Replacement of one OH group in TADDOL ( a,a,a',a'-tetraaryl-1,3-dioxolane-4,5-dimethanol) by an OOH group gives a stable, crystalline chiral hydroperoxy alcohol TADOOH ( {(4R,5R)-5-[(hydroperoxydiphenyl)methyl]-2,2-dimethyl-1,3-dioxolan-4-yl}diphenylmethanol) 3, the crystal structure of which resembles those of numerous other TADDOL derivatives (Fig. 2). The new hydroperoxide was tested as chiral oxidant in three types of reactions: the epoxidation of enones with base catalysis (Scheme 2), the sulfoxidation of methyl phenyl sulfide (Scheme 3), and the Baeyer-Villiger oxidation of bicyclic and tricyclic cyclobutanones, rac-10a ± d with kinetic resolution (Scheme 4, Fig. 3, and Table). Products of up to 99% enantiomer puritiy were isolated (the highest values yet observed for oxidations with a chiral hydroperoxide!). Mechanistic models are proposed for the stereochemical courses of the three types of reactions (Schemes 5 and 6, and Fig. 4). Results of AM1 calculations of the relative transition-state energies for the anionic rearrangements of the exo Criegee adducts of TADOOH to the enantiomeric bicyclo[3.2.0]heptan-6-ones are in qualitative agreement with the observed relative rates (Table and Fig . 5).
A new solid-sate donor-acceptor system based on periodic mesoporous organosilica (PMO) has been constructed. Viologen (Vio) was covalently attached to the framework of a biphenyl (Bp)-bridged PMO. The diffuse reflectance spectrum showed the formation of charge-transfer (CT) complexes of Bp in the framework with Vio in the mesochannels. The transient absorption spectra upon excitation of the CT complexes displayed two absorption bands due to radical cations of Bp and Vio species, which indicated electron transfer from Bp to Vio. The absorption bands slowly decayed with a half-decay period of approximately 10 mus but maintained the spectral shape, thereby suggesting persistent charge separation followed by recombination. To utilize the charge separation for photocatalysis, Vio-Bp-PMO was loaded with platinum and its photocatalytic performance was tested. The catalyst successfully evolved hydrogen with excitation of the CT complexes in the presence of a sacrificial agent. In contrast, reference catalysts without either Bp-PMO or Vio gave no or little hydrogen generation, respectively. In addition, a homogeneous solution system of Bp molecules, methylviologen, and colloidal platinum also evolved no hydrogen, possibly due to a weaker electron-donating feature of molecular Bp than that of densely packed Bp in Bp-PMO. These results indicated that densely packed Bp and Vio are essential for hydrogen evolution in this system and demonstrated the potential of PMO as the basis for donor-acceptor systems suitable for photocatalysis.
diastereoselective syntheses, enantioselective syntheses diastereoselective syntheses, enantioselective syntheses (incl. cis/trans-isomerism) O 0031
-035Preparation of TADOOH, a Hydroperoxide from TADDOL, and Use in Highly Enantioface-and Enantiomer-Differentiating Oxidations.-Chlorination of TADDOL, followed by reaction with urea-hydrogen peroxide complex, generates the title hydroperoxide TADOOH, a powerful reagent for asymmetric oxidation reactions. TADOOH is successfully applied in the base-catalyzed asymmetric epoxidation of enones (I) and (III), in the chemoand stereoselective oxidation of sulfide (V) to the sulfoxide (VI) and in the Baeyer-Villiger oxidation of cyclic ketones (VII) to lactones. All three types of oxidation proceed with moderate to excellent enantioselectivities (up to 99% e.e.). The mode of stereoinduction is studied using mechanistic models for each reaction. -(AOKI, MASAO; SEEBACH, DIETER; Helv. Chim. Acta 84 (2001) 1, 187-207; Lab. Org. Chem., ETH, CH-8092 Zuerich, Switz.; EN)
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