Magnetic Ionic Liquid [ bmim ] [ FeCl 4 ] as an Efficient Catalyst for the Synthesis of 2-Aryl Benzimidazoles and 2-Aryl Benzothiazoles Derivatives

The magnetic ionic liquid (MIL) 1-butyl-3-methylimidazolium tetrachloro ferrate(III) ([bmim][FeCl 4 ]) sufficiently catalyzes the one-pot condensation of 1,2 diaminobenzene or 2aminobenzenethiol with different aromatic aldehydes producing benzimidazoles and benzothiazoles drivatives, respectively. The MIL showed high performance resulting great yields with appropriate reaction time.


INTRODUCTION
Ionic liquids (ILs) have been applied in green catalytic technologies and have been studied widely in reactions early after their discovery because of their individual unique properties such as low vapor pressure, wide liquid range, low flammability, high conductivity, excellent stability and large electrochemical window.ILs can perform specific catalytic processes and using them usually Otherwise, the term "magnetic ionic liquid" (MIL) was recently proposed by Hamaguchi and co-workers in 2004 to introduce ILs with paramagnetic exclusivity 3,4 .The MILs are primarily based on high-spin d 5 Fe(III) in the form of FeCl 4 -or FeBr 4 -with various counter cations.These MILs exhibited a strong response to magnetic fields by their high single-ion magnetic moments 5 .The catalytic activities of MILs have been studied in Friedel crafts acylation 6 , aryl grignard cross coupling of alkyl halides 7 , preparation of 1,2-azidoalcohols 8 , glycolysis of poly(ethylene terephthalate) 9 , "liquid fixed-bed" catalysts in flow application 10 , oxidative desulfurization of fuels 11 and multi-component synthesis of 1-and 5-substituted 1H-tetrazoles 12 , quinazolines 13 and1-amidoalkyl-2-naphthols 14 derivatives.
Basically, heteroaromatic bicycles have a wide range of applications in medicinal chemistry because of their pharmaceutical and biological activities 15 which are responsible for antibacterial and antifungal activity observed for benzimidazole derivatives 16 .The substituted benzimidazoles such as 2-Aryl and 2-alkyl benzimidazoles deliver biological activity against several viruses such as HIV, human cytomegalovirus (HCMV) 17 , Herpes (HVS-1) 18 , and influenza 19 .Besides, 2-Aryl benzothiazoles are important molecules due to their use not only as medicinal agents but also as organic functional materials such as fluorescent dyes and liquid crystals 20 .Various derivatives of benzothiazoles are also used as radioactive amyloid imaging agents 21 .Thus, the synthesis of these organic compounds has received considerable attention in diverse areas of chemistry.So far, a number of various synthetic methods have been developed to uncover a variety of new reagents for the preparation of benzimidazoles and benzothiazoles.The most commonly-used synthetic approaches to produce benzimidazoles typically entail the condensation of benzene-1,2-diamine with carbonyl compounds, such as aldehydes, carboxylic acids and their derivatives [22][23][24] .In addition, there are several reports on benzimidazoles synthesis via the reductive cyclization of benzene-1,2-diamine with aldehydes 25 , cyclization of benzene-1,2-diamine derivatives with aryl isothiocyanates 26 , and Baker's yeast reduction of 2, 4-dinitroacyl anilines 27 .Traditional methods for the synthesis of benzothiazoles typically involve the condensation of 2-amino thiophenols with aryl aldehydes 28,29 , carboxylic acids 30 , nitriles 31 , acyl chlorides 23,32 , alcohols 33 , or through Jacobson's potassium ferricyanide mediated cyclization of thiobenzanilides 34 .However, most of the reported methods have several drawbacks including low yield, long reaction time, the use of expensive reagents, harsh reaction conditions, tedious workup procedures, involving more than one step in their synthesis, and co-occurrence of several side reactions 35 .
In this paper the catalytic activity of [bmim][FeCl 4 ] was practically investigated for the one-pot synthesis of benzimidazoles and benzothiazoles, respectively.

EXPERIMENTAL General
All compounds were purchased from Aldrich and Merck companies and used as received without further purification.Progress of the reaction was monitored by TLC on Merck DC-Alufolien plates pre-coated with silica gel F 254 .Melting points were recorded using a Thermo Fisher Scientific IA 9200 instrument.

Synthesis of MIL [bmim][FeCl 4 ]
The magnetic ionic liquid was synthesized following the same way remarked in literature [36].5 mmol [bmim]Cl (0.870g) and 5 mmol FeCl 3 (0.825g) were added to a round bottom flask and stirred with a magnet for 15 minutes.The resulting ionic liquid was dissolved in ethyl acetate (10 mL) and the solution was centrifuged after filtration in order to separate any possible residue of inorganic salts.Afterwards, ethylacetate was evaporated and the obtained dark brown liquid, butyl methyl imidazolium tetrachloroferrate(III) was dried under vacuum at 80 ºC overnight.

General procedure for the synthesis of benzimidazoles and benzthiazoles in the presence of [bmim][FeCl 4 ]
A mixture of 2-aminobenzenethiol or 2aminobenzenthiol (1 mmol), aryl aldehyde (1 mmol), [bmim][FeCl 4 ] (0.5 mmol) and ethanol (5 mL) was placed in a round bottom flask and refluxed for the given times recorded in Table 2.After completion of the reaction observed by TLC (using n-hexane/ ethylacetate (7:3) as eluent), the mixture was cooled to room temperature, the precipitate was filtered and washed with water for several times.The resultant product purified by column chromatography and characterized by comparison of their physical data

RESULTS AND DISCUSSIONS
In order to investigate the catalytic ability of the MIL, the condensation of benzene-1,2diamine with 4-nitrobenzaldehyde was chosen as the model reaction in different conditions.As seen in Table 1, the most appropriate condition among the candidate ones giving excellent yield in a proper time is the 7 th process using ethanol as solvent under reflux condition.
Then, to study the scope and limitation of these optimized procedures a wide variety of aryl benzaldehydes were exerted and the results showed a highly effective performance of the catalyst in the preparation of 2-Aryl Benzimidazoles and 2-Aryl Benzothiazoles (Scheme 1, Table 2).
It has been assumed that, magnetic ionic liquids could act as efficient catalysts due to the synergic effect of the cation and anion [37].The catalytic operation of [bmim][FeCl 4 ] is explained in the proposed mechanism (Scheme 2).Initially, the carbonyl group in aldehyde is activated by the [bmim] cation and meanwhile [FeCl 4 ] " anion interacts with the hydrogen in X (NH 2 or SH) of either benzene-1,2-diamine or 2-aminobenzenethiol.Hence, the nucleophilic attack and ring closure becomes easier to obtain the intermediate (c).Ultimately, the product is formed.

CONCLUSION
We have established an efficient method demonstrating the high catalytic activity of the magnetic ionic liquid [bmim][FeCl 4 ] optimizing the synthesis of 2-Aryl Benzimidazole and 2-Aryl Benzothiazole derivatives, respectively.Furthermore, the synthesis procedure exhibited well to excellent yields under semi-mild conditions.

a
All products are known and were identified by comparing their physical and spectral data with literature b Isolated yields