Unprecedented Concomitant Formation of Cu₂O–CD Nano-Superstructures During the Aerobic Oxidation of Alcohols and Their Catalytic Use in the Propargylamination Reaction: A Simultaneous Catalysis and Metal Waste Valorization (SCMWV) Method

Abstract

Copper–cyclodextrins (CDs)-catalyzed aerobic oxidation of alcohols under aqueous conditions and a concomitant formation of Cu₂O–cyclodextrin nano-superstructures (Cu₂O–CD nps) during the reaction are reported. The use of affordable copper and cyclodextrin combination for aerobic oxidation precluding organic solvents makes it a benign methodology. Intriguingly, a diverse array of Cu₂O–CD nps with unique morphologies was obtained by varying copper salts, cyclodextrins, and bases. The nano-superstructures were characterized by different techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy, differential scanning calorimetry-thermogravimetric analysis, scanning electron microscopy, time of flight secondary-ion mass spectrometry, and transmission electron microscopy to confer their authenticity. Interestingly, the nano-superstructures showed promising catalytic efficiency for a one-pot three-component propargylamination reaction. The used particles were found to be recoverable and recyclable for propargylamination for up to three cycles, with no loss of catalytic activity. Moreover, the concomitant formation of Cu₂O–CD nanostructures and their self-segregation during an aerobic oxidation reaction under homogenous conditions is a first-of-its-kind method depicting simultaneous catalysis and metal waste valorization (SCMWV). Overall, this new approach of reaping the benefits of homogenous metal catalysis and simultaneously sequestrating the metal into a high-value product might pave the way to develop many such SCMWV protocols in future.

Introduction

Aerobic oxidations are all important upcoming green organic reactions, which utilize cheap and abundant molecular oxygen to synthesize valuable organic compounds.¹ Amongst them, the oxidation of alcohols² is an indispensable transformation, well known with N-oxide radicals (2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), 9-azabicyclo[3.3.1]nonane N-oxyl, etc.),³ nanocarbons,⁴ graphene/graphite oxide,⁵ supported metals,⁶ and metal catalysts.⁷ Unlike the conventional oxidation methods, aerobic oxidations delineate the need for extra steps to treat and dispose oxidant waste, as they generate the nontoxic side product water. For this reason, they are not only regarded as economical but also environmentally beneficial. Unfortunately, despite the substantial benefits, several factors, such as fire (or explosion) hazards of organic solvents, the need for modified plant designs, along with specialized equipment, limit its large-scale utility. Additionally, requisites such as elevated temperatures and pressures (to compensate the “limiting oxygen concentrations”) for achieving significant conversions escalate the overall cost of the process. Thus, on account of these drawbacks, aerobic oxidations are seldom preferred in the pharmaceutical and fine chemical industry.⁸ In this context, from the perspective of green chemistry, to make oxidations safe and sustainable, the use of benign solvents such as water can be a promising alternative. Owing to its abundance, nontoxicity, and nonflammability water has been explored as solvent by many research groups and proven to be an excellent cost-effective replacement for the toxic solvents.⁹ The other aspects such as the ability to enhance reactivity and selectivity make it a promising candidate. However, the solubility of organic substrates negates the advantages and makes it difficult to channel the full utility. In such cases, the use of supramolecular catalysts such as cyclodextrins (CDs)¹⁰ emulates the solubility issues.

Cyclodextrins, also known as “Shardinger’s dextrins” are toroid-shaped cyclic oligosaccharides comprised of several glycopyranose units linked by 1,4-glycosidic bonds. They possess a hydrophilic outer surface and a hydrophobic cavity. The cavity is accountable for the selective reversible binding of molecules (inclusion phenomenon) driven predominantly by hydrophobic^11a,11c and noncovalent interactions.^11b The fascinating feature of cyclodextrins akin to enzymes has been widely exploited for various reactions, such as oxidations,¹² reductions,¹³ protections,¹⁴ cycloadditions,¹⁵ and epoxide ring opening.¹⁶ Besides these, several other uses of native/modified CDs include capping agents of metal nanoparticles and semiconductor nanocrystals, templates for nanocluster composites, nonporous metal organic frameworks, and molecular recognition. Furthermore, from the standpoint of biomimicking catalysis, CD–metal complexes mimicking the active sites of metalloenzymes is an upcoming area of interest which is vastly expanding, especially the catalysis by the copper–CD dyad.¹⁷ Moreover, many essential molecular oxygen activator enzymes found in some aerobic organisms are known to house the copper cofactor. Also, copper is an inexpensive, non-noble metal widely known for its catalytic versatility.¹⁸ Therefore, considering the advantageous characteristics of copper–CD and the limitations of aerobic oxidations, we decided to exploit the same combination for biomimetic oxidation of alcohols under aqueous conditions. During such an effort, we serendipitously discovered the unusual concomitant formation of Cu₂O–CD nano-superstructures (Cu₂O–CD nps) while carrying out the aerobic oxidation of alcohols in water (Scheme 1).

Scheme 1. Comparison of Previous Methods and Our Report.

Generally speaking, in metal catalysis, homogenous reactions are considered superior to heterogeneous reactions because of their performance tunability, scope for improvement through ligand design and better understanding of the mechanisms at molecular levels. Unfortunately, factors such as the generation of copious metal waste, high energy consumption, and the use of hazardous solvents make the homogenous reactions uneconomical and prevent their significant potential for industrial utility. Even the recently emerging alternate methods that make use of alternate solvents, such as ionic liquids,¹⁹ scCO₂,²⁰ and switchable systems (fluorous solvent systems),²¹ to separate the homogenous catalysts via multiphase catalysis²² are less tangible, as the procedures involved are complex and cumbersome. Under such circumstances, the discoveries of new methods which are conducive for catalysis and facilitate the spontaneous metal recovery are highly desirable. Therefore, with a view to the same purpose, we present herein a novel approach of simultaneous catalysis and metal waste valorization (SCMWV), wherein a possible solution to the burgeoning problems of metal waste recovery and disposal are provided. Moreover, this report validates the concept of “simultaneous waste valorization” which might pave way for the development of greener and sustainable protocols.

Results and Discussion

Initially, we started our investigation using piperonyl alcohol as a model substrate with the combination of copper(II) acetate and β-cyclodextrin, Na₂CO₃ as base, and with TEMPO as an additive under aqueous conditions at room temperature. The substrate took 24 h for complete conversion to afford piperonal in 91% yield (Table 1, entry 1) without any over oxidation side products. With this preliminary result, we screened various bases, cyclodextrins, and copper salts to achieve the best conversions of alcohol. K₂CO₃ (1 equiv) was found to be most effective of all of the bases tried and was continued for further study (Table 1, entry 6). Reducing the equivalents of base has drastically reduced the product yields (Table 1, entries 7–10), whereas no further change in yields was observed when the amount was increased to 2 equiv (Table 1, entry 11). Although the yields were promising with various cyclodextrins during the initial screening (Table 1, entries 12–15), β-CD was selected for further study taking into account the cost of benefit. Of the various copper salts tested, copper acetate afforded the best conversions (Table 1, entries 16–18). The optimum amount of copper and CD for obtaining the maximum substrate conversion was found to be 10 mol % each. An increase of the amount of copper–CD gave almost same yield, whereas a decrease in the amount resulted in lowering of yield. Nevertheless, the reactions without either one of them, that is, copper, CD, or TEMPO, showed meager progress (Table 1, 19–21).

Table 1. Optimization of Reaction Conditions for On-Water Aerobic Oxidation of Alcohols with Copper–Cyclodextrins^a.

entry	Cu salt	base	cyclodextrin (CD)	yield (%)b
1	Cu(OAc)2·H2O	Na2CO3	β-CD	91
2	Cu(OAc)2·H2O	K3PO4	β-CD	82
3	Cu(OAc)2·H2O	Cs2CO3	β-CD	85
4	Cu(OAc)2·H2O	Et3N	β-CD	82
5	Cu(OAc)2·H2O	NaOH	β-CD	81
6	Cu(OAc)2·H2O	K2CO3	β-CD	95
7	Cu(OAc)2·H2O	K2CO3 (0.8 equiv)	β-CD	86
8	Cu(OAc)2·H2O	K2CO3 (0.6 equiv)	β-CD	72
9	Cu(OAc)2·H2O	K2CO3 (0.4 equiv)	β-CD	65
10	Cu(OAc)2·H2O	K2CO3 (0.2 equiv)	β-CD	40
11	Cu(OAc)2·H2O	K2CO3 (2 equiv)	β-CD	98
12	Cu(OAc)2·H2O	K2CO3	α-CD	89
13	Cu(OAc)2·H2O	K2CO3	γ-CD	90
14	Cu(OAc)2·H2O	K2CO3	HP−β-CD	89
15	Cu(OAc)2·H2O	K2CO3	β-CD sulfate sodium salt	88
16	CuCl2	K2CO3	β-CD	86
17	CuBr2	K2CO3	β-CD	88
18	CuSO4·5H2O	K2CO3	β-CD	83
19		K2CO3	β-CD	5c
20	Cu(OAc)2·H2O	K2CO3		25c
21	Cu(OAc)2·H2O	K2CO3	β-CD	10c,d

^aReaction conditions: Cu salt (10 mol %), CD (10 mol %), and TEMPO (10 mol %), 1 equiv base used.

^bisolated yield.

^cGC conversion.

^dWithout TEMPO.

These results indicated that Cu, CD, and TEMPO are essential for the smooth conversion of alcohols. Overall, the best combination to achieve maximum conversions turned out to be 10 mol % Cu(OAc)₂·H₂O, 1 equiv of K₂CO₃, 10 mol % of β-CD and 10 mol % of TEMPO. Thus, with the set of optimized conditions in hand, we further proceeded to test various substrates for their conversions. A variety of functional groups, such as CH₃O, Cl, OH, NO₂, CH₃, and alkynyl were tolerated to afford the products in good to moderate yields (96–65%). The electronic effects played a major role in the reactivity of the substrates, which can be clearly seen in Table 2. The electron-donating groups afforded products in better yields when compared to the electron withdrawing groups, a trend that is comparatively similar to that observed in earlier reports. The other substrates such as heterocyclic and secondary alcohols also underwent smooth conversions to afford the respective aldehydes in good yields (90–78%). Unfortunately, the catalyst showed very poor activity toward aliphatic alcohols. Surprisingly, in all of the reactions tried in Table 1, the end of the reaction was accompanied a significant amount of an “orange-reddish precipitate”. All of these reactions followed a typical color change pattern, wherein an initial homogenous blue solution (characteristic of copper) slowly transformed to a hazy bluish hue, which after 20 min turns green and finally after an hour into an orange-reddish mixture, as shown in Figure 1. All of the precipitates obtained were analyzed by several characterization techniques, such as X-ray diffraction (XRD) and scanning electron microscopy (SEM), to validate their composition and morphology. The powder X-ray diffraction (PXRD) spectra (Figure 2) recorded for the sample (conditions: Table 1, entry 6) showed peaks for the lattice planes [1 1 0], [1 1 1], [2 0 0], [2 2 0], and [3 1 1], which were indexed to Cu₂O (JCPDS: 05-0667). The XRD spectra of all of the different samples obtained in Table 1 exhibited peaks corresponding to both CD and Cu₂O (Figures S1−S3, SI). A deviation of CD peaks when compared to those of the native CD in all of the samples endorsed the formation of a Cu₂O–CD composite. The shift is apparently because of the interaction of CDs with Cu₂O. The XRD for the sample obtained under the conditions without cyclodextrin (Table 1, entry 20) showed patterns corresponding to both oxides of copper, that is, Cu₂O and CuO (Figure 2). Surprisingly, a year-old precipitate, when checked by powder XRD, did not display any peaks corresponding to CuO (Figure S4, SI). On the other hand, precipitate formation was not seen when each of the precursors, that is, TEMPO, alcohol, and base, alone were employed for the reaction along with copper and CD. On the basis of these results and also recalling the fact that CDs are nonreducing sugars, it can be concluded that Cu and CD interactions play a crucial role in preventing over oxidation of the formed oxides and the reduction of copper metal might be because of the oxidation of alcohols mediated by TEMPO/oxygen. Furthermore, to understand the surface morphology of the precipitates, we proceeded to analyze them by the scanning electron microscopy (SEM) technique. As shown in (Figure 3), when CDs such as β-CD, α-CD, randomly methylated β-CD, and γ-CD are employed for the reaction, different morphological structures of spherical shape (α, β, γ-CD) and rosylike shapes were formed (Figure 3A–D). The other examples wherein potassium carbonate was incrementally varied from 20 mol % to 1 equiv, various structures resembling cabbages and urchins were obtained. Whereas, only spherical aggregates were formed with 1 equiv of Na₂CO₃, K₃PO₄, and Cs₂CO₃. Surprisingly, the shapes of the particles were discrete in the cases of CuCl_2, CuSO₄·5H₂O, and CuBr₂ (Figure S5, SI). From the above SEM studies, it can be concluded that the morphologies and surface structure of the supernanostructures can be tuned by changing the CDs, copper salts, and bases. In continuation to the characterization of the precipitates, we further proceeded for the X-ray photoelectron spectroscopy (XPS) analysis to establish the oxidation state of copper.

Table 2. Substrate Study for On-Water Aerobic Oxidation of Alcohols with Copper–Cyclodextrin^a.

^aReaction conditions: Cu (10 mol %), CD (10 mol %), TEMPO (10 mol %).

^bisolated yield.

Figure 1.

Reaction color: (a) just after addition of reagents, (b) after 20 min, and (c) after an hour to 24 h.

Figure 2.

XRD of (a) Cu₂O–CD precipitate, (b) β-cyclodextrin, (c) without CD showing Cu₂O + CuO, and (d) Cu₂O–CD precipitate.

Figure 3.

SEM image of Cu₂O−CD nano-superstructures with different cyclodextrins: (A) β-CD, (B) α-CD, (C) γ-CD, and (D) randomly methylated-β-CD.

As shown in Figure 4, a sharp peak at 932.8 eV corresponding to Cu^I 2p_3/2 of Cu₂O was observed. A shoulder at 934.5 eV corresponding to Cu^II 2p_3/2 along with satellite peaks at 943.5 and 963.5 eV, which are diagnostic of a Cu^I (3d⁹ shell),²³ were also observed. The peaks corresponding to cyclodextrin (carbon) were also noticeable in the XPS survey (Figure 4). The presence of CD in the sample was also supported by the time of flight secondary-ion mass spectrometry (SIMS-TOF) analysis. A peak of m/z = 1173 provided a strong basis for the presence of β-CD, which can be assigned to the single charged K-ion adduct of the intact β-CD molecule. The spectra also showed secondary ion peaks at m/z = 163 and 325, attributable to one and two glycopyranose units of CD, respectively (Figure S6, SI).

Figure 4.

XPS of Cu₂O–CD superstructures (a) survey and (b) Cu(I) peak.

Besides these studies, to establish the distribution of copper and CD in the formed particles, the SEM energy-dispersive X-ray spectroscopy (EDX) mapping was carried out (Figure 5). The actual distribution of carbon in the sample cannot be distinguished in the mapping because of the conductive carbon tape used in the sample preparation. The Cu map follows a spherical distribution in the CD matrix, with the O map coinciding as well. Thus, strongly suggesting that Cu and O are evenly distributed and the superstructures are consistently composed of Cu₂O–cyclodextrin. Likewise, the transmission electron microscopy (TEM) analysis of the same sample has proved that the particles are high-density compact metal aggregates (Figure 6).

Figure 5.

Scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDX) and elemental mapping data images of Cu₂O–CD superstructure.

Figure 6.

TEM images of Cu₂O–CD superstructures (A) after 24 h and (B) after 1 h.

Overall, on the basis of the above-discussed characterization and control experiment studies, we propose a mechanistic rationale for the formation of particles (Scheme 2). At first (step 1),¹⁷ the dissolved copper and CD might be forming a complex in the solution, followed by reduction of copper with subsequent alcohol oxidation mediated by TEMPO (step 2) to afford Cu₂O nanoparticles stabilized by CDs via capping. Many of such initially formed particles may aggregate to afford the superstructures (step 3).

Scheme 2. Plausible Mechanism for the Formation of Hybrid Cu₂O–CD Nano-Superstructures during Aerobic Oxidation.

Steps: (1) Cu–CD interaction in aqueous solution, (2) capping of Cu₂O by CDs, and (3) formation of Cu₂O–CD aggregates.

Even though the above studies provided insights to understand the superstructures’ formation, it is equally important to quantify the amount of copper converted into the oxide. As revealed by thermogravimetric analysis (TGA) studies (Figure S7, SI), the precipitates obtained were hydrated; therefore, an indirect method was adopted to estimate the copper conversion. After careful separation of precipitate from the solution, the filtrate was subjected to inductively coupled plasma (ICP) analysis to find out the amount of unreacted copper. The amount of copper converted to Cu₂O was back-calculated and was found to be 82.5% (copper conversion to the oxide) in a typical run.

However, the same indirect method could not be adopted to quantify the amount of cyclodextrin present in the hybrid, as the freeze-dried recovered CD from the solution was highly hygroscopic and made the task unfeasible. To our surprise, the XPS and EDAX analysis of the obtained composite did not show any presence of potassium thus confirming that much of the potassium has remained in the aqueous phase of the reaction mixture. The same aqueous mixture containing the potassium carbonate salt was reused for the alcohol oxidation by charging it with cyclodextrin, alcohol, and copper. The oxidized product obtained in the reaction was 93% consistent to the native one (Figure S8, SI). Therefore, the base present in the aqueous phase was also found be recyclable for further reactions.

Having completed the particle synthesis and aerobic oxidation, our attention was drawn toward finding a utility for the as-synthesized particles. A thorough literature survey revealed that Cu₂O is used as the catalyst for various organic transformations. Therefore, we chose to test the same for a three-component propargylamines synthesis. In a typical study, benzaldehyde, piperidine, and phenylacetylene were taken and heated at 100 °C under solvent-free conditions (Scheme 3).

Scheme 3. Cu₂O–CD-Catalyzed Propargylamines Synthesis.

To our delight, we found that the particles successfully catalyze the reaction to afford the product in 85% (isolated yield). As the reaction was carried under neat conditions, it was possible to reuse the catalyst just by recovering the crude product by addition of ethyl acetate. After decanting the solvent, the round bottom flask containing the catalyst was air-dried and checked for the propargylamination reactions. Likewise, we were able to recycle the Cu₂O–CD catalyst for three cycles with no loss in catalytic activity, affording the products with only slight variation of yields (Figure 7). The ICP analysis of the ethyl acetate extracts were carried out to check for any leaching of the copper from into the solution. The analysis revealed that only a negligible amount of copper has leached into the solution (7.2 ppm). Additionally, the surface and XRD analysis of the used catalyst (third cycle) was carried out to probe for any compositional and structural changes. The SEM analysis did not show any morphological changes thus confirming the intactness of the particles (Figure S5, O, SI). No significant change in peaks of the XRD pattern also endorsed that the catalysts did not undergo any change (Figure S9, SI). All of these studies indicated a possible surface-mediated catalysis.

Figure 7.

Recyclability of Cu₂O–CD for propargylamines synthesis.

Conclusions

In conclusion, we have developed a sustainable protocol for the aerobic oxidation of alcohols catalyzed by copper–cyclodextrins under aqueous conditions at room temperature. A serendipitous concomitant formation of Cu₂O–CD nano-superstructures during the aerobic oxidation precluding any extra reducing agents is reported for the first time. The morphological tuning of particles with variation of copper salts, CDs, and bases was thoroughly investigated. The formed particles were systematically characterized and shown to be efficient catalysts for one-pot three-component propargylamination. They were also recyclable up to three cycles with no loss of catalytic activity to consistently afford the product in good yields. Various techniques showed the evidence for the intactness of the formed structures thus supporting propargylation to be taking place via a heterogeneous pathway. Apparently, this study has provided substantial insights regarding the metal waste valorization. Even though prevention of waste is always better than treating it, with the current existing knowledge, waste prevention is not always possible, and yet many reactions are still in practice due to the nonavailability of alternate processes. In this context, the discovery of new organic reactions, wherein the generated waste is “simultaneously valorized” while substrates are undergoing conversion, might pave the way to make organic reactions greener. Further studies to develop such similar simultaneous catalysis metal waste valorization (SCMWV) protocols are underway in our group.

Experimental Section

Materials and Methods

All reagents and starting materials were obtained commercially from sources and were used without additional purification. Thin layer chromatography (TLC) was performed on silica (Silica Gel 60 F254) precoated aluminum plates, and the products were visualized by a UV lamp (PHILIPS TUV 8W lamp) and I₂ stain. Powder X-ray diffraction (PXRD) patterns were recorded on a (Shimadzu XRD-6100 using Cu Kα radiations = 1.5405 Å) powder diffractometer instrument. Transmission electron microscopy studies were performed using a Philips CM200 transmission electron microscope at 100 kV. ICP-atomic emission spectroscopy was performed using the ARCOS model instrument from M/s Spectro, Germany. The ¹H and ¹³C NMR were recorded in CDCl₃ using the residual solvent peak as a reference on an Avance III and Bruker NMR spectrophotometer at 400 and 101 MHz, respectively. In most cases, column chromatography was not required. Passing the crude product through a short pad of silica gel afforded the analytically pure product.

General Reaction Procedure for Oxidation of Alcohols

To a solution of copper acetate (20 mg, 10 mol %) in water (10 mL), β-cyclodextrin (113 mg, 10 mol %) was added. The solution turned to transparent blue after 5 min. To this, potassium carbonate (138 mg, 1 mmol) was added and stirred for another 10 min (solution turned into hazy blue color). Later, alcohol (1 mmol) and TEMPO (31 mg, 20 mol %) were added and vigorously stirred under air at room temperature till complete conversion of the starting material as seen by TLC. After completion of the reaction, the residue was filtered and ethyl acetate (3 × 10 mL) was added to extract the product. The combined organic layers were dried over anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was purified on a short pad of silica gel using petroleum ether/ethyl acetate (most cases filter column) to obtain the pure product.

General Reaction Procedure for Propargylamines Synthesis

A mixture of benzaldehyde (102 μL, 1 mmol), piperidine (119 μL, 1.2 mmol), phenylacetylene (165 μL, 1.5 mmol), and Cu₂O–CD (50 mg) were stirred in a round bottom flask at 100 °C for 1 h. After completion of the starting material, as monitored by TLC, the reaction was cooled to the room temperature. To this, ethyl acetate (3 × 5 mL) was added, stirred at room temperature for 5 min, and later centrifuged to separate the catalyst. The combined organic layers were dried over anhydrous sodium sulfate, concentrated under reduced pressure to afford the crude product. The crude was purified on a short pad of silica (petroleum ether/ethyl acetate) to obtain the pure product.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00898.

• ¹H NMR and ¹³C NMR copies; differential scanning calorimetry-TGA analysis; SEM images; X-ray diffraction patterns of Cu₂O–CD; SIMS-TOF spectra (PDF)

The authors declare the following competing financial interest(s): A.V.K. declares that the aerobic oxidation process, synthesis of Cu₂O−CD nano-superstructures concomitantly during the oxidation process and their utility for organic transformations are filed for Indian Patents.