Silver nanoparticles supported over mesoporous alumina as an efficient nanocatalyst for N-alkylation of hetero (aromatic) amines and aromatic amines using alcohols as alkylating agent

Accepted Manuscript Regular Article Silver nanoparticles supported over mesoporous alumina as an efficient nanocatalyst for N-alkylation of hetero (aromatic) amines and aromatic amines using alcohols as alkylating agent Priyanka Paul, Piyali Bhanja, Noor Salam, Usha Mandi, Asim Bhaumik, Seikh Mafiz Alam, Sk. Manirul Islam PII: DOI: Reference:

S0021-9797(16)31080-3 http://dx.doi.org/10.1016/j.jcis.2016.12.072 YJCIS 21906

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

9 September 2016 28 December 2016 30 December 2016

Please cite this article as: P. Paul, P. Bhanja, N. Salam, U. Mandi, A. Bhaumik, S. Mafiz Alam, Sk. Manirul Islam, Silver nanoparticles supported over mesoporous alumina as an efficient nanocatalyst for N-alkylation of hetero (aromatic) amines and aromatic amines using alcohols as alkylating agent, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.12.072

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Silver nanoparticles supported over mesoporous alumina as an efficient nanocatalyst for N-alkylation of hetero (aromatic) amines and aromatic amines using alcohols as alkylating agent Priyanka Paul,a Piyali Bhanja,b Noor Salam,c,d,‡ Usha Mandi,c,‡ Asim Bhaumik,b,* Seikh Mafiz Alama,* and Sk. Manirul Islamc,* a

Department of Chemistry, Aliah University, Newtown, Kolkata-700156, West Bengal, India

b

Department of Material Science, Indian Association for the Cultivation of Science, Kolkata -

700032, India c

d

Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India.

Department of Chemistry, The University of Burdwan, Burdwan, 713104, W.B., India.

Abstract We have successfully embedded rhombohedral crystallites of silver nanoparticles (AgNPs) over mesoporous alumina (Ag@Al2O3) material for the first time. The Ag@Al2O3 has been characterized by powder X-ray diffraction (PXRD), ultra high resolution transmission electron microscopy (UHR-TEM), scanning electron microscopy (SEM), N2 adsorption/desorption isotherm, Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible absorption spectra (UV-Vis), and thermogravimetric analysis (TGA). The PXRD confirms the presence of the rhombohedral phase of Ag nanoparticles. The agglomeration of the silver nanoparticle having a dimension of 80-90 nm has been clearly observed from UHR-TEM images. This novel nanocatalyst showed excellent catalytic activity towards the N-alkylation of hetero (aromatic) amines and aromatic amines using alcohols as the alkylating agent. The catalyst is heterogeneous 1

in nature and can be recovered and recycled more than five reaction cycles without any noticeable loss in its catalytic activity. Keywords: Mesoporous material, alumina, silver nanoparticles, N-alkylation reaction, hetero (aromatic) amines. ‡ *

These authors contribute equally to this manuscript. Authors to whom correspondence should be addressed.

Prof. Sk. Manirul Islam, Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India. Phone: +91-33-2582-8750, Fax: +91-33-2582-8282, E-mail: [email protected] Prof. Asim Bhaumik, Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata-700032, India, Phone: +91-33-2473-4971; Fax: +91-33-2473-2805, Email: [email protected] Dr. Seikh Mafiz Alam, Department of Chemistry, Aliah University, Newtown, Kolkata700156, West Bengal, India, Phone: +91-8902214131, Fax: +91-33-2986-0252, E-mail: [email protected]

Introduction The development of C-N bond is of great importance in synthetic organic chemistry because the nitrogen-containing compounds especially amines are the key intermediates in agrochemicals, bioactive molecules, pharmaceuticals and natural products [1]. Various methods have been developed for C-N bond formation [2]. But the transition metal catalyzed N-alkylation reactions of amines with alcohols, namely the borrowing hydrogen or hydrogen auto transfer methodology [3, 4] have recently become a new way to produce the useful amine derivatives. For the reaction with amines using alcohols as an alkylating reagent is an atom-economical [5]

2

and environment friendly approach because alcohols are commercially available at low cost and water is the only sustainable by-product produced during this reaction. This interesting transformation was first reported by Grigg [6] and Watanabe [7]. After that, many homogeneous transition metal-catalyzed N-alkylation reactions of amines have been studied [8]. However, these catalysts suffer from some difficulties, such as recovery, recycle of costly metals and specially handling of metal complexes. Therefore, the developments of highly efficient, easily recoverable and recyclable heterogeneous catalysts that can overcome these drawbacks are quite demanding for the N-alkylation of amines with alcohols. Recently, Nalkylation of amines using alcohols has been employed over various heterogeneous catalysts containing transition metals such as Pd [9], Ru [10] and Au [11]. Some silver grafted solid materials also exhibit high catalytic activity towards N-alkylation reactions of amines with alcohols [12]. Thus, the N-alkylation of amines with alcohols using silver nanomaterial is very important from the view point of benign and sustainable chemistry. 2-(N-Alkylamino)benzothiazole are very important building blocks in many biologically active compounds [13,14]. They also exhibit different pharmacological and physiological activities [15]. Various transition metal-based catalytic systems have been employed for the synthesis of these compounds [16]. But, these procedures undergo from the multistep preparation of the starting materials, low functional group tolerance and use of expensive metal catalysts. Thus, the production of 2-(N-alkylamino)benzothiazoles is still a challenge in organic synthesis. In view of the importance of 2-aminobenzothiazole derivatives, we also investigated the Nalkylation of other heteroaromatic amines with alcohols. Moreover, benzazoles such as benzimidazoles, benzoxazoles and benzothiazoles have also been found broad application in 3

pharmaceuticals [17]. Thus nowadays the synthesis of these heterocyclic compounds is most important for drug discovery in modern chemistry. Mesoporous metal oxides found several useful applications in the different frontline areas of research today [18]. Among these metal oxides, Al2O3 is the most promising material because of its high surface area, large pore volume, special size, inexpensive and easy to handle. For this reason, it has great importance in catalysis [19], electronics [20], optics [21] and so on. Catalytic activity of a nanomaterial is largely dependent on the size and shape of the nanoparticles and its suitable stabilizing media [22]. In this context, supported silver nanoparticles have also attracted considerable interest in the past few years, as catalysts for a number of reactions [23]. Here, we have prepared a new and atom-economical catalytic system composed of Ag@Al2O3 as a novel recyclable solid catalyst for a one-pot N-alkylation of heterocyclic amines and aromatic amines with alcohols under milder reaction conditions. It is revealed that Ag@Al2O3 material shows high catalytic activity due to the silver nanoparticles and the acid sites present at the alumina surface. We have determined the catalytic activity of Ag@Al2O3 towards N-alkylation reaction of 2-aminobenzothiazoles, 2-aminopyridines, 2-aminopyrimidines, benzazoles and anilines with alcohols as alkylating agent under mild reaction conditions for the synthesis of value added heteroaryl amine derivatives.

Experimental Materials Ammonium chloride (NH4Cl), sodium salicylate (C7H5NaO3), ammonia solution (25% aqueous) and silver nitrate (AgNO3) were obtained from E-Merck, India. Anhydrous aluminium chloride (anh.AlCl3), tris (hydroxymethyl) aminomethane (TRIS) and sodium borohydride 4

(NaBH4) were obtained from Spectrochem, India. All other reagents and substrates were purchased from Sigma-Aldrich. Characterisation techniques The FT-IR spectra of the samples were recorded on KBr pellets using a Perkin-Elmer FT-IR 783 spectrophotometer. UV-Vis spectra of the samples were obtained on a Shimadzu UV-2401 PC doubled beam spectrophotometer having an integrating sphere attachment. Scanning electron microscopic (SEM) images of the nanomaterials were recorded on a JEOL JEM 6700F fieldemission scanning electron microscope. Transmission electron microscopic (TEM) analysis was carried out with the help of a JEOL JEM 2010 transmission electron microscope operating at 200 kV. Powder X-ray diffraction (PXRD) pattern of the Ag@alumina nanocatalyst was obtained by using a Bruker D8 Advance X-ray diffractometer using the Ni-filtered Cu Kα (λ= 0.15406 nm) radiation. Thermogravimetric analysis (TGA) of the samples was carried out by using MettlerToledo TGA-DTA 851e thermal analyzer under N2 flow. After completion of the N-alkylation of hetero (aromatic) amines and aromatic amines using alcohols, the reaction products were analyzed through a Varian 3400 gas chromatograph equipped with a 30 m CP-SIL 8CB capillary column and a flame ionization detector. All reaction products were identified by matching the GC retention time with authentic samples. Dodecane was used as the internal standard for the GC analysis. Synthesis of mesoporous alumina Mesoporous alumina has been synthesized following a literature procedure [24]. In a typical synthesis, firstly 2.0 g of ammonium chloride was added to a 20 mL aqueous solution 5

consisting of 1.6 g sodium salicylate and this mixture was stirred for 15 minutes. After adding 25% aqueous ammonia solution (4 mL) to this mixture it was stirred for another 30 minutes. In another beaker, 1.33 g of anhydrous aluminium chloride was dissolved in 5 mL distilled water and this aqueous solution was slowly added to the above solution. By the addition of ammonia solution, the pH was adjusted to 10. Now this mixture was stirred for 3 h. Finally, the mixture was hydrothermally treated at 393 K for one day. The resultant white solid was collected by filtration, thoroughly washed with distilled water and dried well at room temperature under vacuum. Then the as-synthesized material was calcined at 773 K for 6 h to obtain the templatefree mesoporous alumina. Synthesis of colloidal silver nanoparticles In a typical synthesis, 1% of aqueous solution of silver nitrate (0.1 mL) was added to 10 mL of water containing 0.06 g TRIS and stirred for 2 minutes. After that, an aqueous solution of sodium borohydride (0.08%, 0.25 mL) was mixed to the above solution dropwise under constant stirring. The stirring was continued for another 10 min. Resulting silver nanocolloides were stored at 4 oC for the next step. Synthesis of Ag@Al2O3 nanocatalyst 100 mg of mesoporous alumina was dispersed in a 10 mL of TRIS stabilized Ag-NPs and the resulting mixture was stirred for 1 h at room temperature. Then the mixture was centrifuged. A black coloured material was obtained, which suggested the loading of Ag-NPs on the surface of mesoporous alumina. Then the resulting composite was cooled, washed with distilled water and dried at room temperature. The loading of Ag-NPs onto alumina was further confirmed by

6

spectroscopic analyses. The strategy for the synthesis of Ag@Al2O3 nanocatalyst is shown in Scheme 1.

Anh. AlCl 3, H2O

C7H5NaO3, H2O NH4Cl 25% aqu. NH3, Calcine

AgNO3

Al2O3 Al2O3

H2O, TRIS NaBH4 Ag-NPs

Ag@Al2O3

Scheme 1. Synthesis of Ag@Al2O3 nanomaterial. General procedure for the N-alkylation reactions over Ag@Al2O3: In a typical experiment, a mixture of 1.0 mmol of substrate (2-aminobenzothiazole or aniline or aminopyridine or aminopyrimidine), benzyl alcohol (1.0 mmol), Cs2CO3 (2.0 mmol), toluene (5 mL) and 20 mg of Ag@Al2O3 nanocatalyst was taken in a 50 mL round bottomed flask and stirred at 100°C for 10 h. In another typical experiment, a mixture of ortho-substituted (-NH2 or –OH or -SH) aniline (1.0 mmol), benzyl alcohol (1.5 mmol), Cs2CO3 (2.0 mmol) toluene (5 mL) and 25 mg of Ag@Al2O3 nanocatalyst was taken in a 50 mL round bottomed flask and stirred for 120°C for 30 h. Both reactions were carried out under N2 atmosphere. The progress of the catalytic reactions was monitored through the GC analysis of the reaction mixtures at different time intervals. On completion of the reaction, the reaction mixture was cooled and filtered. Then the mixture was passed through anhydrous Na2SO4 and dried under

7

vacuum. The residue was purified by silica-gel column chromatography to afford the corresponding desired product. The products were identified by GC analysis [29,30,33,34].

Results and discussion The mesoporous alumina supported silver nanocatalyst, Ag@Al2O3 has been characterized thoroughly by powder XRD, electron microscopy, FT-IR and UV-Visible spectroscopic studies together with elemental microanalysis and thermal analysis. Characterization of Ag@Al2O3 nanomaterial X-ray difraction analysis The wide angle powder X-ray diffraction pattern of both mesoporous Al2O3 and Ag@Al2O3 are shown in Figure 1. As noticed from Figure 1 (red) that sharp peaks are observed in the case of Ag@Al2O3 sample which signifies the highly crystalline nature. The characteristic crystal planes are well matched with the (012), (104), (110), (113), (024), (116), (214) and (030) crystal planes of the rhombohedral phase of Ag nanoparticles, analogous to the hematite α-Fe2O3 [25].

8

Figure 1. The wide angle powder XRD pattern of Al2O3 (black) and Ag@Al2O3 (red). Surface area and porosity measurement To measure the specific surface area of Ag@Al2O3 material nitrogen sorption analysis has been carried out at 77 K temperature in the presence of nitrogen gas as adsorbate molecule after outgassing the sample for 12 h at 413 K. The N2 adsorption/desorption isotherm of Ag@Al2O3 sample is shown in Figure 2. This isotherm can be classified as type IV with a very small hysteresis loop. At low-pressure region (P/P0 = 0.1-0.6) a steady increase in N2 uptake is observed instead of sharp capillary rise corresponding to the classical type IV isotherm and slight increasing fashion of isotherm at high-pressure region (P/P0 = 0.6-1.0), suggested the presence of wide range of mesopores in the material [26]. The BET (Brunauer-Emmett-Teller) surface area and pore volume of Ag@Al2O3 material are obtained to be 414 m2 g-1 and 0.2638 cc g-1, respectively. With the help of NLDFT (non-local distribution functional theory) method the pore size distribution plot has been drawn and this is shown in the inset of Figure 2. The three different sizes of mesopores are found in this pore size distribution plot for Ag@Al2O3 sample 9

which are 2.5, 3.1 and 3.7 nm. The De Boer statistical thickness (t-plot) represents that the total BET surface area of the Ag@Al2O3 material is acquired only due to mesoporosity as the micropore contribution to the surface area is almost negligible.

Figure 2. N2 adsorption/desorption isotherms of Ag@Al2O3 material where fill circle indicates the adsorption points and empty circle represents desorption points. The pore size distribution plot is shown in the inset of Figure 2. Microscopic analysis The ultra high resolution transmission electron microscopy (UHR-TEM) images of Ag@Al2O3 material is demonstrated in Figure 3. As seen from Figure 3a, the mesopores (white spot) with a diameter of ~3.0 nm are spread over throughout the whole specimen in a disordered manner. Figure 3a and 3b represents the UHR-TEM images of the Ag@Al2O3 sample at two different magnified scales i.e. 100 nm and 1000 nm, respectively. The agglomeration of the silver nanoparticle having a dimension of 80-90 nm has been clearly observed from Figure 3b, 10

sugggestinng tthe proopeer grrafttingg off Ag A oontoo thhe ppareent meesopporoouss aluuminiuum oxxidee maaterriall. The T ED DAX X ppatteern off Agg@Al2O3 mate m eriall haas bbeeen oobtaaineed ffrom mT TEM M aanaalyssis w wheere alll deesirred eleemeents likke A Ag, A Al aand O is preesennt ((Figguree 3cc). Froom AA AS annalyysis thee A Ag conntennt iin tthe Agg@A Al2O3 cattalyyst w wass foound too bee caa. 11.5 wt% %. In aaddditioon tto tthatt, thhe ffieldd em misssioon sscannninng eleectroon miccrooscoopicc (F FE--SE EM)) im magges of Agg@A Al2O3 materriall arre sshow wn in Figurre 4 att tw wo diff ffereent maagnnificcationss. A As nnotiicedd frrom m Fiigurre 44a, tthe agggloomeeratted silvver nannoppartticlee haavinng a ddiam meteer oof 2280--3000 nnm hhass beeen graafteed too A Al2O3 ssuppporrt.

Figgurre 33. Thhe UH HR--TE EM imaagees oof A Ag@ @All2O3 m mateeriaal att tw wo ddifffereent m maggnificaatioons:: 1000 nm m (a) annd 1 µ m ((b),, annd thhe E ED DAX X paatteern of A Ag@A Al2O3 m matteriial ((c)..

11

F Figu uree 4.. Thhe F FE-SEM iimaages off Ag@ A @All2O3 mate m erial att tw wo ddiffeerennt m maggnifficaatioons. FT T-IR R sp pecctrooscoopicc sttud diess F Figuure 5 dissplaayedd tthe FT T-IR R sspectraa oof m messopporoous Al A 2O3 aandd thhe corrressponndinng Agg@A Al2O3 maaterrialss. B Bothh m mesopoorouus Al2O3 annd A Ag@ @A Al2O3 shooweed aalm mostt sim millar ppeaaks at 34550 aandd 166400 cm m-1. Thhesee peeakks ccoulld bbe aassiigneed dduee to thee sttretcching andd beenddingg viibraatioons of OH H grrouups, resspeectivvelyy [227]. Inn puure meesopporrouss A Al2O3 tw wo chharaacteeristtic vibbratiionnal ppeaaks m-1 aree obbserveed ddue to thee Al-O A O viibraatioon m moddes in AlO O6 octtaheedra and AlO O4 at 77799 annd 66099 cm tetrraheedrra [[28]]. T Thee occtahheddral annd tetrraheedrral Al--O vibbrattionnal moodees aare shiifteed tto llow wer waavellenggth in thee sppecctrum of Agg@A Al2O3. Thhis sm mall shiift inddicaates thee ccoorrdinnatiion,, i.ee., an inteeracctioon bbetw weeen tthe silvver nannoppartticlees w withh thhe m messoporoous Al2O3 maaterriall.

12

Al2O3

Ag@Al2O 3

Figure 5: FT-IR spectra of Al2O3 and Ag@Al2O3 materials. UV-Visible spectroscopic studies The UV-Vis absorption spectra of the mesoporous Al2O3 and Ag@Al2O3 have been recorded and shown in Figure 6. An absorption band at 270 nm is observed for mesoporous [24]. In the case of Ag@Al2O3, it displays similar absorption bands at 270 nm as that of mesoporous Al2O3 together with an additional band at 400 nm region. The lower energy absorption band observed around 400 nm could be assigned to the surface plasmon resonance of the silver nanoparticles [12b].

13

Figure 6: UV-Vis spectra of Al2O3 and Ag@Al2O3. Thermal analysis We have carried out thermogravimetric analysis at 10 °C per min temperature ramp under air flow to estimate the thermal stability of Ag@Al2O3 material. In Figure 7 the TGA/DTA plots of Ag@Al2O3 in the temperature range 25-500 °C is shown. The first weight loss up to 150 °C could be attributed to the evaporation of adsorbed free water molecule bound at the outer and inner surface of Ag@Al2O3. This weight loss is associated with an endotherm in the DTA plot. Above this temperature region the further weight loss of ca. 3.8 wt% could be assigned to the further condensation of the surface defected sites with an associated mild endothermic peak (Figure 7b). Beyond this temperature no considerable weight loss is noticed. Thus this TGA/DTA analysis suggested that Ag@Al2O3 material possess considerably good thermal stability up to 500 °C. 14

Figure 7. TGA (a) and DTA (b) plots of Ag@Al2O3 under air flow. Catalytic activity Tiny nanoparticles dispersed over high surface area porous nanomaterials showed excellent catalytic activity in many areas of industrial processes and thus they are extensively studied. The catalytic performance of the Ag@Al2O3 nanocatalyst has been investigated in the N-alkylation of heteroaromatic amines and aromatic amines using alcohols as the source of alkyl group under N2 atmosphere. Catalytic N-alkylation reaction of 2-aminobenzothiazoles over Ag@Al2O3 The catalytic activity of our Ag@Al2O3 nanocatalyst has been studied on the N-alkylation of 2-aminobenzothiazole using benzyl alcohol as the alkylating agent (Scheme 2). We have optimized the reaction conditions for the N-alkylation reactions by varying solvent, base, and catalyst. To determine the best solvent for this N-alkylation reaction we have carried out the reaction in various solvents, like toluene, ethanol, 1,4-dioxane, H2O and DMF and their 15

respective yields are shown in Table 1. Now, from Table 1 it is observed that the reaction gives ~5% N-alkylated product (Table 1, entry 1) in toluene as solvent when mesoporous Al2O3 was used as catalyst. But in the presence of Ag@Al2O3 the maximum yield of 98% of product was obtained in toluene (Table 1, entry 2). Next, to optimize the base for the N-alkylation of 2aminobenzothiazole with benzyl alcohol, different bases, such as NaOtBu, Cs2CO3, K2CO3, NaOH, K3PO4 and DABCO was used and the corresponding yields were shown in Table 2. It is observed from Table 2 that without using any base only 7% yield was observed (Table 2, entry 1). In the presence of weak base K2CO3, 27% product was obtained (Table 2, entry 2) and in the presence of strong base NaOH, lower conversion of substrates and thus lower yield of product (39%) were obtained (Table 2, entry 3). whereas the use of bases like NaOtBu and Cs2CO3 give 92% and 98% of products respectively (Table 2, entries 4 and 5). Hence among the bases, Cs2CO3 is chosen as the most suitable base for the N-alkylation reaction of 2-aminobenzothiazole with benzyl alcohol. Furthermore, no products were obtained when the catalyst was absent in the reaction medium (Table 2, entry 8). This result suggested that this N-alkylation reaction over Ag@Al2O3 is purely catalytic in nature. OH S NH 2 N

Base, Solvent

S NH

+ 100°C, 10 h N2, Ag@Al2O3

N

Scheme 2: Catalytic N-alkylation reaction of 2-aminobenzothiazole with benzyl alcohol. Table 1: Optimization of solvent for the N-alkylation of 2-aminobenzothiazole with benzyl alcohola

16

a

Entry

Catalyst

Solvent

Yield (%)

1

Al2O3

Toluene

~5.0

2

Ag@Al2O3

Toluene

98.0

3

Ag@Al2O3

Ethanol

57.0

4

Ag@Al2O3

1,4-dioxane

48.2

5

Ag@Al2O3

H2 O

32.5

6

Ag@Al2O3

DMF

39.5

Reaction conditions: 2-aminobenzothiazole (1 mmol), benzyl alcohol (1 mmol), catalyst (20 mg,

1.5 wt% Ag loading), Cs2CO3 (2 mmol), solvent (5 mL), temperature 100 °C, time 10 h. Table 2: Optimization of base and catalyst for the N-alkylation reaction of 2aminobenzothiazole with benzyl alcohola

a

Entry

Catalyst

Base

Yield (%)b

1

Ag@Al2O3

-

<7

2

Ag@Al2O3

K2CO3

27.0

3

Ag@Al2O3

NaOH

39.0

4

Ag@Al2O3

NaOtBu

92.0

5

Ag@Al2O3

Cs2CO3

98.0

6

Ag@Al2O3

K3PO4

56.0

7

Ag@Al2O3

DABCO

17.0

8

-

Cs2CO3

-

Reaction conditions: 2-aminobenzothiazole (1 mmol), benzyl alcohol (1 mmol), catalyst (20 mg,

1.5 wt% Ag loading), base (2 mmol), toluene (5 mL), temperature 100 °C, time 10 h.

17

Table 3: Effect of silver source on the N-alkylation of 2-aminobenzothiazolea, aminopyridinea, anilinea and ortho-phenylenediamineb Entry

Silver

N-alkylation

source

reaction of 2-

N-alkylation N-alkylation reaction of

reaction of

reaction of ortho-

aniline

phenylenediamine

aminobenzothiazole aminopyridine

a

N-alkylation

Yield (%)

Yield (%)

Yield (%)

Yield (%)

1

None

No reaction

No reaction

No reaction

No reaction

2

Al2O3

~5.0

~3.0

Trace

Trace

3

Ag metal

21.2

16.4

10.2

12.2

4

AgNO3

35.4

22.3

20.5

~3.6.

5

Ag@Al2O3

98.0

95.2

96.3

91.2

Reaction conditions: amine (1 mmol), benzyl alcohol (1 mmol), catalyst (20 mg, 1.5 wt% Ag

loading), Cs2CO3 (2 mmol), toluene (5 mL), temperature 100 °C, time 10 h. bReaction conditions: amine (1 mmol), benzyl alcohol (1.5 mmol), catalyst (25 mg, 1.5 wt% Ag loading), base (2 mmol), toluene (5 ml), temperature 120 °C, time 30 h. The N-alkylation reactions of 2-aminobenzothiazoles, aminopyridines, anilines and ortho-phenylenediamine did not occur in absence of the catalyst (Table 3, entry 1). It was observed from Table 3 that in presence of only Al2O3, 2-aminobenzothiazole and aminopyridine gave a low yield of products whereas aniline and ortho-phenylenediamine gave trace amount of products (Table 3, entry 2). Again, we performed the N-alkylation reactions of 2aminobenzothiazoles, aminopyridines, anilines and ortho-phenylenediamine with Ag metal and AgNO3, the corresponding yields are not so very high (Table 3, entries 3 and 4). Now, when we 18

carried out the same reactions with our catalyst Ag@Al2O3, then the high yield of products (9198%) were obtained (Table 3, entry 5). Hence, the N-alkylation reactions were carried out using Ag@Al2O3 nanocatalyst as it showed excellent catalytic activity. After optimization of the reaction conditions, the standard N-alkylation reaction of 2aminobenzothiazoles with benzyl alcohols were carried out using toluene as best solvent and Cs2CO3 as base in presence of 20 mg of Ag@Al2O3 as catalyst at 100°C temperature for 10 h under N2 atmosphere. Now, these optimized reaction conditions are extended to the substituted 2-aminobenzothiazoles with substituted benzyl alcohols, which is shown in Table 4. N-alkylation of

2-aminobenzothiazole

with

benzyl

alcohol

produced

the

corresponding

2-

(alkylamino)benzothiazole in 98% yield (Table 4, entry 1). The electron-donating or withdrawing groups present in both the substrates vary the yields of the reactions. It is observed from Table 4 that the electron-withdrawing chloro group present in 2-aminobenzothiazole (Table 4, entry 2) gave 98% yield whereas the presence of electron-donating methyl and methoxy groups in 2-aminobenzothiazoles (Table 4, entries 3 and 4) gave comparatively higher yield of the desired products of 94% and 95% respectively. Moreover, the substrate benzyl alcohols with electron-withdrawing chloro and bromo groups produced 80%-88% yield of products (Table 4, entries 5 and 6) whereas the electron-donating methoxy group afforded the good results with 99% yield (Table 4, entry 7). As seen from this table that turn over numbers (TONs) for all the substrates are very high and varies from 321-357. Table 4: Catalytic N-alkylation of 2-aminobenzothiazoles with benzyl alcohols using Ag@Al2O3a

19

Entry

Amine

1

S

Alcohol

Product OH

NH 2

Yield (%) 98

Conversion (%)b 100

TONc

98

100

357

94

99

354

95

99

354

88

95

339

80

90

321

99

100

357

357

S NH

N

N

2

S

Cl

OH

NH 2

Cl

S NH

N

N

3

H 3C

S

OH

NH 2

H3C

S NH

N

N

4

H3CO

S

OH

NH2

H 3CO

S NH

N

N S

5

OH

NH 2

NH

Cl

N

Cl

S

N S

6

OH

NH 2

NH

Br

N

Br

S

N S

7

OH

N

OCH 3

S

NH 2

NH

H3CO

N a

Reaction conditions: amine (1 mmol), benzyl alcohol (1 mmol), catalyst (20 mg, 1.5 wt% Ag

loading), Cs2CO3 (2 mmol), toluene (5 mL), temperature 100 °C, time 10 h. bConversions are obtained from the GC analysis. cTurnover number (TON) = (% of conversion x mmol of substrate used)/(mmol of Ag in the catalyst used). Catalytic N-alkylation of anilines with benzyl alcohols over Ag@Al2O3 20

We continued the N-alkylation of aniline with benzyl alcohol after establishing the optimized reaction conditions (Tables 1 and 2) with toluene as a solvent, using Cs2CO3 as a base and Ag@Al2O3 as nanocatalyst at 100°C temperature under N2 atmosphere (Scheme 3). Now, we carried out the reactions of anilines and benzyl alcohols in the standard reaction conditions, which are summarized in Table 5. Herein, Table 5, entry 1 showed that the N-alkylated product of aniline with benzyl alcohol produced in a good yield of 96%. The reaction of aniline with 4methylbenzyl alcohol and 4-methoxybenzyl alcohol gave the corresponding N-alkylated product in 98% and 93% yield, respectively (Table 5, entries 2 and 3). Again, aniline gave 82% yield of the corresponding N-alkylated product with the halogen-substituted benzyl alcohol (Table 5, entry 4). With the electron-donating methyl and methoxy groups present at the para positions of anilines (Table 5, entries 5 and 6), the corresponding yields were 99% and 98% respectively. A product yield of greater than 99% was obtained in the reaction of 4-chloro aniline with benzyl alcohol (Table 5, entry 7). Very high TONs are observed for all aniline derivatives.

NH2

OH

+

Cs2CO 3, Toluene NH 100°C, 10 h N 2, Ag@Al2O 3

Scheme 3: Catalytic N-alkylation reaction of aniline with benzyl alcohol. Table 5: Catalytic N-alkylation of anilines with benzyl alcohols using Ag@Al2O3a Entry

Amine

Alcohol

Product

1 NH2

OH

NH

21

Yield (%)b

Conversion (%)

TON

96

99

354

2 NH2

H 3C

98

100

357

93

98

350

82

90

321

99

100

357

98

100

357

>99

100

357

CH 3

OH

NH

3 NH2

H 3CO

OCH 3

OH

NH

4 NH2

Cl

Cl

OH

NH

5 H 3C

NH 2

OH

H3C

NH

6 H3CO

NH2

OH H 3CO

NH

7 Cl

NH 2

OH

Cl a

NH

Reaction conditions: amine (1 mmol), benzyl alcohol (1 mmol), catalyst (20 mg, 1.5 wt% Ag

loading), Cs2CO3 (2 mmol), toluene (5 mL), temperature 100 °C, time 10 h. Catalytic N-alkylation reaction of aminopyridines and aminopyrimidines with benzyl alcohols using Ag@Al2O3 After investigating the N-alkylation reactions of 2-aminobenzothiazoles and anilines with benzyl alcohols, the present protocol was also applied to aminopyridines and aminopyrimidines. The catalytic N-alkylation reaction of 2-aminopyridine with benzyl alcohol is shown in Scheme 4. Now, the optimized reaction conditions (Table 1 and Table 2) are also applied to the N22

alkylation reactions of heteroamines, like aminopyridines and aminopyrimidines with benzyl alcohols. The resultant products were given in Table 6. The N-alkylation of 2-aminopyridine with benzyl alcohol yielded 95% of N-alkylated product (Table 6, entry 1). From Table 6, it was observed that 4,6-dimethyl-2-aminopyridine gave 90% of product on reaction with benzyl alcohol (entry 2) and 2-chloro-3-aminopyridine afforded 96% of the desired product (entry 3). Furthermore, 2-aminopyrimidine produced 97% of the corresponding product on reaction with benzyl alcohol, which was shown in Table 6, entry 4. The yield of the N-alkylation reaction of 4,6-dimethyl-2-aminopyrimidine with benzyl alcohol was 98% (Table 6, entry 5). Again, the electron-withdrawing bromo substituted benzyl alcohol and the electron-donating methoxy substituted benzyl alcohol produced 95% and 96% of the desired N-alkylated products, respectively (Table 6, entries 6 and 7).

NH2 N

OH

+

Cs 2CO 3, Toluene NH 100°C, 10 h N 2, Ag@Al2O 3

N

Scheme 4: Catalytic N-alkylation reaction of 2-aminopyridine with benzyl alcohol. Table 6: Catalytic N-alkylation of aminopyridines and aminopyrimidines with benzyl alcohols using Ag@Al2O3a Entry

Amine

Alcohol

Product

1 NH2

OH

N

NH N

23

Yield (%)

Conversion (%)

TON

95

100

357

2

H 3C

H3C

90

98

350

96

99

354

97

100

357

98

100

357

95

99

354

96

100

357

OH NH 2

NH

N

N

H 3C

H 3C

3 NH2

OH NH

N Cl

N Cl

N

4

N

OH

NH2

NH

N

N

5

H 3C

H 3C

N

OH

N

NH 2

NH

N

N

H 3C

6

H 3C

H 3C H 3C

N

OH

N

NH 2

NH

Br

N

N

H 3C

7

H 3C

H 3C H 3C

N

OH

N

NH 2 N

NH

H3CO

H 3C

a

Br

N

OCH 3

H3C

Reaction conditions: amine (1 mmol), benzyl alcohol (1 mmol), catalyst (20 mg, 1.5 wt% Ag

loading), Cs2CO3 (2 mmol), toluene (5 mL), temperature 100 °C, time 10 h. Catalytic N-alkylation reaction of ortho-substituted (-NH2 or -OH or -SH) anilines with benzyl alcohols using Ag@Al2O3 24

To check the catalytic activity of our silver nanocatalyst in the N-alkylation of hetero (aromatic) amines, the extent catalytic effect of this nanocatalyst with ortho-substituted (-NH2 or -OH or -SH) anilines with benzyl alcohols under the previously optimized reaction conditions was studied, which is already showed in Tables 1 and 2. The catalytic N-alkylation reaction of ortho-phenylenediamine with benzyl alcohol was shown in Scheme 5. Now, the scope of the Nalkylation reaction was extended to the other ortho-substituted anilines, like 2-aminothiophenol and 2-aminophenol with benzyl alcohols. The catalytic reaction of o-phenylenediamine with the alkylating agent benzyl alcohol was performed to obtain the desired product 2phenylbenzimidazole in 91%yield (Table 7, entry 1). Further, several electron-withdrawing or electron-donating substituents on benzyl alcohols were studied to afford the corresponding products in 84%-73% yields, respectively (Table 7, entries 2-5). Moreover, the catalytic reaction of 2-aminothiophenol with benzyl alcohol produced 95% of the desired product of 2phenylbenzothiazole (Table 7, entry 6). Again, 2-aminothiophenol on reaction with functionalized alcohols, such as 2-bromobenzyl alcohol, 2-methylbenzyl alcohol, and 3-methoxy benzyl alcohol produced 97%, 75% and 99% of yields, respectively (Table 7, entries 7-9). Moreover, N-alkylation reaction of 2-aminophenol with benzyl alcohol generated the corresponding product 2-phenylbenzoxazole in 90% yield (Table 7, entry 10). Here also we observed very high TONs for all the ortho-substituted anilines. NH 2 + XH

X= NH, S, O

Cs 2CO 3, Toluene

N

120°C, 30 h N2, Ag@Al2O3

X

OH

Scheme 5: Catalytic N-alkylation reaction of ortho-substituted (-NH2 or -OH or -SH) aniline with benzyl alcohol. 25

Table 7: Catalytic N-alkylation of ortho-substituted (-NH2 or –OH or -SH) anilines with benzyl alcohols using Ag@Al2O3a Entry

1

Amine

Alcohol

NH 2

Product

H N

OH

NH 2

2

Yield (%)b

Conversion (%)

TON

91

99

283

84

92

263

80

90

257

70

79

226

73

82

234

95

100

286

97

100

286

N

NH 2

H N

OH

Cl

NH 2

3

Cl N

NH 2

Br

NH 2

4

N

NH 2

CH 3

NH 2

N

N

OCH 3 NH 2

S

NH 2 SH

OCH 3 N

OH

SH

7

H3C

H N

OH

NH 2

6

Br

H N

OH

NH 2

5

H N

OH

N

OH Br

S

26

Br

NH 2

8

CH 3

SH

NH 2

9

N

OH

N

OH

S

N

OH

99

100

286

90

98

280

O

OH

a

237

OCH 3

OCH 3

NH2

83

S HC 3

SH

10

75

Reaction conditions: amine (1 mmol), benzyl alcohol (1.5 mmol), catalyst (25 mg, 1.5 wt% Ag

loading), base (2 mmol), toluene (5 ml), temperature 120 °C, time 30 h.

Comparison with other reported systems To show the efficiency and versatility of our present Ag@Al2O3 nanocatalyst in the Nalkylation of hetero (aromatic) amines and aromatic amines using alcohols, we have compared the catalytic activity of Ag@Al2O3 with other reported systems [29-35]. The catalytic Nalkylation reaction of 2-aminobenzothiazole gave 92-97% of the N-alkylated product in the presence of ruthenium (II), iron phthalocyanine and ruthenium (II) carbonyl complex catalyst [29-31], whereas using our Ag@Al2O3 nanocatalyst 98% of the desired product was obtained. The catalytic N-alkylation reaction of aniline resulted 92% and 94% yields in the presence of Ru(II) - CNN pincer complex and cobalt(II) - PNP pincer complex, respectively [32-33], but our Ag@Al2O3 nanocatalyst gave 96% of the N-alkylated product. Similarly, the N-alkylation reaction of 2-aminopyridine and o-phenylenediamine produced the desired product in high yields in the presence of Ag@Al2O3 nanocatalyst compared to other catalysts [29,31,32,34,35]. 27

Hence from the above discussion we can conclude that Ag@Al2O3 gave comparatively higher yields of different N-alkylated products that mean, higher catalytic activity than the literature study mentioned [29-35]. Test for Heterogeneity of the Ag@Al2O3 nanocatalyst To verify whether silver is being leached out from the Al2O3 support into the reaction mixture, controlled reaction for the N-alkylation of aniline with benzyl alcohol has been carried out. A typical hot filtration test of the N-alkylation of aniline with benzyl alcohol was performed under optimized reaction conditions. After 4 h of the reaction, the entire reaction mixture was filtered under hot conditions and the reaction was further continued for another 4 h with a part of the filtrate under the same reaction conditions in the absence of catalyst. In the absence of catalyst the yields of N-alkylated product remained unaltered at 51.8 and 52.1 after 4 and 8 h reaction times, respectively. From the above observation, it can be concluded that the N-alkylation of aniline over Ag@Al2O3 is catalytic in nature and the reaction has been catalysed heterogeneously. Recyclability of the Ag@Al2O3 nanocatalyst We have studied the recyclability of Ag@Al2O3nanocatalyst for the N-alkylation of 2aminobenzothiazole, aminopyridine, aniline and o-phenylenediamine with benzyl alcohol (Figure 8). The Ag@Al2O3nanocatalyst can be easily recovered from the reaction mixture by centrifugation. Then it was washed with distilled water and then by acetone to remove the products of the reaction. It was then dried in air and used as the recycled catalyst in the next runs. As seen from Figure 8 that the Ag@Al2O3 can be successfully recycled five times without any

28

appprecciabble losss in thhe catalytic acttivity. Maargiinall deecreease inn prodductt yiieldd coouldd bee atttribbutted to tthe parrtiaal looss of tthe cattalyyst aamoounnt dduriing thee reccycclinng pproccesss.

F Figu ure 8: Reecycclinng eefficcienncyy off Agg@A Al2O3 in thee N--alkkylaatioon oof aaminness. Cooncllusiion n Inn cooncclussionn, w we havve preeparred ann effficiientt reeusaablee hheteeroggeneeouus ccataalysst bbaseed oon graaftinng oof ssilvver nnannopaartiiclees att thhe suurfaacee off meesopporrouss allum minaa mate m eriall (A Ag@ @A Al2O3) aandd it shooweed exccelllentt ccataalyttic acttiviity forr tthe N-alkkylaatioon of am minnobeenzzothhiazzolees, annilinne, am minoopyrridiiness, aand am minoopyyrim midinees inn a susstaiinabble waay. Thiis pporoouss caatalyyst is verry aactiive andd reeusaablee foor thhe synntheesiss off 2-ssubbstittuteed bbenzzim midaazooles, beenzothhiazzolees aand bennzooxazzolees. Thiis ccataalyttic pprocesss iss grreenn, aatom m-eeconnom miccal aandd ennvirronm mennt-ffrieendlly. Furrtheer, tthe catalyyst is aair stabble annd thhe acttivee sittes do noot deecoompposee oor leeachh oout ffrom m tthe porouus m materiaal w whiich sugggests thaat oour cattalyyst is rrobbustt annd hheteerogenneous in natturee. H Hennce thee silveer nnanoocatalyyst

29

Ag@Al2O3 reported herein may contribute significantly for a wide range of catalytic Nalkylation reactions in future.

Acknowledgements PB acknowledges CSIR, New Delhi for a senior research fellowship. S.M.I. acknowledges the Department of Science and Technology (DST-SERB, Project No. SB/ S1/PC107/2012 dated 10/06/2013), New Delhi, India, University Grant Commission (UGC, Project F. No 43-180/2014(SR) dated 25th July 2015), New Delhi, India and Department of Science and Technology, West Bengal (DST-W.B., Project Sanction F. No-811(Sanc.)/ST/P/S&T/4G-8/2014, dated 04/01/2016) for funding. We gratefully acknowledge the DST and UGC, New Delhi, Govt. of India, for award of grant under FIST, PURSE and SAP program to the Department of Chemistry, University of Kalyani. UM is thankful to the UGC, New Delhi, India for her senior research fellowship. NS is grateful to the Science and Engineering Research Board (SERB), DST, Govt. Of India, for his National Post-Doctoral Fellowship Scheme (File No.: PDF/2015/000460). AB wishes to thank DST for funding through DST-UKIERI research grant.

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Graphical Abstract

Silver nanoparticles supported over mesoporous alumina as an efficient nanocatalyst for N-alkylation of hetero (aromatic) amines and aromatic amines using alcohols as alkylating agent Priyanka Paul, Piyali Bhanja, Noor Salam, Usha Mandi, Asim Bhaumik,* Seikh Mafiz Alam* and Sk. Manirul Islam*

A novel heterogeneous alumina supported silver nanocatalyst (Ag@Al2O3) has been designed and it showed excellent catalytic activity for the N-alkylation of hetero (aromatic) amines and aromatic amines with alcohols as the alkylating agent.