Highly Selective Synthesis of Hydrazoarenes from Nitroarenes via Polystyrene-Supported Au-Nanoparticle-Catalyzed Reduction: Application to Azoarenes, Aminoarenes, and 4,4′-Diaminobiaryls

Abstract

A selective synthesis of hydrazoarene from nitroarene and its application are reported. Using polystyrene (PS) resins as solid supports for Au nanoparticles (AuNPs), polystyrene-supported Au nanoparticles (AuNPs@PS) were synthesized and characterized. In the presence of AuNPs@PS (1.0 mol %) as a catalyst, nitroarenes afforded corresponding hydrazoarenes (up to 99%) with high selectivity (up to 100%) under mild reaction conditions (NaBH₄, 50% aq. EtOH, and room temperature). Depending on the reaction conditions (the amount of NaBH₄, the substituent of nitroarenes, and the sequential addition of HCl), nitroarenes were converted to corresponding azoarenes (up to 95%), aminoarenes (up to 99%), and 4,4′-diaminobiaryls (up to 99%). Our easily recyclable catalytic system using a solid-phase reaction vessel provides an attractive synthetic method in an eco-friendly and sustainable manner.

Introduction

Au nanoparticles (AuNPs) have received continuous attention as nanosized material-based catalysts because they confer the advantages of both homogeneous and heterogeneous catalysis.^1,2 AuNPs have a very high surface-to-volume ratio to enhance effective collisions between reactants and catalysts. At the same time, AuNPs are easily separated from the reaction mixture because they are insoluble.³ The catalytic activity of Au particles is directly related to their nanometer length scale.⁴ Considering that Au has long been known as an inert metal, this is a very interesting feature.

To minimize size growth of AuNPs,⁵ metal oxides,⁶⁻⁹ graphene derivatives,^10,11 porous silicas,¹² and polymers^13,14 have been commonly used as solid supports. These supports can increase the catalytic activity through the synergistic effect between supports and AuNPs,¹⁵ as well as the chemical stability in reaction solutions by inhibiting the aggregation and precipitation of AuNPs.⁴ Polystyrene (PS) offers the advantages of supporting AuNPs of various sizes, functional group diversity, and chemical stability and is easy to handle and recover through simple filtration.¹⁶ In particular, the use of solid-phase reaction vessels^17,48 with polystyrene-supported AuNPs (AuNPs@PS) can contribute to eco-friendly and sustainable chemistry by maximizing catalyst recovery and reuse. Reported examples using AuNPs@PS as a catalyst in organic reactions include lactonization of diol,¹⁸ oxidation of 1,4-dioxane to 1,4-dioxan-2-ol,¹⁹ reduction of nitro to amine,²⁰ Suzuki–Miyaura coupling,²⁰ and hydration of nitrile to amide.²¹

The reduction of nitroarenes is an interesting reaction because it proceeds through a more complicated route than expected and can produce a variety of intermediates, such as hydrazoarenes, azoarenes, and aminoarenes. These are essential intermediates in the manufacture of pharmaceuticals and dyes.^22,23 According to the mechanism proposed by Haber,²⁴⁻²⁷ nitroarenes are reduced to aminoarenes via the direct route through nitrosoarenes and N-arylhydroxylamines or the condensation route through azoxyarenes, azoarenes, and hydrazoarenes. Since these various species can be present in the reaction mixture at the same time, selective reduction toward a specific product is of paramount importance for the success of this reaction. Therefore, several cases have been reported in which newly developed nanosized material-based catalysts were applied to the selective reduction of nitroarenes to hydrazoarenes,²⁸ azoarenes,^15,29 azoxyarenes,³⁰ aminoarenes,³⁰⁻³² and N-arylhydroxylamines.^30,33

Among them, hydrazoarenes are useful intermediates that can be converted to azoarenes, aminoarenes, 4,4′-diaminobiaryls, and 2-aminodiarylamines in one simple step.³⁴ Particularly, 4,4′-diaminobiaryls generated from hydrazoarenes via 5,5-sigmatropic rearrangement contain the biphenyl moiety, one of the most important building blocks in chemistry.³⁵ However, relatively few studies have been reported that address the synthesis of hydrazoarenes from nitroarenes as the main products compared to examples of azoarenes and aminoarenes. In this paper, we reported the selective synthesis of hydrazoarenes from corresponding nitroarenes via catalytic reduction by AuNPs@PS (1.0 mol %). Reactions were carried out under mild reducing conditions (NaBH₄, 50% aq. EtOH, room temperature) in a solid-phase reaction vessel.

Results and Discussion

To synthesize AuNPs@PS, anion-exchange PS resins were selected since we expected that cations (RNMe₃⁺) of resins interact with anions (BH₄^– and AuCl₄^–) to help AuNPs form at the PS resin. Among the resins, PS1 was chosen because it has been used as a solid support for metal NPs in several studies.^{21,33,36−39} Also, we thought that a smaller particle size and lower cross-linking of PS can provide a higher catalytic performance to the AuNPs@PS hybrid based on the previous report.⁴⁰ Therefore, we chose PS2 and PS3, and detailed properties of PS1–3 obtained from commercial sources are shown in Table S1.⁴⁸

Following the previous protocol for polystyrene-supported metal NPs,^{21,33,36−39} AuNPs@PS1–3 were synthesized in two steps using PS1–3 (Scheme 1). The first step was the anion exchange of chloride to borohydride, and the second step was to reduce chloroauric acid to AuNPs in DMF at 100 °C. DMF is a polar aprotic solvent. Polar solvents can induce more swelling of PS1–3 with quaternary ammonium functional groups than nonpolar solvents.⁴¹ An aprotic solvent was preferred to avoid an acid–base reaction caused by a protic solvent. The reaction temperature (100 °C) was determined by comparing rate constants of synthesized AuNPs@PS hybrids through the initial screening by a pseudo-first-order kinetic model used in our previous work¹¹ (data not shown).

Scheme 1. Preparation of Polystyrene-Supported Au Nanoparticles.

Synthesized AuNPs@PS1–3 were examined by scanning electron microscopy (SEM), and the overall sizes of AuNPs@PS1–3 were 831, 122, and 69 μm, respectively (Figure 1). High-magnification SEM images of the AuNPs@PS1–3 surfaces are presented in Figure S2.⁴⁸ Additionally, the hybrids were ground with a mortar and pestle and measured by field-emission transmission electron microscopy (FE-TEM) to determine the sizes of NPs. Average particle sizes of Au in corresponding AuNPs@PS1–3 were 9.4, 17.6, and 6.8 nm (Figure 2). Among the three hybrids, AuNPs@PS3 exhibited the smallest overall size and average AuNP size. Prior to subsequent experiments, the loading of Au on every hybrid was determined by inductively coupled plasma mass spectrometry (ICP-MS), and it was confirmed that the Au loading on AuNPs@PS1–3 is 0.05 mmol/g. This indicates that Au was quantitatively loaded on PS1–3 during the second step in Scheme 1.

Figure 1.

SEM images of (a) AuNPs@PS1 (∼831 μm), (b) AuNPs@PS2 (122 ± 28 μm), and (c) AuNPs@PS3 (69 ± 16 μm).

Figure 2.

FE-TEM images and Au nanoparticle size distributions of (a) AuNPs@PS1 (9.4 ± 3.2 nm), (b) AuNPs@PS2 (17.6 ± 5.0 nm), and (c) AuNPs@PS3 (6.8 ± 2.2 nm).

The AuNPs@PS1–3 hybrids of the same weight (50 mg) were tested to study the reaction conditions for hydrazobenzene (2a, Table 1, entries 1–3). The substrate was nitrobenzene (1a, 0.2 M), and NaBH₄ (10 equiv.) was used as a reducing agent in 50% aq. EtOH at room temperature. The amount of Au on 50 mg of AuNPs@PS1–3 is 1.0 mol % substrate 1a (0.25 mmol). Conversion yields were calculated by ¹H NMR based on the weight of crude products and the molar ratio of each included species.^42,48 All three catalysts produced hydrazobenzene (2a) with very high yields (entry 1, AuNPs@PS1, 97%; entry 2, AuNPs@PS2, 99%; entry 3, AuNPs@PS3, 99%) and selectivity (100%), but AuNPs@PS3 exhibited the shortest reaction time (4 h) and was selected as the optimal catalyst (entry 3). The highest catalytic performance of AuNPs@PS3 is thought to be due to the smallest overall size and average AuNP size and the lowest cross-linking of PS among three hybrids. Interestingly, AuNPs@PS2 with the largest average AuNP size showed a slight increase in the yield (entry 2, 99%) compared to AuNPs@PS1 (entry 1, 97%). Although these results are very limited, they support that a smaller particle size and lower cross-linking of PS can give a higher catalytic performance to the AuNPs@PS hybrid. Using half as much NaBH₄ (5 equiv.) as that in entry 3 still afforded hydrazobenzene (2a) with high yield (99%) and selectivity (100%) in entry 4. Even with 25 mg of AuNPs@PS3 (entry 5) and the increased reaction time from 6 to 10 h, the yield (98%) and selectivity (100%) were almost maintained compared to entry 4. Interestingly, 2.5 equiv. of NaBH₄ gave azobenzene (3a) with a 76% yield and 97% selectivity (entry 6). The results for entries 4 and 6 reveal that hydrazobenzene (2a) or azobenzene (3a) can be selectively synthesized by controlling the amount of NaBH₄ as a reducing agent. When only a single solvent was used—EtOH or H₂O—different main products were obtained depending on the solvent (entry 7, EtOH, 4a; entry 8, H₂O, 5a), but the yield (entry 7, 59%; entry 8, 55%) and selectivity (entry 7, 63%; entry 8, 70%) were relatively low compared to 50% aq. EtOH. The results for entries 7 and 8 indicate that the composition of the solvent is a very important factor in controlling the selectivity of the reaction. Additionally, it was confirmed that no reduction reaction of nitrobenzene (1a) occurred without AuNPs@PS3 (entry 9) or with PS3-BH₄ alone (entry 10). This shows that AuNPs are a key site among the AuNPs@PS3 hybrids, which exhibits the catalytic activity. When 1.0 mol % HAuCl₄ was used, azoxybenzene (4a) was obtained as the main product (entry 11). Since the reaction was a reducing system, perhaps, colloidal AuNPs were generated, and the reaction from nitrobenzene (1a) to azoxybenzene (4a) proceeded.

Table 1. Study on the Reaction Conditions for Reduction of Nitrobenzene (1a) under NaBH₄ to Hydrazobenzene (2a), Azobenzene (3a), Azoxybenzene (4a), and Aniline (5a).

					yield (%)b
entry	catalysta	NaBH4 (equiv.)	solvent	time (h)	2a	3a	4a	5a	selectivity (%)c
1	AuNPs@PS1	10	50% aq. EtOH	24	97				100
2	AuNPs@PS2	10	50% aq. EtOH	24	99				100
3	AuNPs@PS3	10	50% aq. EtOH	4	99				100
4	AuNPs@PS3	5.0	50% aq. EtOH	6	99				100
5d	AuNPs@PS3	5.0	50% aq. EtOH	10	98				100
6	AuNPs@PS3	2.5	50% aq. EtOH	24	2	76			97
7	AuNPs@PS3	5.0	EtOH	24	5	29	59		63
8	AuNPs@PS3	5.0	H2O	24	20	4		55	70
9		5.0	50% aq. EtOH	24	nre
10	PS3-BH4–	5.0	50% aq. EtOH	24	nr
11f	HAuCl4	5.0	50% aq. EtOH	24	1	7	68		89

^a50 mg of catalysts (1.0 mol %) was used.

^bYields were determined by ¹H-NMR spectroscopy.

^cSelectivity was calculated based on the major product.

^d25 mg of AuNPs@PS3 (0.5 mol %) was used.

^enr means no reaction.

^f1.0 mol % HAuCl₄ was used.

The optimal catalyst, AuNPs@PS3, was examined by TEM energy-dispersive X-ray spectroscopy (EDS) for elemental analysis (Figure 3), and high-resolution X-ray diffraction (HR-XRD) (Figure 4), high-resolution TEM (HR-TEM) (Figure 5a), and selected area electron diffraction (SAED) (Figure 5b) were employed for the crystalline structure analysis. TEM EDS mapping images showed C, N, and Au atoms at a single point of the hybrid. Based on the results of XRD, HR-TEM, and SAED, the crystalline nature of AuNPs@PS3 exhibited a face-centered cubic structure.

Figure 3.

TEM-EDS mapping images of AuNPs@PS3: C (green), N (blue), and Au (red).

Figure 4.

XRD pattern of AuNPs@PS3.

Figure 5.

HR-TEM images: (a) d-spacing and (b) SAED pattern of AuNPs@PS3.

The mechanism that occurs on the surface of AuNPs as a catalyst is not yet known. The Langmuir–Hinshelwood model might be one way of explaining the bimolecular reaction on the surface of AuNPs.^43,44 According to the description of this model, BH₄^– is adsorbed on the surface of AuNP in AuNPs@PS3 to form AuNP-H^–. After nitrobenzene (1a) is adsorbed here again, the reduction reaction proceeds by AuNP-H^–.^28,45

Hydrazobenzene (2a), azobenzene (3a), azoxybenzene (4a), nitrosobenzene (6a), and N-phenylhydroxylamine (7a) were reacted under the conditions of (i) and (ii) to trace the reduction pathway of nitrobenzene (1a) (Scheme 2). The reaction conditions (i) and (ii) are the same as those of entries 4 and 8 of Table 1, respectively, and the difference between the two conditions is whether 50% aq. EtOH or H₂O is used as a solvent. Hydrazobenzene (2a) did not afford aniline (5a), and hydrazobenzene (2a) was recovered quantitatively in both conditions (Scheme 2a,b). Azobenzene (3a) provided hydrazobenzene (2a, 1 h, 99%) in 50% aq. EtOH (Scheme 2c), but no reaction occurred without the catalyst, and 99% of azobenzene (3a) was recovered. Azoxybenzene (4a) and nitrosobenzene (6a) both provided hydrazobenzene (2a) in 50% aq. EtOH (Scheme 2d, 1.5 h, 98%; Scheme 2e, 3 h, 99%). Interestingly, N-phenylhydroxylamine (7a) directly afforded hydrazobenzene (2a, 3 h, 82%) without any side products in 50% aq. EtOH (Scheme 2f), while aniline (5a) did not change to any other compounds (data not shown). This is probably because N-phenylhydroxylamine (7a) is in equilibrium with nitrosobenzene (6a) to produce azoxybenzene (4a), and then azoxybenzene (4a) is reduced to hydrazobenzene (2a) through the condensation route. However, N-phenylhydroxylamine (7a) yielded hydrazobenzene (2a, 4%), azobenzene (3a, 69%), azoxybenzene (4a, 4%), and aniline (5a, 23%) in H₂O (Scheme 2g). When only H₂O was used as a solvent, the reaction proceeded through both direct and condensation routes, and aniline (5a) was produced from N-phenylhydroxylamine (7a) rather than hydrazobenzene (2a). Similar solvent-dependent changes between the direct and condensation routes of nitrobenzene reductions have been found in a recent paper.³⁰

Scheme 2. Reductions of (a) Hydrazobenzene (2a) under (i), (b) Hydrazobenzene (2a) under (ii), (c) Azobenzene (3a) under (i), (d) Azoxybenzene (4a) under (i), (e) Nitrosobenzene (6a) under (i), (f) N-Phenylhydroxylamine (7a) under (i), and (g) N-Phenylhydroxylamine (7a) under (ii).

Reaction conditions: (i) AuNPs@PS3 (1.0 mol %), NaBH₄ (5 equiv.), 50% aq. EtOH, rt; (ii) AuNPs@PS3 (1.0 mol %), NaBH₄ (5 equiv.), H₂O, rt.

The proposed mechanism for synthesis of hydrazobenzene (2a) from nitrobenzene (1a) under the conditions of (i) is shown in Scheme 3. Reduction of N-phenylhydroxylamine (7a) to aniline (5a) seems to be slower than condensation of nitrosobenzene (6a) and N-phenylhydroxylamine (7a) to azoxybenzene (4a). The fact that nitrosobenzene (6a) and N-phenylhydroxylamine (7a) were not observed by ¹H NMR indicates that condensation to azoxybenzene (4a) proceeds very quickly. The recently published literature also showed a similar phenomenon.²⁸ Thus, the reaction proceeded through the condensation route, and the route was stopped at hydrazobenzene (2a) so that 2a could be obtained with a high yield (99%) and selectivity (100%).

Scheme 3. Proposed Mechanism.

Various nitroarenes 1 with 1.0 mol % AuNPs@PS3 calculated based on the moles of Au were tested to determine the scope and limitations of the reaction (Table 2). Nitroarenes 1a–g afforded 14 corresponding hydrazoarenes 2a–g (up to 99%). In general, the synthesis of substituted hydrazobenzenes 2b–g proceeded more slowly than hydrazobenzene (2a), so 10 equiv. of NaBH₄ was used. A trend was observed that showed shorter reaction times and higher yields in the order of m- (2bB, 16 h, 98%; 2 dB, 4 h, 99%), o- (2bA, 24 h, 98%; 2dA, 24 h, 85%), and p-substituted hydrazoarenes (2bC, 24 h, 89%; 2dC, 24 h, 69%). A similar tendency was reported in the previous literature.¹⁵ It is thought that the nitro group and its intermediates in m-substituted nitroarenes are less affected by the electronic effect of substituents in the synthesis of hydrazoarenes. Among m-substituted nitroarenes, longer reaction times and lower yields were observed in the order of F, Cl, and Br (2dB, 4 h, 99%; 2eB, 6 h, 78%; 2fB, 24 h, 69%). The steric effect of substituents seems to be also important in this reaction.

Table 2. Synthesis of hydrazoarenes 2^a.

^aIsolated yields.

^b5.0 equiv. of NaBH₄ was used.

To investigate further scopes and limitations, azoarenes 3 were synthesized by adjusting the amount of NaBH₄ (Table 3). Reactions were carried out using 5.0 equiv. of NaBH₄ in the presence of AuNPs@PS3 (1.0 mol %). Among methylated azobenzenes 3bA–C, m-methylated azobenzene 3bB showed the lowest yield (39%). Although the chemical yield of p-methylated azobenzene 3bC (18 h, 88%) was slightly lower than that of o-methylated azobenzene 3bA (24 h, 94%), less hydrazoarenes were observed on the TLC plate during the reaction, so additional p-substituted azoarenes 3dC, 3eC, and 3gC were synthesized. Seven azoarenes 3 were synthesized (up to 95%). Interestingly, it was found that o- (1iA), m- (1iB), and p-nitrophenol (1iC) gave high yields of corresponding aminoarenes (5iA, 99%; 5iB, 98%; 5iC, 99%) in a relatively short reaction time (2 h). This is probably because the reaction proceeded through the direct route due to the repulsive effect between corresponding nitrosophenolate and hydroxyaminophenolate groups, which can be generated from nitrophenols (1iA, pK_a = 7.23; 1iB, pK_a = 8.36; 1iC, pK_a = 7.15)⁴⁶ under basic conditions (pH = 11.2). The fact that p-nitrobenzoic acid (1jC, pK_a = 3.44)⁴⁷ afforded p-aminobenzoic acid (5jC, 6 h, 17%) supports this possibility because p-nitrobenzoic acid (1jC) can also be converted to carboxylate intermediates in the reaction media. These limited exceptions toward aminoarenes suggest that the type of substituent is an important factor in determining the direct or condensation route.

Table 3. Synthesis of Azoarenes 3 or Aminoarenes 5^a.

^aIsolated yields.

^b2.5 equiv. of NaBH₄ was used.

^cConversion yield determined by ¹H-NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard.

The recyclability test of the AuNPs@PS3 catalyst was performed using nitrobenzene (1a) and a solid-phase reaction vessel. In a total of 10 cycles, hydrazobenzene (2a) was synthesized without a significant decrease in yields (99–90%) (Figure 6). After the recyclability test, AuNPs@PS3 was filtered, and any traces of Au in the filtrate were measured by ICP-MS: they were found to be below the detection limit.⁴⁸ This proves that no Au leached from the PS3 solid support during the reaction and that the reaction followed a heterogeneous catalytic pathway. It also indicates that the loading of Au on the recovered catalyst was maintained. However, the average size of AuNPs on the recovered catalyst increased from 6.8 to 9.8 nm as shown in Figure S12.⁴⁸ In the recyclability test, the decrease in yields might be due to the size growth of AuNPs. The same reaction strategy was also employed for gram-scale synthesis of hydrazobenzene (2a).⁴⁸ It exhibited a slightly decreased chemical yield (84%) but can still be applied for industrial mass production.

Figure 6.

Recyclability of AuNPs@PS3.

The one-pot synthesis of 4,4′-diaminobiaryls 8 from corresponding nitroarenes 1 was carried out (Scheme 4). Nitroarenes 1 were reacted with AuNPs@PS3 for an appropriate time under the conditions of entry 4 in Table 1 followed by addition of concentrated hydrochloric acid. After 4 h, corresponding 4,4′-diaminobiaryls 8 were successfully synthesized (up to 99%).

Scheme 4. One-Pot Synthesis of 4,4′-Diaminobiaryls 8 from Nitroarenes 1.

Conclusions

In conclusion, a novel synthetic method for hydrazoarenes (with up to 99% yield and 100% selectivity) from nitroarenes using polystyrene-supported Au nanoparticles as catalysts (1.0 mol %) was developed. It is notable that the amount of NaBH₄, the type of solvent, and the substituent of nitroarenes play integral roles for selective reduction toward a specific product. As an application of the developed method, nitroarenes afforded azoarenes (up to 95%), aminoarenes (up to 99%), and 4,4′-diaminobiaryls (up to 99%) depending on the amount of NaBH₄, the substituents of nitroarenes, and the sequential addition of HCl, respectively. Our new catalytic system using recyclable catalysts, mild reducing conditions, green solvents, and solid-phase reaction vessels provides eco-friendly, sustainable, and industrially favorable synthetic methods for hydrazoarenes, azoarenes, aminoarenes, and 4,4′-diaminobiaryls. The synthetic potential of this method for a continuous flow chemistry system is now under investigation.

Experimental Section

Preparation of PS-BH₄

In a round-bottom flask (100 mL), distilled water (30 mL) was added to sodium borohydride (100 mg) and PS1–3-Cl (4.0 g). After stirring for 5 h at room temperature, the reaction mixture was filtered under reduced pressure, washed with distilled water until pH ∼7, and then washed with acetone. The obtained resin (PS1–3-BH₄) was dried in vacuo for at least 6 h.

Preparation of AuNPs@PS1–3

In a round-bottom flask (50 mL), a stock solution of HAuCl₄·3H₂O in DMF (50 mM, 1.5 mL) was added to PS1–3-BH₄ (1.5 g) and DMF (13.5 mL). The mixture was vigorously stirred for 1 h at 100 °C until the pale-yellow surface of PS1–3 turned completely reddish purple. Thereafter, the resin beads were filtered under reduced pressure, washed with distilled water, and then washed with acetone. The obtained AuNPs@PS1–3 were dried in vacuo for at least 6 h.

Calculation of Au Loading on AuNPs@PS1–3

To determine the initial concentration of Au in ppb, a stock solution of HAuCl₄·3H₂O in DMF (50 mM, 1.5 mL) was added to DMF (13.5 mL) in a 20 mL vial and measured by ICP-MS. After completion of the AuNPs@PS1–3 preparation, the drained filtrate in the 20 mL vial was also measured by ICP-MS to confirm the concentration of unreacted Au in ppb. The Au loading on AuNPs@PS1–3 was calculated based on the equation below {n_Au: mole of added HAuCl₄·3H₂O (mmol), m_AuNPs@PS: weight of synthesized AuNPs@PS1–3 (g), c₁: initial concentration of Au (ppb), c₂: concentration of unreacted Au (ppb)}:

1

General Procedure for Hydrazoarenes 2

In a solid-phase reaction vessel (5.0 g), sodium borohydride (47.3 mg, 1.25 mmol), AuNPs@PS3 (50 mg, 1.0 mol %), and 50% aq. ethanol (1.25 mL) were added together. Then nitrobenzene (1a, 25.7 μL, 0.25 mmol) was added to the reaction mixture. The reaction was monitored by TLC. After stirring for 6 h at room temperature, the filtrate was drained using water and dichloromethane for recovery of AuNPs@PS3. The filtrate was extracted with dichloromethane (10 × 3 mL). The organic layer was washed with brine (3 × 3 mL), dried over anhydrous MgSO₄, filtered, and concentrated in vacuo. The residue was purified on silica gel (petroleum ether:DCM = 10:1 → 1:1) to afford hydrazobenzene (2a) as a pale-yellow solid (22.8 mg, 99% yield).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00402.

• Chemicals, analytical equipment, reaction vessels, experimental details, and NMR spectra for synthesized compounds (PDF)

The authors declare no competing financial interest.