Design of silver(I)-heteropolyacids: toward the molecular control of reactivity in organic chemistry

Abstract

A series of silver polyoxometalates (POMs) have been prepared, characterized and studied as catalysts. By tuning the amount of silver and POM acidity, a very effective and reusable catalyst was obtained, allowing the synthesis of various furans and pyrroles in high yields under mild conditions.

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1 Introduction

The design and control of chemical systems over multiple-length scales from the molecule to the crystal represent one of the greatest challenges in science,¹ especially with specific applications in mind. This could be achieved by associating chemical entities through non-covalent interactions.² By selecting the appropriate building blocks and the nature of the interaction, one can thus tune the so-obtained supramolecular structures, leading to new chemical and physical properties.³

Catalysis is now reaping benefits from this concept, especially in homogeneous catalysis, in which new catalysts with dual properties have been obtained by self-assembling different species.⁴ In heterogeneous catalysis, the rational design of multifunctional catalysts is still a challenge,⁵ although few self-assembled catalysts have begun to emerge.^5,6

In this context, polyoxometalates (POMs) hold a special position, due to their preparation by self-assembly offering numerous variations of their macrostructural organization (secondary and tertiary structures) through tailoring size, structure and elemental composition of the primary unit (Keggin, Wells–Dawson, Anderson, etc…). Consequently, POMs exhibit a large diversity of properties, leading to various applications from medicine⁷ to magnetism,⁸ and more recently, as heterogeneous⁹ and homogeneous¹⁰catalysts in organic synthesis.

In such structures, Keggin-type POMs are readily obtained by condensing two different oxoanions, i.e. silicate or phosphate, the tetrahedron of which serves as a condensing point to assemble tungstate or molybdate in extremely stable and compact structures. Depending on the conditions used, these assemblies form heteropolyanions or their conjugated acids, the so-called heteropolyacids (HPAs) such as H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, and H₄SiW₁₂O₄₀. The latter exhibits strong Brönsted acidities, due to weak interactions between the protons and the large counter-anion, as for liquid superacids.¹¹

We thus wondered if we could tune and amplify Keggin POM reactivity by replacing step by step these protons with metals known for their catalytic properties in homogeneous catalysis. To the best of our knowledge, only substituted iron,¹²copper,¹³ and silver,¹⁴titanium, tin, bismuth or ruthenium¹⁵POMs have been reported and used as monofunctional heterogeneous catalysts. Such a conceptually novel strategy would provide catalysts with at least dual activity, offering new opportunities in organic chemistry by combining two or more chemical properties on the same POM catalyst.¹⁶ This would lead to new versatile and bi(multi)functional POMs as catalytic and recoverable tools toward sustainable organic chemistry.

Due to our interest in coinage metals in organic synthesis,¹⁷ we have chosen silver and prepared a series of Ag–H–POMs and applied them, for the first time, to a typical reaction, i.e. the rearrangement of alkynyloxiranes to furans.¹⁸ This reaction proceeds under homogeneous conditions in methanol using a mixture of silver triflate and p-toluenesulfonic acid as catalysts (Scheme 1, top). The acid co-catalyst increases the selective oxirane opening caused by the nucleophilic attack of methanol, while silver salts promote the cyclization into furan of the major opened product (Scheme 1, bottom). Ag–H–POMs combining both silver and acid in a single catalyst would thus be perfect for this transformation, and their inherent heterogeneous nature would facilitate recovery of both the product and the catalyst.

Scheme 1 Mechanism for the Ag- and acid catalyzed rearrangement of alkynyloxiranes to furans.

We report herein the synthesis, the characterization of HPA–silver catalysts and the evaluation of their performance in the synthesis of furans.

2 Results and discussion

Four silver-substituted Keggin POMs were easily prepared by the so-called “titration method”¹⁹ from the parent H₄SiW₁₂O₄₀ HPA. This exchange method requires very mild conditions compared to the hydrothermal synthesis, based on the condensation of all the building blocks—usually metal oxides––in water in an autoclave at high temperature, under autogeneous pressure. The former allowed for a better control of the organization of the Keggin units at the microscale level.^6d,20

The chemical composition and the crystallinity of the so-obtained Ag_xH_4−xSiW₁₂O₄₀ materials were determined by several physico-chemical techniques. All these materials exhibited infrared bands at similar frequencies, close to those of the parent HPA (see ESI†). These bands were typical from a Keggin structure, assigned in agreement with the literature²¹ as follows: 1010–1020 cm⁻¹ (ν_as Si–O), 963–975 cm⁻¹ (ν_as W=O,), 870–880 cm⁻¹ (ν_as W–O–W inter-octahedral), 728–742 cm⁻¹ (ν_as W–O–W intra-octahedral). The bands located at around 1600 and 3500 cm⁻¹ corresponded to the presence of water in the coordination sphere (secondary structure). These infrared data confirmed the retention of the Keggin structure throughout the Ag–H–POM series.

To investigate more in depth the structural transformations accompanying Ag doping, the materials were also examined by powder X-ray diffraction (XRD). The XRD patterns of the Ag–H–POMs exhibited the (220), (310), (222), (400), (411), (420), (332) and (510) main reflections typical from the body-centered cubic secondary structure present in the parent H₄SiW₁₂O₄₀ acid.²² In addition, a continuous shift toward higher 2θ diffraction angles, while ranging from H₃AgSiW₁₂O₄₀ to HAg₃SiW₁₂O₄₀viaH₂Ag₂SiW₁₂O₄₀, was observed, consistent with a cationic exchange.²³ A refinement study of the dominant 25.5–26.5° reflection revealed this effect (Fig. 1). This shift showed that doping the SiW₁₂O₄₀⁴⁻ secondary structure with Ag⁺ cations viaproton exchange induced the crystallization of a Keggin structure possessing the same symmetry as the parent material but with a contracted unit cell.

Fig. 1 Refined XRD pattern of dominant reflection for all Ag_xH_4−xSiW₁₂O₄₀ polyoxometalates.

These data showed that a single crystalline phase of the Ag–H–POM was formed, whatever the amount of Ag⁺ introduced. These results are in line with those from Haber et al. showing that silver salts of tungstophosphoric acid were obtained with high crystallinity when the silver content was high.²³

The microstructures of Ag_xH_4−xSiW₁₂O₄₀ polyoxometalates were investigated by SEM to assess the powder XRD refined data. Fig. 2 shows the morphology of Ag₂H₂SiW₁₂O₄₀ crystals, which exhibit a well-defined dodecahedral shape, with ten microns in size. The dodecahedral shape was also observed for all Ag–H–POMs and is a characteristic feature for analogous materials.²⁴ At higher magnification of the POM crystal, the elemental composition was obtained by EDX analysis. An extremely homogeneous distribution of Si, W, O, and Ag elements throughout the material was observed by statistical EDX mapping within a crystal (Fig. 3). The atomic ratios calculated from EDX analysis were equal to the expected stoichiometries for Ag₂H₂SiW₁₂O₄₀ and Ag₄SiW₁₂O₄₀, with, respectively, W/Ag = 6 and W/Ag = 3. These observations agreed with the XRD diffraction study, thus confirming the presence of a single crystalline phase for these Ag–H–POMs.

Fig. 2 SEM images of Ag₂H₂SiW₁₂O₄₀ (left) and Ag₄SiW₁₂O₄₀ (right).

Fig. 3 EDX mapping of Ag, O, Si, and W elements within the Ag₂H₂SiW₁₂O₄₀ POM crystal.

The chemical composition of Ag_xH_4−xSiW₁₂O₄₀ (x = 0–4) materials was further confirmed by titration of the remaining protons, using our H/D exchange isotope technique.²⁵

With the full characterization of these Ag–H–POMs in hand, we then evaluated their potential as bifunctional heterogeneous catalysts in the rearrangement of alkynyloxiranes to furans.¹⁸ Each one was applied to the transformation of the alkynyloxirane 1a into the corresponding furan 2a (Table 1).

Table 1 Screening of reaction conditions for the Ag–H–POM catalyzed transformation of alkynylepoxide 1a

Entry	Catalyst (x)	Yield 2a (%) t = 2 h^a	Yield 2a (%) t = 5 h^a	Yield 2a (%) t = 24 h^a
a Yields of 2a were calculated from 1H NMR integration relative to naphthalene as an internal standard.
1	x = 0	5	11	9
2	x = 1	69	100	—
3	x = 2	92	99	—
4	x = 3	61	96	—
5	x = 4	<5	46	87

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The parent and purely acidic H₄SiW₁₂O₄₀ material furnished approximately 10% of furan (entry 1), while the main product was the alcohol resulting from epoxide opening (60%). As expected, the POMs combining Brönsted acidity and silver(I) were effective catalysts, quantitatively leading to the furan within 5 h, whatever the H⁺/Ag⁺ ratio (entries 2–4). In contrast, the reaction only proceeded slowly in the presence of the non-acidic Ag₄SiW₁₂O₄₀ POM and required 24 h to be nearly complete (entry 5).

These results were perfectly consistent with those obtained under homogeneous conditions, since the presence of both silver and protons was required to achieve the rearrangement to furan.¹⁸

Interestingly, the rate of furan formation was much higher for the three Ag–H–POMs containing 1, 2 or 3 Ag⁺ cations around the Keggin unit. Among them, Ag₂H₂SiW₁₂O₄₀ POM proved to be the most efficient catalyst giving the furan in 92% yield after only 2 h of reaction, whilst 61% and 69% were reached with, respectively, Ag₃HSiW₁₂O₄₀ and AgH₃SiW₁₂O₄₀ (entry 3 vs. 2 and 4). Both the acidity and the silver content thus represent key parameters for performing the reaction. While too many acid sites had a detrimental effect on the reaction efficiency, the absence of these acid sites dramatically reduced the reaction rate. Ag-containing POM having an appropriate molecular composition has therefore to be tuned for the targeted reaction.

It is noteworthy that the leaching test revealed unambiguously the heterogeneous nature of the catalysts, as no conversion was observed using the filtrate after removal of the solid catalyst.²⁶

With these conditions in hand, we briefly investigated the scope of this heterogeneous rearrangement compared to the homogeneous version (Table 2). The general trend of this screening showed that an Ag₂H₂SiW₁₂O₄₀ catalyst was at least as efficient in terms of yields and reaction rates as the combination of silver trifluoromethanesulfonate and p-toluenesulfonic acid (entries 1–3 and 5–7).¹⁸ However, the homogeneous conditions were more effective in the transformation of 1-(hex-1-ynyl)-7-oxabicyclo[4.1.0]heptane 1d to 2d (entry 4). Under Ag₂H₂SiW₁₂O₄₀ catalysis, this epoxide gave the expected furan 2d with only 40% yield, while an 80% yield was obtained under homogeneous conditions. An unexpected regioselectivity at the epoxide opening step was responsible for the difference, as shown by NMR monitoring. After 10 min, the ratio between the opening products A and B (Scheme 1) was, respectively, 50 : 50 and 11 : 89 for the heterogeneous and homogeneous versions, respectively. Since it has been shown that Ag-catalysts could only readily cyclize the B regioisomer,¹⁸ the overall yield could only be at best 50% starting from a 50 : 50 mixture of regioisomers. Similar reasons may also explain the slight difference for the transformation of 1f to 2f (entry 6).

Table 2 Scope of the Ag₂H₂SiW₁₂O₄₀-catalyzed rearrangement of alkynyloxiranes and alkynylaziridines to furans and pyrroles compared to the homogeneous version.¹⁸

Entry	Furans or Pyrroles		Heterogeneous conditions^a		Homogeneous conditions^b
			Yield^c	Time/h	Yield^d	Time/h
a Yields of 2a were calculated from 1H NMR integration relative to naphthalene as an internal standard. Ag2H2SiW12O40 (5 mol%/Ag). b AgOTf/pTsOH (5 mol%). c Yields of 2a were calculated from 1H NMR integration relative to naphthalene as an internal standard. d Isolated yields. e Degradation occurs leading to unidentified by-products. f 10 mol% catalyst loading. g Reactions run at reflux.
1		2a	92	2	92	3
2		2b	89	1.75	90	1.75
3		2c	84	5	72	3
4		2d	40	24	80	24
5		2e	7^e	2.5	Trace	24
6		2f	41	24	51	2
7		2g	70	24	81	16
8		2h	51^f	48	61^f	16^g

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Interestingly, analogous aziridines could also be rearranged, for the first time, with such catalysts. The aziridine 1h was converted to the pyrrole 2h, although longer contact time and higher catalyst loading (10 mol%) were necessary (entry 8). It is worth noting that the homogeneous version required heating with this starting material.

An important and useful feature of any heterogeneous catalyst is its recovery and recyclability. Thus, we engaged the optimized bifunctional catalyst, i.e.Ag₂H₂SiW₁₂O₄₀, into a recycling process in the rearrangement of 1a into furan 2a (Fig. 4). For each recycling run, the catalyst was recovered from the reaction mixture by filtration, washed with n-pentane and dried in an oven before the next run. Two to three successive runs could be performed without loss in activity if the reaction time was increased to 5 or 24 h. The furan yields were mostly quantitative in each run (100%, 100% in runs 1–2 for 5 h, and 100%, 100%, 96% in runs 1–3, Fig. 4). At the fourth run, an activity decrease was observed. Nonetheless, the furan was still obtained with high yield after 24 h.²⁷ This could so far not be explained since control experiments did not show significant modification of the catalyst structure²⁸ nor a real mass loss of the catalyst after each run due to filtration.

Fig. 4 Ag₂H₂SiW₁₂O₄₀ (5 mol%/Ag) recycling and reuse for the transformation of alkynyloxirane 1a in furan 2a.

Conclusions

In conclusion, we designed bifunctional POMs exhibiting both Brönsted acidity and metallic properties, and applied them in an organic transformation as catalytic and recoverable tools toward a more sustainable chemistry. Following a rational approach, a catalyst, having an optimal molecular composition i.e. proper acidity versus silver content (Ag₂H₂SiW₁₂O₄₀), could easily be prepared and used for the transformation of alkynyloxiranes to furans.

Further synthesis and applications of bifunctional HPAs containing transition-metal cations are ongoing in our laboratory.

Experimental

General experimental details

All starting materials were commercial and were used as received; alkynyloxiranes 1a–g and alkynylaziridine 1h were synthesized according to the procedure reported elsewhere.^18a,29Furans 2a, 2b–g were already fully characterized in the literature.^18a,b The reactions were monitored by thin-layer chromatography carried out on silica plates (silica gel 60 F₂₅₄) using UV-light and molybdophosphoric acid/Ce(SO₄)₂ · 4H₂O for visualization. Column chromatographies were performed on silica gel 60 (0.040-0.063 mm) using mixtures of diethyl ether and n-pentane or ethyl acetate and cyclohexane as eluents. Evaporations of solvents were conducted under reduced pressure at temperatures less than 30 °C. FT-IR measurements were performed neat on an ALPHA FT-IR apparatus. ¹H and ¹³C NMR spectra were recorded on an Avance 300 spectrometer at 300 and 75 MHz, respectively. Chemical shifts δ and coupling constants J are given in ppm and Hz, respectively. Chemical shifts δ are reported relative to residual solvent as an internal standard (chloroform-d1: 7.24 ppm for ¹H and 77.23 ppm for ¹³C). Electrospray (ES-MS) low/high-resolution mass spectra were obtained from the mass spectrometry department of the Service Commun d'Analyses, Institut de Chimie, Strasbourg.

General procedure for the preparation of silver-containing POMs. 12-Tungstosilicic acid hydrate (H₄SiW₁₂O₄₀ · 25H₂O; 5.33 g; 1.6 μmol) was dissolved in 20 mL of distilled water under vigorous stirring. The appropriate amount of an aqueous solution of AgNO₃ (0.1 mol L⁻¹) was then added to the former solution at room temperature under vigorous stirring. The metal content, x in Ag_xH_4−xSiW₁₂O₄₀, can therefore be properly adjusted. The resulting solution was then allowed to age overnight. Removing the water under vacuum led to a solid, further dried in an oven at 393 K for 12 h before use.

1-(5-Benzyloxypent-1-ynyl)-9-oxabicyclo[6.1.0]nonane 1b. Prepared following the procedure reported by Blanc et al.^18a in 24% overall yield (403 mg, 1.35 mmol, 3 steps) as a colorless oil from cyclooctanone (718 mg, 5.69 mmol). Rf = 0.62 (cyclohexane/EtOAc 10%). IR (neat) ν_max 2926, 2854, 1495, 1469, 1453, 1364, 1325, 1298, 1281, 1264, 1201, 1158, 1103, 1077, 1026, 1002, 926, 878, 847, 834, 821 cm⁻¹. ¹H NMR (300 MHz, CDCl₃): δ 7.30–7.19 (m, 5H), 4.44 (s, 2H), 3.48 (t, J = 6.0, 2H), 2.94 (dd, J = 7.3, J = 4.3, 1H), 2.27 (t, J = 7.0, 2H), 2.12–2.01 (m, 2H), 1.74 (q, J = 6.7, 2H), 1.64–1.09 (m, 10H). ¹³C NMR (75 MHz, CDCl₃): δ 138.5, 128.4, 128.4, 127.6, 127.6, 127.6, 82.7, 80.0, 73.0, 63.8, 63.7, 54.0, 30.8, 28.8, 27.1, 26.4, 26.1, 25.9, 25.2, 15.6. HRMS (ESI, positive mode): m/z: calcd for C₂₀H₂₆O₂ 321.182, found 321.183 [M + Na]⁺.

1-(Hept-1-ynyl)-9-(phenylsulfonyl)-9-azabicyclo[6.1.0]nonane 1h. Prepared following the procedure described by Andersson et al.²⁹ in 45% (158 mg, 0.44 mmol) as a colorless oil from 1-(hept-1-yn-1-yl)cyclooct-1-ene (200 mg, 0.98 mmol). Rf = 0.36 (cyclohexane/EtOAc 10%). IR (neat) ν_max 2928, 2858, 1737, 1467, 1447, 1415, 1373, 1327, 1290, 1242, 1158, 1090, 1071, 1025, 1006, 932, 886, 842, 810 cm⁻¹. ¹H NMR (300 MHz, CDCl₃): δ 7.95 (d, J = 6.1, 2H), 7.57 (d, J = 5.0, 1H), 7.50 (t, J = 6.1, 2H), 3.08 (dd, J = 11.3, J = 3.9, 1H), 2.24 (t, J = 7.0, 2H), 2.15–1.95 (m, 2H), 1.79–1.20 (m, 16H), 0.90 (t, J = 6.9, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 140.2, 132.9, 128.8, 127.6, 87.6, 76.6, 75.5, 51.7, 45.6, 32.4, 31.0, 28.1, 26.9, 26.0, 25.9, 25.9, 25.8, 25.5, 22.1, 18.9, 14.0. HRMS (ESI, positive mode): m/z: calcd for C₂₁H₂₉NO₂S 366.202, found 366.207 [M + Li]⁺.

General procedure 1 for the synthesis of furans from alkynyloxiranes. A mixture of alkynyloxirane (0.45 mmol), naphthalene (0.05 mmol, internal standard) and Ag_xH_4−xSiW₁₂O₄₀ (x = 0, 1, 2, 3, 4, 5 mol%/Ag) in dichloromethane (4.5 mL) and methanol (0.5 mL) was stirred at room temperature for an appropriate time. Samples (0.2 mL) were taken to assess the amount of furan and the kinetics of the reaction. After completion of the reaction, the solvent was removed in vacuo. The product was separated from the catalyst by filtration and washed with n-pentane. The product thus obtained was charged on a small silica gel column and eluted with n-pentane to afford pure furan. The recovered catalyst was finally dried in an oven for 10 min at 120 °C prior to reuse. The amount of catalyst was calculated to have 5 mol% of silver in each test. Reactions were monitored by NMR and yields were assessed relative to the internal standard (naphthalene).

2-(3-(Benzyloxy)propyl)-4,5,6,7,8,9-hexahydrocycloocta[b]furan 2b. Prepared following the general procedure 1 in 89% (89 mg, 0.3 mmol) as a colorless oil from 1b (100 mg, 0.34 mmol). Rf = 0.43 (n-pentane/diethyl ether 5%). IR (neat) ν_max 2924, 2849, 1571, 1495, 1475, 1452, 1409, 1362, 1307, 1280, 1248, 1209, 1144, 1101, 1075, 1027, 983, 961, 906, 882, 860, 801 cm⁻¹. ¹H NMR (300 MHz, CDCl₃): δ 7.28–7.18 (m, 5H), 5.63 (s, 1H), 4.43 (s, 2H), 3.43 (t, J = 6.4, 2H), 2.60 (m, 4H), 2.40 (t, J = 6.1, 2H), 1.85 (q, J = 6.4, 2H), 1.67–1.35 (m, 8H). ¹³C NMR (75 MHz, CDCl₃): δ 152.1, 149.7, 138.6, 128.4, 128.4, 127.6, 127.6, 127.5, 118.6, 107.9, 72.9, 69.5, 28.8, 28.3, 27.4, 26.1, 25.9, 25.4, 24.7, 23.8. HRMS (ESI, positive mode): m/z: calcd for C₂₀H₂₆O₂ 321.182, found 321.183 [M + Na]⁺.

General procedure 2 for the synthesis of pyrrole from alkynylaziridine. A mixture of alkynylaziridine (0.45 mmol), naphthalene (0.05 mmol, internal standard) and Ag₂H₂SiW₁₂O₄₀ (10 mol%/Ag) in dichloromethane (4 mL) and methanol (0.45 mL) was stirred at room temperature for an appropriate time. Samples (0.2 mL) were taken to assess the amount of pyrroles. The kinetics of the reaction was followed by NMR and yield was assessed relative to the internal standard (naphthalene).

2-Pentyl-1-(phenylsulfonyl)-4,5,6,7,8,9-hexahydro-1H-cycloocta[b]pyrrole 2h. Prepared following the general procedure 2 in 51% (26 mg, 0.07 mmol) as a colorless oil from 1h (50 mg, 0.14 mmol). Rf = 0.55 (cyclohexane/EtOAc 10%). IR (neat) ν_max 2923, 2853, 1535, 1447, 1409, 1361, 1311, 1289, 1265, 1235, 1182, 1140, 1092, 1073, 1051, 1027, 988, 952, 865, 814 cm⁻¹. ¹H NMR (300 MHz, CDCl₃): δ 7.60–7.51 (m, 3H), 7.45 (t, J = 7.8, 2H), 5.81 (s, 1H), 2.90 (t, J = 6.1, 2H), 2.75 (t, J = 7.6, 2H), 2.40 (t, J = 5.9, 2H), 1.65–1.51 (m, 6H), 1.44–1.22 (m, 8H), 0.88 (t, J = 6.9, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 141.2, 136.9, 133.2, 131.9, 129.4, 126.3, 126.0, 113.2, 77.4, 31.8, 31.6, 30.1, 30.0, 29.1, 29.0, 26.8, 26.6, 25.9, 24.2, 22.7, 14.3. HRMS (ESI, positive mode): m/z: calcd for C₂₁H₂₉NO₂S 366.202, found 366.294 [M + Li]⁺.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c1cy00154j

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