C-H and C-Si Functionalization of Furan Derivatives: Palladium-Catalyzed Homocoupling and Arylation Reactions

Abstract

Palladium-catalyzed arylation reactions of benzofuran derivatives are shown to take place at the carbon-hydrogen bond or carbon-silicon bond adjacent to the oxygen atom. A variety of furan derivatives are obtained in good yields.

Key words

C-H arylation - benzofuran - palladium - homocoupling reaction - cross-coupling reaction

References and Notes

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7

Fractional addition of silver was shown to improve the yield of the coupling product in the reactions of thiophene derivatives.^¹b,²c

10

Experimental Procedure for the Homocoupling Reaction of 1 To a 50 mL Schlenk tube equipped with a magnetic stirring bar were added PdCl_{2 (}PhCN)₂ (5.8 mg, 0.015 mmol), DMSO (3 mL), benzofuran (1, 55 µL, 0.5 mmol), and AgF (127 mg, 1.0 mmol) in one portion, and the resulting mixture was stirred at r.t. for 3 h. Additional AgF (2 × 1.0 mmol) were then added, and stirring was continued for further 3 h and 5 h, respectively. The reaction mixture was passed through a Celite pad to remove a solid residue, and the cake was washed repeatedly with Et₂O. The filtrate was washed with H₂O twice (2 × 50 mL) and brine (50 mL). Then, the organic layer was dried over anhyd MgSO₄ and concentrated under reduced pressure to leave a crude solid, which was purified by chromatography on silica gel to afford 33 mg of 2 (56%).
2,2′-Bibenzofuran ( 2) ^¹¹ Mp 202-203 ˚C. ^¹H NMR (500 MHz, CDCl₃): δ = 7.17 (s, 2 H), 7.25-7.37 (m, 4 H), 7.55 (d, J = 8.0 Hz, 2 H), 7.63 (d, J = 8.2 Hz, 2 H). ^¹³C NMR (125 MHz, CDCl₃): δ = 103.7, 111.3, 121.4, 123.3, 125.1, 128.5, 147.7, 155.1. IR (KBr): 1441, 1256, 1173, 1049, 876, 804, 750 cm^-¹.
Experimental Procedure for the C-Si Arylation Reaction of 7 and 3c To a 25 mL Schlenk tube equipped with a magnetic stirring bar were added, PdCl_{2 (}PPh₃)₂ (10.5 mg, 0.015 mmol), DMSO (1 mL), 2-trimethylsilylbenzofuran (7, 95 mg, 0.5 mmol), and ethyl 4-iodobenzoate (6c, 165 mg, 0.6 mmol) under argon atmosphere. The mixture was heated in an oil bath at 100 ˚C, KF (7.3 mg, 0.125 mmol), and AgNO₃ (21.2 mg, 0.125 mmol) were added in one portion, and the resulting mixture was continued for 1 h. Addition of AgNO₃/KF (0.125 mmol) and stirring for 1 h at 100 ˚C were repeated for further 5 times. After cooling to r.t., the mixture was passed through a Celite pad, which was washed with EtOAc repeatedly. The filtrate was washed with H₂O twice. Then the organic layer was dried over anhyd MgSO₄ and concentrated under reduced pressure to leave a crude solid, which was purified by chromatography on silica gel to afford 99 mg of 4 (75%).^¹²