Catherine M. Mitchell

CHEMOSELECTIVE METHYLTRIOXORHENIUM(VII)-CATALYZED SULFOXIDATIONS WITH HYDROGEN PEROXIDE

Abstract The selective oxidation of sulfides to sulfoxides by hydrogen peroxide under methyltrioxorhenium(VII) catalysis was examined; a high selectivity of sulfoxide over sulfone was achieved, except in the presence of water, which enhanced sulfone formation.

Diastereoselective epoxidation of allylic alcohols with hydrogen peroxide catalyzed by titanium-containing zeolites or methyltrioxorhenium versus stoichiometric oxidation with dimethyldioxirane: Clues on the active species in the zeolite lattice

Abstract Chiral, acyclic allylic alcohols 1 are epoxidized chemoselectively to the epoxy alcohols 2 by hydrogen peroxide, catalyzed by titanium-containing zeolites (TS-1, Ti-beta). For substrates with 1,3-allylic (A 1,3 ) strain, a high diastereoselectivity is observed with preference for the threo isomer, while derivatives with 1,2-allylic (A 1,2 ) strain or no allylic strain give a low threo diastereomeric excess. Comparison of the diastereomeric ratios of the titanium-containing zeolites with those for meta -chloroperbenzoic acid shows a good correspondence which suggests that the active species for the oxygen transfer in the epoxidations for zeolites is peracid- rather than peroxo-type. Comparison of the diastereomeric ratios achieved with the three-membered ring peroxide oxidants dimethyldioxirane and MTO/UHP (metal peroxo complex) disfavor the peroxo species since significantly lower thrio diastereoselectivities for substrates with 1,3-allylic strain were obtained. Direct coordination of the allylic alcohol through a metal alcoholate bond is unlikely because of the different diastereomeric ratios obtained for the heterogeneous and homogeneous titanium species with allylic alcohols that possess 1,2-allylic strain. Moreover, the number of coordination sites at the titanium atom in the zeolite framework is limited for steric reasons and the constrained space around the active center in the zeolite lattice presents severe geometrical problems for the stereoelectronically controlled linear S N 2-type alignment of the oxygen donor (metal-activated peroxide bond) and the acceptor (metal-alcoholate-bonded substrate) during the epoxidation process.

Titanium-catalysed oxidation of thianthrene 5-oxide in heterogeneous (zeolites) versus homogeneous media : electrophilic versus template-mediated oxygen transfer

Abstract Large differences in the ratio of sulfide versus sulfoxide oxidation of thianthrene 5-oxide ( SSO ) have been observed for heterogeneous versus homogeneous Ti IV catalysts; whereas the Ti-doped zeolites (heterogeneous) oxidise by means of an electrophilic species, sulfoxide coordination (template effect) for Ti(O i Pr) 4 (homogeneous) masks the true electrophilic character of the metal oxidant.

Structure, Reactivity, and Selectivity of Metal-Peroxo Complexes Versus Dioxiranes

Dioxiranes, MTO/H2O2, and MTO/UHP are becoming increasingly important as efficient and selective oxidants in organic chemistry and numerous publications have appeared on each of them. Here we present a comprehensive review, comparing the chemistry of these reagents with particular emphasis on the oxidation of π bonds, heteroatoms, and σ bonds. We will discuss structural aspects and on this basis, will outline the reactivity and selectivity of these oxidants, along with the mechanistic aspects of the oxygen-transfer process.

The selective catalytic oxidation of silanes to silanols with H2O2 activated by the Ti-beta zeolite

Ti-beta catalyses the oxidation of small- and medium-sized silanes to the corresponding silanols by aqueous (30%) H2O2 as oxygen donor with high conversions and excellent selectivity (no disiloxane).