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Synlett 2021; 32(04): 387-390
DOI: 10.1055/s-0040-1707246
DOI: 10.1055/s-0040-1707246
cluster
Radicals – by Young Chinese Organic Chemists
Direct Deoxygenative Intramolecular Acylation of Biarylcarboxylic Acids
We thank the National Natural Science Foundation of China (Grant Nos. 21971108, 21971111, 21702098, 21732003 and 21672099), the Natural Science Foundation of Jiangsu Province (Grant No. BK20190006), the Fundamental Research Funds for the Central Universities (Grant No. 020514380176), Jiangsu Six Peak Talent Project, and start-up funds from Nanjing University for financial support.Further Information
Publication History
Received: 23 June 2020
Accepted after revision: 19 July 2020
Publication Date:
21 August 2020 (online)
Published as part of the Cluster Radicals – by Young Chinese Organic Chemists
Abstract
A photocatalyzed intramolecular cyclization is developed for the synthesis of fluorenones. In this photoredox reaction, triphenylphosphine is used as an inexpensive and effective deoxygenative reagent for biarylcarboxylic acids to give acyl radicals, which quickly undergo intramolecular radical cyclization. Reactions in the presence of air and continuous flow photoredox technology demonstrate the generality and practicality of this process.
Key words
photocatalytic - deoxygenation - triphenylphosphine - radical cyclization - flow chemistrySupporting Information
- Supporting information for this article is available online at https://doi.org/10.1055/s-0040-1707246.
- Supporting Information
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Fluorenones 2a–o; General Procedure Biarylcarboxylic acid 1 (0.1 mmol), Ir[dF(CF3)ppy]2(dtbbpy)PF6 (2.3 mg, 0.02 mmol), Ph3P (78.6 mg, 0.3 mmol), K2CO3 (10.4 mg, 0.075 mmol) and DCE (2 mL) were added to a 10 mL Schlenk tube equipped with a magnetic stir bar. The tube was placed at a distance of ~5 cm from 2 blue LEDs (45 W) under air, and the resulting solution was stirred for 24 h. After the reaction was complete (monitored by TLC), the mixture was concentrated under vacuum to remove DCE and the residue was purified by silica gel column chromatography (hexane/EtOAc = 20:1 to 5:1) to afford the product 2. 9H-Fluoren-9-one (2a) Compound 2a (12.6 mg, 70%) was synthesized using the general procedure and isolated as a yellow solid; mp 81–82 °C. 1H NMR (500 MHz, CDCl3): δ = 7.66 (d, J = 7.3 Hz, 2 H), 7.53–7.47 (m, 4 H), 7.29 (td,
J = 7.3, 1.2 Hz, 2 H). MS (EI): m/z (%) = 180.0 (100) [M+]. 2-Methyl-9H-fluoren-9-one (2b) Compound 2b (14 mg, 72%) was synthesized using the general procedure and isolated as a yellow solid; mp 90–91 °C. 1H NMR (500 MHz, CDCl3): δ = 7.63 (d, J = 7.3 Hz, 1 H), 7.47–7.44 (m, 3 H), 7.39 (d, J = 7.5 Hz, 1 H), 7.29–7.23 (m, 2 H), 2.37 (s, 3 H). MS (EI): m/z (%) = 194.1 (100) [M+].
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Flow Chemistry; General Procedure Biarylcarboxylic acid 1a (198 mg, 1 mmol), PPh3 (786 mg, 3 mmol), K2CO3 (69 mg, 0.5 mmol, 120 mesh), and Ir[dF(CF3)ppy]2(dtbbpy)PF6 (11.7 mg, 0.01 mmol) were placed in a round-bottom flask (100 mL). DCE (30 mL) was added under air and the mixture was pumped at a rate of 82 mL min–1 using a peristaltic pump under irradiation with 2 blue LEDs (45 W). After 8.5 h, the reaction was complete. The mixture was concentrated under vacuum to remove DCE and the residue was purified by silica gel column chromatography (hexane/EtOAc = 20:1 to 5:1) to afford the product 2a.