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Mild and Highly Efficient Transfer Hydrogenation of Aldehyde and Ketone Catalyzed by Rubidium Phosphate

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Mild and Highly Efficient Transfer Hydrogenation of Aldehyde and Ketone Catalyzed by Rubidium Phosphate J. Cent. South Univ. (2016) 23: 1603−1610 DOI: 10.1007/s11771-016-3214-x Mild and highly efficient transfer hydrogenation of aldehyde and ketone catalyzed by rubidium phosphate HUANG Yun-jing(黄云敬), YANG Wei-jun(阳卫军), QIN Ming-gao(秦明高), ZHAO Hao-liang(赵昊良) College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China © Central South University Press and Springer-Verlag Berlin Heidelberg 2016 + Abstract: Rubidium phosphate can be more conveniently obtained by extracting trace Rb from the salt lake brine. Rb3PO4 was found to be an excellent heterogeneous catalyst for transfer hydrogenation. Rb3PO4 lost 70% of its active sites after adsorbing water, but the remaining was not affected. The reductions of aldehydes and ketones, when promoted by Rb3PO4, were allowed at room temperature. The activities of substrates at room temperature followed a descending order of 2,6-dichlorobenzaldehyde> 4-bromobenzaldehyde>benzaldehyde>acetophenone>anisaldehyde>butanone. A new catalytic cycle postulating a six-membered cyclic transition state for the reductions of aldehydes and ketones was proposed. These results exploited the catalytic usage of Rb3PO4 and worth in industrial application. Key words: rubidium phosphate; transfer hydrogenation; heterogeneous catalysis; cyclic transition state not to mention their reusability. To find a more efficient 1 Introduction and environmental friendly catalyst for transfer hydrogenation, mild basic Rb3PO4 was used to promote Hydrogenation of carbonyl compounds, which is the reduction (Scheme 1) in this work. Now, Rb3PO4 can important in both organic synthesis and industrial be more conveniently obtained by our new process of application [1−4], was performed with hazardous extracting trace Rb+ from the salt lake brine [16]. With molecular H2 and specifically treated catalysts under the similar structure as Rb3PO4 [17], anhydrous K3PO4 very rigorous conditions before the Meerwein-Ponndorf- was also used as catalyst to go comparing with Rb3PO4. Verley (MPV) reaction was reported [5]. Although MPV Surprisingly, the experiment results indicated that the reaction is carried out at mild temperature by using catalytic property of rubidium phosphate is very high. secondary alcohols such as 2-propanol as H donors, large The reaction can be generated at room temperature. A amounts of Al alkoxides are required to catalyze the new catalytic cycle postulating a six-membered cyclic reduction with high yields [6]. Nickel phthalocyanine [7], transition state for the reductions of aldehydes and ruthenium [8], iridium [9], rhodium [10], and gold [11] ketones is proposed. have also been reported to promote H transfer efficiently. However, these precious metals are costly and toxic, and 2 Experimental the ligands are insufficiently active [12]. Alkali carbonates are often used as additives in All chemicals were analytically pure. Rb3PO4 and catalytic transfer hydrogenation. BACKVALL [13] K3PO4 were obtained from Shanghai Energy Lithium reported that with base additives RuCl2(PPh3)2 could Industrial Co., Ltd. They were calcined at 600 °C before form active dihydride catalyst RuH2(PPh)3. Base use and their structures after calcined had been additives have accelerated the reaction greatly. The confirmed as Rb3PO4/K3PO4 by XRD. Catalytic transfer importance of the base is illustrated by KOH and NaOH hydrogenation was typically carried out using 6.0 mmol used alone without any transition metal or ligands for the carbonyl substrate and 50.0 mL of 2-propanol as reduction of some aromatic aldehydes and ketones reductant and solvent. The mixture was placed in a [14−15]. It is known to all that KOH and NaOH could two-neck round-bottom flask and heated in air with a react with alcohols which is the H donator in this reflux device whereupon 0.4 mmol calcined Rb3PO4 or reaction. So, when the reaction is finished, KOH and 4.0 mmol calcined K3PO4 was added. Liquids were NaOH dissolved in the solvent are hard to be separated, sampled (0.1 mL each time) at regular time intervals and Foundation item: Project(21576074) supported by the National Natural Science Foundation of China Received date: 2015−05−26; Accepted date: 2015−08−30 Corresponding author: YANG Wei-jun, Professor, PhD; Tel: +86−731−88821449; E-mail: [email protected] 1604 J. Cent. South Univ. (2016) 23: 1603−1610 Scheme 1 Catalytic transfer hydrogenation of carbonyl compounds analyzed by gas chromatography. Gas chromatography analysis was performed on a Shimadzu GC-2014 equipped with a 0.5 mm i.d. _25 m PEG20000 capillary column and flame ionization detector. The products were identified by comparison with an authentic sample through gas chromatography and mass spectrometry. To test the reusability of Rb3PO4, the used sample was recovered by filtration after reaction and washed with 2-propanol. It was reused for another reaction after drying. 3 Results and discussion Fig. 2 Conversion of 2,6-dichlorobenzaldehyde promoted by dry Rb PO and wet Rb PO (Reaction conditions: 6.0 mmol 3.1 Operational conditions for Rb PO 3 4 3 4 3 4 substrate, 50.0 mL of 2-propanol as solvent, 0.4 mmol Rb PO , Oxygen and H O in the air, which may be adsorbed 3 4 2 70 °C, air) on the surface cations, poison the catalyst. Although nitrogen environment can reduce the content of the that water adsorption may significantly affect its catalytic interaction between oxygen and H O in the air and the 2 activity. As a result, Rb PO lost nearly 70% of active catalysts, it also rendered the operation more 3 4 sites (Fig. 2). Then, the exposure time was prolonged to complicated. Therefore, comparative experiments were 2.0 h, during which Rb PO totally turned into a conducted (Fig. 1) in order to find out whether nitrogen 3 4 concentrated solution because of copious water protection was necessary. adsorption. The Rb PO solution was used to promote H One system was opened to the air with a 3 4 transformation, the catalytic activity of which remained reflux device and the other one was protected by N2 (Fig. 1). Nitrogen protection plays little effect in the partially with even more water adsorbed. After 1.0 h, reaction as shown. 27% of 2,6-dichlorobenzaldehyde was converted into The influences of water on the catalytic properties corresponding alcohols. of moisture-vulnerable anhydrous alkali metal Equimolar anhydrous K3PO4 was used as catalyst to compounds with extremely high hygroscopicity were go comparing with Rb3PO4. After 1.0 h, only 14% of also evaluated (Fig. 2). 2,6-dichlorobenzaldehyde was converted. So, even Rb3PO4 was fully saturated in air, its catalytic activity Anhydrous Rb3PO4, which was exposed in air for 20 min on purpose, became wet and viscous, suggesting was higher than that of anhydrous K3PO4. Then, 4.0 mmol K3PO4 (ten times of Rb3PO4) was used to promote the reaction, with other conditions being the same as those shown in Fig. 2. After 1.0 h, 98% conversion was observed. K3PO4 that had been exposed in air for 2.0 h and turned into concentrated solution was then used to catalyze the reaction. After another 1.0 h, 33% of 2,6-dichlorobenzaldehyde turned into corresponding alcohols. Water exerted similar effect on the activity of K3PO4 to that on Rb3PO4. 3.2 Reductions of aldehydes and ketones To demonstrate the efficiency of Rb PO , 3 4 Fig. 1 Conversion of 2,6-dichlorobenzaldehyde in air and N2 comparative experiments were carried out on every (Reaction conditions: 6.0 mmol substrate, 50.0 mL of substrate by using K3PO4 as hydrogenation catalyst 2-propanol as solvent, 0.4 mmol Rb3PO4, 70 °C) (Table 1). Different amounts of catalyst were used to J. Cent. South Univ. (2016) 23: 1603−1610 1605 Table 1 Reductions of aldehydes and ketones with 2-propanol Entry Substrate Catalyst Temperature/°C Time/h Conversion/% Product Selectivity/% Cl a 1 2,6-dichlorobenzaldehyde K3PO4 80 2.0 100 100 OH Cl Cl b 2 2,6-dichlorobenzaldehyde Rb3PO4 80 2.0 100 100 OH Cl Cl a 3 2,6-dichlorobenzaldehyde Rb3PO4 20 15.0 42.0 100 OH Cl a 4 4-bromobenzaldehyde K3PO4 60 2.5 52.1 Br 95 OH b 5 4-bromobenzaldehyde Rb3PO4 60 5.0 56.2 Br 95 OH a 6 4-bromobenzaldehyde Rb3PO4 20 15.0 31.4 Br 95 OH a 7 Benzaldehyde K3PO4 80 8.5 82.6 100 OH a 8 Benzaldehyde Rb3PO4 80 3.0 85.8 100 OH a 9 Benzaldehyde Rb3PO4 20 15.0 58.7 100 OH a 10 Anisaldehyde K3PO4 80 5.0 48.9 98 a 11 Anisaldehyde Rb3PO4 80 3.0 56.3 98 a 12 Acetophenone K3PO4 80 9.0 37.0 100 a 13 Acetophenone Rb3PO4 80 4.0 42.3 100 a 14 Butanone K3PO4 80 32.0 13.8 100 a 15 Butanone Rb3PO4 80 10.0 14.9 100 a 16 Butanone Rb3PO4 20 40.0 2.3 100 aReaction conditions: 6.0 mmol substrate, 50.0 mL 2-propanol as solvent, 4.0 mmol catalyst, air; b0.4 mmol catalyst. promote the hydrogenation of aldehydes and ketones to 3.3 High efficiency of Rb3PO4 shorten the reaction time. IVANOV et al [5] reported that alcohols could be The rate of reaction was increased by electron- adsorbed on base catalysts via their hydroxyl hydrogen withdrawing groups on the benzene ring (Entries 1−6, bound to the oxygen of base catalysts and their hydroxyl Table 1) and decreased by electron-donating groups oxygen bound to the surface cation of base catalysts, and (Entries 10 and 11, Table 1). The reduction of butanone that aldehydes and ketones could be absorbed through was much slower due to reversible H transformation. The their oxygen atoms bound to the surface cation of base reduction did happen at room temperature very slowly, catalysts.
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