A green and recyclable CuSO4·5H2O/ionic liquid catalytic system for the CO2-promoted hydration of propargyl alcohols: an efficient assembly of α-hydroxy ketones
Graphical abstract
Introduction
The construction of the carbonyl compounds is one of the vital topics in organic chemistry, which has been intensively researched to this day [1], [2], [3]. Especially, great efforts have been devoted for the efficient synthesis of ketones, which are considered as one of the most basic but extremely important carbonyl structures [4], [5], [6]. Theoretically, the direct hydration of alkynes that adding H2O to the C≡C bonds followed by the keto-enol tautomerism is an ideal way to provide ketones for its merits such as high atom-economy, green & sustainability and easy operability. Therefore, numerous catalytic systems have been developed for this strategy [7], [8], [9]. Among them, the classical Hg(II) salts/H2SO4 system used to be a milestone that performed good catalytic activity for this hydration [10], [11]. However, the employment of toxic mercury and corrosive acid blocked its further applications in modern green and sustainable society. Over the past several decades, alternative systems containing Cu [12], [13], [14], Ag [15], [16], [17], [18], [19], [20], Zn [21], [22], [23], Au [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], Fe [41], [42], [43], Co [44], [45], [46], [47], Ru [48], [49], [50], Pd [51], [52], [53], Pt [54], In [55], [56], Sn [57], [58], Hf [59]salts, etc. have been established and remarkable progress has been achieved for the conversion of most alkynes. However, for a series of specific alkynes that contain hydroxyl groups, namely propargyl alcohols, seldom systems were reported to show general and excellent catalytic activity. This might be attributed to the side reactions known as Meyer-Schuster and Rupe rearrangements which would inevitably occur under the acidic circumstances required by the most abovementioned systems [60], [61]. Probably for this reason, the traditional Hg(II) salts/H2SO4 system developed 100 years ago is still one of the most general tools for the hydration of propargyl alcohols even nowadays [62], [63], [64]. From another perspective, the hydration of propargyl alcohols is a reaction of great significance and high-value since the obtained products, α-hydroxy ketones, are a series of important organics which have been widely applied in biological, pharmaceutical and synthetic chemistry [65], [66], [67]. As a consequence, it is urgently desirable for the investigations on this specific hydration process without toxic catalytic materials and unfavorable rearrangements.
Very recently, a CO2-promoted hydration process of propargyl alcohols and H2O quickly emerged and attracted attentions from the researchers, in which CO2 was firstly cyclized with the propargyl alcohols and then released through the in-situ hydrolysis of the corresponding products (Table 1) [68], [69], [70], [71]. Particularly, both of the cyclization and hydrolysis could be accomplished in basic circumstance, so the rearrangements occurred under the acidic conditions would be effectively restrained. Thus, this indirect hydration process might be a potential substituted strategy for the traditional direct hydration method in the cases of propargyl alcohols. However, until now only a few catalytic systems were reported for this process (Table 1), which could be divided into two series: the systems of metal salts/organic bases (1 and 2, Table 1) and the systems containing ionic liquids (ILs) (3 and 4, Table 1). Generally, the first series were appealing since the employed materials were commercially-available and easily-accessible, while they were suffered from the disadvantages of high metal loading and usage of additional ligands or volatile solvents [68], [69]. In the other series, Liu et al designed and synthesized several task-specific ILs for this reaction, which successfully avoided the usage of volatile solvents, additives or even metal salts [70]. However, these ILs were not commercially-available and the corresponding catalytic systems still required high catalytic amount, elevated CO2 pressure and prolonged reaction time to reach high yields for most substrates. Based on this, our group combined these two strategies together and developed the “metal salt + IL” catalytic systems, which successfully inherited the merits from both sides. One representative work was the AgOAc combined with a carboxylic IL, which employed ppm level of Ag loading while exhibited excellent catalytic activity, recyclability, green and sustainability [71]. However, during this research, we noticed the catalytic performance of this Ag system would be significantly restrained even by the sunlight, which largely limited its practical applications. Moreover, the usage of relatively rare and sensitive silver salts would also increase the cost for the production of desire products. Comparing with the rare and sensitive Ag compounds, Cu salts exhibited more stable, economic and greener properties which exactly offset the current problems that the Ag-catalyzed system faced. However, due to the inherent characteristics, Cu salts generally behaved much lower activity than the Ag ones, which led to their less reports on the target hydration reaction. By far, the only example was a Cu2O/1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) system, in which the Cu loading was up to 40 mol% with strong base DBU and volatile CH3CN employed [69]. Consequently, utilization of green and economical Cu salts to make highly efficient catalytic system was still urgently desirable for the CO2-promoted hydration of propargyl alcohols.
Herein, the economical and stable CuSO4·5H2O was combined with a commercially available and easily accessible 1-butyl-3-methylimidazolium acetate ([C4C1im][OAc]) IL for the catalysis of CO2-promoted hydration of propargyl alcohols under low CO2 pressure (0.2–1 bar) without any additives or traditional volatile solvents. Notably, this system inherited the advantages of both metal salts and ILs, which simultaneously behaved excellent catalytic activity, recyclability and sustainability. Particularly, a new record of turnover number (TON) was achieved by this system.
Section snippets
Materials
Unless otherwise noted, commercially available Cu salts, ILs and starting materials including propargyl alcohols were obtained from Sigma-Aldrich, Aladdin, TCI, Alfa, Macklin in China and used as received. The DBU-based ILs and [N4444][OAc] were synthesized according to the reported procedures [72], [73]. The CO2 (99.999%) and the mimetic flue gas (20 vol% of CO2 and 80 vol% of N2) used for purging and catalysis were supplied by Wuhan Xiangyun Industry and Trade Co., Ltd.
General analytic methods
With regard to the
Results and discussions
Firstly, the catalytic systems utilized for the CO2-promoted hydration were studied based on the substrate of 2-methyl-3-butyn-2-ol (1a), as shown in Table 2. Initially, the blank experiment was carried out which indicated that the reaction couldn’t proceed without catalysis (entry 1). Moreover, no desired product was detected when the copper salt was individually used (entry 2). In the case that only IL of [C2C1im][OAc] catalyzed this hydration, a poor yield of 16% was obtained (entry 3).
Conclusion
In conclusion, a green and efficient CuSO4·5H2O/[C4C1im][OAc] system was developed for the CO2-promoted hydration of diverse propargyl alcohols under 1 bar of CO2 or mimetic flue gas (20 vol% of CO2). Notably, although a theoretically less active Cu salt was employed herein, this system reached the lowest metal loading ever reported. Moreover, it is the first Cu catalytic system which could be recyclable and reused for at least 6 times with yields higher than 85%. Particularly, a new record of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 22102127, 21950410754). F.V. acknowledges the Chinese Central Government for an “Expert of the State” position in the program of Thousand talents. We appreciate the Innovation and Entrepreneurship Training Program of School of Materials Science and Engineering, Wuhan University of Technology in 2021.
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