Lanthanide–surfactant-combined catalysts for the allylation of benzaldehyde with tetraallyltin in aqueous solutions

Journal of Alloys and Compounds 451 (2008) 418–421

Lanthanide–surfactant-combined catalysts for the allylation of benzaldehyde with tetraallyltin in aqueous solutions Karen Deleersnyder, Danzhao Shi, Koen Binnemans, Tatjana N. Parac-Vogt ∗ Katholieke Universiteit Leuven, Department of Chemistry, Laboratory of Coordination Chemistry, Celestijnenlaan 200F, B-3001 Leuven, Belgium Available online 18 April 2007

Abstract Metal–surfactant-combined catalysts having the general formula M(DOS)x (DOS = dodecylsulfate), x = 1–3, can be used for the efficient allylation of benzaldehyde with tetraallyltin in water. Due to the formation of micelles the reaction proceeds in the absence of organic solvents. Examination of a series of lanthanide(III) and divalent transition-metal dodecylsulfates as catalysts for the reaction between benzaldehyde and tetraallyltin revealed that the highest yields can be obtained when lanthanide(III) and copper(II) salts are used. Within 6 h at room temperature nearly quantitative conversion of benzaldehyde to 1-phenyl-3-buten-1-ol was achieved in the presence of 10 mol% of Cu(DOS)2 or Yb(DOS)3 . The influence of the amount of catalyst and tetraallyltin on the reaction yield was investigated. Different alkylsulfates, alkylsulfonates and arylsulfonates metal salts were prepared and tested as catalysts in the allylation reaction. The highest yields were obtained with ytterbium(III) alkylsulfates and ytterbium(III) arylsulfonates, while ytterbium(III) alkylsulfonates were found to be poor allylation catalysts. The results show that both the nature of the metal cation as well as the length and the nature of anionic surfactant chain are crucial for the activity of the metal–surfactant-combined catalysts in water. © 2007 Elsevier B.V. All rights reserved. Keywords: Lanthanides; Lewis acids; Micellar catalysis; Allylation; Surfactants; Tetraallyltin

1. Introduction The current concern about environmental issues stimulates research focused on organic reactions in water [1]. Since water is a cheap, benign, safe, and environmentally friendly solvent, the use of water as a medium for organic reactions could greatly contribute to the development of environmentally friendly processes. However, the poor solubility of many organic compounds in water limits the widespread use of water as a solvent for organic reactions. Moreover, the traditional Lewis acids like AlCl3 or FeCl3 , which are of great interest due to their high reactivities and selectivities, are readily decomposed in the presence of water and must be handled under strictly anhydrous conditions. During the last decade Kobayashi and co-workers have demonstrated that rare earth triflates, such as Sc(OTf)3 , Y(OTf)3 , and Yb(OTf)3 are water-stable Lewis acids which can be used to perform carbon–carbon bond-forming reactions in water [2,3].

∗

Corresponding author. Fax: +32 16 327992. E-mail address: [email protected] (T.N. Parac-Vogt).

0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.161

Although the rare earth triflates were found to efficiently catalyze several reactions in aqueous solutions, certain amounts of watersoluble organic solvents such as tetrahydrofuran, ethanol, or acetonitrile had to be combined with water in order to solubilize the organic reactants. To overcome these solubility problems, an anionic surfactant can be added to the Lewis acid [4,5]. This approach led to the development of Lewis acid–surfactantcombined catalysts (LASCs), which are usually composed of water-stable cationic Lewis acids and an anionic surfactant as the counter ion [6]. These systems act both as a catalyst to activate the substrate molecules and as a surfactant forming micelles in which the reactants are dissolved. Although there have been several reports on the use of LASCs in aldol and Diels–Alder reactions [6–9], the use of LASCs for other types of reactions still remains largely unexplored. The Lewis acid catalyzed allylation of aldehydes using allylstannanes or silanes is a versatile synthetic tool. Allylation of a carbonyl group introduces simultaneously a hydroxyl group and a double carbon–carbon bond into the substrate. The reaction can be performed under enantioselective conditions [10]. Some successful procedures published in literature use a Lewis acid catalyst in either pure organic solvent or in a water–organic

K. Deleersnyder et al. / Journal of Alloys and Compounds 451 (2008) 418–421

solvent mixture [3,11]. A mesoporous silica-supported Lewis acid catalyst was also reported [12]. The allylation of aldehydes has been performed in ionic liquids as well [13]. There are also reports of water or methanol promoted allylations, without the use of a catalyst [14], but these non-catalyzed procedures require relatively long reaction times. Reactions of benzaldehyde with tetraallyltin in water have been carried out using scandium(III) dodecylsulfate [6], but frequently a Br¨onsted acid was added to shorten the reaction period [15]. Although this latter method gave good yields after a short reaction period, its widespread application may be limited due to the high price of scandium salts and due to the use of a Br¨onsted acid such as hydrochloric acid, which is unfavourable from an environmental point of view. In an attempt to perform efficient and environmentally friendly allylation reactions, we combined the advantages of Lewis acid–surfactant combined catalysts to perform allylation of benzaldehyde in the absence of any acid or organic solvent. The use of lanthanide(III) sulfonates such as lanthanide(III) tosylates and lanthanide(III) nosylates, as cheap, recyclable and environmentally friendly catalysts has been recently demonstrated in our laboratory [16]. In this study we report our results on the use of lanthanide(III) and copper(II) alkylsulfates for the allylation of benzaldehyde with tetraallyltin in aqueous solutions. 2. Experimental Reagents were obtained from Aldrich Chemical., Acros Organics or Rhodia Electronics and Catalysis, and used without further purification. All reaction mixtures were analysed with a ThermoFinnigan Trace GC (GC-FID). The reaction yields were determined with ChromCard software. 1 H NMR spectra were run on a Bruker Avance 300, operating at 300 MHz. Elemental analyses were performed on a CE Instruments EA-1110 elemental analyzer.

2.1. General procedure for the synthesis of metal–surfactant-combined catalysts Sodium dodecylsulfate (2.07 g, 7.19 mmol) was dissolved in water (70 mL) at 70 ◦ C. To this was added a solution of CuCl2 ·2H2 O (0.613 g, 3.60 mmol) in water (1 mL) at the same temperature. The mixture was cooled to room temperature and left in the fridge for a few days. The resulting precipitate was filtered on a crucible, washed thoroughly with water and dried in a vacuum oven at 50 ◦ C overnight to afford the pure Cu(OSO3 C12 H25 )3 . The other transition metal complexes were prepared by using the same procedure. To a stirred aqueous solution (50 mL) of sodium dodecylsulfate (6 mmol, 1.73 g), a solution of YbCl3 ·6H2 O (2 mmol, 0.77 g) in water (20 mL) was added dropwise at room temperature. The solution was left to stir for about 1 h. The precipitate was then filtered on a crucible, washed thoroughly with water and dried in a vacuum oven at 50 ◦ C over night to afford the pure Yb(OSO3 C12 H25 )3 . The other lanthanide(III) complexes were prepared in an analogous manner by using their respective sodium salts. In the synthesis of Ln(OSO3 C10 H21 )3 and Ln(O3 SC6 H4 C12 H25 )3 the resulted precipitate had to be filtered immediately after formation to prevent it from being re-dissolved in water. In the synthesis of Yb(O3 SC12 H25 )3 and Yb(O3 SC16 H33 )3 the sodium salt had to be dissolved in water at 50 ◦ C. Then the mixture was cooled to room temperature and the resulting precipitate was filtered.

Scheme 1. Preparation of Ln(DOS)3 complexes. temperature. After a given period of time the reaction mixture was extracted with ethylacetate. The organic phase was diluted in acetone and analyzed by gas chromatography to obtain the yield of the reaction.

3. Results and discussion Transition metal and lanthanide(III) dodecylsulfates were prepared by a metathesis reaction between sodium dodecylsulfate (NaDOS) and the corresponding metal chloride (Scheme 1) [17]. The elemental analysis results were in good agreement with the formula Ln(DOS)3 ·nH2 O (Ln = La, Eu, Gd, Yb, etc.) and M(DOS)2 ·nH2 O (M = Co, Cu, Ni). Upon drying the crystalline solid at 50 ◦ C in vacuo, some crystallization water is lost, but the complexes still contain one to three molecules of water. The reaction of benzaldehyde with tetraallyltin was chosen as a model reaction (Scheme 2). As it can be seen in Table 1, the allylation reaction performed in the absence of any catalyst (blank reaction) resulted in low yields. Low yields were also obtained in the presence of ytterbium(III) tosylate, Yb(TOS)3 , as the catalyst. Low yields obtained with the strong Lewis acid ytterbium(III) tosylate, which cannot form micellar systems, imply that formation of a hydrophobic reaction medium facilitates the allylation reaction. Similarly, low yields obtained in the presence of sodium dodecylsulfate, which forms micelles in solution, but acts as a weak Lewis acid suggest that merely presence of a micellar system is not sufficient for the efficient allylation of benzaldehyde. In contrary, all metal dodecylsulfate complexes exhibited catalytic activity, but noticeable differences in the reactivity were observed. The differences in Lewis acidity between the metal cations can be explained by the hydrolysis constants (Kh ) and exchange rate constants for substitution of inner-sphere water ligands, expressed as the water exchange rate constant (WERC) [18]. Kobayashi and co-workers have suggested that metals should have a pKh in the range from about 4 to 10 and WERC values greater than 3.2 × 106 M−1 s−1 in order to be effective catalysts in aldol reactions [1e]. The low WERC values for Co2+ (2 × 105 M−1 s−1 ) and Ni2+ (2.7 × 104 M−1 s−1 ) explain their low Lewis acidic activity for the allylation of benzaldehyde in water. In contrary, Na+ has a fast WERC (1.9 × 108 M−1 s−1 ) but its pKh value is too large (14.18), resulting in inefficient hydrolysis and low Lewis acidity. Inspection of pKh and WERC values for copper(II) and lanthanide(III) shows that they are in the optimal range (pKh values from 7.5 to 8.5, and the WERC values from 6 × 107 to 7 × 108 M−1 s−1 ).

2.2. General procedure for the allylation of benzaldehyde To a solution of the respective lanthanide(III) or transition metal(II) catalyst (10 mol%, 0.025 mmol) in water (1.5 mL) was added benzaldehyde (0.25 mmol; 25 ␮L) and tetraallyltin (0.075 mmol; 18 ␮L). The mixture was stirred at room

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Scheme 2. LASC catalyzed reaction of benzaldehyde with tetraallyltin.

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K. Deleersnyder et al. / Journal of Alloys and Compounds 451 (2008) 418–421

Table 1 Influence of the metal ion on the conversion of benzaldehyde to 1-phenyl-3buten-1-ol in the presence of 0.3 equiv. of tetraallyltin

Table 3 Effect of the ytterbium(III) dodecylsulfate loading on the allylation of benzaldehyde in the presence of 0.3 equiv. of tetraallyltin

LASC

Conversiona (%)

Catalyst load (mol%)

Conversiona (%)

None Yb(TOS)3 NaDOS Co(DOS)2 Ni(DOS)2 Cu(DOS)2 La(DOS)3 Gd(DOS)3 Yb(DOS)3

30 35 53 64 48 93 69 74 78

1 2 5 10 20 50

47 60 71 78 92 98

a

a

10 mol% of M(DOS)x , 4 h, room temperature.

As can be seen from Tables 1 and 2, the yield of the reaction increases with decreasing ionic radius of the lanthanide(III) ion. Previous studies involving reactions catalyzed by lanthanide(III) triflates, lanthanide(III) tosylates and scandium(III) dodecylsulfate revealed a similar trend [6,19,16a]. Our NMR results suggest that in strongly coordinating solvents such as water, the first coordination sphere of all lanthanide(III) dodecylsulfates will be the same, since the coordinating solvent replaces DOS ligand to form strong Lewis-acidic species such as [Ln(H2 O)n ]3+ (n = 8–9). The smaller ionic radius of trivalent lanthanide ion leads to a greater charge to size ratio (Z/r), which in turn results in greater polarizing power of the lanthanide(III) center. This polarization increases the electrophilicity of the carbonyl carbon, making this bond more susceptible to nucleophilic attack by tetraallyltin. The effect of catalyst concentration on the reaction yields was examined by varying the concentration of the catalyst from 1 to 50 mol%. The results presented in Table 3 show that when 20 mol% of Yb(DOS)3 catalyst was used instead of 10 mol%, the yield of 1-phenyl-3-buten-1-ol increased from 78 to 92%. On the other hand, increasing the catalyst concentration from 20 to 50 mol% resulted in only 6% yield improvement. This proves that catalytic amounts (less than 20 mol%) are sufficient for this reaction. The effects of the allylation agent concentration on the extent of the reaction were examined by comparing the reaction yields obtained in the presence of 0.25 and 0.3 equiv. of tetraallyltin. Because tetraallyltin contains four allyl groups, in theory every Table 2 Influence of the nature of the metal ion in the dodecylsulfate complex on the extent of benzaldehyde conversion to 1-phenyl-3-buten-1-ol LASC

None NaDOS Cu(DOS)2 La(DOS)3 Gd(DOS)3 Yb(DOS)3

˚ Ionic radius of the metal (A)

Conversion (%) After 4 h

After 6 h

30 53 93 69 74 78

53 65 95 82 90 93

– 0.97 0.72 1.06 0.94 0.86

The reaction was performed in the presence of 0.3 equiv. of tetraallyltin and 10 mol% of M(DOS)x at room temperature.

Four hours at room temperature.

molecule of tetraallyltin should react with four molecules of benzaldehyde, resulting in an atom-economic reaction. However, when the experiments were performed with 0.3 equiv. of tetraallyltin it was noticed that a small excess of tetraallyltin results in a rather large difference in the reaction yields: using 0.3 equiv. of tetraallyltin instead of 0.25 in the Yb(DOS)3 catalyzed reaction resulted in the increase of 1-phenyl-3-buten-1-ol yield from 73 to 90%. In order to examine the importance of the surfactant anions for the efficiency of LASC catalyst, some ytterbium(III) complexes containing alkylsulfate and alkylsulfonate anions with different alkyl chain length were prepared and used as catalysts in the allylation of benzaldehyde. Only the ytterbium(III) complexes wherefrom the sodium salt was commercially available were prepared and tested. In the reaction of benzaldehyde with 10 mol% of catalyst, results after 4 h at room temperature indicate that ytterbium(III) dodecylsulfate (78%) is a more efficient catalyst than ytterbium(III) dodecylsulfonate (38%). By changing the alkyl group it turned out that the highest yield is achieved by using ytterbium(III) dodecylsulfate. This trend is similar to the trend observed for aldol reactions catalyzed by scandium(III) alkylsulfates and scandium(III) alkylsulfonates [7]. The mixtures of the Lewis acid–surfactant-combined catalyst and the organic substrates form dispersions, and the particle size and stability of these dispersions depend on the size and the nature of the surfactant anions. Higher yields of benzaldehyde allylation in the presence of ytterbium(III) alkylsulfate compared to the yields obtained with ytterbium(III) alkylsulfonate suggest that a better hydrophobic reaction field should be formed in the presence of the former catalyst. This prompted us to explore the lanthanide(III) 4-alkylbenzenesulfonate catalysts for the allylation of benzaldehyde. Within 4 h, Ln(C12 H25 C6 H4 SO3 )3 (Ln = La, Gd, Yb) catalysts effected benzaldehyde conversion of ca. 50–55%, compared to ca. 35% yields obtained with Ln(C12 H25 SO3 )3 under the same conditions. This finding can be attributed to the ␲–␲ interaction between the molecules of the surfactant containing the aromatic group and benzaldehyde, which may lead to the stabilization of the substrates in the hydrophobic cavities [4]. 4. Conclusions In conclusion, we have shown that allylation of benzaldehyde can be achieved in high yields in pure water by using lanthanide(III) or copper(II) dodecylsulfate catalysts. Similar

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to the scandium(III) dodecylsulfate [6a] we can assume that the metal complexes that were studied in this paper form stable dispersion systems in water and at the same time act as catalyst to activate the substrate molecules. This allows the use of water as the reaction medium, which is both economically and environmentally beneficial.

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Acknowledgments T.N.P.V. is a Postdoctoral Fellow of the FWO-Flanders (Belgium). Financial support has been provided by the K.U. Leuven (VIS/01/006.01/2002/-06/2004 and GOA 03/03) and by the FWO-Flanders (project G.0117.03). The authors thank Rhodia Electronics and Catalysis for a generous gift of rare earth oxides. [12]

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