Synthesis of 6-Thiocyanatophenanthridines by Visible-Light- and Air-Promoted Radical Thiocyanation of 2-Isocyanobiphenyls

Abstract

A convenient, efficient, and metal-free synthesis of 6-thiocyanatophenanthridines by visible-light- and air-mediated, eosin Y-catalyzed, sequential radical cyclization and aromatization of 2-isocyanobiphenyls with ammonium thiocyanate is reported. Advantageously, the protocol utilizes inexpensive, clean, and sustainable natural resources such as visible light and atmospheric oxygen at room temperature in a one-pot procedure.

Key words

radical reaction - photochemistry - cyclization - photoredox catalysis - thiocyanation - phenanthridines

• References

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• 20 6-Thiocyanatophenanthridines 2; General Procedure A mixture of the appropriate isocyanobiphenyl 1 (1 mmol), NH₄SCN 2 (1.0 mmol), eosin Y (2 mol%), and CH₃CN (3 mL) in a flask open to the air was stirred at r.t. for 8–16 h with irradiation by green LEDs (2.50 W, λ = 535 nm) (Scheme 2). When the reaction was complete (TLC), H₂O (5 mL) was added and the mixture was extracted with EtOAc (3 × 5 mL). The organic phases were combined, dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The resulting crude product was purified by chromatography [silica gel, hexane–EtOAc (4:1)]. Phenanthridin-6-yl Thiocyanate (2a) Yellowish solid; yield: 198 mg (84%); mp 152−154 °C. ¹H NMR (400 MHz, CDCl₃): δ = 8.90 (d, J = 8.4 Hz, 1 H), 8.63 (d, J = 8.4 Hz, 1 H), 8.51 (d, J = 8.0 Hz, 1 H), 8.22 (d, J = 7.6 Hz, 1 H), 7.81 (m, 1 H), 7.75–7.66 (m, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 158.7, 140.8, 134.8, 131.1, 130.6, 129.2, 129.0, 128.3, 126.6, 125.0, 122.7, 121.7, 120.6, 98.4. HRMS (EI); m/z calcd for C₁₄H₈N₂S: 236.0408; found: 236.0405. 8-Methoxyphenanthridin-6-yl Thiocyanate (2f) Yellow solid; yield: 231 mg (87%); mp 145−147 °C. ¹H NMR (400 MHz, CDCl₃): δ = 8.58 (d, J = 9.2 Hz, 1 H), 8.47–8.44 (m, 1 H), 8.30 (d, J = 2.4 Hz, 1 H), 8.26–8.24 (m, 1 H), 7.74–7.71 (m, 2 H), 7.51 (dd, J = 9.2, 2.4 Hz, 1 H), 4.02 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 157.6, 151.7, 140.0, 131.1, 129.4, 129.1, 128.2, 125.2, 124.2, 122.0, 121.5, 121.3, 109.0, 98.6, 55.7. HRMS (EI): m/z calcd for C₁₅H₁₀N₂OS: 266.0514; found: 266.0510. 8-Cyanophenanthridin-6-yl Thiocyanate (2m) Orange solid; yield: 185 mg (71%); mp 186−189 °C. ¹H NMR (400 MHz, CDCl₃): δ = 9.25 (s, 1 H), 8.74 (d, J = 8.8 Hz, 1 H), 8.52 (d, J = 8.0 Hz, 1 H), 8.26 (d, J = 8.4 Hz, 1 H), 8.01 (d, J = 8.4 Hz, 1 H), 7.93–7.78 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 152.0, 141.4, 137.1, 133.4, 131.7, 131.5, 131.1, 130.0, 124.0, 123.6, 122.3, 120.1, 118.2, 110.4, 97.4. HRMS (EI): m/z calcd for C₁₅H₇N₃S: 261.0361; found: 237.0364.
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