Abstract: The di-nitrogen Schiff base ligands were synthesized by the condensation of 4-methylaniline with 2-pyridinecarboxaldehyde, 6-methyl-2-pyridinecarboxaldehyde, or 6-isopropyl-2-pyridinecarboxaldehyde, respectively. The Schiff base ligands were used as the additives for the epoxidation of three olefins, cyclohexene, styrene, and 1-octene, catalyzed by methyltrioxorhenium (MTO) with 30% H₂O₂ as oxidant. The effects of the solvent, reaction temperature, and structure of the Schiff base ligands on the epoxidation were investigated. Methanol was the best solvent for the catalytic system made up of above Schiff base ligands and MTO. The addition of the Schiff base ligands increased the epoxide selectivity remarkably. Both the conversion of cyclohexene and the selectivity of epoxide could reach up to 100% when the reaction was run at –10 ^oC for 12 h in methanol. The strong coordination capacity of the Schiff base is in favor of the augmentation of the selectivity for epoxide in the reaction. For these Schiff base ligands, the coordination capacity was determined by the electronic and steric properties of the substitute at 6-position in the pyridine ring. The alkyl group with small steric effect and strong electron donation could increase the coordination capacity, which increased the selectivity for the epoxidation reaction.

Key words: methyltrioxorhenium, 2-pyridinecarboxaldehyde, Schiff base, olefin, epoxidation