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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
a   Department 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
b   Division of Chemistry, Faculty of Science and Technology, Rajabhat Rajanagarindra University, Chachoengsao 24000, Thailand
,
Nawasit Chotsaeng
a   Department 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 
c   Department 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  ∗
a   Department 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
› Author AffiliationsThis research by King Mongkut’s Institute of Technology Ladkrabang (KMITL) received funding support from the National Science, Research and Innovation Fund (NSRF), Thailand (FRB660065/0258-REKRIS/FF66/21). C.K. thanks the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Ministry of Higher Education, Science, Research and Innovation.
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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

Supporting Information

    Supporting information for this article is available online at https://doi.org/10.1055/a-2158-7980.
  • Supporting Information


Publication History

Received: 30 July 2023

Accepted after revision: 22 August 2023

Accepted Manuscript online:
22 August 2023

Article published online:
28 September 2023

© 2023. Thieme. All rights reserved

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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, JC–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.