Dendritic Fluoroalcohols as Catalysts for Alkene Epoxidation with Hydrogen Peroxide

摘要

Fluorinated alcohols such as 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) are known to enhance the rate and selectivity of various reactions involving positively or partially positively charged transition states. High ionizing power, strong hydrogen-bond donor ability, and low nucleophilicity account for the observed effects. Quite remarkable accelerations (up to ca. 10~5) have been achieved by using TFE-and in particular HFIP-as the solvent in the epoxidation of olefins with aqueous hydrogen peroxide used as the terminal oxidant. The same holds for certain Baeyer-Villiger-type oxidations of ketones with aqueous H2O2, which proceed by cationic rearrangements of peroxidic ketone-H2O2 adducts. In the oxidation of thioethers with aqueous H2O2, fluoroalcohol solvents provide remarkable selectivities for sulfoxide formation, with basically no overoxidation to sulfones.