Asymmetric Oxidation Of Sulfides Catalyzed By In-situ Generated V And Ti Complexes Of Chiral Schiff Bases

Optically chiral sulfoxides are important chiral auxiliary reagents and chiral synthons in asymmetric synthesis, the bioactive group of many drugs, and also used as chiral ligands in enantioselective catalysis. Therefore, the preparation of chiral sulfoxides has significant academic and application values. During the last twenty years, various methods have been developed for the preparation of chiral sulfoxides. The asymmetric oxidation of sulfides catalyzed by chiral Schiff base-metal (vanadium and iron) systems has been considered as one of the most promising methods due to the following advantages:using hydrogen peroxide as oxidant, under mild conditions, and in high yields.In this thesis, the Schiff base ligands, (S)-2-(N-(3,5-bis(a,a-dimethylbenzyl) salicylidene)amino)-3-methyl-1-butanol (L3), (S)-2-(N-(3,5-bis(α,α-dimethylbenzyl)salicylic dene)amino)-3,3-dimethyl-l-butanol (L4), and (S)-2-(N-(3,5-bis(a,a-dimethylbenzyl)salicyl idene)amino)-3-phenyl-l-propanol (L5), were prepared from 3,5-dicumenylsalicylaldehyde and inexpensive chiral amino alcohols. All prepared Schiff base ligands were characterized by IR,1H NMR, 13C NMR and MS. It was found that the catalyst system of VO(acac)2/L4 displayed excellent catalytic activity under optimal reaction conditions. When the loading amount of the precatalyst was decreased from 1 mol% to 0.1 mol%, the yield of sulfoxides still remained 75% and the enantioselectivity was 53% ee.The catalytic properties of the catalyst systems of Ti(Oi-Pr)4/(L3, L4 and L5) were studied by varying reaction conditions, such as oxidants, solvents, the ratios of ligands and Ti(Oi-Pr)4, the loading amount of the precatalyst and the temperature. The catalytic results were compared with those obtained from the catalyst system of Ti(Oi-Pr)4 and the conventional ligands (S)-2-(N-(3,5-ditertbutylsalicylidene)amino)-3-methyl-l-butanol (L1) and (S)-2-(N-(3,5-ditert butylsalicylicdene)amino)-3,3-dimethyl-1-butanol (L2). As a result, the enantiomeric excess of (S)-sulfoxide was up to 72.8% and the yield was 88.6% for the catalyst system of Ti(Oi-Pr)4/L4 in oxidation of thioanisole using H2O2 as oxidant in CH2Cl2 at 0℃. The experimental evidence suggests that enantioselectivity of the sulfoxides obtained is directly generated from asymmetric oxidation of sulfides, instead of the further kinetic resolution of sulfoxides.