Direct Deoxygenative Intramolecular Acylation of Biarylcarboxylic Acids

Yantao Li; Wentao Xu; Chengjian Zhu; Jin Xie

doi:10.1055/s-0040-1707246

Synlett, Table of Contents

Synlett 2021; 32(04): 387-390
DOI: 10.1055/s-0040-1707246

cluster

Radicals – by Young Chinese Organic Chemists

Direct Deoxygenative Intramolecular Acylation of Biarylcarboxylic Acids

Yantao Li

^aState Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, Chemistry and Biomedicine Innovation Center (ChemBIC), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. of China Email: cjzhu@nju.edu.cn Email: xie@nju.edu.cn

,

Wentao Xu

^aState Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, Chemistry and Biomedicine Innovation Center (ChemBIC), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. of China Email: cjzhu@nju.edu.cn Email: xie@nju.edu.cn

,

Chengjian Zhu∗

^aState Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, Chemistry and Biomedicine Innovation Center (ChemBIC), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. of China Email: cjzhu@nju.edu.cn Email: xie@nju.edu.cn

^bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Shanghai 200032, P. R. of China

,

Jin Xie∗

^aState Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, Chemistry and Biomedicine Innovation Center (ChemBIC), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. of China Email: cjzhu@nju.edu.cn Email: xie@nju.edu.cn

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Abstract

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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 chemistry

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References

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16 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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18 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.

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