Synlett 2021; 32(10): 999-1003
DOI: 10.1055/a-1467-5585
letter

Tetrabutylammonium Iodide (TBAI) Catalyzed Electrochemical C–H Bond Activation of 2-Arylated N-Methoxyamides for the Synthesis of Phenanthridinones

Kripa Subramanian
,
Subhash L. Yedage
,
Kashish Sethi
,
K.S. would like to thank the University Grants Commission of India (UGC) for providing a Senior Research Fellowship under the Basic Scientific Research (BSR) program vide No. [F.25-1/2014-15, F.7- 227/2009 dt. 16th Feb, 2015]


Abstract

An electrochemical method for the synthesis of phenanthridinones through constant-potential electrolysis (CPE) mediated by Bu4NI (TBAI) is reported. The protocol is metal and oxidant free, and proceeds with 100% current efficiency. TBAI plays a dual role as both a redox catalyst and a supporting electrolyte. The intramolecular C–H activation proceeds under mild reaction conditions and with a short reaction time through electrochemically generated amidyl radicals. The reaction has been scaled up to a gram level, showing its practicability, and the synthetic utility and applicability of the protocol have been demonstrated by a direct one-step synthesis of the bioactive compound phenaglaydon.

Supporting Information



Publication History

Received: 31 January 2021

Accepted after revision: 25 March 2021

Accepted Manuscript online:
25 March 2021

Article published online:
08 April 2021

© 2021. Thieme. All rights reserved

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  • 13 5-Methoxyphenanthridin-6(5H)-one2d (2a); Typical Procedure A 25 mL pear-shaped three-necked undivided cell equipped with Pt plate anode (30 × 15 mm) and a Cu plate cathode (30 × 15 mm) was charged with a solution of N-methoxybiphenyl-2-carboxamide (1a; 0.5 mmol, 113.6 mg) and TBAI (0.1 mmol, 36.9 mg) in DMF (15 mL). The cell was then connected to a regulated DC power supply, and constant-potential electrolysis was carried at 2.5 V and 70 °C until 2 F mol–1 electric charge was consumed (~5 h). The mixture was constantly stirred during the electrolysis. The resulting mixture was diluted with EtOAc and washed twice with H2O. The organic layers were collected, dried (Na2SO4), filtered, and concentrated in vacuo. The crude product was purified by column chromatography [silica gel, PE–EtOAc (9.5:0.5)] to give a white solid; yield: 101.4 mg (90%); mp 102–104 °C. 1H NMR (400 MHz, CDCl3): δ = 8.57 (d, J = 8.1 Hz, 1 H), 8.28 (dd, J = 8.3, 4.0 Hz, 2 H), 7.79 (t, J = 7.8 Hz, 1 H), 7.69 (d, J = 8.3 Hz, 1 H), 7.60 (q, J = 7.9, 7.4 Hz, 2 H), 7.41–7.32 (m, 1 H), 4.14 (s, 3 H). 13C NMR (100 MHz, CDCl3): δ = 157.3, 135.8, 133.0, 132.6, 130.0, 128.5, 128.1, 126.3, 123.2, 121.9, 118.6, 112.6, 62.7. GC/MS (EI, 70 eV): m/z (%) = 225.0 (39.0), 195.0 (100), 180.0 (28.7), 166.05 (58.2), 152.05 (16.0), 140.05 (28.6), 83.4 (13.7), 76.0 (13.2), 40.0 (13.9).