Metal-Free Greener Syntheses of Pyrimidine ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Metal-Free Greener Syntheses of Pyrimidine Derivatives Using a Highly Efficient and Reusable Graphite Oxide Carbocatalyst under Solvent-Free Reaction Conditions Binoyargha Dam, Ramen Jamatia, Ajay Gupta, and Amarta Kumar Pal* Department of Chemistry, Centre for Advanced Studies, North-Eastern Hill University, Umshing, Mawkynroh, Shillong−793022, India S Supporting Information *

ABSTRACT: Graphite oxide, a green metal-free carbocatalyst, has been successfully exploited for the library synthesis of biologically active pyrimidine derivatives. Reaction was carried out under solvent-free reaction conditions (SFRC) thereby reducing the application of hazardous chemicals and solvents. The present catalytic system eliminates the risk of metal contamination in the product which is viable for pharmaceutical industries and showed better catalytic activity under sustainable conditions compared to other classical catalytic systems. The catalyst, being heterogeneous in nature, can be easily recycled and reused up to nine consecutive runs without much decrease in catalytic activities thereby increasing sustainability of the procedure. Diversity in the formation of pyrimidine moieties has been exhibited with the tolerance of a large number of functional groups establishing the generality of this reaction. A few other cutting edge advantages of the present one-pot multicomponent methodology are high atom economy, low catalyst loading, milder reaction conditions, higher yield of the desired product, simple work up procedure, easy handling of the catalyst, etc. The present methodology showed good results in gram scale conditions thereby indicating its applicability in academic as well as industrial settings in the near future. KEYWORDS: Metal-free carbocatalyst, Graphite oxide, Solvent-free reaction conditions, Pyrimidine, Recyclable catalyst, Multicomponent reaction

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INTRODUCTION Sustainable chemistry commonly known as “green chemistry” is a philosophy of modern day chemical engineering and research which deals with the development of environment friendly procedures1 for synthesizing biologically significant compounds. Recently the field of catalysis has emerged as the heart of many chemical protocols because it lowers the activation energy and makes the reaction feasible. Application of clean and reusable catalytic material is one of the principles of green chemistry.2−5 So, from this perspective, the search for sustainable, environmentally benign, and efficiently reusable catalytic systems has become vital. Owing to high natural abundance of carbon, development of carbon materials as green catalysts is one of the hot topics of research around the world.6 The significance of graphene (an allotrope of carbon) in engineering, nanotechnology, and electrochemistry has been documented by the Nobel prize of 2010.7,8 Graphene because of its astonishing thermal, optical, mechanical, electronic properties, large surface area, and biocompatibility has emerged as a new heterogeneous catalyst.9−17 To date, graphene and related carbon materials have been used mainly as supports for catalytically active transition metals.18−21 But, after Bielawski’s application of © 2017 American Chemical Society

pristine graphite oxide in the oxidation of alcohols to corresponding carbonyl compounds, it gained worldwide attention and was being applied in many organic reaction involving C−C22 and C−heteroatom bond formation,23 alkyne hydration,24 thiol oxidation,25 oxidative coupling of amines to imines,26 etc. The high reactivity of graphite oxide is due to the presence of various functional groups like hydroxyl (−OH), epoxy (−O−), and carboxyl (−COOH) on its surface,27 which prompted us to apply this carbocatalyst in a multicomponent reaction, which in turn very efficiently drove the reaction toward completion. Because of growing concerns about the environment, various research groups recently have focused on sustainable synthesis.28−39 Therefore, the application of nanomaterials as recyclable catalysts in these sustainable syntheses is very demanding.30−35 Multicomponent reactions (MCRs) have been acknowledged as essential tools for the greener synthesis of biologically important complex molecules eliminating the isolation of intermediates and reducing the number of discrete Received: August 1, 2017 Revised: September 19, 2017 Published: October 17, 2017 11459

DOI: 10.1021/acssuschemeng.7b02626 ACS Sustainable Chem. Eng. 2017, 5, 11459−11469

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ACS Sustainable Chemistry & Engineering chemical steps and waste products.36−41 Pyrimidine and its derivatives are an important class of heterocyclic compound which exist in the core of many biologically active natural products. They possess several pharmaceutical activities like anticancer,42 antibacterial,43 antileishmanial,44 analgesic,45 antidiabetic,46 antiallergic,47 antifungal,48 antipyretic,49 herbicidal,50 calcium channel blocker,51 central nervous system depressant,52 etc. Structures of some biologically active molecules containing pyrimidine blocks are shown in Figure 1. Afloqualone, epirizole, lamivudine, and minoxidil were found

toluenesulfonic acid monohydrate,63 boric acid,64 and silica sulfuric acid;65 (ii) hygroscopic, difficult to handle, costly, and unrecyclable catalysts like AlCl3,66 FeF3,67 [bmim]BF4,68 molecular iodine,69 and guanidine chloride;70 (iii) heterogeneous metal catalyst like nickel nanoparticles,71 nano-WO3supported sulfonic acid,72 copper nanoparticles, etc.,73 because of which problems like waste disposal and contamination of metals in desired products are arising thereby necessitating tedious work up for the removal of metal impurities from the final product. Moreover most of the above-mentioned catalytic systems required application of toxic solvents, but it is always recommended to develop a process in the absence of any solvents, which is an important viewpoint of green chemistry.74 Recently, Raj and group57 synthesized the present compound by a ball milling technique but limited substrate variation (only alkylacetoacetates as β-dicarbonyl compounds) was explored thereby limiting their methodology. Again Sahu and group reported synthesis of the said compound by using chitosan,75 but for homogenizing the catalyst with the reaction mixture as well as to increase its efficiency, they used 2% acetic acid solution which is not environmentally benign; moreover, the reaction was not extended to benzoimidazole and thiazole derivatives, and neither was the reaction compatible with aliphatic, heteroaromatic aldehydes nor with aldehydes possessing multiple carbonyl groups. The research work described herein addresses the abovementioned issues and develops an ecofriendly and sustainable protocol using metal-free recyclable graphite oxide carbocatalyst under solvent-free reaction conditions. A few other advantages of this current methodology are shorter reaction time, higher yield of the desired product, easy handling and excellent reusability of the catalyst, high substrate variation, etc.

Figure 1. Structures of some biologically active pyrimidine molecules.

to exhibit anti-inflammatory,53 analgesic,53 anti-HIV,54 and antihypertensive55 properties, respectively. Because of these properties, various synthetic methodologies have been developed for pyrimidine analogues over the years.56−60 Each method has its own advantages, but many of these procedures lack application under sustainable conditions due to the use of (i) corrosive catalysts like base, 61 sulfamic acid,62 pScheme 1. Synthesis of Graphite Oxide

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RESULTS AND DISCUSSION Preparation and Characterization of Catalyst. In our present attempt to develop a metal-free greener synthesis of fused pyrimidine derivatives, we choose graphite oxide as a heterogeneous catalyst. Graphite oxide was prepared from graphite powder according to modified Hummer’s method (Scheme 1).76,77 Prepared graphite oxide was then characterized by several spectral techniques like Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), electron dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM), powder XRD, Raman spectroscopy, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). First to determine functional groups present in the synthesized catalyst we carried out an FT-IR analysis (Figure 2). In FT-IR spectra the appearance of peaks at around 3386,

graphite oxide was performed by EDX and it showed presence of carbon and oxygen (Figure 3B). Weight percent and atomic percent of carbon and oxygen were found to be 27.25%, 72.75%, 33.29%, and 66.71%, respectively. To further gain information regarding structure of catalyst we took HR-TEM images of graphite oxide, and it showed the presence of distinct multilayered graphite oxide sheet (Figure 3C). Selected area electron diffraction (SAED) pattern of sample shows characteristic hexagonal pattern for graphite oxide (Figure 3D).77 We used the powder XRD technique to prove the formation of graphite oxide. The absence of a peak around 2θ = 26 (002 plane) and the appearance of the peaks at around 2θ = 42.30 (100 plane) and 9.36 (001 plane) indicated the oxidation of graphite to graphite oxide (Figure 4).79 Since Raman analysis is

Figure 4. XRD of graphite oxide.

a very helpful method for characterization of carbon based materials, we also carried out this analysis for our synthesized graphite oxide. It revealed two prominent characteristic Raman modes. One is a G-band at around 1589.0 cm−1 which is because of stretching of C−C bonds common to sp2 carbon network and the other one is a D-band at around 1360.8 cm−1 which is due to the chaos in that network (Figure 5). ID/IG, the

Figure 2. FT-IR spectra of graphite oxide.

1732, and 1047 cm−1 indicated presence of hydroxyl, carboxyl, and epoxy functional groups, respectively.78 Then in order to get a fair idea regarding structure and shape of graphite oxide, SEM images were taken. Figure 3A shows SEM image of graphite oxide which indicates that it possess a sheet like structure. Following that an elemental analyses of synthesized

Figure 5. Raman spectra of graphite oxide.

intensity ratio of D and G bands, was also calculated, and its value was found to be 1.06 which designates the introduction of anarchy in the π-network of graphite.79 TGA analysis of the prepared graphite oxide is shown in Figure 6 (pink line), and its TGA thermograph shows weight loss in two steps. First, below 120 °C a weight loss of around 10% due to the loss of the intercalated water molecules is displayed, and the second

Figure 3. (A) SEM image of graphite oxide. (B) EDX image of graphite oxide. (C) HR-TEM image of graphite oxide (200 nm). (D) SAED pattern of graphite oxide. 11461


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Scheme 2. Retrosynthetic Pathway for the Synthesis of Targeted Pyrimidine Derivatives

derivatives 4. This reaction leads to the formation of one stereogenic center and three new σ bonds in a single operation. We evaluated one-pot three component reaction of 4cyanobenzaldehyde 1{1}, dimedone 2{1}, and 2-aminobenzothiazole 3{1} (Scheme 3). The reaction containing Figure 6. TGA of graphite oxide and graphite.

Scheme 3. Pilot Reaction

drastic weight loss of around 85% at above 180 °C signifies decomposition of various functional groups characteristic for graphite oxide. A TGA thermograph of graphite was also analyzed and compared with that of graphite oxide. The thermograph of graphite (blue line) shows very negligible loss of weight in the same temperature range as mentioned in the literature.79 For further characterization of prepared graphite oxide we also performed an XPS analysis. In the survey spectrum of graphite oxide (Figure 7A) two significant peaks corresponding to O and C are observed, which are indicative of oxidation of graphite to graphite oxide. In the spectrum of C 1s, the peak at 284.6 eV is attributed to the sp2 hybridized carbon. The other three peaks at 286.6, 287.8, and 288.6 eV are because of three main functional groups of graphite oxide, namely the hydroxyl (C−OH), epoxide (C−O−C), and the carboxyl (HOCO) groups, respectively (Figure 7B).80 From the above analysis it can be concluded that graphite oxide is successfully synthesized from graphite powder. In an attempt to design a fresh multicomponent pathway for the synthesis of fused pyrimidine derivatives, we devised a cascade approach as delineated in the retrosynthetic pathway in Scheme 2. Based on the retrosynthetic breakdown, we envisaged that reaction of 1 and 2 would lead to the formation of i followed by addition of 3 to give the desired pyrimidine

1:1:1 mixture of above-mentioned reactants was analyzed under various conditions. Initially we focused toward optimization of catalyst concentration and time required to carry out the reaction. A set of reactions was first carried out in the absence of any catalyst at 100 °C, and it was found that the reaction showed no positive result even after 6 h of stirring. Following that, we thought of adding catalyst to the reaction mixture and various sets of reactions were carried with different catalyst concentrations ranging from 5 to 30 mg at 100 °C for 2 h. It was observed that with increasing the catalyst concentration, yield of the product increases. Maximum yield was obtained when 20 mg of catalyst was used. Further increase in the catalyst loading (30 mg) and time (3 h) had no significant effect on the yield of the reaction (Table 1). Then, we examined the minimum time required for this transformation. Likewise, reaction time was reduced gradually; it was noticed that at 20 min the best conversion was achieved, and on

Figure 7. (A) Survey spectrum of graphite oxide. (B) Deconvulated XPS peaks of graphite oxide. 11462


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ACS Sustainable Chemistry & Engineering Table 1. Standardization of Catalyst Concentration and Time sl no. 1 2 2 3 4 5 6 7 8 9 10 11 12 13 a

amount of catalyst (mg)

time (min)

5 10 15 20 20 30 20 20 20 20 20 20

120 360 120 120 120 120 180 120 90 60 40 20 10 5

solvent

temperature (°C)

yield (%)a

100 100 100 100 100 100 100 100 100 100 100 100 100 100

8 10 48 70 85 96 96 96 96 96 96 96 84 55

Figure 9. Temperature standardization.

Isolated yield.

We have also compared the catalytic activity of graphite oxide with that of other solid acid and base catalysts (Table 2).

further decreasing the time, lower yield was encountered (Table 1). Next, the effect of solvents and temperature on the pilot reaction was evaluated. It was observed that the reaction proceeds moderately in the presence of various solvents (5 mL) like toluene, tetrahydrofuran (THF), chloroform, N,Ndimethylformamide (DMF), acetonirile, water, and ethanol (EtOH) under refluxing conditions, but when the reaction was carried out under solvent-free reaction conditions, maximum yield was obtained at 100 °C (Figure 8). The better catalytic

Table 2. Comparison of Catalystsa sl no.

catalyst (20 mg)

time (min)

yield (%)b

1 2 3 4 5 6

CaO A-21 SBA-15 A-15 MK-10 graphite oxide

120 120 120 120 120 20

55 53 58 96

a

4-Cyanobenzaldehyde (1 mmol), dimedone (1 mmol), 2-aminobenzothiazole (1 mmol), 60 °C, SFRC. bIsolated yield.

Catalysts like calcium oxide (CaO) and amberlyst-A21 (A 21) showed no positive results even after 2 h of stirring, but SBA15, montmorillonite K-10, and amberlyst-15 provided poor yield compared to that of graphite oxide. It is important to mention that from TGA analysis it was found that synthesized graphite oxide contains 10% of water which is because of moisture present on the surface of the catalyst. In order to confirm whether this water has any role in the reaction or not we carried out the pilot reaction only in the presence of water without addition of any catalyst, and after stirring the reaction for 2 h, it showed no positive result. This observation proved that there is no role of water in the reaction. Based on the proposed retrosynthetic scheme, the optimized reaction conditions were tested on numerous starting materials for the combinatorial metal-free synthesis of a fused pyrimidine derivatives. A wide range of aldehydes 1{1−15}, β-dicarbonyl compounds 2{1−3}, and amine sources 3{1−3} (Figure 10) were well tolerated under the optimized reaction parameters providing excellent yields of desired pyrimidine derivatives within a shorter period of time (Table 3, Scheme 4). The scope of various aldehydes on the desired reaction was also evaluated. The reaction progressed smoothly with both electron withdrawing and electron donating aromatic aldehydes. The position of the substituent on the phenyl ring of aryl aldehydes showed no significant effect on the yield and time of the reaction. It has also been observed that heteroaromatic aldehydes, aliphatic aldehydes, and other sensitive aldehydes like N,N-dimethylamino-, 4-chloro-3-nitro-, 2,4,5-trimethoxy-, and aldehydes containing multiple carbonyl groups were very

Figure 8. Solvent standardization.

activity under SFRC can be explained by two factors: (a) in absence of solvents, there is no dilution effect and the heat required for energy of activation is directly provided to the reactant molecules and (b) superior dispersal of active reagent sites provides better contact between the catalyst and reactant molecules. Now, to choose the optimum reaction temperature, a model reaction was carried out under SFRC at various temperatures ranging from 20 to 120 °C. Here, 60 °C was found to be the most favorable reaction temperature because of the higher product yield and lower reaction time (Figure 9). Further increase in temperature had no significant effect on the product yield. 11463


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Figure 10. Diversity of reagents.

Table 3. Synthesis of Various Pyrimidine Derivativesa sl no.

aldehydes

β-dicarbonyl compounds

amine source

product

time (min)

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

1{1} 1{1} 1{1} 1{2} 1{2} 1{2} 1{3} 1{4} 1{4} 1{4} 1{4} 1{5} 1{5} 1{6} 1{7} 1{8} 1{9} 1{10} 1{11} 1{11} 1{12} 1{13} 1{14} 1{15}

2{1} 2{2} 2{2} 2{2} 2{2} 2{1} 2{2} 2{1} 2{1} 2{2} 2{2} 2{2} 2{2} 2{2} 2{3} 2{3} 2{2} 2{2} 2{2} 2{3} 2{2} 2{2} 2{1} 2{1}

3{1} 3{1} 3{2} 3{1} 3{2} 3{2} 3{2} 3{1} 3{2} 3{2} 3{3} 3{1} 3{2} 3{2} 3{3} 3{3} 3{3} 3{3} 3{2} 3{2} 3{2} 3{2} 3{1} 3{1}

4{1,1,1} 4{1,2,1} 4{1,2,2} 4{2,2,1} 4{2,2,2} 4{2,1,2} 4{3,2,2} 4{4,1,1} 4{4,1,2} 4{4,2,2} 4{4,2,3} 4{5,2,1} 4{5,2,2} 4{6,2,2} 4{7,3,3} 4{8,3,3} 4{9,2,3} 4{10,2,3} 4{11,2,2} 4{11,3,2} 4{12,2,2} 4{13,2,2} 4{14,1,1} 4{15,1,1}

20 20 20 20 20 25 25 35 35 35 35 30 30 30 20 35 25 40 45 45 30 30 25 25

96 94 95 92 93 92 91 88 88 87 88 87 87 85 92 89 92 86 85 83 86 88 90 93

Scheme 4. Graphite Oxide Catalyzed Synthesis of Pyrimidine Derivatives

much compatible with the reaction parameters, furnishing desired pyrimidine derivatives in good yield. It is noteworthy to mention that, to create a diverse range of pyrimidine frameworks when we used various amine sources and βdicarbonyl compounds, the reaction went smoothly without showing any difficulties. Structures of all the fused pyrimidine derivatives were deduced from elemental and spectral analyses. FT-IR spectra of compound 4{1,1,1} showed absorption peaks at 2228, 1630, and 1605 cm−1 which may be due to −CN, −CO, and −C N stretching. In the 1H NMR spectrum of 4{1,1,1}, eight aromatic protons appear between δ 7.57 and 7.01. The methine proton was observed as a singlet at δ 6.57. Four methylene protons of the dimedone residue appeared as AB system at δ 2.48 as a singlet and as multiplet between δ 2.30 and 2.16. The two methyl groups of dimedone were observed as two singlets at δ 1.08 and 0.90 ppm. The plausible mechanistic path way is shown in Scheme 5. It is believable that initially, catalyst graphite oxide increases the

a Aldehyde (1 mmol), β-dicarbonyl compounds (1 mmol), amines (1 mmol), 20 mg of graphite oxide (catalyst), 60 °C, SFRC. bIsolated yield.

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was complete, the crude reaction mixture was dissolved in 10 mL of chloroform and graphite oxide was separated from it by centrifugation and filtration. This separated graphite oxide was then repeatedly washed with deionized water and ethanol, followed by diethyl ether and dried. The catalytic activity of recycled graphite oxide was tested under optimal reaction conditions, and to our outmost delight, it was found that recycled graphite oxide furnished the product in good yield. Likewise this catalyst can be reused in nine more consecutive runs (Figure 11) without much decrease in catalytic activity.

Scheme 5. Probable Mechanism for the Formation of Fused Pyrimidine Derivatives Using Graphite Oxide

electrophilicity of carbonyl groups of aldehydes 1{1−15},81−83 which is attacked by β-dicarbonyl compounds 2{1−3} (in enolic form) to form Knoevenagel adduct B84 via dehydration of intermediate A. To prove this assumptions, we tested the pilot reaction with acidic and basic graphite oxide (acidic and basic graphite oxide have been synthesized according to the procedure reported by Zhang et al.83). It was observed that basic graphite oxide furnished better result within short period than acidic graphite oxide. It may be due to the fact that, in acidic graphite oxide the reaction proceeds through enolization but in the case of basic graphite oxide it follows the carbanion pathway. Therefore, it has been concluded that the acidic proton of the carboxylic acid of simple graphite oxide activates the carbonyl group of the aldehydes. Amine sources 3{1−3} then attacks intermediate B in Michael fashion to form another intermediate C, which finally undergoes intramolecular cyclization followed by dehydration to give the desired product 4. In order to test industrial applicability of this procedure we also carried out a gram scale reaction (Scheme 6). For that 4-

Figure 11. Reusability chart.

The chemical composition and surface morphology of reused graphite oxide was studied by using powder XRD, TGA, SEM, HR-TEM, and EDX analyses (Figures S1, Supporting Information), and the obtained results were in good agreement with that of freshly prepared graphite oxide. Finally we compared the result of our present catalytic process with that of other reported methodologies, as shown in Table 4. For nonmetal acid catalysts (entry 1, 2, 5), the methodologies utilized toxic solvents. However, in entry 3 (boric acid) where water was used as solvent, poorer yield for some of the derivatives was found. For entries 4 and 6, although the reaction was reported under SFRC, they required prolonged reaction time for completion (c.f. present methodology). Moreover, all the above-mentioned catalysts were nonrecyclable. In the case of entries 7 and 8 (metal catalysts), the catalysts used were hygroscopic, homogeneous, and difficult to handle. Some heterogeneous nanoparticle mediated reactions for the synthesis of pyrimidine derivatives were also reported (entries 9 and 10), but they lose their applicability under sustainable conditions due to the use of expensive metals in the catalytic system (entry 9) or application of ionic liquids as solvents (entry 10). Therefore, in comparison to these methodologies, the present protocol utilizes a cheap, metal-free, heterogeneous recyclable catalyst for the library synthesis of pyrimidine derivatives under sustainable and economic conditions.

Scheme 6. Gram Scale Reaction for Synthesis of Pyrimidine Derivatives

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cyanobenzaldehyde 1{1} (10 mmol), dimedone 2{1} (10 mmol), 2-aminobenzothiazole 3{1} (10 mmol), and graphite oxide (200 mg) were taken and stirred under SFRC at 60 °C for 20 min. After that, the reaction was stopped, catalysts were separated, and reaction mixture was purified by column chromatography. The yield of desired product was found to be 88% indicating its applicability in industry. Then in order to make the procedure economically and environmentally more viable, we also carried out a reaction to inspect reusability of graphite oxide. As soon as the reaction

CONCLUSION In conclusion, we have developed a proficient, metal-free, graphite oxide catalyzed procedure for the synthesis of pyrimidine derivatives under solvent-free reaction conditions. Easy handling of the catalyst, high reusability, shorter reaction time, and solvent-free reaction conditions are the plus points of this methodology which makes the process ecofriendly, sustainable, and green. Moreover, high tolerance of this 11465


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ACS Sustainable Chemistry & Engineering Table 4. Comparative Study of the Present Methodology with Others Reported in the Literature sl no. 1 2 3 4 5 6 7 8 9 10 11 a

catalyst used sulfamic acid p-toluenesulfonic acid monohydrate boric acid silica sulfuric acid molecular iodine guanidine chloride AlCl3 FeF3 nano-WO3-supported sulfonic acid copper nanoparticles graphite oxide

nature of catalyst

reaction condition

nonmetal nonmetal nonmetal nonmetal nonmetal nonmetal metal metal metal metal nonmetal

solvent: CH3CN (80 °C) solvent: CH3CN (rt) solvent: water (rt) SFRC (110 °C) solvent: CH3CN (refluxed) SFRC (110 °C) SFRC (60−70 °C) SFRC (100 °C) SFRC (100 °C) solvent: ionic liquid−ethylene glycol (rt) SFRC (stirred at 60 °C)

yielda

time (min) 15−20 10−45 8−30 300−360 10−15 120 66−120 30 12−20 10−15 20−45

62

90−95 85−9863 75−9564 86−9465 69−8469 88−9270 60−8966 70−9767 90−9572 86−9873 83−96 (present method)

Isolated yield.

procedure toward various functional groups, easy work up, exceptionaly high yields of desired products, and good result in the gram scale reaction are added advantages for its application to academic and industrial purposes.

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EXPERIMENTAL PROCEDURE

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected].

Preparation of Graphite Oxide. Preparation of graphite oxide from graphite powder was carried out according to a modified Hummer’s method. A 1 g portion of graphite (Sigma-Aldrich) and 0.5 g of sodium nitrate was added to 20 mL of concentrated sulfuric acid (98%). The resulting solution was then kept in an ice bath, and potassium permanganate (4 g) was added to it slowly over a span of 1 h under stirring conditions to avoid any explosion. After completion of addition, the reaction mixture was allowed to stir for 1 h more following which, it was heated to 45 °C and the stirring was continued for 1 h more. During that time a thick brown paste was obtained. A 20 mL portion of deionized water was then added to it and heated at 45 °C for another 30 min. Finally 180 mL more deionized water was added to the reaction mixture, followed by dropwise addition of hydrogen peroxide (30%) until the color of the solution turns yellowish brown from dark brown. The prepared graphite oxide was recovered by centrifugation, washed with deionized water (3 × 10 mL), ethanol (3 × 10 mL), and diethyl ether (3 × 10 mL). At last it was dried under vacuum and obtained as a yellowish brown powder. General Procedure for the Synthesis of Pyrimidine Derivatives 4{1,1,1−15,1,1}. A mixture of aldehydes 1{1−15}, βdicarbonyl compounds 2{1−3}, and amines 3{1−3} in the molar ratio of 1:1:1 was taken in a round-bottom flask, and 20 mg of graphite oxide was added to it. The reaction mixture was then placed on a preheated oil bath (60 °C) and stirred under solvent-free reaction conditions until completion of reaction (monitored by TLC). The reaction mixture was then cooled to temperature and the crude reaction mass was dissolved in chloroform (10 mL). To recover the catalyst, the chloroform layer was centrifuged and filtered. From the residue portion graphite oxide was separated, and the filtrate was washed with water (3 × 10 mL), brine (1 × 10 mL), and dried over anhydrous sodium sulfate. The reaction mixture was then concentrated under vacuum and purified by column chromatography using an ethyl acetate−hexane mixture as the desired eluent. Procedure for Catalyst Recycling. Following centrifugation and filtration, residue containing graphite oxide was washed with deionized water (2 × 5 mL), ethanol (2 × 5 mL), and diethyl ether (2 × 5 mL) and dried under vacuum. Recovered graphite oxide (desired catalyst) was then reused in another reaction under similar experimental conditions.

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General remarks, characterization of reused graphite oxide, spectral data of compounds, 1H and 13C spectra of compounds (PDF)

ORCID

Amarta Kumar Pal: 0000-0001-7838-3804 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Department of Chemistry, NorthEastern Hill University (NEHU), and the University Grant Commission (UGC) for supporting this work under the Special Assistance Programme (SAP) (sanctioned no. SERC/F/0293/ 2012-13) and DST-Purse programme. We are thankful to SERB for financial support (sanctioned no. EMR/2016/ 005089) and also thankful to the Sophisticated Analytical and Instrumentation Facility (SAIF) of North-Eastern Hill University, ACMS-IIT Kanpur, and the central instruments facility (CIF) of IIT-Guwahati. We also take this opportunity to thank the Institute of Advanced Study in Science and Technology (IASST), Guwahati, for powder XRD analyses. We are also grateful to UGC-non-NET fellowship for offering financial support.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02626. 11466


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