A retro Baeyer–Villiger reaction: electrochemical reduction of [60]fullerene-fused lactones to [60]fullerene-fused ketones

Abstract

A highly efficient electrochemical reduction of [60]fullerene-fused lactones to [60]fullerene-fused ketones, a formal process of retro Baeyer–Villiger reaction, has been achieved for the first time. The electrochemically generated dianionic [60]fullerene-fused lactones can be transformed into [60]fullerene-fused ketones in the presence of acetic acid in 85–91% yields. Control experiments have been performed to elucidate the reaction mechanism. The products have been characterized with spectroscopic data and single-crystal X-ray analysis. Moreover, the electrochemical properties have also been investigated.

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Introduction

The Baeyer–Villiger oxidation is one of the most important transformations in organic synthesis, because valuable esters and lactones can be obtained directly from the corresponding ketones (Scheme 1a).¹ However, to the best of our knowledge, the retro Baeyer–Villiger reaction, that is, the direct reduction of esters/lactones to ketones accompanied by the elimination of only one oxygen atom via either a deoxygenative (–O) or dehydrative (–H₂O) pathway, has never been reported and remains a challenging task.^2,3

Scheme 1 (a) Baeyer–Villiger oxidation. (b) Retro Baeyer–Villiger reaction of C₆₀-fused lactones.

Over the past few decades, fullerene derivatives have attracted much attention due to their potential for application in the fields of biomedical and materials science.⁴ Therefore, a great diversity of synthetic protocols for functionalizing fullerenes have been developed by chemists.^5,6 Among the numerous methods, electrosynthesis has been demonstrated as a novel and efficient strategy due to its mild reaction conditions, good regioselectivity, and relatively high yields.⁶

It has been shown that electrochemically generated fullerene anions, especially singly bonded fullerene dianions, can be readily prepared and used as building blocks in the regioselective synthesis of fullerene derivatives with novel addition patterns.⁶ In an attempt to protonate dianionic [60]fullerene (C₆₀)-fused lactones with acetic acid (AcOH), C₆₀-fused ketones 2,⁷ rather than the expected tetrahydrofullerenes,⁶ⁱ can be surprisingly obtained in high yields (Scheme 1b). This is the first time the direct reduction of lactones to ketones, which is a formal retro reaction of Baeyer–Villiger oxidation, has been realized. Herein, we report this unprecedented retro Baeyer–Villiger reaction of C₆₀-fused lactones by the electrochemical approach.

Results and discussion

The employed C₆₀-fused lactone 1a was synthesized according to our previous procedure.⁸ Cyclic voltammetry (CV) of 1a in o-dichlorobenzene (ODCB) containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) showed that the first redox was an irreversible one-electron transfer process with an E_pc of −0.60 V (A) versus a saturated calomel electrode (SCE), and the second redox was chemically quasi-reversible on the CV timescale with E_pc at −1.14 V (B) (Fig. 1a), indicating that the compound underwent a chemical reaction process after receiving one electron. The heterolytic cleavage of the C₆₀–O bond occurred to provide the ring-opened radical anion 1a˙⁻, in which the negative charge and unpaired electron were distributed on the fullerene skeleton and/or the carbonyl group, respectively (vide infra), once 1a acquired one electron. Upon acceptance of the second electron, a singly bonded dianionic species 1a²⁻, in which one negative charge was located at the carboxylate group and another one was distributed on the fullerene cage, was formed.⁹ These ring-opened structures were further confirmed by the visible/near-infrared (Vis/NIR) study of 1a˙⁻ and 1a²⁻, which were obtained by controlled potential electrolysis (CPE) at −0.90 V and −1.34 V, respectively. The Vis/NIR spectra of 1a˙⁻ and 1a²⁻ (Fig. 1b) showed strong absorption bands at λ = 986 and 652 nm, which were in excellent agreement with those of the singly bonded anions of a C₆₀-fused oxazoline (λ = 963, and 645 nm),^9c a C₆₀-fused sultone (λ = 983 and 648 nm),^9d and a C₆₀-fused indoline (λ = 966 and 648 nm).^9e

Fig. 1 (a) Cyclic voltammograms of compound 1a (1.0 mM) shown within different potential windows. The CVs recorded in ODCB containing 0.1 M TBAP starting from 0.0 V toward the negative potential with a scan rate of 50 mV s⁻¹. The arrows indicate the scan direction for the cyclic voltammetric measurements. (b) Vis/NIR spectra of 1a˙⁻ (red) and 1a²⁻ (black) in ODCB (0.25 mM).

Controlled potential electrolysis of 1a (0.015 mmol) in 15.0 mL of anhydrous ODCB solution containing 0.1 M TBAP was carried out at −1.34 V to obtain 1a²⁻ under an argon atmosphere at ambient temperature (∼25 °C). With an aim to protonate 1a²⁻, AcOH (10 equiv.) was added, and the reaction mixture was stirred at room temperature for 30 min. To our surprise, an intriguing product, C₆₀-fused ketone 2a, was obtained in 91% yield. Importantly, this unexpected dehydrative retro Baeyer–Villiger reaction could be extended to other C₆₀-fused lactones, and the results are summarized in Table 1. C₆₀-fused lactones with electron-donating groups including the methyl and methoxy groups as well as electron-withdrawing groups such as the chloro and carbonyl groups at different positions of the aromatic ring afforded 2a–g in excellent yields of 86–91%. Detailed comparisons of these results showed that the electronic properties (entries 1–4 vs. entries 5–7) and locations (entry 1 vs. entry 2, entry 3 vs. entry 4, and entry 5 vs. entry 6) of the substituents on the phenyl ring had little effect on the product yields, indicating that the ring-closure of 1a–g²⁻ to afford 2a–g was a highly efficient process. In addition, when the di-substituted substrate with two methoxy groups was employed, the corresponding product 2h could also be obtained smoothly in 85% yield. Finally, C₆₀-fused lactone 1i with no substituent on the phenyl ring gave the simplest C₆₀-fused ketone 2i in 90% yield.

Table 1 Results for the reaction of dianionic 1a–i²⁻ with AcOH^a

Entry	C60-fused lactone 1	Potential (V)	Product 2	Yield^b (%)	Entry	C60-fused lactone 1	Potential (V)	Product 2	Yield^b (%)
a All the reactions were performed with 0.015 mmol of 1a–i^2− and 0.150 mmol of acetic acid at room temperature (∼25 °C) for 30 min under an argon atmosphere. b Isolated yield.
1		−1.34		91	6		−1.31		90
2		−1.38		90	7		−1.38		88
3		−1.38		89	8		−1.36		85
4		−1.38		91	9		−1.36		90
5		−1.38		86

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The structures of products 2a–i were unambiguously characterized by MALDI-TOF MS, ¹H NMR, ¹³C NMR, FT-IR, and UV-vis spectrometry. All mass spectra of these products exhibited the correct [M]⁺ peaks. Their ¹H NMR spectra displayed the expected chemical shifts as well as the splitting patterns for all protons. The ¹³C NMR spectra of 2a–i exhibited no more than 30 peaks in the range of 135–159 ppm for the 58 sp²-carbons of the fullerene cage and two peaks at 70–80 ppm for the two sp³-carbons of the fullerene skeleton, consistent with the C_s symmetry of their molecular structures. Their UV-vis spectra exhibited a peak at 430–432 nm, which corresponds to the diagnostic absorption of 1,2-adducts of C₆₀ at the [6,6]-junction. The structures of products were unambiguously confirmed by the single-crystal X-ray diffraction analysis of 2f as an example (Fig. 2).¹⁰

Fig. 2 ORTEP diagram for one enantiomer of 2f with thermal ellipsoids shown at 50% probability. The toluene molecule is omitted for clarity.

During the screening of the added acids, it was intriguingly found that different amounts of trifluoroacetic acid (TFA) afforded different products. When 1a²⁻ was treated with 1 equiv. of TFA, 2a could also be obtained in 90% yield, but required a long reaction time of 12 h. However, when 1a²⁻ was reacted with 3 equiv. of TFA for only 3 min, hydrofullerene 3a was obtained in 89% yield (Scheme 2a). The structure of 3a was established by its spectral data, particularly the singlet at δ_H = 6.89 ppm for the diagnostic fullerenyl proton in its ¹H NMR spectrum.^{6f,i,j,9a,e,11} Additional control experiments showed that treatment of 3a with 1 equiv. of sodium hydride (NaH) in a mixture of ODCB and CH₃CN (4 : 1) at room temperature under an argon atmosphere provided 2a in 71% yield (Scheme 2b). The reported pK_a values of TFA, t-BuC₆₀H, PhCO₂H, and AcOH in DMSO were 3.45, 5.7, 11.1, and 12.3, respectively.¹² Although their corresponding pK_a values in ODCB or a mixture of ODCB and CH₃CN are unavailable, it is reasonable to assume that the relative pK_a values of the same order are retained in these solvent systems. Therefore, it is expected that TFA would first protonate the carboxylate anion and then the fullerenyl anion. When only 1 equiv. of TFA was added, the carboxylate anion of 1a²⁻ would be preferably protonated, and subsequent intramolecular cyclization by the attack of the fullerenyl anion to the formed carboxyl group afforded C₆₀-fused ketone 2a. In comparison, when excess amounts (3 equiv.) of TFA were added, both the carboxylate anion and the fullerenyl anion were protonated to give hydrofullerene 3a as the most stable 1,2-adduct. On the other hand, 1 equiv. of NaH would selectively deprotonate the more acidic fullerenyl proton rather than the carboxyl group of 3a, followed by a cyclization process to provide 2a.

Scheme 2 Control experiments.

Based on the above results and previous literature,⁹ a plausible reaction mechanism for the formation of 2 is depicted in Scheme 3. Firstly, C₆₀-fused lactone 1 is electrochemically reduced with a cleavage of the C–O bond to generate ring-opened dianionic 1²⁻. Since AcOH is the weakest acid in the order of the above-mentioned acids (TFA, t-BuC₆₀H, PhCO₂H and AcOH), only the carboxylate anion of dianion 1²⁻ seems to be protonated even in the presence of excess AcOH to give monoanion 4. Finally, intermediate 4 undergoes intramolecular cyclization accompanied by the removal of the hydroxide ion, which is assisted by the neutralization with another molecule of AcOH, to provide product 2.

Scheme 3 Proposed reaction mechanism for the formation of C₆₀-fused ketones from C₆₀-fused lactones.

We also explored the possibility for the retro Baeyer–Villiger reaction of C₆₀-fused lactones by utilization of their radical monoanions with 1a as an example. The irreversible first redox process in the CV of 1a (Fig. 1a) hinted that its lactone moiety would rupture to provide the ring-opened 1a˙⁻ after receiving one electron. The Vis/NIR spectrum of 1a˙⁻ showed significantly lower intensities at 986 and 652 nm than that of 1²⁻ at the same concentration (Fig. 1b), suggesting that only some of 1a˙⁻ had a ring-opened structure with the negative charge distributed on the fullerene skeleton. The synthesis of 2a by the reaction of 1a˙⁻ with 10 equiv. of AcOH was attempted, yet 2a could be obtained in only 54% yield (Scheme 4), much lower than that (91%) from the reaction of 1²⁻. The exact reaction pathway leading to 2a is not clear and currently under investigation. Therefore, the retro Baeyer–Villiger reaction of C₆₀-fused lactones was much more efficiently achieved through their dianionic intermediates rather than with their radical monoanionic species.

Scheme 4 Synthesis of 2a by the reaction of 1a˙⁻ with AcOH.

The half-wave reduction potentials of C₆₀-fused ketones 2a–i and hydrofullerene 3a along with those of C₆₀ were investigated by CV and are summarized in Table 2. All of their electrochemical properties were quite similar and showed two reversible redox processes. As shown in Table 2, the first reduction potentials of products 2a–i and 3a were more negative than that of C₆₀, indicating that they possess higher LUMO energy levels than C₆₀ and may have potential for application in organic photovoltaic devices as acceptors.¹³

Table 2 Half-wave reduction potentials (V) of C₆₀ and compounds 2a–f and 3a^a

Compd	E 1	E 2
a Versus ferrocene/ferrocenium. Experimental conditions: 1.0 mM compound and 0.1 M TBAP in anhydrous ODCB; reference electrode: SCE; working electrode: Pt disc; auxiliary electrode: Pt wire; scan rate: 50 mV s^−1.
C60	–1.076	–1.460
2a	–1.121	–1.503
2b	–1.127	–1.522
2c	–1.125	–1.520
2d	–1.128	–1.522
2e	–1.106	–1.495
2f	–1.104	–1.500
2g	–1.103	–1.490
2h	–1.117	–1.499
2i	–1.111	–1.483
3a	–1.130	–1.518

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Conclusions

In summary, we have achieved a highly efficient synthesis of various C₆₀-fused ketones from C₆₀-fused lactones for the first time via electrochemical reduction, an unprecedented dehydrative retro Baeyer–Villiger reaction. The present protocol shows advantages of mild reaction conditions, a short reaction time, excellent product yields, and remarkable functional group tolerance. Moreover, control experiments have been performed to elucidate the plausible reaction mechanism for the formation of C₆₀-fused ketones. The electrochemical properties of the synthesized C₆₀-fused ketones have been characterized and may be utilized in solar cell devices.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We are thankful for financial support from the National Natural Science Foundation of China (21572211) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures and characterization data, the NMR spectra, and CVs of 2a–i and 3a (PDF). X-ray crystallographic data for 2f (CIF). CCDC 1856172. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8sc05089a

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