Catalytic oxidation of secondary alcohols by molybdenum complexes derived from 4-acyl pyrazolone in presence and absence of an N-based additive: Conventional versus microwave assisted method

Abstract

Four dioxidomolybdenum(VI) complexes, [MoVIO2(bp-bhz)(MeOH)] (1), [MoVIO2(bp-inh)(MeOH)] (2), [MoVIO2(bp-nah)(MeOH)] (3) and [MoVIO2(bp-fah)(MeOH)] (4) have been isolated with ONO tridentate Schiff base ligands, H2bp-bhz (I), H2bp-inh (II), H2bp-nah (III) and H2bp-fah (IV) [where ligands I–IV are derived from 4-benzoyl-3-methyl-1-phenyl-2-pyrazoline-5-one (Hbp) and isonicotinoyl hydrazide (Hinh), benzoyl hydrazide (Hbhz), nicotinoyl hydrazide (Hnah) or furoyl hydrazide (Hfah), respectively] and characterized. Structures of complexes [MoVIO2(bp-bhz)(MeOH)] (1) and [MoVIO2(bp-nah)(DMSO)]·0.5H2O (3a) have been confirmed by single crystal X-ray study. Using conventional and microwave-assisted methods, these complexes have been tested as catalysts for the oxidation of secondary alcohols (1-phenylethanol, 2-propanol and 2-butanol) in the presence of 30% H2O2 as an oxidant. Various parameters such as amounts of catalyst, oxidant, solvent and temperature of the reaction mixture have been taken into consideration for the maximum conversion of substrates. Under the atmospheric optimized reaction conditions, secondary alcohols give high yield of the corresponding ketones selectively. Addition of an N-based additive reduces the reaction time considerably. Microwave irradiation effect on the reaction time and yield has been taken into consideration to conclude that microwave method is more efficient compared to conventional liquid phase method.