双氮席夫碱配体对甲基三氧化铼催化烯烃环氧化反应的影响

 利用 2-吡啶甲醛、6-甲基-2-吡啶甲醛或 6-异丙基-2-吡啶甲醛与对甲基苯胺缩合制得双氮席夫碱配体, 考察了席夫碱配体以及溶剂和温度对甲基三氧化铼 (MTO) 催化不同结构烯烃环氧化反应的影响. 结果表明, 这些席夫碱配体与 MTO 构成的催化剂体系在甲醇溶剂中的催化性能最好, 双氮配体能显著提高环氧化反应的选择性. 当以甲醇为溶剂, 环己烯为底物, 在 –10 oC 反应 12 h 时, 环己烯转化率和环氧化物选择性均可达 100%. 席夫碱的配位能力越强, 越有利于提高环氧化物选择性, 而其配位能力取决于吡啶环中 6-位取代基的电子和立体结构. 给电子能力较强和空间位阻较小的烷基对应的配体的配位能力较强.

Effects of Di-nitrogen Schiff Base Ligands on Olefin Epoxidation Catalyzed by Methyltrioxorhenium

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.