Palladium-Mediated Synthesis of Phenanthridines: The First Report of Palladium Insertion into Imidoyl Selenides

Abstract

Imidoyl phenyl selenides have been shown to be precursors to phenanthridines, when treated with Pd(PPh₃)₄ under reflux in toluene in the presence of a base, giving reasonable yields. It is proposed that insertion of palladium into the C-Se bond is followed by cyclisation onto a phenyl ring and subsequent aromatisation by elimination of HPdSePh.

heterocycles - palladium - phenanthridines - imidoyl sel­enides - cyclisations

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Typical Procedure for the Synthesis of Phenanthridines To a mixture of Pd(PPh₃)₄ (83 mg, 0.072 mmol) and Et₃N (0.13 ml, 0.90 mmol) was added the imidoyl selenide 2e (80 mg, 0.18 mmol) in toluene (2 mL). The mixture was refluxed for 48 h, and after cooling to r.t., H₂O (10 mL) was added. The reaction mixture was extracted with CH₂Cl₂ (2 × 10 mL). The combined organic phases were washed with a sat. aq solution of NaHCO₃ (10 mL), brine (10 mL), dried (MgSO₄), filtered, and the solvent removed under reduced pressure. The crude product was further purified by column chromatography (neutral alumina, 20% CH₂Cl₂ in hexane) to yield the desired 6-(4-chlorophenyl)phenanthridine (1e, 24 mg, 46%) as colourless crystals; mp 152-154 ˚C (Lit.^¹¹ mp 160-161 ˚C). IR (KBr): 2924, 1608, 1485, 1361, 1091, 829, 752, 721 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 7.54 (2 H, m, ArH), 7.64 (1 H, m, ArH), 7.70 (3 H, m, ArH), 7.77 (1 H, m, ArH), 7.88 (1 H, m, ArH), 8.06 (1 H, dd, J = 0.8, 8.4 Hz, ArH), 8.23 (1 H, dd, J = 1.2, 8.4 Hz, ArH), 8.63 (1 H, dd, J = 1.6, 8.4 Hz, ArH), 8.72 (1 H, d, J = 8.4 Hz, ArH). ^¹³C NMR (100 MHz, CDCl₃): δ = 122.0 (ArCH), 122.4 (ArCH), 123.8 (ArC), 125.0 (ArC), 127.2 (ArCH), 127.3 (ArCH), 128.5 (ArCH), 128.7 (2 × ArCH), 129.0 (ArCH), 130.4 (ArCH),130.7 (ArCH), 131.1 (2 × ArCH), 133.5 (2 × ArC), 134.9 (ArC), 143.8 (ArCCl), 160.0 (ArC=N). LRMS-FAB: m/z (%) = 290 (67) [M + H⁺], 289 (38), 176 (22), 155 (24), 154 (100), 138 (25), 137 (48), 136 (61). HMRS: m/z calcd for C₁₉H₁₃NCl: 290.0736; found: 290.0732.