The design of new ligands and transition metal compounds for the oxidation of organic compounds

摘要

A review of metal-mediated epoxidation is given. Jacobsen's catalyst and theSharpless asymmetric epoxidation catalyst are discussed. The origins of enantioselectivityare explained using stereochemical models.Several new salen-type ligands were synthesized based on biphenol andbinaphthol. The synthesis of these ligands and their subsequent coordination totransition metals were described. The transition metal complexes were structurallycharacterized by X-ray diffraction of single crystals. The manganese (III) complexeswere evaluated for catalytic activity in epoxidation reactions. Despite the fact that thesemany of these complexes were optically active, little asymmetric induction was observedin any of the epoxidation reactions.The investigation of a soluble nickel salen complex for the epoxidation of olefinsled to the discovery of a new heterogeneous catalyst for the epoxidation of ?,?-unsaturated carboxylic acids. Nickel salen complexes, upon reaction with commercialbleach, yield a fine black powder, which we identified as nickel oxide hydroxide-aknown but poorly characterized nickel peroxide containing species. The reaction of an aqueous nickel (II) source with commercial bleach also yields nickel oxide hydroxide.This material was characterized by X-ray diffraction (XRD), transmission electronmicroscopy (TEM), differential scanning calorimetry (DSC) and thermal gravimetricanalysis (TGA). Extremely broad peaks in the X-ray diffraction pattern suggested thatthis material consisted of particles with a very small diameter and this was confirmed byTEM. This insoluble material was found to function as a heterogeneous catalyst for theepoxidation of ?,?-unsaturated carboxylic acids in the presence of sodium hypochlorite.The high activity of this catalyst in the epoxidation of certain olefins is due in part to itssmall particle size, which increases the overall surface area of this heterogeneouscatalyst. Large particles of nickel oxide hydroxide were prepared and the catalyticactivity was comparatively less.The oxidation of several other organic substrates was also explored using thiscatalyst. Both primary and secondary alcohols can be oxidized with our nickel-basedsystem. Primary alcohols go through an aldehyde intermediate which is then in turnoxidized to the carboxylic acid.