二溴和二碘取代的手性Schiff碱-钒(Ⅳ)络合物催化硫醚不对称氧化

Asymmetric Oxidation of Sulfides Catalyzed by Vanadium(Ⅳ) Complexes of Dibromo-and Diiodo-Functionalized Chiral Schiff Bases

由廉价的手性氨基醇与3,5-二溴或3,5-二碘水杨醛缩合得到配体,配体与VO(acac)2按一定比例络合形成络合物催化剂,考察了室温下该催化剂对芳基甲基硫醚不对称氧化反应的催化性能.结果表明,当VO(acac)2/配体摩尔比为1/2,并且以H2O2作为氧源时,催化剂具有较高的活性和中等至很高的对映选择性.与(S)-苯丙氨醇和(R)-亮氨醇衍生得到的配体相比,由(S)-缬氨醇得到的配体具有更高的对映选择性.在缓慢滴加H2O2的条件下,以3,5-二碘水杨醛和(S)-缬氨醇缩合得到的Schiff碱为配体,以苯甲硫醚和对溴苯甲硫醚为底物时,产物的ee值分别为88%和92%.研究表明,与Fe(acac)3/Schiff碱体系不同,向VO(acac)2/Schiff碱催化体系中加入羧酸或羧酸盐类化合物并不能改善催化剂的催化性能.

The catalyst system of VO（acac）2 and Schiff base ligands derived from 3,5-dibromo- or 3,5-diiodosalicylaldehyde and inexpensive chiral amino alcohols was prepared. This catalyst displayed good yields and moderate to high enantioselectivity for the asymmetric oxidation of aryl methyl sulfides at room temperature when 1% catalyst （VO（acac）2/ligand molar ratio of 1：2） and H2O2 oxidant were used. The ligands derived from （S）-valinol exhibited considerably higher enantioselectivity than those ligands derived from （S）-phenylalaninol and （R）-leucinol. The enantiomeric excess values were improved up to 88% for methyl phenyl sulfoxide and 92% for methyl p-bromophenyl sulfoxide by slow dropwise addition of H202 with the ligand prepared from 3,5-diiodosalicylaldehyde and （S）-valinol. The present study showed that the catalytic efficiency of VO（acac）2/Schiff base systems could not be improved by the addition of carboxylic acids or carboxylate salts.