CatalysisScience &Technology

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PAPER View Article OnlineView Journal | View Issue

Catal. Sci. TechnoThis journal is © The Royal Society of Chemistry 2014

Materials Science Division, CSIR-North East Institute of Science and Technology,

Jorhat 785006, Assam, India. E-mail: dipakkrdutta@yahoo.com;

Fax: +91 376 2370011

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cy00639e

Cite this: Catal. Sci. Technol., 2014,

4, 1047

Received 27th August 2013,Accepted 10th December 2013

DOI: 10.1039/c3cy00639e

www.rsc.org/catalysis

Efficient three-component coupling reactionscatalyzed by Cu0-nanoparticles stabilized onmodified montmorillonite†

Bibek Jyoti Borah, Subrat Jyoti Borah, Lakshi Saikia and Dipak Kumar Dutta*

The in situ generation of Cu0-nanoparticles into the nanopores of modified montmorillonite and their

catalytic performance in the three component (A3) coupling reactions of aldehyde, amine and alkyne to

synthesize propargylamines have been described here. The modification of montmorillonite was carried

out with HCl under controlled conditions for generating nanopores into the matrix and these pores act

as a “host” for the in situ generation of Cu0-nanoparticles. The Cu0-nanoparticles were generated in the

nanopores of the acid activated clay mineral matrix by the successful loading of a Cu(CH3COO)2 metal

precursor using an incipient wetness impregnation technique followed by reduction with NaBH4. The

TEM study reveals that Cu0-nanoparticles of below 10 nm in size are evenly distributed on the support.

The synthesized Cu0-nanoparticles exhibit a face centered cubic (fcc) lattice. The Cu0-nanoparticles

serve as an efficient green and heterogeneous catalyst for three-component coupling via C–H alkyne-

activation to synthesize propargylamines with excellent yields (82–94%) and 100% selectivity under mild

reaction conditions without requiring any additives or an inert atmosphere. The nanocatalysts can be

recycled and reused several times without significant loss of their catalytic activity.

1. Introduction

Recently, in the route of the development of green processesfor the synthesis of fine chemicals and bio-active molecules,the concept of atom economy/efficiency finds considerableattention.1 In this regard, an increasing number of multi-component reactions have been designed for the synthesis ofdiverse complex molecules through a combination of three ormore starting materials in one-pot, which reduces waste andincreases safety as well as reaction yield.2 Among the differentmulticomponent reactions, the catalytic coupling reaction ofaldehyde, amine and alkyne (so-called A3 coupling) is one ofthe best examples, where propargylamines are produced asthe main product. Propargylamines are versatile intermediatesfor the preparation of various nitrogen-containing compoundsand are the key components of biologically active pharmaceu-ticals and natural products.3 Propargylamines derivatives havegained much importance after the pivotal discovery ofrasagiline (Azilect™), which is molecularly a propargylaminederivative used as a monotherapy in early Parkinson's disease,and some propargylamine derivatives are also used as potentanti-apoptotic agents and neuroprotective agents.4 Several

classical methods are available for the synthesis ofpropargylamines through the addition of metallated alkynesto an imine. Metallated alkynes are generated by using differ-ent strong bases such as alkylmetals, metallated amides,alkoxides and hydroxides.5 However, these metal alkynylidesare relatively difficult to handle, because they are moisturesensitive and require strictly controlled reaction conditionsotherwise they form large amounts of waste materials.In recent years, various mid- and late transition metal cata-lyzed couplings of aldehydes, amines and alkynes havebecome a powerful tool that facilitates the coupling of alkynesto imines (generated in situ) via terminal alkyne C–H activa-tion in a more efficient manner.3e–h,6 Among the differentmetals, Cu(I) and Cu(II) complexes, and also various simpleand inexpensive copper halide salts have been extensively uti-lized as catalysts for the A3-coupling reaction with secondaryamines.7 However, most of these catalytic systems are homo-geneous in nature, and only a few reports on heterogeneouscatalysts are available in the literature. Li and Wang reportedan efficient and reusable heterogeneous Cu(I) silica basedorganic–inorganic composite material for the A3-couplingreaction under solvent free conditions.8 Li, Wang and co-workers prepared a silica supported complex of Cu(I) and anN-heterocyclic carbene and used it to achieve a high yield inthe A3-coupling reaction.9 Besides silica based materials, othersupports like molecular sieves, zeolites, etc., have also beenused for immobilizing Cu metal.6d,10 But these catalytic systems

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often show several disadvantages. Most of the catalyst supportsare synthetic and require laborious and time-consuming effortsin their synthesis as well as surface functionalization, whichmay poison the catalytic active sites of the supports and theparticles, moreover, molecular sieves or zeolites, like meso-porous matrixes in general, exhibit low or restricted pore sizes.

In the last decade, supported metal nanocatalytic systemshave been considered as a sustainable and competitive alter-native to conventional catalysis, due to their high surfacereactivity and high surface-to-volume ratio of metal nano-particles, which enhances their activity and selectivity andmore importantly they satisfied the stringent criteria of aheterogeneous catalyst.11 Nowadays, nanoparticles of metalssuch as Cu, Au, Fe, In, etc., have been used as substitutes forbulk metal in the various C–H activation reactions in order toreduce catalyst loading, reaction time and also to minimizethe undesired products.2a,3g,6a,11,12 Particularly, Cu0-nano-particles supported on different supports have been exten-sively utilized in the 1,3-dipolar cycloaddition of azides andterminal alkynes to synthesize 1,2,3-triazoles.13 But, limitednumbers of reports are available in the literature whichdescribe the use of supported Cu0-nanoparticles in the A3

coupling reaction.12a,14 Albaladejo et al. reported an efficientcatalytic system based on Cu0-nanoparticles supported onTiO2 for the A3 coupling reaction and widely illustrated thesubstrate tolerance as well as recyclability of the catalyst.12a

Although, homogeneous and heterogeneous catalysts studiedin A3 coupling reactions show good activity, there are stillsome drawbacks which restrict their acceptance, such as highprices of the used precious metals and tedious catalyst prepa-ration methods. In addition, some catalytic systems usedenvironmentally unacceptable substances, harsh reactionconditions, long reaction times, and required additives orinert atmospheres. Therefore, the development of a novel,efficient and recoverable catalytic system for A3 couplingreactions is a great challenge for researchers. With these pre-cedents in mind and encouraged by our recent works,15

herein, we have reported a well-defined heterogeneous cata-lytic system i.e. Cu0-nanoparticles supported on modifiedmontmorillonite for A3 coupling reaction under mild reactionconditions. These nanoparticles maintain their catalytic effi-ciency for several cycles and serve as a potential reusable cat-alyst. The modified montmorillonite is considered to beenvironmentally benign, cheap, easily available and a robust

1048 | Catal. Sci. Technol., 2014, 4, 1047–1054

Table 1 Surface properties of different montmorillonite based support/cat

Samples

Surface properties of suppor

Specific surface area(m2 g−1)

Avera(nm)

AT-Mont. 422 4.28

Cu0-Mont. 299 5.181 255 6.432 210 7.253 192 8.53

support/stabilizer material for the synthesis of different metalnanoparticles.15 Laszlo et al. reported interesting montmoril-lonite based catalytic systems for organic synthesis.16 Thismontmorillonite was collected from an indigenous source,i.e. natural deposits from the western part of India, and waspurified and activated with mineral acid under controlledconditions to generate a matrix with a high surface area andwhich contained micro- and mesopores with diameters in therange 0–10 nm on the surface. These nano range pores act asa “host” for the in situ generation of Cu0-nanoparticles andalso limit the growth of the particles up to a desired range.The other added advantages of this catalyst support is that itrequires no surface functionalization and the pore size canbe customized by applying controlled acid activation.15a–d

2. Experimental section2.1. Preparation of support

Purified Na+-montmorillonite (10 g) was added into a 250 mLthree necked round bottom flask and 200 mL of 4 Msulphuric acid was added to it. The resulting dispersion wasrefluxed for 1 h. After cooling, the supernatant liquid wasdiscarded and the activated montmorillonite was repeatedlywashed with deionised water and finally dried in a hot airoven at 50 ± 5 °C overnight to obtain the solid product. Theactivated montmorillonite was designated as AT-Mont. Thesurface characterizations of activated montmorillonite arepresented in Table 1.

2.2. Preparation of Cu0-nanoparticles

0.5 g of AT-Mont. was added into a 100 mL beaker and15 mL of the (0.254 mmol) aqueous solution of Cu(CH3COO)2was added slowly under vigorous stirring. The stirringwas continued for another 6 h followed by evaporation tothe dried solid in a rotary evaporator. 0.3 g of the dryclay–Cu(CH3COO)2 composite was dispersed in 50 mL of waterin a 100 mL two necked round bottom flask and 10 mL of anaqueous solution of NaBH4 (134 mg, 3.54 mmol) was thenadded slowly over 15 min in a nitrogen environment underconstant stirring. The reaction started immediately and thecolour changed from blue to black, due to conversion ofCu(II) into Cu0-nanoparticles. The black solid mass was recov-ered and washed with distilled water several times and then

This journal is © The Royal Society of Chemistry 2014

alysts

t/catalysts

ge pore diameter BJH pore size distribution(nm)

Specific pore volume(cm3 g−1)

3.77 0.58

3.81 0.483.90 0.384.27 0.323.89 0.30

Fig. 1 N2 adsorption–desorption isotherm and BJH pore size distributioncurve (inset) of AT-Mont.

Fig. 2 Powder XRD pattern of Cu0-Mont.

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dried in a desiccator for 12 h. The sample thus prepared wasdesignated as Cu0-Mont.

2.3. General procedure for the one-pot synthesis ofpropargylamines

The aldehyde (1 mmol), amine (1 mmol), alkyne (1.2 mmol),20 mg of catalyst and 5 mL of toluene were put in a 25 mLround bottom flask and the reaction mixture was refluxed at110 °C for a stipulated time period. The progress of the reac-tions was monitored by TLC. After completion of the reaction,the solid catalyst was separated from the mixture by filtrationthrough a sintered funnel (G-3) and the solvent was removedunder low pressure in a rotavapor. The recovered catalyst waswashed with acetone (30 mL), dried in a desiccator and storedfor another consecutive reaction run. The crude productobtained was then purified by silica gel column chromatogra-phy using ethyl acetate and hexane as eluents. The productswere characterized by (1H, 13C) NMR and mass spectrometryand all gave satisfactory results [ESI†].

3. Results and discussion3.1. Characterization of support

Naturally occurring montmorillonite is considered to be acheap, robust, easily available and environmentally benigncatalyst and catalyst support.15a–f Purified montmorillonitewas modified with a mineral acid treatment under con-trolled conditions in order to generate a high surface areaand porous matrix. The characterization of the modifiedmontmorillonite (AT-Mont.) was thoroughly carried out withthe help of different sophisticated analytical instrumentslike Powder-XRD, FTIR, N2 adsorption–desorption, SEM, 29Siand 27Al MAS-NMR studies and is well documented in ourearlier reports.15a–f The parent montmorillonite (ParentMont.) is 2 : 1 layered dioctahedral aluminosilicate and con-sists of two tetrahedral silicate sheets which are bonded toeither side of an octahedral aluminate sheet having a basalreflection at 7.06° 2θ corresponding to a basal spacing of12.5 Å.17 The powder XRD reveals that after 1 h of acid acti-vation, the layered structure is disrupted and becomes anamorphous high silica containing matrix [ESI†]. During acidactivation, the surface area (up to 422 m2 g−1) (Table 1) aswell as pore volume (~0.6 cm3 g−1) increased due to theleaching of Al from the octahedral sites of the montmoril-lonite matrix, which also introduced permanent porosity onthe clay surface. The clay mineral matrix contained micro-(<2 nm) and mesopores (2–50 nm) with average pore diame-ters of ~4.28 nm and also the differential volume versus porediameter plot, i.e. BJH plot, indicated a relatively narrowpore size distribution and these nano range pores wereadvantageously utilized for the in situ synthesis of variousmetal nanoparticles as reported by us recently.15a–f The shapeof the N2 adsorption–desorption isotherm (Fig. 1) wasof type-IV with a H3 hysteresis loop at P/Po ~ 0.4–0.9, indi-cating a mesoporous solid. The FTIR, 29Si and 27Al MAS-NMRmeasurements and SEM-EDX studies also confirmed

This journal is © The Royal Society of Chemistry 2014

that during acid activation, nano range pores are formed onthe surface of the clay mineral matrix due to Al leachingand predominant amounts of Si compared to Al werepresent on the surface [ESI†].15a–e

3.2. Characterization of Cu0-nanoparticles

The evidence of formation of the Cu0-nanoparticles (Cu0-Mont.)was obtained from powder XRD analysis. Fig. 2 shows threebroad peaks of 2θ values 43.3, 50.3 and 74.1° which areassigned to the (111), (200) and (220) indices of the fcc latticeof metallic Cu. The XRD peaks for any other oxidation statesof Cu were not observed, which could be attributed to thepresence of Cu only in its zero oxidation state.

The TEM image (Fig. 3) of the nanoparticle compositeshows that dispersed Cu0-nanoparticles were formed in the

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Fig. 3 TEM and HRTEM (inset) images of Cu0-nanoparticles.

Fig. 4 N2 adsorption–desorption isotherm and BJH pore sizedistribution curve (inset) of Cu0-Mont.

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micro- and mesopores of AT-Mont. The supported Cu0-nano-particles are spherical in shape, well separated from eachother and with sizes of below 10 nm. From the TEM study, itwas also observed that some of the Cu0-nanoparticles werefound to be larger than the pore size of the support whichmay be due to the presence of Cu0-nanoparticles on the outersurface of the support rather than inside the pores. TheHRTEM image (Fig. 3 (inset)) of a single Cu0-nanoparticleshowed regular lattice planes inside the nanoparticles andthese planes are continuously extended to the whole particlewithout any stacking faults or twins, indicating its singlecrystalline nature.

SEM-EDX analysis also substantiated the formation ofCu0-nanoparticles in the well-tuned pores of AT-Mont. Fur-thermore, EDX analysis indicates that Cu is present on thesurface of the modified montmorillonite along with the otherelements of clay [ESI†].

Although the encapsulation of the Cu0-nanoparticles intothe nanopores of the modified clay mineral changed some ofthe textural parameters of the materials, from Fig. 4, it isobserved that Cu0-Mont. exhibits a similar type of isothermand hysteresis loop to that of AT-Mont. This reveals that thehighly ordered porous structure of clay is still maintainedeven after the encapsulation of the Cu0-nanoparticles. Anappreciable decrease in specific surface area and specificpore volume after supporting Cu0-nanoparticles (Table 1)might be due to clogging of some of the pores by the Cu0-nanoparticles. In addition, the presence of Cu0-nanoparticlesmay cause complexities in the porosity measurements withnitrogen sorption, because the electrostatic forces betweenan adsorbate (i.e. nitrogen) and a metallic surface may affectthe measured values to some extent. However, the increase inpore diameter may be due to the rupture of some of the

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smaller pores to generate bigger ones during the formationof the Cu0-nanoparticles in the pores.

The Cu contents in Cu0-Mont., as analyzed by ICP-AES,reveals the presence of 0.05 mol% of Cu in the catalyst.

3.3. Catalytic activity

In order to determine the best reaction conditions (i.e. sol-vent, reaction time, etc.) that are required to afford excel-lent yields of propargylamines, a series of three-componentreactions were carried out using benzaldehyde, piperidineand phenylacetylene as the model substrates. After screen-ing a wide range of reactions, we found that our catalyticsystem (i.e. Cu0-Mont.) is most efficient for the title reac-tion in toluene as a solvent at refluxing temperature. Thereactions required 3 h time for completion. A control experi-ment under the above conditions but in the presence ofonly the acid activated clay (AT-Mont.) as catalyst was doneand gave no trace of product after 24 h. Using the opti-mized reaction conditions, we explored the versatility andlimitations of some commercially available aldehydes,secondary amines and alkynes as substrates as well as theefficiency of our catalyst for the said three-componentcoupling to synthesise propargylamine derivatives and theresults are summarized in Table 2. Firstly, phenylacetyleneand piperidine were allowed to couple with different aro-matic aldehydes (e.g. benzaldehyde, 4-methoxybenzaldehyde,4-chlorobenzaldehyde, 4-methylbenzaldehyde, 3-bromobenzaldehyde,2-hydroxybenzaldehyde) and gave the corresponding propar-gylamines in very good to excellent yields. The substituent ofthe aldehyde seemed to have pronounced effects both on thereaction time and on the isolated yield. The model reaction,i.e. the coupling between benzaldehyde, phenylacetyleneand piperidine gave a 94% isolated yield (entry 1). In thecase of aldehydes containing an electron donating groupe.g. 4-methoxybenzaldehyde, 4-methylbenzaldehyde and

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Table 2 Cu0-nanoparticle catalyst A3 coupling reaction of aldehyde, amine and alkyne

Entry Alkyne Aldehyde Amine Product Time (h) Yielda (%)

1 394b

90c

88d

2 5 89

3 5 87

4 3 92

5 390b

88c

85d

6 5 87

7 3 82

8 3 87

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Table 2 (continued)

Entry Alkyne Aldehyde Amine Product Time (h) Yielda (%)

9 3 89

10 390b

87c

84d

11 5 85

Reaction conditions: aldehyde (1 mmol), amine (1 mmol), alkyne (1.2 mmol), 20 mg Cu0-Mont. (0.05 mol %), solvent: toluene, temperature:110 °C. Yields are given for isolated products.a The reaction products were free from copper as checked by the analytical method followed fromVogel's Textbook of Macro and Semimicro Qualitative Inorganic Analysis; revised by G. Svehla, Fifth Edition, 1979, Longman Group Limited,London, page no.: 215–217. Methodology: 100 mg of each of the reaction products (from entries 1 or 2) were dissolved in 10 mL of mediumconcentrated HNO3 (8 M). The solution was diluted by adding 10 mL of distilled water followed by passing H2S gas through the solution for aperiod of 5 minutes. No black precipitate was observed. Hence, reaction products were free from Cu. The detection concentration limit is: 1 in5 × 106. Selectivity: 100%.b 1st run. c 2nd run. d 3rd run.

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2-hydroxybenzaldehyde, it was observed that they requiredrelatively longer times and gave slightly lower yields com-pared to aldehydes containing electron withdrawing groups,e.g., 4-chlorobenzaldehyde and 3-bromobenzaldehyde. Wealso used an aliphatic alkyne, i.e., 3-butyne-1-ol and allowedit to react with two different iminium ions which were gener-ated in situ from benzaldehyde and piperidine (entry 8) andbenzaldehyde and morpholine (entry 9), it was found that theyformed the desired propargylamines in a high yield. In anotherstrategy, we compared the synthesis of propargylamines bykeeping the benzaldehyde and phenylacetylene constantand using different secondary amines such as piperidine,morpholine, and diethyl amine. The results obtained fromthese studies revealed that an aliphatic amine, i.e. diethylamine(entry 7), shows a lower conversion than the cyclic amines,i.e. piperidine and morpholine (entries 1 and 10). Next, weexplored our methodology for the synthesis of symmetricalbis-propargylamine derivatives by coupling dialkyne, viz.1,6-heptadiyne, with benzaldehyde and piperidine (entry 11)and as expected, the desired product formed with a goodisolated yield (85%) but the reactions needed a comparativelylonger time, i.e. 5 h, for completion.

In order to show the efficiency as well as the practicalapplicability of the catalyst (Cu0-Mont.), we scaled up thereaction of benzaldehyde to 10 mmol with 10 mmol of piperi-dine and 13 mmol of phenylacetylene under similaroptimised reaction conditions. The reaction proceeded wellwith a 90% isolated yield and 100% selectivity at 5 h.

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3.4. Recyclability of the catalyst

For practical applications of heterogeneous catalysts, the levelof reusability is a very important factor. The recyclability ofour catalyst for the three component coupling reactions wasinvestigated (Table 2) in the coupling of phenylacetylene withbenzaldehyde and piperidine (entry 1), 4-chlorobenzaldehydeand piperidine (entry 5) and benzaldehyde and morpholine(entry 10). The catalyst was recovered by a simple filtrationtechnique after each experiment. The recovered catalyst waswashed with acetone, dried in a desiccator and reuseddirectly with fresh reaction mixture and without further puri-fication for the desired coupling products synthesis up to a3rd run. The results (Table 2, entries 1, 5 and 10) show thatin each case the catalyst remains active for several runs with-out significant loss in efficiency.

The recovered catalyst was further investigated through N2

adsorption–desorption, powder XRD studies and HRTEManalysis. The identical shape of the isotherms and the hyster-esis loop reveal that the characteristics of the catalysts areretained even after successive reactions (ESI†). The specificsurface areas of the recovered catalysts decrease compared to299 m2 g−1 of the freshly prepared catalyst (Table 1). Thedecrease in surface area of the recovered catalysts after eachreaction may be due to blockage of the pores by the reactantmolecules. In the HRTEM analysis of the recovered Cu0-Mont.(after 3rd run), it was found that a few particles aggregate,but the sizes are still below 10 nm (Fig. 5). In the powder XRD

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Fig. 5 HRTEM image of recovered Cu0-Mont. after 3rd run.

Scheme 1 Tentative reaction mechanism of A3 coupling reaction.

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of the recovered catalyst, the appearance of low intense broadpeaks at around 2θ values 36 and 54° along with other char-acteristic peaks of metallic Cu indicates the formation of asmall amount of CuO after the 1st run of the reaction(ESI†.18 However, the efficiency of the recovered catalyst wasnot affected much though a fraction of Cu(0) is converted toCuO. This may be because Cu(II) is also known for exhibitinghigh alkynophilicity. These experimental results are adequateenough to establish the robust character of the catalyst andalso prove that the reactions occur in a heterogeneous process.

A tentative reaction mechanism for the three component(A3 coupling) synthesis of propargylamines catalyzed by Cu0-nanoparticles under heterogeneous conditions is proposed.The tentative mechanistic scheme is pictorially illustrated inScheme 1. It is proposed that, in the first step, a terminalalkyne is coordinated to Cu0-nanoparticles supported onAT-Mont., which activates the C–H bond. As a result, thecorresponding copper–alkylidine complex is formed on thesurface of the AT-Mont. This is a very favourable step becauseCu metal is well known to exhibit high alkynophilicity forterminal alkynes.12a,15a In the second step, an addition of thecopper–alkylidine complex to the iminium ion generatedin situ from the reaction between the aldehyde and the amineoccurs to give the desired product, and the catalyst is regene-rated. The only theoretical by-product formed from the reac-tion is water. The regenerated catalyst continues the catalyticcycle until the completion of the reaction. The drastic reduc-tion in catalyst amount requirement as well as reaction timein these reactions is possibly due to the presence of highlyactive Cu0-nanoparticles as the catalyst. The presence of arobust and high surface area (422 m2 g−1) modified montmo-rillonite as the support for the Cu0-nanoparticles also mighthave a beneficial effect on the rate and selectivity of the reaction.

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4. Conclusion

Nanoporous modified montmorillonite was found to be anexcellent eco-friendly support material for the in situ syn-thesis of Cu0-nanoparticles. The Cu0-nanoparticles werespherical in shape and had sizes in the range of 0–10 nm.These nanoparticles demonstrated high catalytic activity inpromoting the three-component coupling of aldehyde,amine and alkyne via C–H alkyne-activation to synthesizepropargylamines with excellent yields and selectivity undermild reaction conditions without requiring any additives oran inert atmosphere. Further, the nanocatalysts were reusedfor new batch of reactions without a significant loss intheir activity under similar conditions. The operational sim-plicity, robustness of the catalyst as well as utilization ofenvironmentally benign indigenous materials as the catalystsupport and use of mild reaction conditions, make itattractive for the large scale synthesis of biologically activepropargylamine molecules.

Acknowledgements

The authors are grateful to Dr. R. C. Boruah, Director, CSIR-North East Institute of Science and Technology, Jorhat,Assam, India, for his kind permission to publish the work.The authors also thank Council of Scientific and IndustrialResearch (CSIR), New Delhi, India, for financial support(XIIth FYP Network projects: BSC 0112, CSC 0125, CSC 0135and In-house project: MLP-6000/1). SJB thanks UGC, NewDelhi for JRF.

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