Baeyer–Villiger oxidation of ketones with hydrogen peroxide catalyzed by cellulose-supported dendritic Sn complexes

Catalysis Communications 8 (2007) 1202–1208 www.elsevier.com/locate/catcom

Baeyer–Villiger oxidation of ketones with hydrogen peroxide catalyzed by cellulose-supported dendritic Sn complexes Cuilin Li, Jiaqing Wang, Zhiwang Yang, Zhongai Hu, Ziqiang Lei

*

Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China Received 13 October 2006; received in revised form 9 November 2006; accepted 10 November 2006 Available online 14 November 2006

Abstract Cellulose supported dendritic Sn complexes were prepared by solid phase synthetic methodology. A series of cycloketones and acyclic ketones including 2-andamantanone, cyclohexanone, 4-methylcyclohexanone, 4-tert-butylcyclohexanone, 3-methyl-2-pentanone, 4methyl-2-pentanone and cyclopentanone were oxidized by hydrogen peroxide in a reaction catalyzed by cellulose-supported dendritic Sn complexes, affording the corresponding lactones or esters with the conversion of 70–99% and the product selectivity of 100%. The catalysts can be recycled for several times without any significant decline in catalytic activity.  2006 Elsevier B.V. All rights reserved. Keywords: Baeyer–Villiger oxidation; Cellulose-supported dendrimer; Hydrogen peroxide; Solid phase synthesis

1. Introduction The Baeyer–Villiger oxidation of ketones is a reaction of considerable interest in organic chemistry because the products, lactones or esters, are important synthetic intermediates in the agrochemical, chemical and pharmaceutical industries [1,2]. However, the oxidants used in the traditional Baeyer–Villiger oxidation are organic peroxy acids which potentially produce large amounts of harmful waste. Much recent effort has been devoted to research trying to find chemically green oxidants along with the recyclable catalysts [3,4]. Dendrimer chemistry is one of the most fascinating and rapidly expanding areas of modern chemistry. Considering the unique structures of most of the dendrimers, immobilized metal complexes on the peripheral of these dendrimers should be ideal catalysts for a series of organic transformations [5–7]. However, solution-phase synthesis of dendrimers is often challenging requiring long reaction *

Corresponding author. Tel./fax: +86 931 7970359. E-mail address: [email protected] (Z. Lei).

1566-7367/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.11.012

times and tedious purifications under high vacuum. This dramatically limits the preparation and the applications of the dendrimer–metal complexes. Several groups have reported the polymer supported dendrimers, which were prepared by solid-phase synthesis methodology [8–10]. Solid-phase methodology enables reactions to be driven to completion with the advantages of simple purification in that only filtration and washing are required to remove large excesses of reagents [11]. In a previous paper, we reported the Sn-palygorskite catalyst which shows promising catalytic activities for the Baeyer–Villiger oxidation of ketones with hydrogen peroxide [12]. In this paper, we report the Baeyer–Villiger oxidation of ketones with hydrogen peroxide catalyzed by cellulose-supported dendritic Sn complexes which were cleanly and efficiently prepared. We prepared the cellulose-supported dendritic Sn complexes P-PAMAM-HBA (1.0–3.0G)-Sn (II) (where P = cellulose, PAMAM = polyamidoamine, and HBA = 4-Hydroxybenzaldehyde, G = generation) by solid phase synthesis methodology. The procedure for the preparation of the polymer-supported dendrimers is much simpler than that of the earlier

C. Li et al. / Catalysis Communications 8 (2007) 1202–1208

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literature [11]. After the completion of each steps only required filtering and washing the reaction mixture, avoiding the most difficult procedure for the purification of the dendrimers. The procedure not only allows convenient separation of the dendrimers, but also generates high-metal loading for the later oxidation reaction. Furthermore, cellulose is a natural and environmentally friendly product abundant in the world and this allows us to prepare the catalysts in large scale in low cost.

repeatedly. The first generation dendrimer (P-PAMAM (1.0G)) was obtained. After having repeated the two reactions of Michael addition and amidation, the polymer-supported dendrimers from second generation to third generation were also prepared. The reactions are shown in Scheme 1.

2. Experimental

First generation polymer-supported dendrimer PPAMAM (0.5 g) was added to the solution of HBA (0.5 g) in methanol (20 ml). The mixture was stirred at 50 C for 24 h, then filtered and the product was repeatedly washed with methanol and dried. The first generation ligand (P-PAMAM-HBA (1.0G)) was obtained. The first generation ligand was added to the solution of SnCl2 Æ 2 H2O (1.0 g) in THF (20 ml). The mixture was stirred at 50 C for 24 h, then filtered and the product was repeatedly washed with THF. The first generation tin complex (P-PAMAM-HBA (1.0)-Sn (II)) was obtained. The reaction is shown in Scheme 2. The other generation’s ligands and complexes were prepared using the similar procedure.

2.1. Materials and instrumentations Cellulose was obtained commercially and extracted with THF for 24 h using a Soxhlet apparatus. Ethylenediamine (EDA) and methyl acrylate (MA) were distilled before use. SnCl2 Æ 2H2O and other reagents were obtained and used without further purification. IR spectra were recorded in KBr disks with a Nicolet AVATAR 360 FT-IR spectrophotometer. Metal content was measured on an American ICPV-5600 analytic instrument. XPS analyses were obtained with the PHI-5702/ESCA/SAM System equipped with an Al K (29.35 eV) X-ray source. The binding energy (BE) of the C1s peak at 284.8 eV was taken as an internal standard. The reaction products of oxidation were determined and analyzed using Trace GC/MS 2000 system with 3 m · 0.25 mm SE-54 column and Shimadzu GC-16. A gas chromatograph with a 3 m · 3 mm OV-17 column. The structures of the products were determined by comparing the MS spectrometry and fragmentation patterns of the products with those of the standard compounds. 2.2. Synthesis of the polymer supported polyamidoamine (P-PAMAM (1.0–3.0G)) Cellulose (1.0 g) was added to the solution of SOCl2 (5 ml) in 1,4-dioxane (20 ml), and the mixture was refluxed for 48 h under stirring with a magnetic stirrer, then distilled to eliminating the excess SOCl2 and 1,4-dioxane. The resulting cellulose was dried in vacuum at 90 C for 24 h. The chlorinated cellulose and K2CO3(0.30 g) were added to the solution of EDA (0.5 ml) in methanol (20 ml). The mixture was stirred at 50 C for 24 h, then filtered and the solid was washed with copious amount of methanol. Michael addition of MA to amino groups on the surface: The cellulose having surface amino groups as an initiat site was added to the solution of MA (1.0 ml) in methanol (20 ml). The mixture was stirred at 50 C for 24 h, then filtered and the product was thoroughly washed with methanol. The amidation of resulting terminal ester groups on the surface: After Michael addition of MA the cellulose powder was added to the solution of EDA (2.0 ml) in methanol (20 ml). The mixture was stirred at 50 C for 24 h, then filtered and the product was washed with methanol

2.3. Synthesis of the complexes (P-PAMAM-HBA (1.0–3.0G)-Sn (II))

2.4. Ketone oxidations Oxidation of ketones was carried out in a 10 ml glass reactor. A typical procedure for the Baeyer–Villiger oxidation is as follows: 2-Adamantanone (15 mg, 0.1 mmol) and 30% hydrogen peroxide (2.0 eq) were dissolved in 3 ml of organic solvent. Cellulose-supported dendritic Sn complexes (3 mg) were added, the mixture was heated to 70– 90 C and stirred at this temperature for 15 h. The reaction products were identified by GC-MS analysis. Other cycloketones and chain ketones were also oxidized in this oxidation system to give the corresponding lactones or esters. 3. Results and discussion The ligands and the complexes were characterized by IR, XPS and ICP. Fig. 1 shows the IR spectra of cellulose (a), P-PAMAM (2.0G) (b) and P-PAMAM-HBA (2.0G)-Sn (II) (c) (where P = cellulose, PAMAM = polyamidoamine, HBA = 4-Hydroxybenzaldehyde and G = generation). The bands at 1429, 1314, 1136 and 1059 cm 1, which are characteristic absorptions of O–H and C–OH bonds of cellulose, were changed. The new bands in the IR spectra of cellulose-supported dendrimer at 1650 cm 1 and 1550 cm 1 are ascribed to C=ONHR vibration. The absorptions at 1100 cm 1 are characteristic of C–N vibration. All these changes suggested that polyamidoamine have grafted on cellulose. The new bands at 1182 cm 1 and 1044 cm 1 of the complexes, which are attributed to C–OH, identified that HBA has reacted with surface amino groups. In the process of the preparation of the ligands, we find that the color of the ligand gradually becomes yellow (the cellulose

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SOCl2

OH O NH

EDA

Cl

NH

K2CO3

NH2 NH2

O

OCH3

MA

NH EDA

N

NH

MA

N

OCH3

NH

O

O

(0.5G)

NH2 (1.0G)

O

OCH3 NH2

O NH

N O NH

NH O

N

O O

NH EDA

NH

NH

N

NH

NH O

N

NH2 NH2

O O

OCH3

NH

N

O

OCH3

N NH

(2.0G)

OCH3

O

NH2

O (1.5G)

MA

EDA

2.5G

3.0G

Scheme 1. The procedure for the preparation of the polymer-supported dendrimers.

NH2

O

O

NH NH

N

CH

OH

CH

OH

NH HBA

N

NH

N

NH

NH

NH2

O

O

N

(1.0G)

O SnCl2.2H2O

CH

O Sn Cl

CH

O Sn Cl

N NH

NH

N NH O

N

Scheme 2. The procedure for the preparation of the ligand of first generation.

supported dendrimer is white). All these results show that cellulose-supported ligands and their complexes have been prepared. It can be seen from Table 1 that metal content of the complexes decreased with the increase of the generation. From the structures of the ligands, we know that Sn (II) ions can coordinate with the peripheral phenol groups.

The theoretical metal content of the complexes should increase with the generation, but from Table 1 we can see that metal content of the complexes decreased with the generation, this may be due to the steric hindrance of higher generation dendrimer. The ligands and their complexes were also investigated by XPS. Here we take the second generation ligand and

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Table 2 XPS date of the complex P-PAMAM-HBA (2.0G)-Sn (II), ligand P-PAMAM-HBA (2.0G) and the salt SnCl2 Æ 2H2O

b

XPS peaks

%T

c

Sn3d5 Cl2p N1s

a

O1s

2000

1500

1000

500

Binding energy (eV)

DEb (eV)

SnCl2 Æ 2H2O

Ligand

Complex

487.5 198.6 – – – – – – –

– – 401.7 400.3 399.3 397.7 532.9 531.9 529.9

487.0 199.2 402.9 401.6 400.0 399.0 533.1 532.1 530.8

0.5 +0.6 +1.2 +1.3 +0.7 +1.3 +0.2 +0.2 +0.9

The binding energy is referred to C1s = 284.80 eV.

Wavenumber Fig. 1. IR spectra of cellulose (a), P-PAMAM (2.0G) (b) and P-PAMAMHBA (2.0G)-Sn (II) (c).

Table 1 ICP data of complexes P-PAMAM-HBA (1.0–3.0G)-Sn (II) Catalyst

ICP data (%)

(1.0G)-Sn(II) (2.0G)-Sn(II) (3.0G)-Sn (II)

20.00 15.50 11.30

the corresponding complex as the example (Table 2 and Fig. 2). Compared with SnCl2 Æ 2H2O, the binding energy of the Sn3d5/2 in the complex decreases by 0.5 eV; the decrease of Sn3d5/2 binding energy means the increase of its election density. The oxygen peak in the complexes can be divided into three components including C–O–C, CONH2 and C–O–H. The binding energies of O1s of the complex increases by 0.2 eV, 0.2 eV and 0.9 eV, respectively, compared with those of the corresponding ligand.

The nitrogen peak in the complexes could be divided into four components including tertiary amino group, Schiff base, amide group and secondary amine. The binding energies of N1s of the complexes increase by 1.2 eV, 1.3 eV, 0.7 eV and 1.3 eV, respectively, compared with those of the corresponding ligand. This means the electrons in the nitrogen atom may flow into the tin atom to form an N– Sn–O coordination bond. Compared with SnCl2 Æ 2H2O, the binding energy of the Cl2p in the complex increases by 0.6 eV; the change of Cl2p binding energy means the decrease of its election density. The XPS analysis results further confirmed the formation of the cellulose-supported dendritic Sn complexes. A series of oxidation reactions were carried out to evaluate the catalytic activity of these newly synthesized complexes. The reactions were carried out for 15 h using different generation of the complexes as catalysts, respectively, to investigate the implications of dendrimer generation on the Baeyer–Villiger oxidation reactions: adamantanone was chosen as test substrate for the oxidation with hydrogen peroxide. As shown in Table 3, the conversion is near 100% for the first and second generation of cellulose-supported

LCL60.SPE

4100

LCL56.SPE

4500

4000 3900 4000

c/s

c/s

3800 3700 3500

3600 3500 3400 415

410

405

400

Binding Energy (eV)

395

3000 415

410

405

400

395

Binding Energy (eV)

Fig. 2. XPS spectra of N1sof complex P-PAMAM-HBA (2.0G)-Sn (II) (left) and ligand P-PAMAM-HBA (2.0G) (right) (Ref. C1s = 284.80 eV).

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Table 3 Baeyer–Villiger oxidation of 2-adamantanone catalyzed by different generation of cellulose-supported dendritic Sn complexes

Cat. H2O2 O Catalyst

O

O

Conversion (%)

1.0G 2.0G 3.0G

Selectivity (%)

98 >99 85

100 100 99

Reaction condition: 2-adamantanone 0.1 mmol, cat. 3 mg, 30% H2O2 (2.0 eq to the ketones), 1,4-dioxane 3 ml, 15 h at 90 C.

dendritic Sn complexes. The conversion decreased to about 85% for the third generation of cellulose-supported dendritic Sn complexes. The substrate conversion results

indicate that the catalytic activity decreased with the generation of the complexes and this may be due to the steric hindrance and the lower metal content of higher generation of the dendrimer. Table 3 also shows that by using 1–3 generation of cellulose-supported dendritic Sn complexes, the selectivity to the product remains to be high (99–100%). Several cyclic and acyclic ketones such as 2-andamantanone, cyclohexanone, 4-methylcyclohexanone, 4-tertbutylcyclohexanone, 3-methyl-2-pentanone, 4-methyl-2pentanone, and cyclopentanone were also oxidized using cellulose-supported dendritic Sn complexes at 70–90 C in different organic solvents in this oxidation system. As shown in Table 4, 2-andamantanone was oxidized with hydrogen peroxide in 1,4-dioxane at 90 C using 3 mg of the catalyst, affording the required product with 99% conversion. For cyclohexanone and cyclopentanone, the oxidations were carried out in 1,2-dichloroethane at 70 C. In this reaction condition, 99% and 70% conversions

Table 4 Baeyer–Villiger oxidation of ketones catalyzed by P-PAMAM-HBA (2.0G)-Sn (II) Substrate

Solvent

Temperature (C)

Conversion (%)

Selectivity (%)

1,4-Dioxane

90

99

100

Product

O

O

1,2-Dichloroethane

O

70

99

100

O

O O

1,2-Dichloroethane

O

70

70

100

O O

Ethanol

O

70

85

100

O O

CH3

CH3

Ethanol

O

70

99

100

O O

C(CH3)3

C(CH3)3 O

Chorobenzene

90

99

100

CH3 CH2 CH C CH3

O CH3 CH2 CH O C

O CH3 CH CH2 C CH3 CH3

CH3

CH3

CH3

Chorobenzene

90

99

100

O CH3 CH CH2 O C CH3 CH3

Reaction condition: substrate 0.1 mmol, 30% H2O2 (2.0 eq to the ketones), P-PAMAM-HBA (2.0G)-Sn(II) 3 mg, solvent 3 ml, 15 h.

C. Li et al. / Catalysis Communications 8 (2007) 1202–1208 O

100

Conversion (%)

1207

P-PAMAM-HBA

80

+H2O2

Sn2+

O

£- H+

O¦Ä

60

¦Ä

GC - MS

40

O

20

P-PAMAM-HBA

Established by IR

O

P-PAMAM-HBA

Sn

1

2

3

4

5

O

Recycling of catalyst (time) Fig. 3. The effect of recycling times on the conversion. Reaction condition: 2-adamantanone 0.1 mmol, P-PAMAM-HBA (2.0G)-Sn (II) 3 mg, 30% H2O2 (2.0 eq to the ketones), 1,4-dioxane 3 ml, 15 h at 90 C.

O OH

2+

0

Sn2+

Criegee adduct O

+ H+

2O

£- H

Fig. 4. Catalytic mechanism for the Baeyer–Villiger oxidation catalyzed by cellulose-supported dendritic Sn complexes.

4. Conclusions were achieved, respectively, with the catalyst. The catalyst is also active for the oxidation of chain aliphatic ketones like 3-methyl-2-pentanone and 4-methyl-2-pentanone. The oxidation conversion for these acyclic ketones is also very high (99%). The product selectivity remains 100% for all the ketones investigated. The outstanding character of this catalytic system is that the acyclic ketones can be oxidized in high conversion and selectivity and that some of the oxidation can be carried out in ethanol which is a relatively less harmful organic solvent. P-PAMAM-HBA (2.0G)-Sn (II) possesses promising catalytic activity for the Baeyer–Villiger oxidation of cycloketones. In particular, the oxidation reaction time was shorter than that mentioned in the literature [3–12]: within 15 h the reaction has completed. These complexes do not dissolve in the ordinary organic solvents, so the reaction is carried out under heterogeneous conditions. After the completion of the oxidation, the cellulose-supported complexes were easily separated by filtration. The catalysts can be recycled for five times without the obvious decrease of their catalytic activity. The results are shown in Fig. 3. We confirmed that the oxidation reaction did not occur in the absence of the catalyst. Moreover, the reaction did not proceed in the presence of P-PAMAM-HBA and exhibited lower catalytic activity in the presence of PPAMAM-Sn (II). We come to a conclusion that the activity point of the catalyst was Sn (II) ions that coordinated with the dendritic ligands. A possible reaction mechanism using P-PAMAM-HBASn (II) as the catalyst for the Baeyer–Villiger oxidation is shown in Fig. 4. Firstly, the ketone is coordinated to the Lewis-acid tin centre, and thereby, the carbonyl group is activated. Hydrogen peroxide subsequently attacks the more eletrophilic carbonyl carbon atom. After the rearrangement, the lactone product is replaced by a new substrate molecule, a mechanism that is analogous to that for peracids, which Corma et al. [3] have confirmed by means of 18O labeling experiment, infrared spectroscopy; and gas chromatography–mass spectrometry.

In summary, P-PAMAM-HBA (1.0–3.0G)-Sn (II) was prepared by a simple procedure. The complexes are shown to act as highly active catalysts for the Baeyer– Villiger oxidation of ketones using environmentally friendly 30% hydrogen peroxide as oxidant and natural product cellulose as the supporter, which can be obtained in large scale with low cost. This oxidation procedure is promising both for cyclic and acyclic ketones and is much cleaner than those of the traditional BV oxidation as it is carried out without the use of peracids which would produce harmful by-products. The catalyst preparation does not involve the use of any expensive materials. The catalyst can also be recycled. Further work exploring the utility of these novel catalysts is on-going in our laboratory. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 20474052) and Specialized Research Fund for the Doctoral Program of High Education (No. 20050736001) from the Ministry of Education. We also thank Key Laboratory of EcoEnvironment-Related Polymer Materials (Northwest Normal University), Ministry of Education, for financial support. References [1] G.C. Krow, Org. React. 43 (1993) 251. [2] R. Bernini, A. Coratti, G. Fabrizi, A. Goggiamani, G. Fabrizi, A. Goggiamani, Tetrahedron Lett. 44 (2003) 8991. [3] A. Corma, L.T. Nemeth, M. Renz, S. Valencia, Nature 412 (2001) 423. [4] G. Strukul, Nature 412 (2001) 388. [5] D. Astruc, F. Chardac, Chem. Rev 101 (2001) 2991. [6] A. Cordova, K.D. Janda, J. Am. Chem. Soc 123 (2001) 8248. [7] R. Laurent, A.M. Caminade, J.P. Majoral, Tetrahedron Lett. 46 (2005) 6503.

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