Nanocomposite of MoS2-RGO as facile ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Nanocomposite of MoS2‑RGO as Facile, Heterogeneous, Recyclable, and Highly Efficient Green Catalyst for One-Pot Synthesis of Indole Alkaloids Ashish Bahuguna,† Suneel Kumar,† Vipul Sharma,† Kumbam Lingeshwar Reddy,† Kaustava Bhattacharyya,‡ P. C. Ravikumar,*,†,§ and Venkata Krishnan*,† †

School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Mandi, Himachal Pradesh 175005, India ‡ Chemistry Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra 400085, India § National Institute of Science Education and Research, Bhubaneswar, Odisha 752050, India S Supporting Information *

ABSTRACT: A nanocomposite comprised of MoS2-RGO having unique structural features was developed by using a facile preparation strategy and demonstrated to be a highly efficient heterogeneous catalyst for the synthesis of indole alkaloids in water. The catalyst could be recycled six times without significant loss of its activity. Green chemistry matrix calculations for the reaction showed high atom economy (A.E. = 94.7%) and small E-factor (0.089). Using this nanocomposite as catalyst, four naturally occurring indole alkaloids, Arundine, Vibrindole A, Turbomycin B, and Trisindole, were synthesized along with their other derivatives in excellent yields.

KEYWORDS: Nanocomposite, Heterogeneous catalyst, Green chemistry, Organic transformation, Indole alkaloids

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INTRODUCTION Organic transformations using heterogeneous catalysts have played a crucial role in synthetic chemistry for the past few decades.1 Heterogeneous catalysis has many advantages over homogeneous catalysis,2,3 such as easy recyclability, can be operated at elevated temperature and pressure ranges, and high industrial relevance (i.e., about 85% of all industrial catalytic processes are catalyzed by heterogeneous catalysts), because of which it has become the preferred choice of chemists across the world. Nanocomposites of metals with graphene-based materials are some examples of heterogeneous catalysts, which have demonstrated their wide applicability.4 Nanocomposite-mediated organic transformation has recently emerged as an important and growing field in chemistry and materials science, and has attracted the attention of several researchers working in this area. Yet most of the organic reactions using nanocomposites have been performed in nongreen and toxic organic solvents.5 In addition, use of adverse reaction conditions, such as high temperature and pressure as well as longer reaction time and low percentage of yields, has prompted researchers to develop catalysts, which could overcome these problems. Hence, there is a need to develop a new kind of catalyst, which could resolve these issues pertaining to sustainability. Use of green solvents and ambient reaction conditions is the prerequisite need of green and sustainable chemistry. Green © 2017 American Chemical Society

chemistry refers to the practice of designing environmentally friendly reactions using safe and less hazardous chemicals, reducing waste, less energy consumption, and producing maximum yields.6 Oxides and sulfides of several metals, such as Zn, Cd, Mo, Cu, Fe, Ru, etc., have been used as an active catalyst for various organic transformations.7−11 There are also few reports in the literature12−14 where researchers have developed various heterogeneous catalysts using nanocomposites of various metal oxides and sulfides with graphene or reduced graphene oxide, which could minimize the use of toxic organic solvents and performed reactions at ambient conditions. Recently, Rawat et al. developed a protocol for the synthesis of 3-substituted indole by using ZnO-RGO nanocomposite as a heterogeneous catalyst.12 The same group later developed another methodology for the synthesis of aminoindolizines using CuI-carbon spheres-based nanocomposites under green conditions.13 Verma et al. performed Ritter and other multicomponent reactions using iron oxide-supported sulfonic acid nanocatalysts.14 In addition to this chitosan, supported ruthenium was also used as a heterogeneous catalyst for hydration of nitriles.15 Received: March 2, 2017 Revised: July 11, 2017 Published: August 15, 2017 8551

DOI: 10.1021/acssuschemeng.7b00648 ACS Sustainable Chem. Eng. 2017, 5, 8551−8567

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ACS Sustainable Chemistry & Engineering Molybdenum metal is considered relatively safe for living beings16 in comparison to other heavy metals, and no toxicity has been reported so far.17 In fact, molybdenum has been found as an essential trace element for the proper growth of plants and animals.18 Sulfides of molybdenum (e.g., MoS2, etc.) have been employed as efficient catalysts for various reactions such as hydrogen evolution,19 desulfurization in petrochemistry,20 reductive alkylation,21 hydrogenolysis,22 and so on. Recently, nanocomposite of MoS2 with reduced graphene oxide (RGO) has attracted the attention of researchers from various research areas such as photocatalysis,23 dye-sensitized solar cells,24 biosensing,25,26 photovoltaics,24 supercapacitors,27 etc. Organic transformations using nanocomposites have been emerging as a thrust area of chemistry and nanomaterials. MoS2-RGO and CdS nanocomposite was found to be an efficient photocatalyst for the reduction of 4-nitrophenol.28 Two-dimensional MoS2-graphene composites were also used as a supports for Pt electrocatalysts in methanol oxidation.29 Although MoS2-RGO nanocomposites have been used as catalysts for different reactions in various other fields of materials, to the best of our knowledge, there exists no report on its use for organic transformation reactions. MoS2 possesses two-dimensional (2D) structure where Mo atom is sandwiched between two layers of S atoms, which are held together by van der Waal forces.30 These MoS2 nanosheets supported over RGO nanosheets result in 2D−2D contact in nanocomposite with high surface area,31 and the 2D nature of one atom thick carbon sheet of graphene makes it an ideal catalyst for various applications, including organic transformations. Indole-based organic structures have been found in various natural products and drug molecules.32−38 Among them, bis-indole alkaloids have been found to show a large range of medicinal activities such as antibacterial,39 antimalarial,40 antifungal,41 antileishmanial,42 anticancer,43 and anti-inflammatory,44 etc. Considering the drawbacks mentioned above with regard to performing reactions in organic solvents, we ventured to develop a heterogeneous catalyst composed of nontoxic and non-noble materials, which can perform organic reactions in water (a green solvent) at ambient conditions with good yields even at short reaction times. In this regard, considering the unique properties of graphene-based materials and the 2D transition metal dichalcogenides, we have prepared MoS2-RGO nanocomposite, a novel high surface area 2D−2D hybrid material, using a facile hydrothermal method and have demonstrated it as a highly efficient heterogeneous green catalyst for the synthesis of indole alkaloids, which are pharmaceutically relevant natural products. We have also performed detailed investigations using multiple substrates and varying the reaction parameters to better understand the activity of the prepared catalyst and its mechanism of action. In addition, we have also demonstrated the recyclability of the catalyst by taking a model reaction. Finally, two different gram-scale reactions were performed to show the potential and industrial applicability of the developed reaction. On the basis of the results obtained and insights gained from this work, we have finally proposed some strategies for the development of heterogeneous catalysts for performing organic transformation reactions under green conditions.

Figure 1. XRD patterns of MoS2 and MoS2-RGO nanocomposite.

peaks at 2θ = 14°, 32°, and 57°, which can be assigned to the (002), (100), and (110) reflection planes, respectively. The (002) reflection plane corresponds to the d-spacing of about 0.62 nm, and the broader nature of this diffraction peak indicates the presence of layered MoS2 having lamellar structure. However, MoS2-RGO nanocomposite also displays the same diffraction peaks as that of pure MoS2; in addition to these peaks, the characteristics diffraction peak due to RGO around 2θ = 24° can also be seen, confirming the presence of RGO. The Raman spectrum of the as-prepared MoS2-RGO nanocomposite is presented in Figure 2 along with the spectrum of

Figure 2. Raman spectra of MoS2 and MoS2-RGO nanocomposite.

MoS2 precursor. MoS2 shows the presence of prominent Raman peaks around 280, 332, and 376 cm−1, which could be assigned to E1g, A1, and E2g1 vibrational modes of S atoms with respect to Mo atom, respectively. The Raman peak around 454 cm−1 is a typical peak of MoS2 and assigned as 2LA (M). The MoS2-RGO nanocomposite also exhibits the same characteristic Raman peaks as that of MoS2, and, in addition to these peaks, this composite also displays Raman peaks around 1344 and 1584 cm−1, which originate due to the D-band and G-band of RGO, respectively. The presence of the D-band and G-band confirms the presence of reduced graphene oxide (RGO) in MoS2-RGO nanocomposite. Thus, Raman results also confirm the successful fabrication of MoS2-RGO nanocomposite.

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RESULTS AND DISCUSSION Synthesis and Characterization of the Catalyst. The crystal phase of the as-prepared nanocomposite and MoS2 precursor was analyzed by powder X-ray diffraction (XRD), and the data are presented in Figure 1. MoS2 shows diffraction 8552


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Figure 3. SEM Images (a) GO sheets, (b) MoS2, (c) MoS2-RGO nanocomposite, and (d) EDAX data of MoS2-RGO nanocomposite.

surface chemical composition and valence state in nanocomposite, and the obtained data are presented in Figure 5. The survey spectrum (Figure 5a) of MoS2-RGO nanocomposite confirms the existence of C-1s, O-1s, Mo-3d, and S-2p in nanocomposite. The deconvoluted C-1s spectrum (Figure 5b) displays the binding energy peaks at 284.4 and 287.9 eV, which could be assigned to C−C bonds in RGO and C−O−C bonds, respectively.45 It is noteworthy to mention here that the characteristic binding peak for CO (287.41 eV) almost diminishes, signifying the reduction of GO to RGO during the hydrothermal synthesis process. Therefore, it could be concluded the formation of the RGO from the GO during the hydrothermal synthesis. The deconvoluted O-1s spectrum (Figure 5c) shows the presence of two peaks at 531.0 and 529.1 eV, wherein the peak at 531.0 eV could be attributed to lattice oxygen contribution from RGO and the peak at 529.1 eV corresponds to the nonstoichiometric peak due to O. Figure 5d shows the Mo-3d spectrum, wherein two binding energies can be observed at 231.7 and 228.2 eV, which corresponds to the Mo 3d3/2 and Mo 3d5/2, respectively. The S-2p spectrum in Figure 5e presents two binding peaks at 162.4 and 161.1 eV. These binding energy peaks in case of Mo-3d and S-2p signify the presence of Mo4+ and S2− ions in MoS2-RGO nanocomposite. The two peaks of the S2− could be due to the S from the MoS2 and the other S of MoS2 linked with the RGO, that is, possessing an electronic interaction with the O of the RGO. The O being more electronegative than the S will draw the electron clouds toward itself; thereby in all probabilities the 161.1 eV peak could be

The morphology of bare GO, MoS2, and MoS2-RGO nanocomposite was investigated by scanning electron microscopy (SEM), and the obtained images are presented in Figure 3. The SEM image of GO sheets (Figure 3a) shows wrinkled morphology due to the presence of various oxygen-containing functional groups on its surface, while the MoS2 micrograph (Figure 3b) shows sheet-like morphology. In the SEM image of MoS2-RGO nanocomposite (Figure 3c), layered sheet-like MoS2 can be seen on RGO nanosheets (GO reduced to RGO during hydrothermal synthesis). It can be evidenced from the SEM images that the layered MoS2 is well adhered to RGO nanosheets to form nanocomposite as confirmed by other characterization techniques as well. The presence of all constituent elements (Mo, S, C, and O) in MoS2-RGO nanocomposite has been confirmed by energy dispersive X-ray spectrum EDAX analysis (Figure 3d). Furthermore, the transmission electron microscopy (TEM) analysis was also performed on GO, MoS2, and MoS2-RGO nanocomposite to evaluate their surface morphology at nanometer level, and the obtained data are presented in Figure 4. Herein, Figure 4a and b shows GO nanosheets and layered MoS2, respectively. In MoS2-RGO nanocomposite, layered MoS2 well adhered on the surface of RGO nanosheets can be seen clearly in Figure 4c. The presence of all constituent elements Mo, S, C, and O was further confirmed by the EDAX analysis (Figure 4d), which reveals the successful formation of MoS2-RGO nanocomposite. X-ray photoelectron spectroscopy (XPS) studies were performed with MoS2-RGO nanocomposite to investigate the 8553


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Figure 4. TEM images of (a) GO sheets, (b) MoS2, (c) MoS2-RGO nanocomposite, and (d) EDAX data of MoS2-RGO nanocomposite.

the nanocomposite decreases due to the presence of RGO. On the basis of a literature procedure,47 the relative content of the constituent compounds of the nanocomposite could be estimated from the weight loss observed in the TGA curves. In the case of MoS2-RGO (1:1) nanocomposite, the corresponding values determined were 44.35% MoS2 and 55.65% of RGO. Hydrophilicity study of ZnO-RGO nanocomposite was carried out in an earlier study12 by contact angle measurement, in which ZnO-RGO nanocomposite showed a 74° water contact angle. Inspired from the above study, we measured the contact angle for MoS2-RGO nanocomposite, and it was found to be 67° (Figure 7), which indicates a slightly more hydrophilic nature of MoS2RGO nanocomposite in comparison to previously reported ZnO-RGO nanocomposite.12 Hydrophilicity favors the water molecules to get attached on the surface of the catalyst and thereby enhance the reactivity. Catalytic Activity Studies. The catalytic potential of MoS2RGO nanocomposite has been explored for the green synthesis of various bisindolylmethanes (BIM) and trisindolylmethanes (TIM), as these molecules are of high significance and are found in the skeleton of various natural products and drugs.48−50 The catalytic activity of MoS2-RGO nanocomposite was examined on indole and benzaldehyde as an initial model reaction by using 5 wt % of catalyst to obtain a phenyl bisindolylmethane at room temperature in water as reaction medium (Scheme 1). NMRbased confirmation of the product was also verified by the singlecrystal X-ray data of 3a (CCDC no. 1517905). Optimization of

assigned the S possessing interaction with RGO. Mo is having two oxidation states here, and the binding energy of the Mo2+ (MoS2) is at 228.2 eV, which is in line with the previous literature.45 The other oxidation state of Mo is that of Mo5+, which suggests that either during the hydrothermal synthesis a part of the MoS2 is oxidized to the Mo2O5 or the Mo also has a very strong interaction with the O of RGO.46 The O-1s peak is that of the O of RGO, which is at 530.0 eV. However, the nonstoichiometry in the O could be a result of the O being attached to the Mo partially, that is, suggesting a Mo-RGO interaction along with a S-RGO interaction. The elemental composition of MoS2-RGO nanocomposite was also determined from XPS studies, and the obtained results are presented in Table 1. The relative percentage of the constituent elements present in MoS2-RGO nanocomposite matches well with the expected stoichiometry. The thermal stability of MoS2 and MoS2-RGO nanocomposite was studied by thermogravimetric analyses (TGA) in N2 atmosphere, and the obtained data have been presented in Figure 6. MoS2 shows initial weight loss around 100 °C, which is attributed to the evaporation of adsorbed water molecules. Furthermore, MoS2 shows weight loss around 300 °C, which could be attributed to oxidation of MoS2 into MoO3. MoS2-RGO nanocomposite shows continuous weight loss from initial temperature due to loss of cointercalated water molecules. Sudden weight loss at 500 °C occurs due to oxidation of MoS2 and removal of organic groups. Beyond 700 °C, the stability of 8554


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Figure 5. X-ray photoelectron spectra (XPS) for the MoS2-RGO nanocomposite: (a) survey spectrum, (b) C-1s, (c) O-1s, (d) Mo-3d, and (e) S-2p.

reaction conditions is shown in Table 2, where it can be evidenced that the reaction was optimized with GO, MoS2, and MoS2-RGO nanocomposite in water at room temperature and varying different concentrations of water. Because of the slightly acidic nature of GO,51 the reaction proceeded with simple GO also but percentage yields of isolated were very poor (Table 2, entry 1). The reaction then was carried out with MoS2 alone as a catalyst, which also led to the formation of product up to a significant yield (52%) in 6 h (Table 2, entry 2), but further continuation of reaction for a period of 12 h

Table 1. Elemental Composition of MoS2-RGO Nanocomposite As Determined from XPS catalyst

element

peak positions (eV)

atom %

MoS2-RGO

C-1s O-1s S-2p Mo-3d

284.4 and 287.9 531.0 and 529.1 161.1 and 162. 4 228.2 and 231.7

38.55 28.51 21.65 11.29

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Scheme 1. MoS2-RGO Nanocomposite-Catalyzed Synthesis of Bis- and Trisindolylmethanes

ascertain this, we measured the magnetization versus magnetic field (M−H) curves for MoS2 and MoS2-RGO (1:1) nanocomposite, and the obtained data are shown in Figure S1, which reveals the room-temperature magnetic behavior of the catalyst. Furthermore, to explore the generality of this reaction, various aromatic, aliphatic, and heterocyclic aldehydes were examined (Table 3), and most of the aldehydes resulted in good to excellent yields. Furthermore, the electron-donating substituent on the indole ring also favored the reaction to proceed well and produced good yields. The electron-withdrawing atom inside the indole ring structure (e.g., 7-azaindole) disfavored the reaction. Yet interestingly, the electron-withdrawing substituent (such as F, Cl, Br, CN, and NO2) outside of the indole ring structure has no effect on the reactivity for the synthesis of bisindolylmethanes. All of the indoles with electron-withdrawing group were found to give the desired product in excellent yields (Table 3, 3zh−3zl). Moreover, there was no significant effect on reactivity due to the presence of electron-donating or -withdrawing group present on the aldehyde part unlike the indole ring. In addition to this, the reaction proceeded equally good even in the presence of various alkyl groups on the indole nitrogen atom. To our delight, we synthesized four indole alkaloids, Turbomycin B reductant53 (3a), Arundine54 (3y), Vibrindole A55 (3z), and Trisindole (3x),2,56,57 by this method in very good yields. Similarly, various other derivatives of these indole alkaloids were also synthesized (3b−3g and 3y−3za) by our developed protocol. Progress of the reaction (Figure 9) was monitored as a function of time, and it

Figure 6. TGA curves of MoS2 and MoS2-RGO nanocomposite.

(Table 2, entry 3) did not lead to significant improvement in the yield (63%). Furthermore, some unknown side products were formed before consumption of total starting material. Surprisingly, when the same reaction was performed with MoS2-RGO (1:1) nanocomposite, it was completed in just 2 h (Table 2, entry 4) with 97% isolated yield. Subsequently, various other combinations of solvent concentration were examined (Table 2, entries 5−10), and better yields were found with 0.5 M concentration of the solvent for both indole and N-methylindole substrates (Table 2, entries 4 and 8). To ascertain the effect of RGO content on the catalytic activity of the MoS2-RGO nanocomposite, three additional nanocomposites of MoS2-RGO in weight ratios of 1:2, 1:3, and 1:4 were prepared, and their catalytic activities were examined (Table 2, entries 11−16). It could be evidenced that the best yields for the synthesis of bisindolylmethanes were obtained when MoS2-RGO (1:1) nanocomposite was used as catalyst. Moreover, the amount of catalyst optimized for the reaction was found to be 5 wt %, and it could be recycled up to 6 cycles without significant loss in its activity (Figure 8). Recycling of catalyst was congenial due to magnetic properties of MoS2,52 which helps the catalyst to stick to the magnetic stir bar slowly as the reaction proceeds. To

Figure 7. Water contact angle on the surface of MoS2-RGO nanocomposite. 8556


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ACS Sustainable Chemistry & Engineering Table 2. Optimization of MoS2-RGO Nanocomposite-Catalyzed Synthesis of Bisindolylmethanes

entry

R

solvent conc. (M)

catalyst (5 wt %)

time (h)

temp

% yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

H H H H H H Me Me Me Me H Me H Me H Me H

0.50 0.50 0.50 0.50 0.25 0.10 0.50 0.50 0.25 0.10 0.50 0.50 0.50 0.50 0.50 0.50 0.50

GO MoS2 MoS2 MoS2-RGO (1:1) MoS2-RGO (1:1) MoS2-RGO (1:1) MoS2-RGO (1:1) MoS2-RGO (1:1) MoS2-RGO (1:1) MoS2-RGO (1:1) MoS2-RGO (1:2) MoS2-RGO (1:2) MoS2-RGO (1:3) MoS2-RGO (1:3) MoS2-RGO (1:4) MoS2-RGO (1:4) no catalyst

24 6 12 2 2 2 2 4 2 2 2 2 2 2 2 2 48

rt rt rt rt rt rt rt rt rt rt rt rt rt rt rt rt rt

10 52 63 97 85 70 82 93 74 61 87 74 72 62 54 45 0

To extend the scope of the developed catalytic system, we tried to synthesize unsymmetrical bisindolylmethanes. After careful optimization of reaction conditions, we were able to synthesize unsymmetrical bisindoles having electron-withdrawing, electrondonating, as well as halogen substituent in the product (Scheme 2, Table 4). The isolated yields for unsymmetrical bisindolylmethanes were found to be moderate to good, ranging from 61% to 72%. Finally, we examined our catalytic system for the development of ketone-derived bisindolylmethanes. After several permutations and combinations of amount of catalyst and reaction temperature, we were able to optimize the reaction conditions for the synthesis of ketone-derived bisindoles. Slow reactivity of ketones was observed in comparison to the aldehydes for the synthesis of bisindolylmethanes (Scheme 3, Table 5). Because of slow and low reactivity of ketones, reaction was optimized at 45 °C. Prolonged reaction at elevated temperature led to the formation of a series of side products. Among ketones, cyclohexanone, cyclopentanone, and acetone reacted faster and produced moderate to good yields. Yet the reactivity of 9-fluorenone and acetophenone was sluggish in comparison to aliphatic ketones. Surprisingly, no reactivity was observed with benzophenone. Industrial utility of the developed strategy was demonstrated by carrying out a gram-scale reaction (Scheme 4). Starting from 1 g of indole, 0.99 g (95%) of Arundine and 1.02 g (92%) of Vibrindole A natural products were synthesized. These results illustrate the potential of the developed catalyst for industrial applications to perform large-scale synthesis of these molecules of high significance at ambient conditions using water as solvent. For a standard green chemistry reaction, atom economy/ reaction catalysts58−62 for mass efficiency should be high, and environmental factor as well as process mass intensity should be low. When green chemistry matrixes were calculated for our optimized reaction, we found high atom economy (A.E. = 94.7%)/reaction mass efficiency (R.M.E. = 70.9%) and small Efactor (=0.089)/process mass intensity (P.M.E. = 1.089) (refer

Figure 8. Recyclability study of MoS2-RGO nanocomposite catalyst for the synthesis of 3a.

was found that aromatic aldehydes react more efficiently with indole than do the corresponding aliphatic aldehydes. Furthermore, we tried several other benzo-fused electron-rich systems such as 3-methylindole, benzimidazole, indazole, and some five-membered electron-rich heterocycles such as imidazole, furan, and thiophene, but none of them led to the formation of desired products. In the case of 3-methylindole, position-3, which is more nucleophilic in nature, is substituted by a methyl group, but position-2 being less nucleophilic could not facilitate the reaction. Similarly, in the case of imidazole and benzimidazole, the carbon atom is present between the two nitrogen atoms at position-2; hence nucleophilicity might be suppressed by the electron-withdrawing effect of nitrogen at position-3. The specific reason for the other five-membered heterocycles not yielding the desired product could not be unambiguously ascertained. 8557


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ACS Sustainable Chemistry & Engineering Table 3. Substrate Scope with Various Aldehydes

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Figure 9. Progress of the reaction of indole with aldehydes to form bisindolylmethanes: (a) reaction of indole with aromatic aldehydes and (b) reaction of indole with aliphatic aldehydes.

Scheme 2. Synthesis of Unsymmetrical Bisindolylmethanes

Table 4. Optimization of MoS2-RGO Nanocomposite-Catalyzed Synthesis of Unsymmetrical Bisindolylmethanes

entry

solvent conc. (M)

catalyst (wt %)

time (h)

temp

% yield (4aa)

% yield (3a+3w)

1 2 3 4 4 5 6 7 8 9 10

0.50 0.25 0.50 0.25 0.50 0.25 0.25 0.25 0.25 0.25 0.25

4 4 5 5 6 6 6 8 10 20 no catalyst

4 4 4 4 4 4 6 4 4 4 24

rt rt rt rt rt rt rt rt rt rt rt

51 58 52 61 54 63 55 52 48 40 0

48 40 46 37 44 36 43 47 51 59 0

comparison indicates that our developed protocol is mild,

to the Supporting Information for the metrics calculations). The comparison of the activity of the catalyst with various other reported support-based catalyst for the synthesis of 3,3′(phenylmethylene)bis(1H-indole) is shown in Table 6. The

efficient, recyclable, and equally competitive to other reported methods. 8559


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ACS Sustainable Chemistry & Engineering Scheme 3. Ketone-Derived Synthesis of Bisindolylmethanes

Table 5. Optimization of Ketone-Derived Synthesis of Bisindolylmethanes

Table 6. Comparison of Catalytic Activity of MoS2-RGO Nanocomposite with Other Reported Support-Based Catalysts for the Synthesis of 3,3′-(Phenylmethylene)bis(1Hindole) entry 1

entry

solvent conc. (M)

catalyst (wt %)

time (h)

temp (°C)

% yield

1 2 3 4 5 6 7 8 9 10

0.50 0.25 0.50 0.25 0.25 0.25 0.25 0.25 0.25 0.25

5 5 6 6 8 10 10 10 10 20

4 4 4 4 4 4 6 4 4 4

rt rt rt rt rt rt rt 45 60 rt

20 32 23 35 43 48 54 72 57 44

2 3 4 5 6 7 8 9

Scheme 4. Gram-Scale Synthesis of Arundine (3w) and Vibrindole A (3x)

catalyst sulfonated carbon/nano metal−oxide composite mesoporous benzene silica Pd−silica cellulose complex carbohydrate-based tosylsulfonyl hydrazide poly(ammonium methanesulfonate) Cu-tmtppa-based catalyst PAN supported ionic liquid TPPMS/CBr4 MoS2-RGO

temp (°C)

% yield

4h

100

90

63

20 min 3h 12 h

60 100 rt

95 87 84

64 65 66

4h

rt

96

67

2h 4h 4h 2h

90 rt rt rt

92 96 95 97

68 69 70 this work

time

ref

The proposed catalytic cycle (Scheme 5) shows that the initial aldehyde moiety becomes more electrophilic, due to the presence of MoS2 on the surface of RGO, and gets attacked by highly nucleophilic 3-position of indole via a Friedel−Craft pathway and forms a hydroxyalkyated intermediate 4, which gives off an hydroxyl ion to form an intermediate 5, which is known as an enamine 71−73 intermediate. The enamine intermediate 5, being highly electrophilic in nature, acts similar to a Michael acceptor and gets attacked by another indole molecule to form a bis-indolylmethane moiety with the removal of a water molecule, and the catalyst was set free for the next cycle.

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CONCLUSIONS In summary, we report the use of MoS2-RGO nanocomposite as an environmentally benign, highly efficient heterogeneous 8560


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ACS Sustainable Chemistry & Engineering Scheme 5. Mechanism of Catalytic Activity of MoS2-RGO Nanocomposite

FEI Tecnai G2 20 S-twin microscope operating at 200 kV. Energy dispersive X-ray spectra (EDAX) were obtained using the same SEM and TEM instruments. X-ray photoelectron spectroscopic (XPS) measurements were done using the SPECS instrument with a PHOBIOS 100/150 delay line detector (DLD) with 385 W, 13.85 kV, and 138.6 nA (sample current). We have used the Al Kα (1486.6 eV) dual anode as the source. The XPS was taken with a pass energy of 50 eV. As an internal reference for the absolute binding energy, the C-1s peak (284.5 eV) was used. The data obtained from the instrument were processed using the CASA software. The survey peak of the specimens was initially corrected for the C-1s peak and post that the quantitative estimate for the elemental analysis was performed. The XPS peaks were deconvoluted using a combination of Gaussian and Lorentzian curve [GL(30)], and the baseline correction for each spectra was initially carried out using a Shirley background correction with the CASA software. Thermogravimetric (TGA) analyses were carried out by using a PerkinElmer Pyris 1 instrument. The samples were heated from room temperature to 1100 °C at a heating rate of 10 °C min−1 under nitrogen atmosphere with a flow rate of 20 mL min−1. A small amount of the sample (2−3 mg) was kept in a standard platinum crucible, and an empty crucible was used as reference. Contact angle measurements were performed using the Phoenix 300 contact angle instrument. Magnetic properties (M−H curve) were measured using a Quantum design Inc. magnetic property measurement system (MPMS). Melting points were determined using open capillary method. NMR spectra were recorded on a JEOL-USA (JNMECX500) spectrometer in CDCl3-d1 taking TMS (tetramethyl silane) as an internal standard. The NMR chemical shift

catalyst for the synthesis of indole alkaloids in water in a one-pot reaction. The nanocomposite catalyst was prepared using a facile hydrothermal method and has been characterized for its structure, morphology, and surface properties using X-ray diffraction, Raman spectroscopy, SEM, TEM, EDAX, XPS, TGA, and contact angle. Green chemistry matrix calculations showed high atom economy (A.E. = 94.7%) and small E-factor (0.089) for the reactions. Potential industrial use of the catalyst was validated by performing a gram-scale synthesis of natural products. In addition, the recyclability of the catalyst, without any significant loss of activity, was also demonstrated. The developed nanocomposite perhaps can also be used to catalyze several other industrially relevant organic transformation reactions in the future.

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EXPERIMENTAL SECTION

Instrumentation. X-ray diffraction (XRD) measurements were performed using the Rigaku Smart Lab 9 kW rotating anode X-ray diffractometer with Ni-filtered Cu Kα irradiation (λ = 0.1542 nm) at 45 kV and 100 mA in 2θ ranging from 10° to 80° with a scan rate of 2° per minute with stepping size of 0.02°. Raman spectroscopic measurements were done using Horiba LabRAM high resolution UV−vis−NIR instrument using 633 nm laser excitation. Morphology of the samples was investigated by using a scanning electron microscope (SEM), FEI Nova Nano SEM-450, and a transmission electron microscope (TEM), 8561


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evaporator. Purification of the crude reaction mixture was done by column chromatography using a suitable eluent mixture. Isolated yields for each synthesized molecule have been given below in the characterization part. It is noteworthy to mention that, during the process of establishing this green synthesis strategy, we developed an economical and less solvent consuming method to purify the crude reaction mixture. In this method, the crude reaction mixture is triturated with hexane (2 × 3 mL) followed by washing with a 5:95 ratio mixture of EtOAc and hexane (2 × 2 mL). All of the new compounds were characterized by MP, NMR, IR, and HRMS, while all of the known compounds were confirmed by 1H and 13C NMR only. Compound Characterization. 3-((1H-Indol-3-yl)(phenyl)methyl)-1H-indole (Turbomycin B Reductant).76−78 White solid, 74 mg (97%, Rf = 0.57, 30:70 EtOAc/Hex), mp 125−127 °C; 1H NMR (CDCl3, 500 MHz) δ 7.90 (br s, 2H), 7.39−7.33 (m, 6H), 7.29−7.25 (m, 2H), 7.22−7.15 (m, 3H), 7.00 (t, 2H, J = 7.5 Hz), 6.65 (d, 2H, J = 1.4 Hz), 5.88 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 143.9, 136.6, 128.7, 128.2, 127.0, 126.1, 123.6, 121.9, 120.0, 119.7, 119.2, 111.0, 40.1. 1-Methyl-3-((1-methyl-1H-indol-3-yl)(phenyl)methyl)-1H-indole.76,79 Whitish pink solid, 66.5 mg (91%, Rf = 0.72, 20:80 EtOAc/ Hex), mp 182−185 °C; 1H NMR (CDCl3, 500 MHz) δ 7.37 (d, 2H, J = 7.5 Hz), 7.34 (d, 2H, J = 7.5 Hz), 7.29−7.24 (m, 4H), 7.21−7.17 (m, 3H), 6.98 (d, 2H, J = 6.8 Hz), 6.52 (s, 2H), 5.87 (s, 1H), 3.67 (s, 6H); 13 C NMR (CDCl3, 125 MHz) δ 144.4, 137.4, 128.6, 128.2, 128.1, 127.4, 125.9, 121.4, 120.0, 118.6, 118.2, 109.0, 40.0, 32.7. 1-Ethyl-3-((1-ethyl-1H-indol-3-yl)(phenyl)methyl)-1H-indole.80 White solid, 67 mg (90%, Rf = 0.76, 20:80 EtOAc/Hex), mp 162−163 °C; 1H NMR (CDCl3, 500 MHz) δ = 7.37−7.30 (m, 6H), 7.28−7.25 (m, 2 H), 7.23−7.15 (m, 3H), 6.97 (t, 1H, J = 7.5 Hz), 6.57 (s, 2H), 4.06 (q, 4H, J = 7.5 Hz), 1.37 (t, 3H, J = 7.5 Hz); 13C NMR (125 MHz, CDCl3) δ = 144.4, 136.3, 128.7, 128.1, 127.6,126.6, 125.9, 121.1, 120.2, 118.5, 118.2, 109.1, 40.8, 40.2, 15.5. 1-Allyl-3-((1-allyl-1H-indol-3-yl)(phenyl)methyl)-1H-indol. 50 White solid, 68 mg (92%, Rf = 0.80, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.37 (d, 2H, J = 8.2 Hz), 7.34 (d, 2H, J = 6.9 Hz), 7.28−7.24 (m, 4H), 7.21−7.14 (m, 3H), 6.97 (t, 2H, J = 6.8 Hz), 6.56 (s, 2H), 5.96−5.89 (m, 1H), 5.84 (s, 1H), 5.12 (dd, 2H, J = 10.3 and 1.3 Hz), 5.01 (dd, 2H, J = 17.1 and 1.3 Hz), 4.61 (d, 4H, J = 4.8 Hz); 13C NMR (CDCl3, 125 MHz) δ 144.2, 136.8, 133.7, 128.7, 128.2, 127.6, 127.4, 126.0, 121.4, 120.1, 118.7, 118.5, 116.8, 109.5, 48.7, 40.1. 1-Methyl-3-(1-(1-methyl-1H-indol-3-yl)ethyl)-1H-indole.50 Colorless oil; 59 mg (91%, Rf = 0.80, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.36−7.25 (m, 8H), 7.21−7.14 (m, 3H), 6.96 (t, 2H, J = 5.3 Hz), 6.55 (s, 2H), 5.86 (s, 1H), 3.96 (t, 4H, J = 6.9 Hz), 1.78 (sext, 4H, J = 6.8 Hz), 0.86 (t, 6H, J = 6.8 Hz); 13C NMR (CDCl3, 125 MHz) δ 144.4, 136.6, 128.7, 128.1, 127.5, 127.4, 125.9, 121.1, 120.2, 118.4, 118.0, 109.2, 47.8, 40.1, 23.5, 11.5. 1-Butyl-3-((1-butyl-1H-indol-3-yl)(phenyl)methyl)-1H-indole.50 Colorless oil, 58 mg (88%, Rf = 0.86, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.36−7.25 (m, 8H), 7.22−7.14 (m, 3H), 6.95 (t, 2H, J = 6.8 Hz), 6.54 (s, 2H), 5.86 (s, 1H), 3.98 (t, 4H, J = 6.8 Hz), 1.72 (sext, 4H, J = 7.5 Hz), 1.26 (sext, 4H, J = 7.5 Hz), 0.88 (t, 6H, J = 7.5 Hz); 13C NMR (CDCl3, 125 MHz) δ 144.4, 136.6, 128.7, 128.1, 127.5, 127.4, 125.9, 121.1, 120.1, 118.4, 118.0, 109.2, 45.9, 40.2, 32.3, 20.1, 13.7. 1-Hexyl-3-((1-hexyl-1H-indol-3-yl)(phenyl)methyl)-1H-indole.50 Colorless oil, 50 mg (82%, Rf = 0.90, 15:85 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.36−7.26 (m, 8H), 7.21−7.14 (m, 3H), 6.96 (t, 2H, J = 7.5 Hz), 6.54 (s, 2H, J = 6.9 Hz), 5.86 (s, 1H), 3.99 (t, 4H, J = 6.9 Hz), 1.74 (t, 4H, J = 6.2 Hz), 1.24 (br s, 12H), 0.84 (t, 6H, J = 6.2 Hz); 13 C NMR (CDCl3, 125 MHz) δ 144.5, 136.6, 128.7, 128.1, 127.5, 127.4, 125.9, 121.1, 120.1, 118.4, 118.0, 109.2, 46.2, 40.1, 31.3, 30.1, 26.6, 22.5, 14.0. 3-((1H-Indol-3-yl)(4-methoxyphenyl)methyl)-1H-indole. Red solid, 60 mg (89%, Rf = 0.5, 30:70 EtOAc/Hex), mp 197−198 °C; 1H NMR (CDCl3, 500 MHz) δ 7.90 (br s, 2H), 7.38 (d, 2H, J = 8.2 Hz), 7.35 (d, 2H, J = 7.5 Hz), 7.24 (d, 2H, J = 7.5 Hz), 7.16 (t, 2H, J = 6.8 Hz), 6.99 (t, 2H, 7.5 Hz), 6.81 (d, 2H, J = 9 Hz), 6.64 (s, 2H), 5.83 (s, 1H), 3.77 (s, 3H); 13C NMR (CDCl3, 125 MHz) δ 157.8, 136.6, 136.2, 129.6, 127.0, 123.5, 121.8, 120.0, 119.9, 119.1, 113.5, 110.9, 55.2, 39.3.

was reported in ppm relative to 7.26 and 77.00 ppm of CDCl3 solvent as the standards for 1H and 13C spectra, respectively. 1H spectra were recorded in 500 MHz frequencies, and 13C NMR spectra were recorded in 125 MHz frequencies. Coupling constant “J” was calculated in Hz. FT-IR spectra were acquired on a PerkinElmer Spectrum 2 spectrometer. Mass spectra were recorded on an advance Bruker Daltonics (impact HD) UHR-QqTOF (ultra-high resolution Qq-timeof-flight) mass spectrometer. Deionized water (18.2 MΩ cm) used in the synthesis of catalyst was obtained from a double stage water purifier (ELGA PURELAB Option-R7). Single-crystal X-ray data were obtained by using a Agilent Supernova SuperNova E(Dual) diffractometer system. Chemicals. All of the required chemicals were purchased from Aldrich, Fluka, Loba, Merck, and TCI suppliers, and some of them were purified by column chromatography prior to use. For GO synthesis, graphite powder (crystalline, −300 mesh, 99%) was purchased from Alfa Aesar, whereas sodium nitrate (NaNO3), sulfuric acid (H2SO4), potassium permanganate (KMnO4), and hydrogen peroxide (H2O2) were purchased from Merck. Sodium molybdate dihydrate (Na2MoO4· 2H2O) was supplied by Merck. L-Cysteine (98%) and hydrochloric acid (HCl) used in syntheses were purchased from Alfa Aesar and Fischer Scientific, respectively. Synthesis of Graphene Oxide. Graphene oxide (GO) was synthesized from natural graphite flakes using modified Hummers’ method.74 In brief, 1.0 g of graphite powder and 0.5 g of NaNO3 were stirred in 23 mL of concentrated H2SO4 in an ice bath to maintain the reaction temperature below 10 °C. This was followed by the slow addition of 3.0 g of KMnO4 to the reaction mixture with continuous stirring. Subsequently, the reaction mixture was stirred in an oil bath at 35 °C until a brown colored paste was formed in about 4 h. Now, the reaction was terminated by slow addition of deionized water (90 mL), which increased the temperature to 95−98 °C, and the resulting suspension was maintained at this temperature for 15−20 min; subsequently, the suspension was then diluted to about 250 mL by addition of deionized water. This is followed by the addition of 10 mL of H2O2 to remove unreacted KMnO4 in the reaction mixture. To remove the ions of oxidant origin, the mixture was washed with 10% HCl and then with deionized water until the pH value of the filtrate was neutral. Obtained graphite oxide was exfoliated by sonication (1 mg/mL in aqueous solution) and then centrifuged at 4500 rpm for 15 min. The supernatant was decanted and dried with rotary evaporator at 40 °C followed by vacuum drying at the same temperature to obtain GO powder. Synthesis of MoS2 Nanosheets. MoS2 nanosheets were prepared by hydrothermal method using a reported procedure.75 In brief, 0.20 g of Na2MoO4·2H2O and 0.20 g of L-cysteine (0.4 g) were mixed and kept stirring for about 1 h. The reaction mixture then was transferred into a Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h. The obtained product was recovered by centrifugation and washed with deionized water thrice. The final material was dried at 80 °C overnight to obtain MoS2-RGO nanocomposite. Synthesis of MoS2-RGO Nanocomposite. For the preparation of MoS2-RGO nanocomposite, the same hydrothermal strategy was adopted as for the preparation of MoS2 nanosheets. Initially, 0.2 g of as-prepared GO was well dispersed in deionized water with the help of ultrasonication, which was followed by addition of 0.10 g of NaMoO4· 2H2O and 0.10 g of L-cysteine (0.2 g). This reaction mixture was maintained at 180 °C for 24 h in a Teflon-lined stainless steel autoclave. The obtained product was recovered by centrifugation and washed with deionized water and ethanol thrice. The final product was dried at 80 °C overnight to obtain MoS2-RGO nanocomposite. Typical Procedure for the Synthesis of Bisindolylmethanes (3a−3zg). A mixture of indole (1, 0.5 mmol), aldehydes (2, 0.25 mmol), MoS2-RGO catalyst (5 wt %), and water (1.1 mL) was mixed in a 5 mL round-bottomed flask and stirred at ambient temperature. Progress of the reaction was monitored using thin layer chromatography (TLC). After completion of reaction, mixture was decanted to remove water, and ethanol was added to the reaction mixture. Solid catalyst was separated by centrifugation. The organic layer was dried over Na2SO4, and the solvent was removed under reduced pressure using a rotary 8562


Research Article


(CDCl3, 500 MHz) δ 8.14 (d, 2H, J = 8.2 Hz), 8.02 (br s, 2H), δ 7.50 (d, 2H, J = 8.2 Hz), 7.39 (d, 2H, J = 8.2 Hz), 7.33 (d, 2H, J = 8.2 Hz), 7.20 (t, 2H, J = 8.2 Hz), 7.02 (t, 2H, J = 8.2 Hz), 6.68 (d, 2H, J = 2.7 Hz), 5.99 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 151.9, 136.7, 129.6, 126.7, 123.7, 122.4, 119.7, 119.6, 118.2, 111.3, 40.3. 3,3′-(Furan-2-ylmethylene)bis(1H-indole).88,90−92 Dark red solid, 66.3 mg (85%, Rf = 0.80, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.94 (br s), 7.47 (d, 2H, J = 7.5 Hz), 7.35 (d, 3H, J = 7.5 Hz), 7.17 (t, 3H, J = 8.2 Hz), 7.03 (t, 2H, J = 7.5 Hz), 6.87 (d, 2H, J = 2.7 Hz), 6.29 (t, 1H, J = 2.0 Hz), 6.05 (d, 1H, J = 3.4 Hz), 5.94 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 157.0, 141.2, 136.5, 126.7, 123.0, 121.9, 119.7, 119.3, 117.2, 111.0, 110.1, 106.6, 34.0. 3,3′-(Thiophen-2-ylmethylene)bis(1H-indole).76,85 Pink solid, 66.5 mg (82%, Rf = 0.80, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.91(br s, 2H), 7.46 (d, 2H, J = 8.2 Hz), δ 7.35 (d, 2H, J = 8.2 Hz), 7.18− 7.14 (m, 3H), 7.03 (t, 2H, J = 8.2 Hz), 6.92−6.89 (m, 2H), 6.83 (d, 2H, J = 2.0 Hz), 6.16 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 148.6, 136.5, 126.7, 126.4, 125.1, 123.6, 123.1, 122, 119.7, 119.6, 119.3, 111.1, 35.3. Tri(1H-indol-3-yl)methane.76 White solid, 71.4 mg (79%, Rf = 0.70, 60:40 EtOAc/Hex); 1H NMR (DMSO, 500 MHz) δ 10.70 (s, 3H), 7.37 (d, 3H, J = 8.2 Hz), 7.31 (d, 3H, J = 8.2 Hz), 7.00 (t, 3H, J = 7.5 Hz), 6.92 (d, 3H, J = 1.3 Hz), 6.83 (t, 3H, J = 7.5 Hz), 6.03 (s, 1H); 13C NMR (DMSO, 125 MHz) δ 137.0, 127.2, 123.7, 121.1, 119.7, 118.7, 118.4, 111.8. 3,3′-(Phenylmethylene)bis(5-methoxy-1H-indole). Pink solid, 68 mg (93%, Rf = 0.75, 25:75 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.84 (br s, 2H), δ 7.34 (d, 2H, J = 6.9 Hz), 7.29−7.13 (m, 6H), 7.22 (t, 1H, J = 7.5 Hz), 6.83 (d, 1H, J = 2.0 Hz), 6.81 (d, 1H, J = 2.0 Hz), 6.79 (d, 2H, J = 2.1 Hz), 6.66 (d, 2H, J = 2.1 Hz), 5.77 (s, 1H), 3.68 (s, 3H); 13 C NMR (CDCl3, 125 MHz) δ 153.6 143.8, 131.8, 128.7, 128.2, 127.5, 126.1, 124.4, 119.3, 111.9, 111.6, 101.9, 55.8, 40.2. 3,3′-(Phenylmethylene)bis(1-ethyl-5-methoxy-1H-indole). Pink solid, 100.8 mg (92%, Rf = 0.70, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.34 (d, 2H, J = 7.5 Hz), 7.28 (d, 3H, J = 7.5 Hz), 7.20 (t, 3H, J = 8.9 Hz), 6.84 (d, 1H, J = 2.0 Hz), 6.82 (d, 1H, J = 2.0 Hz), 6.77 (d, 2H, J = 2.1 Hz), 6.57 (s, 2H), 5.74 (s, 1H), 4.03 (q, 4H, J = 6.8 Hz), 3.67 (s, 6H), 1.35 (d, 6H, J = 6.9 Hz); 13C NMR (CDCl3, 125 MHz) δ 153.3, 144.4, 131.7, 128.7, 128.1, 127.9, 127.2, 125.9, 117.6, 111.3, 109.8, 102.1, 55.9, 41.0, 40.4, 15.6. 5-Met hoxy-3 -((5 -methoxy-1-methyl-1H-i ndol-3-yl)(4 methoxyphenyl)methyl)-1- methyl-1H-indole.50 Pink solid, 59.5 mg (86%, Rf = 0.71, 25:75 EtOAc/Hex), mp 171.5−172.5 °C; 1H NMR (CDCl3, 500 MHz) δ 7.24 (d, 2H), 7.17 (d, 2H, J = 8.9 Hz), 6.79−6.86 (m, 6H), 6.49 (s, 2H), 5.70 (s, 1H), 3.78 (s, 3H), 3.70 (s, 6H), 3.65 (s, 6H); 13C NMR (CDCl3, 125 MHz) δ 157.7, 153.3, 136.5, 132.8, 129.5, 128.8, 127.7, 117.8, 113.5, 111.3, 109.7, 102.1, 55.9, 55.2, 39.2, 32.8. 1-Ethyl-3-((1-ethyl-5-methoxy-1H-indol-3-yl)(4-methoxyphenyl)methyl)-5-methoxy-1H-indole.50 Pink solid, 60 mg (83%, Rf = 0.77, 20:80 EtOAc/Hex), mp 125.5−126.5 °C; 1H NMR (CDCl3, 500 MHz) δ 7.25 (d, 2H, J = 8.9 Hz), 7.21 (d, 2H, J = 8.9 Hz), 6.81−6.84 (m, 4H), 6.78 (d, 2H, J = 2 Hz), 6.55 (s, 2H), 5.70 (s, 1H), 4.03 (q, 4H, J = 6.8 Hz), 3.79 (s, 3H), 3.68 (s, 6H), 1.35 (t, 6H, J = 6.8 Hz); 13C NMR (CDCl3, 125 MHz) δ 157.7, 153.2, 136.6, 131.7, 129.6, 127.9, 127.2, 117.9, 113.5, 111.2, 109.8, 102.2, 55.9, 55.2, 40.9, 39.5, 15.6. Di(1H-indol-3-yl)methane.93 White solid, 60.3 mg (98%, Rf = 0.55, 15:85 EtOAc/Hex), mp 162−163 °C; 1H NMR (CDCl3, 500 MHz) δ 7.90 (br s, 2H), δ 7.62 (d, 2H, J = 8.2 Hz), 7.36 (d, 2H, J = 7.5 Hz), 7.19 (t, 2H, J = 8.2 Hz), 7.09 (t, 2H, J = 7.5 Hz), 6.94 (d, 2H, J = 2.0 Hz), 4.24 (s, 2H); 13C NMR (CDCl3, 125 MHz) δ 136.4, 127.5, 122.1, 121.8, 119.2, 115.6, 111.0, 21.2. 3,3′-(Ethane-1,1-diyl)bis(1H-indole).93 Colorless oil, 61.1 mg (94%, Rf = 0.67, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.84 (br s, 2H), δ 7.59 (d, 2H, J = 7.5 Hz), 7.34 (d, 2H, J = 8.2 Hz), 7.18 (t, 2H, J = 6.9 Hz), 7.06 (t, 2H, J = 8.2 Hz), 6.91 (d, 2H, J = 2.0 Hz), 4.69 (q, 1H, J = 6.8 Hz), 1.82 (d, 3H, J = 7.5 Hz); 13C NMR (CDCl3, 125 MHz) δ 136.6, 126.8, 121.7, 121.6, 121.2, 119.7, 118.9, 111.0, 28.1, 21.7. 1-Methyl-3-(1-(1-methyl-1H-indol-3-yl)ethyl)-1H-indole.94 Colorless oil, 65.6 mg (91%, Rf = 0.62, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.57 (d, 2H, J = 8.2 Hz), 7.26 (d, 2H, J = 8.2 Hz), 7.18 (t, 2H, J = 6.8 Hz), 7.02 (t, 2H, J = 6.8 Hz), 6.76 (s, 2H), 4.66 (q, 1H, J = 6.8

3-((4-Methoxyphenyl)(1-methyl-1H-indol-3-yl)methyl)-1-methyl1H-indole.50 Brown solid, 62 mg (89%, Rf = 0.58, 25:75 EtOAc/Hex), mp 220−222 °C; 1H NMR (CDCl3, 500 MHz) δ 7.37 (d, 2H, J = 8 Hz), 7.29−7.23 (m, 4H), 7.19 (t, 2H, J = 8 Hz), 6.98 (t, 2H, J = 8 Hz), 6.81 (d, 2H, J = 8.6 Hz), 6.51 (s, 2H), 5.83 (s, 1H), 3.78 (s, 3H), 3.68 (s, 6H); 13 C NMR (CDCl3, 125 MHz) δ 157.8, 137.3, 136.6, 129.5, 128.2, 127.4, 121.3, 120.0, 118.5, 113.5, 109.0, 55.2, 39.2, 32.6. 1-Ethyl-3-((1-ethyl-1H-indol-3-yl)(4-methoxyphenyl)methyl)-1Hindole.50 Yellowish orange solid, 63 mg (88%, Rf = 0.65, 25:75 EtOAc/ Hex), mp 191−192 °C; 1H NMR (CDCl3, 500 MHz) δ 7.36 (d, 2H, J = 8.2 Hz), 7.31 (d, 2H, J = 8.2 Hz), 7.24 (d, 2H, J = 8.2 Hz), 7.17 (t, 2H, J = 6.8 Hz), 6.97 (t, 2H, J = 6.8 Hz), 6.81 (d, 2H, J = 8.2 Hz), 6.56 (s, 2H), 5.82 (s, 1H), 4.05 (q, 4H, J = 7.6 Hz), 3.78 (s, 3H), 1.36 (t, 6H, J = 7.6 Hz); 13C NMR (CDCl3, 125 MHz) δ 157.7, 136.7, 136.3, 129.6, 127.6, 126.6, 121.1, 120.2, 118.5, 118.4, 113.4, 109.1, 55.1, 40.8, 39.3, 15.5. 4-(Di(1H-indol-3-yl)methyl)phenol.81−83 Red solid, 85.7 mg (97%, Rf = 0.60, 40:60 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.88 (br s, 2H), δ 7.37 (d, 2H, J = 8.2 Hz), 7.33 (d, 2H, J = 8.2 Hz), 7.18−7.14 (m, 4H), 6.99 (t, 2H, J = 8.2 Hz), 6.71 (d, 2H, J = 8.2 Hz), 6.61 (d, 2H, J = 2.0 Hz), 5.81 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 136.6, 136.1, 129.7, 126.9, 123.5, 121.8, 119.9, 119.1, 115.0, 111.0, 39.2. 3,3′-(p-Tolylmethylene)bis(1H-indole).70,84,85 White solid, 83.4 mg (95%, Rf = 0.40, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.88 (br s, 2H), δ 7.39 (d, 2H, J = 7.5 Hz), 7.34 (d, 2H, J = 8.2 Hz), 7.22 (d, 2H, J = 8.2 Hz), 7.15 (t, 2H, J = 7.6 Hz), 7.07 (t, 2H, J = 8.2 Hz), 6.99 (t, 2H, J = 7.5 Hz), 6.65 (d, 2H, J = 1.4 Hz), 5.84 (s, 1H), 2.31 (s, 3H); 13 C NMR (CDCl3, 125 MHz) δ 140.9, 136.6, 135.4, 128.8, 128.5, 127.0, 123.5, 121.8, 119.9, 119.8, 119.2, 110.9, 39.7, 21.1. 3-((4-Fluorophenyl)(1H-indol-3-yl)methyl)-1H-indole.80,86 White solid, 49.5 mg (92%, Rf = 0.58, 30:70 EtOAc/Hex), mp 73−75 °C; 1 H NMR (CDCl3, 500 MHz) δ 7.93 (s, 2H), 7.36−7.38 (d, 4H, J = 8.2 Hz), 7.31−7.28 (m, 2H), 7.18 (t, 2H, J = 7.5 Hz), 7.01 (t, 2H, J = 7.5 Hz), 6.96 (t, 2H, J = 8.2 Hz), 6.64 (s, 2H), 5.87 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 162.2, 160.4, 139.6, 136.6, 130.1, 130.0, 126.9, 123.5, 122.0, 119.8, 119.5, 119.2, 115, 114.8, 111.1, 39.4. 3-((4-Fluorophenyl)(1-methyl-1H-indol-3-yl)methyl)-1-methyl1H-indole.76 Brown solid, 57 mg (93%, Rf = 0.71, 20:80 EtOAc/Hex), mp 202−204 °C; 1H NMR (CDCl3, 500 MHz) δ 7.36 (d, 2H, J = 7.5 Hz), 7.31−7.26 (m, 4H), 7.21 (t, 2H, J = 7.5 Hz), 7.01 (t, 2H, J = 8.2 Hz), 6.96 (t, 2H, J = 8.2 Hz), 6.51 (s, 2H), 5.87 (s, 1H), 3.69 (s, 6H); 13C NMR (CDCl3, 125 MHz) δ 162.3, 160.3, 140.1, 137.4, 130.0, 129.9, 128.2, 127.2, 121.5, 119.9, 118.7, 118.1, 115, 114.8, 109.1, 39.3, 32.7. 1-Ethyl-3-((1-ethyl-1H-indol-3-yl)(4-fluorophenyl)methyl)-1H-indole.50 Whitish pink solid, 59.5 mg (92%, Rf = 0.76, 20:80 EtOAc/Hex), mp 197−198 °C; 1H NMR (CDCl3, 500 MHz) δ 7.35−7.25 (m, 6H), 7.18 (t, 2H, J = 7.5 Hz), 6.99−6.93 (m, 4H), 6.55 (s, 2H), 5.87 (s, 1H), 4.06 (q, 4H, J = 7.6 Hz), 1.37 (t, 6H, J = 7.6 Hz); 13C NMR (CDCl3, 125 MHz) δ 162.2, 160.3, 140.1, 136.4, 130.1, 130.0, 127.4, 126.6, 121.3, 120.1, 118.6, 118.1, 114.9, 114.8, 109.2, 40.8, 39.5, 15.5. 3,3′-((2-Chlorophenyl)methylene)bis(1H-indole).70,85,87,88 White solid, 79 mg (85%, Rf = 0.70, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.91 (br s, 2H), δ 7.40−7.34 (m, 4H), 7.25−7.01 (m, 6H), 6.63 (s, 2H), 6.34 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 141.2, 136.7, 133.9, 130.3, 129.5, 127.5, 127.0, 126.6, 123.7, 122.0, 119.8, 119.3, 118.4, 111.0, 36.6. 3,3′-((4-Chlorophenyl)methylene)bis(1H-indole). 70,87,88 White solid, 86.4 mg (93%, Rf = 0.70, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.90 (br s, 2H), δ 7.35 (d, 4H, J = 8.2 Hz), 7.27− 7.16 (m, 6H), 7.01 (t, 2H, J = 8.2 Hz), 6.63 (s, 2H), 5.85 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 142.5, 136.6, 131.7, 130.0, 128.3, 126.8, 123.5, 122.0, 119.8, 119.3, 119.1, 111.0, 39.6. 4-(Di(1H-indol-3-yl)methyl)benzonitrile.78,89 White solid, 83.3 mg (92%, Rf = 0.70, 40:60 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.99 (br s, 2H), δ 7.56 (d, 2H, J = 8.2 Hz), 7.44 (d, 2H, J = 8.2 Hz), 7.37 (d, 2H, J = 8.2 Hz), 7.32 (d, 2H, J = 7.5 Hz), 7.19 (t, 2H, J = 8.2 Hz), 7.02 (t, 2H, J = 6.9 Hz), 6.65 (d, 2H, J = 2.1 Hz), 5.93 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 149.6, 146.9, 144.4, 136.6, 132.2, 129.5, 126.6, 125.3, 123.6, 122.3, 119.5, 118.2, 111.2, 40.3. 3,3′-((4-Nitrophenyl)methylene)bis(1H-indole). 47,75,84 Yellow solid, 91.6 mg (96%, Rf = 0.80, 40:60 EtOAc/Hex); 1H NMR 8563


Research Article


NMR (CDCl3, 500 MHz) δ 7.92 (s, 1H), 7.81 (s, 1H), 7.38−7.33 (m, 4H), 7.29−7.23 (m, 3H), 7.20 (t, 1H, J = 7.5 Hz), 7.16 (t, 1H, J = 7.5 Hz), 7.00 (t, 1H, J = 7.5 Hz), 6.83−6.81 (m, 2H), 6.65 (dd, 2H, J = 16.5 and 1.4 Hz), 5.82 (s, 1H), 3.69 (s, 3H); 13C NMR (CDCl3, 125 MHz) δ 153.7, 143.9, 136.6, 131.7, 128.7, 128.2, 127.4, 127.0, 126.1, 124.4, 123.6, 121.8, 120.0, 119.5, 119.4, 119.2, 111.9, 111.6, 111.0, 101.8, 55.9, 40.2. HRMS (ESI): m/z [M]+ calcd for C24H20N2O, 352.1576; found, 352.1574. 3-((1H-Indol-3-yl)(phenyl)methyl)-5-nitro-1H-indole. Yellow solid, 62.5 mg (68%, Rf = 0.40, 30:70 EtOAc/Hex), mp 201−202; 1H NMR (CDCl3, 500 MHz) δ 11.62 (s, 1H), 10.8 (s, 1H), 8.24 (d, 1H, J = 2.1 Hz), 7.95 (dd, 1H, J = 8.9, 2.0 Hz), 7.52 (d, 1H, J = 8.9 Hz), 7.38−7.34 (m, 3H), 7.30−7.28 (m, 3H), 7.19 (t, 1H, J = 6.8 Hz), 7.09 (s, 1H), 7.04 (t, 1H, J = 7.5 Hz), 6.88−6.84 (m, 2H), 5.98 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 144.3, 140.0, 139.8, 136.6, 128.3, 127.5, 126.4, 126.1, 125.9, 123.6, 121.1, 121.0, 119.1, 118.3, 117.5, 116.5, 116.2, 112.0, 111.5. HRMS (ESI): m/z [M + Na]+ calcd for C23H17N3O2Na, 390.1218; found, 390.1221. 3-((1H-Indol-3-yl)(phenyl)methyl)-5-chloro-1H-indole. Pink solid, 64.1 mg (72%, Rf = 0.50, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 11.03 (s, 1H), 10.81 (s, 1H), 7.36−7.32 (m, 4H), 7.28−7.24 (m, 3H), 7.21 (d, 1H, J = 2.0 Hz), 7.17 (t, 1H, J = 6.9 Hz), 7.04−7.01 (m, 2H), 6.88 (m, 2H), 6.85 (t, 1H, J = 8.2 Hz), 6.80 (d, 1H, J = 2.0 Hz), 5.80 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 144.6, 136.5, 135.2, 128.3, 128.1, 126.5, 125.9, 125.2, 123.5, 123.3, 121.2, 120.9, 119.1, 118.2, 117.8, 117.7, 113.5, 111.5, 110.8. HRMS (ESI): m/z [M]+ calcd for C23H17N2Cl, 356.1080; found, 356.1084. 3-((1H-Indol-3-yl)(phenyl)methyl)-5-bromo-1H-indole.97 Red solid, 64.2 mg (64%, Rf = 0.50, 20:80 EtOAc/Hex), mp 138−140; 1H NMR (CDCl3, 500 MHz) δ 11.06 (s, 1H), 10.8 (s, 1H), 7.40 (s, 1H), 7.35−7.26 (m, 7H), 7.19−7.13 (m, 2H), 7.03 (t, 1H, J = 7.5 Hz), 6.88− 6.81 (m, 3H), 5.83 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 144.6, 136.5, 135.2, 128.3, 128.1, 126.5, 125.9, 125.2, 123.5, 123.3, 121.2, 120.9, 119.1, 118.2, 117.8, 117.7, 113.5, 111.5, 110.8. HRMS (ESI): m/z [M]+ calcd for C23H17N2Br, 400.0575; found, 400.0579. 3,3′-(Cyclohexane-1,1-diyl)bis(1H-indole).78,81,89 White solid, 56.6 mg (72%, Rf = 0.40, 20:80 EtOAc/Hex), mp 117−118; 1H NMR (CDCl3, 500 MHz) δ 7.91 (br s, 2H), 7.55 (d, 2H, J = 8.2 Hz), 7.28 (d, 2H, J = 8.2 Hz), 7.09 (d, 2H, J = 2.0 Hz), 7.05 (t, 2H), 6.88 (t, 2H, J = 8.2 Hz), 2.54 (t, 4H, J = 5.5 Hz), 1.64 (t, 4H, J = 4.8 Hz), 1.58 (t, 2H, J = 4.8 Hz); 13C NMR (CDCl3, 125 MHz) δ 137.0, 126.3, 123.7, 121.9, 121.4, 121.9, 118.5, 111.0, 39.5, 36.8, 26.8, 22.9. 3,3′-(Cyclopentane-1,1-diyl)bis(1H-indole).78,81,89 White solid, 47.3 mg (63%, Rf = 0.50, 20:80 EtOAc/Hex), mp 71−72; 1H NMR (CDCl3, 500 MHz) δ 7.86 (br s, 2H), 7.50 (d, 2H, J = 8.2 Hz), 7.28 (d, 2H, J = 8.2 Hz), 7.09−7.05 (m, 4H), 6.90 (t, 2H, J = 7.5 Hz), 2.51 (t, 4H, J = 6.9 Hz), 1.82 (t, 4H, J = 6.9 Hz); 13C NMR (CDCl3, 125 MHz) δ 131.1, 126.5, 123.4, 121.3, 121.1, 120.9, 118.5, 110.1, 38.6, 29.6, 23.9. 3,3′-((9H-Fluoren-9-ylidene)methylene)bis(1H-indole). White solid, 45.6 mg (46%, Rf = 0.50, 20:85 EtOAc/Hex), mp 237−239; 1H NMR (CDCl3, 500 MHz) δ 7.88 (br s, 2H), 7.82 (d, 2H, J = 7.5 Hz), 7.63 (d, 2H, J = 8.2 Hz), 7.34 (t, 2H, J = 7.5 Hz), 7.30 (d, 2H, J = 7.5 Hz), 7.17 (t, 2H, J = 7.5 Hz), 7.09−7.03 (m, 4H), 6.86−6.82 (m, 4H); 13C NMR (CDCl3, 125 MHz) δ 144.8, 136.8, 135.3, 128.5, 128.4, 128.0, 126.7, 126.2, 125.6, 123.7, 123.0, 121.3, 121.2, 119.3, 118.5, 118.4, 118.2, 118.0, 113.4, 111.8, 119.0, 111.0. HRMS (ESI): m/z [M]+ calcd for C29H20N2, 390.1626; found, 390.1621. 3,3′-(Propane-2,2-diyl)bis(1H-indole).82,83 White solid, 37.1 mg (54%, Rf = 0.40, 15:85 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) δ 7.82 (br s, 2H), 7.44 (d, 2H, J = 8.2 Hz), 7.30 (d, 2H, J = 8.2 Hz), 7.09 (t, 2H, J = 8.2 Hz), 7.02 (d, 2H, J = 1.4 Hz), 6.90 (d, 2H, J = 8.2 Hz), 1.93 (s, 6H); 13C NMR (CDCl3, 125 MHz) δ 137.0, 126,2, 125.4, 121.3, 121.2, 120.4, 118.6, 111.0, 34.8, 29.9.

Hz), 3.67 (s, 6H), 1.78 (d, 3H, J = 6.8 Hz); 13C NMR (CDCl3, 125 MHz) δ 137.2, 126.0, 125.9, 121.2, 120.2, 119.8, 119.7, 118.3, 109.0, 32.6, 27.9, 21.1. 1-Ethyl-3-(1-(1-ethyl-1H-indol-3-yl)ethyl)-1H-indole.95 Colorless oil, 70.4 mg (89%, Rf = 0.67, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) 7.59 (d, 2H, J = 8.2 Hz), 7.32 (d, 2H, J = 8.2 Hz), 7.19 (t, 2H, J = 6.8), 7.04 (t, 2H, J = 6.9 Hz), 6.86 (s, 2H), 4.67 (q, 1H, J = 7.5 Hz), 4.03−3.99 (m, 4H), 1.80 (d, 3H, J = 7.6 Hz), 1.42 (t, 6H, J = 7.5 Hz); 13C NMR (CDCl3, 125 MHz) δ 136.3, 127.4, 124.3, 121.0, 120.3, 120.0, 118.2, 109.1, 40.7, 28.1, 22.1, 15.5. 3,3′-(Ethane-1,1-diyl)bis(1-allyl-1H-indole).50 Colorless oil, 74 mg (87%, Rf = 0.67, 20:80 EtOAc/Hex); 1H NMR (CDCl3, 500 MHz) 7.59 (d, 2H, J = 8.2 Hz), 7.30 (d, 2H, J = 7.5 Hz), 7.19 (t, 2H, J = 7.5 Hz), 7.05 (t, 2H, J = 7.5 Hz), 6.86 (s, 2H), 6.02−5.94 (m, 2H), 5.18 (d, 2H, J = 10.3 Hz), 5.08 (d, 2H, J = 16.5 Hz), 4.72−4.67 (m, 5H), 1.82 (d, 3H, J = 6.8 Hz); 13C NMR (CDCl3, 125 MHz) δ 136.7, 133.8, 133,7, 127.5, 125.0, 124.9, 121.2, 120.5, 119.9, 118.5, 116.8, 109.4, 48.6, 28.1, 22.0. 3,3′-(Phenylmethylene)bis(1-benzyl-1H-indole). White solid, 123.1 mg (98%, Rf = 0.50, 5:95 EtOAc/Hex), mp 137.5−138.5; 1H NMR (CDCl3, 500 MHz) δ 7.40 (d, 2H, J = 8.2 Hz), δ 7.37 (d, 2H, J = 7.5 Hz), 7.29−7.20 (m, 11H), 7.12 (t, 2H, J = 6.8 Hz), 7.03 (d, 2H, J = 7.6 Hz), 6.98 (t, 2H, J = 7.5 Hz), 5.93 (s, 1H), 5.22 (s, 4H); 13C NMR (CDCl3, 125 MHz) δ 144.0, 137.8, 136.9, 128.7, 128.6, 128.2, 127.9, 127.7, 127.3, 126.4, 126.1, 121.6, 120.1, 118.8, 118.7, 109.6, 49.9, 40.22. HRMS (ESI): m/z [M + Na]+ calcd for C37H30N2Na, 529.2307; found, 529.2311. 3,3′-((4-Methoxyphenyl)methylene)bis(1-benzyl-1H-indole). White solid, 131.8 mg (99%, Rf = 0.50, 7:93 EtOAc/Hex), mp 136−137 °C; 1H NMR (CDCl3, 500 MHz) δ 7.40 (d, 2H, J = 7.5 Hz), 7.37, 7.28− 7.19 (m, 10H), 7.11 (t, 2H, J = 7.5 Hz), 7.03 (d, 4H, J = 7.5 Hz), 6.79 (t, 2H, J = 6.8 Hz), 6.82 (d, 2H, J = 8.2 Hz), 6.64 (s, 2H), 5.88 (s, 1H), 5.21 (s, 3H), 3.77 (s, 3H); 13C NMR (CDCl3, 125 MHz) δ 157.8, 137.8, 137.0, 136.3, 129.6, 128.6, 127.8, 127.7, 127.3, 126.4, 121.6, 120.2, 119.1, 118.8, 113.5, 109.6, 55.2, 49.9, 39.4. HRMS (ESI): m/z [M + Na]+ calcd for C38H32N2NaO, 555.2412; found, 555.2418. 3,3′-(Phenylmethylene)bis(6-fluoro-1H-indole).49 Red solid, 79.7 mg (91%, Rf = 0.50, 20:80 EtOAc/Hex), mp 131.5−132.5; 1H NMR (CDCl3, 500 MHz) δ 7.88 (br s, 2H), 7.31−7.19 (m, 7H), 7.00 (dd, 2H, J = 9.6 and 2.0 Hz), 6.74 (dt, 2H, J = 9.6 and 2.0 Hz), 6.59 (d, 2H, J = 1.3 Hz), 5.78 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 160.8, 158.9, 143.5, 136.5, 136.4, 133.7, 130.1, 128.5, 128.4, 128.2, 126.3, 123.7, 123.5, 120.5, 120.4, 119.5, 108.1, 108.0, 40.1. 3,3′-(Phenylmethylene)bis(5-chloro-1H-indole).49 White solid, 92.93 mg (95%, Rf = 0.50, 25:75 EtOAc/Hex), mp 222−224; 1H NMR (CDCl3, 500 MHz) δ 7.95 (br s, 2H), 7.31−7.23 (m, 9H), 7.11 (d, 2H, J = 8.2 Hz), 6.66 (s, 2H), 5.75 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 143.0, 135.0, 128.5, 128.4, 128.0, 126.5, 125.0, 124.9, 122.4, 119.2, 119.1, 112.1, 39.9. 3,3′-(Phenylmethylene)bis(5-bromo-1H-indole).49 Red solid, 111.6 mg (93%, Rf = 0.50, 20:80 EtOAc/Hex), mp 233−234; 1H NMR (CDCl3, 500 MHz) δ 11.08 (s, 2H), 7.42 (d, 2H, J = 1.4 Hz), 7.34−7.31 (m, 4H), 7.29 (t, 2H, J = 7.5 Hz), 7.19 (t, 1H, J = 7.5 Hz), 7.14 (dd, 2H, J = 8.2 and 2.1 Hz), 6.88 (d, 2H, J = 2.0 Hz), 5.85 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 144.3, 135.2, 128.3, 128.2, 126.0, 125.2, 123.4, 121.2, 117.6, 113.5, 110.9. 3,3′-(Phenylmethylene)bis(5-nitro-1H-indole).93 Yellow solid; 101 mg (98%, Rf = 0.50, 50:50 EtOAc/Hex), mp 277−278; 1H NMR (CDCl3, 500 MHz) δ 11.68 (s, 2H), 8.32 (d, 2H, J = 2.0 Hz), 7.97 (dd, 2H, J = 8.9 and 2.7 Hz), 7.53 (d, 2H, J = 8.9 Hz), 7.39 (d, 2H, J = 6.8 Hz), 7.32 (t, 2H, J = 7.5 Hz), 7.22 (t, 2H, J = 7.5 Hz), 7.14 (d, 2H, J = 7.5 Hz), 6.20 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 143.8, 140.2, 139.8, 128.5, 128.2, 127.6, 126.4, 125.8, 120.5, 116.6, 116.2, 112.1, 38.4. 3,3′-(Phenylmethylene)bis(1H-indole-5-carbonitrile).96 Orange solid, 93.2 mg (99%, Rf = 0.50, 35:65 EtOAc/Hex), mp 142−143; 1H NMR (CDCl3, 500 MHz) δ 11.5 (s, 2H), 7.53 (d, 2H, J = 8.2 Hz), 7.39 (t, 4H, J = 8.2 Hz), 7.30 (t, 2H, J = 6.8 Hz), 7.20 (t, 1H, J = 6.8 Hz), 7.13 (s, 2H), 6.03 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ 144.0, 138.2, 128.3, 128.2, 126.7, 124.5, 123.7, 120.7, 118.8, 112.9, 100.3, 38.5. 3-((1H-Indol-3-yl)(phenyl)methyl)-5-methoxy-1H-indole. Pink solid, 55.5 mg (63%, Rf = 0.45, 20:80 EtOAc/Hex), mp 162−163; 1H

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Crystallographic data for 3a, green chemistry matrix calculations, M−H curves of MoS2 and MoS2-RGO nanocomposite, and 1H and 13C NMR data of the synthesized compounds (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

Suneel Kumar: 0000-0002-5259-1792 Vipul Sharma: 0000-0002-4460-4610 Kumbam Lingeshwar Reddy: 0000-0001-6885-1328 Venkata Krishnan: 0000-0002-4453-0914 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Advanced Materials Research Center (AMRC), Indian Institute of Technology Mandi, is gratefully acknowledged for providing all of the necessary laboratory and instrumentation facilities. V.K. acknowledges the financial support from the Department of Science and Technology, India, under Young Scientist Scheme (YSS/2014/000456). P.C.R. acknowledges SERB, India, for the financial support (CS-185/2011). We acknowledge Mr. D. Rambabu for his help in solving crystal structure. A.B. and S.K. acknowledge CSIR and UGC, India, for senior research fellowships, respectively. V.S. and K.L.R. thank MHRD, India, for senior research fellowships. We would like to thank the reviewers for their very helpful suggestions and comments concerning the work.

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