A new and convenient synthesis of 2-arylbenzimidazoles through reaction of benzoylisothiocyanates with ortho -phenylenediamines

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.


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 antihypertensive 4 and antiulcer activities. 5The 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. 6ecently, 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 oxidizing agents and catalysts such as MnO2, 8 PbOAc4, 9 NaHSO3, 10 DDQ, 11 thiamine hydrochloride, 12 FeCl3/Al2O3, 13 HMTA-Br2, 14 an ionic liquid [Pmim]BF4, 15 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 irradiation 17 and ultrasonic irradiation 18 have been employed to achieve easy synthesis of 2-arylbenzimidazoles.For example, Joshi and coworkers have reported the synthesis of 2-arylbenzimidazoles by tetrabutylammonium fluoride mediated reaction of phenylene-1,2diamines with aromatic aldehydes in aqueous medium under the influence of ultrasonic irradiation. 18Niknam and Fatehi-Raviz reacted phenylene-1,2-diamines with aromatic carboxylic acids in the presence of methylsulfonic acid over neutral alumina by microwave irradiation. 19he reactions of benzoylisothiocyanates are of interest to organic chemists from synthetic and mechanistic point of view. 20The reactions of these heterocumulenes often depend on the structure of the isothiocyanates. 21In the present paper, we report 2:1 molar reactions of bezoylisothiocyanates with 1,2-phenylenediamines resulting into formation of corresponding bisthioureas which undergo cyclization to 2-arylbenzimidazoles by refluxing in pyridine.There is a report on formation of 2-N-(acyl)aminobenzimidazoles by the reactions of bezoylisothiocyanates with 1,2-phenylenediamines. 22

Results and Discussion
A 1:2 molar reaction of 1,2-phenylenediamines 1 with benzoylisothiocyanates 2 in benzene afforded the corresponding N,N'-bis(benzoylthiocarbamoyl)-1,2-phenylenediamines 3 in almost quantitative yields.The products were characterized on the basis of their analytical and spectral data (Table 1) with reported values.The IR spectra of the products showed a broad band at 3330-3280 cm -1 corresponding to the N-H linkage, a strong band at 1675±5 cm -1 corresponding to an amide carbonyl absorption, and a strong absorption band around 1150 cm -1 corresponding to the C=S bond.
The products 3a-h, after characterization, were dissolved in pyridine and heated at 120 o C for 15 hours.After distilling off the pyridine and cooling, a solid product was obtained which was washed with water, dried and recrystallized with ethanol.The solid products have been characterized as 2-arylbenzimidazoles 4a-h on the basis of their analytical and spectral data, and undepressed mixed mp with authentic samples. 23he IR spectra of the products 4 showed no absorption in the carbonyl region indicating the removal of both the carbonyl groups in the product.The IR spectra showed a broad band around 3350 cm -1 .Similarly, the 13 C spectra did not show any downfield signal corresponding to the carbonyl carbons.Instead, a low-field signal at around  152 was observed in all products that has been assigned to azomethine carbon.The 1 H NMR spectra exhibited a downfield signal corresponding to one proton.This signal was D2O exchangeable; so assigned to NH proton.These significant spectral features of the products led us to characterize these products as 2arylbenzimidazoles.Finally, the authentic samples of 4a and 4b were prepared according to a reported method for comparing the mp and spectral data.The comparison of spectral data and undepressed mixed mp with the authentic sample confirmed the products as 2arylbenzimidazoles.All compounds gave analysis for C, H, and N in the range of ± 0.4.

Scheme 1
The formation of products 3 has been explained by usual nucleophilic attack of amines 1 on carbon atom of the isothiocyanate linkage in substrates 2 (Scheme 1).On refluxing in pyridine, the removal of one aroylisothiocyanate moiety may lead to the formation of N-(benzoylthiocarbamoyl)-1,2-phenylenediamines 5.The formation of N-(benzoylthiocarbamoyl)-1,2-phenylenediamines 5 from the reaction of amines 1 with isothiocyanates 2 has been reported previously by refluxing in acetonitrile. 22The intermediate product 5 may lead to the formation of 2-arylbenzimidazoles 4 by dethiocyanation to intermediate product 6 and concomitant cyclodehydration.It is worth mentioning that when intermediate products 5 were refluxed in acetonitrile in the presence of DCC, cyclization took place with desulfurization resulting into formation of 2-N-(acyl)aminobenzimidazole indicating the sensitivity of reaction towards solvents and cyclodehydrating agents used in the reaction. 22The present study demonstrates benzoylisothiocyanates as excellent alternatives to aldehydes and carboxylic acids used in other methods for preparation of 2-arylbenzimidazoles as no catalyst is employed in this case.

Conclusions
In summary, the paper reports a new and convenient entry to a biologically important heterocyclic compounds, 2-arylbenzimidazoles and demonstrates the dependence of the outcome of the decomposition of N,N'-bis(benzoylthiocarbamoyl)-1,2-phenylenediamines on solvent.This reaction, to the best of our knowledge, is first report on the formation of 2arylbenzimidazoles from these substrates.

Experimental Section
General.Melting points have been recorded on a Stuart Scientific melting point apparatus and are uncorrected.The IR spectra were recorded on a Perkin-Elmer-781 IR spectrophotometer using KBr disc of the sample.The 1 H and 13 C NMR spectra were recorded in a DMSO-d6 solution at 300 MHz and 75.4 MHz, respectively, on a Brucker TM 300 MHz spectrometer.The mass spectra were recorded on a Finnigan LCQ Deca mass spectrometer by electrospray ionization.The 1,2-phenylenediamines were procured from Aldrich Chemicals and benzoylisothiocynates were prepared by reactions of acid chlorides with potassium thiocyante according to reported methods. 24

General procedure for the preparation of N,N'-bis(benzoylthiocarbamoyl)-1,2phenylenediamines (3a-h).
Benzoyl isothiocyanates (10 mmol) were added to a solution of 1,2phenylenedimines (5 mmols) in 6 mL of benzene under vigorous stirring.After complete addition of isothiocyanate, the reaction mixture was cooled in an ice bath to get the solid product in almost quantitative yield.The products were used as such in next step for cyclization reaction.