Industrial-Quality Graphene Oxide Switched Highly Efficient Metal

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Industrial-Quality Graphene Oxide Switched Highly Efficient Metal- and Solvent-Free Synthesis of #-Ketoenamines under Feasible Conditions Dian Deng, LANG XIAO, Ill-Min Chung, Ick Soo Kim, and Mayakrishnan Gopiraman ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02766 • Publication Date (Web): 15 Jan 2017 Downloaded from http://pubs.acs.org on January 17, 2017

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ACS Sustainable Chemistry & Engineering

Industrial-Quality Graphene Oxide Switched Highly Efficient Metal- and Solvent-Free Synthesis of β-Ketoenamines under Feasible Conditions

Dian Deng,† Lang Xiao,§ Ill-Min Chung,‡,* Ick Soo Kim,†,* Mayakrishnan Gopiraman,‡,*

†

Nano Fusion Technology Research Group, Division of Frontier Fibers, Institute for Fiber

Engineering (IFES), Interdisciplinary Cluster for Cutting Edge Research (ICCER), Shinshu University, Tokida 3-15-1, Ueda, Nagano prefecture, 386-8567, Japan ‡

Department of Applied Bioscience, College of Life & Environment Science, Konkuk

University, 120 Neungdong-ro, Gwangjin-gu, Seoul 05029, South Korea §

Department of Fine Arts, College of New Asia, The Chinese University of Hong Kong,

Hong Kong.

Corresponding author e-mail: [email protected] (M. Gopiraman).

ACS Paragon Plus Environment


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ABSTRACT Metal- and solvent-free industrial-quality graphene oxide (IQGO) based highly efficient carbocatalytic system has been developed for the synthesis of β-ketoenamines. Initially, physicochemical properties of IQGO are briefly discussed by means of various microscopic and spectroscopic techniques. The present system accessed a wide range of substrates to yield β-ketoenamines in an excellent yield (86-100%) with 100% selectivity. Catalytic activity of IQGO is compared with other carbon materials such as carbon nanotubes, carbon nanofibers and graphene nanoplatelets. Cost effective recovery, high level reusability, chemoselective nature, possible scale reaction, and sustainability of IQGO are demonstrated. Based upon experimental results and earlier reports, possible reaction mechanism has been proposed for the synthesis of β-ketoenamines.

KEYWORDS: Graphene oxide, Carbocatalysis, β-Ketoenamines, Reusable, Sustainable


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INTRODUCTION Currently, over 90% of the potential high-value products (fine chemicals and pharmaceuticals) in chemical industries are produced from catalytic processes.1 Transition metal based catalysts (both heterogeneous and homogenous), mainly Ru, Cu, Ni, Pt, Rh, and Pd, are commonly used in the catalytic process.2,3 In addition to high cost, disposal of these metal catalysts often causes environmental problems.4 The U.S. Environmental Protection Agency (EPA) tells that the disposal of metal catalysts is not only expensive but also dangerous.5 In fact, some of the catalytic materials are unusually reactive when they mixed with other wastes which pose unwanted environmental issues. Several points (in addition to the environmental pollution and high cost) should be carefully considered when we use metal-based catalysts: mainly, (1) preparation method, (2) morphology, (3) surface area, (4) pore properties, (5) metal-support interaction, and (6) chemical state of the catalysts. Moreover, after use, the metal recovery from the catalysts is quite complicated and requires high cost techniques.6 In order to overcome these shortcomings, researchers have widely focused on the development of metal-free green synthesis of fine chemicals and pharmaceuticals.7 Owing to the outstanding intrinsic physicochemical properties such as unique structure, huge surface area, good pore interconnectivity and high stability, carbon materials (CMs) such as activated carbons (ACs),8 fullerenes,9 single- and multi-walled carbon nanotubes (SWCNTs and MWCNTs),10,11 and graphene oxide (GO)12 have been demonstrated as efficient carbocatalysts. Among them GO is highly preferred due to low cost, unique 2D structure, rich chemical functionality (solid acid, solid base, redox and defect sites) and



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stability.13 To date, there are several metal-based catalytic systems have been replaced by GO-based carbocatalytic systems. For instant, Lv et al.,14 demonstrated GO as a suitable metal-free carbocatalyst for the oxidation of 5-hydroxymethylfurfural into 2,5-diformylfuran. They concluded that the GO could be an alternate choice for the metal catalysts such as Co/Ce/Ru, Ru, Cu, Cu/V, Mn, Mo/V and V. Huang and co-workers,15 utilized GO as carbocatalyst for aerobic oxidative coupling of amines to imines. Recently, a considerable number of review articles dealing the importance and development of the carbocatalysts have been published.16-23 Formation of β-enamino ketones and esters via enamination of β-dicarbonyl is one of the important and extensively used transformations in organic synthesis.24 The β-enamino ketones and esters have been used as basic units for the formation of biologically active motifs such as peptides, β-amino esters and alcohols, γ-amino alcohols and alkaloids.25 The β-ketoenamines

are

found

to

be

highly

efficient

antitumor,

antibacterial

and

anti-inflammatory agents.26 Metal catalysts mediated direct condensation of β-dicarbonyl compounds with amines is the prime route to obtain these active compounds. Babu et al.,27 prepared Cu/AlO(OH) and utilized as a catalyst for the formation of β-enamino ketones/esters.

Later,

Sun

and

coworkers28

developed

magnetically

recoverable

Ag-medicated catalyst for the synthesis of β-enaminones. Similarly, there are several metal catalysts

including

NaAuCl4,29

Ag

nanoparticles,30

Mg(ClO4)2,31

CoCl2.6H2O,32

Zn(OAc)2.2H2O,33 CeCl3.7H2O,34 Zn(ClO4)2.6H2O,35 ZrCl4,36 SnCl4,37 Cu nanoparticles,38 Cu(II) nitrate trihydrate,39 ZrOCl2.8H2O,40 and InBr341 reported till date. Unfortunately, there is no GO-based carbocatalytic system demonstrated for the synthesis of β-enaminones to date.


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We assumed that the GO could be an alternate choice to the existing metal catalysts for the β-enaminones formation. Herein, we report low-cost industrial quality graphene oxide (IQGO) as an efficient carbocatalyst for the metal- and solvent-free synthesis of β-enamino ketones and esters. The catalytic activity of IQGO has been compared with other CMs including SWCNTs, ACs, GNPs and MWCNTs.

RESULTS AND DISCUSSION TEM image, EDS, Raman and XRD pattern of IQGO are provided in Fig. 1. The TEM image showed that the IQGO is continuous, transparent and pure without any impurities (Fig. 1a). In addition, wrinkles and bends on IQGO at several places could be also noticed. The mean thickness of IQGO was measured to be 0.8-2.0 nm. The wt% of C and O in IQGO was determined to be 63 and 37 respectively [Fig. 1(b)]. The Raman spectra in Fig. 1c confirmed two characteristic Raman features for IQGO, the well-defined D-band at ∼1353 cm–1 and G-band at ∼1557 cm–1. The D-band corresponds to disordered graphite whereas the G-band reflects the ordered state graphite. To calculate IG/ID ratio, baseline correction was performed and, D band and G band were fitted as the sum of a Gaussian function (Fig. 1c). The high IG/ID value (0.88) confirmed the presence of defect sites on IQGO. The XRD pattern of IQGO demonstrated a weak diffraction peak at 26.5° corresponds to well-defined (002) crystal plane of graphite (Fig. 1d). Fig. 2 shows XPS of IQGO. The C 1s and O 1s XPS peaks were noticed at ~284.5 and ~532.5 eV, respectively. In order to study the oxygen functionalities on IQGO, curve fitting was performed on C 1s and O 1s peaks using a Gaussian-Lorentzian peak shape. Both the deconvoluted C 1s and O 1s peaks resulted in five



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obvious peaks and the peak values verified the presence of C–C/C–H, C=O, –COOH, C–OH, –C–O–C– and H2O groups.42 In addition, p→p* shake-up satellite at ~294 eV was also observed. Generally, GO is hydrophobic in nature and the presence of oxygen functional groups would make the GO hydrophilic, as consequences, better dispersion of IQGO could be achieved in reaction medium. As expected, the IQGO demonstrated a very high BET surface area of 767 m2/g with average BJH pore volume and pore size of 1.3 cm³/g and 6.8 nm, respectively (Fig. 3). In the present case, the important futures of IQGO are the smaller size and the presence of oxygen functional groups. According to Tang et al.,12 the catalytic activity of the carbon materials is highly dependent on the carbon-bond structures of carbon materials. Moreover, since they are smaller in size, their giant π-conjugated structures and presence of oxygen groups on carbon materials, and tunable electronic states are all can act as active sites in catalytic reactions. Initially, the reaction conditions were optimized using acetylacetonate (1a) and aniline (2a) as modal substrates (Table 1). In the absence of IQGO, the reaction achieved a very low 49% of the desired product (Z)-4-(phenylamino)pent-3-en-2-one (3a) (Table 1, entry 1). In order to find out the most efficient carbocatalyst, in addition to IQGO, various CMs such as CNFs, SWCNTs, MWCNTs, and GNPs (Table 1, entries 2-6) were tested as catalyst. Among them, both IQGO and SWCNTs gave 3a in better yields of 96% and 98% respectively. The better yield maybe due to the higher surface area of the IQGO (767 m2/g) and SWCNTs (462 m2/g)42 compared to MWCNTs (210 m2/g),43 CNFs (352 m2/g)44 and GNPs (63 m2/g)45. In addition, the small size, better dispersion and the presence of oxygen functionalities would have assisted the IQGO and SWCNTs to achieve the higher yields (Table 1, entries 3 and 4).


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However, IQGO was chosen as a best carbocatalyst over SWCNTs due to its low-cost, easy recovery and non-toxic. Mainly, the cost of IQGO (ACS Materials, 10 g = 980 USD) is about ten times lower than the SWCNTs (Sigma Aldrich, 1 g = 1353 USD). In fact, the large scale production of SWCNTs is very complicated. Subsequently, the amount of IQGO was optimized and found that a 10 mg of IQGO is efficient enough to catalyze the reaction (Table 1, entries 4, 7 and 8). It was noticed that the temperature played a crucial role in the present IQGO system. At low temperatures (27°C and 50°C), the rate of the reaction was very slow (Table 1, entries 9 and 11). Alike, increasing the reaction temperature, from 60°C to 70°C, did not show any significant improvement in the yield (Table 1, entry 10). At 60°C, the IQGO gave a better 96% yield of 3a (Table 1, entry 4). In time optimization, 3 h was found to be the best reaction time since it gave 96% of 3a. No significant improvement in the yield of 3a was noticed even after prolonging the reaction time (from 3 h to 5 h) (Table 1, entries 15 and 16). A series of reactions was performed to understand the importance of oxygen functionalities on IQGO (Table 1, entries 17-20). For this purpose, the IQGO was calcinated under inert atmosphere at 500°C for 1 h to remove most of the oxygen functional groups (C=O, –COOH, and –C–O–C–) and the resultant solid (IQGO-500°C) was used as a carbocatalyst (Table 1, entry 17). However, the IQGO-500°C gave a poor yield of 38%. Moreover, an external O2 supply also did not work well to improve the 3a yield (Table 1, entry 18). Interestingly, under various atmospheres (O2, air and N2), the IQGO gave similar yields (Table 1, entries 19 and 20). The scope of the present IQGO system was extended under the optimized reaction conditions. Results presented in Table 2 confirmed that a wide range of aromatic, aliphatic



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and cyclic amines can be effectively condensed with various symmetric and unsymmetric β-diketones or β-ketoesters. The present catalytic system obtained a better 96% yield of 3a (100% selectivity) compared to 90% of 3a afford by expensive Ag NPs and Ag/HMMS systems.28,30 The substituent present (electron withdrawing and electron donating) in the β-diketones and the aromatic rings of the amines did not significantly affect the yield of the desired products. In the presence of 10mg of IQGO, the acetylacetonate (1a) effectively reacted

with

various

substituted

aromatic

amines

including

p-toluidine

(2b),

4-methoxyaniline (2c), 4-fluoroaniline (2d) and 4-chloroaniline (2e) to obtain the corresponding

β-ketoenamines,

(Z)-4-(p-tolylamino)pent-3-en-2-one

(3b,

96%),

(Z)-4-((4-methoxy-phenyl)amino)pent-3-en-2-one (3c, 96%), (Z)-4-((4-fluorophenyl)amino)pent-3-en-2-one (3d, 95%), and (Z)-4-((4-chlorophenyl)amino)pent-3-en-2-one (3e, 93%), respectively (Table 2, entries 2-6). Alike, the aliphatic and cyclic amines, cyclohexanamine (2g) and butan-1-amine (2h), effectively condense with acetylacetonate (1a) to respectively give (Z)-4-(cyclohexylamino)-pent-3-en-2-one (3g, 100%) and (Z)-4-(butylamino)-pent-3-en-2-one (3h, 99%) in shorter reaction time (Table 2, entries 7 and 8). The aliphatic amines have higher nucleophilicity when compared to aromatic amines which is the obvious reason for the faster reactivity of the aliphatic amines than the aromatic amines (Table 2). Unlike other catalytic systems,27,28 the present IQGO-system worked well for the sterically hindered amine also in a shorter reaction time (Table 2, entries 6 and 22). A good 86% yield of (Z)-4-((2-chloro-phenyl)-amino)pent-3-en-2-one (3f) was afford from the reaction between 2-chloroaniline (2f) and 1a (Table 2, entry 6). Similarly, 2f condensed with 1d

in

the

presence

of

IQGO

to

give

an


excellent

99%

yield

of

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(Z)-4-((2-chlorophenyl)amino)-1,1,1-trifluoropent -3-en-2-one (3v) (Table 2, entry 22). Further, β-ketoesters, methyl 3-oxobutanoate (1b) and ethyl 3-oxobutanoate (1c), were also allowed to react with 2a, 2b, 2g and 2h (Table 2, entries 9-16). The reaction gave the corresponding β-ketoenamines (3i-3p) in an excellent yield (94-100%) with 100% selectivity. Most of the metal-based catalytic systems require longer reaction time when processing 1,1,1-trifluoropentane-2,4-dione (1d), 4-chloroaniline (2e), 2-chloroaniline (2f) and butan-1-amine (2h) as substrates.27 Interestingly, the present study was highly effective when 1d reacts with 2e, 2f and 2h to form corresponding β-ketoenamines (Table 2, entries 21, 22 and 24). The system gave (Z)-4-((4-chlorophenyl)amino)-1,1,1-trifluoropent-3-en-2-one (3u), (Z)-4-((2-chlorophenyl)amino)-1,1,1-trifluoropent-3-en-2-one (3v), and (Z)-4-(butylamino)-1,1,1-trifluoropent-3-en-2-one (3x) in excellent yield of 100, 99 and 89% respectively. Similarly, 1d was condensed with 2a, 2b, 2c, 2d and 2g in the presence of 10mg IQGO to obtain (Z)-1,1,1-trifluoro-4-(phenylamino)pent-3-en-2-one

(3q),

(Z)-1,1,1- trifluoro-4-

-(phenylamino)pent-3-en-2-one (3r), (Z)-1,1,1-trifluoro-4-((4-methoxyphenyl) -amino)pent3-en-2-one

(3s),

(Z)-1,1,1-trifluoro-4-((4-fluorophenyl)amino)pent-3-en-2-one

(3t),

(Z)-4-(cyclohexylamino)-1,1,1-trifluoropent-3-en-2-one (3w), respectively. The yield of 3q, 3r, 3s, 3t, and 3w was calculated to be 100, 100, 100, 100 and 95% (with 100% selectivity) respectively (Table 2, entries, 17-20, 23). Overall, the results confirmed the superior catalytic activity of the present IQGO-system towards the synthesis of β-ketoenamines. The activity of IQGO is better or comparable with other reported metal and non-metal catalysts including Cu/AlO(OH),27

NaAuCl4,29

Ag

nanoparticles,28,30

Mg(ClO4)2,31

CoCl2.6H2O,32

Zn(OAc)2.2H2O,33 CeCl3.7H2O,34 Zn(ClO4)2.6H2O,35 ZrCl4,36 SnCl4,37 Cu nanoparticles,38



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Cu(II) nitrate trihydrate,39 ZrOCl2.8H2O,40 Ag/γ-Fe2O3@TiO246 and GO-SnO247 InBr341. The better activity is obviously due to the three main reasons; (1) the higher surface area of IQGO (767 m2/g), (2) presence of oxygen functionalities (C=O, –COOH, C–OH, –C–O–C–) and interconnected pores on IQGO, and (3) higher level of dispersion of IQGO in the reaction mixture. Mainly, the –COOH on IQGO would have played a significant role as an active site in the synthesis of β-ketoenamines. Density functional theory (DFT) calcinations are very useful to find out the suitable reaction pathway of catalytic reactions.46,47 DFT calculations were previously performed for the synthesis of β-ketoenamines by Babu and co-workers.27 The computed result confirms that the enol form of acetyl acetone is more stable and reactive than the diketone from. Moreover, the activation energy required for the uncatalyzed reaction was found to be very high compared to catalyzed reaction. In particularly, the catalyst active site coordinates to the carbonyl oxygen of 1a followed by the addition of 2a could lead towards the low-energy reaction pathways. Based upon the present results and earlier reports, possible reaction mechanism is proposed for the IQGO catalyzed synthesis of β-ketoenamines (Fig. 4). In the first step, the presence of active defect sites or soli acid groups on IQGO binds with the carbonyl oxygen of 1a and a subsequent addition of aniline in step 2 forms an intermediate 2 as shown in Fig 4. In step 3, the elimination of water molecule from 2 yields the desired product 3a. The better performance of catalysts in large scale reactions is one of the very important factors for industrial applications. Surprisingly, the IQGO worked well for scale reaction (Scheme 1). A 50 mg of IQGO was enough to transform a 100 mmol of the reactants (1a and


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2a) to the corresponding product. It is worth mentioning that the present IQGO system affords 3a in good 93% yield (100% selectivity) after the reaction mixture was stirred under air atmosphere at 60°C for 6 h. Chemoselectivity of the present IQGO system was also studied (Scheme 2). A mixture of 1a (5 mmol), 2a (5 mmol), 2h (5 mmol) and IQGO (10 mg) was magnetically stirred under air atmosphere at 60°C for 3 h. It was found that the IQGO system was favoured for the selective condesation of acetylacetone with butylamine but not with aniline which maybe due to the higher neucleophilic nature of the butylamine. An excellent 99% yield of 3h was obtained, whereas, the yield of 3a was calculated to be 0%; indicating the high level of chemoselevtive nature of IQGO system. To futher, the significant features of IQGO such as recovery, reusability and stability were investigated in detail (Fig. 5). After the reaction, about 95% of IQGO was recovered by simple centifugation method. The recoverd IQGO can be reused at least for 5 cycles without any significant loss in the yield. The IQGO gave 90%, 89%, 88% and 85% of 3a at 2nd, 3rd, 4th and 5th cycle, respectively. The stability of IQGO after 5th cycle was investigated by means of TEM, Raman and SEM-EDS; the results are provided in Fig. 5. In general, due to Van der Waals interaction, GO nanosheets can be easily reassembled to form multilayer agglomerates. However, the TEM image of IQGO after 5th use shows that there is no face-to-face agglomeration between the sheets. Alike pure IQGO, wrinkles and bends at several places can be seen. The Raman spectrum of IQGO after 5th use shows the characteristic G band and D band with ID/IG ratio of 0.89. The SEM-EDS result confirmed that the wt% of C and O in used IQGO was similar to that of fresh IQGO. The results assure



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that the IQGO is easily recoverable, reusable and stable.

CONCLUSIONS Highly efficient IQGO-based carbocatalytic system was developed for the synthesis of β-ketoenamines. The IQGO system is applicable to a wide range of substrates. Chemoselectivity and scale reaction of the IQGO system were found to be very impressive. The carbocatalyst is highly reusable, easily recoverable and stable. Overall, we believe that the advantages (metal-free, solvent-free, mild reaction conditions, easy recovery, reusability, scale reaction, chemoselectivity and high stability) could make the IQGO system an alternate choice to the existing metal and non-metal based catalytic systems.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acssuschemeng.xxxxxxx. Experimental section, 1H and

13

C NMR spectra, GC chromatogram and Mass spectrum

(GC)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M. Gopiraman). *E-mail: [email protected] (I.M. Chung). *E-mail: [email protected] (I.S. Kim). Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This study was supported by Konkuk University KU research professor program. DD would like to thank Epson International Scholarship Foundation for scholarship.



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Table 1 Optimization of reaction conditions for the synthesis of 3aa

entry

catalyst -

amount of catalyst (mg) -

temperature (°C) 60

time (h) 3

GC yield (%)b 49

1 2

CNFs

10

60

3

47

3

SWCNTs

10

60

3

98

4

IQGO

10

60

3

96

5

MWCNTs

10

60

3

51

6

GNPs

10

60

3

61

7

IQGO

5

60

3

88

8

IQGO

15

60

3

96

9

IQGO

10

27

3

39

10

IQGO

10

70

3

98

11

IQGO

10

50

3

67

12

IQGO

10

60

0.5

64

13

IQGO

10

60

1

88

14

IQGO

10

60

2

88

15

IQGO

10

60

4

97

16

IQGO

10

60

5

97

17

IQGO-500°C

10

60

3

38

18[c]

IQGO-500°C

10

60

3

42

19[c]

IQGO

10

60

3

97

20[d]

IQGO

10

60

3

88

a

Reaction condition: diketone (5 mmol), amine (5.2 mmol), solvent-free, air atmosphere. GC yield. c Reaction carried under O2 atmosphere d Reaction carried under N2 atmosphere b



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Table 2 Scope of the IQGO system

3a, (96%)b, (96%)c, (85%)d, (100%)e, 3h

3b, (96%)b, (96%)c, (84%)d, (100%)e, 3h

3c, (96%)b, (96%)c, (86%)d, (100%)e, 3h

3d, (95%)b, (95%)c, (72%)d, (100%)e, 3h

3e, (93%)b, (93%)c, (79%)d, (100%)e, 3h

3f, (86%)b, (86%)c, (74)d, (100%)e, 9h

3g, (100%)b, (100%)c, (87%)d, (100%)e, 1h

3h, (99%)b, (99%)c, (86%)d, (100%)e, 10min

3i, (99%)b, (83%)d, 3h

3j, (99)b, (75%)d, 3h

3k, (95%)b, (95%)c, (81%)d, (100%)e, 30min

3l, (100%)b, (100%)c, (79%)d, (100%)e, 10min


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3m, (99%)b, (79%)d, 3h

3n, (99%)b, (81%)d, 3h

3o, (94%)b, (94%)c, (78%)d, (100%)e, 30min

3p, (99%)b, (99%)c, (81%)d, (100%)e, 10min

3q, (100%)b, (100%)c, (85%)d, (100%)e, 3h

3r, (100%)b, (100%)c, (86%)d, (100%)e, 1h

3s, (100%)b, (100%)c, (88%)d, (100%)e, 1h

3t, (100%)b, (100%)c, (85%)d, (100%)e, 1h

3u, (100%)b, (100%)c, (91%)d, (100%)e, 1h

3v, (99%)b, (99%)c, (82%)d, (100%)e, 1h

3w, (95%)b, (95%)c, (79%)d, (100%)e, 3h

3x, (89%)b, (89%)c, (75%)d, (100%)e, 5min

a

Reaction conditions: diketone (5 mmol), amine (5.2 mmol), IQGO (10 mg), air atmosphere, 60°C. bGC conversion. cGC yield. dIsolated yield. eSelectivity.



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Scheme 1. Scale reaction of IQGO catalyzed condensation of acetyalaceton (1a) and aniline (2a).

Scheme 2. Chemoselectivity of IQGO system.


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Fig. 1. (a) TEM image, (b) EDS, (c) Raman spectrum and (d) XRD pattern of IQGO.

Fig. 2. Deconvoluted XPS (a) C 1s and (b) O 1s peaks of IQGO.



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Fig. 3. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distribution of IQGO.

Fig. 4. Proposed mechanism for IQGO catalyzed synthesis of β-ketoenamines.


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Fig. 5. (a) Reusability test of IQGO. (b) TEM image, (c) SEM-EDS, and (d) Raman spectrum of IQGO after 5th use.



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Table of content (TOC) use only:

Industrial-Quality Graphene Oxide Switched Highly Efficient Metal- and Solvent-Free Synthesis of β-Ketoenamines under Feasible Conditions

Dian Deng,† Lang Xiao,§ Ill-Min Chung,‡,* Ick Soo Kim,†,* Mayakrishnan Gopiraman,‡,*

Synopsis Highly efficient graphene oxide-based carbocatalytic system was demonstrated for the synthesis of β-ketoenamines under mild conditions.


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Highly efficient graphene oxide-based carbocatalytic system was demonstrated for the synthesis of βketoenamines under mild conditions. Graphical Abstract 254x190mm (96 x 96 DPI)


Industrial-Quality Graphene Oxide Switched Highly Efficient Metal

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