Task-specific ionic liquid and CO2-cocatalysed efficient hydration of propargylic alcohols to α-hydroxy ketones† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c5sc00040h Click here for additional data file.

Task-specific ionic liquid and CO2-cocatalysed efficient hydration of propargylic alcohols to α-hydroxy ketones.


Introduction
The selective addition of water to unsaturated bonds is of paramount importance in the production of building blocks for the synthesis of specialty chemicals. 1 Especially the hydration of acetylenic compounds, involving simple addition of a water molecule with 100% atom efficiency for generating carbonyl compounds, has received much attention in the past decades, 2 notably with the requirements of green chemistry and sustainable development. The key issue of the hydration of alkynes relies on the effective activation of the carbon-carbon triple bond, followed by the rapid addition of a water molecule. So far, a variety of metal (e.g., Fe, Au, Ag, Ru)-based catalysts have been developed to replace traditional toxic Hg(II) catalysts for the hydration of alkynes, where cocatalysts such as strong acids and organic ligands are generally required and/or side reactions including the Meyer-Schuster and Rupe rearrangements usually occur in these catalytic systems. 3 The hydration of propargylic alcohols is an efficient and green route to produce a-hydroxy ketones, which are important building blocks for more elaborate molecules; 3a-c,4 however, it is very difficult to do this under mild and metal-free conditions. Recently, Qi et al. reported a CO 2 -promoted route for the hydration of propargylic alcohols to a-hydroxy ketones using silver acetate (Ag 2 COO) and 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) as the catalysts at 120 C and under high CO 2 pressure. 5 Our literature survey indicates that there is no report on the hydration of propargylic alcohols to a-hydroxy ketones under metal-free conditions. Ionic liquids (ILs), possessing unique features such as high thermal and chemical stability, negligible vapor pressure, and tunable properties, have been applied in many areas. 6 Particularly, task-specic ILs have displayed superior performances in catalysis (e.g., hydrolysis reactions, CO 2 conversion) and gas capture via the careful design and selection of novel component ions to endow unique properties upon them. 6f,g,7 For example, CO 2 -reactive ILs have shown excellent performances for the capture and conversion of CO 2 under mild conditions. 6b,7a The supported basic IL [Emim][HCO 3 À ] exhibits a high activity for catalyzing the hydrolysis of propylene carbonate to 1,2propylene glycol. 6f Herein, we report the hydration of propargylic alcohols to ahydroxy ketones cocatalysed by task-specic ILs and CO 2 . The structures of all the ILs used are given in Scheme 1 and in the ESI. † It was discovered that the IL [Bu 4 P][Im] was able to catalyse the hydration of propargylic alcohols in the presence of CO 2 at atmospheric pressure, and various propargylic alcohols could be transformed into the corresponding a-hydroxy ketones in good to excellent yields. Moreover, it was found that both CO 2 and [Bu 4 P][Im] were indispensable for the hydration reactions, and they cocatalyzed these reactions efficiently. In addition, the IL catalyst could be easily recovered and reused without considerable activity loss. To the best of our knowledge, this is the rst example of the efficient hydration of propargylic alcohols under mild and metal-free conditions. Meanwhile it is also the rst time it has been found that CO 2 can serve as a cocatalyst.

Results and discussion
The hydration of 2-methylbut-3-yn-2-ol was carried out both in the absence and presence of ILs, and the results are listed in  (Table 1, entries 10 and 11), which may be ascribed to the very weak interactions between their anions and CO 2 . From the above results, it can be concluded that both CO 2 and the task-specic ILs were indispensable, and their synergistic effects resulted in the formation of the nal products. In addition, the chemical structures (both cations and anions) of the ILs played important roles in the hydration of 2-methylbut-3-yn-2-ol.
Encouraged by the above results, the generality of [Bu 4 P]-[Im]/CO 2 -catalyzed hydration reactions of diverse propargylic alcohols was evaluated. The results indicated that most reactions proceeded smoothly, producing corresponding a-hydroxy ketones in good to excellent yields under experimental conditions (Table 2, entries 1-15). For example, 3-methyl-1-pentyn-3ol afforded the corresponding product in a yield of 77% within 24 h (Table 2, entry 2), comparable to that reported previously using Ag 2 COO and DBU as catalysts at 120 C and 2 MPa of CO 2 pressure (Table 2, entry 16). 5 Moreover, prolonging the reaction time to 48 h, the product yield reached 93% (Table 2, entry 3). Notably, the steric effects of the substituents in the substrates had signicant effects on their activities in the formation of the corresponding a-hydroxy ketones. This was conrmed by the fact that the product yields decreased when the lengths of the substituent chains in the propargylic alcohols increased (Table  2, entries 1, 2, 4, and 6). Moreover, large scale reactions of substrates including 3-methyl-1-nonyn-3-ol, 2-phenyl-3-butyn-2ol and 2-methyl-4-phenyl-3-butyn-2-ol were carried out, and the corresponding a-hydroxy ketones were isolated in yields of 84%, 87%, and 85%, respectively ( respectively), showing that the present system has great potential for applications. In addition, 3-butyn-2-ol, 3,3dimethyl-1-butyne and 3-chloro-3-methylbut-1-yne were also employed as substrates; however, the hydration reactions did not occur, and no products were obtained in these cases. This suggests that the presence of an -OH linked to the C next to the alkynyl C is necessary for the hydration of propargylic alcohols.
To explore the reusability of the IL catalyst, ve catalytic cycles of 2-methylbut-3-yn-2-ol hydration were performed over the [Bu 4 P][Im] catalyst in the presence of CO 2 . It was demonstrated that the product yield almost remained unchanged (  Fig. S8 †), suggesting that [Bu 4 P][Im] was stable and the designed catalytic system was recyclable.
The above results indicate that CO 2 plays an important role in the hydration of various propargylic alcohols. To explore the reaction mechanism, the IL, the IL exposed to CO 2 , and the reaction solution for 2-methylbut-3-yn-2-ol hydration in the presence of atmospheric CO 2 at 80 C for 7 h were examined by NMR analysis. As illustrated in Fig. 1, a new signal appeared at 161.6 ppm in the 13 C NMR spectrum of the IL exposed to atmospheric CO 2 , which was attributed to the carbonyl carbon of carbamate, indicating that CO 2 could react with the anion [ 8a thus facilitating the conversion of this propargylic alcohol. The 13 C NMR analysis indicated that besides 2-methylbut-3-yn-2-ol and 3-hydroxy-3-methyl-2-butanone, the species from CO 2 and the anion of the IL were detectable in the reaction solution. This implies that the IL served as a catalyst for activating CO 2 in the reaction process, and the resultant [Bu 4 P][ImCOO] intermediate played a  Fig. S9 †). However, a-alkylidene cyclic carbonates were not detectable in the reaction solutions for the hydration of various propargylic alcohols. Another control experiment, hydrolysis of a-alkylidene cyclic carbonate in [Bu 4 P][Im], was performed. To our delight, this cyclic carbonate was rapidly hydrolysed at 80 C within 1 h, affording 3-hydroxy-3-methyl-2-butanone in a yield approaching 100%. The above ndings suggest that the a-alkylidene cyclic carbonate formed from CO 2 reacting with 2-methylbut-3-yn-2-ol may be the key intermediate for the formation of 3-hydroxy-3methyl-2-butanone.
On the basis of the experimental results and previous reports, 5,8,9 a possible mechanism for IL and CO 2 -cocatalyzed hydration of propargylic alcohols was proposed, as shown in Scheme 2. CO 2 is rst activated by the anion [Im À ] to form intermediate a, produces e, which subsequently converts to the product a-hydroxy ketone via keto-enol tautomerization and the release of CO 2 . In this reaction process, CO 2 is involved in the formation of the key intermediates (i.e., a-alkylidene cyclic carbonates), and is rapidly released via the hydrolysis of these intermediates. CO 2 is required, but not consumed in the whole reaction process, and plays a catalyst-like role in the formation of the nal product.
In addition, it should be pointed out that CO 2 was able to react with propargylic alcohols catalyzed by [Bu 4 P][Im] in the absence of water, producing a-alkylidene cyclic carbonates at atmospheric pressure. This is a new metal-free catalytic route for the synthesis of a-alkylidene cyclic carbonates through coupling reactions of CO 2 with propargylic alcohols under mild conditions.

Conclusions
In summary, we have developed a green, metal-free and efficient method for the hydration of propargylic alcohols to generate a-hydroxy ketones using [Bu 4 P][Im]/CO 2 as the catalytic system, which enables the reactions to proceed smoothly and to afford good to excellent yields of products. The IL and CO 2 have an excellent synergistic effect on catalyzing the reactions, and CO 2 serves as a cocatalyst. In addition, the multifunctional IL can be easily recovered and reused without obvious loss in its activity. We believe that this kind of highly-efficient and greener CO 2 -IL catalytic system has great potential for applications.