Issue in Honor of Prof Richard R. Schmidt ARKIVOC 2013 (ii) 213-219
A new and convenient synthesis of 2-arylbenzimidazoles through reaction of benzoylisothiocyanates with ortho-phenylenediamines
Girija S. Singh,*a Tarjeet Singh,b and Ram Lakhanb
aChemistry Department, University of Botswana, Private Bag: 0022, Gaborone, Botswana bDepartment of Chemistry, Banaras Hindu University, Varanasi-221 005, India E-mail: singhgs@mopipi.ub.bw
Dedicated to Prof. Richard R. Schmidt on the occasion of his 78th birthday
Abstract
This paper describes the reaction of benzoylisothiocyanates with ortho-phenylenediamines forming the corresponding bisthioureas which undergo cyclization with elimination of thiocyanic acid on refluxing in pyridine leading to the formation of 2-arylbenzimidazoles. The products have been characterized on the basis of satisfactory analytical and spectral (IR, NMR, MS) data, and the mechanism of their formation is proposed.
Keywords: Benzimidazoles, phenylenediamines, bisthioureas, cyclodehydration
Introduction
Benzimidazoles constitute a well-known class of five-membered azaheterocycles possessing different types of biological activity.1 These compounds are reported to have anthelmintic,2 anticancer,3 antihypertensive4 and antiulcer activities.5 The conventional method for the synthesis of 2-arylbenzimidazoles involves reaction of ortho-phenylenediamines with aromatic acids, nitrile, imidates or ortho-esters under strongly acidic condition at very high temperature.6 Recently, the reaction of ortho-phenylenediamine with acid chlorides in the presence of KF/Al2O3 as a catalyst has been reported to afford 2-arylbenzimidazoles.7 Alternatively, the reactions of ortho-phenylenediamines with aromatic aldehydes in the presence of several
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oxidizing agents and catalysts such as MnO2, PbOAc4, NaHSO3, DDQ,11 thiamine
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hydrochloride,12 FeCl3/Al2O3, HMTA-Br2, an ionic liquid [Pmim]BF4, etc. have been employed for the synthesis of 2-arylbenzimidazoles. A three-component reaction of 2bromo/ iodoaniline, aldehydes and sodium azide in the presence of CuCl-TMEDA has been reported to afford 2-arylbenzimidazoles.16 Recently, non-conventional techniques such as microwave irradiation17 and ultrasonic irradiation18 have been employed to achieve easy
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