1 Introduction

Aldehydes and ketones are important classes of chemicals that have been used extensively as synthetic intermediates in the preparation of a variety of fine or special chemicals, such as drugs, vitamins, fragrances, etc. [1]. Up to now, many publications in the open literature have been found related to the synthesis of these types of compounds, and a well-known method for such a synthesis is the oxidation of alcohols [2]. Traditional methods for performing such a transformation generally involve the use of stoichiometric amounts of strong oxidizing reagents (e.g., MnO₂, SeO₂, CrO₃, and RuO₄) [2–4], and suffer from considerable drawbacks, such as high cost, low yield, harsh or delicate reaction conditions, and a large amount of waste by-products. Oxidation with ceric ammonium nitrate [5], H₅IO₆ [6], diaziridinone [7], tert-butyl hydroperoxide [8], trichloroisocyanuric acid [9,10], H₂O₂ [11–13], and other reagents [14–16] have also been developed for this conversion. However, some of the procedures are often associated with one or more disadvantages, such as long reaction times, high temperatures, difficulties in work-up, use of expensive, toxic and moisture-sensitive reagents, and difficulties in recycling the catalyst.

Molecular oxygen has recently been utilized as an attractive oxidant for the oxidation of alcohols for the production of these compounds due to cheap price and environmental friendly nature. Therefore, molecular oxygen-based protocols are powerful methods for oxidation of alcohols and attract a great attention, which make the reactions of choice in large- and industrial-scale applications. A variety of catalytic systems, such as nanopalladium [17], Pd(OAc)₂/DMSO[18], Ru(TPFPP)(CO) [19], PI Ru/TEMPO [20], CeO₃/RuCl₃ [21] VOPO₄/TEMPO [22], VO(acac)₂/DABCO [23], [copper(II)-(N ligand)_n]/TEMPO [24], gold [25], OsHCl(CO)(PiPr₃)₂ [26], and other systems [27–29], have been developed for this conversion. However, these procedures are invariably associated with one or more disadvantages, such as long reaction times, expensive metals, low yields, use of toxic compounds, difficulties in work-up, harmfulness toward the environment, etc. Consequently, the search for new and environmentally benign synthetic methodologies for the oxidation of alcohols that address these drawbacks remains of value and interest.

Ionic liquids (ILs) have attracted considerable attention during the past few years due, in some cases, to their favourable properties, such as very low vapour pressure, wide liquid temperature range, good ionic conductivity, excellent electrochemical properties, and strong ability to dissolve many chemicals [30–33]. Therefore, ILs have recently gained recognition from scholars from various fields, such as analytical and separation science [34], chemical synthesis [35,36], biocatalytic transformations [37], electrochemistry [38], and catalysis [39–41]. In continuation of our interest in exploring green oxidation reactions in ionic liquids, we report herein a new, efficient and environmentally friendly protocol for the selective aerobic oxidation of alcohols to the aldehydes and ketones mediated by indium nitrate (In(NO₃)₃) in ionic liquid 1-dodecyl-3-methylimidazolium iron bromide ([C₁₂mim][FeBr₄]) under mild conditions (Scheme 1). Furthermore, we demonstrate that the catalytic system can be recycled and reused without any significant loss of activity.

2 Experimental

3 Results and discussion

A preliminary survey of the reaction conditions (Table 1) used benzyl alcohol (0.1 mol) as the model substrate in the presence of ionic liquid [C₁₂mim][FeBr₄] and In(NO₃)₃. In the absence of ionic liquid and In(NO₃)₃, no product was detected (Table 1, entry 1). When the reaction was performed in the presence of ionic liquid [C₁₂mim][FeBr₄] (10 mL) or catalyst In(NO₃)₃ (1.0 equiv), the product could be obtained in a higher yield 28% (Table 1, entry 2) or 23% in a shorter time (12 h) (Table 1, entry 3) under the same conditions. The yield can be greatly improved with the combined use of [C₁₂mim][FeBr₄] and In(NO₃)₃ (Table 1, entries 4–11), and it reached its maximum value (92%) within a shorter time (6 h) when the addition of [C₁₂mim][FeBr₄] (30 mL) and In(NO₃)₃ (3.0 equiv.) (Table 1, entry 10). However, upon further addition of [C₁₂mim][FeBr₄] (Table 1, entry 7) or In(NO₃)₃ (Table 1, entry 11) under the same conditions, the yield was not enhanced significantly. These experiments revealed that In(NO₃)₃ and ionic liquid [C₁₂mim][FeBr₄] are crucial for the oxidation, and the lack of any component can lead to a much lower yield. Therefore, the optimal reaction conditions were observed in Table 1, entry 10.

The effects of different ionic liquids, such as [C₈mim][FeBr₄], [C₁₂mim][FeBr₄], [C₁₂mim][FeCl₄], [C₁₂mim][OTf], [C₁₂mim][BF₄], and [C₁₂mim]Br, were tested with In(NO₃)₃ as a catalyst in the reaction (Table 2, entries 1–6), and it was observed that [C₁₂mim][FeBr₄] demonstrated the best performance (Table 2, entry 2). The different catalytic effects of ILs may be attributed to their different abilities to stabilize and dissolve O₂, the catalyst and the substrate. Under our reaction conditions, O₂ may be more soluble in [C₁₂mim][FeBr₄], leading to a higher effective concentration of the oxidant, which increases the encounter probabilities between the substrate and the reactive species, and so the higher rate and yield of the reaction were observed. Furthermore, besides In(NO₃)₃, other types of catalysts were also tested in this model reaction (Table 2, entries 7–11). The results showed that In(NO₃)₃ demonstrated the best performance. In addition, no over-oxidized product (benzoic acid) was detected by ¹H NMR analysis of the crude reaction mixtures in all the cases. Therefore, the combination of [C₁₂mim][FeBr₄] and In(NO₃)₃ was chosen as the optimal conditions for further exploration.

In addition, the catalytic system could be typically recovered and reused for subsequent reactions with no appreciable decrease in yields and reaction rates (Fig. 1). The recycling process involved the removal of the upper layer of product, by a simple extraction with organic solvent. Fresh substrates were then recharged to the under-layer of catalytic system and the mixture was heated to react once again.

With these results in hand, the catalytic system was then applied to various alcohols as summarized in Table 3. It is clear that various types of benzylic and allylic alcohols, both primary and secondary, have been successfully oxidized to the corresponding aldehydes and ketones in good to high conversions and yields (Table 3, entries 1–11), whereas the aliphatic alcohols were less reactive, and the reactions proceeded more slowly, and even under more drastic reaction conditions, the reactions were still incomplete (Table 3, entries 12 and 13). Surprisingly, the oxidation of primary alcohols to the corresponding carbonyl compounds is faster and more efficient than that of secondary alcohols (Table 3, entries 8 and 9). In addition, it was also observed that the electronic nature of the substituents on the aromatic ring has some impact on the reaction rate. Benzylic alcohols, especially those with electron-donating substituents (Table 3, entries 2–5), were more reactive than those with electron-withdrawing substituents (Table 3, entries 6 and 7), providing excellent yields.

4 Conclusions

In conclusion, a new efficient oxidation reaction system using [C₁₂mim][FeBr₄] in combination with In(NO₃)₃ has been developed, which is capable of converting alcohols into the corresponding aldehydes and ketones in good to excellent isolated yield. Mild reaction conditions, simplicity of operation, high yields, stability, easy isolation of products, and the excellent recyclability of the catalytic system are the attractive features of this methodology. The scope, definition of the mechanism and synthetic application of this reaction are currently under study in our laboratory.

Acknowledgements

We thank the Natural Science Foundation of Jiangsu Province (No. BK20140460 and BK20141257) and the University Science Research Project of Jiangsu Province (No. 14KJD150010) for supporting this research.