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

+ 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 Rb3PO4, 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 C l Cl b 2 2,6-dichlorobenzaldehyde Rb3PO4 80 2.0 100 100 OH C l Cl a 3 2,6-dichlorobenzaldehyde Rb3PO4 20 15.0 42.0 100 OH C l 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. It is well-established that rubidium atom when promoted by Rb3PO4, which has never been adsorbs oxygen atom in the substrates stronger than reported hitherto. Under the same conditions (Rb3PO4 as potassium atom does [18]. catalyst), 2,6-dichlorobenzaldehyde, benzaldehyde and To test the postulation that adsorption was a key

4-bromobenzaldehyde were converted into step of the whole reduction process promoted by Rb3PO4 corresponding alcohols slowly, whereas acetophenone or K3PO4, an experiment was set as Entry 2 in Table 1 and anisaldehyde remained stable after 10.0 h, but not and liquids were sampled at regular time intervals after 30 h. According to the conversion rates of all (0.1 mL each time). Then, the liquids were cooled to substrates after 30 h, the activities at room temperature room temperature. The solid catalyst Rb3PO4 swelled followed the descending order of 2,6-dichloroben- dozens of times (about 1.0 mmol Rb3PO4 absorbed zaldehyde>4-bromobenzaldehyde>benzaldehyde>aceto- 2.0 mL reaction solution) that of the original volume in phenone>anisaldehyde>butanone. the early stage. After 2,6-dichlorobenzaldehyde was fully

1606 J. Cent. South Univ. (2016) 23: 1603−1610 converted, Rb3PO4 shrank to its initial volume. A (Scheme 2). Thus, with a higher conductivity than K3PO4, comparative experiment catalyzed by K3PO4 was set as Rb3PO4 could promote H transformation faster.

Entry 1 in Table 1, and K3PO4 barely swelled and also recovered to its original volume after complete 3.4 Preliminary consideration of Rb3PO4/K3PO4 conversion of 2,6-dichlorobenzaldehyde. The swollen catalytic cycle solids may be the intermediate products of Rb3PO4 or There is no report about the catalytic cycle of

K3PO4 adsorbing 2,6-dichlorobenzaldehyde. Therefore, Rb3PO4 promoting transfer hydrogenation so far. by adsorbing aldehydes and ketones more rapidly and RADHAKRISHAN et al [19] added benzaldehyde and efficiently, Rb3PO4 promoted H transformation faster 2,6-dichlorobenzaldehyde simultaneously in the reaction than K3PO4 did. mixture (K3PO4 promoted transfer hydrogenation), and

VORONIN et al [17] reported that Rb3PO4 has a reported that the rate of reaction for each substrate was higher electroconductivity than K3PO4. In our experiment, unaffected by the other substrate [19]. They then alcohols, aldehydes and ketones could be bound to the concluded that benzaldehyde and 2,6-dichloro- exposed alkali-metal atoms and the oxygen atoms in benzaldehyde were transformed into alcohols at different

[PO4] tetrahedra. The high conductivity of Rb3PO4 is sites on K3PO4. Thus, benzaldehyde and 2,6-dichloro- conducive to electron transfer, and electron benzaldehyde will only compete for 2-propanol. transformation is the key step of transfer hydrogenation VORONIN et al [17] reported that the crystal

Scheme 2 Possible catalytic cycle of forming six-membered cyclic transition states for reductions of aldehydes and ketones catalyzed by M3PO4 (M=K, Rb): (a) Reduction process of substrates without substituent groups on benzene ring (when used alone); (b) Reduction process of substrates with substituent groups on benzene ring (when used alone); (c) Reduction process of benzaldehyde and 2,6-dichlorobenzaldehyde (when used together)

J. Cent. South Univ. (2016) 23: 1603−1610 1607 structure of K3PO4 consisted of [KO6] octahedra and 2,6-dichlorobenzyl alcohol in this work. Given the

[PO4] tetrahedra, with layers of potassium chains in distinctive difference, there must be other competitive between. Then, they emphasized that Rb3PO4 had the relationships between 2,6-dichlorobenzaldehyde and same crystal structures as that of K3PO4 [20]. Since benzaldehyde.

Rb3PO4 and K3PO4 have the similar structure, it is Based on our experimental results and the crystal reasonable to investigate their catalytic cycle together. structures of K3PO4 and Rb3PO4, we postulated that the RADHAKRISHAN et al [19] designed all the hydrogenations of benzaldehyde (or acetophenone) and reactions with excessive 2-propanol and a mixture of benzaldehydes (or acetophenone) with substituent groups equimolar benzaldehyde and 2,6-dichlorobenzaldehyde. on the benzene ring went through different six- However, in this case, weak interactions between the two membered cyclic transition states (Scheme 2). (CHA [6] substrates could not be easily observed. also postulated a similar six-membered cyclic transition Therefore, to amplify interactions between the two state of solid base catalyst system) substrates, we increase the use of benzaldehyde to 40 mL In Scheme 2(a), benzaldehyde (or acetophenone), and reduce the use of 2-propanol to 10.0 mL. The when used alone, is adsorbed on the same alkali atom reaction conditions herein were set as follows: 40 mL of with 2-propanol, forming a six-membered cyclic benzaldehyde as solvent, 6.0 mmol 2,6-dichlorobenzal- transition state. Subsequently, acetone is yielded owing dehyde, 10.0 mL of 2-propanol, 4.0 mmol K3PO4 as to the disappearance of the state, while the α carbon of catalyst, 80 °C and air. After 9.0 h, only 7.0% of benzaldehyde (or acetophenone) obtains a hydrogen 2,6-dichlorobenzaldehyde was converted into from that of 2-propanol. The carbon-oxygen double bond 2,6-dichlorobenzyl alcohol. The result of 2 h reaction of of benzaldehyde (or acetophenone) breaks, but oxygen this competitive experiment is shown in Fig. 3. thereof is still linked with the alkali atom. During the next step, the oxygen of benzaldehyde (or acetophenone) obtains a hydrogen from 2-propanol (its hydroxyl hydrogen), forming the corresponding alcohol as a result. After desorption, the catalyst is ready for another cycle. In Scheme 2(b), when 2,6-dichlorobenzaldehyde is used alone, it cannot be adsorbed on the same alkali atom with 2-propanol due to steric effect. Instead, it is adsorbed on the adjacent alkali atoms, forming a different six-membered cyclic transition state. After the state disappears, 2,6-dichlorobenzaldehyde obtains the α carbon hydrogen and hydroxyl hydrogen of 2-propanol, yielding 2,6-dichlorobenzyl alcohol and acetone. After

Fig. 3 Conversion of 2,6-dichlorobenzaldehyde (Reaction desorption, the catalyst can be used for another cycle. conditions: 1−6.0 mmol 2,6-dichlorobenzaldehyde, 50.0 mL of 4-bromobenzaldehyde and anisaldehyde are reduced through the same way as 2,6-dichlorobenzaldehyde. 2-propanol as solvent, 4.0 mmol K3PO4, 80 °C and air; 2− 40 mL of benzaldehyde as solvent, 6.0 mmol When benzaldehyde and 2,6-dichlorobenzaldehyde 2,6-dichlorobenzaldehyde, 10.0 mL of 2-propanol, 4.0 mmol are used together in the reaction mixture, there are four possible situations. Situation 1: Benzaldehyde is K3PO4 as catalyst, 80 °C and air) adsorbed on the same alkali atom with 2-propanol and no 2,6-dichlorobenzaldehyde was reduced much faster 2,6-dichlorobenzaldehyde is adsorbed on the adjacent than benzaldehyde (Table 1), and the transfer alkali atoms. Then, benzaldehyde goes through the steps hydrogenation of 2,6-dichlorobenzaldehyde finished in in Scheme 2(a). Situation 2: 2,6-Dichlorobenzaldehyde is 2.0 h when used alone. Very little 2-propanol (about adsorbed on the adjacent alkali atoms with 2-propanol 3.0%) was observed to be consumed after 9.0 h (in 40 and no benzaldehyde is adsorbed on the same alkali atom mL of benzaldehyde as solvent, and 6.0 mmol with this 2-propanol, and then, 2,6-dichlorobenzaldehyde 2,6-dichlorobenzaldehyde reaction mixture). Thus, goes through the steps in Scheme 2(b). Situation 3: 2-propanol is always adequate for the reduction of Benzaldehyde is adsorbed on the same alkali atom with 2,6-dichlorobenzaldehyde. CHUAH et al held that 2-propanol, and after that, 2,6-dichlorobenzaldehyde is benzaldehyde and 2,6-dichlorobenzaldehyde only adsorbed on the interfacing alkali atom in Scheme 2(c). competed for 2-propanol. Therefore, 2,6-dichlorobenzal- Benzaldehyde got the opportunity to form the dehyde will be fully converted into 2,6-dichlorobenzyl six-membered cyclic transition state with 2-propanol alcohol after 9.0 h as they postulated. But in fact, only because of early binding. The oxygen of 2,6-dichloro- 7.0% of 2,6-dichlorobenzaldehyde turned into benzaldehyde attracts the hydroxyl hydrogen of

1608 J. Cent. South Univ. (2016) 23: 1603−1610 2-propanol and biases the electronic doublet toward state. The reduction of benzaldehyde is accelerated by oxygen in O—H bond. Due to transmission effects, being attracted by anisaldehyde oxygen. In Situation 5, electronic doublet in the α carbon hydrogen bond when benzaldehyde and 4-bromobenzaldehyde are used becomes more biased toward hydrogen. Thus, the as the reactant pairs, 4-bromobenzaldehyde is adsorbed six-membered cyclic transition state which is formed by on the interfacing alkali atom with 2-propanol, and after benzaldehyde and 2-propanol vanishes more quickly, and that, benzaldehyde is adsorbed on the same alkali atom 2,6-dichloro-benzaldehyde waits for another 2-propanol with 2-propanol. 4-bromobenzaldehyde forms a six- to be adsorbed on the interfacing alkali atom. In this way, membered cyclic transition state. The reduction of the reduction of benzaldehyde is accelerated. Situation 4: 4-bromobenzaldehyde is accelerated by being attracted 2,6-dichlorobenzaldehyde is adsorbed on the interfacing by benzaldehyde oxygen. When benzaldehyde and alkali atom with 2-propanol, and after that, benzaldehyde anisaldehyde are used as the reactant pairs, anisaldehyde is adsorbed on the same alkali atom with 2-propanol. Or is adsorbed on the interfacing alkali atom with benzaldehyde and 2,6-dichlorobenzaldehyde are 2-propanol, and after that, benzaldehyde is adsorbed on adsorbed at the same time, while the former is adsorbed the same alkali atom with 2-propanol. Anisaldehyde on the same alkali atom with 2-propanol and the latter is binds 2-propanol and forms a six-membered cyclic adsorbed on the interfacing alkali atom. In the presence transition state. The reduction of anisaldehyde is of electron-withdrawing groups on the benzene ring, accelerated by being attracted by benzaldehyde oxygen. 2,6-dichlorobenzaldehyde binds the α carbon hydrogen In Scheme 2, when overdose of benzaldehyde (electronic doublet is biased toward hydrogen in this (40 mL benzaldehyde as solvent) is used in the reactant C—H bond) of 2-propanol more easily, forming a six- pairs, the excess part occupies most of Rb3PO4/K3PO4 membered cyclic transition state with 2-propanol in the surface, and very little 2,6-dichlorobenzaldehyde is two kinds of conditions. The oxygen of benzaldehyde adsorbed and catalyzed into 2,6-dichlorobenzyl alcohol. attracts the hydroxyl hydrogen of 2-propanol and biases When a mixture of equimolar benzaldehyde and the electronic doublet toward oxygen in O—H bond. 2,6-dichlorobenzaldehyde with excessive 2-propanol Electronic doublet in the α carbon hydrogen bond is were added in the reaction mixture, benzaldehyde and biased more toward hydrogen owing to transmission 2,6-dichlorobenzaldehyde competed for the active effects. Thus, the six-membered cyclic transition state surface sites of Rb3PO4/K3PO4, whereas the interactions which is formed by 2,6-dichlorobenzaldehyde and between them accelerated each other’s reduction. Hence, 2-propanol disappears more quickly, and benzaldehyde there seems no macroscopic interactions between waits for another 2-propanol to be adsorbed on the same benzaldehyde and 2,6-dichlorobenzaldehyde because of alkali atom. As a result, the reduction of 2,6-dichloro- the two opposite effects. benzaldehyde is accelerated. To verify our postulated catalytic cycle, another When benzaldehyde and 4-bromobenzaldehyde or experiment was performed under the following benzaldehyde and anisaldehyde are used as reactant pairs, conditions: 40 mL of benzaldehyde as solvent, 6.0 mmol there are five possible situations. Situations 1−3 are the 2,6-dichlorobenzaldehyde, 10.0 mL of 2-propanol, same as the outcomes of benzaldehyde and 2,6-dichloro- 0.4 mmol Rb3PO4 as catalyst, 80 °C and air. Compared benzaldehyde. In Situation 4, when benzaldehyde and with the experiment above, only 4.0 mmol K3PO4 was 4-bromobenzaldehyde are used as the reactant pairs, replaced by 0.4 mmol Rb3PO4. After 5.0 h, 19.7% of benzaldehyde is adsorbed on the same alkali atom with 2,6-dichlorobenzaldehyde was reduced and 34.3% of it 2-propanol and 4-bromobenzaldehyde is adsorbed on the turned into 2,6-dichlorobenzyl alcohol after 9.0 h. interfacing alkali atom at the same time. The presence of Obviously, 2,6-dichlorobenzaldehyde, when promoted by electron-withdrawing groups on the benzene ring of Rb3PO4, became more competitive in the reaction 4-bromobenzaldehyde and the distance between mixture. The outcomes cannot be ascribed to different 4-bromobenzaldehyde and 2-propanol, the pairs bind active sites indicated by CHUAH et al. The result of this 2-propanol resembling a tug-of-war, the winner of which competitive experiment is shown below in Fig. 4. undergoes reduction that is accelerated by the attraction In our postulated catalytic cycle, trace of the other. When benzaldehyde and anisaldehyde are 2,6-dichlorobenzaldehyde competes for the active sites used as the reactant pairs, benzaldehyde is adsorbed on with excess benzaldehyde. Compared with K3PO4, the same alkali atom with 2-propanol and anisaldehyde is Rb3PO4 promoted the reduction of 2,6-dichlorobenzal- adsorbed on the interfacing alkali atom at the same time. dehyde almost 10 times faster and that of benzaldehyde Due to the presence of electron-donating groups on the only 3 times faster (Table 1). Accordingly, when Rb3PO4 benzene ring of anisaldehyde and the distance between was used, 2,6-dichlorobenzaldehyde bound 2-propanol anisaldehyde and 2-propanol, benzaldehyde binds more competitively and became more prone to forming a 2-propanol and forms a six-membered cyclic transition six-membered cyclic transition state.

J. Cent. South Univ. (2016) 23: 1603−1610 1609

Fig. 4 Conversion of 2,6-dichlorobenzaldehyde (Reaction Fig. 6 Conversion of benzaldehyde and 4-bromobenzaldehyde conditions: 1−40 mL of benzaldehyde as solvent, 6.0 mmol (Reaction conditions: 20.0 mL of 2-propanol, each of substrates

2,6-dichlorobenzaldehyde, 10.0 mL of 2-propanol, 0.4 mmol are 1.0 mmol, 1.0 mmol Rb3PO4, 80 °C and air)

Rb3PO4 as catalyst, 80 °C and air; 2−40 mL of benzaldehyde as solvent, 6.0 mmol 2,6-dichlorobenzaldehyde, 10.0 mL of

2-propanol, 4.0 mmol K3PO4 as catalyst, 80 °C and air)

Moreover, to test the synergist and competed effect of different substrates, interesting experiments were carried out under the following reaction conditions: 20.0 mL of 2-propanol as solvent, 2.0 mmol substrates,

1.0 mmol Rb3PO4, 80 °C and air. When benzaldehyde, 4-bromobenzaldehyde and anisaldehyde were used alone, 62%, 6% and 49% of them were converted after 2.0 h, respectively. When benzaldehyde and 4-bromobenzaldehyde or benzaldehyde and anisaldehyde were used as reactant pairs, their conversions were 73.5% and 3% or Fig. 7 Conversion of benzaldehyde and anisaldehyde (Reaction 83% and 24.6%, respectively. Surprisingly, the conditions: 20.0 mL of 2-propanol, each of substrates are conversion of benzaldehyde increased when used in pairs. 1.0 mmol, 1.0 mmol Rb3PO4, 80 °C and air) The results of these competitive experiments are shown in Figs. 5−7. a result, the conversion of 4-bromobenzaldehyde or As we expected, 4-bromobenzaldehyde or anisaldehyde was decreased and that of benzaldehyde anisaldehyde competed with benzaldehyde for the active was elevated. surface sites of Rb3PO4, and the interactions between them also accelerated the reduction of benzaldehyde. As 3.5 Reuse of Rb3PO4 The possibility of reusing Rb3PO4 was tested (Table 2). The solid base was recovered after reduction of 2,6-dichlorobenzaldehyde, washed with 2-propanol, dried, and thereafter used for another reduction experiment. After being reused 5 times, its catalytic

activity was almost identical to that of fresh Rb3PO4. Reusing catalysts can reduce waste and save resources,

so Rb3PO4 is a green, potentially eligible solid base catalyst for the catalytic transfer hydrogenation of aldehydes and ketones.

Table 2 Reuse of Rb3PO4 to reduce 2,6-dichlorobenzaldehyde Reused times1 2 3 4 5 Fig. 5 Conversion of benzaldehyde, 4-bromobenzaldehyde and Conversion/% 96.8 95.2 96.5 95.7 94.9 anisaldehyde (Reaction conditions: 20.0 mL of 2-propanol, Reaction conditions: 6.0 mmol substrate, 50.0 mL of 2-propanol, 0.4 mmol 2.0 mmol substrates, 1.0 mmol Rb3PO4, 80 °C and air) Rb3PO4, 70 °C, air and 1.0 h.

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