Transition metal-free oxidation of benzylic alcohols to carbonyl compounds by hydrogen peroxide in the presence of acidic silica gel

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Introduction
Oxidation of alcohols to carbonyl compounds is fundamental to organic chemistry, and is currently attracting a lot of interest.Oxidation of alcohols to carbonyl compounds has established itself as an important application in the chemical process industry.Carbonyl compounds such as aldehydes and ketenes are used in the synthesis of many organic compounds, vitamins, complex ligands as well as medicine 1 .Benzaldehyde is a valuable chemical substance, and has widespread applications in chemical industries 2 .However, most of these reagents of transition metal complexes such as chromium and cobalt are often either toxic or hard to prepare.
Due to its importance, several papers in the field have been devoted to this topic.In some of these, a great number of oxidizing agents have been identified for the oxidation of benzyl alcohols 3- 17 .Nevertheless, some of the reported reagents and catalysts suffer from disadvantages such as lack of availability, difficult work-up, long reaction time, cost of metal catalysts, as well as toxicity and high cost of the reagents.Thus, milder and more selective reagents and transition metal-free eco-friendly systems are still in great need.Recently, the use of solid supported reagents has received considerable attention in organic synthesis because of its ease of handling, enhanced reaction time, greater selectivity power, simple workup, and recoverability of catalysts 18 .Among the various heterogeneous catalysts, silica gel is advantageous not only for its low cost but also for its ease of preparation, especially when it is impregnated with hydrochloric acid.Since the reaction is heterogeneous in nature, the catalyst can conveniently be separated through simple filtration.
Over the past few years, the importance of hydrogen peroxide and its derivatives as oxidizing agents has grown considerably.From an environmental viewpoint, in contrast to other oxidizing agents, hydrogen peroxide is the most attractive one, and-as water is the only theoretical byproduct in this process-is an ideal waste-avoiding oxidant, hence its attractiveness as a good oxidant, thanks to its solubility in water as well as its numerous available organic solvents.Moreover, aqueous hydrogen peroxide solution lends itself safely to storage, operation as well as transportation, and is relatively cheap and widely available in the market 19 .Recently Sharma et al. reported oxidation of alcohols to carbonyl compounds using aqueous HBr and hydrogen peroxide in acetonitrile under the reflux condition 20 .Hong Yun Guo has recently reported using H2O2/HCl for the synthesis of substituted benzothiazoles 21 .In 1977, Hurst made a detailed investigation of kinetic parameters for oxidation of hypochlorous acid by hydrogen peroxide and yield formation of excited oxygen (or singlet oxygen, eq. 1) 22 .
Photocatalytic oxidation of benzyl alcohol to benzaldehyde by oxygen is a product of H2O2 23 (Eq.( 2)).

Results and discussion
As part of our research on the development of supported reagents and catalysts for the oxidation of alcohols, we have recently developed a heterogeneous catalytic method based on sodium nitrite in the presence of acidic silica gel for effective aromatization of Hantzsch 1,4-dihydropyridines 24 .Here we describe this catalyst (acidic silica gel) for effective oxidation of benzyl alcohols to corresponding aldehydes and ketones using hydrogen peroxide as an oxidant (Scheme 1).

Scheme 1. Oxidation of benzyl alcohols
In this paper, acidic silica gel along with hydrogen peroxide is found to be a simple metal free oxidation reagent.Silica gel has such advantages as low cost and ease of preparation, and since the reaction is heterogeneous in nature, the catalyst can conveniently be separated with simple filtration.Due to its solubility in water, hydrogen peroxide is a very convenient oxidizing agent.The reaction yield is high, with water as the only by-product, which is environmentally friendly.The catalyst (acidic silica gel) is stable, and can easily be prepared by adding silica gel to aqueous hydrochloric acid and evaporating the solvent.Different types of alcohols were subjected to oxidation with hydrogen peroxide using acidic silica gel as catalyst.The oxidation of alcohols is carried out in acetonitrile as solvent, and the workup simply requires extraction and separation of the heterogeneous catalyst through filtration.Removal of solvent followed by purification of products aldehydes and ketones in high yields.The respective reaction times and yields are given in Table 1.As shown in Table 1, benzylic alcohols are oxidized to corresponding carbonyl compounds in good yields, but the yields for secondary alcohols (Entry 12, 13) are lower using this condition.Some experiments were performed in order to show the utility of the methodology.The oxidation of benzylic alcohols to the corresponding carbonyl compounds was found to be very slow at room temperature, while it could be carried out more efficiently in refluxing acetonitrile.A reaction condition is optimized not only in terms of solvent and temperature, but also with respect to the amount of acidic silica gel and oxidant.In select solvents such as CH2Cl2, THF, DMF and water low yields were obtained.CH3CN was found to be the most suitable solvent for this transformation with the shortest reaction time and good yields (Table 2).

Table 1. Oxidation of alcohols by hydrogen peroxide in the presence
The optimum temperature was 80 o C, and the optimum amount of SiO2-HCl was 0.5 gram for 1 mmole alcohols.No Reactions happened at room temperature or without SiO2-HCl; thus, only the initial materials were recovered.The real catalyst was found to be HCl, where silica gel served only as a shuttle for delivering HCl in solution.Furthermore, when the reaction runs under an inert atmosphere of argon without atmospheric oxygen, it renders good yields of products, which repel oxidation with oxygen.Finally, oxidation of benzylic alcohols over acidic silica gel with O2 under the same reaction condition resulted only in the initial materials.After the activation the catalyst can be used.Accordingly, the mechanism of alcohol oxidation to carbonyl compounds through hydrogen peroxide acidic silica gel is proposed in scheme 2.

Conclusion
As demonstrated above, in this paper, we have successfully extended the application of hydrogen peroxide and acidic silica gel for the oxidation process of alcohols.Due to its simplicity of performance, metal-free oxidation, good yields, and the wide range of benzylic alcohols that can be converted into their corresponding aldehydes and ketones, this method offers a lot of advantages over its rivals, and yet the silica gel can ultimately be recycled after activation.

Experimental
The products were characterized by comparing their physical and spectral data against those obtained from authentic samples.All yields refer to isolated products.IR and NMR spectra were recorded on Perkin Elmer 781, and Broker DPX500.The progress of reactions was monitored by TLC.

Preparation of the catalyst: Acidic silica gel
Chromatographic Silica gel (1 g, 70-230 mesh) was added to 1 mL conc.HCl and stirred at room temperature for 10 min.The mixture was evaporated and dried in vacuum [20].

Typical Experimental Procedure
To a stirred solution of 1 mmol alcohols in 2 mL acetonitrile, 0.5 g acidic silica gel was added.Then 0.5 mL of H2O2 30% was gradually added to the same solution over a period of 5 min.The resulting mixture was stirred under reflux of acetonitrile for the time indicated in Table 1.After completion of the reaction (TLC, ethyl acetate/petroleum ether; 1:4), the heterogeneous mixture was filtered and thoroughly washed with the acetonitrile (2×10 ml).The solvent was evaporated, and the product was purified through short column chromatography over silica gel.

Table 2 .
Optimization of reaction condition a Isolated yield; b without HCl; c recovered SiO2-HCl; d reaction run at rt.