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

Nitchakan Purahong; Sakchai Hongthong; Nawasit Chotsaeng; Chutima Kuhakarn; Jatuporn Meesin

doi:10.1055/a-2158-7980

Synlett, Table of Contents

Synlett 2023; 34(19): 2323-2328
DOI: 10.1055/a-2158-7980

letter

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

Nitchakan Purahong

^aDepartment of Chemistry and Advanced Pure and Applied Chemistry Research Unit (APAC), School of Science, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Road, Ladkrabang District, Bangkok 10520, Thailand

,

Sakchai Hongthong

^bDivision of Chemistry, Faculty of Science and Technology, Rajabhat Rajanagarindra University, Chachoengsao 24000, Thailand

,

Nawasit Chotsaeng

^aDepartment of Chemistry and Advanced Pure and Applied Chemistry Research Unit (APAC), School of Science, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Road, Ladkrabang District, Bangkok 10520, Thailand

,

Chutima Kuhakarn

^cDepartment of Chemistry and Center of Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science, Mahidol University, Rama 6 Road, Bangkok 10400, Thailand

,

Jatuporn Meesin ∗

^aDepartment of Chemistry and Advanced Pure and Applied Chemistry Research Unit (APAC), School of Science, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Road, Ladkrabang District, Bangkok 10520, Thailand

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Abstract

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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

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References

References

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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.

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