79702-79-7Relevant academic research and scientific papers
New Heterogeneous Polyoxometalate Based Mesoporous Catalysts for Hydrogen Peroxide Mediated Oxidation Reactions
Vasylyev, Maxym V.,Neumann, Ronny
, p. 884 - 890 (2007/10/03)
Inorganic-organic hybrid mesoporous materials were prepared by cocrystallization of a "sandwich" type polyoxometalate, [ZnWZn 2(H2O)2(ZnW9O34) 2]12-, and branched tripodal organic polyammonium salts, tris[2-(trimethylammonium)ethyl]-1,3,5-benzenetricarboxylate or 1,3,5-tris[4-(N,N,N-trimethylammonium-ethylcarboxyl)phenyl]benzene trications. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed formation of three-dimensional perforated coral-shaped amorphous materials with the organic cations surrounding polyoxometalate anions. N 2 sorption analysis showed that the hybrid materials have a BET surface area of ～30-50 m2 g-1 and an average pore diameter of 36 A leading to the classification of these materials as mesoporous materials with moderate surface areas. These hybrid materials behaved as very effective and selective heterogeneous catalysts for the epoxidation of allylic alcohols and oxidation of secondary alcohols to ketones with hydrogen peroxide as oxidant. The activity and selectivity of the heterogeneous catalysts based on the hybrid materials was similar to those of homogeneous catalysts based on the same [ZnWZn2(H2O) 2(ZnW9O34)2]12- polyoxometalate.
Titanium-Catalyzed Diastereoselective Epoxidations of Ene Diols and Allylic Alcohols with β-Hydroperoxy Alcohols as Novel Oxygen Donors
Adam, Waldemar,Peters, Karl,Renz, Michael
, p. 3183 - 3189 (2007/10/03)
β-Hydroperoxy alcohols 1-4 serve as effective tridentate oxygen donors for the highly diastereo-selective, titanium-catalyzed epoxidation of ene diols 5a-e. Thus, in contrast to the bidentate tert-butyl hydroperoxide, the usual oxygen donor employed in Sharpless-type epoxidations and known to work poorly for polyhydroxy substrates, the tridentate β-hydroperoxy alcohols efficiently replace the tridentate epoxy diol products 6a-e in the titanium template and thereby the catalytic cycle is sustained by replenishing with efficacy the loaded complex necessary for the oxygen transfer. Irrespective of the substitution pattern of the double bond or the configuration (erythro versus threo) of the diol functionalities in the ene diol substrate, high diastereoselectivities are observed for the epoxy diol products. The high stereochemical control is due to the rigid transition state for the oxygen transfer, which is imposed by the multiple titanium-oxygen bonding and coordination in the titanium template. The observed erythro selectivity for the ene diol derives from the additional bonding of its homoallylic hydroxy group to the titanium center, which fixes the substrate conformation in such a way that the oxygen atom to be transferred approaches from the side of the allylic oxygen functionality (cf. loaded complex A). This additional binding of the bidentate ene diol in the titanium template is also manifested in the enhanced reactivity of the ene diol versus the monodentate allylic alcohols. Nevertheless, the less reactive allylic alcohols also display a high erythro selectivity, provided these monodentate substrates possess 1,2-allylic strain. For the first time a direct, diastereoselective, and catalytic epoxidation of ene diols has been made available for synthetic applications, without recourse to protection group methodology.
Solvent effects in the regio- and diastereselective epoxidations of acyclic allylic alcohols by dimethyldioxirane: hydrogen bonding as evidence for a dipolar transition state
Adam, Waldemar,Smerz, Alexander K.
, p. 3506 - 3510 (2007/10/03)
A mechanistically significant solvent effect is observed in the regioselectivity of the geraniol epoxidation by dimethyldioxirane.In hydrogen bonding solvents (MeOH), the 6,7-epoxide is preferred over the 2,3-epoxide (74:26), which reveals that the more nucleophilic 6,7 double bond (the 2,3 double bond is inductively deactivated by the allylic hydroxy group) is preferentially attacked by the electrophilic dimethyldioxirane.In MeOH, both regioisomeric dipolar transition states are equally well stabilized by interaction through intermolecular hydrogen bonding with solvent molecules.In the nonpolar CCl4, intramolecular hydrogen bonding with the allylic hydroxy functionality favors attack at the 2,3-double bond and proportionally more 2,3-epoxide is formed.Similarly, also the ?-facial selectivity in the dimethyldioxirane epoxidation of methyl-substituted cchiral acyclic allylic alcohols is controlled by intermolecular versus intramolecular hydrogen bonding.Thus, higher threo selectivities are obtained in the nonpolar CCl4 by stabilization of the diastereomeric transition state with minimal allylic strain through intramolecular hydrogen bonding with the allylic hydroxy group.The geometry of the dipolar transition state for the dimethyldioxirane epoxidations is similar to that of m-CPBA, but with apparently a slightly larger (ca. 130 deg) dihedral angle α to relieve 1,2-allylic strain.
