Aerobic oxidation and oxidative bromination in aqueous medium using polymer anchored oxovanadium complex
Graphical abstract
The aerobic oxidation of alcohols in aqueous medium is efficiently catalysed by polymer anchored oxovanadium catalyst. It was also effective for the oxidative bromination reaction with 90–100% selectivity of mono substituted products. This catalyst was reusable upto six times without significant loss of its initial activity.
Introduction
The selective aerobic oxidation and oxidative bromination of organic compounds are useful and fundamental transformations in organic synthesis [1], [2]. In recent years, there has been an increasing interest in developing environmental friendly greener processes which are also economically viable in industrial chemistry [3]. With environmental and economic concerns, heterogeneous catalysis reaction using molecular oxygen as an oxidant is developed as desired green process for selective oxidation of alcohol and oxidative bromination of various organic substrates [4], [5], [6], [7], [8], [9], [10].
Up to now, numerous expensive metal (e.g. palladium and ruthenium) catalyst and toxic organic solvents have been traditionally used to accomplish most of the aerobic oxidation of alcohols [11], [12], [13]. On other hand, conventional bromination reaction requires hazardous and toxic elemental bromine [14], [15], [16]. In order to overcome these problems, several safe systems for the aerobic oxidation and oxidative bromination have been developed. One is the use of inexpensive metal catalyst for the aerobic oxidation and use of bromide ion as a bromide source instead of bromine for oxidative bromination [17], [18], [19], [20], [21].
We have chosen polymer anchored vanadium complex as the heterogeneous catalyst and water as the green solvent for the aerobic oxidation and oxidative bromination. Vanadium exists on the surface of the earth more abundantly than copper, palladium or ruthenium [22]. It plays an important role in aerobic oxidation and oxidative bromination processes. In addition, heterogeneous catalysts have some advantages compared to homogeneous catalyst. Metals supported on materials such as alumina [23], amorphous silicates [24], polymers [25], zeolites [26], and MCM-41 [27] are commonly in use in heterogeneous catalysis. Nowadays functionalized polystyrene anchored catalysts are used to carry out various catalytic organic transformations [28], [29], [30], [31]. Among them chloromethylated polystyrene are widely used as polymer support. These polystyrene anchored metal complexes are inert and reusable catalysts for various organic reactions.
In the chemical processes, traditional organic solvents are used in large quantities, which have led to various environmental and health concerns. As part of green chemistry efforts, a variety of cleaner solvents have been used as replacements [32], [33], [34], [35]. As an alternative solvent, water has been paid extraordinary attention. Apart from being a chemically interesting solvent, water provides a cheap alternative for organic solvents, making it environmentally and economically interesting as well. So, based on the above two concepts, we tried to synthesize a polymer anchored oxovanadium complex which catalyse the aerobic oxidation of primary and secondary alcohols and oxidative bromination of various aromatic amines, aldehydes in aqueous medium.
Herein we report the synthesis and characterization of a polymer anchored oxovanadium complex and illustrate its application for the aerobic oxidation and oxidative bromination reactions in aqueous medium using hydrogen peroxide as an oxidant.
Section snippets
Materials and instruments
All the reagents used were chemically pure and were of analytical reagent grade. The solvents were dried and distilled before use following the standard procedures [36]. Chloromethylated polystyrene was supplied by Sigma–Aldrich chemicals Company, USA. Other reagents were obtained from Merck or Fluka. A Perkin–Elmer 2400C elemental analyzer was used to collect micro analytical data (C, H and N). Vanadium content of the sample was measured by Varian AA240 atomic absorption spectrophotometer
Characterization of polymer anchored oxovanadium complex
The reaction of aldehyde functionalised polystyrene and triethylenetetramine (teta) in methanol leads to the formation of polymer anchored Schiff base ligand. During this process CHO group of the polymer reacts with amine group of the triethylenetetramine (teta) moiety as shown in Scheme 1. The polymer anchored Schiff base ligand, on reaction with VO(acac)2, gave an oxovanadium(IV) complex which we designate as PS-teta–VO. The physicochemical data of the polymer anchored ligand and polymer
Comparison with other reported system
Oxidation of primary, secondary alcohols and oxidative bromination of various aromatic substrates under heterogeneous conditions over a variety of catalysts has been studied (Table 8). Table 8 provides a comparison of the results obtained for our present catalytic system with those reported in the literature [1], [3], [6], [40], [41], [42]. From Table 8, it can be concluded that the present catalyst follows a green pathway and shows excellent catalytic activities as compared to other reported
Test for heterogeneity
The leaching of vanadium from polymer anchored VO complex was confirmed by analysis of the used catalyst (FTIR) as well as the product mixtures (AAS and UV–Vis). Analysis of the used catalyst did not show appreciable loss in the vanadium content as compared to the fresh catalyst. IR spectrum of the recycled catalyst was quite similar to that of fresh sample indicating the heterogeneous nature of this complex. Analysis of the product mixtures showed that if any vanadium was present it was below
Recycling of catalyst
The catalyst remains insoluble in the present reaction conditions and hence can be easily separated by simple filtration followed by washing. The catalyst was washed with dichloromethane and dried at 100 °C. Oxidation of benzyl alcohol was carried out with the recycled catalyst under the optimized reaction conditions. The catalyst was recycled in order to test its activity as well as stability. The obtained results are presented in Fig. 6. As seen from Fig. 6, the recycled catalyst did not show
Conclusion
In the present work, we have developed and characterized an efficient polymer anchored oxovanadium Schiff base complex as the catalyst for the selective aerobic oxidation of primary and secondary alcohols to the corresponding carbonyl compounds via ideal green process, i.e. water as the green solvent and with molecular oxygen as oxidant. This catalyst also shows excellent catalytic activity in oxidative bromination reaction in water at room temperature. Another important factor is the stability
Acknowledgements
SMI acknowledges Department of Science and Technology (ref no: SB/S1/PC-107/2012), Council of Scientific and Industrial Research (02(0041)/11/EMR-II) and UGC, New Delhi, Govt. of India for financial support. RAM acknowledges UGC, New Delhi, India (F no 39-847/2010(SR)) for his Maulana Azad National Fellowship (F1-17.1/2012-13/MANF-2012-13-MUS-WES-9628/SA-III). ASR acknowledges CSIR, New Delhi, India for his senior research fellowship. NS acknowledges UGC, New Delhi, India for his senior
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H bond functionalization reaction.