28664-08-6Relevant articles and documents
Water-resistant solid Lewis acid catalysts: Meerwein-Ponndorf-Verley and Oppenauer reactions catalyzed by tin-beta zeolite
Corma, Avelino,Domine, Marcelo E.,Valencia, Susana
, p. 294 - 304 (2007/10/03)
The catalytic activity of Sn-beta zeolite in the Meerwein-Pondorf-Verley reduction of carbonyl compounds with secondary alcohols as reductants and Oppenauer oxidation of alcohols were performed with quantitative yields to the corresponding product. The catalyst had an excellent activity and selectivity even after four catalytic recycles, and good stereoselectivities to the thermodynamic less favorable cis-alcohol isomer when alkyl-cyclohexanones were used as substrates. A prochiral ketone was reduced within an enantiomeric excess close to 50% when using a chiral alcohol as the reducing reactant. IR studies using cyclohexanone as probe molecule over beta zeolites showed that the more specific Lewis acid sites in the framework of Sn-beta were responsible for its better catalytic activity with respect to Ti- or Al-beta. The order of hydrophobicity of the samples was Ti-beta > Sn-beta > Al-beta. Ti-beta and Sn-beta retained a higher percentage of catalytic activity when water was present in the reaction media. Even with ～ 4 wt % of H2O (0.2 g) in the media, Sn-beta yielded a higher turnover number than Ti- or Al-beta when working in the absence of water.
Ion-molecule complexes in 1,2 alkyl shifts
Gappa, Andrea,Herpers, Ekkehard,Herrmann, Roland,Hülsewede, Volker,Kappert, Wilhelm,Klar, Matthias,Kirmse, Wolfgang
, p. 12096 - 12106 (2007/10/03)
The internal return of neutral leaving groups was studied in rearrangements of polycyclic systems (2-norpinyl → 2-norbornyl, endo- → exo-tricyclo[5.2.1.02.6]dec-8-yl, bicyclo[3.2.0]hept-2-yl → 7-norbornyl, and 4-protoadamantyl → 2-adamantyl). Acid catalysis was applied to 18O-labeled alcohols in aqueous organic solvents, to alcohols in methanol, and to ethers R-O-R′ in alcohols R″-OH. The leaving group was found to attack the migration origin in competition with solvent molecules. Return:exchange ratios were obtained from product distributions, either directly or by kinetic simulation (in cases of partial exchange prior to rearrangement). If departure and return of the leaving group occur on the same side of the carbon framework, return:exchange ratios ranging from 1 to 11.5 were observed. Less internal return was found for bridged than for open carbocations. Migration of the departing molecule to the opposite face (exo ? endo) or to a β carbon is a minor process (return:exchange ～ 0.1), in accordance with previous reports on inverting displacements and allylic 1,3 shifts. These data are rationalized in terms of short-lived ion-molecule (ion-dipole) complexes whose collapse competes with ligand exchange.
