Rhodium-Catalyzed Oxidative Annulation of (2-Arylphenyl)boronic Acids with Alkynes: Selective Synthesis of Phenanthrene Derivatives

Abstract

A rhodium-catalyzed annulative coupling of (2-arylphenyl)boronic acids with alkynes has been developed for the facile construction of phenanthrene frameworks. The reaction proceeded without external bases, and dioxygen worked as a terminal oxidant. Deuterium-labeling experiments indicated the involvement of five-membered rhodacycle intermediates.

Key words

alkynes - arylboronic acids - phenanthrenes - rhodium - oxidative coupling - C–H functionalization

• References and Notes


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• 12 General Procedure for Equation 1 and Scheme 2To a 20 mL two-necked flask were added arylboronic acid 1 (0.25 mmol), alkyne 2 (0.25 mmol), [(Cp*RhCl₂)₂] (0.005 mmol, 3 mg), Cu(OAc)₂·H₂O (0.025 mmol, 5 mg), 1-methylnaphthalene (ca. 50 mg) as internal standard, and DMF (3 mL). The resulting mixture was stirred under air at 100 °C for 2 h. Then, the reaction mixture was cooled to room temperature and filtered through an alumina pad with CH₂Cl₂. After evaporating the solvents, product 3 was isolated by column chromatography on silica gel.9,10-Dipropylphenanthrene (3aa) [CAS Reg. No. 19793-67-0]¹⁰ Purified by column chromatography with hexane as eluent (54.2 mg, 83%); white solid. ¹H NMR (400 MHz, CDCl₃): δ = 1.19 (t, J = 7.3 Hz, 6 H), 1.71–1.84 (m, 4 H), 3.11–3.22 (m, 4 H), 7.57–7.68 (m, 4 H), 8.08–8.17 (m, 2 H), 8.74 (dd, J = 7.2, 2.4 Hz, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 15.0, 24.2, 31.7, 123.1, 124.9, 125.5, 126.7, 130.0, 131.5, 134.0. HRMS: m/z calcd for C₂₀H₂₃ [M + H⁺]: 263.1794; found: 263.1771.For other compounds, see the Supporting Information.
• 13 Procedure for Equation 2To a 20 mL two-necked flask were added arylboronic acid 1a (1.0 mmol, 198 mg), alkyne 4 (0.25 mmol), [(Cp*RhCl₂)₂] (0.01 mmol, 6 mg), Cu(OAc)₂·H₂O (1.0 mmol, 200 mg), 1-methylnaphthalene (ca. 50 mg) as internal standard, and DMF (3 mL). The resulting mixture was stirred under N₂ at 100 °C for 2 h. Then, the reaction mixture was cooled to room temperature and filtered through a Celite pad with CH₂Cl₂. After evaporating the solvents, product 5 was isolated by column chromatography on silica gel.10,10′-Dimethyl-9,9′-biphenanthrene (5aa)Purified by column chromatography with hexane–toluene (95:5, v/v) as eluent and then preparative GPC (55.6 mg, 58%); white solid; mp 268–269 °C. ¹H NMR (400 MHz, CDCl₃): δ = 2.37 (s, 6 H), 7.22 (dd, J = 8.2, 1.1 Hz, 2 H), 7.29 (ddd, J = 8.2, 6.8, 1.1 Hz, 2 H), 7.58 (ddd, J = 8.3, 6.8, 1.4 Hz, 2 H), 7.70–7.80 (m, 4 H), 8.19–8.26 (m, 2 H), 8.81 (d, J = 8.3 Hz, 2 H), 8.85–8.92 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 16.8, 122.7, 123.2, 125.2, 126.0, 126.5, 127.0, 127.0, 127.2, 129.8, 130.4, 131.5, 132.0, 132.1, 134.8. HRMS: m/z calcd for C₃₀H₂₃ [M + H⁺]: 383.1794; found: 383.1796.10,10′-Diethyl-9,9′-biphenanthrene (5ab)Purified by preparative GPC (58.1 mg, 57%); white solid; mp 260–261 °C. ¹H NMR (400 MHz, CDCl₃): δ = 1.03 (t, J = 7.5 Hz, 6 H), 2.75–2.93 (m, 4 H), 7.21–7.30 (m, 4 H), 7.58 (ddd, J = 8.3, 6.3, 1.9 Hz, 2 H), 7.70–7.79 (m, 4 H), 8.20–8.27 (m, 2 H), 8.80 (d, J = 8.3 Hz, 2 H), 8.87–8.93 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 14.3, 24.4, 122.6, 123.4, 125.5, 126.1, 126.4, 126.7, 126.9, 127.9, 129.9, 130.9, 131.1, 132.2, 134.3, 136.7. HRMS: m/z calcd for C₃₂H₂₇ [M + H⁺]: 411.2107; found: 411.2086.
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