z Sustainable Chemistry

Amberlite IR 120H+ Catalyzed N-C/C-N Coupled CylizationStrategy to Give Imidazoles: Design and Fabrication ofOrganic Nanomaterial with AFM ImagingMithun Chakraborty,[a] Barnali Deb,[a] Bapi Dey,[b] Syed Arshad Hussain,[b] Dilip K. Maiti,[c] andSwapan Majumdar*[a]

Sustainable and highly efficient one-pot multicomponentsyntheses of functionalized imidazole derivatives were de-scribed using benzil, aldehydes, ammonium acetate/or aminesin the presence of Amberlite IR 120H+. The products wereobtained in the short period of time with high yields throughthe chromatography-free procedure. The catalyst could be

recycled due to its insolubility in most of the solvents andreused without any noticeable decrease in its catalytic activity.The novel strategy was exploited for synthesis of a designedsubstituted imidazole, fabrication of its nanostructured materi-als on LB films and AFM imaging.

Introduction

Multicomponent reaction (MCR)[1] is an important area ofresearch in the field of organic and medicinal chemistrybecause functional and complex molecules can be achieved inthe fast, efficient, time saving and operationally simple mannerwithout isolation of intermediates. The MCR contributessustainability through simplifying all aspects of organic syn-thesis. Thus, development of a new MCR is desirable for diversesyntheses of functional molecules using readily availableinexpensive ingredients. Recently tri- and tetra-substitutedimidazoles have received considerable attention of researchersbecause of their presence in the bioactive natural products aswell as their therapeutic potentialities.[2] Many of the substi-tuted imidazoles are known as inhibitor of p38 MAP kinase,[2c]

fungicidal,[2d] antibacterial[2e] and antitumor[2f] activities. Manyimidazole derivatives possess rapid fluorescence switching[3]

and semiconducting material properties.[4] These versatileapplicability of imidazole derivatives prompted the syntheticorganic chemists for developing efficient methods to achievewell-designed highly substituted imidazole derivatives. Anumber of catalytic reactions were investigated to afford 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles. The 2,4,5-trisubstituted imidazoles were generally synthesized[5] throughthree component cyclo-condensation of a 1,2-diketone, a-

hydroxyketone or a-keto-monoxime with an aldehyde andammonium acetate in refluxing acetic acid,[5a]using micro-waves,[5b] ionic liquids,[5c] p-toluenesuphonic acid,5d solid sup-ported silica-sulfuric acid,5e heteropolyacids,[5f] InCl3.3H2O,[5g] andNiCl2.6H2O/Al2O3.[5h] On the other hand, the syntheses of 1,2,4,5-tetra-substituted imidazoles[6] were carried out through four-component cyclocondensation using 1,2-diketone, a-hydroxy-ketone or a-keto-monoxime, aldehyde, primary amine andammonium acetate in the presence of HBF4–SiO2,[6a] SiO2-NaHCO3,[6b] and ZrCl4

[6d] under microwave irradiation, hetero-polyacid,[5f] and HClO4–SiO2

[6c] catalyst. Recently nickel (II)catalyzed sp3Ca–H activation of primary amines[6e] for multi C–N bond-forming robust annulation and solvent-free heatingprotocol[6f] with Ce(SO4)2.4H2O were also reported. In spite ofhuge number of literature reports for three or four componentsynthesis of multi-substituted imidazoles, most of them sufferfrom serious drawbacks such as prolong reaction time, co-occurrence of side products, use of elevated temperaturecreated by conventional heating or microwave, reacting withexpensive reagents and toxic metal catalysts. Thus, develop-ment of a new and simple strategy is still desirable usingreadily available ingredients and inexpensive metal-free recycla-ble catalyst for direct construction of poly-substituted imida-zoles with economic viability and sustainability. Current aware-ness, tightening environmental legislation and the interest ingreen chemistry, the chemical industries are driving to inves-tigate efficient solid acid catalysts to replace conventionalmineral acids. The use of solid supports, as either reagents oranchors, contributes significantly to progress and practicinggreen chemistry[7] because they render the reaction moreefficient and environment friendly. Solid acid reagents eitherpolymeric or solid supported have received much attention tothe researcher for implementing these reusable catalysts indifferent synthetic protocols. In this context polystyrene basedsolid acid Amberlite IR 120H+ (Figure 1) has many remarkablefeatures such as air and moisture stability, simple handling,

[a] M. Chakraborty, B. Deb, Prof. S. MajumdarDepartment of ChemistryTripura University, Suryamaninagar 799 022,Tripura (W), INDIAE-mail: smajumdar@tripurauniv.in

[b] B. Dey, Dr. S. A. HussainDepartment of Physics, Tripura University, Suryamaninagar 799 022,Tripura, INDIA

[c] Prof. D. K. MaitiDepartment of Chemistry, University of Calcutta92 A P C Road, Kolkata 700 009, INDIA

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/slct.201601596

Full PapersDOI: 10.1002/slct.201601596

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inexpensive and recyclability with efficiency.[8] It can berecovered by simple filtration of the post reaction mixtures.Therefore utilization of this kind of solid acid reagent generatesless organic and aqueous waste, which minimizes overallenvironmental hazards.

In continuation of our efforts[9] to develop environmentallybenign synthetic methods, herein we report the results of ourinvestigation related to the generality and selectivity in usingamberlite IR-120H+ as a catalyst for synthesizing polyaryl/alkylsubstituted imidazoles from benzil/benzoin, aldehydes andammonium acetate or both ammonium acetate-primary amineas the sources of imidazole nitrogen (Scheme 1).

Results and Discussion

In our initial investigation the efficiency of Amberlite IR 120H+

and other acidic resin such as Amberlyst 15 or Amberlyst 33was examined for synthesis of 2,4,5-triphenyl-1H-imidazole (3 a)using benzil (1), benzaldehyde (2 a) and ammonium acetate amolar ratio of 1:1:2.5 of respectively, under different reactionconditions (Table 1).

No reaction was observed at room temperature in both theaprotic and protic solvent using 5 % (w/w) Amberlite resin(Table 1, entries 1, 2). A mixture of unidentified products(Table 1, entry 3) was generated under refluxing THF. To ourdelight the desired product 3 a was obtained in 92 % yield inrefluxing ethanol after 10 min. (Table 1, entry 4). The yield wasnot significantly improved on carrying out the reaction withincrease amount (10 % w/w) of Amberlite catalyst (Table1,entry 5). The reaction did not proceed in the absence ofethanol (neat reaction) and catalyst (Table1, entries 6–7). Thereaction rate became sluggish and yield was reduced on

decreasing the reaction temperature (entry 8). We also con-ducted study with other nitrogen sources such as ammoniumformate (entry 9) and ammonium carbonate (entry 10). Unfortu-nately results were unsatisfactory, which might be due to theirhigh volatile nature, lower solubility, instability and competitiveside reactions. Two other resins Amberlyst 15 or Amberlyst 33were also examined under the reaction conditions and resultedonly 55 % and 58 % yields of 3 a after 1 h (entries 11,12). Thestriking difference of reactivity can be attributed due to the factthat Amberlite IR 120H + resin is a gel-type cross linkingpolymer bearing –SO3H group in the exterior and in polarsolvent like ethanol the swelling of resin occurs.[10] It causesenhancement of the catalytic potency through significantincrease of active surface area of the resin bead. Thus thereaction was inconsistant in aprotic solvent or under solvent-free conditions. On the other hand, Amberlyst 15 or 33 isbasically a macroreticular resin and consequently showsmoderate catalytic activity.

With the standardized preliminary result (Table1, entry 4) inhand, we turned our attention to carry out the reaction usingother structurally diverse aromatic and aliphatic aldehydes forestablishing the generality of the present protocol. A widerange of aromatic aldehydes bearing electron donating orwithdrawing substituents underwent through the cycloconden-sation reaction in one pot fashion to achieve 2,4,5-trisubsti-tuted imidazoles with high yield. The results are summarized inFigure 2. In our experiments p-tolualdehyde provided bestresults with benzil to produced 3 b in 95 % yield. Presentprotocol was also effective for the substrate benzoin thatproduced 90 % yield of 3 b but took longer reaction time(140 min). It can be seen from the Figure 2 that the functionalgroups like phenolic –OH (3c–d), ether functional groups (3 e,3g–h, 3o–q), -Cl (3i–j), -NO2 (3 k), -NMe2 (3 f), as well as indole(3 n) and thiophene heterocycles (3 m) remained unaffected

Figure 1. Amberlite IR-120H+ resin

Scheme 1. Synthesis of multisubstituted imidazole derivatives.

Table 1. Optimization of reaction conditions for the synthesis of tri-substituted imidazole (3 a) using Amberlite IR 120H+ catalyst.

Entry Catalyst Conditions Yield (%)a

1 Amberlite IR 120H+, 5% w/w THF, 25 oC, 5 h NRb

2 Amberlite IR 120H+, 5% w/w EtOH, 25 oC, 5 h NRb

3 Amberlite IR 120H+, 5% w/w THF, reflux, 1 h Mixture4 Amberlite IR 120H+, 5% w/w EtOH, reflux, 10 min 925 Amberlite IR 120H+, 10% w/w EtOH, reflux, 10 min 936 Amberlite IR 120H+, 5% w/w Neat, 80 oC, 5 h NRb

7 - EtOH, reflux, 5 h NRb

8 Amberlite IR 120H+, 5% w/w EtOH, 60 oC, 4 h 769 Amberlite IR 120H+, 5% w/w EtOH, reflux, 5 h Mixturec

10 Amberlite IR 120H+, 5% w/w EtOH, reflux, 5 h Mixturee

11 Amberlyst 15, 5% w/w EtOH, reflux 1 h 5512 Amberlyst 33, 5% w/w EtOH, reflux 1 h 58

abased on isolated yield, bno reaction, cammonium formate, dammoniumcarbonate.

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under the reaction conditions. Our results revealed that thereaction with aldehydes bearing electron withdrawing groupsoccurred faster than the electron donating aldehydes. Thereactivity of aldehydes bearing an electron withdrawing groupmade the carbonyl carbon more electrons deficient thatrendered the initial attack as well as subsequent reaction withammonia or amine. Unfortunately the reactions with aliphaticaldehyde possessing a-hydrogen produced a complex mixtureof products, which might be due to the presence enolisable a-hydrogen present in the aliphatic aldehydes. In the case of

formaldehyde, 4,5-diphenyl imidazole (3 l) was obtained in only58 % yield. In general the reaction was very clean and noundesirable side product was found on utilization of aromaticaldehydes. At least two moles of ammonium acetate wereneeded to install two nitrogen atoms in the imidazole frame-work; however we found much improved yields in our experi-ments on use of 2.5 moles of ammonium acetate. Based on theoptimized reaction conditions we have also devised a verysimple protocol for the synthesis of 1,2,4,5-tetrasubstitutedimidazoles (4) via four component one pot condensation ofbenzil (1), aldehyde (2), NH4OAc and primary amines. In thiscase, organic amine acts as the source of second nitrogen ofimidazole moiety. Both the aromatic and aliphatic primaryamines furnished tetra-substituted imidazoles in good toexcellent yield (60-85 %). Aniline provided lower yield (4 a-c,60–70 %) than benzyl amine-derived products (4d–h, 77–85 %)because of its lesser nucleophilicity. It also took longer reactiontime to complete the cyclocondensation reaction. All theproducts were characterized by recording FTIR, 1H and 13C NMRand HRMS (new compounds only) spectral data (SI).

The proposed mechanism for the Amberlite IR 120H+

catalyzed transformation of benzil to multisubstitutedimidazole is presented in Scheme 2. Ammonium acetate acts as

a latent source of ammonia. Aldehyde first gets protonated byresin bound acid thereby facilitate the formation of imine X.The second nucleophilic attack by ammonia or primary amineproduces aminal Y. The putative intermediate Y reacts withprotonated 1,2-dione to form an intermediate Z, which followsan acid catalyzed elimination of two molecules of water toafford imidazole 3 or 4.

We next examined the reusability of the Amberlite IR 120H+

catalyst in the reaction of benzil, benzaldehyde and ammoniumacetate under the optimized conditions (Table 1). After thecompletion of the reaction (10 min), the light yellow productwas dissolved in either dichloromethane or ethyl acetate. Theresin was separated by means of filtration and the filtrate

Figure 2. Synthesized tri- and tetra-substituted imidazoles.

Scheme 2. Catalytic cycle of Amberlite IR 120H+ catalyzed MCR’s.

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containing product was isolated through crystallization. Theresidue was washed successively with ethyl acetate andethanol, dried under vacuum and reused for fresh batches ofreaction. The results of five cycled showed that the recoveredcatalyst retains its activity in terms of yields of 2,4,5-phenylimidazole (Figure 3).

One of the aspects of synthesizing these imidazolecompounds is that the development of their innovative nano-fabrication properties to achieve unidirectional packing oforganic nanomaterial for fabrication of smart organic electronicdevices.[11] The extent of electronic or optoelectronic behavior[12]

depend primarily on their chemical structure, high chemicaland thermal robustness, good solubility in common organicsolvents, and should be available in reasonable quantities.Hence, various five- and six-membered heterocycles wereutilized as suitable p-conjugated chromophore backbones.Moreover, heteroatoms may act as auxiliary donors or acceptorsand improve the overall polarizability of the chromophore. Inthis respect, five-membered diazoles, in particular imidazole,seems to be suitable parent p-conjugated backbones.[13]

Imidazole possessing two nitrogen atoms of different electronicnature, represents a robust and stable heterocycle with furtherfunctionalization at positions C-2, C-4, and C-5. In view of theabove mentioned structural features and having in mind theutility of thin films in making the devices we designed,synthesized and investigated the surface morphology ofcompound 3 r assembled onto thin films. Langmuir Blodgett(LB) technique[14, 15] is used for making thin films of controllablesize and uniform surface with a high degree of orientation.Compound 3 r having a long alkoxy chain in the 4-position ofaryl group located at C-2 of imidazole core might act aselectron donor and C=N group in imidazole core or otherphenyl aryls at C-4 or 5 as an electron acceptor. Therefore, itwas expected that some sort of polarization might occur withinthe molecule in a particular supramolecular assembly. The longalkyl chain and N�H of imidazole core was helpful to fabricatesupra-molecular architecture through hydrophobic-hydropho-bic interaction and intermolecular hydrogen bonding, respec-tively. In our experiments, we observed that on spreading outof a dilute solution of pure 3 r in chloroform (0.5 mg/mL) ontoa pure water surface form a thin film, which was transferred to

a solid substrate for further investigation. The film wasprepared at 5mN/m surface pressure. The surface morphologyof 3 r, LB film was imaged in atomic force microscope (AFM).The surface morphology of the tri-substituted imidazole 3 r LBfilm revealed the formation of self-aggregated nanowirematerials (Figure 4A). The width and height of the ultra-long

nanowires are about 100 nm and 6 nm respectively (Figure 4B,C). Besides the layered thin films, such kind of nanowirematerials and their arrays in LB film might be also veryimportant with respect to their unique photo-physical andphotochemical properties. The LB films with well-definednanostructures would be promising candidates for applicationsin electronic devices. Also the size- and morphology-adjustablenanostructures are highly essential for fabricating nanoscalemolecular (opto)-electronic devices, which usually requires awide range of channel lengths to get the optimum gate oroptical modulation. The small molecule (3 r) bearing gluingweak interactions such as hydrogen bonding (donor andacceptor), p-p stacking, hydrophobic and van der Waalsinteractions had shown tremendous potential towards fabrica-tion of several types of organic nanomaterial, which will findconsiderable application in developing futuristic nanomaterialfor building organic electronic devices of ultimate sensitivity.

In conclusion, we have demonstrated an unprecedentedmethod for synthesis of poly-substituted imidazole derivativesusing highly efficient Amberlite IR 120H+ as a catalyst. Themethod provides many obvious advantages such as theavoidance of discharging harmful organic solvent, good yields,environmentally friendly nature and use of the relatively safer“green” technique. Furthermore, it is remarkable that thecatalyst amberlite IR 120H+ could be easily recycled andretained similar reactivity as well as yields after sixth cycles. Anorganic nanomaterial was fabricated simply by tuning thechange in surface pressure, which will find considerableapplication in developing futuristic nanomaterial for buildingsmart organic electronic devices of ultimate sensitivity. Detailsinvestigation on exploration of origin of self-assembly withvariation of alkyl chain length or N-1 substitution and effect ofsurface pressure on such assembly are underway.

Figure 3. Recyclability of Amberlite IR 120H+ catalyst.

Figure 4. (A) AFM image monolayer of pure 3 r at 5 mNm�1, (B) zoomedimage and (C) size distribution curve.

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Supporting information

Details of experimental procedure, compound characterizationdata and copies of 1H and 13C NMR spectra are available insupporting information.

Acknowledgments

The authors are grateful to the Council of Scientific andIndustrial Research (CSIR Project no. 02(0235)/15/EMR-II), Govt.of India for the financial support of this work.

Conflict of Interest

The authors declare no conflict of interest.

Keywords: aggregation, · LBL film, · organic nanostructuredmaterials, · solid acid catalyst, · substituted imidazoles

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Submitted: October 22, 2016

Accepted: December 19, 2016

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