Acetic Acid-Promoted Rhodium(III)-Catalyzed Hydroarylation of Terminal Alkynes

^◊Both authors contributed equally to this work

Abstract

Rhodium(III)-catalyzed hydroarylation of terminal alkynes has not previously been achieved because of the inevitable oligomerization and other side reactions. Here, we report a novel Cp*Rh(III)-catalyzed hydroarylation of terminal alkynes in acetic acid as solvent to facilitate the C–H bond activation and subsequent transformations. This reaction proceeds under mild conditions, providing an effective approach to the synthesis of alkenylated heterocycles in high to excellent yields (31–99%) with a broad substrate scope (37 examples) and good functional-group compatibility. In this transformation, the loading of the alkyne can be reduced to 1.2 equivalents, which indicates the significant role of HOAc in lowering the reaction temperature and suppressing the oligomerization of the terminal alkyne. Preliminary mechanistic studies are also presented.

Key words

acetic acid - rhodium catalysis - hydroarylation - alkynes

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• 11 2-{2-[(E)-2-Phenylvinyl]phenyl}pyridine (3aa); Typical ProcedureA dried Schlenk tube equipped with a magnetic stirrer bar was charged sequentially with [Cp*RhCl₂]₂ (3.1 mg, 0.005 mmol, 2.5 mol%), AgSbF₆ (10.3 mg, 0.03 mmol, 15 mol%), substrate 1a (0.2 mmol), HOAc (1 mL), and ethynylbenzene (2a; 0.24 mmol) under argon. The mixture was then stirred at rt for 12 h. When the reaction as complete, the mixture was diluted with EtOAc (10 mL), filtered through a short pad of silica gel that was washed with EtOAc (30 mL). The filtrate was adsorbed on silica gel and concentrated by rotary evaporation. The crude product was purified by flash chromatography [silica gel (300–400 mesh); PE/EA (9:1)] to give a yellow oil; yield: 48.3 mg (94%).¹H NMR (400 MHz, CDCl₃): δ = 8.76 (d, J = 4.4 Hz, 1 H), 7.80–7.71 (m, 2 H), 7.56 (d, J = 6.5 Hz, 1 H), 7.48–7.36 (m, 5 H), 7.34–7.19 (m, 5 H), 7.06 (d, J = 16.2 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 158.8, 149.4, 139.4, 137.6, 136.2, 135.7, 130.3, 130.2, 128.7, 128.6, 127.7, 127.6, 127.5, 126.6, 126.3, 125.1, 121.9.