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DOI: 10.1039/C3RA43829E (Communication) RSC Adv., 2014, 4, 1567-1569

Transfer hydrogenation of acetophenone in an organic-aqueous biphasic system containing double long-chain surfactants†

Liang Wang a, Hongxia Ma *ab, Li Song b, Lei Li b, Yifei Wang b and Haidong Wang *ab
aSchool of Petrochemical Engineering, Changzhou University, Changzhou, 213164, China. E-mail: jxmahx@mail.zjxu.edu.cn; Fax: +86 (0) 573 83646047
bCollege of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing, 314001, China

Received 22nd July 2013 , Accepted 13th November 2013

First published on 14th November 2013

Abstract

Transfer hydrogenation of acetophenone catalyzed by a water-soluble ruthenium complex, RuCl2(TPPTS)2 [TPPTS: P(m-C6H4SO3Na)3], in the presence of surfactants, was studied. The results showed that the reaction was obviously accelerated by double long-chain cationic surfactants. This was attributed to the formation of vesicles. The active ruthenium catalyst was enriched at the biphasic interface owing to the static electricity attraction between ruthenium anionic species and the positive electric field of the vesicle hydrophilic interface, at the same time the water insoluble substrate was solubilized in the hydrophobic interior core of vesicles.

Catalytic transfer hydrogenation of carbonyl compounds has become increasingly significant in organic synthesis because of many potential advantages.1,2 This reaction can be fulfilled in environmentally friendly media, such as water, and it represents an important direction in green chemistry, aimed at reducing chemical pollution due to organic solvents.3 In such organic–aqueous systems, the use of water-soluble catalysts has been of great interest due to the simplification of product separation and reuse of the catalyst.4–11 But the main drawback of this two-phase system is the low reaction rates due to phase-transfer limitations caused by poor substrate solubility in the aqueous phase.12,13 The addition of surface active compounds can enhance the solubility of the substrates.14–17 Deng and co-workers has reported that the cationic surfactant CTAB(cetyltrimethylammonium bromide) could accelerate the asymmetric transfer hydrogenation of aromatic ketones in aqueous media. They assumed that this should be caused by elevated concentration of hydrogen source sodium formate (HCOONa) around the micelles due to the attraction between formate anionic ions and positive charges around the micelle surface.18

It has well known that surfactant can improve the mass transfer across the interface by forming inclusion complexes with highly hydrophobic substrates.14 We are reporting here the reactivity dependent on the property of the surfactants, in case of transfer hydrogenation of acetophenone with water-soluble RuCl2(TPPTS)2 in the organic–aqueous biphasic system, and the 2-propanol was used as hydrogen source.

With the different surfactants, we sequentially test their performance for the transfer hydrogenation of acetophenone in the organic–aqueous biphasic system. The data listed in Table 1 showed that the conversion was low without any surfactants in the reaction system. However, the conversion was dramatically enhanced when the cationic surfactant (CTAB) was added. But the reaction was obviously inhibited if the anionic surfactant (SDS) and the nonionic surfactant (Tween 80) were used.

Table 1 Two-phase transfer hydrogenation of acetophenone with different surfactantsa,b
a Reaction conditions: [Ru] = 9.7 × 10−4 mol L−1; [TPPTS][thin space (1/6-em)]:[thin space (1/6-em)][Ru] = 3[thin space (1/6-em)]:[thin space (1/6-em)]1; [substrate][thin space (1/6-em)]:[thin space (1/6-em)][catalyst][thin space (1/6-em)]:[thin space (1/6-em)][KOH] = 250[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]14; [surfactant] = 1.0 × 10−3 mol L−1; 2-propanol[thin space (1/6-em)]:[thin space (1/6-em)]water = 1[thin space (1/6-em)]:[thin space (1/6-em)]1; 60 °C; 6 h. b CTAB: cetyltrimethylammonium bromide; SDS: sodium dodecyl sulfate; Tween 80: polysorbate 80.
Surfactant CTAB SDS Tween 80 —
Conversion (%) 61.4 24.5 17.3 37.0

The great promotion of the conversion with cationic surfactants could be attributed to the formation of micelles. In the micelle solution the cationic end of the micelle could orient to the aqueous phase and formed a positive electric field, thus ruthenium catalyst species with negative charge are easily attracted on the micelle hydrophilic surface with positive charge. At the same time acetophenone was solubilized in the hydrophobic interior of micelle. The results was favorable for increasing the interfacial area of organic–aqueous two phases and was effectively minimized the phase transfer energy barrier, thereby this was beneficial to promote the substrate transfer to interface and come into contact with ruthenium catalyst.

Although anionic and nonionic surfactants also had the ability to form micelle and increase the interfacial area of organic–aqueous two phases, the repulsion of hydrophilic group with negative charge in SDS or the electron pair of oxygen atom in Tween 80 for ruthenium complex anion would retard the coordination of the substrate with ruthenium catalyst, thereby the reaction rate decreased.

Based on the abovementioned results, a series of cationic surfactants with different alkyl chain length were synthesized and the effect of these surfactants on the transfer hydrogenation of acetophenone was studied. The data was listed in Table 2.

Table 2 Effect of different cationic surfactants on acetophenone transfer hydrogenationa
Entry Surfactant Conversion (%)
a Reaction conditions: [Ru] = 9.7 × 10−4 mol L−1; [TPPTS][thin space (1/6-em)]:[thin space (1/6-em)][Ru] = 3[thin space (1/6-em)]:[thin space (1/6-em)]1; [substrate][thin space (1/6-em)]:[thin space (1/6-em)][catalyst][thin space (1/6-em)]:[thin space (1/6-em)][KOH] = 250[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]14; [surfactant] = 1.0 × 10−3 mol L−1; 2-propanol[thin space (1/6-em)]:[thin space (1/6-em)]water = 1[thin space (1/6-em)]:[thin space (1/6-em)]1; 60 °C; 6 h.
1 C16H33N(CH3)2C2H5Br 62.4
2 C16H33N(CH3)2C4H7Br 67.9
3 C16H33N(CH3)2C8H17Br 70.7
4 C16H33N(CH3)2C12H25Br 79.6
5 C16H33N(CH3)2C16H33Br 82.9

The data in Table 2 indicated that, when the length of one alkyl chain was fixed, the conversion increased with the lengthening of another alkyl chain. This is owing to increase another alkyl chain length in CTAB, its CMC (critical micelle concentration) would decrease gradually. CMC of CTAB is 9.0 × 10−4 mol L−1, however CMC of DDAB16 (dihexadecyldimethylammonium bromide) is 8.0 × 10−5 mol L−1. Therefore, when the concentration of surfactant in aqueous solution is higher than its CMC, the micelle amount of the double long-chain cationic surfactants is much higher than that of single long-chain cationic surfactants in the same surfactant concentration. Moreover the double long-chain cationic surfactants in aqueous solution could form vesicles as showed in Scheme 1.19 The structure of internal and external aqueous phases in vesicles20 would be more favorable for the enrichment of ruthenium catalysts on the hydrophilic surface of vesicle,21 meanwhile the substrates are more easy solubilized in the hydrophobic interior of bilayer structure formed from the two long alkyl chains of surfactant. Thus the local concentration of acetophenone in vesicles would highly increase.22 In this microenvironment, the transfer distance of acetophenone from the vesicle interior to interface of aqueous phase was shortened. Furthermore, the mass transfer energy barrier of acetophenone to arrive the interface was considerably reduced.


image file: c3ra43829e-s1.tif
Scheme 1 Sketch of vesicle formed by double long-chain cationic surfactants.

From the variation of surface tension with the concentration of DDAB16 solution (see Fig. 1), we found the CMC of DDAB16 to be about 2.0 × 10−4 mol L−1 and 1.4 × 10−4 mol L−1 in aqueous solution and reaction mixture, respectively. The addition of other substances, including ruthenium complex, TPPTS, isopropanol and acetophenone, into the aqueous solution caused the decrease of CMC.


image file: c3ra43829e-f1.tif
Fig. 1 Graph of surface tension to DDAB16 concentration.

The influence of DDAB16 concentration on the conversion in the biphasic catalytic system was studied. The data were plotted in Fig. 2. When the DDAB16 concentration increased from 0 to 11 × 10−4 mol L−1, the conversion rose from 37% to 80%. After DDAB16 concentration reached its CMC, the amount of vesicle would increase with further rising DDAB16 concentration. As the results, the interfacial area between two phases would extend, and the ruthenium complexes concentration would obviously increase in the interfacial layer. This would be more favorable for the coordination of acetophenone with ruthenium complexes, and the reaction would be accelerated. However, when the DDAB16 concentration reached a certain value, it would not obviously influence on the conversion.


image file: c3ra43829e-f2.tif
Fig. 2 Influence of DDAB16 concentration on acetophenone transfer hydrogenation. Reaction conditions: [Ru] = 9.7 × 10−4 mol L−1; [TPPTS][thin space (1/6-em)]:[thin space (1/6-em)][Ru] = 3[thin space (1/6-em)]:[thin space (1/6-em)]1; [substrate][thin space (1/6-em)]:[thin space (1/6-em)][catalyst][thin space (1/6-em)]:[thin space (1/6-em)][KOH] = 250[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]14; 2-propanol[thin space (1/6-em)]:[thin space (1/6-em)]water = 1[thin space (1/6-em)]:[thin space (1/6-em)]1; 60 °C; 6 h.

Conclusions

This investigation has demonstrated that transfer hydrogenation of acetophenone in an organic–aqueous system can be accelerated by addition of a double long-chain cationic surfactant. This could be attributed to the formation of vesicles by the double long-chain surfactant. The active ruthenium catalyst species would be enriched on the hydrophilic surface of the interfacial layer, and the substrates would be solubilized in the hydrophobic interior core of the vesicles. Thus the mass-transfer limitation between two phases was considerably reduced and this reaction was accelerated.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (Grant no. 21371079), the Zhejiang Provincial Natural Science Foundation of China (Grant no. LQ12B01002, LQ12E02007).

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Footnote

† Electronic supplementary information (ESI) available See DOI: 10.1039/c3ra43829e
This journal is © The Royal Society of Chemistry 2014
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