Rh(iii)-catalyzed C–H olefination of N-pentafluoroaryl benzamides using air as the sole oxidant

Rhodium(iii)-catalyzed C–H olefination reaction using air as the sole oxidant.

To a 50 mL Schlenk-type sealed tube equipped with a magnetic stirring bar, were added the benzamide (60.2 mg, 0.2 mmol), [RhCp*Cl 2 ] 2 (6.2 mg, 0.01 mmol, 5 mol%), AgOAc (66.8 mg, 0.4 mmol), Solvent (2.0 mL) and olefine coupling partner (0.5 mmol). The tube was heated to 80 °C for 24 hours and then cooled to room temperature. The reaction mixture was filtered through a pad of Celite and concentrated in vacuo to afford crude products. The yield was determined by 1 H NMR analysis of the crude reaction mixture using CH 2 Br 2 as the internal standard.
To a 50 mL Schlenk-type sealed tube equipped with a magnetic stirring bar, were added the benzamide (60.2 mg, 0.2 mmol), [RhCp*Cl 2 ] 2 (6.2 mg, 0.01 mmol, 5 mol%), O 2 (1 atm), Ligand (0.02 mmol), MeCN (2.0 mL) and olefine coupling partner (0.5 mmol). The tube was heated to 80 °C for 24 hours and then cooled to room temperature. The reaction mixture was filtered through a pad of Celite and concentrated in vacuo to afford crude products. The yield was determined by 1 H NMR analysis of the crude reaction mixture using CH 2 Br 2 as the internal standard.

General procedure for C-H olefination:
To a 350 mL Schlenk-type sealed tube equipped with a magnetic stirring bar, were added the substrate (0.2 mmol), [RhCp*Cl 2 ] 2 (6.2 mg, 0.01 mmol), NaOPiv (28.4 mg, 0.2 mmol), MeCN (2.0 mL) and olefine coupling partner (0.5 mmol). The tube was capped, and heated to 80 °C for 24 hours. After cooled to room temperature, the reaction mixture was filtered through a pad of Celite. The filtrate was concentrated in vacuo to afford crude products, which was purified by flash column chromatography on silica gel to give the pure product.

Procedure for the ring-opening/auxiliary removal:
To an oven dried 10 mL round bottle flask equipped with a magnetic stir bar were added lactam (83.1mg, 0.2 mmol) and 1 mL of anhydrous THF. After cooling to -78℃, LiHMDS (1.0 M in THF, 5.0 equiv.) was added dropwise within 5 minutes.
The mixture was warmed up to -20℃ naturally in 50 minutes. Then Boc 2 O (6.0 equiv.) was added in -78 o C followed by warming up to 0 o C naturally in 2 hours.

The effect of heterocycle in C-H olefination
The acid (20 mmol) was prepared according previous report 3 and treated with oxalyl chloride to afford the acid chloride. And then, 2,3,4,5,6-pentafluoroaniline (22 mmol) in toluene (50 mL) was added and stirred for 24 h under reflux. After cooling to room temperature, the precipitate was filtered off and washed with ethyl acetate, water, and ethanol to give the substrate 1t. Standard condition has been applied for 1t, and the reaction mixture was concentrated in vacuo to afford crude products, which was purified by preparative TLC using EtOAc/Hexane (2/1) as the eluent to afford the major product 2t (28%).