Oxidation of p-chlorotoluene and cyclohexene catalysed by polymer-anchored oxovanadium(iv) and copper(ii) complexes of amino acid derived tridentate ligands

Abstract

3-Formylsalicylic acid (Hfsal), covalently bound to chloromethylated polystyrene (PS) and cross-linked with 5% divinylbenzene reacts with D,L-alanine and L-isoleucine to give the Schiff-base tridentate ligands PS–H₂fsal–D,L-Ala and PS–H₂fsal–L-Ile, respectively. These anchored ligands upon reaction with VOSO₄ and Cu(CH₃COO)₂·H₂O form the complexes PS–[VO(fsal–D,L-Ala)(H₂O)], PS–[Cu(fsal–D,L-Ala)(H₂O)], PS–[VO(fsal–L-Ile)(H₂O)] and PS–[Cu(fsal–L-Ile)(H₂O)]. The structures of these immobilized complexes have been established on the basis of scanning electron micrographs, spectroscopic (infrared, electronic and EPR), thermogravimetric and elemental analysis studies. The oxidation of p-chlorotoluene and cyclohexene has been investigated using these complexes as the catalysts in the presence of H₂O₂ as the oxidant. Reaction conditions have been optimised by considering the concentration of the oxidant, the amount of catalyst used and the temperature of the reaction mixture. Under the optimised conditions, p-chlorotoluene gave a maximum of 14% conversion using PS–[VO(fsal–D,L-Ala)(H₂O)] as the catalyst, with the main products having a selectivity order of: p-chlorobenzaldehyde >> p-chlorobenzylalcohol > p-chlorobenzoic acid > 2-methyl-5-chlorophenol > 3-methyl-6-chlorophenol. The oxidation of cyclohexene with PS–[VO(fsal–D,L-Ala)(H₂O)] proceeds with 79% conversion, which is followed by PS–[VO(fsal–L-Ile)(H₂O)] with 77% conversion, and the oxidation of cyclohexene by Cu-based catalysts occurs with considerably lower conversions (29–32%). The selectivity of the products follows the order: 2-cyclohexene-1-ol > cyclohexene oxide > cyclohexane-1,2-diol > 2-cyclohexene-1-one. Recycling studies indicate that these catalysts can be reused at least three times without any significant loss in their catalytic potential. However, EPR studies indicate that while the polymer supported V(IV)O-complexes do not change after being used, the EPR spectra of the Cu-complexes show significant changes. The corresponding non-polymer bound complexes [VO(fsal–D,L-Ala)(H₂O)], [Cu(fsal–D,L-Ala)(H₂O)], [VO(fsal–L-Ile)(H₂O)] and [Cu(fsal–L-Ile)(H₂O)] have also been prepared in order to compare their spectral properties and catalytic activities. The non-polymer bound complexes exhibit lower conversion, along with lower turn-over frequency as compared to their polymer-bound analogues. Several EPR, ⁵¹V NMR and UV-vis studies have been undertaken to detect the intermediate species, and outlines for the mechanisms of the catalytic reactions are proposed.