Organic Photoredox Catalysis for Pschorr Reaction: A Metal-Free and Mild Approach to 6H-Benzo[c]chromenes

Abstract

An expedient organic photoredox Pschorr reaction has been developed that opens up a synthetic route to 6H-benzo[c]chromenes. The process can be performed under mild conditions by using eosin Y as a photoredox catalyst and acetonitrile as the solvent. The diazonium salts can be either preformed or generated in situ from the corresponding amines with t-BuONO. The process is amenable to gram-sale synthesis of 6H-benzo[c]chromenes, which can be further transformed into both 6H-benzo[c]chromen-6-ones through oxidation or to 6H-benzo[c]chromen-6-amine through sp³ C–H bond amination. The protocol provides an attractive route for the synthesis of a library of 6H-benzo[c]chromes.

Key words

organic photoredox catalysis - Pschorr reaction - diazonium salts - benzochromenes - C–H bond functionalization

• References and Notes


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• 12 6H-Benzo[c]chromenes 2a–o: Typical Procedure A 10 mL reaction vial was charged with a magnetic stirrer bar, the appropriate diazonium salt (0.20 mmol), eosin Y (1.3 mg, 1 mol%), and anhyd MeCN (2 mL). The vial was sealed and the mixture was then bubbled with N₂ for 10 min. After irradiating with 36 W green LEDs for 16 h, the mixture was purified directly by column chromatography (silica gel).
• 13 2,7-Dimethyl-6H-benzo[c]chromene (2i) White crystalline solid; yield: 26.9 mg (64%; 0.2 mmol scale); mp 72–73 °C. IR (film): 2923, 1501, 1459, 1250, 1028, 816, 736 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.58 (d, J = 8.0 Hz, 1 H), 7.55 (d, J = 1.6 Hz, 1 H), 7.29 (dd, J = 8.0, 7.6 Hz, 1 H), 7.14 (d, J = 7.6 Hz, 1 H), 7.06 (dd, J = 8.4, 1.6 Hz, 1 H), 6.91 (d, J = 8.0 Hz, 1 H), 5.16 (s, 2 H), 2.39 (s, 3 H), 2.33 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 152.3, 133.1, 131.3, 130.2, 130.0, 129.9, 129.4, 127.8, 123.9, 122.8, 119.8, 116.8, 65.6, 20.9, 18.5. GC/MS: m/z (%) = 209 (100) [M – H]⁺, 210 (72) [M]⁺. HRMS (CI): m/z [M]⁺ Calcd for C₁₅H₁₄O: 210.1045; found: 210.1048.