Oxidation of Cyclohexene with H2O2 Catalyzed by an Oxovanadium(IV) Quinoxaline Schiff Base Complex

Introduction: A new heterocyclic oxovanadium(IV) quinoxaline Schiff base complex, ((VO)L2) with a square pyramidal geometry around vanadium (IV) (where ‘L’ is the azomethine Schiff base formed by the condensation of 3-hydroxyquinoxaline-2-carboxaldehyde with 2-amono-phenol), has been prepared and characterized. The elemental analysis, molar conductance, magnetic measurement, FT-IR, UV-Visible, EPR, TG-DTA-DTG and cyclic voltametric studies confirmed the coordination of oxovanadium with the two molecules of ligands. Catalytic activity of the oxo-vanadium complex was tested in the liquid phase oxidation of cyclohexene with H2O2 as the oxi-dant. The catalyst has a turn over frequency 251 h-1 with 7.50 × 10-6 mol catalyst, 18.170 × 10-3 mol of H2O2 at 80°C and the cyclohexene conversion was estimated to be 19.10% with product selectiv-ity of cyclohexene oxide (6%), cyclohexanol (34%) and cyclohexanone (60%). Catalytic results yield cyclohexenol and cyclohexenone as the major products, along with a minor cyclohexene oxide product. These results suggest both the allylic C−H bond and the olefinic C=C bond activation dur-ing the process of catalysis by the complex. Methods: Condensation reaction of 3-hydroxyquinoxaline-2-carboxaldehyde and 2-aminophenol in the presence 2 or 3 drops of concentrated HCl resulted in the formation of the azomethine Schiff base. The complexation of the Schiff base with vanadium and the geometry of the resulting complex were determined by employing various physico-chemical measurements like elemental analysis, molar conductance, magnetic measurement, FT-IR, UV-Visible, EPR, TG-DTA-DTG and cyclic voltametry. Catalytic property of the synthesized oxovanadium(IV) quinoxaline Schiff base com-plex in cyclohexene oxidation was studied in a simple glass reactor and the progress of the reaction was monitored gas chromatographically. Results: A square-pyramidal oxovanadium(IV) quinoxaline Schiff base complex, ((VO)L2), was synthesized and characterized. Catalytic oxidation of cyclohexene with the complex gave cyclohex-enol and cyclohexenone as the major products, along with minor cyclohexene oxide. The experi-mental findings predict the formation of both allylic C−H bond and the olefinic C=C bond activation during catalysis. Conversion of cyclohexene, product selectivity and H2O2 efficiency were calcu-lated under various reaction conditions. Conclusion: A new square-pyramidal oxovanadium(IV) quinoxaline complex with the general for-mula (VOL2) was prepared and characterized by elemental analysis, molar conductance, magnetic measurement, FT-IR, UV-Visible, EPR, TG-DTA-DTG and cyclic voltametric studies. The utility of the oxovanadium(IV) complex as a catalyst in the oxidation of cyclohexene in acetonitrile with H2O2 was studied under various reaction conditions. Catalytic observations predicted the formation of cyclohexenol and cyclohexenone as the major reaction products, along with minor cyclohexene oxide. Cyclohexene conversion, product selectivity and H2O2 efficiency values were calculated for this reaction under various conditions. Under the optimized reaction conditions, the oxovanadium complex catalyzed the cyclohexene oxidation with 19.10 % conversion with a turnover frequency of 251 h-1. conclusion: Catalytic activity of a new oxovanadium (IV) quinoxaline Schiff base complex in the liquid phase oxidation of cyclohexene with H2O2 as oxidant was studied. Under the optimized reaction conditions at 30 % HP (18.70 × 10^(-3) mol), temperature (80 °C), time (2 h), catalyst (7.50 × 10^(-6) mol), and cyclohexene (1.97 × 10^(-2) mol) and acetonitrile (30 mL), the complex showed 19.10 % cyclohexene conversion.