Oxidation of phenol and hydroquinone catalysed by copper(II) and oxovanadium(IV) complexes of N,N′-bis(salicyledene)diethylenetriamine (H2saldien) covalently bonded to chloromethylated polystyrene

Reaction between N,N′-bis(salicyledene)diethylenetriamine (H2saldien) covalently bonded to chloromethylated polystyrene cross linked with 5% divinylbenzene (abbreviated as PS-H2saldien, I) and aqueous potassium vanadate at ca. pH 6.5 results in the formation of polymer-anchored complex PS-[VO(saldien)] (1). Cupric acetate on reaction with I in methanol gives PS-[Cu(saldien)] (2). Formations of these complexes were confirmed by IR and electronic spectroscopic techniques, elemental and thermo gravimetric analyses and scanning electron micrographs. Catalytic potential of these complexes have been tested for the oxidation of phenol and hydroquinone using 30% H2O2 as an oxidant. Various reaction parameters such as substrate to oxidant ratio, concentration of catalysts and solvent of the reaction medium have been optimised to get maximum oxidation of phenol to p-benzoquinone selectively. Under optimised conditions, i.e. phenol (1.88 g, 20 mmol), H2O2 (4.56 g, 40 mmol), PS-[Cu(saldien)] (30 mg), temperature (70 °C) and water (10 ml), the phenol conversion was found to be 22.2% and percentage yield of the main products p-benzoquinone and catechol are 14.4% and 7.8%, respectively. PS-[VO(saldien)] exhibits only 3% conversion in water with 100% selectivity towards benzoquinone. Both the catalysts become more selective towards catechol formation along with higher conversion in acetonitrile. The selectivity of p-benzoquinone was also found to be dependent on volume and nature of solvent, and temperature. Hydroquinone oxidation was also found to be pH dependent; carbonate buffer gives quantitative conversion within 30 min at 1:6 substrate to oxidant ratio.

Graphical abstractOxidation of phenol and hydroquinone catalysed by polymer-anchored complexes, PS-[VO(saldien)] and PS-[Cu(saldien)] in water and acetonitrile are reported. Figure optionsDownload full-size imageDownload as PowerPoint slide