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Postępy w stereoselektywnym utlenianiu sulfidów

Skarżewski J., Ostrycharz E.

Wiadomości Chemiczne

2000

[Z] 54, 9-10

725-758

The preparation of optically active sulfoxides, useful chiral auxiliaries and synthetic building blocks is reviewed. Beginning with the progresses in the non-oxidative synthesis of chiral sulfoxides, the main part of the survey is devoted to the advances in stereoselective oxidation of the easily available prochiral sulfides. Firstly, the diastereoselective oxidation of sulfides containing other chiral center is discussed. Remarkable selectivities were obtained in the oxidation with MCPBA, t-BuOCl, and (NaOCl, cat. TEMPO). In the next part the enantioselective oxidation processes are considered. The material is organized according to the character of the oxidant: chiral-stoichiometric methods and chiral catalytic methods. Among stoichiometric chiral oxidants, some of the most important are the Davis oxaziridines. An essential improvement in enantioselectivity was gained by the application of N-(phenylsulfonyl)-3,3-dichlorocamphoryloxaziridine, [(8,8-dimethoxycamphor)sulfonyl]oxaziridine and ethylene ketal of the respective 8-oxocamphor derivative. Much progress was also observed in oxidations with the Ti(IV)-supported chiral reagents. The Kagan and Modena methods were further elaborated. The highest e.es were observed using Ti(Oi-Pr)4/DET/H2O in 1:2:1 ratio [33] or Ti(Oi-Pr)4/DET/i-PrOH (1:4:4) + molecular sieves, both with cumyl hydroperoxide. The non-linear enantioselectivity effects (NLE) were found for these systems as a proof for involvement of the chiral dimeric Ti-species. Uemura made an important modification to this oxidant by replacement of diethyl tartrate with Binol. Also chiral 1,2-diols and C3-symmetric triols were used as chiral inducers with the Ti- supported system. The other approach exploited chiral hydroperoxide obtained via biooxidation of hydrocarbon. Thus prepared oxidant gave enantioselective sulfoxidation with ca. 80 % e.e. In the field of catalytic sulfoxidations an important advancement was made by Bolm and Binewald who discovered the system active even at the 0.01 % mol level of chiral inducer. The catalyst was in situ formed from vanadyl acetylacetonate and (S)-(-)-N-(3,5-di-tert-butylsalicylidene)tert-leucinol and 30% aqueous hydrogen peroxide was applied as a stoichiometric oxidant. The results attained were within 70-80 % e.e. level for simple alkyl aryl sulfoxides, [57] and over 91 % e.e. for the oxidation of di(tert-butyl) disulfide [59]. The catalytic system was further optimized and 75-95 % e.es were observed for the oxidation of thioanisole, 2-aryl-1,3-dithianes and 1,2-bis(arylthio)ethanes [61]. The other successful catalytic oxidant was the Uemura system (Ti(Oi-Pr)4/Binol/H2O, 5 % mol of Binol). In this version the system gave truly enantioselective oxygen transfer (98 % e.e.). Many papers have been published on the application of the biological oxidants and the use of such methods is a feasible alternative to the chemical ones. In particular, the microbiological systems containing fungi Helmithosporium were successfully applied for the preparative oxidation of prochiral thioketals and thioacetals, which could not be oxidized by purely chemical means. Also various bacterial strains of Pseudomonas sp. were used and, e.g. hexyl methyl and cyclohexyl methyl sulfides were oxidized to the respective sulfoxides with high enantioselectivity. Bacterial cyclohexanone monooxygenase was studied most widely among all the purified enzymes used in sulfoxidations. Asymmetric sulfoxidation was also investigated with hydrogen peroxide and vanadium bromoperoxidase and high enantioselectivities were obtained particularly for cyclic sulfides and those having a carboxylic group.