Diels–Alder cycloadditions of N-arylpyrroles via aryne intermediates using diaryliodonium salts

With a strategy of the formation of benzynes by using diaryliodonium salts, a cycloaddition reaction of N-arylpyrroles with benzynes was reported. A wide range of bridge-ring amines with various substituents have been synthesized in moderate to excellent yields (35–96%). Furthermore, with a catalytic amount of TsOH·H2O, these amines can be converted into the corresponding N-phenylamine derivatives easily, which are potentially useful in photosensitive dyes.


Part 3. Synthesis and characterization of products from cycloaddition reactions.
General procedure 2: Synthesis of Diels-Alder reaction products.
To an oven-dried Schlenk tube was added iodonium salts (0.5 mmol, 1 equiv) and substituted pyrrole (if solid, 2.5 mmol, 5 equiv). The tube was degassed with argon for three times. Then the tube was placed in an ice bath. Toluene (4.25 mL) and substituted pyrrole (if liquid, 2.5 mmol, 5 equiv) were added sequentially via syringe. After being stirred for approximately 10 minutes, LiHMDS (0.75 mL (1 M in toluene), 0.75 mmol, 1.5 equiv) was added via syringe and the mixture was allowed to warm up to room temperature gradually. After TLC indicated that the iodonium salts were completely consumed, the reaction mixture was quenched by addition of an aqueous solution of ammonium chloride (5 mL) and extracted with DCM (10 mL) for three times. The combined organic layers were dried with Na 2 SO 4 , filtered and concentrated. The crude products were purified using flash column chromatography on silica gel to afford the desired product.

9-Phenyl-1,4-dihydro-1,4-epiminonaphthalene (3aa)
Prepared according to the general procedure 2 on 0.5 mmol scale and obtained an isolated yield of 85% (93 mg) as a pale brown solid. Spectral data is consistent with that of previous reported. 1

General procedure 3: Synthesis of N-phenylamine derivatives.
To an oven-dried Schlenk tube was added 9-phenyl-1,4-dihydro-1,4-epiminonaphthalene (0.5 mmol, 1 equiv) and TsOH·H 2 O (0.1 mmol, 0.2 equiv). The tube was degassed with argon for three times. Then DCE (4 mL) was added via syringe under room temperature. The mixture was stirred at 80 o C until TLC indicated that the starting materials were completely consumed. Then the reaction mixture was cooled down to room temperature and it was quenched with water (20 mL) and extracted with ethyl ether (10 mL) for three times. The combined organic layers were dried with Na 2 SO 4 , filtered and concentrated. The crude products were purified using flash column chromatography on silica gel to afford the desired product.

N-Phenylnaphthalen-1-amine (4)
Prepared according to the general procedure 3 on 0.5 mmol scale and obtained an isolated yield of 93% (102 mg) as a light gray solid. Spectral data is consistent with that of previous reported. [

General procedure 4: Synthesis of hydrogenation products.
To a stirred solution of 9-phenyl-1,4-dihydro-1,4-epiminonaphthalene (0.5 mmol) in EtOH (20 mL) was carefully added wet Pd/C catalyst (0.05 mmol) under argon atmosphere. The tube was degassed with hydrogen for three times. The mixture was stirred at room temperature until TLC indicated that the starting materials were completely consumed. Then the solid was filtered off and the filtrate was concentrated in vacuo. The crude products were purified using flash column chromatography on silica gel to afford the desired product.