25.17, 28.18, 29.85, 33.35, 51.04, 55.56, 64.60, 65.97, 115.06, 120.55, 125.11, 127.85, 128.41, 129.29, 130.43, 130.67, 136.34, 138.38, 139.29, 139.73, 139.86, 141.46, 141.50, 141.68, 142.10, 142.25, 142.31, 142.44, 142.60, 142.73, 142.84, 143.11, 143.20, 144.60, 144,77, 144.86, 144.97, 145.47, 145.60, 145.93, 146.20, 146.39, 146.53, 146.64, 146.87, 147.99, 148.31, 148.61, 152.22, 156.29, 169.99. LRMS (APCI): m/z calcd for C₇₅H₁₆O₃ [M – C]: 951; found: 951. Compound 2g: 9.5 mg, 16% yield as a brown solid. ¹H NMR (500 MHz, CDCl₃): δ = 0.86–1.88 (m, 16 H), 4.00–4.02 (m, 2 H), 4.30–4.41 (m, 2 H), 5.49 (br s, 1 H), 6.81 (s, 1 H), 7.24–7.27 (m, 1 H), 7.30 (d, J = 6.0 Hz, 1 H), 7.36–7.39 (m, 2 H), 7.42 (t, J = 6.0 Hz, 1 H), 7.54 (d, J = 6.0 Hz, 1 H), 7.61 (t, J = 6.0 Hz, 1 H), 7.80 (d, J = 6.4 Hz, 1 H). ¹³C NMR (125 MHz, CDCl₃): not detected because of the low solubility of 2g in organic solvents. HRMS (ESI): m/z calcd for C₈₈H₃₀O₆ [M – CO + K]: 1193.1730; found: 1193.1783.