Catalytic air oxidation of olefins using molybdenum dioxo complexes with dissymmetric tridentate O,N,S-donor Schiff base ligands derived from o-hydroxyacetophenone and S-benzyldithiocarbazate or S-methyldithiocarbazate

We report the homogeneous catalytic air oxidation of 1-hexene, cyclohexene and styrene using CiS-[MoO2(hap-SBDTC)(solv)] (1b) and CiS-[MoO2(hap-SMDTC)(solv)] (2b), where hap-SBDTC and hap-SMDTC are Schiff bases derived from o-hydroxyacetophenone (hap) and S-benzyldithiocarbazate (SBDTC) or S-methyldithiocarbazate (SMDTC), respectively. Both hap-SBDTC and hap-SMDTC are dissymmetric tridentate 0,N,S-donor Schiff base ligands. The catalytic tests were performed in DMF solvent at 60 degrees C under 1 atm O-2. The olefin conversion was determined using gas chromatography. The percentage conversion of the above-mentioned substrates at the end of 6 h was in the range 86-98%. The final oxidation products were found to be 1-hexanal for 1-hexene, styrene epoxide and phenyl acetaldehyde (81:19) for styrene and cyclohexene epoxide and 2-cyclohexen-1-ol (85:15) in the case of cyclohexene. The oxidation reaction typically followed pseudo-order kinetics; however, a two-stage first order reaction is evident with complex 2b. This is attributed to less steric and electron-donating methyl substitution on S in 2b that possibly imparted a higher reactivity accompanying the formation of an intermediate in a relatively faster reaction step prior to the formation of final oxidation products. A reaction mechanism that explains the experimental results is proposed. (c) 2007 Elsevier B.V. All rights reserved.