Iron-Catalyzed Highly Efficient Aerobic Oxidative Synthesis of Benzimidazoles, Benzoxazoles, and Benzothiazoles Directly from Aromatic Primary Amines Under Solvent-Free Conditions in the Open Air

Abstract

Solvent-free oxidative synthesis of benzimidazoles, benzoxazoles, and benzothiazoles from aromatic primary amines and o-phenylenediamine, o-aminophenol, and o-aminothiophenol has been achieved by using Fe(NO₃)₃ and TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl). This reaction can use air as an economical and green oxidant, and a wide variety of derivatives were obtained in good to excellent yields. The reaction mechanism was proposed and this method provides a direct, practical, and efficient approach for the preparation of substituted benzimidazoles, benzoxazoles, and benzothiazoles.

GRAPHICAL ABSTRACT

Keywords:

• Aerobic oxidation
• benzimidazoles
• benzothiazoles
• benzoxazoles
• iron

Notes

^a Reaction conditions: A mixture of o-phenylenediamine and benzylamine, Fe(NO₃)₃, and TEMPO were mixed in a 5-mL, three-necked flask and then stirred rapidly in the open air for several hours. The reaction progress was monitored by thin-layer chromatography (TLC).

^b Isolated yield.

^a Reaction conditions: A mixture of 4 mmol o-phenylenediamine, 2-aminophenol, or 2-amino-thiophenole and 4.8 mmol aromatic primary amine, 5 mol% Fe(NO₃)₃, and 5 mol% TEMPO were mixed in a 5-mL, three-necked flask and then stirred rapidly open to air at 110 °C for several hours. The reaction progress was monitored by TLC.

^b Isolated yield.

^a Reaction conditions: A mixture of 4 mmol aniline and 4 mmol benzylamine, Fe(NO₃)₃, and TEMPO were mixed in a 5-mL, three-necked flask and then stirred rapidly while open to air at 110 °C for 24 h.

^b Isolated yield.

^a Reaction conditions: A mixture of 4 mmol benzimidazoline, Fe(NO₃)₃, and TEMPO was mixed in a 5-mL, three-necked flask and then stirred rapidly while open to air at 110 °C for 12 h.

^b Isolated yield.