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

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1 Catalysis Communications 8 (2007) 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, orthwest ormal University, Lanzhou , China Received 13 ctober 2006; received in revised form 9 ovember 2006; accepted 10 ovember 2006 Available online 14 ovember 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, 4- methyl-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. Ó 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: address: leizq@nwnu.edu.cn (Z. Lei). 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 ( G)-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 /$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi: /j.catcom

2 C. Li et al. / Catalysis Communications 8 (2007) 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. 2. Experimental 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. SnCl 2 Æ 2H 2 and other reagents were obtained and used without further purification. IR spectra were recorded in KBr disks with a icolet 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 C 1s peak at 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 3m 3 mm V-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 Synthesis of the polymer supported polyamidoamine (P-PAMAM ( G)) Cellulose (1.0 g) was added to the solution of SCl 2 (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 SCl 2 and 1,4-dioxane. The resulting cellulose was dried in vacuum at 90 C for 24 h. The chlorinated cellulose and K 2 C 3 (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 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 Synthesis of the complexes (P-PAMAM-HBA ( G)-Sn (II)) First generation polymer-supported dendrimer P- PAMAM (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 SnCl 2 Æ 2H 2 (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 Ketone oxidations xidation 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 C and stirred at this temperature for 15 h. The reaction products were identified by GC-MS analysis. ther 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 H and C H 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=R vibration. The absorptions at 1100 cm 1 are characteristic of C 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 H, 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

3 1204 C. Li et al. / Catalysis Communications 8 (2007) H SCl 2 Cl EDA 2 K 2 C 3 MA 2 EDA MA (0.5G) (1.0G) 2 EDA (1.5G) (2.0G) 2 MA EDA 2.5G 3.0G Scheme 1. The procedure for the preparation of the polymer-supported dendrimers. (1.0G) 2 2 HBA H H SnCl 2.2H 2 Sn Cl Sn Cl 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

4 C. Li et al. / Catalysis Communications 8 (2007) %T b c a Wavenumber Fig. 1. IR spectra of cellulose (a), P-PAMAM (2.0G) (b) and P-PAMAM- HBA (2.0G)-Sn (II) (c). Table 1 ICP data of complexes P-PAMAM-HBA ( G)-Sn (II) Catalyst ICP data (%) (1.0G)-Sn(II) (2.0G)-Sn(II) (3.0G)-Sn (II) the corresponding complex as the example (Table 2 and Fig. 2). Compared with SnCl 2 Æ 2H 2, the binding energy of the Sn 3d5/2 in the complex decreases by 0.5 ev; the decrease of Sn 3d5/2 binding energy means the increase of its election density. The oxygen peak in the complexes can be divided into three components including C C, C 2 and C H. The binding energies of 1s of the complex increases by 0.2 ev, 0.2 ev and 0.9 ev, respectively, compared with those of the corresponding ligand. Table 2 XPS date of the complex P-PAMAM-HBA (2.0G)-Sn (II), ligand P-PAMAM-HBA (2.0G) and the salt SnCl 2 Æ 2H 2 XPS peaks Binding energy (ev) DEb (ev) SnCl 2 Æ 2H 2 Ligand Complex Sn 3d Cl 2p s s The binding energy is referred to C 1s = ev. 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 1s 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 Sn coordination bond. Compared with SnCl 2 Æ 2H 2, the binding energy of the Cl 2p in the complex increases by 0.6 ev; the change of Cl 2p 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 4100 LCL60.SPE 4500 LCL56.SPE c/s 3700 c/s Binding Energy (ev) Binding Energy (ev) 395 Fig. 2. XPS spectra of 1s of complex P-PAMAM-HBA (2.0G)-Sn (II) (left) and ligand P-PAMAM-HBA (2.0G) (right) (Ref. C 1s = ev).

5 1206 C. Li et al. / Catalysis Communications 8 (2007) Table 3 Baeyer Villiger oxidation of 2-adamantanone catalyzed by different generation of cellulose-supported dendritic Sn complexes Cat. H 2 2 Catalyst Conversion (%) Selectivity (%) 1.0G G > G Reaction condition: 2-adamantanone 0.1 mmol, cat. 3 mg, 30% H 2 2 (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-2- pentanone, and cyclopentanone were also oxidized using cellulose-supported dendritic Sn complexes at 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 (%) Product 1,4-Dioxane ,2-Dichloroethane ,2-Dichloroethane Ethanol Ethanol C( ) 3 C( ) 3 2 C Chorobenzene C Chorobenzene C 2 C Reaction condition: substrate 0.1 mmol, 30% H 2 2 (2.0 eq to the ketones), P-PAMAM-HBA (2.0G)-Sn(II) 3 mg, solvent 3 ml, 15 h.

6 C. Li et al. / Catalysis Communications 8 (2007) Conversion (%) 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% H 2 2 (2.0 eq to the ketones), 1,4-dioxane 3 ml, 15 h at 90 C. GC-MS P-PAMAM-HBA Sn 2+ + H + -H 2 +H 2 2 -H + Ä P-PAMAM-HBA Established by IR Sn 2+ P-PAMAM-HBA Sn 2+ Ä H Criegee adduct Fig. 4. Catalytic mechanism for the Baeyer Villiger oxidation catalyzed by cellulose-supported dendritic Sn complexes. 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 P- PAMAM-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-HBA- Sn (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 18 labeling experiment, infrared spectroscopy; and gas chromatography mass spectrometry. 4. Conclusions In summary, P-PAMAM-HBA ( G)-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 ational atural Science Foundation of China (o ) and Specialized Research Fund for the Doctoral Program of High Education (o ) from the Ministry of Education. We also thank Key Laboratory of Eco- Environment-Related Polymer Materials (orthwest ormal University), Ministry of Education, for financial support. References [1] G.C. Krow, rg. React. 43 (1993) 251. [2] R. Bernini, A. Coratti, G. Fabrizi, A. Goggiamani, G. Fabrizi, A. Goggiamani, Tetrahedron Lett. 44 (2003) [3] A. Corma, L.T. emeth, M. Renz, S. Valencia, ature 412 (2001) 423. [4] G. Strukul, ature 412 (2001) 388. [5] D. Astruc, F. Chardac, Chem. Rev 101 (2001) [6] A. Cordova, K.D. Janda, J. Am. Chem. Soc 123 (2001) [7] R. Laurent, A.M. Caminade, J.P. Majoral, Tetrahedron Lett. 46 (2005) 6503.

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