A Recyclable Supramolecular - Ruthenium Catalyst for the Selective

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A Recyclable Supramolecular - Ruthenium Catalyst for the Selective Aerobic Oxidation of Alcohols on Water: Application to Total Synthesis of Brittonin A Mahendra Ramesh Patil, Anant R. Kapdi, and Vijay Kumar A ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03448 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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ACS Sustainable Chemistry & Engineering

A Recyclable Supramolecular - Ruthenium Catalyst for the Selective Aerobic Oxidation of Alcohols on water: Application to Total Synthesis of Brittonin A Mahendra R. Patil, Anant R. Kapdi, and A. Vijay Kumar* Department of Chemistry, Institute of Chemical Technology, NP Marg, Matunga, Mumbai, Maharashtra, India – 400019, E-mail: [email protected] ABSTRACT: A supramolecule based ruthenium catalyst has been developed for on-water aerobic oxidation of alcohols. The catalyst is synthesised by supporting ruthenium nanoparticles on cyclodextrins modified Graphene Oxides (rGO@Ru-RMβ-CD) via a simultaneous one-pot reduction of ruthenium precursor and graphite oxide in water. The rGO@Ru-RMβ-CD was completely characterized by various techniques such as XRD, TGA, FT-IR, SEM, TEM, XPS to understand the morphology and structure. The catalyst showed promising efficiency with good selectivity for benzylic, propargylic and aromatic alcohols under aqueous conditions. Sensitive functional groups such as –NH2, phenolic–OH were well tolerated under the reaction conditions and exclusively afforded the aldehydes in good to excellent yields with no side products. Moreover, the used catalyst was found to be easily recoverable and recyclable up to five times. Additionally, the developed oxidation methodology has been used as a key step for the total synthesis of natural product – Brittonin A, including the other functional group transformations such as wittig olefination and reduction exclusively in water. Notably, these oxidation and reduction transformations could be carried out by using the developed catalyst under aqueous conditions. This unique ability of the catalyst to switch between oxidation and reduction reactions simply by changing O2 and H2 atmospheres with a balloon assembly exemplifies its versatility. To the best of our knowledge this is a first report showing the total synthesis of a molecule completely on water.

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Keywords: Aerobic oxidation; Alcohols; Brittonin; Cyclodextrin; Reduced graphene oxide; Ruthenium nanoparticles. INTRODUCTION: Oxidations are an important class of organic reactions that are widely used in synthetic organic transformations.1,2 Amongst them, the oxidation of alcohols is of prime importance as they provide access to the building blocks for the synthesis of various natural, unnatural and biologically active molecules.2,3 Recently, the importance of green protocols such as aerobic oxidations3-5 has increased because of the growing concerns for environmental preservation. A variety of methods reported in literature for the aerobic oxidation of alcohols are based on metal complexes/salts of Pd,6 Ru,7 Cu,8 Co,9 V,10 Ir,11 Fe,12 Os,13 Au,14 etc. Other methods including N-oxide radicals (TEMPO, ABNO, etc.),15 supported/immobilized catalysts,16-19 nanoparticles,20,21 nanocarbons,22 graphene/graphite oxide,23-25 and bimetallic catalysts26,27 have also proved to be useful towards the oxidation of alcohols. The aerobic oxidation of alcohols

has been reported under homogeneous,

heterogeneous and with nanocatalysts.3,28-30 In general, aerobic oxidations are deemed benign from the green chemistry point of view as they employ green oxidant i.e. molecular oxygen. The use of inexpensive and abundant molecular oxygen results in the formation of water as by product. However, to carry out large scale aerobic oxidations in flammable solvents, Limiting Oxygen Concentrations are to be maintained to prevent combustion reactions.31-33 Additionally, controlling the over oxidation of aldehydes into acid34-37 and the selective oxidation of alcohols in the presence of oxidisable functional groups38-41 are challenges that need to be addressed. Moreover, very few reports are available in the literarure for aerobic oxidation of alcohol in water.42-46 On account of all these aforementioned drawbacks, the development of safe and highly selective aerobic oxidation protocols that are operatable under ambient pressure conditions using recyclable catalysts in a benign solvent like water


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are highly desirable. To achieve the above said goal one must keep in mind the problems of solubility and incompatibility of organic substrates when water is used as solvent. The employment of supramolecular reagents such as cyclodextrins (CD) under aqueous conditions not only provides a unique and practical alternative to overcome such difficulties but also favours the reaction progress.47 The hollow toroidal shape CDs tender additional properties such as the ability to form stable host-guest inclusion complexes with various organic molecules essentially similar to those mediated by enzymes. Their hydrophobic cavity and hydrophilic exterior suits their use as mass transfer reagents under aqueous conditions.48-50 Because of these features, they are very well exploited for oxidation,51-53 reduction,54,55 cis-trans photoisomerization,56 asymmetric synthesis,57-59 CD-metal complex catalysis,60-63 biphasic reactions,64 etc. and because of their molecular recognition ability, they are also used in sensing,65 drug delivery66,67 and metal ion complexation.68 Recently, Eric Monflier and his group has shown CD and modified CDs as stabilizing agents in the ruthenium nanoparticle systems. The problems pertianing to particle aggregation, low catalytic activity and poor recyclability of nanoparticles were greatly addressed in the reports.69-71 In 2010, Wang and Dong with their co-workers functionalized the cyclodextrin through H-bonding on the surface of reduce graphene oxide (rGO).72 Other reports also showed the derivatization and functionalization of graphene oxide surface to support various metal nanoparticles as well as organocatalysts.73-79 So we planned to use cyclodextrin functionalized rGO to support Ru nps. Based on the literature reports, such nanohybrid catalysts have been developed only with Pt80 and Pd81 metal and have been applied for the βnaphthol sensing and C–C coupling reactions, respectively. Unfortunately, the above discussed catalyst preparations are tedious as the rGO functionalization with CDs and metal deposition are done separately. Thus, considering all the aforementioned potential benefits and pitfalls, we were motivated to develop a ruthenium based nanohybrid catalyst (rGO@Ru-



RMβ-CD) (Figure.1) via an in situ deposition of Ru nps on rGO along with fuctionalization of randomly methylated β-cyclodextrin (RMβ-CD) in water. The synthesized catalyst was further applied for the selective aerobic oxidation of alcohols under aqueous conditions (Scheme 1).

Scheme 1. General reaction scheme for the aerobic oxidation of alcohols.

Figure 1. Graphical representation of the catalyst rGO@Ru-RMβ-CD. EXPERIMENTAL SECTION: All reagents and starting materials were obtained from commercial sources and used without purification. RMβ-CD was purchased from commercial source with an average molecular weight of 1310 g/mol and CH3 degree of substitution 1.6-2.0 mol per anhydroglucose unit. Thin Layer Chromatography (TLC) was performed on silica (Silica Gel 60 F254) pre-coated aluminum plates and the products were visualized by UV lamp (PHILIPS TUV 8W lamp) and I2 stain. X-ray powder diffraction (XRD) patterns were recorded on Shimadzu XRD6100 (CuKα radiations = 1.5405 Å) powder diffractometer instrument with a scanning rate 2° per min and 2 theta (θ) angle ranging from 10° to 80° with current 30 mA and voltage 40 kV. Thermogravimetric analysis (TGA) was carried out using PerkinElmer STA 6000 in nitrogen


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gas flow,

heated from room temperature to 700 oC with a ramp rate of 25 oC/min.

Transmission Electron Microscopy (TEM) studies were performed using a FEI, Tecnai G2, F30 Transmission Electron Microscope with resolution of point 2 Å and line 1 Å at 300 kV. Field emission gun-scanning electron microscopy (FEG-SEM) analysis was done by Tescan MIRA 3 model with secondary electron (SE) detector between 10.0 kV to 20.0 kV. ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) was performed using ARCOS model instrument from M/s Spectro, Germany. GCMS of products

was performed on

GCMS-QP 2010 instrument (Rtx-17, 30 m × 25 mm ID, film thickness (df) = 0.25 µm) (column flow 2 mL min−1, 100 °C to 240 °C at 10 °C min−1 rise). X-ray photoelectron spectra (XPS) was carried out by using PHI 5000 Versa Probe with a monochromatic focused (100µm×100µm) Al Kα X-ray radiation (15 kV, 30 mA) and dual beam neutralization using a combination of argon ion gun and electron irradiation. The progress of reaction was monitored by gas chromatography (GC) on Perkin Elmer Clarus 480 GC equipped with flame ionization detector with a capillary column (Elite-1, 30 m x 0.32 mm x 0.25 µm). Products were purified by column chromatography on silica gel (60-120) mesh using petroleum ether and ethyl acetate. The characterization of products was done by 1H and

13

C-NMR

spectroscopy recorded in CDCl3 and TMS as reference standard on an Avance III and Bruker NMR spectrophotometer at 400 MHz and 101 MHz respectively. Preparation of rGO@Ru-RMβ-CD catalyst Initially graphene oxide (GO) was prepared by modified Hummers method. 5 g of graphite powder (purity ≈ 99.5%, Alfa Aesar) was mixed with 115 mL of concentrated H2SO4 and 2.5 g of NaNO3 in a 2 litre beaker for 30 minutes. The reaction mixture was cooled to 0 oC, under vigourous stirring 15 g of KMnO4 was added portion wise within 1 h keeping the temperature below 10 oC. The mixture was then stirred for another 1 h. The ice bath is removed and the reaction is allowed to cool at room temperature (ca. 1hour) and then the remperature was



adjusted to 35 oC with the aid of a water bath and stirred for another 2h. After this, 230 mL of distilled water was added drop wise (approximately 1 hour, the exothermic reactions needs to be maintained at 98oC throughout the addition). Later, external heating was introduced to maintain the reaction temperature at 98 oC for 20 min. Then allowed to room temperature and stirred for 15 min. Finally, the oxidation reaction was terminated by the addition of 700 mL distilled water and 50 mL of 30% H2O2 solution. The resultant solid product was repeatedly washed with 5% aqueous HCl solution and then with distilled water and dried at 60 oC under vacuum for 12 h. The obtained GO was dialysed using a cellulose membrane bag and further used to make rGO@Ru-RMβ-CD catalyst. A 300 mg of the above dialysed GO was dispersed in 350 mL of deionized water by sonication for 1 h. To the dispersed solution of GO, a 600 mg of RMβ-CD was added and stirred for 2 h. The pH of the solution was adjusted to ~ 9–10 by using liq.NH3 (ca 500 µL) or 0.5 M K2CO3. To this, 40 mg of RuCl3.xH2O was added and stirred for 45 min. Later, NaBH4 solution (290 mg of NaBH4 + 20 mL DI water + 200 µL liq.NH3) was added drop wise with the aid of a dropping funnel (20 to 30 min) which resulted in the formation of a black suspension. The solution was stirred for a period of 2 hours and later heated at 80 oC for 6 hrs. After which, cooling to room temperature afforded a fine black foamy suspension. The black residue was isolated by centrifugation at 6000-7000 rpm and washed with deionized water until washing showed a neutral pH. Then the residue was dried in a vacuum oven at 60 oC for 8-10 hrs and it was crushed into fine powder by using mortar-pestle (yield = 407 mg). The as-made catalyst was characterised by different analytical techniques. Preparation of rGO and rGO@Ru The preparation of rGO and rGO@Ru was carried out by adopting the above described process of rGO@Ru-RMβ-CD catalyst preparation. But, In the case of rGO@Ru, the


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synthesis was carried out in the absence of RMβ-CD. Whereas, rGO was prepared in the absence of both RMβ-CD and RuCl3.xH2O. Typical procedure for alcohol oxidation In a typical procedure for alcohol oxidation a 25 mL of round bottom flask equipped with condenser and a small magnetic bar, a oxygen balloon with a bent tube kept on the top of the condenser. The RBF was charged with Piperonyl alcohol (152 mg, 1 mmol), rGO@Ru-RMβCD catalyst (80 mg, 2 mol% of Ru), K2CO3 (69 mg, 0.5 mmol) of and 4 mL water as reaction solvent. The reaction flask was kept on an oil bath at constant temperature of 85oC and reaction was carried out for 6 h and stirred under oxygen atmosphere with the aid of an oxygen balloon. The progress of reaction was monitored by TLC. After the reaction, catalyst was separated by centrifugation and the product was extracted from the aqueous layer with 3 x 4 mL of EtOAc. The obtained product conversion and selectivity was analyzed by GC using a flame ionization detector with anisole as an internal standard. The combined organic layers were evaporated under reduced pressure and the product was isolated from crude residue by silica gel (mesh 60–120) column chromatography (petroleum ether/EtOAc, 90:10) to afford the pure product. Modified procedure for preparation of 3,4,5-trimethoxy benzyl bromide82 PBr3 (3.25 g, 12 mmol, 1.2 equiv) and pyridine (7.91 mg, 0.1 mmol, 0.1 equiv) was added slowly with the aid of a syringe to a 50 mL round bottom flask containing the solution of 3, 4, 5-trimethoxybenzyl alcohol (1.98 g, 10 mmol, 1.00 equiv) in dry CH2Cl2 (20 mL) under N2 atmosphere on an ice bath 5-10oC) and stirred for 30 min at room temperature. The reaction mixture was then heated to up 40 °C and kept for around 3h for the reaction completion (monitored by TLC, mobile phase 10 % EtOAc in petroleum ether). The reaction was cooled to room temperature, quenched with water (10 mL) and extracted with CHCl3 (3×10 mL).



The organic layer was washed with brine (2×10 mL), separated and dried over anhydrous NaSO4. Combined organic layers were evaporated at reduced pressure to furnish the crude product which was utilised without any further purification for next steps. The product was stored in a refrigerator at 2-8 oC to avoid the product decomposition (the faint yellow compound decomposes to a dark brown solid). Note: for NMR characterization a short pad silica gel (mesh 60–120) column chromatography was carried out and the product was confirmed by 1H and 13C NMR. Procedure for preparation of benzyltriphenylphosphonium bromide (wittig salt) A 0.98 mL of benzyl bromide (6 mmol, 1 equiv) was added to the solution of PPh3 (1.6 g, 6.12 mmol, 1.02 equiv) in toluene (30 mL). The resulting mixture was stirred at reflux condition until the formation of a white solid (wittig salt) in approximately 3-4 h. The reaction mixture was cooled to room temperature and the white solid was filtered and washed thoroughly with toluene. The obtained white solid was dried in a vacuum desiccator for 12 h and characterised by mass spectrometry analysis for authentication. Procedure for preparation of (3,4,5-trimethoxybenzyl)triphenylphosphonium bromide (wittig salt)83 3,4,5-trimethoxy benzyl bromide (0.783 g, 3 mmol, 1 equiv) was added to the solution of PPh3 (0.802 g, 3.06 mmol, 1.02 equiv) in toluene (15 mL). The resulting mixture was stirred and refluxed till the formation of a white solid (1-2 hours for the salt to precipitate out completely). The crude product was filtered and the solid was washed with toluene and recrystallized from EtOH. The product was characterized by mass spectrometry. General procedure for wittig olefination A 100 mL round bottom flask equipped with a magnetic bar was charge with wittig salt (1.8 mmol), a homogeneous solution of wittig salt into 40 mL of ethanol:water (1:4) was prepared


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under vigorous stirring. The potassium tertiarybutoxide (3.6 mmol) was added portion wise within 1-2 min followed by addition of aldehyde (1.6 mmol). Reaction was stirred at room temperature up to the rection completion. The progress of reaction was monitored by TLC. After the reaction completion 40 mL of brine solution was added into the reaction mixture and product was extracted 3 X 15 mL of ethyl acetate. The organic layers were combined and evaporated under reduced pressure. The product was isolated as mixture of cis and trans olefin through flash chromatography on 60-120 silica gel and take it further for reduction. The ratio of cis and trans olefin and product conformation by 1H NMR spectroscopy. General procedure for reduction of olefin by rGO@Ru-RMβ-CD A 25 mL round bottom flask equipped with a small magnetic bar was charged with mixture of cis and trans olefin (0.5 mmol), 50 mg (1.5 mol% of Ru) of rGO@Ru-RMβ-CD catalyst, 36 mg (0.25 mmol) of K2CO3 and 4 mL of water as reaction solvent. The reaction mixture was purged with N2 gas 2-3 times prior to the introduction of H2 gas followed by flushing the flask 1-2 times with H2 gas. The reaction was continued under hydrogen atmosphere (with the aid of a hydrogen balloon) at 60oC (oil bath temperature). The progress of reaction was monitored by gas chromatography. After the completion of the reaction, catalyst was separated by centrifugation and the product was extracted from aqueous layer with 3×5 mL of ethyl acetate. The organic layers were combined and evaporated under reduced pressure. The product was isolated by column chromatography on silica gel (mesh 100-200) using a mixture of EtOAc/petroleum ether as eluent. Procedure for preparation of cis and trans- dehydrobrittonin A 25 mL round bottom flask equipped with a small magnetic bar was charged with 198.2 mg (1 mmol) of 3,4,5-trimethoxy benzyl alcohol, 80 mg (2 mol% of Ru) of rGO@Ru-RMβ-CD catalyst, 69 mg (0.5 mmol) of K2CO3 and 6 mL water. Reaction was kept in oxygen atmosphere with the aid of an oxygen balloon. The reaction was kept on an oil bath at



constant temperature of 85oC for 8-10 h under magnetic stirring. The progress of reaction was monitored by TLC and completion of alcohol was analysed by GC. Then reaction mixture was allowed to cool at rt and it was stirred at 15-20 oC for 15 min. After that, 628.1 mg (1.2 mmol) of wittig salt and 224.4 mg (2 mmol) of t-BuOK was added in to the reaction mixture at 15-20 oC and it was stirred at 35-40 oC for 2-3 h until the complete consumption of aldehyde (monitored by TLC, mobile phase 30% EtOAc in petroleum ether). After the completion of the reaction, the catalyst was separated by centrifugation and the product was extracted from aqueous layer with 3×5 mL of CHCl3. The organic layer was combined and evaporated under reduced pressure. The two isomers, cis and trans- dehydrobrittonin (seen as distinct spots on TLC) was isolated from crude residue by silica gel (mesh 100–200) column chromatography (petroleum ether: EtOAc, 90:10, and characterized by NMR and mass spectrometry. (Overall isolated yield of the mixture i.e. cis and trans- dehydrobrittonin was 85% yield with 40% cis isomer and 60% of trans isomer). Procedure for preparation of Brittonin A Method A (dehydrobrittonin as starting material): A 25 mL round bottom flask equipped with a small magnetic bar was charged with 180 mg (0.5 mmol) of dehydrobrittonin [mixture of cis and trans- dehydrobrittonin (40:60) purified by flash chromatography from the crude residue of dehydrobrittonin], 50 mg (1.5 mol% of Ru) of rGO@Ru-RMβ-CD catalyst, 36 mg (0.25 mmol) of K2CO3 and 4 mL of water as reaction solvent. The reaction mixture was purged with N2 gas 2-3 times prior to the introduction of H2 gas followed by flushing the flask 1-2 times with H2 gas. The reaction was kept at constant temperature of 60oC for 8-10 h under hydrogen atmosphere with the aid of a hydrogen balloon. The progress of reaction was monitored by GC. After the completion of the reaction, catalyst was separated by centrifugation and the product was extracted from aqueous layer with 3×5 mL of CHCl3. The organic layers were combined and evaporated under reduced pressure. The product was


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isolated in 90% yield by column chromatography on silica gel (mesh 100-200) using a mixture of EtOAc/petroleum ether as eluent. Method B (one-pot procedure): Following the above described oxidation procedure for the preparation of dehydrobrittonin from 3, 4, 5-trimethoxy benzyl alcohol, without isolating dehydrobrittonin from the reaction mixture, reduction was carried out simply by replacing the oxygen balloon with hydrogen balloon. After the removal of oxygen balloon, the reaction mixture was flushed with N2 gas 2-3 times prior to the introduction of H2 gas followed by flushing the flask 1-2 times with H2 gas. Then, the reaction was kept at constant temperature of 60oC and reaction for 8-10 h under stirring in hydrogen atmosphere with aid of a hydrogen balloon. The progress of the reaction was monitored by GC and product was confirmed by NMR and mass spectroscopy after isolation (in 78% yield) by column chromatography on silica gel (mesh 100-200) using a mixture of EtOAc/petroleum ether as eluent. RESULTS AND DISCUSSION: Unlike the cumbersome preparation procedures, the rGO@Ru-RMβ-CD was synthesized by a simple in situ method as illustrated in scheme 2. Initially, graphite oxide (GO) was synthesized by Hummers method84,85 (a chemical top down method for synthesis of graphene oxide) and carefully dialyzed to ensure the complete removal of metal debris. The reduction of graphene oxide by NaBH486 and NH2NH2.H2O87 is well known in literature and reports show that rGO generated by NaBH4 method has more oxygen functionalities on the surface as compared to rGO generated by NH2NH2.H2O method.88,89 So, a one-pot NaBH4 mediated reduction procedure was adopted to reduce ruthenium and graphene oxide in the presence of RMβ-CD. Herein RMβ-CD acts as capping agent for Ru nps to control the size of the nanoparticles90 and simultaneously it intercalates between the layers of graphene oxide and



Scheme 2. Schematic illustration for the preparation of rGO@Ru-RMβ-CD catalyst. functionalizes the surface of reduced graphene oxide through H-bonding.72 The incorporated CDs not only serve as capping agents but also play an important role as phase transfer catalysts in water.91 Prolonged heating of the solution at 80 oC for six hours was necessary during catalyst preparation for the reduction of graphene oxide to rGO. The arrangement of stacking assemblies of rGO intercalated by RMβ-CD resulted in the formation of thick foam. The synthesized catalyst was centrifuged, dried and then ground in a mortar, which resulted in a fine black powdered (refer supporting information for preparation of the rGO@Ru-RMβCD catalyst Figure S1). It was further tested for the aerobic oxidation of alcohols in aqueous medium and successively studied for the catalyst recyclability. Both the native and recycled catalysts were completely characterized by techniques such as FT-IR, SEM, TEM, XRD, TGA, ICP-AES and XPS for establishing the structure and morphological changes.


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Catalyst characterisation XRD The X-ray diffraction patterns of the obtained GO, rGO, rGO@Ru, rGO@Ru-RMβ-CD and 5th recycled rGO@Ru-RMβ-CD catalyst are shown in Figure 2. GO shows a characteristic (0 0 1) peak centered at 2θ = 10.7o, corresponding to an interplanar spacing of 0.84 nm which resulted from the insertion of hydroxy and epoxy groups between graphite sheets as a result of graphite oxidation.92,93 rGO shows the broad and weak (0 0 2) reflection peak centered at 2θ = 24.7o and (0 0 1) reflection peak centered at 2θ = 10.6o, corresponding to an interplanar spacing of 0.37 nm and 0.85 nm respectively. This might be due to the partial reduction of GO by using NaBH4 which leaves some oxygen functionalities on the surface of rGO.94,95 But in the case of rGO@Ru, the peak at 2θ = 10.7o has completely disappeared, which might be due to the reduction of oxygen functionalities by ruthenium metal akin to transfer hydrogenation of ketones, olefins, etc.96,97 The XRD patterns of rGO@Ru, rGO@Ru-RMβCD and 5th recycled rGO@Ru-RMβ-CD shows the characteristic (0 0 2) reflection peak centered at 2θ = 24.7o and (1 0 0) reflection peak centered at 2θ = 43.1o for rGO. In rGO@Ru-RMβ-CD spectrum the (0 0 2) reflection peak at 2θ = 23.21o is broad and shifted from 2θ = 24.9o. This might be attributed to the high degree of rGO layers exfoliation and due to intercalation of RMβ-CDs, which results in the increase of interlayer spacing from 0.33 to 0.39 nm. The fifth recycled rGO@Ru-RMβ-CD catalyst spectra shows a same peak shifting 2θ = 25.65o [(0 0 2) reflection] with decreased interlayer spacing because of CDs defunctionalisation.



Figure 2. XRD analysis. Thermogravimetric analysis TGA was further used to determine the amount of RMβ-CD molecules on the surface of rGO. From Figure 3 it is found that a rapid weight loss occurs in the range of 315–380 oC temperature in the case of the native rGO@Ru-RMβ-CD catalyst because of the thermal decomposition of the RMβ-CD. Thus, the amount of RMβ-CD molecules incorporated in rGO@Ru-RMβ-CD catalyst was found to be 42.33wt% from the weight loss, which means that a significant amount of RMβ-CDs are available for the phase transfer activity for carrying out organic reaction in water. In the case of catalyst after 1st run a weight loss in the range of 318-375 oC, showed that 25.16 wt % of RMβ-CD retained on the surface of rGO. But, the 5th run catalyst showed no signs of thermal degradation corresponding to CDs. This shows that CDs are coming out of the surface in the successive runs and are going into the aqueous phase after the fifth cycle.


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Figure 3. TGA analysis. FTIR Spectroscopy Figure 4 shows FTIR spectra of RMβ-CD, rGO@Ru and rGO@Ru-RMβ-CD which was recorded to get an idea about the functional group interaction between RMβ-CD and rGO. The FTIR spectra shows C-O-C stretching vibrations at 1152 cm–1 which could be attributed to 1- 4 glycosidic ether linkage in the RMβ-CD that can be distinctly seen in the RMβ-CD curve. The other peaks at 1026 cm-1 for C–C or C–O stretching, 1079 cm-1 for O–H bending, 1568 cm−1 for C=C stretching of aromatic rings, 1724 cm−1 for C=O stretching of acid or ester functionalities, 1442 cm−1 for CH2 bending vibration and 2923 cm–1 for CH2 stretching vibration. It was also noted that the catalyst spectrum has peaks corresponding to the individual IR spectra of both RMβ-CD and rGO@Ru. Importantly, spectra showed a very broad peak 3700-3225 cm–1 for O-H stretching vibration with a red-shift in the region of free O-H functionality, it might be attributed to the strong H-bonding between the RMβ-CD and rGO. Thus, all the above experimental facts and IR data suggest that RMβ-CD functionalized the surface of rGO and formed a layer by layer assemblies through H-bonding.



Figure 4. FTIR analysis of rGO@Ru-RMβ-CD, RMβ-CD and rGO@Ru. SEM and TEM To check the surface morphology of the rGO@Ru-RMβ-CD catalyst we had performed the SEM and TEM analysis. For elemental mapping EDX analysis was also carried out. In figure 5 (a-d), SEM images show a flaky types of layers stacked on each other with an uneven topological surface, which is due to the hollow film layered structure of the catalyst. The elemental mapping, EDX analysis [Figure 5(e)] suggest that approximately 2.98 % of ruthenium supported on the rGO@Ru-RMβ-CD catalyst. Further detailed study of surface morphology was investigated by using HR-TEM analysis. Figure 6 shows the HR-TEM images [Figure 6(a-d)] of rGO@Ru-RMβ-CD and the particle size-distribution histogram of Ru NPs [Figure 6(e)] in the rGO@Ru-RMβ-CD. It can be seen that a very fine and homogeneously dispersed Ru NPs were supported on the surface of CD functionalized rGO with a very narrow particle size distribution. The size distribution histogram of Ru NPs [Figure 6(e)] confirmed that the diameter of Ru NPs in the range of 2.5 to 8 nm, with a mean diameter of 4.73 nm. HR-TEM image of recycled rGO@Ru-RMβ-CD


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Figure 5. FEG-SEM images (a-d) and EDX (e) of the rGO@Ru-RMβ-CD catalyst.

Figure 6. (a-d) HR-TEM images and (e) Histogram of native rGO@Ru-RMβ-CD, (f) HRTEM image of recycled rGO@Ru-RMβ-CD catalyst.



catalyst [Figure 6(f)] shows some particle agglomeration with increase in diameter of particle size. X-ray photoelectron spectroscopy XPS spectra was recorded for native and recycled rGO@Ru-RMβ-CD catalyst to confer the chemical state of the ruthenium in the catalyst and to study the oxygen functionalities on the surface of rGO (Figure 7). The deconvolution of C1s peak of rGO@Ru-RMβ-CD catalyst [Figure 7(b) and 7(d)] shows the peaks at 284.7, 286.3, 287.6 and 288.7 eV, corresponding to the binding energies of C–C/C–H, C–O–C/C–OH, C=O and COOH groups respectively.98-100 In the native rGO@Ru-RMβ-CD catalyst BE of C–O–C/C–OH is 286.3 eV which is corresponding to BE of C–O–C (epoxy/alkoxy/ether). But in the case of recycled rGO@RuRMβ-CD catalyst it was shifted to 285.6 eV with decreased intensity fairly corresponding to BE of C–OH.101 This might be attributed to the decreased percentage of C-O-C ether linkage (which present in the CD) in the recycled catalyst. This also shows that defunctionalization of CDs result in the formation of more C–OH functionalities on the surface of recycled catalyst. The peak shrinking at 284.6 eV corresponding to the BE of C-C/C-H of C1s spectrum of recycled catalyst that is attributable to removal of CDs. The C1s spectrum of the native catalyst did not show the intensity of the π → π* satellite peak at 291.5 eV, which supports the successful functionalization on the surface of rGO.102 The XPS spectrum of the native and recycled rGO@Ru-RMβ-CD catalyst in Ru 3p region [Fig. 7(a) and 7(c)] showed BE of Ru 3p3/2 at 461.2 eV and Ru 3p1/2 at 483.4 eV, which correlates to the photoemissions from metallic Ru (0).103,104 In the recycled catalyst, the Ru 3p3/2 region shows a small hump at 469.8 eV, corresponding to the hydrous RuO2 (RuO2.xH2O (or) RuOxHy).105,106 This can be due to the formation of a thin layer of hydrous oxide (RuOxHy) on the surface of Ru nanoparticles. From the catalytic point of view, this hydrous and defective RuOxHy oxide formation is a huge boon for the oxidation of alcohols, as the hydrous oxide is more active


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when compared to RuO2 and Ru metal.107-109 Therefore during reaction, the formation of a thin layer of hydrous RuO2 on the surface of Ru nanoparticles are responsible for the aerobic oxidation of alcohols in our developed catalyst.

Figure 7. XPS spectra for the peaks of Ru 3p and C1s of native rGO@Ru-RMβ-CD (a-b), Recycled rGO@Ru-RMβ-CD (c-d). ICP-AES analysis The amount of Ru present in the prepared catalyst was found to be 2.5% from the ICP-AES analysis. To determine the possible leaching of ruthenium from the catalyst, ICP-AES analysis of the native reaction mixture was carried out. The leached amount from the support was found to be 2.35 ppm, which is significantly low when compared to the previously prepared rGO@Ru catalysts without CD modifications in whose cases the Ru leaching was as high as 9.85 ppm. We also found that ruthenium leaching increases up to 6.25 ppm when the equivalents of K2CO3 was increased from 0.3 - 1.0 as an additive in the reaction. To find out



the amount of Ruthenium present intact on the support after the five cycles, Ru was deliberately leached from the recycled rGO@Ru-RMβ-CD catalyst and was found to be 2.22% when compared to 2.5% of the native rGO@Ru-RMβ-CD catalyst. Aerobic Oxidation of alcohols To investigate the catalytic application of the as made catalyst, we tested the same for the aerobic oxidation of alcohols. Optimization studies were carried out with Piperonyl alcohol as the model substrate for aerobic oxidation of alcohols in water. The blank oxidation reaction with GO alone as catalyst gave only 30% yield (Table 1, entry 1). Whereas, rGO and rGO@RMβ-CD as catalysts showed no progress (Table 1, entries 2, 3). The oxidation reaction with rGO@Ru-RMβ-CD in the presence of K2CO3 afforded the product in 93% yield (Table 1, entry 4), whereas the same catalyst rGO@Ru-RMβ-CD gave only 80% yield (Table 1, entry 5) without the base. On the other hand, the reaction with Ru nanoparticles on rGO (Ru@rGO) gave only 75% yield after 24 h (Table 1, entry 6). Mild bases viz. Na2CO3 and K2CO3 were chosen for the base study taking into account of the expensiveness of salts such as Cs2CO3, Rb2CO3. The other harsh alkali bases such as NaOH, KOH were not considered in order to avoid harsh reaction conditions. Of the two bases tried, 0.5 equivalent of K2CO3 gave better yields when compared to Na2CO3 (Table 1, entries 7–11). Different solvents such as toluene, acetonitrile, ethanol, ethyl acetate were also screened along with water. The reaction in toluene showed very little progress affording only 30% yield of the product after 24h (Table 1, entry 12) apparently because of the competitive inclusion of toluene by cyclodextrin. In case of ethanol 85% of yield of the product was obtained (Table 1, entry 13), whereas acetonitrile and ethyl acetate gave moderate to less yields (Table 1, entries 14, 15). Finally, the other conditions such as time, temperature and oxygen/air were also optimised. A decrease of the temperature prolonged the reaction time to 24 h (Table 1, entry 16). The reaction under nitrogen atmosphere afforded only 10% yield (Table 1, entry 18), whereas


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Table 1. Optimization of reaction conditionsa O

Catalyst, Base

O

OH

O

O

Water, O2/air/N2, Time/Temp.

O 2a

1a entry

H

catalyst

O 2/ air/N2

base (equvi.)

solvent

temp/ time/ yield b/ o h C %

Catalyst screening study 1

GO

O2

K2CO3 (0.5)

water

85

24

30

2

rGO

O2

K2CO3 (0.5)

water

85

24

NR

3

rGO@RM -CD

O2

K2CO3 (0.5)

water

85

24

NR

4

rGO@Ru-RM -CD

O2

K2CO3 (0.5)

water

85

12

93

5

rGO@Ru-RM -CD

O2

water

85

12

80

6

rGO@Ru

O2

K2CO3 (0.5)

water

85

18

75

7

rGO@Ru-RM -CD

O2

Na2CO3 (0.3)

water

85

12

75

8

rGO@Ru-RM -CD

O2

Na2CO3 (0.5)

water

85

12

83

9

rGO@Ru-RM -CD

O2

Na2CO3 (1.0)

water

85

12

88

10

rGO@Ru-RM -CD

O2

K2CO3 (0.3)

water

85

12

90

11

rGO@Ru-RM -CD

O2

K2CO3 (1.0)

water

85

12

92

-

Base Study

Solvent study 12

rGO@Ru-RM -CD

O2

K2CO3 (0.5)

toluene

85

12

30

13

rGO@Ru-RM -CD

O2

K2CO3 (0.5)

ethanol

85

12

85

14

rGO@Ru-RM -CD

O2

K2CO3 (0.5)

acetonitrile

85

12

75

15

rGO@Ru-RM -CD

O2

K2CO3 (0.5)

ethyl acetate

85

12

50

Time, temp and atmospheric condition study

a

16

rGO@Ru-RM -CD

O2

K2CO3 (0.5)

water

60

24

60

17

rGO@Ru-RM -CD

O2

K2CO3 (0.5)

water

70

16

78

18

rGO@Ru-RM -CD

Air

K2CO3 (0.5)

water

85

12

70

19

rGO@Ru-RM -CD

N2

K2CO3 (0.5)

water

85

12

10

Reaction Conditions: Piperonyl alcohol (1mmol, 152 mg), O2 balloon, Catalyst (80 mg), base, solvent (4 mL). bIsolated yield.



under open air conditions only 70% yield of the product was obtained (Table 1, entry 19).With the optimised conditions in hand, we proceeded to test the substrate scope. Different

benzylic alcohols with various substituents such as Cl, NO2, NH2, OMe,

phenolic-OH (Table 2) were tried under the optimised conditions. The yields of the product were good to excellent (94-88%) and showed maximum selectivity (>99%). Substrates with electron donating groups gave good yields when compared to electron withdrawing groups and their reaction completion times were also lesser than the ones with the electron withdrawing groups (Table 2, entry 2). The benzylic secondary alcohols were also tested, which showed the smooth conversion with >99% selectivity affording the products in 92% and 87% isolated yield (Table 2, entry 3). Interestingly, the catalyst showed the selective oxidation of propargylic alcohols with good conversion and afforded moderate yields (7835%) in which the aryl-substituted gave superior yields when compared to the aliphatic substituted alkynols (Table 2, entry 4).The alpha-keto substituted secondary alcohol i.e. benzoin gave the product in 90% yield (Table 2, entry 5). The oxidation of the anthroquinone alcohol aloe emodin wasn’t that promising, only 30% of the oxidised product was isolated with the rest being unconsumed starting material (entry 6; Table 2). It is noteworthy to mention that the catalyst showed efficient catalytic activity for the selective oxidation of the alkynols and benzylic alcohols in the presence of other sensitive functional groups which are susceptible to oxidation. The preservation of alkynyl, phenolicOH, amino functionalities and the absence of any over oxidised products endorses the efficiency, selectivity and mildness of the protocol. Having completed the study of benzylic and propargylic alcohols, we further proceeded to test the allylic and aliphatic alcohols (Table 2). The allylic alcohol i.e. cinnamyl alcohol showed smooth conversion into cinnamaldehyde with good selectivity and afforded in 75% isolated yield (Table 2, entry 8).


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Table 2. Substrate Scope for aerobic oxidation of benzylic, propargylic and aliphatic alcoholsa substrate

entry

time/ conv.b/ h %

product

OH 1

O

sel.c/ yieldd / % %

O O

H

O

12

98

>99

93

R=H R = m-Cl R = p-Me R = p-NO2 R = m-NO2 R = o-NH2 R = o-OH R = p-OH R = 3,4-dimethoxy R = 2,5-dimethoxy

12 8 8 16 12 10 12 12 16 18

94 96 93 65 72 94 96 90 96 93

97 >99 99 >99 >99 98 >99 99 >99 >99

88 91 89 60 70 84 90 86 90 82

R = 3,4,5-trimethoxy

18

95

>99

85

R1 = Ph, R2 = CH3

8

98

>99

92

R1 = Ph, R2 = Ph

10

92

>99

87

R1 = CH3, R2 = H R1= Ph, R2 = H R1= Ph, R2 = Ph

24 16 16

80 96 94

63 99 >99

35e 70 76

12

94

>99

90

24

-

-

30f

12

85

98

75

24

55

>99

30

24

40

60

24e

24

16

>99

15e

24

-

-

NRg

12

-

-

NR

H

O OH

2

O

H

R

H

R

OH

O

3 R2

R1

OH 4

R2

R1

O

R2 R1

R2 R1

O

O

5 OH OH O

O

OH

OH O

OH O H

OH

6 O

O

O

OH

8

H H

OH 9 O

10 HO

O OH

O

11

O 12

13

a

OH

H O OH

H

Reaction Conditions: alcohol (1mmol), rGO@Ru-RMβ-CD (80 mg, 2 mol% Ru), K2CO3 (0.5 mmol), b c water (4 mL), O2 balloon, 85 oC. Conversion of alcohol determined by GC analysis. Selectivity of d e product determined by GC with an appropriate internal standard. Isolated yield. GC yield and confirmed by GC-MS analysis. f100 mg of rGO@Ru-RMβ-CD, 95 oC. g75 oC. NR: no reaction.



The present catalytic system shares some of the similarity with previously reported aerobic oxidation of alcohols in water. In the activation of benzylic alcohols, our catalyst showed similar trend as the previous reported oxidations such as homogeneous Pd,42 Au-clusters on PVP,110

Ru/C-β-CD,111

Cu- pytl-β-CD,112

Cu/TEMPO/TPGS-750-M,113 polymer

supported Pd catalyst,46 VOSO4/4,4’-t-Bubpy,114 FeCl3-imine@SiO2,115 MOF-derived Co,43 quasi-homogeneous Cu.116 Unfortunately, the reactions in the case of aliphatic alcohols were less promising. The reactions of several aliphatic alcohols were sluggish and afforded low yields of the aldehydes (Table 2, entry 9–11). The aliphatic alcohols such as 1-butanol and 1octanol were non-progressive (Table 2, entry 12, 13). When compared to catalysts such as homogeneous Pd,42 Au/NiO doped with copper,117 FeCl3-imine@SiO2,115 which are shown to work for the aliphatic alcohols oxidation, our system was less active for the aliphatic ones. Finally, to extend the substrate scope, the oxidation of heteroaromatic alcohols (Table 3) was tried. The heterocyclic alcohols showed good conversion along with maximum selectivity and afforded the products in good to moderate yield (Table 3, entry 1–3). Table 3. Substrate scope for aerobic oxidation of heteroaromatic alcohola

a

Reaction Conditions: alcohol (1mmol), rGO@Ru-RMβ-CD (80 mg, 2 mol% Ru), K2CO3 (0.5 mmol), water (4 mL), O2 balloon, 85oC. bConversion of alcohol determined by GC

analysis. cSelectivity of product determined by GC with an appropriate internal standard. d

Isolated yield. eGC yield.


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Having completed the substrate scope of the aerobic oxidation reaction, we further extended the study for hydrogenation reactions using rGO@Ru-RMβ-CD. For this, we checked the hydrogenation of alkenes by rGO@Ru-RMβ-CD under aqueous conditions with molecular hydrogen. Interestingly, we were able to reduce several alkenes bearing various functional groups such as methoxy, chloro and pyridyl, which afforded products in good to excellent yields. The heterocyclic pyridine containing substrate was also obtained in good yields (Table 4, entry 5). Table 4. Substrate scope for olefin reductiona R2 R1

H2 Balloon rGO@Ru-RM -CD, K2CO3

R2 R1

H2O, 60oC Mixture of Cis and T rans Olefin

a

Reaction Conditions: mixture of cis and trans olefin (0.5 mmol), rGO@Ru-RMβ-CD (50

mg, 1.5 mol% Ru), K2CO3 (0.25 mmol), water (4 mL), H2 balloon, 60oC. bIsolated yield.



The methoxy substituted derivatives were obatined in good yields when compared to the other substrates tested (Table 4, entries 2, 4, 6). A notable feature of the catalyst is the reaction of halogenated compounds which showed no signs of the dehalogenation product for the halogen substituted derivatives. This proved that the halogen funtionalities are tolerated by the catsalyst under the employed conditions (Table 4, entry 3). The success of both oxidation and reduction reactions by the catalyst thus endorsed that the catalyst is versatile in performing dual type of transformations. Catalysts recycle study Encouraged by these results, we further expanded our study to test the recyclability of the catalyst. We found that the catalyst can be easily recovered by centrifugation and the oxidised product could be extracted from aqueous layer using diethyl ether or ethyl acetate. The aqueous layer was used successively for the next cycle just by charging it with a fresh reactant and recovered catalyst. Likewise, we were able to recycle the catalyst up to five cycles (Figure 8). The yields were consistently same for three runs, but were slightly decreased in the further next cycles. Moreover, we also observed that the time of aerobic alcohol oxidation has increased in the successive runs after third cycle. This might be due to the defunctionalisation of RMβ-CD from the surface of rGO which results in the deprivation of substrates’ access to catalytic sites. Thus, the leaching of cyclodextrins cannot be considered as a disadvantage of this process as CDs are non-toxic and inexpensive.


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Figure 8. Recyclability of rGO@Ru-RMβ-CD catalyst for aerobic oxidation of Piperonyl alcohol in water. Plausible reaction mechanism Based on the above results, we propose a plausible reaction mechanism for the aerobic oxidation of alcohol by the heterogeneous Ru catalyst (Scheme 3). The formation of a thin layer of ruthenium oxide on the surface of Ru nps takes place in the presence of O2 atmosphere as evidenced from the XPS spectrum of recycled catalyst [Figure 7(c)]. In the hydrous environment, ruthenium hydroxyl species generated on the surface of ruthenium oxide undergo the ligand exchange between alcohol substrate to form Ru-alcoholate.118 Then, β-elimination of the alcohol takes place to afford the corresponding carbonyl compound and ruthenium hydride species. The hydride species is then reoxidized by molecular oxygen followed by decomposition to generate the oxy form of ruthenium on the surface. A similar mechanism has also been observed earlier in the heterogeneous ruthenium mediated alcohol oxidations.119



Scheme 3. Plausible reaction mechanism of alcohol oxidation by using rGO@Ru-RMβ-CD catalyst. Total synthesis of Brittonin A Having completed the catalyst recyclability study, our interest was drawn towards applying the developed methodology to the synthesis of a natural product. A recently isolated Bibenzyl molecule Brittonin A from Dendrobium secundum, T. Frullania brittoniae subsp. truncatifolia (F. F. muscicola)120 caught our attention. A literature survey revealed that the recent syntheses of the same uses Ru nps relying on hydrogenation reactions under high pressure conditions.121 The other report by Alonso et al used organic solvents and other pyrophoric reactants.120 The recent synthesis of Batatasin III and other analogues also relies on wittig and olefin reduction in flammable solvents.122 Keeping in mind of these lacunae, we were motivated to use the developed oxidation protocol as a key step for the synthesis of Brittonin A. Our synthesis strategy involved the oxidation of the alcohol followed by wittig olefination and a final reduction step to complete the synthesis. For this, we attempted a sequential alcohol oxidation followed by wittig olefination to synthesize the dehydrobrittonin


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in a one-pot process. The reaction proceeded smoothly to afford dehydrobrittonin in 85% yield. Further reduction of the alkene furnished Brittonin A in 90% yield (Scheme 4). To our delight we also found that the reduction step can be carried out in a one-pot fashion without isolating the alkene precursor to afford the final product Brittonin A in 78% isolated yield (overall one-pot steps i.e. oxidation + wittig + reduction) (Scheme 5). In both the cases, a simple filter column procedure was sufficient to obtain the analytically pure products without any cumbersome purification steps. Surprisingly, we also found that our developed catalyst was able to carry out both the reduction and oxidation reactions in a simple balloon set-up under aqueous conditions precluding any specialized equipment. Additionally, these telescoped processes are advantageous as they avoid the unnecessary purification steps and toxic solvents, thereby making the whole process benign from the green chemistry point of view.

Scheme 4. Total synthesis of Brittonin A in water by using rGO@Ru-RMβ-CD as catalyst for sequential oxidative Wittig and olefin reduction.



Scheme 5. One-pot total synthesis of Brittonin A in aqueous medium by using rGO@RuRMβ-CD as catalyst.

CONCLUSION: In conclusion we developed a novel rGO@Ru-RMβ-CD catalyst and characterized by various spectroscopic techniques to elucidate the structure and morphology. It displayed a very good catalytic activity for aerobic oxidation of alcohols in aqueous medium. The catalyst showed good selectivity for a wide range of substrates in presence of sensitive functionalities and could be reused up to five cycles. The application of total synthesis of Brittonin A in aqueous medium was also demonstrated with oxidation as the key step followed by olefin reduction by the same catalyst. The catalyst’s versatility to perform both oxidation and reduction by switching H2 and O2 atmospheres might be pivotal to develop new methods of tandem assisted one-pot oxidation-reduction reactions in near future. Associated content Supporting Information Copies of 1H-NMR & 13C-NMR; Schematic of catalyst preparation. Author Information Corresponding Author Tel.: +91 22 33612614. E-mail: [email protected] ORCID A. Vijay Kumar: 0000-0001-9753-0590


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Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS MP is grateful to CSIR, New Delhi, India for providing a Senior Research Fellowship (SRF). AVK is thankful to Department of Science and Technology, Government of India for the INSPIRE Faculty Award [IFA12-CH40] and research funding. We are thankful to Pooja Ayare and Karishma Singh for carying out the preliminary experiments. We also acknowledge the Department of Pharmaceutical Sciences and Technology, ICT (for NMR facillity) Department of Chemistry, ICT (for XRD, DSC-TGA and SEM) for carrying out the analysis.

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natural allelopathic compound Batatasin-III and synthetic analogues. J. Nat. Prod. 2017, 80, 2001−2011. DOI: 10.1021/acs.jnatprod.7b00129

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Synopsis: A ruthenium catalyst for an on-water aerobic oxidation of alcohols and reduction of alkenes is developed and used for the total synthesis of Brittonin A.


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A Recyclable Supramolecular - Ruthenium Catalyst for the Selective

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