22520-29-2Relevant articles and documents
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.
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.
