DBU-Mediated Dimerization: Facile Access to 9,9′-Bifluorenylidenes and Isoindigos

Abstract

The present work describes the 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-promoted self-dimerization of 9-chlorofluorenes and 3-chlorooxindoles for the preparation of the corresponding 9,9′-bifluorenylidene and isoindigo derivatives in moderate to good yields of 29–97%. The reactions proceed readily in a short reaction time under mild and metal-free conditions. Scale-up syntheses (5.0 mmol) of selected 9,9′-bifluorenylidene derivatives proceed in decent yields, highlighting the synthetic utility of the reported protocol.

Key words

9,9′-bifluorenylidenes - isoindigos - 9-chlorofluorenes - 3-chlorooxindoles - dimerization

Publication History

Received: 30 July 2023

Accepted after revision: 22 August 2023

Accepted Manuscript online:
22 August 2023

Article published online:
28 September 2023

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• 23 9,9′-Bifluorenylidenes (2); General Procedure To a solution of 9-chlorofluorene 1 (0.5 mmol) in DMSO (2 mL) was added DBU (0.15 mL, 1.0 mmol). The reaction mixture was stirred at room temperature for 2 h. After completion of the reaction, the mixture was diluted with EtOAc (20 mL) and washed with H₂O (2 × 30 mL) to remove DMSO. The EtOAc layer was then washed with brine, dried over anhydrous Na₂SO₄, and filtered. The solvent was evaporated under reduced pressure and the crude residue was purified by column chromatography using silica gel and 10% DCM in hexane as the eluent. Di-tert-butyl [9,9′-Bifluorenylidene]-2,2'-diyl(E)-dicarbamate (2c) Yield: 0.0535 g (38%); orange solid, mp 183–184 °C. ¹H NMR (500 MHz, CDCl₃): δ = 8.46–8.28 (m, 4 H), 7.64–7.51 (m, 4 H), 7.43–7.34 (m, 2 H), 7.28 (td, J = 7.4, 1.0 Hz, 2 H), 7.22–7.19 (m, 2 H), 7.14 (td, J = 7.6, 1.2 Hz, 2 H), 1.53 (s, 18 H). ¹³C NMR (126 MHz, CDCl₃): δ = 153.8, 141.5, 140.8, 138.6, 138.1, 136.9, 136.9, 129.4, 126.9, 126.3, 121.3, 120.4, 119.8, 119.6, 80.4, 28.7. HRMS (ESI): m/z [M + H]⁺ calcd for C₃₆H₃₅N₂O₄: 559.2957; found: 559.2554. Di-tert-butyl [9,9′-Bifluorenylidene]-2,2'-diyl(Z)-dicarbamate (2c) Yield: 0.0648 g (46%); red solid; mp 178–180 °C. ¹H NMR (500 MHz, CDCl₃): δ = 8.23 (d, J = 7.8 Hz, 2 H), 8.15 (t, J = 1.2 Hz, 2 H), 7.50 (d, J = 7.5 Hz, 2 H), 7.46–7.37 (m, 4 H), 7.27–7.20 (m, 2 H), 7.13–7.07 (m, 2 H), 6.51 (s, 2 H), 1.52 (s, 18 H). ¹³C NMR (126 MHz, CDCl₃): δ = 152.8, 141.3, 141.1, 138.9, 138.4, 137.5, 136.4, 129.1, 126.5, 126.3, 120.3, 119.5, 116.6, 81.3, 28.6. HRMS (ESI): m/z [M + H]⁺ calcd for C₃₆H₃₅N₂O₄: 559.2957; found: 559.2580. 2,2′,7,7′-Tetrakis(allyloxy)-9,9′-bifluorenylidene (2f) Yield: 0.1354 g (97%); brown solid; mp 159–161 °C. ¹H NMR (500 MHz, CDCl₃): δ = 7.95 (d, J = 1.4 Hz, 4 H), 7.43 (d, J = 5.0 Hz, 4 H), 6.86 (dd, J = 4.9, 1.4 Hz, 4 H), 6.02 (ddt, J = 10.4, 6.3, 3.2 Hz, 4 H), 5.38 (d, J = 1.0 Hz, 4 H), 5.25 (dd, J = 6.3, 0.9 Hz, 4 H), 4.49 (d, J = 3.2 Hz, 8 H). ¹³C NMR (126 MHz, CDCl₃): δ = 157.7, 141.7, 139.6, 135.3, 133.4, 119.9, 117.9, 116.9, 112.9, 69.4. HRMS (ESI): m/z [M + H]⁺ calcd for C₃₈H₃₃O₄: 553.2379; found: 553.2371.
• 24 Isoindigos (4); General Procedure To a solution of 3-chlorooxindole 3 (0.5 mmol) in 1,4-dioxane (2 mL) was added DBU (0.15 mL, 1.0 mmol). The reaction mixture was stirred at room temperature for 0.5–1 h. After completion of the reaction, the mixture was diluted with H₂O (20 mL) and extracted with EtOAc (2 × 20 mL). The EtOAc layer was then washed with brine, dried over anhydrous Na₂SO₄ and filtered. The solvent was evaporated under reduced pressure and the crude residue was purified by column chromatography using silica gel and 20% EtOAc in hexane as the eluent. (E)-1,1′-Diallyl-[3,3′-biindolinylidene]-2,2′-dione (4a) Yield: 0.0255 g (57%); red solid, mp 161–162 °C. ¹H NMR (500 MHz, CDCl₃): δ = 9.20 (dd, J = 8.1, 1.2 Hz, 2 H), 7.34 (td, J = 7.7, 1.2 Hz, 2 H), 7.06 (td, J = 7.8, 1.2 Hz, 2 H), 6.78 (dd, J = 7.8, 1.1 Hz, 2 H), 5.88 (ddt, J = 17.2, 10.3, 5.2 Hz, 2 H), 5.31–5.20 (m, 4 H), 4.43 (d, J = 5.2 Hz, 4 H). ¹³C NMR (126 MHz, CDCl₃): δ = 167.8, 144.6, 133.5, 132.5, 131.3, 130.1, 122.5, 121.7, 117.7, 108.6, 42.4. HRMS (ESI): m/z [M + H]⁺ calcd for C₂₂H₁₉N₂O₂: 343.1447; found: 343.1443. (E)-1,1′-Diallyl-5,5′-difluoro-[3,3′-biindolinylidene]-2,2′-dione (4g) Yield: 0.0395 g (40%); black solid, mp 176–177 °C. ¹H NMR (500 MHz, CDCl₃): δ = 9.12 (dd, J = 10.8, 2.7 Hz, 2 H), 7.08 (td, J = 8.4, 2.7 Hz, 2 H), 6.70 (dd, J = 8.6, 4.5 Hz, 2 H), 5.90–5.82 (m, 2 H), 5.37–5.13 (m, 4 H), 4.42 (dt, J = 5.2, 1.7 Hz, 4 H). ¹³C NMR (126 MHz, CDCl₃) δ = 167.5, 158.66 (d, J_C–F = 237.6 Hz), 141.0, 134.0, 131.0, 122.2 (d, JC–F = 9.9 Hz), 119.2 (d, JC–F = 24.5 Hz), 118.0, 117.8 (d, JC–F = 27.7 Hz), 108.8 (d, JC–F = 8.1 Hz), 42.6. HRMS (ESI): m/z [M + H]⁺ calcd for C₂₂H₁₇F₂N₂O₂: 379.1258; found: 379.1255.